Effect of dissolved oxygen and nutrient levels on heavy metal contents and fractions in river surface sediments

Distribution, ecological risk, and sediment-influencing mechanisms of heavy metals in surface sediments along the intertidal gradient in typical mangroves in Hainan, China

The relative importance of each sediment physicochemical property to sediment heavy-metal (HM) contents has not yet been quantitatively evaluated. Differences in the HM contents of mangrove surface sediments among the high, middle, and low intertidal zones, and their quantitative relationships to sediment physicochemical properties, were investigated in Dongzhaigang and Qinglan Harbor reserves, Hainan, China. In both reserves, the Cu and Ni concentrations increased significantly from the low to high intertidal zones; the patterns of change in the Mn and Pb contents were opposite in the two reserves. The Cr concentration was significantly lower and the Pb concentration was significantly higher in the dry season than in the wet season. Ecological risks of HM were higher in Dongzhaigang than in Qinglan Harbor. Regression and redundancy (hierarchical partitioning) analyses showed that the sediment total sulfur, nitrogen and potassium contents and pH were key factors affecting the HM contents of mangrove surface sediments.

Decomposition of waterside plants greatly affects the transformation and mobility of sedimentary antimony in water-sediment systems after emergency treatment: A microcosm study

Polyferric sulfate (PFS) coagulation has proven to be effective in addressing antimony (Sb) water pollution accidents; however, the impact of waterside plant decomposition on its effectiveness has not been adequately elucidated. This study investigated the effects of Alternanthera philoxeroides (AP) and Digitaria sanguinalis (DS) decomposition on Sb cycling after PFS treatment. Without plant decomposition, the Fe(OH)3 hydrolysate-associated Sb remained stable, and the sediment continued to exhibit Sb sink properties. Plant residue decomposition facilitated sedimentary Sb release, and DS decomposition had a greater impact than AP decomposition. The strong decomposition phases triggered abiotic/biotic reduction processes, leading to Fe(OH)3 dissolution and subsequent Sb(V) release. Concurrently, sulfate reduction and dissolved organic matter (DOM) release regulated Sb mobility. In addition, Sb(V) reduction occurred, and Sb(III) was elevated in the overlying water. The Sb(III) levels gradually decreased during the later aerobic stages, however, did not completely disappear within a short timeframe. Furthermore, the role of the sediment as an Sb sink was significantly hindered, maintaining relatively high levels of dissolved Sb. Sedimentary Sb speciation analysis revealed that plant decomposition induced a shift in Fe-oxyhydroxide-bound Sb to more bioavailable and stable fractions. Our results indicate that plant residue decomposition easily deteriorates PFS efficiency and increases the risk of secondary Sb pollution in water-sediment systems.

Adsorption and distribution of heavy metals in aquatic environments: The role of colloids and effects of environmental factors

The study investigated the distributions of heavy metals (Cd, Cr, Cu, Mn, and Pb) between dissolved fraction (<0.7 µm) and particles (>0.7 µm) during the adsorption process. The dissolved fraction was further separated into truly dissolved (<3 kDa) and colloidal (3 kDa-0.7 µm) fractions. Significant metal adsorption occurred on the colloids, resulting in their aggregation into particles, which in turn influenced the particle adsorption kinetics. Colloids could either accelerate or inhibit the transformation of metal ions into particulates, depending on their stability. Competitive metals for colloids (Pb and Cr) were more susceptible to the effects of colloids than other elements. DOM was the predominant environmental factor influencing colloid behavior. The XDLVO theory showed that DOM enhanced the negative charge of colloids and made the colloid surface more hydrophilic, inhibiting the aggregation of colloids. DOM resulted in substantial increases in the concentrations of colloidal Pb and Cr from 0.31 μg/L and 4.58 μg/L to 20.52 μg/L and 43.51 μg/L, respectively, whereas the increment for less competitive metals (Cd and Mn) was smaller. These findings suggest that the distribution of heavy metals is influenced not only by adsorption from particles and ions but also by the complex dynamics of colloids.

Hydrodynamics and dissolved organic matter components shaped the fate of dissolved heavy metals in an intensely anthropogenically disturbed estuary

Anthropogenic activities and natural erosion caused abundant influx of heavy metals (HMs) and organic matter (OM) into estuaries characterized by the dynamic environments governed by tidal action and river flow. Similarities and differences in the fate of HM and OM as well as the influences of OM on HMs remain incomplete in estuaries with seasonal human activity and hydrodynamic force. To address this gap, dissolved HMs (dHMs) and fluorescence dissolved OM (FDOM) were investigated in the Pearl River Estuary, a highly seasonally anthropogenic and dynamic estuary. It aimed to elucidate the effects of hydrodynamic conditions and DOM on the seasonal fate of dHMs via the multivariate statistical methods. Our findings indicated dHMs and FDOM exhibited consistently higher levels in the upper estuarine and coastal waters in both seasons, predominantly controlled by the terrestrial/anthropogenic discharge. In the wet season, dHMs and humic-like substances (HULIS) were positively correlated, showing that dHMs readily combined with HULIS. This association led to a synchronous decrease offshore along the axis of the estuary and the transport following the river plume in the surface affected by the salt wedge. Contrarily, dHMs were prone to complex with protein-like components impacted by the hydrodynamics during the dry season. Principal component analysis (PCA) results revealed the terrestrial/anthropogenic inputs and the fresh-seawater mixing process were the most crucial factors responsible for the fate of dHM in wet and dry seasons, respectively, with DOM identified as a secondary but significant influencing factor in both seasons. This study holds significance in providing valuable insights into the migration, transformation, the ultimate fate of dHMs in anthropogenically influenced estuaries, as well as the intricate dynamics governing coastal ecosystems.

Land use/cover drive functional patterns of bacterial communities in sediments of a subtropical river, China

The bacterial community in sediment serves as an important indicator for assessing the environmental health of river ecosystems. However, the response of bacterial community structure and function in river basin sediment to different land use/cover changes has not been widely studied. To characterize changes in the structure, composition, and function of bacterial communities under different types of land use/cover, we studied the bacterial communities and physicochemical properties of the surface sediments of rivers. Surface sediment in cropland and built-up areas was moderately polluted with cadmium and had high nitrogen and phosphorus levels, which disrupted the stability of bacterial communities. Significant differences in the α-diversity of bacterial communities were observed among different types of land use/cover. Bacterial α-diversity and energy sources were greater in woodlands than in cropland and built-up areas. The functional patterns of bacterial communities were shown that phosphorus levels and abundances of pathogenic bacteria and parasites were higher in cropland than in the other land use/cover types; Urban activities have resulted in the loss of the denitrification function and the accumulation of nitrogen in built-up areas, and bacteria in forested and agricultural areas play an important role in nitrogen degradation. Differences in heavy metal and nutrient inputs driven by land use/cover result in variation in the composition, structure, and function of bacterial communities.

Effect of pH, Temperature, and Salinity Levels on Heavy Metal Fraction in Lake Sediments

Heavy metals (HMs) in aquatic environments are characterized by high toxicity, a propensity for bioaccumulation, and non-degradability, and pose significant risks to biological communities. Previous studies of HMs in lakes have shown that the physical and chemical characteristics of the lake water may control both the migration of HMs in the sediments and the concentration of heavy metals in the lake water. In fact, the change in aquatic environments changes the heavy metal fraction in the sediment, which controls the release of HMs. In this paper, we investigated the effects of the pH, temperature, and salinity levels of overlying water on the chemical fraction of Cu and Zn in Wuliangsuhai Lake surface sediments. The results show that lower water pH and higher water salinity and temperature could increase Cu and Zn release from the sediment. An increase in pH led to changes in the speciation of solid fractions of Zn, namely increases in the residual fraction and decreases in the organic matter and sulfide, whereas acid-extractable and Fe-Mn oxide fractions remained largely the same. Increases in temperature and salinity led to opposite changes in the speciation of solid fractions, namely decreases in the residual fraction and increases in the organic matter and sulfide and Fe-Mn oxide fractions, whereas acid-extractable fractions remained largely the same. The effect of pH, temperature, and salinity on Cu fractions in the solids was much smaller. According to the ratio of the secondary phase to the primary phase (RSP), acidic, high-temperature, and high-salt conditions increase the release risks of Zn. Changes in water temperature have the greatest influence on the risk of Zn and Cu release from sediments, followed by the influence of salinity changes.

A microcosm evaluation of metal cycling in an urbanized contaminated estuary varying with oxic-hypoxic-anoxic-reoxic transition: Behavior, fluxes, and mechanism

Water hypoxia and metal pollution are commonly co-existed in urbanized estuaries. This study focuses on the effect of an extended dissolved oxygen (DO) full-life dynamics (86 days) on metal behavior across the sediment-water interface through laboratory microcosms from two typical zones in Pearl River Estuary. Combining our time-series results of concentrations and fluxes, it showed that Co, Ni, and Zn consistently presented a release-precipitation-release trajectory with an oxic-hypoxic-anoxic-reoxic transition, characterized with highly variable behavior in the hypoxic-anoxic hotmoments. In parallel, changing DO dynamics significantly activated a repartitioning process of Co, Ni, and Zn among several species and elevated their risk in sediments, promoting the formation of more labile species in the 0-10 mm hotspots, where metals sensitively responded. Over DO transition, metal cycling was tightly co-related with Fe, Mn, and S elements. It was found that Mn was dominated in low oxygen-hypoxic period, but switched to S and Fe in anoxic stage, limiting sustained metal liberation to overlying water. Enlarging this experiment to practice, released Zn fluxes from sediments in hypoxic summer could contribute about ∼2.0% to their stocks in water column, while increase to 20% (1 m bottom water) in highly-stratified zones. This study has certain significance in understanding the long-term metal behavior and fate in estuarine regions, even lakes and reservoirs.

Heavy metals in centralized drinking water sources of the Yangtze River: A comprehensive study from a basin-wide perspective

Water quality in the Yangtze River Basin (YRB) has received considerable attention because it supplies water to 400 million people. However, the trends, sources, and risks associated with heavy metals (HMs) in water of centralized drinking water sources (CDWSs) in the YRB region are not well understood due to the lack of high-frequency, large-scale monitoring data. Moreover, research on the factors affecting the transportation of HMs in natural water are limited, all of which significantly reduce the effectiveness of CDWSs management. Therefore, this study utilized data on 11 HMs and water quality from 114 CDWSs, covering 71 prefecture-level cities (PLC) in 15 provinces (cities), to map unprecedented geospatial distribution of HMs in the YRB region and examine their concentrations in relation to water chemistry parameters. The findings revealed that the frequency of detection (FOD) of 11 HMs ranged from 28.59% (Hg) to 99.64% (Ba). The mean concentrations are ranked as follows: Ba (40.775 μg/L) > B (21.866 μg/L) > Zn (5.133 μg/L) > V (2.668 μg/L) > Cu (2.049 μg/L) > As (1.989 μg/L) > Mo (1.505 μg/L) > Ni (1.108 μg/L) > Sb (0.613 μg/L) > Pb (0.553 μg/L) > Hg (0.002 μg/L). Concentrations of Zn, As, Hg, Pb, Mo, Sb, Ni, and Ba exhibited decreasing trends from 2018 to 2022. Human activities, including industrial and agricultural production, have led to higher pollution levels in the midstream and downstream of the river than in its upstream. Additionally, the high concentrations of Ba and B are influenced by natural geological factors. Anion concentrations and nutrient levels, play a significant role in the transport of HMs in water. Probabilistic health risk assessment indicates that As, Ba, and Sb pose a potential carcinogenic risk. Additionally, non-carcinogenic risk to children under extreme conditions should also be considered.

Predictions of heavy metal concentrations by physiochemical water quality parameters in coastal areas of Yangtze river estuary

Due to the degradation-resistant and strong toxicity, heavy metals pose a serious threat to the safety of water environment and aquatic ecology. Rapid acquisition and prediction of heavy metal concentrations are of paramount importance for water resource management and environmental preservation. In this study, heavy metal concentrations (Cr, Ni, Cu, Pb, Zn, Cd) and physicochemical parameters of water quality including Temperature (Temp), pH, Oxygen redox potential (ORP), Dissolved oxygen (DO), Electrical conductivity (EC), Electrical resistivity (RES), Total dissolved solids (TDS), Salinity (SAL), Cyanobacteria (BGA-PE), and turbidity (NTU) were measured at seven stations in the Yangtze river estuary. Principal Component Analysis (PCA) and Spearman correlation analysis were employed to analyze the main factors and sources of heavy metals. Results of PCA revealed that the main sources of Cr, Ni, Zn, and Cd were steel industry wastewater, domestic and industrial sewage, whereas shipping and vessel emissions were typically considered sources of Pb and Cu. Spearman correlation analysis identified Temp, pH, ORP, EC, RES, TDS, and SAL as the key physicochemical parameters of water quality, exhibiting the strongest correlation with heavy metal concentrations in sediment and water samples. Based on these results, multiple linear regression as well as non-linear models (SVM and RF) were constructed for predicting heavy metal concentrations. The results showed that the results of the nonlinear model were more suitable for predicting the concentrations of most heavy metals than the linear model, with average R values of the SVM test set and RF test set being 0.83 and 0.90. The RF model showed better applicability for simulating the concentration of heavy metals along the Yangtze river estuary. It was demonstrated that non-linear research methods provided efficient and accurate predictions of heavy metal concentrations in a simple and rapid manner, thereby offering decision-making support for watershed managers.

Pollutant bioaccumulation in sentinel fish chronically exposed in Iguaçu river reservoirs (Southern Brazil) and human health risk of fish consumption

Bioaccumulation studies in fish mark the initial phase of assessing the risk of chemical exposure to biota and human populations. The Iguaçu River boasting a diverse endemic ichthyofauna, is grappling with the repercussions of human activities. This study delved into the bioaccumulation of micropollutants, the early-warning effects on Rhamdia quelen and Oreochomis niloticus in the Segredo Reservoir (HRS) and the potential risk of human exposure. Two groups of caged fish in three sites of the reservoir were exposed during the autumn-winter and spring-summer, while a third group (O. niloticus) underwent a twelve-month exposure, and inorganic and organic chemicals analysis in water, sediment, and biota. Additionally, metallothionein expression and genotoxicity were employed as biomarkers. PAHs, PCBs, Al, Cu, Fe, and As in water and DDTs, Cu, Zn, and As in sediment surpassed the thresholds set by Brazilian regulations, where DDT exhibited bioaccumulation in muscle, alongside metals in liver, kidney, gills, and muscle tissues. R. quelen showed metallothionein expression whereas DNA damage and NMA frequencies were elevated in target organs and in brain and erythrocytes of O. niloticus during summer. In this species the DNA damage in liver was remarkable after twelve months. Target Hazard Quotients and Cancer Risk values shedding light on the vulnerability of both children and adults. The reservoir's conditions led to heightened sensitivity to micropollutants for R. quelen species. The data presented herein provides decision-makers with pertinent insights to facilitate effective management and conservation initiatives within the Iguaçu Basin.

The decomposition of algae has a greater impact on heavy metal transformation in freshwater lake sediments than that of macrophytes

Heavy metal (HM) pollution is a major concern in freshwater ecosystem management. The different types of endogenous organic matter and the way their decomposition affects HM transformation in freshwater lakes is not well understood. An ex situ mesocosm study was conducted to compare HM transformation in sediments during anaerobic decomposition of cyanobacterial bloom biomass (CBB) and submerged cyanobacterial vegetation in Lake Taihu, known as Potamogeton malaianus (PM). Microbial community structures were examined through Illumina sequencing of 16S rDNA. Results indicate that Zn had a remarkably higher amount of potential mobile fraction than other heavy metals (Cr, Pb, Cu, Ni, and Cd) detected in sediments, especially in sediments collected from CBB-dominated areas (approximately 150 mg kg-1). CBB decomposition has caused a significant increase in exchangeable Zn content in sediments and a decrease in reducible Zn that was three times greater than PM decomposition. Additionally, oxidizable Zn content declined during CBB decomposition but increased during PM decomposition. Furthermore, the relative abundance of the main fermentative bacteria and some sulfate-reducing bacteria genera (e.g., Desulfomicrobium) were significantly associated with the HM content of exchangeable and reducible fractions during CBB decomposition. Overall, the findings indicate that Zn is more susceptible to endogenous organic matter decomposition than other metals in freshwater lakes, and the impacts of CBB decomposition on the transformation of heavy metals in sediment are greater than that of submerged macrophyte decomposition.

Influence of mineralized organic carbon in marine sediments on ecological heavy metal risk: Bohai Bay case study

The organic matter in sediments can mineralize over time, which impacts the morphology of the heavy metals therein, which in turn affects the assessment of the risks posed by heavy metals. We used the sediments of Bohai Bay as the study object and analyzed the effects of different organic carbon mineralization levels on the concentrations of heavy metals (Cr, Pb, Cu, Zn, and Cd) using water extraction and potassium permanganate oxidation. The mean concentrations of Cd, Pb, Cu, and Zn in Bohai Bay were within the limits recommended by the World Health Organization. The proportions of the active and inert organic carbon fractions were 61.72% and 32.94%, respectively. Organic carbon mineralization most strongly impacted Cd and Pb levels, with releases accounting for 47.92% and 25.75%, respectively, of the oxidizable fractions. The release of all heavy metals, except for Cr, increased with increases in organic carbon mineralization, and heavy metals were released at a maximum rate of 12.94% when the organic carbon was highly mineralized, whereas Cr was released at a maximum of 0.023% during the first stage of organic carbon mineralization. In terms of spatial distribution, the concentration of mineralizable organic carbon in the sediments of the estuaries was substantially higher than that in other marine areas. Estuary sediments were more easily affected by organic carbon mineralization; therefore, the heavy metals in the oxidizable fraction of the estuarine region were more easily transformed into unstable heavy metal forms, posing high risk levels. Therefore, this study highlights the effects of organic carbon mineralization on heavy metal morphology and stability, when evaluating the ecological risk of heavy metals in marine sediments.

Nutrients in overlying water affect the environmental behavior of heavy metals in coastal sediments

Excessive nutrients in aquatic ecosystems are the main driving factors for eutrophication and water quality deterioration. However, the influence of nutrients in overlying water on sediment heavy metals is not well understood. In this study, the effects of nitrate nitrogen (NO3-N) addition and phosphate addition in the overlying water on the environmental behaviors of chromium (Cr), copper (Cu), and cadmium (Cd) in coastal river sediments were investigated. Fresh estuary sediments and synthetic saltwater were used in microcosm studies conducted for 13 d. To determine the biological effect, unsterilized and sterilized treatments were considered. The results showed that the diffusion of Cr and Cu was inhibited in the unsterilized treatments with increased NO3-N. However, under the NO3-N sterilized treatments, Cr and Cu concentrations in the overlying water increased. This was mostly related to changes in the microbial regulation of dissolved organic carbon and pH in the unsterilized treatments. Further, in the unsterilized treatments, NO3-N addition considerably increased the concentrations of the acid-soluble (Cr, Cu, and Cd increased by 5%-8%, 29%-41%, and 31%-42%, respectively) and oxidizable (Cr, Cu, and Cd increased by 10%, 5%, and 14%, respectively) fractions. Additionally, compared with that in the unsterilized treatments, Cu and Cd concentrations in P-3 treatments decreased by 7% and 63%, respectively. By producing stable metal ions, microorganisms reduced the amount of unstable heavy metals in the sediment and heavy metal concentration in the overlying water, by considerably enhancing the binding ability of phosphate and heavy metal ions. This study provides a theoretical basis for investigating the coupling mechanisms between heavy metals and nutrients.

Remediation of Pb(II) and Cd(II) in polluted waters with calcium thioglycolate-modified straw biochar

The pollution of water bodies by heavy metals (HMs) such as Pb(II) and Cd(II) poses a serious environmental risk. Herein, rice straw biochar (RBC) modified with calcium thioglycolate was used to remove Pb(II) and Cd(II) from aqueous solutions. The adsorption performance of the modified biochar was investigated via adsorption kinetics and isotherm model fitting. Furthermore, scanning electron microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to elucidate the modification and adsorption mechanisms. The results showed that the modification process loaded sulfur-containing functional groups, calcium carbonate, and calcium oxalate crystals on the biochar surface, considerably enhancing its complexation performance and ion-exchange capacity. The equilibrium adsorption amounts for Pb(II) and Cd(II) reached 124.92 and 65.44 mg g-1 in unary systems, respectively; they reached 121.34 and 39.43 mg g-1 in a binary Pb(II) and Cd(II), respectively. Moreover, the optimal adsorption conditions were as follows: pH = 6, temperature = 25 °C, dosage = 0.8 g L-1, and contact time = 2 h. In the binary Pb(II) and Cd(II) system, the adsorption process obeyed the Langmuir competitive adsorption model, which means that one adsorption site on the modified biochar was effective for only one heavy-metal ion, and the modified biochar was more selective for Pb(II) than for Cd(II). The adsorption mechanism, which was dominated by chemisorption, mainly involved complexation, precipitation, ion exchange, and cation-π interactions. Meanwhile, adsorption and desorption experiments indicated that the modified biochar exhibited satisfactory recycling performance, demonstrating its feasibility as an inexpensive and efficient heavy-metal adsorbent for polluted water.

Bioaccumulation and Health Risk Assessment of Metals in Small-Sized Fish (Rhodeus sinensis, Ctenogobius giurinus) and Mussel (Cristaria plicata) from a River Reservoir, Southwest China

In order to assess the bioaccumulation and health risk of metals in a river reservoir, concentrations of copper (Cu), zinc (Zn), lead (Pb), cadmium (Cd), arsenic (As), and mercury (Hg) in the water, sediments, two small-sized fish, and a freshwater mussel from the Zhoubai reservoir were examined. The results indicated that all of these metals conform with class one of environmental quality standards for surface water (State Environmental Protection Administration of China, GB 3838-2002). There were no significant differences for total metal concentrations in sediment between the three sampling sites (p > 0.05), but the bioavailable concentrations in S3 were the lowest. The Cd was dominated with exchangeable fraction and showed considerable risk. All metal concentrations except for Pb in Rhodeus sinensis were significantly higher than those in Ctenogobius giurinus (p < 0.05). The metal concentrations in Cristaria plicata showed a similar pattern of bioavailable metals in sediment, indicating that the metal concentrations in aquatic organisms were determined by the bioavailable forms of metals. Negative correlations were observed between the size of fish and concentrations of Cu, Zn, Pb, Cd, and As. However, significant positive correlations were found between the size of mussel and concentrations of Cd (p < 0.01), As (p < 0.05), and Hg (p < 0.01). Zn had the highest BCF values in fish and mussel. The aquatic organisms showed lower ability of metal bioaccumulation from the sediment. Low values of target hazard quotient (THQ), hazard index (HI), and carcinogenic risk (CR) indicated that these metals do not pose a health risk to public through fish and mussel consumption in this study area.

Roles of dissolved organic matter (DOM) in shaping the distribution pattern of heavy metal in the Yangtze River

Dissolved organic matter (DOM) strongly influences the solid-liquid partitioning and migration characteristics of heavy metals, yet little is known about the metal distribution and risk with the participation of DOM in large riverine systems. This study investigated the spatiotemporal distribution of 14 heavy metals and DOM along the entire Yangtze River (over 6000 km), and highlighted the critical roles of DOM in regulating the environmental behaviors of heavy metals. Significant spatial variations of metal contents were observed, with the river source and lower reach remarkably different from the upper-middle reaches. Heavy metals in the Yangtze River were mainly from the natural sources with minor anthropogenic disturbance. We found DOM could promote the conversion of metals from solid to liquid phase and DOM with higher aromaticity showed higher metal affinities. Although low ecological risks were observed in the Yangtze River, potential risks of metal leaching warrant attention, especially for As, Cd and Sb in the middle-lower reaches with higher DOM content and aromaticity. This study established a source-to-sea investigative approach to evaluate the influences of DOM features on metal partitioning, which is crucial for the risk control and sustainable management of large rivers.

Nutrient and heavy metal dynamics in the coastal waters of St. Martin's island in the Bay of Bengal

Seasonal variation observations were conducted in the coastal waters of St. Martin's Island in the Bay of Bengal to examine the influence of physical processes and the distribution pattern of nutrients in the ocean water. Pollution evaluation indices, health index and statistical techniques were incorporated to assess the heavy metal contamination. Two seasons, cool dry winter and pre-monsoon hot, were considered for sampling from 12 stations around the island. The Cool dry winter season has higher nutrient concentrations than the Pre-monsoon Hot season. The concentration of nutrients appeared as follows: Silicate > Nitrate > Ammonia > Phosphate > Nitrite. PCA and Pearson's Correlation showed that fresh water from nearby rivers, deep water upwelling, and, in some situations, modest anthropogenic sources are crucial. Hence, low DO and phosphate levels during the pre-monsoon hot season indicate there is a planktonic process like photosynthesis prevailing. The island's north-western and south-eastern regions have higher nutrient concentrations, which may be seasonal and due to wind action. Pb, Cu, As, Cr, Cd, and Zn were also considered to comprehend the island's geo-chemical perspectives and ecological and human health risks. The Pre-monsoon Heavy Metal Pollution Index (HPI) and Heavy Metal Evaluation Index (HEI) demonstrated that some places are much higher than the threshold limit, even though no significantly higher value was detected in the cool winter season. The Nemerow Index, the Total Ecological Risk Index (TERI), indicated that heavy metal contamination was severe to moderate and low to moderate. Finally, Pearson's correlation showed the association between physical and chemical characteristics, similar to PCA and Pearson's correlation for nutrients and heavy metals. Thus, this research may help shed light on the state of the seas around St. Martin's Island. This study may also provide explicit insights for the authority to take the necessary measures to preserve marine ecology and the associated terrestrial ecosystem.

Comparison of spatiotemporal burial and contamination of heavy metals in core sediments of two plateau lakes with contrasting environments: implication for anthropogenic-driven processes

Investigating the impacts of climatic factors and human activities on sedimentary records of heavy metal (HM) contamination in lakes is essential for decision-making in global environmental monitoring and assessment. Spatiotemporal distributions of grain size (GS) and HM (Al, Cr, Mn, Ni, Cu, Zn, As, and Pb) concentrations have been conducted in core sediments that are collected from two adjacent plateau fault-bound lakes in southwest China with contrasting environments, i.e., deep oligotrophic Lake Fuxian (FX) and shallow hypertrophic Lake Xingyun (XY). Results showed that the average value of d50 in FX (4.61 μm) was lower than that in XY (8.35 μm), but the average concentrations of HMs (except Cr and Mn) in XY were higher than those in FX. Heavy metal burial rates (HMBR) were mainly controlled by sediment accumulation rates (SARs) rather than HM concentrations. The correlation coefficients between GS and HM concentrations became strong as the increasing water depths were associated with a stable sedimentary environment. Time-integrated enrichment factors (EF) and source identification of HMs between FX and XY represented that Cr, Ni, and Cu originated from natural sources but Mn, Zn, As, and Pb from anthropogenic sources, respectively. Regardless of FX and XY, the transition times of HMs from natural to anthropogenic sources occurred in the mid-1960s. Comparison of qualification impacts of climatic factors and human-induced factors on increased anthropogenic HMBR by the partial least squares path modeling (PLS-PM) implied that socio-economic activities, such as population density (PD) and gross domestic product (GDP), provided higher contributors to increased anthropogenic HMBR in XY (0.23/0.71) than FX (0.11/0.18). The comparative results of this study provided new insights into environmental monitoring and management of HM contamination for adjacent lakes with contrasting environments.

Effects of sediment resuspension and changes in water nutrient concentrations on the remobilization of lead from contaminated sediments in Klity Creek, Thailand

As Pb-containing sediments in Klity Creek have had negative impacts on the area for more than 20 years, the Supreme Court ordered the Pollution Control Department (PCD) of Thailand to remediate the site. In response to the court order, the PCD decided to reduce the contamination level by dredging the sediments of the creek. Therefore, this study is the first investigation to be conducted on the coupled effects of sediment resuspension caused by dredging and changes in water nutrient concentrations upon the remobilization of Pb from sediments into the water column. The Pb concentrations and speciation in both the water and sediments collected from upstream and downstream regions of the contaminated area were determined. The results showed that the total Pb concentrations in the water taken from all sampling sites in both the dry and wet seasons were lower than the national standard (50 μg/L), and a very low mobility index was found for Pb. The highest total Pb concentration in the sediments (6930 mg/kg) from the downstream site was 23.7- to 30.4-fold greater than those of the sediments collected from the upstream site. The predominant Pb species (organic and residual Pb fractions) in the sediments collected during the dry season were identified. However, carbonate- and Fe-Mn oxide-bound Pb fractions were mainly found in the sediments collected in the wet season. The diffusive gradients in thin films (DGT)-labile Pb concentrations, which reached 2.1 mg/L, indicated potential toxicity to aquatic organisms. A total of nine resuspension scenarios generalizing all changes in water nutrient concentrations in addition to sediment resuspension due to dredging were constructed. The results confirmed that sediment resuspension alone could remobilize Pb from the sediments into the water at levels from 0.06 to 16.9 μg/L. Sediment resuspension in water contaminated with 1 mg/L phosphate (PO₄^3-) led to the dissolution of 28.4-73.0 μg/L Pb in the water column. Nitrate (NO₃^-) did not significantly remobilize Pb from the sediments into the water. The high ionic strength and activity coefficient of PO₄^3- in the water were expected to cause the retention of dissolved Pb in the water and enhance the remobilization of Pb from the sediments due to the association of Pb with PO₄^3- in the water.

Improved Calculations of Heavy Metal Toxicity Coefficients for Evaluating Potential Ecological Risk in Sediments Based on Seven Major Chinese Water Systems

Several methods have been used to assess heavy metal contamination in sediments. However, an assessment that considers both composite heavy metal speciation and concentration is necessary to accurately study ecological risks. This study improved the potential ecological risk index method and calculated the toxicity coefficients of seven heavy metals: Arsenic (As), Cadmium (Cd), Chromium (Cr), Copper (Cu), Nickel (Ni), Lead (Pb), and Zinc (Zn). The newly calculated toxicity coefficients were validated by using previously published heavy metal distribution data of the Henan section of the Yellow River. The calculation procedure is based on the principle that the abundance of heavy metals in the environment and their bioavailable forms affect the toxicity of heavy metals. The toxicity coefficients for the seven heavy metals were calculated as follows: As = 10, Cd = 20, Cr = 5, Cu = 2, Ni = 5, Pb = 5, Zn = 1. Ecological risk assessment of the Henan section of the Yellow River using the improved toxicity coefficients revealed that the ecological risk of Cd and total heavy metals is higher than previous calculations, reaching the strength and moderate risk levels, respectively. The improved potential ecological risk index method is more sensitive to heavy metal pollution and thus provides a better indication of ecological risk. This is a necessary improvement to provide more accurate pollution assessments.

P/Pb transport at the interface of water and Al-substituted ferrihydrite: Effect of P/Pb loading sequence

Sediment mineral such as Al-substituted ferrihydrite plays a critical role for contaminant transport in the river systems. Heavy metals and nutrient pollutants often coexisted in the natural aquatic environment, and they may enter the river at different time frames, altering the fate and transport of each other subsequently discharged into the river. However, most studies focused on the simultaneous adsorption of co-existing pollutants instead of their loading sequence. In this study, the transport of P and Pb at the interface of Al-substituted ferrihydrite and water was investigated under different P and Pb loading sequences. The results showed that preloaded P provided additional adsorption sites for the following adsorption of Pb, with enhanced Pb adsorption amount and accelerated adsorption process. Moreover, Pb preferred to be bounded with the preloaded P to form P-O-Pb ternary complexes rather than directly reacted with Fe-OH. The formation of the ternary complexes effectively prevented the release of Pb once adsorbed. However, the adsorption of P was slightly affected by the preloaded Pb, and most of the P were adsorbed onto Al-substituted ferrihydrite directly with the formation of Fe/Al-O-P. Moreover, the release process of the preloaded Pb was significantly inhibited by the following adsorbed P due to the formation of Pb-O-P. Meanwhile, the release of P was not detected from all P and Pb loaded samples of different adding sequence due to the high affinity between P and the mineral. Thus, the transport of Pb at the interface of Al-substituted ferrihydrite was seriously influenced by the adding sequence of Pb and P, while the transport of P was not sensitive to the adding sequence. The results provided important information for the transport of heavy metal and nutrients in river system with different discharging sequence, and offered new insights to further understand the secondary pollution in multi-contaminated river.

Shifts in the bacterial community caused by combined pollutant loads in the North Canal River, China

A typical anthropogenically disturbed urban river polluted by a combination of conventional pollutants (nitrogen and phosphorus pollution) and heavy metals was investigated along a 238 km stretch. Changes in the bacterial community were evaluated using high-throughput sequencing, and the relationships between bacteria, heavy metals, and conventional pollutants were investigated. There was large spatial heterogeneity in the bacterial community along the river, and bacterial diversity in the upstream and midstream sections was much higher than in the downstream section. Heavy metals and conventional pollutants both exhibited close correlations with bacterial diversity and composition. For instance, potential fecal indicator bacteria, sewage indicator bacteria and pathogenic bacteria, such as Ruminococcus and Pseudomonas, were closely associated with Cu, Zn, and NH₄⁺-N. Rather than conventional pollutants, heavy metals were the main driving factors of the microbial community characteristics. These results confirm that bacterial communities play a crucial role in biogeochemical cycles. Therefore, heavy metals could be used as biomarkers of complex pollution to indicate the pollution status of riverine ecosystems and contribute to the restoration of habitats in anthropogenically disturbed urban rivers.

Saltwater intrusion regulates the distribution and partitioning of heavy metals in water in a dynamic estuary, South China

The mixing processes of fresh-salt water in estuarine and coastal regions have a substantial impact on the characteristics of heavy metals. A study was conducted in the Pearl River Estuary (PRE), located in South China, to examine the distribution and partitioning of heavy metals and the factors that influence their presence. Results showed that the hydrodynamic force, caused by the landward intrusion of the salt wedge, was the major contributor to the aggregation of heavy metals in the northern and western PRE. Conversely, metals were diffused seaward at lower concentrations along the plume flow in surface water. The study found that some metals, including Fe, Mn, Zn and Pb, were significantly higher in surface water than in bottom water in eastern waters, but the reverse was true in the southern offshore area, where limited mixing hindered the vertical transfer of metals in the water column. The partitioning coefficients (K_D) of metals varied, with Fe exhibiting the highest K_D (1038 ± 1093 L/g), followed by Zn (579 ± 482 L/g) and Mn (216 ± 224). The highest K_D values of metals in surface water were observed in the west coast, while the highest K_D in bottom water was found in eastern areas. Furthermore, re-suspension of sediment and the mixing of seawater and freshwater offshore, caused by seawater intrusion, resulted in the partitioning of Cu, Ni and Zn towards particulate phases in offshore waters. This study provides valuable insights into the migration and transformation of heavy metals in dynamic estuaries influenced by the interaction of freshwater and saltwater and highlights the importance of continued research in this field.

Comparative study on metal concentrations in water, sediments, and two fish species (Cyprinus carpio and Pelteobagrus fulvidraco) from the Wujiang River, China

In order to assess the metal pollution in the Wujiang River, concentrations of Cu, Zn, Fe, Mn, Pb, Cd, As, and Hg in the water, sediments, and two fish species Cyprinus carpio and Pelteobagrus fulvidraco from the middle reaches (Tuomugang, TMG) and lower reaches (Wulong, WL) of the Wujiang River were examined. The results indicated that all metal concentrations were lower than the values for grade one water quality according to the Environment Quality Standard for Surface Water of China (GB 3838-2002). The bioavailable fraction concentrations of Zn, Fe, Mn, Pb, and Hg in WL were significantly higher than those in TMG (p < 0.05), indicating that these metals in sediments of WL have higher bioavailability and mobility. The Cu, Zn, Fe, Pb, As, and Hg were mainly related to the residual fraction, while the Mn and Cd were mainly associated with the non-residual fraction. The risk assessment code (RAC) and the secondary phase to the primary phase (RSP) values indicated that Mn and Cd have a high risk of secondary release. The mean metal concentrations in the liver of the two fish species were higher than those in muscle. The higher metal concentrations of fish in WL suggested that bioaccumulation of metals in fish could be influenced by metal bioavailability. No identical relationships between metal concentrations and fish length were manifested in the present study. The values of target hazard quotient (THQ) and hazard index (HI), and carcinogenic risk (CR) of metals for the consumption of C. carpio and P. fulvidraco indicated that the anglers would likely not experience significant non-carcinogenic risk, but the carcinogenic risk of As cannot be ignored. Thanks to prohibited commercial fishing in the Wujiang River, the metal pollution will probably not pose a health risk to the general public for wild fish consumption.

Ecological and environmental risks of heavy metals in sediments in Dingzi Bay, South Yellow Sea

As a special geographical location between rivers and oceans, coastal estuaries always face severe heavy metal contaminations, especially in semi-closed bay. In this study, the spatial distribution, chemical fraction, ecological risks, and potential sources of heavy metals (Pb, Cr, Cu, As, Cd, Zn, and Ni) in surface sediments and sediment cores were investigated in Dingzi Bay, Shandong Peninsula. The I_geo values and modified potential ecological risk index (MRI) indicated that Cd and As presented high environmental risks in the surface and sediment cores. The high concentration sites were mainly located in the middle and the mouth of the Dingzi Bay. The source identification indicated that most heavy metals in surface sediments originated from shipping and aquaculture, while As and Ni from industrial pollution. The correlation coefficients showed that high proportion of fine particle, TN, TOC, TP, and AVS in surface sediments could significantly elevate the bioavailability of most heavy metals.

Evaluation of metal contamination in surface sediments and macroalgae in mangrove and port complex ecosystems on the Brazilian equatorial margin

This study evaluated metal contamination in surface sediments and macroalgae of mangroves and port complexes on the Brazilian equatorial margin. Samples were collected between August 2020 and February 2021 at seven points in a mangrove swamp under the influence of port activity and at two points without port activity. Metal concentrations in the macroalgae and sediments were determined using inductively coupled plasma‒optical emission spectrometry. All macroalgal species bioaccumulated metals, as demonstrated by their bioaccumulation factors. The geochemical contamination indices indicated that the estuarine complex was influenced by port activity as moderately contaminated by Pb, Cr, Mn, and Fe and considerably contaminated by Zn and Cu. The enrichment factor confirmed significant mineral enrichment of Zn and Cu in this environment. The concentrations of the metals in the sediment followed the order Fe > Mn > Cr > Zn > Cu > Pb at most sampling points. Cladophoropsis membranacea recorded the highest bioaccumulation values for Pb (0.44), Rhizoclonium africanum for Zn (1.08), Cr (0.55), and Fe (0.30), and Bostrychia radicans for Mn (2.22). The bioaccumulation pattern of metals in the most abundant macroalgal species followed the order Bostrychia radicans (Mn > Zn > Cu > Cr > Pb > Fe) and Rhizoclonium africanum (Zn > Mn > Cr > Cu > Pb > Fe).

Study on release and occurrence of typical metals in corrosion products of drinking water distribution systems under stagnation conditions

Metal contaminants in corrosion products of drinking water distribution systems (DWDS) can be released into potable water under specific conditions, thereby polluting drinking water and posing a health risk. Under stagnation conditions, the release characteristics, occurring forms, and environmental risks of ten metals were determined in loose and tubercle scale solids of an unlined cast iron pipe with a long service history, before and after immersion. Most Al, As, Cr, Fe, and V in corrosion scales existed in the residual fraction, with the released concentration and pollution risk being low. Since more than 59% of Ca in pipe scales existed in the exchangeable fraction, Ca release was high. Although the Pb and Cd content of corrosion solids was low, a high proportion of Pb and Cd was present in non-residual fractions with high mobility. Sudden severe Pb or Cd pollution events in DWDS could result in high pollution and environmental risk levels. The total content and released amount of Mn and Zn in corrosion scales were both high. Therefore, while special attention should be paid to Mn and Zn, Pb and Cd also present a high risk in pipe scales, despite their low concentrations. During stagnation immersion, metal release from powdered pipe scales occurred via the processes of mass release, re-adsorption into scales, and slow release until equilibrium was reached. The levels of metal re-adsorption into scales were much higher than the concentrations dissolved into bulk water. However, the amount of metal re-adsorption into tubercle scale blocks was less. Importantly, these findings highlight that during DWDS operation, the sudden release of metal pollutants caused by pipe scale breakage should be avoided.

The impact of river sand mining on remobilization of lead and cadmium in sediments - A case study of the Jialing River

Due to the fast pace of urbanization worldwide, industrial sand mining activities have imposed great pressure on the environment, and consequently, these activities have led to serious environmental problems in aquatic ecosystems. However, the current understanding of the effect of sand mining on heavy metal remobilization in river sediments remains incomplete. The present study employed sediment quality guidelines (SQGs) and the sequential extraction (SE) and diffusive gradients in thin films (DGT) techniques to comprehensively investigate the effect of sand mining on the remobilization process of heavy metals in the aquatic system of the Jialing River. The SQGs results indicated that stations (S1 to S4) with sand mining disturbance exhibited Pb and Cd accumulation in surface sediments. Both C_total-Pb (61.78-122.04 mg·kg^-1) and C_total-Cd (0.85-3.96 mg·kg^-1) were higher than CSQGI (60 mg·kg^-1 for Pb and 0.5 mg·kg^-1 for Cd) and TEC (35.8 mg·kg^-1 for Pb and 0.99 mg·kg^-1 for Cd) limitation in most of sand mining stations. Pb and Cd were mainly bounded in the acid-soluble/exchangeable fraction (F1) and oxidizable fraction (F3) of the surface sediments. At the four stations with sand mining disturbance, about 5-10 folds of DGT-labile Pb and Cd were released in deep sediments (-9 to -12 cm), and Pb and Cd exhibited a transport trend from the sediments into the overlying water, while the above phenomenon was not observed at the two stations without sand mining activities. Correlation analysis revealed that DGT-labile Pb and Cd were suitably correlated with the F1 and F3 fractions, indicating that the acid-soluble/exchangeable and oxidizable fractions were the main sources leading to Pb and Cd remobilization in the sediments. A potential mechanism explanation may be that (1) intense sediment stirring could result in remobilization of the weakly bound fraction, which is related to the contribution of the F1 fraction, and (2) Cd/Pb experienced a corelease process with sulfur due to O₂ introduction (elevation of the dissolved oxygen level) attributed to sediment evacuation, which is related to the contribution of the F3 fraction. The above results suggested that sand mining in the Jialing River should be paid high attention to prevent heavy metal pollution in aquatic ecosystem.

The influence of alkali-modified biochar on the removal and release of Zn in bioretention systems: Adsorption and immobilization mechanism

Generally, Zn in stormwater runoff is considered as low toxicity, but in the senarios of roads and zinc-based materials roof runoff, the concentration of Zn becomes extremely high and cannot be ignored. Bioretention systems are used to remove heavy metals from stormwater runoff, while Zn adsorption is insufficient by conventional filler and is prone to secondary release when exposed to acid rain or high salinity runoff. This study integrated batch experiments and density functional theory calculation to investigate the mechanisms of how KOH-modified biochar (KBC) influences the removal and release of Zn in bioretention systems. The results revealed that KBC adsorbed 89.0-97.5% Zn in the influent, the main adsorption mechanism were complexation and precipitation, and precipitation is more important. In addition, 67% of Zn was immoblized as the residual form by KBC. In acidic and saline runoff, KBC reduced Zn secondary release by 43.6% and 37.08% compared to the results in the absence of KBC, which was attributed to the convertion of most dissolved Zn in acidic and saline runoff into residual Zn. Therefore, KBC has a considerable application potential not only to decontaminate the runoff of roads and Zn-containing roofs, but also to deal with secondary Zn release in acid rain or under the treatment of snow-melting agents.

Source, Distribution and Potential Risk of Antimony in Water and Sediments of Danjiangkou Reservoir: Impact from Dam

Danjiangkou Reservoir is the water source of the mid-route of the South-to-North Water Transfer Project. The source, distribution and potential risk of antimony in its water and sediments are rarely reported. In this study, symmetrical investigation results demonstrated that the concentration of antimony in the Han sub-reservoir and water in front of the dam fluctuated at about 0.9 mg L^-1, while it was relatively higher and increased with the distance from the dam in Dan sub-reservoir water, with an annual average of 0.93~3.15 mg L^-1. In recent years, the concentration of antimony in the Danjiangkou Reservoir showed a downward trend, and the difference between the Han and Dan sub-reservoirs decreased significantly. The antimony in the sediments in the reservoir was primarily derived from the inflowing rivers, and it was higher in the Dan sub-reservoir than in the Han sub-reservoir. The concentration of antimony in the water in the reservoir was considerably higher than the background value in the watersheds, indicating that there is an external input with decreasing input intensity. The content of antimony in the sediments in the reservoir and its inflow rivers was substantially higher than the background value of watersheds, indicating that there is a certain degree of enrichment. In addition, the antimony mining industry in the water source area poses a risk to the water safety of the reservoir. Antimony is not a conventional pollutant. Consequently, the collection of antimony monitoring results is a challenging task. Additionally, this study fills the gap in regional antimony research. Furthermore, the ecological risk assessment of antimony in China is still in its infancy. Unquestionably, the study of the temporal and spatial distribution of antimony concentration will be beneficial for the protection of water sources in relevant regions.

Assessing the environmental risk and mobility of cobalt in sediment near nonferrous metal mines with risk assessment indexes and the diffusive gradients in thin films (DGT) technique

The Jialing River is the tributary of the Yangtze River with the largest drainage area. In recent years, the Jialing River has suffered a series of environmental problems, such as discharge of industrial effluent and sand mining activities, which have severely threatened the aquatic ecosystem of the river. In the present study, we employed risk assessment indexes, sequential extraction and the diffusive gradients in thin films (DGT) technique to assess environmental risks and study the remobilization of cobalt (Co) in sediments. The potential ecological risk index and risk assessment code results demonstrated that Co may pose a low environmental and ecological risk to the local aquatic environment. However, BCR sequential extraction showed that the sum of the F1, F2 and F3 fractions of Co still accounted for over 50% of the Co in the study areas, indicating that sediments may be a source of Co release. The DGT results showed an increasing trend for DGT-labile Co in deep sediments (-8 cm to -12 cm), and the calculated flux values ranged from 0.08 to 15.54 ng cm²·day^-1, indicating that Co tends to transfer across the sediment-water interface at all sampling sites. Correlation analysis showed that F1-Co, F2-Co and F3-Co are the fractions readily captured by DGT and can be used for predicting Co remobilization in sediment. Sand mining activities contribute substantially to the release of Co from the F1 and F3 fractions as a result of strong stirring of sediments and introduction of oxygen into the sediments. The reductive dissolution of iron (Fe) and manganese (Mn) hydroxides or oxides causes the release of Co and Fe/Mn in the sediment, which leads to Co release from the reducible fraction. The above work suggests that sand mining in the Jialing River should be reasonably regulated to prohibit illegal sand mining activities.

Improvement of Water Quality by Light-Emitting Diode Illumination at the Bottom of a Field Experimental Pond

Remediation of water quality by stimulating algal photosynthesis using light-emitting diodes (LEDs) has attracted attention, but few studies have examined this in outdoor freshwater environments. To understand the effects of LED illumination on water quality, the dissolved oxygen (DO), temperature, pH, and electric conductivity were monitored over 5 months in three depressions with or without a red/blue LED light at the bottom of an experimental pond. The effects of the blue LED on water quality were evident in the period with less rainfall after the change of water quality to an equilibrium state; DO and pH were higher, and EC was lower for the blue LED than for the control. The diel changes of these variables were also lower for the blue LED. The effects of the red LED on DO and pH were also evident, but to a lesser extent compared to those of the blue LED. A vertical mixing of water associated with a nighttime cooling of the surface water was suggested by a rapid DO increase after a temperature decrease in the control. Such internal water circulation and an inflow of water after rainfall might have obscured the LED effects in the rainy period. The bottom water of the blue LED had a higher density and species richness of phytoplankton than that of the control at the end of the experiment. A lower density of phytoplankton and higher nutrient concentrations in the red LED might have been due to a higher density and feeding activity by zooplankton. Our results confirmed the applicability of LED illumination in stimulating algal photosynthesis, and in improving the oxygen condition of the bottom water in freshwater ponds.

Effects of Light-Emitting Diode Illumination on Sediment Surface Biological Activities and Releases of Nutrients and Metals to Overlying Water in Eutrophic Lake Microcosms

The release of nutrients and metals from the sediment to the overlying water induced by oxygen depletion is an important issue in eutrophic aquatic systems. Effects of light-emitting diode (LED) illumination on oxygen conditions and release of nutrients and metals from the sediment were examined by comparing with those effects of aeration in microcosms using water and sediment of Lake Taihu, China. Periphyton with filamentous algae developed on the sediment surface in the LED (blue wavelength) treatment. Dissolved oxygen became rapidly saturated and gradually supersaturated in the aeration and LED treatments, respectively, but remained low in the control. A thicker oxic layer developed on the sediment for the LED than aeration but was poorly developed with a blackened surface in the control. Invertebrate burrows were distributed deeper and the bacterial community was more dominated by aerobic species in the LED, indicating deeper penetration of oxygen into the sediment. Nutrients (e.g., N and P) and some metals (e.g., Hg, As, and Mn) in water were lower for the LED and aeration than in the control; nutrients and other solutes that increased electric conductivity (e.g., Ca, Mg) were lower for the LED than aeration. These results suggest that LED can effectively oxygenate the bottom water by stimulating algal photosynthesis and benthic invertebrate activity, resulting in greater retention of nutrients and metals in/on sediment compared to aeration.

Heavy metals speciation and distribution of microbial communities in sediments from the abandoned Mo-Ni polymetallic mines, southwest of China

Chemical fractions of heavy metals (Mo, Ni, Cu, Zn, Fe, Mn, Pb, Cd, and Cr) and compositions of bacteria and fungi in surface sediments from the Mo-Ni polymetallic mine area were analyzed. The results indicated that the mean concentrations of Mo, Ni, Cu, Zn, and Cd were higher than their background values. The mean percentage of Cr in residual fraction was much higher than that of other heavy metals. Mo, Cu, Zn, Fe, and Pb were mainly associated with oxidizable fraction. The dominant proportions of Mn and Cd were found in exchangeable fraction with mean percentages of 93.46% and 54.50%, respectively. According to RAC classification and potential ecological risk index (PERI), the Cd with high bioavailability had a very high environmental risk. The MisSeq sequencing results of bacteria and fungi revealed that microbial communities discrepantly respond to different sampling sites. The most abundant phylum of bacteria and fungi were Proteobacteria and Ascomycota, respectively. The bioavailable heavy metals including Mo-B, Pb-B, and Cd-B were recognized to have important influences on both dominant bacterial and fungal communities. The present study manifested that the bioavailability of heavy metal is very important to assess the potential environmental risk and plays a key role in shaping microbial structure.

Dissolved oxygen drives the environmental behavior of heavy metals in coastal sediments

In this study, the impacts of dissolved oxygen (DO) on dynamics concentrations of heavy metals (Cu, Cd, Cr, and Pb) from estuary sediments were investigated in a 49-day laboratory simulation. The exchange flux method, Bureau Communautaire de Référence (BCR) sequential extraction procedure, and risk assessment code (RAC) were used to analyze the behavior of heavy metals. The results indicated that oxic environments promoted the concentrations of Cu and Cd in overlying water compared to the anoxic environments. The exchange fluxes showed that the diffusion of Cu, Cd, Cr, and Pb from sediments was the predominant process in the first 9 days, and a metastable equilibrium state was gradually reached in the later period under anoxic conditions. However, oxic conditions extended the time required to reach metastable equilibrium for Cu over the sediment-water (overlying water) interface (SWI). Although the reducible fractions of Cu, Cd, and Pb accounted for a large proportion of their total levels, the release ability of Cu, Cd, and Pb was limited by the high content of sulfide under anoxic conditions. The RAC values indicated that anoxic environments increased the proportion of acid-soluble fraction. The information obtained from this study highlights the potential risk for re-release of heavy metal from sediments under different redox conditions.

Impact of water-sediment regulation on the concentration and transport of dissolved heavy metals in the middle and lower reaches of the Yellow River

Anthropogenic activities in river basins, especially large-scale water conservancy projects, have notably impacted the physical, chemical and ecological environments of estuaries and coastal areas. In this paper, the effects of water and sediment regulation (WSR) on the concentration and transport of heavy metals in the Yellow River were studied based on a continuous daily heavy metal survey in both the middle reaches (Xiaolangdi station) and lower reaches (Lijin station) of the Yellow River during the WSR period in 2019. The results indicated that the variation in the water oxidation-reduction environment of the Xiaolangdi reservoir during the WSR process exerted an important impact on the concentrations of dissolved Cu, Cd, Pb, Cr and As at the Xiaolangdi station but exerted almost no influence on the concentration of dissolved Ni. At Lijin station, the dissolved heavy metal content first increased and then decreased in the first stage, which mainly depended on the release of heavy metals from resuspended sediments. In the second stage, the heavy metal content gradually decreased due to adsorption onto fine particles discharged from the reservoir. The dissolved heavy metal flux during the water-sediment regulation scheme (WSRS) period accounted for 16.9-33.4% of the annual total dissolved heavy metal flux. WSRS changed transport of water and sediment. The dissolved heavy metal concentrations at the Xiaolangdi station were mainly controlled by the discharge of water and sediments from the Xiaolangdi reservoir, while the dissolved heavy metal concentration at the Lijin station was largely affected by the sediments resuspended from downstream riverbeds and the water and sediment scheduling mode of the Xiaolangdi reservoir. Dissolved heavy metal transportation was highly influenced by the WSR process within a short time. Human intervention, especially WSRS operation, apparently alters the natural states of both the mainstream and estuarine environments of the Yellow River.

The Accumulation and Transformation of Heavy Metals in Sediments of Liujiang River Basin in Southern China and Their Threatening on Water Security

Heavy metal (HM) pollution in sediments is tightly related to the security of water quality in rivers, but the accumulation and conversion of HMs are poorly researched, so that a field study was conducted as an example in the Liujiang River Basin. Seven HMs were analyzed to determine between the overlying water and sediments. Moreover, the regulation of HMs speciation and environmental factors in their accumulation and conversion were identified. The obtained results suggested the HM concentrations in water are far below the primary standard of water quality, but in sediments, the contents of Cd and Zn are significantly higher than their corresponding baseline of soil. Only Cd and Pb are dominantly in non-residual form (carbonate-bound fraction and reducible fraction, respectively). The non-significant correlations suggested pH and Eh may be hard to influence HMs in water, while the significant correlations highlighted the regulations of Eh, organic matter and mean grain size on the accumulation of metals in sediments. The opposite correlations between EC, TDS, pH and Cd confirmed the emission of acid wastewater contributed to the accumulation of Cd in sediment. The conversion of metals between water and sediments were found to be significant only in specific forms of Cd, As, Cu, Zn and Pb, suggesting the conversion of HMs in sediments should be largely regulated by their specific forms. The very high risk disclosed by the higher values of Eri and RI are only found upstream, while the higher risk of Cd should be treated as a critical environmental threat.

Accumulation and ecotoxicological risk assessment of heavy metals in surface sediments of the Olt River, Romania

Heavy metal pollution of river freshwater environments currently raises significant concerns due to the toxic effects and the fact that heavy metal behavior is not fully understood. This study assessed the contamination level of eight heavy metals and trace elements (Cr, Ni, Cu, Zn, As, Pb, Cd, and Hg) in the surface sediments of 19 sites in 2018 during four periods (March, May, June, and October) in Olt River sediments. Multivariate statistical techniques were used, namely, one-way ANOVA, person product-moment correlation analysis, principal component analysis, hierarchical cluster analysis, and sediment quality indicators such as the contamination factor and pollution load index. The results demonstrated higher contents of Ni, Cu, Zn, As, Pb, Cd, and Hg, with values that were over 2.46, 4.40, 1.15, 8.28, 1.10, 1.53, and 3.71 times more, respectively, compared with the national quality standards for sediments. We observed a positive significant statistical correlation (p < 0.001) in March between elevation and Pb, Ni, Cu, Cr, and Zn and a negative correlation between Pb and elevation (p = 0.08). Intermetal associations were observed only in March, indicating a relationship with river discharge from spring. The PCA sustained mainly anthropogenic sources of heavy metals, which were also identified through correlation and cluster analyses. We noted significant differences between the Cr and Pb population means and variances (p < 0.001) for the data measured in March, May, June, and October. The contamination factor indicated that the pollution level of heavy metals was high and significant for As at 15 of the 19 sites. The pollution load index showed that over 89% of the sites were polluted by metals to various degrees during the four periods investigated. Our results improve the knowledge of anthropogenic versus natural origins of heavy metals in river surface sediments, which is extremely important in assessing environmental and human health risks and beneficial for decision-maker outcomes for national freshwater management plans.

Influence of land use class and configuration on water-sediment partitioning of heavy metals

Influence of land use and population characteristics on solid-liquid partitioning of heavy metals in aquatic ecosystems is little understood. This study hypothesised that the partitioning of heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) between water and sediments is influenced by different land use classes, their configuration patterns including patch density, Shannon's diversity index, largest patch index, and splitting index and population density. Relationships between variables were investigated from different distances to the stream network (sub-catchment and riparian scales) and considering land use patterns within individual land use classes and individual sub-catchments as a whole (class and landscape levels, respectively). The study outcomes confirmed that the influence of land use and configuration on metals partitioning is scale independent. However, population density increases metal bioavailability at the riparian scale compared to the sub-catchment scale. Agricultural lands discharge the highest fractions of dissolved metals at both spatial scales (eigenvectors = 0.409 - sub-catchment, and -0.533 - riparian, whilst metals have opposite loadings). Positive relationships between splitting index and metal partitioning confirmed that the division of anthropogenic land uses into smaller patches reduces water pollution. However, high fragmentation of forested areas increases the fraction of soluble metals. Further, high patch density and patch diversity are beneficial for controlling the solubility of some metals. Configuration metrics at the landscape level fundamentally reproduce the patterns of the largest land use type and are not effective for assessing metal partitioning. Therefore, analyses at the class level are preferred. This research investigation contributes essential knowledge to improve land use management strategies and, thereby, help safeguard urban waterways.

Adsorption-desorption behavior of heavy metals in aquatic environments: Influence of sediment, water and metal ionic properties

Limited knowledge of the combined effects of water and sediment properties and metal ionic characteristics on the solid-liquid partitioning of heavy metals constrains the effective management of urban waterways. This study investigated the synergistic influence of key water, sediment and ionic properties on the adsorption-desorption behavior of weakly-bound heavy metals. Field study results indicated that clay minerals are unlikely to adsorb heavy metals in the weakly-bound fraction of sediments (e.g., r = -0.37, kaolinite vs. Cd), whilst dissociation of metal-phosphates can increase metal solubility (e.g., r = 0.61, dissolved phosphorus vs. Zn). High salinity favors solubility of weakly-bound metals due to cation exchange (e.g., r = 0.60, conductivity vs. Cr). Dissolved organic matter does not favor metal solubility (e.g., r = -0.002, DOC vs. Pb) due to salt-induced flocculation. Laboratory study revealed that water pH and salinity dictate metal partitioning due to ionic properties of Ca²⁺ and H⁺. Selectivity for particulate phase increased in the order Cu>Pb>Ni>Zn, generally following the softness (2.89, 3.58, 2.82, 2.34, respectively) of the metal ions. Desorption followed the order Ni>Zn>Pb>Cu, which was attributed to decreased hydrolysis constant (pK1 = 9.4, 9.6, 7.8, 7.5, respectively). The study outcomes provide fundamental knowledge for understanding the mobility and potential ecotoxicological impacts of heavy metals in aquatic ecosystems.

First long-term assessment of metals and associated ecological risk in subtidal sediments of a human-impacted SW Atlantic estuary

The main objective of this article was to profile the metal accumulation, sources, pollution levels and trends during 6 years in a SW Atlantic coastal system (Bahía Blanca estuary). Subtidal sediment samples were taken from six stations with different human impacts, and chemical element analyses were performed using ICP-OES. As a result, metals tended to increase in time and differences were observed between a site profoundly impacted by sewage waters and the rest of the sampling stations. Values range from background levels to those considered toxic for the marine biota, as in the case of Cd and Cu. Besides, the geochemical analyses exhibited low to moderate pollution with probable adverse biological effects. Finally, the physicochemical parameters of the water column like pH and DO significantly decreased in time in all stations and temperature correlated with some metals, indicating a potential interaction.

Distribution and partitioning of heavy metals in water and sediments of a typical estuary (Modaomen, South China): The effect of water density stratification associated with salinity

Many estuaries have undergone severe saltwater intrusion in addition to simultaneously experiencing serious heavy metal pollution. To explore the effect of water density stratification associated with saltwater intrusion on the behaviour of heavy metals (Cr, Co, Ni, Cu, Zn, As, Pb, and Cd) in water and sediments, a field survey was conducted in a typical estuary (Modaomen). The content, distribution, and mobility of heavy metals were investigated, as well as the influence of environmental factors on their future. The results showed that Modaomen estuary was characterised by a notable variation in salinity along the estuary, presenting total freshwater upstream, high salinity stratification water in the mouth, and saltwater offshore. Dissolved metals presented a prominent gradient vertically, with 1.2-2.1 times higher in bottom water than in surface water and the highest contents in the highly-stratified bottom water. Elevated salinity and restricted mixing induced by water stratification were likely the causes of this outcome. The distribution of heavy metals in sediments was greatly governed by grain size, Fe/Mn (hydr)oxides, total organic carbon, salinity, and dissolved oxygen. Comprehensive evaluation, combined with total contents and chemical fractions of heavy metals, indicated that internal release from sediments contributed a considerable part to the higher levels of heavy metals in bottom water, particularly for Zn and Pb, which was fully consistent with their status in water body, and elevated salinity and lack of oxygen were likely the primary driving factors. During the phase-partition processes between bottom water and sediments, partitioning coefficients were markedly lower in the highly stratified zone, implying that saltwater intrusion facilitated the mobility and repartitioning processes of metals. Because of increased levels and toxicity of heavy metals in water and extended residence time during saltwater intrusion, the potential damage to the estuarine ecosystem should receive more attention.

Removal effects of different emergent-aquatic-plant groups on Cu, Zn, and Cd compound pollution from simulated swine wastewater

Aquatic plants play effective in removing heavy metal (HM) as a prominent factor of bioremediations, however, there are still knowledge gaps in species selection and configuration for high removal efficiency (RE) of compound HM and ornamental value. In this study, seven emergent-aquatic-plant species were configured into seven groups and planted in a simulated swine wastewater (SW) with Cu, Zn, and Cd for 75 days in summer. REs of Cu, Zn, and Cd were 45.06-86.93%, 42.40-87.22%, and 73.85-85.52% at day 75, respectively. Higher REs were observed from day 30-45 for Cu and Zn, whereas days 15-30 for Cd. The synergistic removal of Zn and Cu or Zn and Cd was almost observed (p < 0.05). The configuration of G5 (S. tabernaemontani, I. sibirica, and P. cordata) was generally efficient roles in the removal at day 45, with REs of 85.14%, 87.06%, and 83.56% for Cu, Zn, and Cd, respectively. The dry weight of roots, water NH₄⁺-N, temperature, pH, and dissolved oxygen acted on heavy-metal removal. During days 45-75, concentrations of Cu, Zn, and Cd in G5 were 0.52-0.66, 0.54-0.65, and 0.23-0.33 mg L^-1. The former two were below the limits of Grade Ⅱ (1.0 mg L^-1) and the latter was above the limits of Grade Ⅴ (0.1 mg L^-1; GB3838-2002). Thus, G5 could be optimal for Cu and Zn removal from simulated SW, however, efficient Cd removal is required to ensure efficient SW recycling.

Rapid monitoring of heavy metal pollution in lake water using nitrogen and phosphorus nutrients and physicochemical indicators by support vector machine

A novel method of predicting heavy metal concentration in lake water by support vector machine (SVM) model was developed, combined with low-cost, easy to obtain nutrients and physicochemical indicators as input variables. 115 surface water samples were collected from 23 sites in Chaohu Lake, China, during different hydrological periods. The particulate concentrations of heavy metals in water were much higher than the dissolved concentrations. According to Nemerow pollution index (Pi), pollution degrees by Fe, V, Mn and As ranged from heavy (2 ≤ Pi < 4) to serious (Pi ≥ 4). The concentrations of most heavy metals were the highest during the medium-water period and the lowest during the dry season. Non-metric Multidimensional Scaling Analysis confirmed heavy metal concentrations had slight spatial difference but relatively large seasonal variation. Redundancy Analysis indicated the close associations of heavy metals with nutrient and physicochemical indicators. When both nutrient and physicochemical indicators were used as input variables, the simulation effects for most elements in total and particulate were relatively better than those obtained using only nutrient or only physicochemical indicators. The simulation effects for As, Ba, Fe, Ti, V and Zn were generally good, based on their training R values of 0.847, 0.828, 0.856, 0.867, 0.817 and 0.893, respectively, as well as their test R values of 0.811, 0.836, 0.843, 0.873, 0.829 and 0.826, respectively; and meanwhile, in both the training and test stages, these metals also had relatively lower errors. The spatial distribution of heavy metals in Chaohu Lake was then predicted using the fully trained SVM models.

Water-sediment interactions and mobility of heavy metals in aquatic environments

The adsorption-desorption behaviour of heavy metals in aquatic environments is complex and the processes are regulated by the continuous interactions between water and sediments. This study provides a quantitative understanding of the effects of nutrients and key water and sediment properties on the adsorption-desorption behaviour of heavy metals in riverine and estuarine environments. The influence levels of the environmental factors were determined as conditional regression coefficients. The research outcomes indicate that the mineralogical composition of sediments, which influence other sediment properties, such as specific surface area and cation exchange capacity, play the most important role in the adsorption and desorption of heavy metals. It was found that particulate organic matter is the most influential nutrient in heavy metals adsorption in the riverine environment, while particulate phosphorus is more important under estuarine conditions. Dissolved nutrients do not exert a significant positive effect on the release of heavy metals in the riverine area, whilst dissolved phosphorus increases the transfer of specific metals from sediments to the overlying water under estuarine conditions. Furthermore, the positive interdependencies between marine-related ions and the release of most heavy metals in the riverine and estuarine environments indicate an increase in the mobility of heavy metals as a result of cation exchange reactions.

Spatial distribution characteristics and health risk assessment of heavy metals in surface sediment of the Hai River and its tributaries in Tianjin, China

To assess the spatial distribution characteristics and health risk of heavy metals (Cu, Zn, Ni, Cd, Pb, and Cr) in surface sediment of the Hai River and its tributaries in Tianjin, China, 32 surface sediment samples were collected. All the heavy metals mainly occurred in residue, except Cd. Cd primarily existed in the exchangeable fraction and posed a high risk to the aquatic environment. The mean values of pollution index followed a decreasing trend of Cu > Cd > Ni > Pb > Cr > Zn. The results of health risk assessment showed that the heavy metals were not a threat to local residents and Cr and Pb were the main contributors to the health risk. The carcinogenic risk posed by Cr was two orders of magnitude higher than that posed by Cd. A self-organizing map divided the 32 sites into three clusters and more attention should be paid to cluster 3. The results will be conducive to understanding the heavy metal pollution patterns and implementing effective and accurate management programs.

Integrating resilience with functional ecosystem measures: A novel paradigm for management decisions under multiple-stressor interplay in freshwater ecosystems

Moving beyond monitoring the state of water quality to understanding how the sensitive ecosystems "respond" to complex interplay of climatic and anthropogenic perturbations, and eventually the mechanisms that underpin alterations leading to transitional shifts is crucial for managing freshwater resources. The multiple disturbance dynamics-a single disturbance as opposed to multiple disturbances for recovery and other atrocities-alter aquatic ecosystem in multiple ways, yet the global models lack representation of key processes and feedbacks, impeding potential management decisions. Here, the procedure we have embarked for what is known about the biogeochemical and ecological functions in freshwaters in context of ecosystem resilience, feedbacks, stressors synergies, and compensatory dynamics, is highly relevant for process-based ecosystem models and for developing a novel paradigm toward potential management decisions. This review advocates the need for a more aggressive approach with improved understanding of changes in key ecosystem processes and mechanistic links thereof, regulating resilience and compensatory dynamics concordant with climate and anthropogenic perturbations across a wide range of spatio-temporal scales. This has relevance contexting climate change and anthropogenic pressures for developing proactive and adaptive management strategies for safeguarding freshwater resources and services they provide.

Habitat heterogeneity induces regional differences in sediment nitrogen fixation in eutrophic freshwater lake

Biological nitrogen fixation (BNF) in sediments is an important source of bioavailable nitrogen in aquatic systems. However, the effect of habitat change caused by eutrophication on nitrogen fixation within sediments is still unclear. In this study, nitrogen fixation rates and diazotroph diversities in sediments with heterogeneous ecological status in one eutrophic lake were investigated by using an isotope tracer method and sequencing of nitrogen-fixing (nif) genes. The results showed that both nitrogenase activity (NA) and nifH abundance in sediments of blooms area were higher than those in vegetation-dominated habitats. Correlation analysis showed that NA was correlated closely to nifH abundance, dissolved sulfide, and iron. The diazotrophic assemblage contained mainly Proteobacterial sequences belonging to Cluster I and III, and the variations of diazotrophic community could be explained by total nitrogen content, total phosphorus content, organic matters, sulfides, ammonium and iron content. Moreover, the co-occurrence network analysis showed the Alphaproteobacteria shaped the major interactions in diazotrophic community, and sediment properties had stronger effect on diazotrophic community in cyanobacteria-dominated habitat. This study revealed that habitat heterogeneity in eutrophic lakes shaped different succession of BNF in sediments and cyanobacterial blooms significantly improved the nitrogen-fixing activity in sediments, which broadened our understanding of nitrogen cycle and nutrient management in eutrophic freshwater lakes.

Interaction analysis of hydrochemical factors and dissolved heavy metals in the karst Caohai Wetland based on PHREEQC, cooccurrence network and redundancy analyses

In this study, to clarify the interaction between dissolved heavy metals and the coexisting chemical factors in karst wetland waters, surface water samples were collected from the Caohai Wetland during a water year, and the hydrochemistry and heavy metal pollution characteristics of the samples were analyzed. The main influencing factors of heavy metals in different water periods were identified through a cooccurrence network analysis. To further analyze the influence mechanism of these main influencing factors, the forms of heavy metals in the water were simulated with PHREEQC software, and the effects of these main influencing factors on the forms were analyzed by redundancy analysis. The results show that Ca²⁺ was the main cation in the wetland water, while the main anion was HCO₃^-. The hydrochemical facies of the Caohai Wetland in the wet and dry seasons were Ca-Mg-SO₄-HCO₃ and Ca-HCO₃, respectively. Cd was the main pollutant in the Caohai Wetland, with Cd levels seriously exceeding the standards. The characteristics of the karst water in the Caohai Wetland are apparent. The cooccurrence network analysis shows that pH, dissolved oxygen (DO), electrical conductivity (EC), SO₄^2- and HCO₃^- are the main factors regulating heavy metals. The results of morphological simulation and analysis were used to explore the mechanism of action of these factors. These data provide geochemical information useful for water quality assessment and management plans on heavy metal pollution.

Effects of heavy metals speciations in sediments on their bioaccumulation in wild fish in rivers in Liuzhou-A typical karst catchment in southwest China

Although fish are widely confirmed to be susceptible to heavy metals (HMs) contamination in sediments, this bioconversion haven't been detailed. This is especially the case in karst areas, where HMs are less stably retained in the sediments and are more bioavailable. Therefore, we surveyed representative karst rivers in Liuzhou, China, in order to study the relationship between the speciations of seven HMs in the sediments with their bioaccumulation in wild fish. The results showed that the HMs in sediments are all below their permissible exposure limit (PEL), but Cd and Zn are significantly higher than soil basline. Most HMs are in residual fraction, while their exchangeable fractions are present in extremely low proportions. The concentration of Zn, Cr and Cd in some fish are above their maximum recommended limit (MRL). The concentrations of most of the HMs in the fish are significantly correlated with the levels in the sediments and given the higher correlation coefficients for their carbonate-bound phase, this phase can be seen to play a critical role in HMs bioconversion. However, the presence of this phase in low proportions enables other phases, especially oxidizable form, to play a greater role in HMs bioaccumulation. Apart from Do, HMs in the fish samples are significantly correlated with multiple environmental factors, demonstrating environmental fluctuations can manipulate HMs bioconversion from sediments; however, their significance depend heavily on the proportion of particular species. HMs in reducible and oxidizable fraction are more important in regulating, rather than promoting, their bioconversion during environmental fluctuations. Fluctuations in EC, TDS and pH can increase the impacts of HMs in carbonate-bound fraction on their bioconversion. Given the higher background values of EC and TDS and lower pH values during the monsoon period, careful attention should be paid to the increased bioconversion of HMs in karst rivers during this season.