The accumulation and potential ecological risk of heavy metals in microalgae from a eutrophic lake (Taihu Lake, China)

Temporal-spatial characteristics and sources of heavy metals in bulk deposition across China

With the rapid development of industries, agriculture, and urbanization (including transportation and population growth), there has been a significant alteration in the emission and atmospheric deposition of heavy metal pollutants. This has consequently given rise to a range of ecological and environmental health issues. In this study, we conducted a comprehensive two-year investigation on the temporal and spatial distribution characteristics of heavy metals in atmospheric deposition across China based on the Nationwide Nitrogen Deposition Monitoring Network (NNDMN). The atmospheric bulk deposition of Lead (Pb), Arsenic (As), Nickel (Ni), Selenium (Se), Chromium (Cr) and Cadmium (Cd) were 6.32 ± 1.59, 4.49 ± 0.57, 1.31 ± 0.21, 1.05 ± 0.16, 0.60 ± 0.06 and 0.21 ± 0.03 mg m-2 yr-1, respectively, with a large variation among the different regions of China. The order for atmospheric deposition flux was Southwest China > Southeast China > North China > Northeast China > Qinghai-Tibet Plateau and rural area > urban area > background area. The concentrations of heavy metals in bulk deposition exhibit seasonal variation with higher levels observed during winter compared to summer and spring, which are closely associated with anthropogenic activities. The Positive Matrix Factorization (PMF) results indicated that combustion, industrial emissions and traffic are the primary contributors to atmospheric deposition of heavy metals. The single factor pollution index (Pi) of heavy metals is consistently below 1, and the composite pollution index (Ni) is 0.16 across China, indicating that atmospheric heavy metal deposition is at a pollution-free level. The comprehensive potential ecological risk index of heavy metals is 11.8, with Cd exhibiting the highest single factor potential ecological risk index at 7.09, suggesting that more attention should be paid to Cd deposition in China. The present study reveals the spatial-temporal distribution pattern of atmospheric heavy metals deposition in China, identifying regional source characteristics and providing a theoretical foundation and strategies for reducing emissions of atmospheric pollutants.

Seasonal impact of acid mine drainage on water quality and potential ecological risk in an old sulfide exploitation

Sulfides are usually associated with deposits of metals and coal. The reactive wastes from their exploitation, typically stored in piles and tailings dams, are often the mining sector's primary source of environmental problems. The surrounding river waters can present signs of acid mine drainage, responsible for aquatic ecosystem degradation. So, the main target of the present study is to investigate the impact of this process on the water's environmental quality and potential ecological risk. The study area is located at the Iberian Pyrite Belt, in an old sulfide exploitation, closed without environmental rehabilitation measures. The results exhibit high sulfate concentrations (410,601 mg/L) and potentially toxic elements, with prominence of Fe (134,000 mg/L), overcoming many other extreme cases of AMD pollution. The Ficklin diagram exposes that most samples are classified as "high-acid, high-metal." Two of them have extreme classifications (high-acid, extreme-metal). The pH value is well below the acceptable range for the environmental quality of superficial waters (5-7), measuring at a minimum of 0.84. Regarding seasonal variability, the study showed a higher degree of contamination in dry conditions (e.g., 4,420 mg/L of Cu), while the rainy month had lower concentrations of PTE (186.8 mg/L of Cu for the same sampling point). In addition, the water does not accomplish the environmental objectives established by the EU Water Framework Directive. According to the new approach developed based on a scale adjustment, the potential ecological risk index studied indicates that most sampled sites present strong, very strong, and even extremely potential ecological risk. With a typical Mediterranean climate, the region suffers from water scarcity, predicting increasingly in the future more degrading scenarios for water environmental quality. Consequently, urgent mitigation and remediation measures are necessary to improve and preserve water quality and fulfill the objectives of the United Nations Sustainability Development Goals.

Ecological risk characteristics of sediment-bound heavy metals in large shallow lakes for aquatic organisms: The case of Taihu Lake, China

Heavy metal contamination in the surface sediments of large shallow lakes in China is becoming increasingly serious. However, more attention has been paid to the human health risk of heavy metals in the past, while little consideration has been given to aquatic organisms. Taking Taihu Lake as an example, we explored the spatial and temporal heterogeneity of the potential ecological risks of seven heavy metals (Cd, As, Cu, Pb, Cr, Ni, and Zn) to species at different taxonomic scales using an improved species sensitivity distribution (SSD) method. The results showed that all six heavy metals, except Cr, were exceeded to some extent compared to background levels, with Cd being the most severe exceedance. Based on the hazardous concentration for 5% of the species (HC₅), Cd had the lowest HC₅ value, implying the highest ecological risk of toxicity. Ni and Pb had the highest HC₅ values and the lowest risk. Cu, Cr, As and Zn were at a relatively moderate levels. For the different groups of aquatic organisms, the ecological risk of most heavy metals was generally lower for vertebrates than for the whole species. The risk for invertebrates and algae was higher than that for all species. Zn and Cu had the highest potentially affected fractions (PAFs) for all classification cases, with mean PAFs of 30.25% and 47.2%, respectively. Spatially, the high ecological risk of sediment heavy metals was significantly related to the spatial characteristics of the type and intensity of human activities in the catchment. Administratively, the environmental quality standards for freshwater sediments proposed by America and Canada are insufficient to protected against the ecological risks of heavy metals in Taihu Lake. In the absence of such standards, China urgently needs to establish an approptiate system of environmental quality standards for heavy metals in lake sediments.

Pollution, ecological risk, and source identification of potentially toxic elements in sediments of a landscape urban lagoon, China

Given the great importance of Yundang lagoon (China), a detailed evaluation and source identification of multiple potentially toxic elements (PTEs) is required. Low concentrations of the PTEs were found in the Diversion canal, while high in the Main canal, Inner lagoon, and Outer lagoon. Evaluation results indicated that the pollution of PTEs was widespread, and that the extremely high eco-risks and evident toxicity were owing to the great contributions of Hg and Cd. Positive matrix factorization model demonstrated that the PTEs were from both natural and different types of anthropogenic sources. TOC played a critical role in the PTEs. It was also found that the limited environmental carrying capacity and the poor hydrological condition of the lagoon may still accumulate the pollution in a progressive fashion. These findings provide a detailed information on making effective strategies of new directions for long-term prevention of PTEs pollution in the landscape urban lagoon.

TiO2@MOF Photocatalyst for the Synergetic Oxidation of Microcystin-LR and Reduction of Cr(VI) in Aqueous Media

The coexistence of pollutants presents a great challenge to the implementation of photocatalysts. In this work, a novel MIL-101(Fe)/TiO2 composite prepared by in situ growth of MIL-101(Fe) on TiO2 was developed for the synergetic oxidation of MC-LR and Cr(VI) reduction. The heterojunction material shows elevated photocatalytic behavior under ultraviolet compared with the unary pollutant system. Furthermore, quenching experiments and electron spin resonance confirm that the enhanced photodegradation behavior is related to the synergistic effect between the photocatalytic reduction and oxidation process, in which MC-LR consumes the holes and Cr(VI) captures electrons, followed by efficient charge separation through the conventional double-transfer mechanism between MIL-101(Fe) and TiO2. This investigation provides a deeper understanding of the construction of MOFs/semiconductor heterojunctions for the pollutants removal in multi-component contaminants system.

Microalgal biosorption of heavy metals: A comprehensive bibliometric review

Heavy metals in the effluents released from industrial establishments pose risks to the environment and society. Prevalent organisms such as microalgae in industrial wastes can thrive in this harmful environment. The connection of the metal-binding proteins of the microalgal cell wall to the metal ions of the heavy metals enables microalgae as an ideal medium for biosorption. The current literature lacks the review of various microalgae used as biosorption of heavy metals from industrial effluents. This work aims to comprehensively review the literature on the use of microalgae as a biosorption for heavy metals. The study summarizes the application of different microalgae for heavy metals removal by identifying the various factors affecting the biosorption performance. Approaches to quantifying the heavy metals concentration are outlined. The methods of microalgae to generate biocompounds to enable biosorption of heavy metals are itemized. The study also aims to identify the materials produced by microalgae to facilitate biosorption. The industrial sectors with the potential benefit from the adoption of microalgal biosorption of heavy metals are recognized. Moreover, the current challenges and future perspectives of microalgal biosorption are discussed.

Identifying spatio-temporal dynamics of trace metals in shallow eutrophic lakes on the basis of a case study in Lake Taihu, China

In shallow eutrophic lakes, metal remobilization is closely related to the resuspension and eutrophication. An improved understanding of metal dynamics by biogeochemical processes is essential for effective management strategies. We measured concentrations of nine metals (Cr, Cu, Zn, Ni, Pb, Fe, Al, Mg, and Mn) in water and sediments during seven periods from 2014 to 2018 in northern Lake Taihu, to investigate the metal pollution status, spatial distributions, mineral constituents, and their interactions with P. Moreover, an automatic weather station and online multi-sensor systems were used to measure meteorological and physicochemical parameters. Combining these measurements, we analyzed the controlling factors of metal dynamics. Shallow and eutrophic northern Lake Taihu presents more serious metal pollution in sediments than the average of lakes in Jiangsu Province. We found chronic and acute toxicity levels of dissolved Pb and Zn (respectively), compared with US-EPA "National Recommended Water Quality Criteria". Suspended particles and sediment have been polluted in different degrees from uncontaminated to extremely contaminated according to German pollution grade by LAWA (Bund/Länder-Arbeitsgemeinschaft Wasser). Polluted particles might pose a risk due to high resuspension rate and intense algal activity in shallow eutrophic lakes. Suspended particles have similar mineral constituents to sediments and increased with increasing wind velocity. Al, Fe, Mg, and Mn in the sediment were rarely affected by anthropogenic pollution according to the geoaccumulation index. Among them, Mn dynamics is very likely associated with algae. Micronutrient uptake by algal will affect the migration of metals and intensifies their remobilization. Intensive pollution of most particulate metals were in the industrialized and down-wind area, where algae form mats and decompose. Moreover, algal decomposition induced low-oxygen might stimulate the release of metals from sediment. Improving the eutrophication status, dredging sediment, and salvaging cyanobacteria biomass are possible ways to remove or reduce metal contaminations.

Peak Chromium Pollution in Summer and Winter Caused by High Mobility of Chromium in Sediment of a Eutrophic Lake: In Situ Evidence from High Spatiotemporal Sampling

To study the mechanisms of chromium (Cr) mobilization in sediments of eutrophic lakes, monthly sampling was performed in the Meiliang Bay of Lake Taihu, China, combined with laboratory experiments. High-resolution dialysis and diffusive gradients in thin film (DGT) sampling techniques were used. Results indicated that in July 2016 and January 2017, the concentrations of soluble Cr and DGT-labile Cr(VI) in the overlying water exceeded both drinking and fishery water quality standards, resulting from the high mobility of Cr in sediments. In July (summer), the high concentration of soluble Cr (134.04 ± 7.20 μg/L) detected in the anaerobic sediments was primarily caused by the complexation of Cr(III) with dissolved organic matter (DOM). This mechanism was supported by an observed simultaneous increase of soluble Cr and DOM under simulated anaerobic conditions. In January (winter), the high concentrations of soluble Cr (97.55 ± 9.65 μg/L) and DGT-labile Cr(VI) (25.83 ± 1.25 μg/L) in aerobic sediments were primarily caused by reoxidation of Cr(III) by Mn(III/IV) oxides as evidenced by the lowest concentrations of soluble and DGT-labile Mn(II). This study sheds light on the full-year variation and mechanisms of Cr mobilization in eutrophic lake sediments and suggests the urgent need for remediation of Cr pollution especially for winter.

High resolution spatiotemporal sampling as a tool for comprehensive assessment of zinc mobility and pollution in sediments of a eutrophic lake

To assess zinc (Zn) pollution risk from sediments, this study investigated the monthly changes of dissolved Zn and labile Zn in sediment-overlying water profiles in a eutrophic bay (Meiliang Bay) of Lake Taihu (China) using high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) at a 4 mm vertical resolution. In February and March, Mn oxides reduction caused high concentrations of DGT-labile Zn (14 ∼ 235 μg L^-1), as evidenced by the significant correlation between DGT-labile Zn and DGT-labile Mn in sediments. In June and July, algal blooms reduced concentrations of dissolved Zn via algal assimilation. From August through October, concentrations of dissolved Zn in overlying water (338 ∼ 1023 μg L^-1) exceeded the water quality limit for fisheries in China (100 μg L^-1). This was attributed to reductive dissolution of Mn oxides in sediments caused by algal degradation followed by complexation of dissolved organic matter (DOM), which was identified in a simulated algal bloom experiment. In the winter, decreased Zn mobility was mainly attributed to adsorption by Mn oxides. It was concluded that enhanced Zn pollution risk from sediments is worthy of concern especially during algal degradation in eutrophic lakes.

Ecological risk assessment of potentially toxic elements (PTEs) in the soil-plant system after reclamation of dredged sediment

The Shayinghe River is an important tributary of the Huaihe River in China. The concentrations of potential toxic elements (PTEs), such as As, Hg, Pb, Cu, Zn, Cr, and Cd in soils and dominant vegetation from mud-dumping areas reclaimed with dredged sediments, and areas without mud-dumping (non-mud-dumping areas) adjacent to the Shayinghe River were investigated. The potential ecological risk index (ERI) and transfer coefficient (TC) were calculated and evaluated. The results showed that the potential toxic element concentrations in all sampling areas decreased in the order of (Zn, Cr) > (Cu, Pb) > As>Cd > Hg. Additionally, the total concentrations of the PTEs significantly increased from mud-dumping to the non-mud-dumping areas. Ecological risk assessment of soils showed that all the elements did not exhibit apparent ecological risks (except for As). This element exhibited a moderate ecological risk, implying that the dredging of sediments could increase the potential ecological risk of individual PTEs. Additionally, the PTEs in some of dominant plants in mud-dumping areas did not increase, indicating no apparent accumulation. The principal component analysis (PCA) showed that the principal components of the PTEs in plants from dredged sediments were different from those in the agricultural soil, indicating the influence of dredging on PTEs. Finally, Cd, Zn, and Hg had higher transfer coefficients (up to 6), even if their concentrations were relatively low, suggesting potential ecological risks to the plants.

Fractions and transformation of organic phosphorus in sediments from a eutrophic lake in China

Several organic phosphorus compounds (Po) in sediment from a representative eutrophic lake were surveyed using a sequential fractionation procedure, which included microbial biomass phosphorus (Biomass-P), fulvic acid phosphorus (FA-P), humic acid phosphorus (HU-P), and residual phosphorus (Res-P). In addition, several organic compounds including orthophosphate monoesters, orthophosphate diesters, and pyrophosphate were simultaneously measured using ³¹P nuclear magnetic resonance (³¹P NMR). Results showed that Po contributed over 50% of total phosphorus (TP), and the average concentration of Po species generally decreased from Res-P > FA-P > HU-P > Biomass-P. Additionally, the relative proportions of phosphorus compounds in the sediment followed the decreasing order of orthophosphate monoesters > orthophosphate diesters > pyrophosphate. In general, Po was the dominant phosphorus species. Residual P was not a single species but comprised of a group of species, and tended to be stable. Although orthophosphate monoesters had the highest concentrations and ratios in Po, orthophosphate diesters displayed a more distinct remineralization trend. Principal component analysis (PCA) coupled with correlation analysis suggested that a greater amount of orthophosphate diesters resided in Res-P, than HU-P or FA-P.

Spatial Evaluation of Heavy Metals Concentrations in the Surface Sediment of Taihu Lake

With regard to the size of China's freshwater lakes, Taihu Lake ranks third and it plays an important role in the supply of drinking water, flood prevention, farming and navigation, as well as in the travelling industry. The problem of environmental pollution has attracted widespread attention in recent years. In order to understand the levels, distribution and sources of heavy metals in sediments of Taihu Lake, random selection was carried out to obtain 59 samples of surface sediment from the entire lake and study the concentrations of Pb, Cd, Cu, Zn, Cr and Ni. Toxic units were also calculated to normalize the toxicities caused by various heavy metals. As a result, Cd and Cu in sediment were considered lower than the effect range low (ERL) at all regions where samples were gathered, while Pb and Ni were categorized into ERL-effect range median (ERM) at over 22% of the regions where samples were obtained. Nevertheless, all average concentrations of the samples were below the level of potential effect. According to the findings of this research, significant spatial heterogeneity existed in the above heavy metals. In conclusion, the distribution areas of heavy metals with higher concentrations were mainly the north bays, namely Zhushan Bay, Meiliang Bay as well as Gonghu Bay. The distribution areas of Cu, Zn, Cr and Ni with higher concentration also included the lake's central region, whereas the uniform distribution areas of those with lower concentrations were the lake's southeast region. In addition, it was most probable that the spatial distribution of heavy metals was determined by river inputs, whereas atmospheric precipitation caused by urban and traffic contamination also exerted considerable effects on the higher concentrations of Pb and Cd. Through evaluating the total amount of toxic units (ΣTU), it was found that higher toxicity existed primarily in the north bays and central region of the lake. If the heavy metals were sorted by the reduction of mean heavy metal toxic units in Taihu Lake in descending order, it would be Pb, Cr, Ni, Cd, Zn and Cu. Generally speaking, these result of analyses are conducive to alleviating the contamination of heavy metals in Taihu Lake.