Variations in size and composition of colloidal organic matter in a negative freshwater estuary

Scallop farming impacts on dissolved organic matter cycling in coastal waters: Regulation of the low molecular weight fraction

To elucidate the impacts of scallop farming on the biogeochemical characteristics of low molecular weight (LMW, <1 kDa) dissolved organic matter (DOM), samples collected from a bay scallop mariculture area (MA) and its surrounding areas were determined for absorption and fluorescence spectroscopy after microfiltration and centrifugal ultrafiltration. The values of absorption coefficient a350 showed a spatial variation trend of inshore area (IA) > MA > non-mariculture area (NMA) for both bulk (<0.7 μm) and LMW fractions. Four fluorescent components, namely two protein-like components (tryptophan-like C1 and tyrosine-like C2) and two humic-like components (microbial humic-like C3 and terrestrial humic-like C4), were identified. Scallop farming influenced DOM transformation by altering phytoplankton abundance and promoting microbial degradation. In July, the net contributions of phytoplankton to the spectroscopy parameters of LMW-DOM in the surface seawater were 11.0% for a350, 4.3% for C1, 0.8% for C2, 0.6% for C3 and 3.0% for C4, respectively; the corresponding values of bulk DOM in the surface seawater were 24.3% for a350, 20.1% for C1, 5.9% for C2, 2.0% for C3, 2.9% for C4, respectively. Compared with NMA, the contributions of microbial degradation to a350 in MA's surface seawater increased by 9.0% for LMW-DOM and 6.9% for bulk DOM in July; however, the effects on different fluorescent components varied. In August, compared with NMA, the contributions of microbial degradation to spectroscopy parameters in the bottom water of MA decreased by 35.7% for a350, 6.3% for C2, 1.3% for C3, and 4.4% for C4 for LMW-DOM fraction; for bulk DOM, the corresponding contribution decreased by 10.8% for C1. These variations indicate that protein-like substances from scallop aquaculture are easily degraded into LMW substances, while humic-like substances degradation diminishes over time.

The influences of Yellow River input and nutrient dynamics on colloidal Fe migration in the Bohai Sea, China

The coupling relationship between the <1 kDa, 1-3 kDa, 3-10 kDa, 10-100 kDa, and 100 kDa-0.45 μm Fe fractions and the environmental factors in the Bohai Sea (BS) was investigated. The 1-100 kDa Fe in the surface water exhibited a non-conservative phenomenon during the river-sea mixing process, which was related to the removal of colloidal Fe via flocculation during this process. For the bottom water, the ligands released by the sediments may form additions to the <100 kDa Fe. The COC and DOC were mainly closely related to the behavior of the Fe in the bottom water. The <1 and 3-10 kDa Fe was mainly significantly positively correlated with the DOC, while the <100 kDa-0.45 μm Fe was significantly negatively correlated with the DOC. <100 kDa LMW colloidal Fe exhibited more synergistic behavior with easily absorbed ammonium salts.

Spatial distribution, compositional pattern, and source apportionment of colloidal trace metals in the coastal water of Shandong Peninsula, northeastern China

The Shandong Peninsula (SP) is the largest peninsula in China hosting rich economic and agricultural activities. In this study, we investigated the behavior of dissolved Mn, Fe, Cu, Zn, Cd, and Pb and their colloidal phases in the coastal and estuarine areas of SP. Pb and Zn had the highest contamination factors of 0.22-10.15 and 0.90-4.41, respectively. The <1 kDa accounted for 23-57 % of the total dissolved phase. Mn, Fe, Cu, Zn, Cd, and Pb were more likely to bind to 100 kDa-0.45 μm colloids (21-57 %). For colloidal Fe and Cu, the adsorption-release behavior had more significant effects on their dynamics. In contrast, the changes in colloidal Mn, Cd, and Pb were mainly controlled by the combined influence of temperature, dissolved oxygen, and microbial activity. However, the 1-3 kDa Zn exhibited a greater pH-dependent dispersion and was significantly positively correlated with it.

Non-conservative behavior of organic matter and its interaction with metals in an equatorial estuary, Brazil

Droughts are becoming more intense and frequent in the Brazilian semiarid because of El Niño and global climate changes. The Jaguaribe River estuary is a semiarid ecosystem that experiences a reduction in freshwater discharges due to droughts and river damming. The decrease in freshwater fluxes has increased metal availability through the water residence time increase in the Jaguaribe River estuary. Then, this study aimed to evaluate the dissolved organic matter quality and its interaction with metals in the Jaguaribe River estuary after a severe drought period. It was performed through carbon analyses, fluorescence spectroscopy, ultrafiltration technique, and determinations of metals by ICP-MS. Optical analysis showed that the dissolved organic carbon (DOC) was preponderantly composed of terrestrial-derived humic compounds, while the low ratio between the particulate organic carbon (POC) and chlorophyll-a indicated that POC was predominantly phytoplankton-derived. DOC and POC presented non-conservative removal during the estuarine mixing. DOM and dissolved elements were mostly distributed within the LMW fraction and presented a low percentage in the colloidal fraction. Li, Rb, Sr, Mo, and U showed conservative behavior, while Cu, Fe, Cr, and V had non-conservative behavior with a significant positive correlation with DOM, suggesting DOM as a relevant driver of metal availability at the Jaguaribe River estuary even during the rainy season.

Distinct variations in fluorescent DOM components along a trophic gradient in the lower Fox River-Green Bay as characterized using one-sample PARAFAC approach

Variations in molecular weight distributions of dissolved organic matter (DOM) and PARAFAC-derived fluorescent components were investigated along a transect in the seasonally hypereutrophic lower Fox River-Green Bay using the one-sample PARAFAC approach coupling flow field-flow fractionation for size-separation with fluorescence excitation-emission matrix (EEM) and PARAFAC analysis. Concentrations of dissolved organic carbon and nitrogen, chromophoric-DOM, specific UV absorbance at 254 nm, and humification index all decreased monotonically from river to open bay, showing a strong river-dominated DOM source and a dynamic change in DOM quality along the river-lake transect. The relative abundance of colloidal DOM (>1 kDa) derived from ultrafiltration exhibited minimal variation, averaging 71 ± 4 % of the bulk DOM, across the entire estuarine transect although the colloidal concentration decreased in general. Using the one-sample EEM-PARAFAC approach, the identified major fluorescent components were distinct between stations along the river-estuary-open bay continuum, with four components in river/upper-estuary but three components in open bay waters. Among the four common fluorescent components (C475, C410, C320 and C290), the most abundant and refractory humic-like component, C475, behaved conservatively and its relative abundance (%ΣFmax) remained fairly constant (50 ± 4 %) along the transect, while the semi-labile humic-like component, C410, consistently decreased from river to estuary and eventually vanished in open Green Bay. In contrast, the two autochthonous protein-like components (C320 and C290) increased from river to open bay along the trophic gradient. The new results presented here provide an improved understanding of the diverse and fluctuating characteristics in DOM composition, lability, and estuarine mixing behavior across the river-lake interface and demonstrate the efficacy of the one-sample PARAFAC approach.

Compositional and spectroscopic analysis of dissolved organic matter samples from Everglades periphyton and water

Periphyton is a ubiquitous niche in aquatic environments and can be a significant source of dissolved organic matter (DOM) production and leaching, especially in such environment as the Everglades, a slow-water flow wetland in Florida, USA. We employed an array of methods, including compositional analysis, 3-dimensional excitation emission matrix (3-D EEM) fluorescence spectroscopy, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, to perform quantitative and qualitative analyses on the DOM produced by periphyton and DOM in surrounding surface water and periphyton overlying water for comparison purposes. Higher dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) contents in periphyton pore water than surface water and periphyton overlying water indicated the remarkable contribution from periphyton-produced DOM. Higher total protein, carbohydrate, and thiol contents in periphyton pore water than in surface water and periphyton overlying water underscored the possibility of periphyton pore water DOM leached from periphyton. These results agreed with 3-D EEM and ATR-FTIR analyses that showed the prevalence of possible microbial source of periphyton pore water DOM as indicated by higher fluorescence index (FI) than surface water and periphyton overlying water. Similarly, the size-fractionated DOM from surface water demonstrated terrestrial sources, and periphyton pore water demonstrated microbial sources regardless of their differences in size based on their FI values. The types of periphyton affect the production and composition of DOM, as evidenced by higher total protein, carbohydrate, and chlorophyll-a (Chl-a) contents in floating mat on the water surface than in epiphyton attached to submerged phytoplankton, probably because the former is photo-synthetically more productive than the latter due to different light availability. This study provided fundamental information on periphyton DOM that is essential for further investigating its role in carbon cycle and its biogeochemistry.

Exploring the optical properties and molecular characteristics of dissolved organic matter in a large river-connected lake (Poyang Lake, China) using optical spectroscopy and FT-ICR MS analysis

River-connected lakes are complicated and dynamic ecosystems due to their distinctive hydrological pattern, which could significantly impact the generation, degradation, and transformation processes of dissolved organic matter (DOM) and further regulate DOM chemistry in lakes. However, the molecular compositions and characteristics of DOM in river-connected lakes are still poorly understood. Thus, here the spatial variations of optical properties and molecular characteristics of DOM in a large river-connected lake (Poyang Lake) were explored via spectroscopic techniques and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results showed high degree of spatial heterogeneity of DOM chemistry (variations in DOC concentrations, optical parameters, and molecular compounds) in Poyang Lake, and the diversity at the molecular level was primarily caused by the heteroatom compounds (N- and S- containing). Compared with classic lakes and rivers, DOM compositions of the river-connected lake had distinctive characteristics (differences in the AI_mod and DBE values, and CHOS proportions). And the composition characteristics of DOM between the southern and northern parts of Poyang Lake were different (such as the lability and molecular compounds), suggesting the changes of hydrologic conditions may affect the DOM chemistry. In addition, various sources of DOM (autochthonous, allochthonous, and anthropogenic inputs) were identified agreeably based on optical properties and molecular compounds. Overall, this study first characterizes the DOM chemistry and reveals its spatial variations in Poyang Lake at the molecular level, which could improve our understanding of DOM in large river-connected lake systems. Further studies are encouraged to investigate the seasonal variations of DOM chemistry under different hydrologic conditions in Poyang Lake to enrich the knowledge of carbon cycling in river-connected lake systems.

Hydroxyl radicals in natural waters: Light/dark mechanisms, changes and scavenging effects

Hydroxyl radicals (•OH) are the most active, aggressive and oxidative reactive oxygen species. In the natural aquatic environment, •OH plays an important role in the biogeochemistry cycle, biotransformation, and pollution removal. This paper reviewed the distribution and formation mechanism of •OH in aquatic environments, including natural waters, colloidal substances, sediments, and organisms. Furthermore, factors affecting the formation and consumption of •OH were thoroughly discussed, and the mechanisms of •OH generation and scavenging were summarized. In particular, the effects of climate change and artificial work on •OH in the largest natural aquatic environment, i.e., marine environment was analyzed with the help of bibliometrics. Moreover, Fenton reactions make the •OH variation more complicated and should not be neglected, especially in those areas with suspended particles and sediments. Regarding the •OH variation in the natural aquatic environment, more attention should be given to global change and human activities.

Mobilization of colloids during sediment resuspension and its effect on the release of heavy metals and dissolved organic matter

Natural colloids are important in mobilizing pollutants in aquatic environments. This study investigated the mobilization and aggregation of natural colloids during the sediment resuspension and re-sedimentation processes using nanoparticle tracking analysis. The metals and organic matter in overlying water were divided and examined in dissolved (<0.45 μm), colloidal (3 kDa - 0.45 μm), and truly dissolved (<3 kDa) forms. Excitation emission matrix-parallel factor analysis (EEM-PARAFAC) was used to characterize the dissolved organic matter (DOM). In overlying water, most natural colloids were < 200 nm before resuspension. An evident mobilization of colloids and an increase in colloid size were observed during resuspension. The formation of particles (>0.45 μm) and decreases of small colloids (<200 nm) indicated that resuspension promoted the aggregation of colloids. Mobilization of colloids was accompanied by increases in concentrations of Fe, Al, and organic carbon in colloidal fractions, which could be related to the formation of mineral-organic complexes under an oxic environment. The release of DOM from sediments mainly contributed to the truly dissolved humic-like fraction, and colloidal organic carbon accounted for, on average, 20 % of the total dissolved organic carbon (DOC). Fe and Al had the highest colloidal proportions as they are major compositions of inorganic colloids. Substantial removal of dissolved Al, Fe, Pb, and Zn occurred when colloids aggregated in the overlying water. Although the adsorption of suspended particles may also decrease the concentrations of dissolved metals, the increased proportions of colloidal metals indicated a possible role of colloids in this process. These findings provide insight into the behavior of colloids during the resuspension process and indicate that the aggregation of colloids could promote the removal of dissolved matter.

Microbial source tracking identifies sources of contamination for a river flowing into the Yellow Sea

The excessive input of nutrients into rivers can lead to contamination and eutrophication, which poses a threat to the health of aquatic ecosystems. It is crucial to identify the sources of contaminants to develop effective management plans for eutrophication. However, traditional methods for identifying pollution sources have been insufficient, making it difficult to manage river health effectively. High-throughput sequencing offers a novel method for microbial community source tracking, which can help identify dominant pollution sources in rivers. The Wanggang River was selected for study, as it has suffered accelerated eutrophication due to considerable nutrient input from riparian pollutants. The present study identified the dominant microbial communities in the Wanggang River basin, including Proteobacteria, Actinobacteria, Bacteroidetes, Cyanobacteria, Verrucomicrobia, and Firmicutes. The Source Tracker machine-learning classification system was used to create source-specific microbial community fingerprints to determine the primary sources of contaminants in the basin, with agricultural fertilizer being identified as the main pollutant source. By identifying the microbial communities of potential pollution sources, the study determined the contributing pollutant sources in several major sections of the Wanggang River, including industry, urban land, pond culture, and livestock land. These findings can be used to improve the identification of pollution sources in specific environments and develop effective pollution management plans for polluted river water.

Spatial and environmental characteristics of colloidal trace Cu in the surface water of the Yellow River Estuary, China

Dynamic variations in chemical composition and size distribution of dissolved copper (Cu) along the river-sea interface in the Yellow River Estuary (China) were investigated. On average, ~64% and ~8% of bulk dissolved Cu (<0.45 μm) were partitioned in the <1 kDa fraction and 1-100 kDa, respectively. The other 28% were in the 100 kDa-0.45 μm colloids, which indicates that this fraction may dominate the overall morphology of colloidal Cu. The <3 kDa Cu fraction was susceptible to environmental parameters and the >3 kDa fraction was related to the behavior of dissolved organic carbon. 1-100 kDa Cu migrated more violently than >100 kDa Cu and tended to be a stable polymer, with stability increasing towards the sea. The source of <1 kDa Cu was complex and may be supplemented by the decomposition of small molecular colloids and the addition of the sediments or particles ligands.

Partitioning and transformation of organic and inorganic phosphorus among dissolved, colloidal and particulate phases in a hypereutrophic freshwater estuary

Phosphorus (P) loadings to the Great Lakes have been regulated for decades, but re-eutrophication and seasonal hypoxia have recently been increasingly reported. It is of paramount importance to better understand the fate, transformation, and biogeochemical cycling processes of different P species across the river-lake interface. We report here results on chemical speciation of P in the seasonally hypoxic Fox River-Green Bay system and variations in sources and partitioning of P species along the aquatic continuum. During midsummer when productivity is generally high, phosphate and dissolved organic P (DOP) were the major species in river water while particulate-organic-P predominated in open bay waters, showing a dynamic change in the chemical speciation of P along the river-bay transect with active transformations between inorganic and organic P and between colloidal and particulate phases. Colloidal organic P (COP, >1 kDa) comprised 33‒65% of the bulk DOP, while colloidal inorganic P was generally insignificant and undetectable especially in open bay water. Sources of COP changed from mainly allochthonous in the Fox River, having mostly smaller sized colloids (1-3 kDa) and a lower organic carbon to phosphorus (C/P) ratio, to predominantly autochthonous in open bay waters with larger sized colloids (>10 kDa) and a higher organic C/P ratio. The observed high apparent distribution coefficients (K_d) of P between dissolved and particulate phases and high-abundant autochthonous colloidal and particulate organic P in the hypereutrophic environment suggest that, in addition to phosphate, colloidal/particulate organic P may play a critical role in the biogeochemical cycling of P and the development of seasonal hypoxia.

Colloidal toxic trace metals in urban riverine and estuarine waters of Yantai City, southern coast of North Yellow Sea

The environmental characteristics of colloidal toxic trace metals Cd, Cu and Pb in riverine and estuarine waters collected from two urban rivers of Yantai City in eastern China, the Guangdang and Xin'an Rivers, were investigated using a modified centrifugal ultrafiltration (CUF) method in conjunction with acid extraction and inductively coupled plasma mass spectrometry. The target metals in dissolved pool were divided into four CUF fractions, i.e. <1 kDa, 1-3 kDa, 3-10 kDa and 10 kDa-0.2 μm, and the results showed that colloidal Cd, Cu and Pb were dominated by 1-10 kDa (1-3 and 3-10 kDa), 1-3 kDa and 10 kDa-0.2 μm fractions, respectively. The coagulation/flocculation of low-molecular-weight (1-10 kDa) colloidal Cd and Cu in the estuaries was obvious and strong, while the enrichment of dissolved Pb in the 10 kDa-0.2 μm fraction may be mainly related to its biogeochemical interactions with Fe-oxides, which is easy to occur in macromolecular colloids. In addition, the actual molecular weight cutoffs (MWCOs) of the three used CUF units with nominal MWCOs of 1, 3 and 10 kDa were determined to be 4.9, 8.5 and 33.9 kDa, respectively, indicating that membrane calibration is essential for explaining the actual fraction of dissolved trace metals and verifying the integrity of ultrafiltration membrane. Overall, the results in this study provide a further understanding of the heterogeneity in biogeochemical features, migration and fate of toxic trace metals in aquatic ecosystems, especially that of the river-sea mixing zone.

Variations in Colloidal DOM Composition with Molecular Weight within Individual Water Samples as Characterized by Flow Field-Flow Fractionation and EEM-PARAFAC Analysis

Fluorescence excitation emission matrices (EEM) and parallel factor (PARAFAC) analysis have been widely used in the characterization of dissolved organic matter (DOM) in the aquatic continuum. However, large sample sets are typically needed for establishing a meaningful EEM-PARAFAC model. Applications of the EEM-PARAFAC technique to individual samples require new approaches. Here, flow field-flow fractionation (FlFFF) combined with offline EEM measurements and PARAFAC analysis was used to elucidate the dynamic changes in DOM composition/optical properties with molecular weight within individual samples. FlFFF-derived size spectra of ultrafiltration-isolated colloidal DOM show that peak-C related fluorophores (E_x/E_m= 350/450 nm) are present mostly in the 1-3 kDa size range, while peak-T associated fluorophores (E_x/E_m = 275/340 nm) have a bimodal distribution with peaks in both the 1-3 kDa and the >100 kDa size fractions. The integrated EEM spectra from FlFFF size-fractionated subsamples closely resembled the bulk EEM spectra, attesting to the convincing comparability between bulk and FlFFF size-fractionated EEMs. The PARAFAC-derived DOM components are distinctive among individual samples with the predominant components being humic-like in river water, but protein-like in a highly eutrophic lagoon sample. This compelling new approach combining FlFFF and EEM-PARAFAC can be used to decipher the dynamic changes in size spectra and composition of individual DOM samples from sources to sinks or across the redox/hydrological/trophic interfaces.

Response of bacterial community in composition and function to the various DOM at river confluences in the urban area

River confluences result in mixture and transformation of dissolved organic matter (DOM), influencing the phylogeny of microbial community, furthermore, the integrity and function of river systems. The relationship between the microbial community and DOM is complex, especially in the confluence zone. Previous reports focused on shifts in the different bacterial community in response to exposure to the same terrestrial DOM. However, the transformation of bacterial community induced by convergent DOM remains unknown. This study showed the shifts of DOM components at the junction via excitation-emission matrices parallel factor analysis. Metabolic differences were also determined via phylogenetic investigation of communities by reconstruction of unobserved states. The results demonstrated a direct link between the microbial metabolism and DOM biodegradation during the heterotrophic process. In response to diverse DOM conditions, the taxonomic composition and metabolic function of the microbial community presented significant differences. Different taxa may be involved in metabolizing various DOM components. As indicative bacteria that are closely associated with DOM components, Proteobacteria (Sphingomonas) are significant for microbial utilization and were important during the DOM-degrading process. Compared with other conditions, the abundance of carbon metabolism was higher in convergences where urban rivers joined with estuary or source water. Furthermore, humic-like DOM, converging in the confluence zone, induced a more active lipid metabolism. This study applied techniques that capture the diversity and complexity of bacterial communities and DOM, and provides new insight on the basis of the interaction between bacterial communities and DOM in confluence processes of biogeochemical significance.

Dissolved Organic Matter and Associated Trace Metal Dynamics from River to Lake, Under Ice-Covered and Ice-Free Conditions

The present study investigates the changes in dissolved organic matter (DOM) composition and its influences on trace metal dispersion from the Shuya River (SR) in the Petrozavodsk Bay of Lake Onega during ice-covered and ice-free periods. Humic substances (HS) found in the SR dominated the composition of DOM through the river-bay-lake continuum in both periods. When the bay was ice-covered, both the aromaticity and the size of HS varied in the water column according to a horizontal stratification and decreased in the bay, while under ice-free conditions, they decreased along the river-lake gradient, suggesting in both cases a decrease in the proportion of HS with high aromatic character. These findings were associated with an overall decrease in the proportion of HS components that have the highest molecular masses. The quantification of metal bound to HS revealed that these characteristics were associated with a decrease in the binding capacity of the HS for Fe and Al but not Cu while dispersing in the bay to the lake. Pb was found to bind on HS, but its behavior in the bay could not be related to the HS dispersion nor to the changes in HS properties.

Fluorescence of Size-Fractioned Humic Substance Extracted from Sediment and Its Effect on the Sorption of Phenanthrene

Phenanthrene (Phe) is a toxin and is ubiquitous in the environment. The sediment humic substances (HS) that bind Phe affect the fate, transport, degradation, and ecotoxicology of Phe. This study investigated Phe sorption constants on size-fractioned HS extracted from river sediment. Fractions were identified as HHS (10 kDa to 0.45 μm), MHS (1-10 kDa), and LHS (<1 kDa). A fluorescence quenching (FQ) method was used to determine the Phe log K_HS on size-fractioned HS; the values ranged from 3.97 to 4.68 L/kg-C. The sorption constant (log K_HS) is a surrogate of the binding capacity between HS and Phe, where a high log K_HS reduces the toxicity and degradation of Phe. The log K_HS values on HHS and MHS were significantly higher than the values on LHS (p = 0.015). The SUVA₂₅₄ values of HHS and MHS were also significantly higher than the LHS value (p = 0.047), while fluorescence index (FI) and S_275-295 values were significantly lower than the LHS values (p < 0.005). The HHS and MHS had a higher aromaticity and more terrestrial sources than LHS. The log K_HS had a significant correlation with the selected optical indicators (p < 0.002), which suggested that the HS-bound Phe was positively affected by high aromaticity, terrestrial sources, and HS molecular weight. The results demonstrated that optical methods successfully obtained log K_HS and the chemical properties of fractioned HS as well as the influenced factors of log K_HS. Moreover, even the LHS had a capacity to bind with Phe.

Molecular weight-dependent spectral and metal binding properties of sediment dissolved organic matter from different origins

The metal binding potential of dissolved organic matter (DOM) is highly related with its inherent properties such as molecular weight (MW). Here sediment DOMs with different origins, i.e., algae- and macrophyte-dominated sediment DOM (named as ASDOM and MSDOM, respectively), were size-fractionated into low MW (LMW-, <1 kDa) and high MW (HMW-, 1 kDa~0.45 μm) fractions, with the spectral and metal binding properties in different MW fractions exploring via total organic carbon, absorption spectroscopy, fluorescence parallel factor (PARAFAC) analysis, and Cu(II) titration techniques. The MSDOM contained more organic carbon, lower specific UV absorbance, lower fluorescence index, higher humification index, and lower biological index compared to the ASDOM. As for the MW-fractionated samples, the humic- and fulvic-like fluorophores were mainly distributed in the LMW-DOM, while the protein-like ones were located richly in the HMW-DOM. Thus, obvious MW- and origin-dependent heterogeneities in abundance and spectral properties were observed for sediment DOMs. One humic-like, one fulvic-like, and two protein-like fluorescent components were identified by PARAFAC analysis, with different components exhibiting different variable patterns in response to Cu(II) addition. Irrespective of sample sources, humic- and fulvic-like components had higher condition stability constants (logK_M > 4.96) than protein-like components (logK_M < 4.86). As for the MW-fractionated samples, the HMW-DOM, especially the HMW humic-/fulvic-like components, exhibited higher metal binding potential than the bulk and LMW counterparts. This study highlighted the importance of HMW-DOM in manipulating the behavior, fate, and bioavailability of heavy metals in lake sediments.

Controls on rare-earth element transport in a river impacted by ion-adsorption rare-earth mining

Rare-earth elements (REEs) are known to be a group of emerging pollutants, but the geochemistry of REEs in river waters in ion-adsorption rare-earth mining areas has attracted little attention. In this study, samples of the <0.45 μm and 0.22-0.45 μm (large colloids) water fractions and acid-soluble particles (ASPs) were collected from a river impacted by ion-adsorption rare-earth mining activities. The roles of ligand complexation, colloid binding, and particle adsorption in REE transport and distribution were also investigated. Results showed higher concentrations of REEs in the <0.45 μm fraction of all sampling sites (3.30 × 10^-2-9.42 μM) compared with that in the control site (1.21 × 10^-3 μM); this fraction was also characterized by middle REE enrichment at upstream sites, where REEs are mainly controlled by the <0.22 μm fraction (55%-94% of the species found in the <0.45 μm fraction) and ligand complexation (REE³⁺, REE(SO₄)⁺, and REE(CO₃)⁺). At downstream sites, heavy REE enrichment was observed, which was largely determined by binding to large colloids (68%-83% of the species found in the <0.45 μm fraction) and adsorption to particles (>90% of the acidified bulk water). Furthermore, REE patterns indicated that the REE-associated large colloids were mineral or mixed mineral-organic matter (OM) at upstream sites and OM-dominated or functionalized at downstream sites. The particles were mainly coated by inorganic matter substances (e.g., Fe/Al oxyhydroxides). In summary, our results reveal that REE patterns provide a useful tool to study the fate of REEs in ion-adsorption rare-earth mining catchments.

Dynamic molecular size transformation of aquatic colloidal organic matter as a function of pH and cations

Knowledge of the dynamic changes in molecular size of natural colloidal organic matter (COM) along the aquatic continuum is of vital importance for a better understanding of the environmental fate and ecological role of dissolved organic matter and associated contaminants in aquatic systems. We report here the pH- and cation-dependent size variations of COMs with different sources (river and lake) quantified using flow field-flow fractionation (FIFFF), fluorescence spectroscopy and parallel factor analysis (PARAFAC), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and zeta potential analysis. Increasing pH caused a decline in molecular sizes and an obvious size transformation from the >10 kDa to 5-10 kDa and further to 1-5 kDa size fraction, whereas the opposite trend was observed for increasing cation (e.g., Ca²⁺ and Cu²⁺) abundance. Compared with lakewater COM, the riverwater COM exhibited a greater pH-dependent dispersion but less extent in cation-induced aggregation, demonstrating that the dispersion and aggregation dynamics were highly dependent on COM source and solution chemistry (e.g., pH and cations). Based on ATR-FTIR analysis, the extensive dissolution of C=O and C-O functional groups resulted in a greater pH-dependent dispersion for river COM. Fluorescence titration revealed that, despite their similar cation-induced aggregation behavior, the binding constants of all the PARAFAC-derived components for Cu²⁺ were 1-2 orders of magnitude higher than those for Ca²⁺ (logK_M: 4.54-5.45 vs. 3.35-3.70), indicating a heterogeneous nature in cation-DOM interactions. The greater extent of decline in zeta potential for lake COM suggested a Ca-induced charge neutralization and aggregation mechanism. However, for Cu-induced aggregation, chemical complexation was the predominant pathway for the river COM, with higher binding constants, while charge neutralization and chemical complexation co-induced the aggregation of lake COM. Thus, natural COMs may have different environmental behavior along the aquatic continuum and further affect the fate and transport of contaminants in aquatic environments.

Sorption of 17β-estradiol to the dissolved organic matter from animal wastes: effects of composting and the role of fulvic acid-like aggregates

Steroid estrogens, such as 17β-estradiol (E2), in animal manure pose a potential threat to the aquatic environment. The transport and estrogenicity of estrogens influence the sorption of estrogens to dissolved organic matter (DOM) in animal manure, and composting treatment alters the structure and composition of the manure. The objectives of the present study were to identify the contribution of the molecular composition of DOM of composted manure to the sorption of E2 and then elucidate the dominant mechanisms involved in the interaction of E2 with manure-derived DOM. The excitation-emission matrix (EEM) spectra and atomic force microscopy (AFM) showed that composting significantly altered the chemical composition and structure of DOM. A decrease in the atomic ratios of oxygen (O)/carbon (C) occurred in conjunction with the formation of DOM aggregates in the composted manure, indicating that the hydrophilicity and polarity of the DOM decreased after composting. Composting increased the sorption coefficients (K_DOC-E2) for E2 to DOM, and K_DOC-E2 was positively correlated with the proportion of the fulvic acid (FA)-like fraction and molecular weight (MW) fractions of the DOM (range of 1.0 × 10³-7.0 × 10³ Da and 7.0 × 10³-1.4 × 10⁴ Da). Specifically, E2 showed a tendency for sorption to medium-sized FA-like molecules of DOM aggregates in composted manure. Hydrophobic forces and π-π binding appeared to be the main mechanisms underlying the aforementioned interaction.