Root-mediated acidification, phosphatase activity and the phosphorus-cycling microbial community enhance phosphorus mobilization in the rhizosphere of wetland plants.

Rhizoremediation of wetland plants is an environmentally friendly strategy for sediment phosphorous (P) removal, the basic underlying principle of which is the complex interactions between roots and microorganisms. This study investigated the immobilization and mobilization mechanisms of P in the rhizosphere of wetland plants using high-resolution spatial visualization techniques and metagenomic sequencing. Two-dimensional visualization of the spatial distribution of P, iron (Fe) and manganese (Mn) indicated that the sequestration of Fe-oxides rather than Mn-oxides caused the depletion of labile P, resulting in an increase in the Fe-adsorbed P fraction. Plants altered the rhizospheric environments and P-cycling microbial community to mobilize low-availability P from sediments. Mineral P solubilization and organic P mineralization were enhanced by local acidification and increased phosphatase activity, respectively. Microbial P mobilization also increased with increasing relative abundances of P solubilization and mineralization genes (gcd and phnW) and decreasing P transportation genes (ugpA, ugpB, and pit) genes in the rhizosphere. These processes led to the remobilization of 10.04 % of inorganic P, and 15.23 % of organic P, in the rhizosphere during the incubation period. However, the resupply of P via the above processes did not compensate for the depletion of rhizospheric P via root uptake and mineral sequestration. Our results provide novel insights into the mechanisms of rhizospheric P cycling, which will help to inform future phytoremediation strategies.

High arsenic pollution of the eutrophic Lake Taihu and its relationship with iron, manganese, and dissolved organic matter: High-resolution synchronous analysis.

Arsenic (As) is a metalloid that can accumulate in eutrophic lakes and cause adverse health effects to people worldwide. However, the seasonal process and dynamic mechanism for As mobilization in eutrophic lake remains effectively unknown. Here we innovatively used the planar optodes (PO), high-resolution dialysis (HR-Peeper) combined with fluorescence excitation-emission matrix coupled with parallel factor (EEM-PARAFAC) analysis technologies. We synchronously investigate monthly O2, As, iron (Fe), manganese (Mn), and naturally occurring dissolved organic matter (DOM) changes in sediments of Lake Taihu at high resolution in field conditions. We find high As contamination from sediments with 61.88-327.07 µg m-2 d-1 release As fluxes during the algal bloom seasons from May to October 2021. Our results show that an increase in DOM, mainly for humic-like components, resulting in high electron transfer capacity (ETC), promoted the reductive dissolution of Fe and Mn oxides to release As. Partial least square-path modeling (PLS-PM) and random forest modeling analysis identified that Mn oxide reductive dissolution directly accelerated sediments As contamination, which is the crucial factor. Understanding crucial factor controlling As release is especially essential in areas of eutrophic lakes developing effective strategies to manage As-rich eutrophic lake sediments worldwide.

Seasonal variations in spatial distribution, mobilization kinetic and toxicity risk of arsenic in sediments of Lake Taihu, China.

This study investigated seasonal variations in spatial distribution, mobilization kinetic and toxicity risk of arsenic (As) in sediments of three representative ecological lakes in Lake Taihu. Results suggested that the bioavailability and mobility of As in sediments depended on the lake ecological types and seasonal changes. At the algal-type zones and macrophyte-type zones, elevated As concentrations were observed in April and July, while these occurred at the transition areas in July and October. The diffusion flux of soluble As ranged from 0.03 to 3.03 ng/cm2/d, indicating sediments acted as a source of As. Reductive dissolution of As-bearing iron/manganese-oxides was the key driver of sediment As remobilization. However, labile S(-II) caused by the degradations of algae and macrophytes buffered sediment As release at the algal-type and macrophyte-type zones. Furthermore, the resupply ratio was less than 1 at three ecological lakes, indicating the resupply As capacity of sediment solid phase was partially sustained case. The risk quotient values were higher than 1 at the algal-type zones and transition areas in July, thereby, the adverse effects of As should not be ignored. This suggested that it is urgently need to be specifically monitored and managed for As contamination in sediments across multi-ecological lakes.

Cyanobacterial blooms increase the release of vanadium through iron reduction and dissolved organic matter complexation in the sediment of eutrophic lakes.

Vanadium (V), a hazardous environmental contaminant, can be highly toxic to aquatic or even human life. Nonetheless, knowledge of its redox geochemistry and mobility in sediments, especially those of eutrophic lakes, remains limited. In this study, we combined in situ high-resolution sampling and laboratory simulation experiments for monitoring soluble and labile V to reveal the mobilization mechanism of V in the sediment of Lake Taihu. The results showed that the concentration of soluble V (1.18-5.22 µg L-1) exceeded the long-term ecotoxicology limitation proposed by the government of the Netherlands. The highest value appeared in summer (July to September), with an average concentration of 3.87 µg L-1, which exceeded the short-term exposure limit. The remobilization of V in summer was caused by the combined effect of the reduction of Fe(hydr)oxides and dissolved organic matter (DOM) complexation, which accelerated the release of associated Fe-bound V and increased the solubility of DOM-V. Additionally, V showed high mobility in winter, owing to the species of V(Ⅲ)/V(Ⅳ) being oxidized to V(Ⅴ) with higher solubility. It is noteworthy that the elevated remobilization of V in sediments increases the risk of V release from sediments, which poses the threat of water V pollution in Lake Taihu.

Mechanisms of antimony release from lacustrine sediments with increasing temperature.

Antimony (Sb) is more mobile in lacustrine sediments with seasonal warming. However, the mechanisms of Sb mobility in sediments are still unclear, especially considering the interactions among Sb, iron (Fe), manganese (Mn), and dissolved organic matter (DOM). In this study, high-resolution dialysis (HR-Peeper) and multi-spectral techniques simultaneously investigated changes in Sb, Fe, Mn, and DOM in two different ecological types (algal and grass) sediments with increasing temperature. We found that the dissolved Sb rapidly increased with the increase in temperature. The oxidation of Sb(III) to Sb(V) by Fe/Mn oxides in oxygen (O2) rich overlying water and surface sediment layers was one of the reasons for Sb concentration enhancement in pore water. Further, using excitation-emission matrix and parallel factor analysis (EEM-PARAFAC), synchronous fluorescence (SF) spectroscopy, fourier transform infrared (FTIR) spectroscopy, and two-dimensional correlation spectroscopy (2D-COS) revealed that complexation with DOM was the other reasons for Sb concentration increasing in sediments. This was demonstrated by the similar distribution pattern and significant correlation between Sb and tryptophan-like components. Titration experiments further revealed that Sb was more stably bound to tryptophan-like components in the aromatic C-H (660 cm-1), alcoholic C-O (1115 cm-1), alkene CC (1615 cm-1), and carboxylic acid OH (3390 cm-1) groups. The tryptophan-like components from the algae region had a higher binding force than that from the macrophytes region. Our study effectively promotes an understanding of Sb mobilization in lacustrine sediments.

Effects on the migration and speciation of heavy metals by combined capping and biochemical oxidation during sediment remediation.

Capping and oxidation by lanthanum-modified bentonite (LMB) and calcium nitrate (CN) has a dual effect of deep phosphorus (P)/arsenic (As) clearance and surface P/As blockade. However, little information is available on the effect of LMB and CN on heavy metals. In this study, we hypothesize that LMB and CN exerted the same synergistic effect on heavy metals as P and As. We verified this through Rhizon samplers, diffusive gradients in thin films technology (DGT) and planar optode (PO) methods. The results showed that individual and combined LMB and CN treatments temporarily decreased but eventually increased the dissolved oxygen of the sediment-water interface (SWI). DGT-labile sulfide in the surface 110 mm sediment, soluble Fe(II) and DGT-labile Fe(II) in the surface 80 mm sediment were eliminated within 30 days by CN and LMB + CN treatments. A temporary sharp increase in soluble Fe, Mn, Co, and DGT-labile Mn, Co, Cu, and Ni was observed in CN and LMB + CN groups probably due to sulfide oxidation and carbonate dissolution. LMB + CN group showed a less-intense increase in DGT-labile metals and less metal release than the CN group (inferred from the total metal content). This indicates that LMB and CN had a synergistic effect on heavy metals. When using the LMB + CN treatment, LMB partly adsorbed and blocked metal release in sulfide and carbonate bound forms and finally transformed them into Fe and Mn oxides and residual forms. We suggest that CN should be combined with capping agents (at an appropriate pH) to compact sediments and block metal exchange at the SWI.

Sediment arsenic remediation by submerged macrophytes via root-released O2 and microbe-mediated arsenic biotransformation.

Arsenic (As)-contaminated water restoration is extremely challenging because As remobilization from sediments can result in episodic or long-term release of As to the overlying water. In this study, by combining high-resolution imaging techniques with microbial community profiling, we examined the feasibility of utilizing the rhizoremediation of submerged macrophytes (Potamogeton crispus) to decrease As bioavailability and regulate its biotransformation in sediments. Results showed that P. crispus considerably decreased the rhizospheric labile As flux to lower than 4 pg cm-2 s-1 from larger than 7 pg cm-2 s-1, suggesting its effectiveness in promoting As retention in sediments. Iron plaques induced by radial oxygen loss from roots decreased the mobility of As by sequestering it. Additionally, Mn-oxides may act as an oxidizer for the oxidation of As(III) to As(V) in the rhizosphere, which can further increase the As adsorption owing to the strong binding affinity between As(V) and Fe-oxides. Furthermore, microbially mediated As oxidation and methylation were intensified in the microoxic rhizosphere, which decreased the mobility and toxicity of As by changing its speciation. Our study demonstrated that root-driven abiotic and biotic transformation contribute to As retention in sediments, which lays a foundation for applying macrophytes to the remediation of As-contaminated sediments.

A combined study on Vallisneria spiralis and lanthanum modified bentonite to immobilize arsenic in sediments.

Submerged plants and lanthanum-modified bentonite (LMB) have important applications for the remediation of contaminated sediments; however, their combined effect on arsenic (As) removal has not been comprehensively evaluated. In this study, the physicochemical properties and changes in soluble As in sediments treated with LMB, Vallisneria spiralis (V. spiralis), and LMB + V. spiralis were observed at three time points (days 15, 35, and 66), and the changes in microbial and As species in sediments on day 66 were analyzed. LMB + V. spiralis treatment was the most effective for As removal. On day 66, the average concentrations of soluble As at a depth of 0-100 mm decreased by 12.71%, 48.81%, and 59.73% following treatment with LMB, V. spiralis, and LMB + V. spiralis, respectively. Further analysis showed that LMB is more effective at removing As(V) than V. spiralis, while V. spiralis is more effective at removing As(III), and the combination of LMB + V. spiralis is more effective for removing both As(III) and As(V) than individual LMB and V. spiralis treatments. LMB + V. spiralis enhanced the transformation of mobile As to Fe2O3/oxyhydroxide-bound As in sediments and the activity of As-oxidizing microorganisms. LMB promoted the growth of V. spiralis and enhanced the removal of As. This study indicates that this combination is an effective method for removing mobile As from sediments, and could effectively inhibit the release of As from sediments to overlying water.

Decrease in macrofauna density increases the sediment phosphorus release and maintains the high phosphorus level of water column in Lake Taihu: A case study on Grandidierella taihuensis.

Internal phosphorus (P) loading can increase the P level in the water column and further sustains cyanobacterial blooms. This study focused on the role of benthic fauna bioturbation in affecting the sediment P release and the P level of water column in a eutrophic lake, Lake Taihu. The macrofauna density decreased from 4766.56 ± 10541.80 ind/m2 in 2007 to 345 ± 447.63 ind/m2 in 2020 due to the frequent bottom-water hypoxia in Lake Taihu. The reduced macrofauna density majorly resulted from Grandidierella taihuensis, Limnodrilus hoffmeisteri, and Tanypus chinensis larvae, and their total density decreased by approximately 97% in 2020 compared to 2007. G. taihuensis, one of the major benthic faunas, was further used as a representative to investigate the effects of bioturbation on sediment P release using high-resolution sampling and imaging techniques. The results show that G. taihuensis can increase the O2 penetration depth by more than 20 mm through bio-irrigation, and causes the redox conditions in burrows and surrounding sediments to change dramatically within a few minutes due to the intermittent ventilation. Subsequent oxidation of the soluble Fe(II) led to the formation of Fe-oxide bound P in the surface sediments, thereby increasing the P retention in the sediments. When the G. taihuensis density was 1563 ind/m2 at the sampling site, approximately 0.12 g m-2 yr-1 P can be retained in sediments. As previous studies have shown that L. hoffmeisteri and T. chinensis played a similar role in increasing the P retention in sediments through their bioturbation activities, the sharp decline in benthic fauna density and burrowing activities in Lake Taihu should be an important reason for maintaining the high P level in the water column by decreasing the P retention in sediments.

Mechanistic insights into trace metal mobilization at the micro-scale in the rhizosphere of Vallisneria spiralis.

Mobilization of trace metals in the rhizosphere of macrophytes is controlled by root-driven chemical changes, especially the steep gradients of O2 and pH from the rhizosphere to bulk sediments. Here, the O2 and pH dynamics, and the distribution of trace metal, in the rhizosphere of Vallisneria spiralis were obtained using planar optodes and diffusive gradients in thin films, respectively. Radial O2 loss (ROL) and acidification occurred on all visible roots of V. spiralis and exhibited highly spatiotemporal dynamics depending on the root growth and various environmental conditions. Trace metals showed different mobilization mechanisms in the rhizosphere. ROL and produced Fe(III) (oxyhydr)oxides decreased the mobility of Fe, As, Co, V and W in the rhizosphere. However, Mn, Ni and Cu exhibited greater mobility in the rhizosphere than bulk sediments as a result of the oxidation of metal sulfide and proton-induced dissolution of minerals. In particular, Co and Ni presented increased activity at the interface between rhizosphere and bulk sediment, which was attributed to the redox dissolution processes of Fe and Mn as a result of ROL and rhizosphere acidification. These results provide new insights into the roles of macrophyte root-induced O2 and pH changes in controlling trace metal mobility in sediments.

Behavior of iron and other heavy metals in passivated sediments and the coupling effect on phosphorus.

In situ passivation, which is easy to operate and affordable, is one of the most commonly used methods for sediment phosphorus (P) remediation. Understanding the behavior of iron and other heavy metals in passivated sediments is important for alleviating lake eutrophication and for ensuring drinking water safety. In this study, we investigated the behavior of P, Fe, Mn, Cd, Co, and Pb in lanthanum modified bentonite (LMB, Phoslock®) and polyaluminum chloride (PAC)-passivated sediments using intact sediment cores. Rhizon sampler and diffusive gradients in thin films technology (DGT) were respectively used to collect soluble and labile substances in sediment; a modified sequential selective extraction method was used to characterize metal forms. Results showed that LMB reduced soluble reactive phosphorus (SRP) at sediment depths of 0 ~ -15 mm and DGT-labile P flux at 0 ~ -50 mm. Correlation between DGT-labile P and Fe (R2 = 0.71) indicated that P mobility in the LMB group was affected by the behavior of Fe. PAC decreased SRP at sediment depths of 0, -5, -10, -15, -20, -25, and -50 mm with removal rates of 100%, 90%, 45%, 35%, 81%, 89%, and 100%, respectively. DGT-labile P flux was decreased by PAC at 0 ~ -10 mm and -50 ~ -110 mm, but increased at -10 ~ -50 mm; this is a result of synthetical effect by Al flocs adsorption and Fe(III) reductive dissolution. LMB decreased Cd, Co, and Pb in LMB layer in carbonate, reducible, and oxidizable forms. PAC decreased Cd mobility but caused the transformation of Co and Pb from reducible to other forms because of Fe(III) reductive dissolution. Those results indicate that sedimentary Fe plays an important role in in situ passivation. We suggest modifying passivators to Fe(II) adsorbents and increasing DO permeability of sediment to promote the formation of an Fe(III) passivation layer and hence the effectiveness of P control.

Phosphorus acquisition strategy of Vallisneria natans in sediment based on in situ imaging techniques.

Phosphorus (P) availability is closely related to the distributions of pH, O2 and phosphatase activities in the rhizosphere of plants growing in soils and sediments. In this study, the P uptake processes and mechanisms of Vallisneria natans (V. natans) during two vegetation periods (i.e., week three and six) were revealed using three noninvasive 2D imaging techniques: planar optode (PO), diffusive gradients in thin films (DGT) and zymography. The results showed that increased phosphatase activity, O2 concentration and root-induced acidification were observed together in the rhizosphere of root segments and tips. In week three, when V. natans was young, the flux of DGT-labile P accumulated more in the rhizosphere in comparison with the bulk sediment. This was because increased phosphatase activity (of up to 35%) and root-induced acidification (with pH decreasing by up to 0.25) enhanced P acquisition of V. natans by the third week. However, the flux of DGT-labile P turned to depletion during weeks three to six of V. natans growth, after Fe plaque formed at the matured stage. The constant hydrolysis of phosphatase and acidification could not compensate for the P demand of the roots by the sixth week. At this stage, Fe plaque become the P pool, due to P fixation with solid Fe(III) hydroxides. Subsequently, V. natans roots acquired P from Fe plaque via organic acid complexation of Fe(III).

O2 distribution and dynamics in the rhizosphere of Phragmites australis, and implications for nutrient removal in sediments.

Root-triggered microscale variations in O2 distribution in the rhizosphere of young Phragmites australis are important for nutrient removal in sediments. In this study, the micro-scale O2 dynamics and the small-scale changes of soluble reactive phosphorus (SRP) and ammonium (NH4+) in the rhizosphere of P. australis were investigated using planar optodes and high-resolution dialysis (HR-Peeper), respectively. Results suggested that root O2 leakage has a highly variable distribution depending on the stage of root growth, the site of O2 leakage gradually shift from the entire emerging main roots to the main root tip and subsequently shifted the emerging lateral roots. The O2 concentration increased in the rhizosphere with increasing light intensity and O2 levels in the overlying water. Continuous O2 release from the lateral roots causes the formation of iron plaque on the surface of lateral roots, which reduce the mobility of P by adsorption of iron plaque in the rhizosphere. The oscillation of oxic-anoxic root zones improves nitrogen removal through the processes of anammox, heterotrophic denitrification and nitrification. This work from the micro-scale demonstrates that the O2 concentration is the spatio-temporal variations in the rhizosphere, and it presents an important role for nutrient removal in sediments.

The decomposition of macrozoobenthos induces large releases of phosphorus from sediments.

Lake eutrophication and algal blooms may result in the mortality of macrozoobenthos. However, it is still not clear how macrozoobenthos decomposition affect phosphorus (P) mobility in sediments. High-resolution dialysis (HR-Peeper) and the diffusive gradients in thin films (DGT) technique were used in this study to assess the dissolved organic matter (DOM), dissolved/DGT-labile iron (Fe), P, and sulfur (S(-II)) profiles at a millimeter resolution. The decomposition of Bellamya aeruginosa significantly increased the internal loading of sediments P. The Fe(III) and sulfate were reduced under anaerobic conditions and promoted P desorption from sediments. This was supported by the significant increase in DGT-labile S(-II) and dissolved/DGT-labile P, Fe(II) and the significant positive correlation between Fe and P on day 8. The simultaneous increase in DOM and soluble reactive phosphorus (SRP) and the significant positive relationship between these factors were observed during the decomposition of B. aeruginosa. This suggested that complexation of DOM with metals may promotes the release of P from sediments.

A holistic understanding of cobalt cycling and limiting roles in the eutrophic Lake Taihu.

Cobalt (Co) cycling is often dominated by its role as a micronutrient in marine, while little is known on its cycling in a shallow eutrophic lake. Monthly sampling was performed in eutrophic Meiliang Bay of Lake Taihu, combining two laboratory control experiments and in situ Co limitation bioassay experiments. The high-resolution dialysis and the diffusive gradients in thin films technique were used to detect dissolved and labile Co, respectively. The positive correlations between dissolved/labile Co and Mn in the sediments for 6 or 7 months demonstrated that the mobility of Co in the sediments was primarily controlled by Mn redox cycling in the field. However, it is unexpected that the dissolved and labile Co only showed a small change over one year irrespective of the significant fluctuation in dissolved/labile Mn, with the concentrations being as low as 1.08 ± 0.22 µg/L and 0.246 ± 0.091 µg/L for dissolved and labile Co in the surface 20 mm sediment, respectively. Cyanobacterial bloom simulation and aerobic-anaerobic-cyanobacterial addition experiments indicated that the level of Co in the sediment-overlying water system was strongly regulated by cyanobacterial uptake, followed by the degradation of Co-enriched cyanobacterial biomass, which offset the influence of Mn redox cycling on Co mobility in the sediment. The significant enhancement of Microcystis spp. biomass by Co addition further indicated that Co was the potential limiting nutrient for cyanobacterial blooms. This work provides new ideas for better management strategies of eutrophication in shallow lakes.

High cadmium pollution from sediments in a eutrophic lake caused by dissolved organic matter complexation and reduction of manganese oxide.

Eutrophication and metal pollution are global environmental problems. The risk of metal pollution is high in the eutrophic lakes because of high mobility of metal in sediments. However, the mechanism of cadmium (Cd) mobility in sediments is still unclear. Here we study the mobilization of Cd in sediments from the eutrophic Lake Taihu via monthly field monitoring of mobile Cd using diffusive gradient in thin films (DGT) and high resolution dialysis (HR-Peeper) techniques. We found a high mobility of Cd in sediments in February and March, resulting from reductive dissolution of Mn oxide mediation by high microbial activities, as shown by the similarities in distribution patterns of DGT-labile Cd and Mn. A two orders of magnitude increase in dissolved Cd concentrations (about 28 µg L-1) was observed in May and June, with dissolved Cd concentrations in overlying water about 110 times higher than the criteria continuous concentration set by Environmental Protection Agency. Hourly changes were found to coincide and correlate between dissolved Cd and dissolved organic matter (DOM) under simulated anaerobic conditions, strongly suggesting that the sudden outbreak of Cd pollution observed in the field resulted from the complexation of DOM with Cd in sediments. This was further supported by the NICA-Donnan model that more than 71% of dissolved Cd in the pore water in May and June was present as Cd-DOM complexes. Three components of DOM including humic-, tryptophan-, and tyrosine-like components in the sediments in June was identified using the fluorescence excitation emission matrix-parallel factor analysis. We found that Cd was stable complexed with tyrosine-like component. The Fourier transform infrared and two-dimensional correlation spectroscopy further revealed that Cd was bound to phenolic OH, alkene CC, alcoholic CO, aromatic CH, and alkene CH groups. Our study effectively promotes the understanding of Cd mobilization in sediments and highlights the risk of sudden Cd pollution events in the eutrophic lakes.

Mechanism of phosphorus mobility in sediments with larval (Propsilocerus akamusi) bioturbation.

The effects of chironomid larval (Propsilocerus akamusi) bioturbation on sediment phosphorus (P) mobility were studied over the course of 34 days using the indoor larval cultivation method on in situ sediment cores. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to record fine-scale changes of soluble and DGT-labile P and iron (Fe) concentrations in the sediment. The larval-driven irrigation of the overlying water into their burrows significantly increased the oxygen penetration depth (OPD) and redox state (Eh) in sediments. In addition, the soluble and DGT-labile P and Fe decreased with the increase of OPD and Eh in larval-bioturbated sediments. The greatest decrease in the mean concentration of SRP, soluble Fe, and DGT-labile P in the Propsilocerus group was observed on Day 15 of the experiment, with a decrease by over half of the mean concentration of the control group. Furthermore, two-dimensional measurements of DGT-labile P concentration showed notable reductions of DGT-labile P around larval burrows. The DGT-induced fluxes in sediments (DIFS) model also exhibited a much longer response time (420 s) and a much higher rate of P adsorption (0.002 s-1) in the bioturbation sediments than those in the control sediments (116 s and 0.009 s-1, respectively). A significant correlation was shown for DGT-labile P and DGT-labile Fe. We conclude that Fe(II) oxidation and its enhanced adsorption were the major mechanisms responsible for the decrease of soluble and DGT-labile P in sediments.

Mobility of chromium in sediments dominated by macrophytes and cyanobacteria in different zones of Lake Taihu.

To study the mechanisms of chromium (Cr) mobilization in sediments of lakes with different ecotypes, seasonal sampling was performed in the macrophyte-dominated East Taihu (MDET) and cyanobacteria-dominated Meiliang Bay (CDMB) in Lake Taihu. Concentrations of labile Cr(VI) and dissolved Cr were assessed using diffusive gradients in thin films (DGT) and high-resolution dialysis passive sampling devices, respectively. Results indicated that in pore water the dissolved Cr concentrations and in sediments total Cr and Cr fractions concentrations (dissolved, exchangeable and carbonate fraction (F1), Fe-Mn oxide fraction (F2), organic/sulfide fraction (F3)) were lower in MDET than in CDMB. However, the highly toxic DGT-labile Cr(VI) concentrations were higher in MDET sediments than in CDMB sediments. In the two lake zones, the seasonal variations in concentrations of dissolved Cr and DGT-labile Cr(VI) were significant, while total Cr and Cr fractions showed negligible changes (except F1 fraction). In summer, the high mobility of dissolved Cr (MDET: 103.42±10.82µg/L; CDMB: 108.99±4.24µg/L) were mainly caused by dissolved organic matter complexing with Cr(III). In winter the high mobility of dissolved Cr (MDET: 100.27±22.04µg/L; CDMB: 102.01±8.81µg/L) and DGT-labile Cr(VI) (MDET: 28.26±3.73µg/L; CDLZ: 25.82±2.26µg/L) were primarily caused by the oxidization of Cr (III) by Mn(III/IV) oxides. This study establishes the mechanisms for seasonal variation of Cr mobilization in different lake ecological zones, highlighting the urgent need for remediation of Cr pollution, especially in macrophyte-dominated lake zones.

Zinc pollution in zones dominated by algae and submerged macrophytes in Lake Taihu.

Zinc (Zn) contamination in lake zones dominated by algae and macrophytes in Lake Taihu was analyzed through diffusive gradient in thin films (DGT) and dialysis (HR-Peeper) methods. It was found that in both zones Zn contamination varied by season. In July and October, dissolved Zn was present in high concentrations, and in July, high concentrations of labile Zn were found in sediments. In July, reductive dissolutions of Fe/Mn oxides likely played a key role in the release of Zn, which was confirmed by both zones having the lowest percentage of the reducible fraction of Zn in July. Complexation of dissolved organic matter (DOM) with Zn may be responsible for the observed increase in the dissolved Zn concentration in October. This conclusion was supported by noting that October had the highest percentages of Zn-DOM complexes (25.3% and 34.4%) in the algae- and macrophytes-dominated zones, respectively. However, in January, low dissolved and labile Zn contents were observed in sediments in the two zones, suggesting that the decrease of Zn in sediments was caused by the adsorption of Fe/Mn oxides.

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.