Evaluation of heavy metal mobilization in creek sediment: Influence of RAC values and ambient environmental factors

Sediment heavy metal speciation of Hirakud Reservoir-a Ramsar site in Mahanadi River in India

Heavy metal speciation is an important tool for the assessment of sediment quality. This work was conducted to investigate the geochemical occurrence, distribution, and spatial variability of sediment heavy metals in the Hirakud Reservoir (a Ramsar site) of the Mahanadi River in India. Estimation based on a single-extraction (speciation) method revealed the dominance of Fe-Mn-bound (39.33%) fractions suggesting the potential mobility of heavy metals. Co-dominance of residual (35.03%) and organic matter-sulfide (23.02%) fractions indicate lattice-bound associations of elements under natural conditions and suggest anthropogenic organic input contribution respectively. The heavy metals distribution was spatially affected (p < 0.05). While, Ag, Cd, Hg, and Mo displayed extremely severe enrichment (EF > 50) and very strong geo-accumulation conditions (Igeo > 5); Cd and Hg displayed very high ecological risk (ERF > 320). However, the contamination factor for all heavy metals except Cd and Hg showcased low contamination (CF < 1). The principal component and cluster analysis revealed that the source of Mn, Mo, Hg, and Ag was mainly from anthropogenic or biogenic origin. The Fe and Al however displayed signs of being derived from multiple sources. However, the risk assessment code (RAC) results suggest that As exhibited a medium to very high risk (11 < RAC < 30) of bio-availability. Thus, the results of this study can be used for the formulation of strategies for the reduction of anthropogenic loads, planning for sediment quality management, and regular monitoring to curb the rising pollution issues of the reservoir.

Environmental impact and carbon recovery in coal gasification slag after Separation-Oxidation-Acid washing (SOA) process

Coal gasification slag (CGS) is a challenging solid waste due to the presence of highly toxic heavy metals, which pose significant risks to environmental and human health. CGS cannot be freely reused or disposed of, creating considerable obstacles to solid waste resource utilization. This study presents a novel method for heavy metal removal from CGS through a separation-oxidation-acid washing (SOA) process, which effectively recycles residual carbon (RC) while minimizing the risk of heavy metal leakage. The chemical morphology, leaching patterns, and environmental risks of heavy metals in CGS following the SOA process were investigated, demonstrating effective separation and removal. The ignition loss (LOI) in CGS exceeded 45% after treatment, significantly surpassing the original value. Removal rates for Sb, Pb, and As from coal gasification fine slag (CGFS) were 88.6%, 75.88%, and 79.35%, respectively, while rates for Sb, As, and Ni from coal gasification coarse slag (CGCS) were 56.65%, 63.24%, and 50.8%. The risk assessment codes (RAC) for Cu, Co, and Sb in CGFS were reduced to low-risk levels of 3.18%, 10.30%, and 25.21%, respectively, and the relative leaching ratios (RLR) for Co (0.162%), Cu (0.006%), and Ni (0.180%) in CGCS exhibited notable reductions, suggesting that the applied process significantly mitigates the environmental risk and leaching potential of these heavy metals. Overall, this study contributes to the clean production of coal gasification enterprises by offering an environmentally friendly strategy for heavy metal removal and enhancing resource utilization through RC recovery.

The Use of Diatomite-Based Composites for the Immobilization of Toxic Heavy Metals in Industrial Wastes Using Post-Flotation Sediment as an Example

Composite materials based on diatomite (DT) with the addition of biochar (BC), dolomite (DL), and bentonite (BN) were developed. The effect of chemical modification on the chemical structure of the resulting composites was investigated, and their influence on heavy metal immobilization and the ecotoxicity of post-flotation sediments was evaluated. It was demonstrated that the chemical modifications resulted in notable alterations to the chemical properties of the composites compared to pure DT and mixtures of DT with BC, DL, and BN. An increase in negative charge was observed in all variants. The addition of BC introduced valuable chemically and thermally resistant organic components into the composite. Among the chemical modifications, composites with the addition of perlite exhibited the lowest values of negative surface charge, which was attributed to the dissolution and transformation of silicon compounds and traces of kaolinite during their initial etching with sodium hydroxide. The materials exhibited varying efficiencies in metal immobilization, which is determined by both the type of DT additive and the type of chemical modification applied. The greatest efficacy in reducing the mobility of heavy metals was observed in the PFS with the addition of DT and BC without modification and with the addition of DT and BC after the modification of H2SO4 and H2O2: Cd 8% and 6%; Cr 71% and 69%; Cu 12% and 14%; Ni 10% and Zn 15%; and 4% and 5%. In addition, for Zn and Pb, good efficacy in reducing the content of mobile forms of these elements was observed for DT and DL without appropriate modification: 4% and 20%. The highest reduction in ecotoxicity was observed in the PFS with the addition of DT and BC, followed by BN and DL, which demonstrated comparable efficacy to materials with DT and BN.

Alteration of soil microbiomes in an arsenic and antimony co-contamination zone after dam failure

Arsenic (As) and antimony (Sb), two toxic metal(loid)s, behave similarly and commonly occur in mine tailings. Yet, responses of microbes to As and Sb co-contamination in tailings dam failure-affected area remain limited. Herein, soil microbiomes (archaea, bacteria and fungi) across two contrasting sites (tailing-contaminated farmland and nearby undisturbed forestland) at a Sb-Au mining district in Chizhou, China were investigated by high-throughput sequencing. Results showed that As and Sb occurred mainly in the residual form, accounting for 55.82 % and 52.04 %, respectively. The bioavailable form was 12.77 % and 10.39 % in contaminated farmland compared to 13.31 % and 11.66 % in undisturbed forestland, respectively. Contrary to archaea and fungi, bacterial alpha-diversity significantly increased in contaminated farmland. The taxa-taxa interactions in archaea were most robust, followed by bacteria; and fungi were the weakest, which was corresponding to the habitat niche breadth. Microbial communities were affected by the deterministic processes with a modified stochasticity ratio (MST) value of 36.36 %, whereas more stochasticity (MST = 49.71 %) was raised in contaminated farmland than in undisturbed forestland (MST = 36.98 %). The microbial function based on taxonomy-based inference indicated that nitrogen and carbon metabolisms associated with archaea and bacteria increased in contaminated farmland, as well as plant pathogen, wood saprotroph and endophyte related with fungi. The turnover of soil microbiomes was tightly correlated with As and Sb speciation. Collectively, this study reveals that the soil microbial survival strategies to As-Sb co-contamination after dam failure, providing guidance for the development of bioremediation and tailings management strategies.

Fractionation, spatial distribution, ecological and health risk assessment of cobalt and nickel in surface sediment of a bay along the southeast coast of China

The fractionation and distribution of two potentially toxic elements (Co and Ni) were investigated in surface sediments to explore the pollution in Xiamen Bay, a special zone experiencing rapid economic growth and enormous environmental pressure. Relatively high concentrations were observed in nearshore areas with frequent human activities. The dominant fractions for Co and Ni were found to be residual, followed by exchangeable phase. Spatial differences in mobility and bioavailability inferred from chemical fractionations were more pronounced for Ni. Multiple evaluation methods including geo-accumulation index, risk assessment code, modified potential ecological risk index, etc., consistently indicated that pollution levels and ecological risks in the entire bay were generally classified as medium-low. However, non-carcinogenic risks of Co for children and carcinogenic risks of Ni for adults exceeded safety thresholds. Terrestrial weathering processes and industrial activities primarily contributed to the presence of these elements, while their distributions were mainly influenced by organic matter.

Speciation distribution and leaching behavior of heavy metals in coal gasification fine ash: Influence of particle size, carbon content and mineral composition

In this study, the occurrence and distribution of heavy metals in coal gasification fine ash (CGFA) with different particle sizes were investigated to ensure safer disposal and utilization strategies for CGFA. These measures are critical to sustainable industrial practices. This study investigates the distribution and leachability of heavy metals in CGFA, analyzing how these factors vary with particle size, carbon content, and mineral composition. The results demonstrated that larger CGFA particles (>1 mm) encapsulated up to 70 % more heavy metals than smaller particles (<0.1 mm). Cr and Zn were present in higher concentrations in larger CGFA particles, whereas volatile elements such as Zn, Hg, Se, and Pb were found in relatively higher contents in finer CGFA particles. At least 70 % of Hg in CGFA was present in an acid-soluble form of speciation, whereas Cd, Zn, and Pb were mostly present in a reducible form of speciation, which could be attributed to the presence of franklinite. More than 40 % of Cd and Zn in fine CGFA particles exist in an acid-soluble form. With the exception of CGFA_1.18, Se in CGFA mainly existed in an oxidizable form at a ratio of 60 %-80 %. This could be attributed to the presence of bassanite particles as well as the higher affinity of Se for S. In contrast, Cr, Cu, and As were mostly present in residual speciation forms owing to their parasitism in quartz, sillimanite, and amorphous Fe solid solution in CGFA. Additionally, the study revealed that there was no significant relationship between heavy metal content, leaching behavior, and carbon content in CGFA. Based on combined analyses using toxicity characteristic leaching procedure (TCLP) leaching concentrations and risk assessment code (RAC) results, it is recommended to focus on the environmental risks posed by Cd, Cr, Pb, Zn, and Hg in CGFA during their modification and utilization processes.

Enhanced multi-metals stabilization: Synergistic insights from hydroxyapatite and peroxide dosing strategies

Sediment contamination by heavy metals is a pressing environmental concern. While in situ metal stabilization techniques have shown promise, a great challenge remains in the simultaneous immobilization of multi-metals co-existing in contaminated sediments. This study aims to address this challenge by developing a practical method for stabilizing multi-metals by hydroxyapatite and calcium peroxide (HAP/CaO2) dosing strategies. Results showed that dosing 15.12 g of HAP/CaO2 at a ratio of 3:1 effectively transformed labile metals into stable fractions, reaching reaction kinetic equilibrium within one month with a pseudo-second-order kinetic (R2 > 0.98). The stable fractions of Nickel (Ni), Chromium (Cr), and lead (Pb) increased by approximately 16.9 %, 26.7 %, and 21.9 %, respectively, reducing heavy metal mobility and ensuring leachable concentrations complied with the stringent environmental Class I standard. Mechanistic analysis indicated that HAP played a crucial role in Pb stabilization, exhibiting a high rate of 0.0176 d-1, while Cr and Ni stabilization primarily occurred through the formation of hydroxide precipitates, as well as the slowly elevated pH (>8.5). Importantly, the proposed strategy poses a minimal environmental risk to benthic organisms exhibits almost negligible toxicity towards Vibrio fischeri and the Chironomus riparius, and saves about 71 % of costs compared to kaolinite. These advantages suggest the feasibility of HAP/CaO2 dosing strategies in multi-metal stabilization in contaminated sediments.

Use of selected amendments for reducing metal mobility and ecotoxicity in contaminated bottom sediments

The aims of the study were 1) to assess the suitability of selected amendments for reducing the mobility of metals in sediments by evaluating their effects on metal sorption capacity, and 2) to assess the ecotoxicity of sediment/amendment mixtures. Three different amendments were tested: cellulose waste, biochar, and dolomite. The efficiency of metal immobilization in mixtures was dependent on pH, which increased with concentrations of amendment. The higher negative charge observed for dolomite and cellulose waste corresponded with greater attraction of cations and enhanced metal sorption. For cellulose waste, the highest values of the Q parameter were attributed to the presence of OH groups, which corresponded with the highest immobilization of metals. Biochar reduced the negative surface charge, which highlights the importance of additional factors such as high specific surface area and volume of pores in metal immobilization. All amendments increased the SSA and VN2, indicating a higher number of sorption sites for metal immobilization. Most bioassays established a reduction of the ecotoxicity for amendments. Mixtures with dolomite (25%, 45% doses) and biochar (45% dose) were low toxic. Mixtures with cellulose waste were toxic or highly toxic. The mobility of metals from contaminated sediments can be limited by reused industry side products, which could contribute to further closing the circular economy loop.

Spatial-temporal variation, source apportionment and risk assessment of lead in surface river sediments over ∼20 years of rapid industrialisation in the Pearl River Basin, China

Lead (Pb) is a highly toxic element and is not essential to the human body. Lead pollution caused by human activities and a high geological background is considered a global environmental issue. According to the China Geochemical Baseline (CGB) project, the Pearl River Basin had the highest Pb content in alluvial sediments of 30 first-level basins in China. For this reason, it is of great significance to determine the temporal and spatial variations in Pb and their influencing factors in the Pearl River Basin. In this study, 956 stream sediment samples collected in the 1980 s (early stage) and 129 river sediment samples collected from 2008 to 2010 (late stage) were used to study the background value and spatial-temporal variation characteristics of Pb in river sediments in the Pearl River Basin. The Pb source apportionment and an ecological risk assessment were also carried out. The background value of Pb (36.2 mg·kg-1) in the river sediments of the Pearl River Basin was significantly higher than that in China (22.1 mg·kg-1). The parent rocks determine the Pb background in sediments and the high Pb background areas mainly comprised carbonate rocks and acid volcanic rocks. Over 20 years of rapid industrialisation, the average Pb increased from 43.3 to 68.3 mg·kg-1 in the Pearl River Basin. The BCR analysis revealed that Pb mainly existed in the reducible phase (48 % on average) and residue phase (42 % on average). The enrichment factor and geo-accumulation index indicated that the late-stage sediments experienced more Pb pollution than the early-stage sediments. However, the risk assessment code (RAC) showed that there was a low ecological risk of Pb in the late-stage sediments. The factor analysis results for the two rounds of data were significantly different. The Pb content in early-stage sediments was closely related to Al2O3 and Zr, while Pb in the late-stage sediments was mainly related to Zn, As, Sb, Au and Hg, indicating that the increase in Pb in the later samples was mainly influenced by human activities. The Pb isotope composition of the late-stage sediments confirmed that low Pb content was mainly controlled by natural sources, while high Pb content was significantly affected by anthropogenic sources. Combining the results of spatial-temporal variation, chemical speciation and source apportionment indicated that the rapid rise of Pb in late-stage sediments in certain areas could be attributed to mining and smelting activities during the process of industrialisation over 20 years. The anthropogenic exogenous Pb could be immobilised by Fe-Mn (hydro)oxides when it entered the soil, so although there was a high background the ecological risk of Pb in river sediments was low. In the future, Pb pollution control and remediation needs to be strengthened in the Pearl River Basin to avoid the outbreak of potential ecological risks linked to Pb.

Distribution, Ecological Risk, and Source Identification of Heavy Metal(loid)s in Sediments of a Headwater of Beijiang River Affected by Mining in Southern China

In this study, the contents of eight heavy metal(loid)s (As, Pb, Zn, Cd, Cr, Cu, Sb and Tl) in 50 sediment samples from a headwater of Beijiang River were studied to understand their pollution, ecological risk and potential sources. Evaluation indexes including sediment quality guidelines (SDGs), enrichment factor (EF), geo-accumulation index (Igeo), risk assessment code (RAC) and bioavailable metal index (BMI) were used to evaluate the heavy metal(loid)s pollution and ecological risk in the sediments. Pearson's correlation analysis and principal component analysis were used to identify the sources of heavy metal(loid)s. The results showed that the average concentration of heavy metal(loid)s obviously exceeded the background values, except Cr. Metal(loid)s speciation analysis indicated that Cd, Pb, Cu and Zn were dominated by non-residual fractions, which presented higher bioavailability. The S content in sediments could significantly influence the geochemical fractions of heavy metal(loid)s. As was expected, it had the most adverse biological effect to local aquatic organism, followed by Pb. The EF results demonstrated that As was the most enriched, while Cr showed no enrichment in the sediments. The assessment of Igeo suggested that Cd and As were the most serious threats to the river system, while Cr showed almost no contamination in the sediments. Heavy metal(loid)s in sediments in the mining- and smelting-affected area showed higher bioavailability. According to the results of the above research, the mining activities caused heavier heavy metal(loid)s pollution in the river sediment. Three potential sources of heavy metal(loid)s in sediment were distinguished based on the Pearson's correlation analysis and PCA, of which Cd, Pb, As, Zn, Sb and Cu were mainly derived from mining activities, Cr was mainly derived from natural sources, Tl was mainly derived from smelting activities.

Source apportionment of heavy metal(loid)s in sediments of a typical karst mountain drinking-water reservoir and the associated risk assessment based on chemical speciations

As important place for water storage and supply, drinking-water reservoirs in karst mountain areas play a key role in ensuring human well-being, and its water quality safety has attracted much attention. Source apportionment and ecological risks of heavy metal(loid)s in sediments of drinking-water reservoir are important for water security, public health, and regional water resources management, especially in karst mountain areas where water resources are scarce. To expound the accumulation, potential ecological risks, and sources of heavy metal(loid)s in a drinking-water reservoir in Northwest Guizhou, China, the surface sediments were collected and analyzed based on the combined use of the geo-accumulation index (Igeo), sequential extraction (BCR), ratios of secondary phase and primary phase (RSP), risk assessment code (RAC), modified potential ecological risk index (MRI), as well as the positive matrix factorization methods. The results indicated that the accumulation of Cd in sediments was obvious, with approximately 61.9% of the samples showing moderate to high accumulation levels, followed by Pb, Cu, Ni, and Zn, whereas the As and Cr were at low levels. A large proportion of BCR-extracted acid extractable and reducible fraction were found in Cd (72.5%) and Pb (40.3%), suggesting high bioavailability. The combined results of RSP, RAC, and MRI showed that Cd was the major pollutant in sediments with high potential ecological risk, while the risk of other elements was low. Source apportionment results of heavy metal(loid)s indicated that Cd (75.76%) and Zn (23.1%) mainly originated from agricultural activities; As (69.82%), Cr (50.05%), Cu (33.47%), and Ni (31.87%) were associated with domestic sources related to residents' lives; Cu (52.36%), Ni (44.57%), Cr (34.33%), As (26.51%), Pb (24.77%), and Zn (23.80%) primarily came from natural geological sources; and Pb (47.56%), Zn (22.46%) and Cr (13.92%) might be introduced by mixed sources of traffic and domestic. The contribution ratios of the four sources were 18.41%, 36.67%, 29.48%, and 15.44%, respectively. Overall, priority control factors for pollution in relation to agricultural sources included Cd, while domestic sources are primarily associated with As. It is crucial to place special emphasis on the impacts of human activities when formulating pollution prevention and control measures. The results of this study can provide valuable reference and insights for water resources management and pollution prevention and control strategies in karst mountainous areas.

Geochemical distribution and mineralogy of heavy metals in the gasification residue of coal-waste activated carbon-slurry: Insights into leaching behavior

Here, a novel approach to the detoxification and reuse of waste activated carbon (WAC) through co-gasification with coal-water slurry (CWS) is proposed. To evaluate the harmlessness to the environment of this method, the mineralogical composition, leaching characteristics, and geochemical distribution of heavy metals were investigated, enabling the leaching behavior of heavy metals in gasification residues to be explained. The results showed that the gasification residue of coal-waste activated carbon-slurry (CWACS) contained higher concentrations of Cr, Cu, and Zn, while those of Cd, Pb, As, Hg, and Se were well below 100 μg/g. Further, the spatial distributions of Cr, Cu, and Zn in the mineral phases of the gasification residue of CWACS were relatively uniform overall, and no obvious regional enrichment was observed. The leaching concentrations of various heavy metals in the gasification residues of the two CWACS samples were all lower than the standard limit. Following the co-gasification of WAC with CWS, the stability of the heavy metals in the environment was enhanced. Meanwhile, the gasification residues of the two CWACS samples showed no environmental risk for Cr, low environmental risk for Pb and Hg, and only a moderate environmental risk for Cd, As, and Se.

Remediation of copper and lead contaminated sediments using iron-based granule biochar: mechanisms and enzyme activity

In recent years, there has been a growing concern about heavy metal contamination in sediments. In this study, iron-based granular biochar (MGB) is prepared to remediate Cu and Pb contaminated sediments. Characterizations via scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) show that the rough surface of MGB with abundant pore structures and a large number of oxygen-containing functional groups that facilitate stabilization of Cu and Pb in sediments. Potential mobility and bioavailability of Cu and Pb are investigated using BCR sequential extraction in the 35 day remediation experiment. The XPS results indicate that FeOOH and C-OH play a crucial part in stabilizing heavy metals. Large affinity of FeOOH for Pb allows it to occupy a proportion in F₂ while C-OH is attractive to Cu. Changes of pH, organic matter (OM), and available phosphorus (AP) in sediments after adding MGB as well as the relationship between changes and the stable solidification of Cu and Pb are explored. The stable solidification of heavy metals effectively reduces the available phosphorus in sediments. Magnetic and particle properties of the material are used to reduce the impact of MGB aging on sediment environment and separate it from the remediated sediment. Finally, 3% of MGB significantly enhanced the sediment catalase activity in the biological enzyme activity experiment. All findings indicate that MGB is a green and environmentally friendly sediment remediation material with satisfactory potential in synergistically stabilizing heavy metals and phosphorus.Highlights The complexation of FeOOH with Pb on the surface of MGB fixes it to the reduced stateThe C-OH on the surface of MGB is more attractive to Cu than PbMGB effectively mitigates the release of bioavailable phosphorus from sediments to overlying water.

Environmental risk assessment of potentially toxic elements in Doce River watershed after mining sludge dam breakdown in Mariana, MG, Brazil

Faced with a potential risk of a colossal amount of sludge released into the Doce River basin in the most shocking Brazilian mining disaster, we proposed to assess the environmental risk from a new perspective: Understanding the mobilization of potentially toxic elements (PTE) with the geochemical fractions. Soil and sediment samples were taken in nine sites throughout the basin and characterized. The environmental risk was assessed from the PTE sequential extraction in three fractions: soluble, reducible, and oxidizable, in addition to the pseudo-total concentration. The potential mobile fraction (PMF) showed a considerable PTE mobilization from the soil and sediment samples. Principal component statistical analysis indicated the sludge as the single source of PTE. The risk assessment depended on the fractional distribution and the PTE enrichment degree in the affected samples. The fractional distribution contributed mainly to Mn, Sb, and Pb mobility, with PMF of 96%, 81%, and 100%, respectively. The mobilization of Cd, Co, Ag, Ni, Pb, Zn, and Cu was predominantly related to the degree of enrichment. The risk assessment from the geochemical fractions pointed to the magnitude of the disaster and the dispersion of PTE with severe effects on the affected populations. Therefore, more strongly enforced regulations in the basin are needed, in addition to the urgent use of more secure containment dams. It is also essential to emphasize the transferability of the design of this study to other environmental units in mining disaster conditions.

Evaluating the release risk of potentially toxic elements from sediments in the New Zhuzhao River Estuary of Nansi Lake, using high-resolution technology and sequential extraction

Potentially toxic elements (PTEs) re-release from sediment is an essential process in the sediment-water interface (SWI), especially for the influent river estuary as an important accumulation site. In this study, the diffusive gradient in thin films (DGT), high-resolution dialysis (HR-peeper) technique, and BCR sequential extraction were employed to evaluate the release risk of PTEs (As, Cu, Pb, Zn, Cd) in the New Zhuzhao River Estuary of Nansi Lake. Results showed that Cd existed primarily in the non-residual fraction (accounting for 59.87%), and the residual fractions of As, Cu, Pb, and Zn accounted for a greater proportion (12.65 to 33.07%). The mobility of Cd was the highest with a risk assessment code of 33.53% reaching the medium risk category. The resupply capacity calculated by C_DGT/C_Dis showed that As was the largest, with an average value of 0.43, indicating the strongest release capacity of As from the sediment to pore water. Furthermore, the diffusive fluxes using DGT and HR-peeper showed that As possesses a much higher potential to release upward overlying water than other elements.

Distribution and ecological risk assessment of heavy metals in sediments of Dajiuhu Lake Wetland in Shennongjia, China

The rapid development of modern society has resulted in discharge of large, heavy metal quantities into wetlands that have been continuously accumulating, causing severe pollution. Dajiuhu, located in the Shennongjia Forest District of Hubei Province in China, is a wetland of significant value internationally, serving as a model wetland ecosystem with heightened scientific research value. In this study, 27 surface sediment samples from nine sub-lakes in Dajiuhu were collected in August 2020. The concentrations of Cd, Cr, Cu, Ni, Pb, and Zn in the sediments were determined. The heavy metal occurrence and speciation characteristics were analyzed by an improved BCR (European Community Bureau of Reference) extraction method. Four methods were used to evaluate heavy metals' pollution degree and ecological risk. The possible source of heavy metals was inferred using correlation analysis and principal component analysis. The heavy metal content in the lake sediments of Dajiuhu wetland was from the highest to the lowest concentration as follows: Zn [Formula: see text] Cr [Formula: see text] Ni [Formula: see text] Pb [Formula: see text] Cu [Formula: see text] Cd. The average Cd content exceeded the national nature reserve threshold values, while the other heavy metals measured were below their respective threshold values. However, due to the occurrence of Pb and Cd in different forms, they still pose certain pollution and ecological risk to the lake wetlands. On the other hand, Zn, Cr, Ni, and Cu do not pose an ecological risk in the lakes of the Dajiuhu wetland. The spatial distribution of heavy metal content in the nine sub-lakes did vary significantly. Regarding the heavy metal sources in the lake sediments, Ni, Cr, and Cu originate from natural factors, and Cd and Pb have mainly anthropogenic origins. In contrast, Zn has both natural and anthropogenic origins. This study provides further insights into the study of heavy metal pollution in lake wetlands. It provides a framework and a direction for managing heavy metal pollution in the Dajiuhu wetland.

Metal contamination in sediments of dam reservoirs: A multi-facetted generic risk assessment

The quality of bottom sediments is a key factor for many functions of dam reservoirs, which include water supply, flood control and recreation. The aim of the study was to combine different pollution indices in a critical generic risk assessment of metal contamination of bottom sediments. Both geochemical and ecological indices reflected that sediment contamination was dominated by Zn, Pb and Cd. The ecological risk indices suggested a high riks for all three metals, whereas human health risks were high for Pb and Cd. An occasional local contamination of sediments with Cr and Ni was revealed, although at levels not expected to cause concerns about potential ecological or health risk. Sediments from the Rybnik reservoir for Cu only revealed a high potential ecological risk. EF turned to be as being the most useful, whereas TRI (∑TRI) was the most important ecological index. All multi-element indices suggested similar trends, indicating that Zn, Pb and Cd taken altogether had the greatest impact on the level of sediment contamination and posed the greatest potential ecological and health risks to organisms. The use of sequential BCR extraction and ecotoxicity analyses allowed for a multi-facetted generic risk assessment of metals in sediments of dam reservoirs.

Mobility and risk assessment of heavy metals in benthic sediments using contamination factors, positive matrix factorisation (PMF) receptor model, and human health risk assessment

Metal pollution in benthic sediments was fractionated and modelled to quantify the risk of anthropogenic activities on river ecosystems. In this study, the individual contamination factor (ICF) and the global contamination factor (GCF) were used to measure the contamination levels in the sediments. On the other hand, the mobility factor (MF) was used to quantify the mobility of heavy metals in benthic river sediments. The factors used to assess pollution in benthic sediments employ bioavailable fractions of heavy metals, which have a greater chance of release into aquatic sediments and hence are more dangerous to the environment. Heavy metal mobility (MF) is highest in the post-monsoon season for Zn, Pb, Cu, and Co; Fe in winter; Mn in pre-monsoon; and Cd in monsoon. This means that heavy metals accumulate in benthic sediments during the post-monsoon season when river flows are less turbulent. ICF and GCF data show that pollution levels are higher post-monsoon than the rest season levels. Sediment samples were further subjected to the positive matrix factorization (PMF) model, which identified four factors that explained the variation in the study: factor 1 is concerned with anthropogenic Cu, Cd, and Co pollution, while factors 2, 3, and 4 are concerned with Fe, Mn, and Zn pollution. Finally, the total cancer risk (TCR) and hazard index (HI) are employed to quantify the risk to human health from accidental ingestion and dermal exposure. According to the risk outcomes from probabilistic and deterministic approaches, river exposure is dangerous to human health, with dermal absorption being the most significant concern of the exposure paths.

Effects of hydrological connectivity project on heavy metals in Wuhan urban lakes on the time scale

Metal pollution in lakes threatens the ecological environment and human health. When environmental conditions change, heavy metals (HMs) in lake sediments can cause secondary pollution. At present, the implementation of the Hydrological Connectivity Project (HCP) is a significant means of lake governance. In this study, the accumulation, potential ecological risk, and sources of HMs in Four lakes (Houguan Lake, Tangxun Lake, Moshui Lake, and Chen Lake) in Wuhan city were compared before and after the completion of the HCP. The results indicated that the HCP reduced the enrichment factor of HMs and the potential ecological risk in the heavily polluted Moshui Lake but caused secondary pollution in the less polluted Houguan Lake. Moreover, the degree of purification of lakes that took a longer time to complete the HCP (Moshui Lake) was significantly higher than that of lakes with a shorter HCP completion time (Tangxun Lake). Water exchange caused by the HCP leading to exchange of the primary pollution source between Houguan Lake and Moshui Lake to a certain extent. This study provides a reference for evaluating the implementation effect of the HCP on HM pollution in lakes and for future governance planning.

Immobilisation of metals from bottom sediments using two additives and thermal treatment

A major problem associated with the land-based management of bottom sediments is their contamination with metals. The aims of the study were: 1) to use two additives for the immobilisation of metals; and 2) to evaluate the effect of three combustion temperatures on metal content and bottom sediment properties. The mixtures were prepared using contaminated bottom sediment and the following waste materials: cellulosic waste and biomass ash. In the second experiment, the bottom sediment samples were subjected to a thermal process, and three temperatures were chosen 500/800/950 °C. Overall, the addition of cellulosic waste and biomass ash to acidic, metal-contaminated bottom sediments significantly improved the properties of the resulting mixtures, including an increase in the pH value, sorption capacity, macronutrient content, and a decrease in the content and mobility of metals (Cd, Zn, Pb, Cr). The study confirmed the effectiveness of the thermal process on a significant reduction in the ecotoxicity of the sediments, a reduction in total content of elements, and a decrease in their leachability, and thus mobility, with increasing process temperature. The study results revealed that the converted contaminated bottom sediments can be effectively managed, provided that further studies on their technical application are carried out.

Heavy metals speciation in surface sediments of the Cross River Estuary, Gulf of Guinea, South East Nigeria

The speciation of heavy metals (Ni, Cr, Cu, Zn, Pb, and Cd) was studied in surface sediments of the Cross River Estuary (CRE), Gulf of Guinea, South East Nigeria. Pb (~56 %), Cd (~71 %), Zn (~67 %), and Cr (~76 %) were mainly available in non-residual phases, suggesting potential bioavailability. High contents of Ni and Cu in residual phase indicated immobilization of these metals in aluminosilicate minerals. Cd was the most polluted heavy metal with the highest bioavailability risk. Bayesian Network model results revealed that sedimentary organic carbon (OC) from terrestrial C₃ plants controlled the contents and variability of Pb and Zn, while the input of terrestrial soil OC strongly influenced Cu and Ni. However, Cd and Cr were dominantly influenced by sediment pH, while Ni was mainly influenced by sediment salinity. Strong interdependency between Cd and total nitrogen (TN) suggested that nitrogen might increase Cd bioavailability upon release from sediments.

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.

Spatial Distributions and Intrinsic Influence Analysis of Cr, Ni, Cu, Zn, As, Cd and Pb in Sediments from the Wuliangsuhai Wetland, China

The spatial distributions of Cr, Ni, Cu, Zn, As, Cd and Pb (potentially toxic elements, PTEs) in sediments and intrinsic influence factors from the Wuliangsuhai wetland of the Hetao Irrigation District, China were studied in this work. The results showed that excluding Zn, the total contents of other PTEs were higher than the background values, of which As (39.26 mg·kg^-1) and Cd (0.44 mg·kg^-1) were six-fold and seven-fold higher, respectively. Especially, the high levels of Cd (70.17%), Pb (66.53%), and Zn (57.20%) in the non-residual fraction showed high bioavailability and mobility. It indicated that PTEs can enter the food chain more easily and produce much toxicity. Based on I_geo, ICF, and MRI, the contamination of As was the most serious in the middle areas (MDP) of the wetland, and its risk was up to moderately strong. Cd and Pb posed moderate and considerate risk, respectively. Furthermore, 29.50% and 55.54% risk contribution ratio of As and Cd, respectively, showed that they were the dominant contaminants. In addition, the positive correlation between sand, OM, and total contents and chemical fractions of PTEs by using PCM, RDA, and DHCA indicated that physicochemical properties could significantly influence the spatial distributions of PTEs. The work was useful for assessing the level of pollution in the study area and acquiring information for future and possible monitoring and remediation activities.

Effect of application mode (capping and amendment) on the control of cadmium release from sediment by apatite/calcite mixture and its phosphorus release risk

In this research, the influence of application mode (capping and amendment) on the control of cadmium (Cd) liberation from sediment by apatite/calcite mixture and its phosphorus release risk were investigated. The results showed that calcite addition had a limited effect on the speciation of Cd in sediment, but apatite addition had a significant impact on the fractionation of Cd in sediment. Apatite amendment could effectively immobilize the most readily mobilized Cd by transferring the acid-soluble fraction to the reducible and residual fractions. Apatite addition also could effectively reduce the concentration of toxicity characteristic leaching procedure (TCLP)-leachable Cd in sediment, and apatite had a much higher reduction efficiency of TCLP-leachable Cd than calcite. Apatite/calcite mixture capping could reduce the risk of Cd liberation from sediment into the overlying water, and the controlling efficiency of apatite/calcite mixture capping was higher than that of apatite/calcite mixture amendment. The effect of apatite/calcite mixture addition on the concentration of reactive soluble phosphorus (SRP) in the overlying water was limited. The introduction of calcite into the apatite capping layer could lower the risk of phosphorus release from apatite to the overlying water as compared to single apatite capping. However, the apatite/calcite mixture capping layer still had a relatively high risk of phosphorus liberation into the overlying water. Results of this work suggest that apatite/calcite mixture has a high potential to be used as a capping material to control Cd release from sediment from the perspective of controlling efficiency and application convenience.

The influence of anthropogenic activities on heavy metal pollution of estuary sediment from the coastal East China Sea in the past nearly 50 years

Historical records of eight heavy metals (HMs: Cu, Zn, Cr, Ni, Pb, As, Hg and Cd) were analyzed in two dated sediment cores collected from the coastal East China Sea (Meishan Bay, MSB) to investigate the influence of anthropogenic activities on estuary sediment from 1972 to 2019. The sedimentary records of HMs in the two cores reflected the development of economy and change of energy consumption in China's east coastal areas. The contents of most HMs in sediments had no change or showed a downward trend before the 1980s. After the 1980s, the contents of HMs showed an increasing trend, mainly because of the rapid development of aquaculture, industry, and agriculture. The distribution of HMs outside the bay (OB) was mainly affected by industry, shipping, and agriculture (vegetable planting), while HMs in sediments inside the bay (IB) was affected by shipping, agriculture, and aquaculture. Principle component analysis (PCA) showed that Cd, Cr, and Ni could be attributed to industrial discharge, while As, Cu, Pb, and Zn were associated with shipping, agriculture, and aquaculture. Sediments were slightly polluted with Cd (I_geo:0.10-0.71, CF:1.90-7.74) and Ni (I_geo:0.08-0.92, CF:1.25-12.55), and seriously polluted with Hg (I_geo:0.95-1.76, CF:13.43-85.65). This study provides insights for the local governments to raise awareness of HM pollution in the coastal East China Sea and formulate corresponding pollution control measures.

Stabilization of heavy metals in municipal solid waste incineration fly ash via hydrothermal treatment with coal fly ash

The environmental risk of heavy metals in hazardous municipal solid waste incineration fly ash (FA) is one of the most important concerns for its safely treating and disposing. This study investigated the stabilization behavior of heavy metals in FA using coal fly ash (CFA) as an additive via hydrothermal treatment. The effects of water washing pre-treatment and FA/CFA ratio on leaching behavior, speciation evolution, and risk assessment of heavy metals were studied. The results showed that 96.6-98.0 % of Cl can be effectively removed by water washing pre-treatment and hydrothermal treatment. Most heavy metals (Cr, Cu, Ni, Pb and Zn) (>91.5 %) were stabilized in the hydrothermal product, rather than transferred to liquid phase. Tobermorite can be synthesized by adjusting Ca/Si ratio with the addition of CFA. The heavy metals were transferred into more stable residue fractions with increasing CFA addition, which resulted in the significant reduction of leaching concentrations and risk assessment code (RAC) of heavy metals. Among, the product with 30% CFA exhibited the most superior performance with the lowest leaching concentrations of heavy metals and RAC was at no risk level (<1). In addition, the economic performance of hydrothermal treatment exhibited a potential advantage by comparing with FA-to-cement, FA-to-glass slags and FA-to-chelating agent & cement solidification/stabilization. Therefore, the hydrothermal treatment coupled with water washing pre-treatment would be a promising method for the detoxification of FA, as well as synergistic treatment of FA and CFA.

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.

Vertical Distribution and Chemical Fractionation of Heavy Metals in Dated Sediment Cores from the Saronikos Gulf, Greece

The Saronikos Gulf is under a lot of anthropogenic pressure, such as the urban expansion of the metropolitan area of Athens, the port of Piraeus and marinas, industrial activities, and tourism. Heavy metal pollution has been a major environmental problem in the area for many decades. Sedimentary cores have proven to be an invaluable indicator of heavy metal pollution, as they can reveal not only the current metal inputs but also the evolution of pollution over time, and with the appropriate geochemical analyses, they can provide information on the potential toxicity of metals. In this study, the temporal evolution and the chemical speciation of eleven elements were examined in sediment cores from Elefsis Bay and the Inner Saronikos Gulf, with an emphasis on the emerging environmental hazards (V and Ag). The results showed extensive pollution of the sediments by Ni, Cr, Cu, Zn, As, Mo, Cd, and Pb from the 1910s and 1960s in Eastern and Western Elefsis Bay, respectively. A significant decrease of the sediment enrichment in V, Ni, Cr, Cu, Zn, As, Cd, Pb, and Ag since 2000 was observed in the part of the Inner Saronikos Gulf that is mainly influenced by the WWTP of Athens. However, a toxicity assessment using the metal contents of the surface sediments showed that most of the trace elements studied still pose a moderate to high risk of toxicity to benthic ecosystems. The present study highlighted the urgent need for focused research and the management of trace element inputs, particularly Ag in the Inner Saronikos Gulf, where severe sediment modification was evident.

Two-compartment membrane electrochemical remediation of heavy metals from an aged electroplating-contaminated soil: A comparative study of anodic and cathodic processes

In this study, two-compartment membrane electrochemical remediation (MER) based on the anode process and the cathode process strategies were compared for treating a multi metal -contaminated soil. Remediation effect, as well as energy consumption and risk evaluation of the two strategies under different current density conditions of electroplating-contaminated soil suspension were performed, the following conclusions were drawn. MERs based on both the anode and cathode processes exhibited a synergetic effect because the DC electric field and extractants dissolved more metals from the soil phase into the liquid phase of the suspension compared to a usual soil washing treatment. The maximum Cr, Cu, and Ni removal efficiencies of MERs based on the anode process were 79.5%, 86.2%, and 85.0%, respectively, compared to 27.5%, 72.5%, and 65.9% based on the cathode process. Risk assessment results showed lower soil environmental risk after MER based on the cathode process than after MER based on the anode process. In this study, MER based on the cathode process as an evolving soil remediation strategy was found to present high simultaneous remediation ability for soil heavy metals and leaching materials, showing its advantages of environmental friendliness and economic effectiveness.

Heavy metals in a typical city-river-reservoir system of East China: Multi-phase distribution, microbial response and ecological risk

The rapid construction of artificial reservoirs in metropolises has promoted the emergence of city-river-reservoir systems worldwide. This study investigated the environmental behaviors and risks of heavy metals in the aquatic environment of a typical system composed of main watersheds in Suzhou and Jinze Reservoir in Shanghai. Results shown that Mn, Zn and Cu were the dominant metals detected in multiple phases. Cd, Mn and Zn were mainly presented in exchangeable fraction and exhibited high bioavailability. Great proportion and high mobility of metals were found in suspended particulate matter (SPM), suggesting that SPM can greatly affect metal multi-phase distribution process. Spatially, city system (CiS) exhibited more serious metal pollution and higher ecological risk than river system (RiS) and reservoir system (ReS) owing to the diverse emission sources. CiS and ReS were regarded as critical pollution source and sink, respectively, while RiS was a vital transportation aisle. Microbial community in sediments exhibited evident spatial variation and obviously modified by exchangeable metals and nutrients. In particular, Bacteroidetes and Firmicutes presented significant positive correlations with most exchangeable metals. Risk assessment implied that As, Sb and Ni in water may pose potential carcinogenic risk to human health. Nevertheless, ReS was in a fairly safe state. Hg was the main risk contributor in SPM, while Cu, Zn, Ni and Sb showed moderate risk in sediments. Overall, Hg, Sb and CiS were screened out as priority metals and system, respectively. More attention should be paid to these priority issues to promote the sustainable development of the watershed.

Geochemical fractionation, bioavailability, and potential risk of heavy metals in sediments of the largest influent river into Chaohu Lake, China

As the largest tributary flowing into Chaohu Lake, China, the Hangbu-Fengle River (HFR) has an important impact on the aquatic environment security of the lake. However, existing information on the potential risks of heavy metals (HMs) in HFR sediments was insufficient due to the lack of bioavailability data on HMs. Hence, geochemical fractionation, bioavailability, and potential risk of five HMs (Cr, Cu, Zn, Cd, and Pb) in HFR sediments were investigated by the combined use of the diffusive gradient in thin-films (DGT), sequential extraction (BCR), as well as the physiologically based extraction test (PBET). The average contents of Cd and Zn in the HFR Basin were more than the background values in the sediments of Chaohu Lake. A large percentage of BCR-extracted exchangeable fraction was found in Cd (8.69%), Zn (8.12%), and Cu (8.05%), suggesting higher bioavailability. The PBET-extracted fractions of five HMs were all almost closely positively correlated with their BCR-extracted forms. The pH was an important factor affecting the bioavailability of HMs. The average DGT-measured contents of Zn, Cd, Cr, Cu, and Pb were 28.07, 7.7, 3.69, 2.26, 0.5 μg/L, respectively. Only DGT-measured Cd significantly negatively correlated with Eh, indicating that Cd also had a high release risk under reducing conditions, similar to the risk assessment results. Our results could provide a reference for evaluating the potential bioavailabilities and ecological hazards of HMs in similar study areas.

Spatial variations and potential risks of heavy metals in sediments of Yueqing Bay, China

In this study, we determined the spatial variations and potential risks of heavy metals in the sediments of Yueqing Bay by assessing the relationship between metal concentrations and sediment physiochemical factors. We found higher sediment metal concentrations in the inner bay than in the central and outer bay, particularly with respect to Hg, Cu, and Pb concentrations. According to the sediment quality guidelines, the heavy metals had a toxicity incidence probability of 21%. Assessments of heavy metal contamination using the geo-accumulation index and potential ecological risk index suggest that Cr, As, Pb, and Hg likely pose low ecological risks, while Cu, Zn, and Cd were identified as priority pollutants and may pose moderate ecological risks to the ecosystem. Multivariate statistical analysis inferred the high influence of sediment texture, total organic carbon (TOC), and petroleum hydrocarbons (PHCs) on the distribution and fate of metals in sediment.

Efficiency and comprehensive risk assessment of soil Pb and Cd by washing technique with three biodegradable eluents

Soil washing with environmentally friendly eluents is a rapid remediation technique for farmland polluted by heavy metals. In this study, polyepoxysuccinic acid (PESA), ethylenediamine tetra (methylene phosphonic acid) sodium (EDTMPS), and phosphonyl carboxylic acid copolymer (POCA) were applied to remedy paddy and arid soils polluted by Pb and Cd. At the same time, ethylenediaminetetraacetic acid (EDTA) was used as a control eluent. PESA showed comparable removal of soil Pb and Cd (over 80.0%) with EDTA, and EDTMPS and POCA removed two heavy metals by 35.2-50.3%. For labile fractions, PESA significantly removed Pb by 93.5-96.7% and Cd by 84.9-90.3% in two soils. EDTMPS and POCA removed Pb by 75.5-85.8% in two soils, while they only removed Cd by 11.7-42.2% in paddy soil, and 76.3-81.7% in arid soil. The risks of total heavy metal concentrations were reduced from the high risk to low risk in paddy soil, and to considerable risk in arid soil, while only dropped to considerable or even had no change by EDTMPS and POCA leaching. The risks of the two soils reduced from high to low or considerable level after PESA washing based on labile fraction change, and to considerable or high level after EDTMPS and POCA leaching, respectively. Therefore, PESA is an ecological benefit eluent for remediating the farmland polluted by heavy metals, and the risk assessment based on labile fraction more easily identifies the dynamic change of heavy metal during the washing process.

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.

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.

Assessment methodology applied to arsenic pollution in lake sediments combining static and dynamic processes

The fraction transformation from stable to mobile forms in sediments is continuous, slow, and spontaneous chain reactions causing static risks to the aquatic system. However, this process may change into abrupt, rapid, and dynamic paths when certain physicochemical conditions changed. Using the Delayed Geochemical Hazard (DGH) model, comprehensive methods combing both static and dynamic risk assessment were therefore conducted to evaluate the aforementioned processes. By applying these methods, arsenic (As) pollution in surface sediments of the Baiyangdian Lake (BYD Lake) was investigated thoroughly as a case study area. The results showed that the total As concentrations in those sediment samples ranged from 4.87 to 17.94 mg/kg, with an average of 8.75 mg/kg. In a fraction, Fe and Mn were observed to pose effects on the surface-adsorbed (As_S) and residual fractions (As_R) with the coefficient analysis. The static risk assessment showed that both the contamination and ecological risk are at a low level in the total content but a low to moderate risk in the fraction. The dynamic risk assessment posted the potential transformation paths of As in the sediments, indicating a trend of potential DGH burst in 45.24%-78.57% of the BYD Lake. In summary, this study provides a methodology for the risk assessment of arsenic that may extend to other heavy metal(loid)s combining static and dynamic processes in sediments.

Impact of ancient iron smelting wastes on current soils: Legacy contamination, environmental availability and fractionation of metals

Past and present metallurgical activity is the origin of the metallic contamination of some current soils. The purpose of this research is to assess the environmental risk of ancient Fe smelting wastes to the terrestrial compartment. For this purpose, two study sites were investigated in Bourgogne-Franche Comté (France). For each site, the soil contamination (Co, Cu, Fe, Mn, Ni and Zn) and the mobility of each metal from the slag to the topsoils were assessed. The principal results show that the topsoils are particularly enriched in Fe and Mn compared to the reference soils. The bulk chemistry of the slag showed high Fe and Mn content related to the mineralogy of slags, in which the minerals include fayalite, spinel, wustite and glass. In the topsoils, we also observed newly formed minerals (clay minerals, goethite and hematite), which were absent in the reference soils. The presence of slag microfragments in soils and the partial weathering of slags, which contributed to the release of metals in the soils, can explain the contribution of slags to the current contamination of soils. The extensive study of a depth profile from Puisaye showed a low vertical diffusion of the released metal in the heap substratum. We also investigated the fractionation of metals in soils and their environmental availability. The results showed that Mn is generally present in reducible forms or associated with the residual fraction but is less adsorbed to the organic matter (OM) or present in easily exchangeable forms. In contrast, the low extractability of Fe indicates that it is mostly bound to the residual (i.e., mineral) fraction. Based on the easily exchangeable metal concentrations measured in soils, low to medium ecological risks were identified at the sites investigated.

Speciation and environmental risk of heavy metals in biochars produced by pyrolysis of chicken manure and water-washed swine manure

This study was conducted to investigate the speciation, bioavailability and environmental risk of heavy metals (HMs) in chicken manure (CM) and water-washed swine manure (WSM) and their biochars produced at different pyrolysis temperatures (200 to 800 °C). As the pyrolysis temperature increased, the remaining proportion, toxicity characteristic leaching procedure (TCLP), HCl and diethylenetriamine pentaacetic acid (DTPA) of HMs gradually declined. This result proved that the speciation of HMs in chicken manure biochars (CMB) and water-washed swine manure biochars (WSMB) was influenced by pyrolysis temperature. The proportions of stable fractions were enhanced with increased pyrolysis temperature and weakened the HM validity for vegetation at 800 °C. Finally, the results of the risk assessment showed that the environmental risk of HMs in CMB and WSMB decreased with increasing pyrolysis temperature. Therefore, pyrolysis at 800 °C can provide a practical approach to lessen the initial and underlying heavy metal toxicity of CMB and WSMB to the environment.

Geogenic enrichment of potentially toxic metals in agricultural soils derived from black shale in northwest Zhejiang, China: Pathways to and risks from associated crops

Agricultural soils derived from black shale are typically enriched in potentially toxic metals. This is a serious problem, both in terms of the ecological environment and human health. To assess the levels of potentially toxic metals, 90 paired soil-crops samples were collected from the Anji Country, western Zhejiang province, a typical exposed black shale area in China. Concentrations and bioavailability of potentially toxic metals in the soil-crops system were measured, and the associated potential risks were further evaluated. Results showed the enrichment of potentially toxic metals (i.e. Cd, Pb, Cu, Zn and Ni) in the soil and crop samples, especially a significant accumulation of Cd. Sequential extraction data indicated that Cd in soils derived from black shale was the second most dominant element in the exchangeable fraction (mean at 33.42%) and possessed high bioavailability, whereas Pb was mostly retained in the residual fraction (mean at 76.34%) and exhibited low mobility. The total concentration as well as mobility and bioavailability of Cd were the highest in the sampled soils. This resulted in a high potential ecological risk in areas with agricultural soils derived from black shale, which could eventually jeopardize the health of local residents through various exposure pathways. Overall, our findings provide a scientific basis for developing suitable management strategies to mitigate the exposure to potentially toxic metals in high risk areas.

Soil metal pollution assessment in Sarcocornia salt marshes in a South American estuary

Soil metal pollution in two Sarcocornia salt marshes within the Bahía Blanca estuary (Argentina, South America) was evaluated through pseudo-total and bioavailable metal levels and pollution indexes. Soil conditions were also studied. The pseudo-total metal concentrations were similar in both salt marshes and followed the same decreasing order: Fe > Zn > Cu > Cr > Ni > Pb > Cd. Bioavailable metals presented different patterns between salt marshes. The percentages of the bioavailable fraction varied between 28 and 80%, being higher than 60% for Cu, Zn and Pb. Organic matter ruled the distribution of all metals, except Pb. Using shale average concentration as background level, indexes did not show pollution nor enrichment, whereas using as background levels local values, anthropogenic enrichment was found for all metals and most metals showed moderate metal pollution. Our results showed that bioavailable metals levels and indexes using local background values provide an adequate assessment of metal pollution in salt marsh soils.

Study on heavy metals' influence on buried cultural relics in soil of an archaeological site

The protection of cultural relics in a chariot pit in the ancient ruins has become an important issue. In order to study the spatial distribution, speciation of heavy metals in soil at a cemetery where an ancient chariot was buried and the influence heavy metals have had on the cultural relics. Heavy metals in soil samples surrounding the cultural relic were analyzed via atomic absorption spectrometry (AAS) using a wet digestion method combined with a three-step sequential extraction procedure. Results for seven heavy metals (Cu, Pb, Zn, Cr, Fe, Mn and Sn) and the speciation of six heavy metals (Cu, Pb, Zn, Cr, Fe and Mn) indicated that their contents exceeded local soil background values, indicating that the soil around the site had been contaminated to a certain degree. Analysis of their speciation showed that Mn mainly existed in the form of a reducible fraction in the soil, while the other elements were mainly in the form of a residual fraction. Correlation analysis showed that, except for the HOAc soluble fraction, a significant positive correlation existed between the content of the different forms and their total amounts. Heavy metals present in the soil surrounding the ancient relic predominantly influence the relic due to their related chemical reactions. In particular, copper and iron have a significant influence on corrosion of the cultural relics in this chariot pit.

Physico-chemical properties of sediments governing the bioavailability of heavy metals in urban waterways

Bioavailability is a critical facet of metal toxicity. Although past studies have investigated the individual role of sediment physico-chemical properties in relation to the bioavailability of heavy metals, their collective effects are little-known. Further, limited knowledge exists on the contribution of nutrients to metal bioavailability. In this study, the influence of physico-chemical properties of sediments, including total organic carbon (TOC), total phosphorus (TP), total nitrogen (TN), cation exchange capacity (CEC), specific surface area (SSA), and mineralogical composition to metal bioavailability is reported. The weak-acid extraction method was used to measure Cd, Cr, Cu, Ni, Pb and Zn as the potentially bioavailable fraction in sediments in an urban creek. The results confirmed that Cu has strong selectivity for organic matter (r = 0.814, p < 0.01). Cr bioavailability was influenced by either sediment mineralogy, nutrients, CEC or SSA. Zn, Ni and Pb showed strong affinity to mineral oxides, though their preferred binding positions were with nutrients, particularly organic matter (r = 0.794, 0.809, and 0.753, p < 0.01, respectively). The adsorption of Cd was strongly influenced by the competition with other metals and its bioavailability was weakly influenced by ion exchange (CEC: r = 0.424, p < 0.01). The study results indicate that nitrogen and phosphorus compounds can elevate metal bioavailability due to complexation reactions. Generally, the estuarine area was more favourable for the adsorption of weakly-bound metals. This is concerning as estuaries generate high biogeochemical activity and are economically important.

Magnetic biochar reduces phosphorus uptake by Phragmites australis during heavy metal remediation

Magnetic biochar has been widely used in the removal of aquatic pollutants due to its strong adsorption capacity and recyclability. However, the nutrient deficiency caused by magnetic biochar reduces plant performance and limits its use. The effects of magnetic biochar (derived from either eucalyptus wood or pig manure compost) on soil Cd, Zn, and Pb bioavailability to Phragmites australis L. (reed) and soil microbial community were investigated in a pot experiment. We also examined treatments of magnetic biochar with P supplementation and unmodified biochar with Fe addition to elucidate the mechanism by which magnetic biochar affects plant growth. We found that the addition of magnetic biochar significantly reduced the concentrations of available heavy metals in soil and inhibited heavy metal uptake by reeds. It also promoted the formation of iron plaque on reed roots to inhibit metal translocation. However, compared to unmodified biochar, magnetic biochar reduced reed performance, as indicated by the reduced plant biomass and photosynthetic ability, and it also reduced the biomass of soil bacteria and fungi. This was due to the interception of P by the iron plaque and the reduced concentration of soil available P. Collectively, although magnetic biochar exhibited a strong potential for heavy metal remediation, P supplementation is recommended to maintain plant performance and soil health when applying magnetic biochar.

Transcription profiling-guided remodeling of sulfur metabolism in synthetic bacteria for efficiently capturing heavy metals

Heavy metal contamination is becoming a global problem threatening human health. Heavy metal removal by engineered microbes by cellular adsorption and uptake is a promising strategy for treatment of heavy metal contamination. However, this strategy is confronted with limited heavy metal-capturing elements. In this study, we performed a transcription profiling-guided strategy for construction of heavy metal-capturing synthetic bacteria. Transcription profiling of a heavy metal-tolerating Cupriavidus taiwanensis strain revealed up-regulation of sulfur metabolism-related operons (e.g., iscSAU and moaEDAB) by Pb²⁺ and Cd²⁺. A synthetic Escherichia coli strain, EcSSMO, was constructed by design of a synthetic sulfur metabolism operon (SSMO) based on iscSAU/moaEDAB. Biochemical analysis and X-ray photoelectron spectroscopy (XPS) revealed that the synthetic bacteria had remodeled sulfur metabolism and enhanced heavy metal-tolerating capacity, with higher surviving EcSSMO cells than the surviving control cells Ec0 (not containing SSMO) at 50 mg/L of Pb²⁺ and Cd²⁺ (>92 % versus <10 %). Moreover, EcSSMO exhibited much higher heavy metal-capturing capacity than Ec0, removing>90 % of Pb²⁺ and Cd²⁺ at 5 mg/L of Pb²⁺ and Cd²⁺, and >40 % of both heavy metals even at 50 mg/L of Pb²⁺ and Cd²⁺. This study reveals emphasizes feasibility of transcription profiling-guided construction of synthetic organisms by large-scale remodeling metabolic network.

Characterization of heavy metals in textile sludge with hydrothermal carbonization treatment

Presence of heavy metals in sludge can severely limit land application due to their bioavailability. The current work studied distribution and risk as well as leaching toxicity of heavy metals in textile sludge treated with hydrothermal carbonization (HTC) at different conditions. Treatment temperature and time can significantly affect characteristics of heavy metals in sludge. For the treatment at 220 °C and 3 h, the content of Cu, Cr, Mn, and Zn existed in form F1 + F2 (water soluble and bound to carbonate and Fe-Mn oxides) dropped by 4.7, 7.1, 8.8, and 7.3%, while the content of Cu, Cr, and Mn in form F4 (bound to quartz, feldspars, etc) increased by 12.9, 19.1, and 10.6%, respectively. This effectively lowered the bioavailability and leaching rate of heavy metals in sludge. Addition of weak alkaline Al(OH)₃ could efficiently force the transformation of F1 to F4, possibly because of the increased pH value of sludge. HTC processing might be an effective way of fixing heavy metals in textile sludge.

The factors influencing sludge incineration residue (SIR)-based magnesium potassium phosphate cement and the solidification/stabilization characteristics and mechanisms of heavy metals

Magnesium potassium phosphate cement (MKPC) is prepared from MgO and KH₂PO₄ through an acid-base reaction and has been widely used in the rapid repairs of building structures and the solidification/stabilization (S/S) of heavy metals (HMs). The use of sludge incineration residue (SIR) rich in phosphorus resources to prepare SIR-based MKPC can achieve the reclamation of SIR and efficient HM S/S. Herein, based on the exploration of the optimal MKPC magnesia/phosphate ratio (M/P), the effects of SIR and HMs on the performance of the matrix and its interaction mechanism were comprehensively investigated. The results indicated that the compressive strength of the SIR-based MKPC increased first and then decreased with the gradual increase of SIR incorporation; the optimal was reached at 40.31 MPa when the SIR incorporation was 5 wt%. The peak signal and crystal lattice of Pb₂(PO₄)₃ indicated that there is a mixed effect between HMs (in SIR) and KH₂PO₄. The Visual MINTEQ analysis results also indicated that HMs are precipitated as HM phosphates. The formation of HM phosphates not only increases the M/P (with 30 wt% SIR, M/P increased by 0.019), affecting the microstructure and changing the compressive strength of the matrix, but also promotes the transformation of HMs from the bioavailable to the more stable residual forms. The residual forms of the six HMs were all above 84% after S/S. Therefore, the SIR-based MKPC preparation significantly immobilized the HMs; particularly, the leaching toxicities of Cu (96.6%) and Zn (96.3%) were alleviated.

Distribution, risk and bioavailability of metals in sediments of Lake Yamdrok Basin on the Tibetan Plateau, China

Total contents of metals in soil and sediments on the Tibetan Plateau of China have been widely analyzed, but existing information is insufficient to effectively evaluate metal ecological risk because of a lack of metal bioavailability data. In this study, distribution, potential risk, mobility and bioavailability of metals in sediments of Lake Yamdrok Basin in Tibet of China were explored by combined use of total digestion, sequential extraction and the diffusive gradient in thin-films (DGT). Average concentrations of Cr, Ni, Cu, Zn, As, Cd and Pb in surface sediments were 31.25, 30.31, 22.00, 45.04, 31.32, 0.13 and 13.39 mg/kg, respectively. Higher levels of metals were found near the inflowing rivers. Residual form was dominant in Cr, Ni, Zn, Cd and Pb, and reducible form was dominant in As and Cd. Metals in surface sediments showed a low enrichment degree overall, but Cd and As had higher ecological risk levels than the other metals. Furthermore, there was a larger average proportion of exchangeable form of As (20.4%) and Cd (9.0%) than the other metals (1.7%-3.3%), implying their higher mobility and release risk. Average DGT-labile concentrations of Cr, Ni, Cu, Zn, As, Cd and Pb were 0.5, 4.5, 0.7, 25.1, 60.0, 0.22 and 1.0 µg/L, respectively. The DGT-labile As was significantly correlated with extractable As forms (p< 0.01), suggesting that extractable As in sediments acts as a "mobile pool" for bioavailable As. These results suggest potential risks of As and Cd, especially As, deserve further attention in Lake Yamdrok Basin.

Leaching behaviors and speciation of cadmium from river sediment dewatered using contrasting conditioning

Chemical conditioning is an effective strategy for improved river sediment dewatering affecting both the dewatering efficiency and subsequent resource utilization of the dewatered cake. Two types of conditioning agents, polyaluminium chloride (PAC)/cationic polyacrylamide (PAM) (coagulation precipitation conditioning agent, referred to as P-P conditioning) and ferrous activated sodium persulfate (advanced oxidation conditioning agent, referred to as F-S conditioning) were examined. With increasing leach liquid to solid (L/S) ratio the concentration of Cd for the real time leachates from the dewatered cakes decreased, but the leaching ratio of Cd in both P-P and F-S dewatered cakes increased. With the same L/S, the leaching ratio was reduced for both types of conditioning, as compared to no conditioning, with the leaching ratio being least with F-S conditioning. The leaching ratio of Cd in the dewatered cake with L/S of 100 L kg^-1 was reduced from 21.3% of the total Cd present for the un-conditioned sediment to 12.5% upon P-P conditioning and 11.6% upon F-S conditioning. Furthermore, the different conditioning methods affected the Cd speciation in the dewatered cakes reducing the easy-to-leach speciation of exchangeable and carbonate-bound Cd species and increasing the potential-to-leach speciation of iron-manganese oxide and organically bound Cd species and also the difficult-to-leach species. Risk assessment indicates that the risk due to Cd leaching from the dewatered cakes at L/S of 100 L kg^-1 was reduced from high risk to medium risk after P-P and F-S conditioning with reduced bioavailability.

Environmental risk assessment in livestock manure derived biochars

Livestock-manure-derived biochar is one of major products obtained from the pyrolysis of livestock manure. This study quantitatively assesses the pollution level and ecological risks associated with heavy metals in livestock manure and the biochar produced by its pyrolysis. The geo-accumulation index (GAI) values of heavy metals in livestock manure were significantly decreased (P < 0.05) and indicated to be at the grade of uncontaminated expected for Zn in pig-manure-derived biochar (PMB, 0.94, 800 °C) via pyrolysis. Therefore, Zn should be paid more attention in PMB. The risk factors (E r i ) result shows that heavy metals in biochars were significantly decreased (P < 0.05) with increasing pyrolysis temperature. Potential ecological risk index values revealed that the integrated risks from the heavy metals were significantly decreased (P < 0.05) after pyrolysis. Similarly, the risk assessment code values indicated that the risks from the heavy metals in livestock-manure-derived biochars were significantly decreased (P < 0.05) after pyrolysis. In summary, pyrolysis represents an effective treatment method for livestock manure and can provide an effective method to reduce the risks of environmental pollution.