Occurrence, speciation and fate of mercury in the sewage sludge of China

Elimination of methylmercury production potential in excessive sludge in wastewater treatment plants by sulfur addition

Mercury ions (Hg(II)) in wastewater can accumulate and transform into the highly neurotoxic methylmercury (MeHg) in activated sludge. The release of MeHg can have severe environmental consequences, making the treatment of MeHg-contaminated sludge a pressing concern. In this study, we found that all the collected activated sludge samples, from different wastewater treatment plants in four cities, had the potential for Hg methylation. The Hg-methylating capacity reached a maximum level of 0.70-0.92 μg/g volatile suspended solids after 48 h of exposure to 5 μg/L Hg(II) and showed an average MeHg production rate of 4.8±0.5%. Accordingly, a sludge treatment method involving the addition of elemental sulfur (S0) for a short-term or long-term duration (3 or 180 days, respectively) was proposed. The results demonstrated that this treatment approach effectively mitigated and potentially eliminated MeHg formation by simultaneously reducing Hg bioavailability and Hg-methylating bioactivity. We found that bioavailable Hg(II) ions were converted to a secondary phase similar to insoluble HgS after S0 addition treatment, leading to a decrease in Hg bioavailability in sludge. The enhancement of Hg and extracellular polymeric substances (EPS) complexation via the increasing amount of thiol groups in EPS also reduced the Hg bioavailability after the long-term treatment. Furthermore, the long-term S0 addition significantly reduced the abundance of Hg-methylators with hgcA gene and promoted the growth of Hg-reducers with merA gene, which ensured the complete elimination of MeHg production potential of the excessive activated sludge. Our findings demonstrated that the proposed S0-addition sludge treatment is a promising and safe biotechnology for treating Hg-contaminated sludge. This approach has the potential to contribute significantly to the mitigation of MeHg pollution within environmental contexts.

A Comprehensive Exploration on Occurrence, Distribution and Risk Assessment of Potentially Toxic Elements in the Multi-Media Environment from Zhengzhou, China

Road dust (RD), roadside soils (RS) and river surface sediments (sediments) are important materials for evaluating contaminant levels in urban areas. This study aims to investigate the contaminant characteristics, pollution levels and ecological risks of RD, RS and sediments of potentially toxic elements (PTEs), including Cr, Ni, Cu, Zn, As, Cd, Hg and Pb, in the central urban area of Zhengzhou. Results reveal that RD shows a higher concentration of PTEs when compared to the other two environments. The spatial distribution characteristics suggest that PTEs in RD, RS and sediments may come from different sources. The geo-accumulation index (I_geo) was used to describe that the RD was moderately to extremely contaminated with Cd and Hg, while both RS and sediments were significantly enriched with Cd and Hg. For RD, RS and sediments, the potential ecological risk (RI) demonstrates a high potential ecological risk from Cd and Hg. Overall, PTEs in Zhengzhou road dust present a moderate risk.

Understanding of mercury and methylmercury transformation in sludge composting by metagenomic analysis

Municipal sewage especially the produced sewage sludge is a significant source releasing mercury (Hg) to the environment. However, the Hg speciation especially methylmercury (MeHg) transformation in sewage sludge treatment process remains poorly understood. This study investigated the transformation of Hg speciation especially MeHg in sludge composting. The distribution of Hg transformation related gene pairs hgcAB and merAB, and their putative microbial hosts were comprehensively analyzed. Both Hg (from 3.16±0.22 mg/kg to 3.20±0.19 mg/kg) and MeHg content (from 4.77±0.64 ng/g to 4.36±0.37 ng/g) were not obviously changed before and after composting, but about 19.69% of Hg and 27.36% of MeHg were lost according to mass balance calculation. The metagenomic analysis further revealed that anaerobes (Desulfobacterota and Euryarchaeota) were the mainly putative Hg methylators especially carrying high abundance of hgcA gene in the initial periods of composting. Among the 151 reconstructed metagenome-assembled genomes (MAGs), only 4 hgcA gene carriers (Myxococcota, Firmicutes, Cyclobacteriaceae, and Methanothermobacter) and 16 merB gene carriers were identified. But almost all of the MAGs carried hgcB gene and merA gene. The merA gene was widely distributed in genomes, which indicated the widespread functionality of microbes for reducing Hg(II) to Hg(0). The hgcA carrying microbes tends to present the similar metabolic pathways including methanogenesis and sulfur metabolism. Besides, both the irregular distribution of hgcA in various species (including Actinobacteria, Archaea, Bacteroidetes, Desulfobacterota, Euryarchaeota, and Nitrospirae, etc.) and opposite evolution trends between hgcA gene abundance and its host genome abundance can be an indication of horizontal gene transfer or gene deletions of hgcA during composting. Our findings thus revealed that sludge composting is not only a hotspot for Hg speciation transformation, but also a potential hotspot for MeHg transformation.

Influence of dissolved organic matter on methylmercury transformation during aerobic composting of municipal sewage sludge under different C/N ratios

Current knowledge about the transformation of total mercury and methylmercury (MeHg) in aerobic composting process is limited. In this study, the composition and transformation of mercury and dissovled organic matter (DOM) in aerobic composting process of municipal sewage sludge were were comprehensively characterized, and the differences among the three C/N ratio (20, 26 and 30) were investigated. The main form of mercury in C/N 20 and 26 was organo-chelated Hg (F3, 46%-60%); while the main form of mercury in C/N 30 was mercuric sulfide (F5, 64%-70%). The main component of DOM in C/N 20 and 26 were tyrosine-like substance (C1, 53%-76%) while the main fractions in C/N 30 were tyrosine-like substance (C1, 28%-37%) and fulvic-like substance (C2, 17%-39%). The mercury and DOM varied significantly during the 9 days composting process. Compared to C/N 20 and 26, C/N 30 produced the less MeHg after aerobic composting process, with values of 658% (C/N 20), 1400% (C/N 26) and 139% (C/N 30) of the initial, respectively. Meanwhile, C/N 30 produced the best compost showed greater degree of DOM molecular condensation and humification. Hg fraction had been altered by DOM, as indicated by a significant correlation between mercury species and DOM components. Notably, C/N 30 should be used as an appropriate C/N ratio to control the methylation processes of mercury and degration of DOM.

A review of microplastic fibres: generation, transport, and vectors for metal(loid)s in terrestrial environments

The laundering of synthetic fabrics has been identified as an important and diffuse source of microplastic (<5 mm) fibre contamination to wastewater systems. Home laundering can release up to 13 million fibres per kg of fabric, which end up in wastewater treatment plants. During treatment, 72-99% of microplastics are retained in the residual sewage sludge, which can contain upwards of 56 000 microplastics per kg. Sewage sludge is commonly disposed of by application to agricultural land as a soil amendment. In some European countries, application rates are up to 91%, representing an important pathway for microplastics to enter the terrestrial environment, which urgently requires quantification. Sewage sludge also often contains elevated concentrations of metals and metalloids, and some studies have quantified metal(loid) sorption onto various microplastics. The sorption of metals and metalloids is strongly influenced by the chemical properties of the sorbate, the solution chemistry, and the physicochemical properties of the microplastics themselves. Plastic-water partition coefficients for the sorption of cadmium, mercury and lead onto microplastics are up to 8, 32, and 217 mL g^-1 respectively. Sorptive capacities of microplastics may increase over time, due to environmental degradation processes increasing the specific surface area and surface density of oxygen-containing functional groups. A range of metal(loid)s, including cadmium, chromium, and zinc, have been shown to readily desorb from microplastics under acidic conditions. Sorbed metal(loid)s may therefore become more bioavailable to soil organisms when the microplastics are ingested, due to the acidic gut conditions facilitating desorption. Polyester (polyethylene terephthalate) should be of particular focus for future research, as few quantitative sorption studies currently exist, it is potentially overlooked from density separation studies due to its high density, and it is by far the most widely used fibre in apparel textiles production.

Mercury emission profile for the torrefaction of sewage sludge at a full-scale plant and application of polymer sorbent

We evaluated mercury (Hg) behavior in a full-scale sewage sludge torrefaction plant with a capacity of 150 wet tons/day, which operates under a nitrogen atmosphere at a temperature range of 250-350 °C. Thermodynamic calculations and monitoring results show that elemental Hg (Hg⁰) was the dominant species in both the pyrolysis gas during the torrefaction stage and in the flue gas from downstream air pollution control devices. A wet scrubber (WS) effectively removed oxidized Hg from the flue gas and moved Hg to wastewater, and an electrostatic precipitator (ESP) removed significant particulate-bound Hg but showed a limited capacity for overall Hg removal. Hg bound to total suspended solids had a much higher concentration than that of dissolved Hg in wastewater. Total suspended solid removal from wastewater is therefore recommended to reduce Hg discharge. Existing air pollution control devices, which consist of a cyclone, WS, and ESP, are not sufficient for Hg removal due to the poor Hg⁰ removal performance of the WS and ESP; a further Hg⁰ removal unit is necessary. A commercial packed tower with sorbent polymer catalyst composite material was effective in removing Hg (83.3%) during sludge torrefaction.

Heavy metal(loid)s in sewage sludge in China: concentrations and spatial-temporal variations

Sewage sludge (SS) production in China has increased rapidly, accompanying the fast expansion of its sewage treatment capacity. Heavy metals (HMs) in SS have been a great concern, hampering the utilization and disposal of SS. In this study, heavy metal (HM) contents in SS from throughout China were determined. The median values of HMs in SS decreased in the order Zn > Cu > Cr > Pb > Ni > As > Hg > Cd. The general attainment rates of HMs in SS are satisfying (> 90%). Combining the present data with those obtained from references, spatial distributions and temporal trends of HMs in SS were analyzed. Depending on the specific HM element, the spatial variation trend might be decreasing trends from south to north and from east to west of China. The element-specific hot spots of SS with relatively high HM contents were identified. Analysis of the historical data in different time intervals reviewed obvious decreasing trends in HM contents of SS in China, indicating the well implementation of more and more stringent environmental regulations.

Fate of mercury and methylmercury in full-scale sludge anaerobic digestion combined with thermal hydrolysis

Methylmercury (MeHg) is one of the highly toxic and bio-accumulated forms of mercury. Its presence in wastewater treatment processes has been evidenced in recent studies. Considering its enrichment in sewage sludge and the ecological risk associated with its land application, this study investigated the fate of mercury and MeHg in full-scale anaerobic digestion combined with Cambi thermal hydrolysis based on one-year sampling. Results showed that the advanced anaerobic digestion could increase the total mercury (THg) content from 4.35 ± 0.43 mg/kg in raw sludge to 6.37 ± 1.05 mg/kg in digested sludge, and the MeHg content decreased from 1.61 to 8.94 ng/g in raw sludge to 0.21-2.03 ng/g after anaerobic digestion. The demethylation of MeHg was dominant in both thermal hydrolysis and anaerobic digestion; it was mostly derived from the physico-chemical impacts such as chemical decomposition in thermal hydrolysis and precipitation in anaerobic digestion. Although the reported microbial methylators, such as Methanosarcina and Clostridia, were dominant in anaerobic digestion, the relative abundances of hgcA and merA were relatively low and did not correlate with the MeHg profiles. Thus, microbial methylation or demethylation seems negligible in terms of MeHg transformation.

Leaching behavior and transformation of total mercury and methylmercury from raw and lime-conditioned sewage sludge under simulated rain

Mercury (Hg) that leaches from municipal sewage sludge (MSS) landfill under natural rain is of increasing concern. The column leaching experiments were conducted to investigate the leaching characteristics of total mercury (THg) and methylmercury (MeHg) as well as pH, total organic carbon (TOC), and total suspended solids (TSS) in the raw sludge (RS) and lime-conditioned sludge (LCS) under simulated rain with different acidities (pH 6.5 and 2.9). Results showed the release of MeHg in the leachates presented different patterns from THg. And the final amounts of MeHg in the MSS columns were 1.49 (RS at pH = 6.5), 1.88 (RS at 2.9), 1.97 (LCS at pH = 6.5), and 2.06 times (LCS at pH = 2.9) higher than the initial amounts, suggesting methylation of inorganic Hg (IHg) occurred in the leaching process. The leaching efficiencies of THg and MeHg in RS was lower than that in LCS, indicating lime was more favorable for the release of THg and MeHg. And lower values of pH of the simulated rain promoted the release of THg and MeHg from RS while the opposite was true for LCS. This study provides a better understanding of the release and biogeochemical transformations of Hg in MSS.

Implementation of Modified Acacia Tannin by Mannich Reaction for Removal of Heavy Metals (Cu, Cr and Hg)

The modified tannin by Mannich reaction was investigated for wastewater treatment. The removal of heavy metals, such as copper, chromium and mercury, in industrial wastewater was evaluated through the coagulation–flocculation technique, using modified Acacia tannin (MAT) as a coagulant agent. The successful tannin modification was evaluated by infrared spectopometry (FTIR), nuclear magnetic resonance (NMR); monitoring the removal of heavy metals was performed by atomic absorption (AA) and a direct mercury analyzer (DMA). Additionally, the parameters of water quality, total suspended solids (TSS), turbidity and chemical oxygen demand (COD) were assessed. Different doses of MAT were evaluated (375 ppm, 750 ppm, 1250 ppm and 1625 ppm) and three different levels of pH (4, 7 and 10). The highest percentages of removal obtained were copper 60%, chromium 87%, mercury 50%–80%, COD 88%, TSS 86% and turbidity 94%, which were achieved with the dose of 375 ppm of MAT at pH 10. The coagulation–flocculation process with the modified Acacia tannin is efficient for the removal of conventional parameters and for a significant removal of the metals studied.