Transformation Of Heavy Metals Research Articles

Mechanism of high content Mn & Ni synchronous stable solidification in Fe-spinel glass-ceramics from battery slag

New-energy industry produce large amount of emerging high-speed battery jarosite slag, containing Mn and Ni, which is large in quantity and harmful. Harmless treatment is in urgent need. We research on the curing mechanism of heavy metals migration and transformation during phase via iron clusters transformation in glass-ceramics. Based on first-principles and density functional theory, regulating electronic structure of Mn2+ and Ni2+ by Cr3+. After clarifying the interaction among curing compatibility, capacity and stability, the structure-activity relationship among electronic & molecular structure and leaching concentration of heavy metals was established. The results show that CaNiSi2O6 is difficult to cure Ni2+, the formation of iron clusters promotes the conversion of multiphase to Fe-Spinel A; Mn2+, Ni2+ went into the octahedron in Fe-Spinel A cannot be stabilized; Cr3+ promotes migration of Mn2+, Ni2+ into the tetrahedral, turning Fe-Spinel A into Fe-Spinel B, improves curing compatibility of Mn2+, Ni2+; Cr3+ also adjust the bond structure, which enhances solid solution attraction to Mn2+ and Ni2+, and capacity of Mn2+ and Ni2+ increases from 19%, 30%–147%, 195%; improving compatibility and capacity of Fe-Spinel reduces leaching concentration of Mn2+ and Ni2+ from 1.32, 0.51 mg/L to 0.48, 0.03 mg/L. This work realizes the harmless disposal of recycling Mn- and Ni-rich battery slag. The solidified body can also be reused as eco-friendly resource.

How atmospheric deposition contribute to watershed heavy metals contamination in coastal watersheds in China: A case study of Laizhou Bay

Atmospheric deposition is a significant source of heavy metal (HM) pollution. In order to understand the migration and transformation process of atmospheric HMs within the watershed and quantify the amount transported offshore by rivers, the Soil and Water Assessment Tool (SWAT) was developed to trace the migration of HMs from atmospheric deposition. The model simulates HMs in three forms: dissolved, adsorbed, and granular. It quantifies the movements of Cd, Cr, Cu, Hg, Pb, and Zn from atmospheric deposition into the sea via rivers in five coastal watersheds in East China and analyzes the effects of meteorological factors, vegetation cover, and slope on non-point pollution of these metals by Spearman correlation analysis. The results showed that the annual flux of HMs from atmospheric deposition to the sea through rivers accounted for 5 %–69 % of the total rivers flux. Among meteorological factors, precipitation demonstrated the strongest correlation with the monthly loads of HMs entering rivers from atmospheric deposition. Additionally, HMs loads entering rivers from atmospheric deposition were more closely related to vegetation cover than topographic slope. This model provides a new approach to distinguishing the flux of atmospheric HMs entering offshore waters through rivers. The findings will deepen our understanding of the migration and transformation of HMs from atmospheric deposition, enhance the ability to control offshore HMs pollution, and reduce the ecological risks associated by HMs.

Sedimentation and transformation of heavy metals during the transport from the Yellow River estuary to the sea: Evidence from surface sediments of the Yellow River subaqueous delta

Estuarine and coastal areas are one of the ultimate sinks for terrestrial heavy metals which play a vital role on the aquatic ecosystem. This study examined heavy metals contents and speciations in Yellow River subaqueous delta surface sediment, to characterize their sedimentation and transformation during transport from estuary to the sea. The results showed that higher concentrations were found in the seaward and the shear front affected areas. The surface sediments were generally not contaminated by heavy metals, except for Cd was highly enriched, while the post-standardization spatial distribution reflected the effect of estuarine processes as a “filter” that effectively intercepting heavy metals. The dominated phase was residual fraction for Cr, Cu, Zn, Cd, while reducible fraction for Ni and carbonate fraction for Pb. The transformation of heavy metal species was mainly influenced by the changes in the sediment components such as carbonate minerals, organic matter and FeMn oxides.

Transformation of typical heavy metals during preparation of porous carbon from coal gasification fine slag

To clarify the fate of typical heavy metals during preparation process of porous carbon, we produced porous carbon using a two-step strategy and investigated the transformation behavior of heavy metals including V, Cr, Mn, Cu, Zn and Ba in the process. A modified Community Bureau of Reference (BCR) sequential extraction procedure was implemented to reveal the occurrence state of heavy metals. The results clearly showed that the transformation of selected heavy metals in activated carbon was mostly determined by the occurrence state, volatile nature and operating procedure. In addition to the new aluminosilicate minerals, abundant pore structure also had evident dependence on solidifying heavy metals in porous carbon. The content of heavy metals in activated carbon were considerably lower than YKFS, and V, Cr and Cu were more accumulated in porous carbon. It is also important to note that Zn possessed markedly lower content and REI value (3.61–56.17 %) in activated carbon and porous carbon owning to the volatile nature of Zn and acid solubility of Zn-base minerals. The Risk Assessment Code (RAC) evaluation conducted that the acid leaching operation effectively decreased the hazardous properties of heavy metals. The research results can provide significant guidance for value-added utilization of gasification fine slag in an environmentally friendly.

Predictive Machine Learning Model to Assess the Adsorption Efficiency of Biochar-Heavy Metals for Effective Remediation of Soil-Plant Environment.

Biochar is crucial for agricultural output and plays a significant role in effectively eliminating heavy metals (HMs) from the soil, which is essential for maintaining a soil-plant environment. This work aimed to assess machine learning models to analyze the impact of soil parameters on the transformation of HMs in biochar-soil-plant environments, considering the intricate non-linear relationships involved. A total of 211 datasets from pot or field experiments were evaluated. Fourteen factors were taken into account to assess the efficiency and bioavailability of HM-biochar amendment immobilization. Four predictive models, namely linear regression (LR), partial least squares (PLS), support vector regression (SVR), and random forest (RF), were compared to predict the immobilization efficiency of biochar-HM. The findings revealed that the RF model was created using 5-fold cross-validation, which exhibited a more reliable prediction performance. The results indicated that soil features accounted for 79.7% of the absorption of HM by crops, followed by biochar properties at 17.1% and crop properties at 3.2%. The main elements that influenced the result have been determined as the characteristics of the soil (including the presence of different HM species and the amount of clay) and the quantity and attributes of the biochar (such as the temperature at which it was produced by pyrolysis). Furthermore, the RF model was further developed to predict bioaccumulation factors (BAF) and variations in crop uptake (CCU). The R2 values were found to be 0.7338 and 0.6997, respectively. Thus, machine learning (ML) models could be useful in understanding the behavior of HMs in soil-plant ecosystems by employing biochar additions.

Advances in Research on Bacterial Oxidation of Mn(II): A Visualized Bibliometric Analysis Based on CiteSpace.

Manganese (Mn) pollution poses a serious threat to the health of animals, plants, and humans. The microbial-mediated Mn(II) removal method has received widespread attention because of its rapid growth, high efficiency, and economy. Mn(II)-oxidizing bacteria can oxidize toxic soluble Mn(II) into non-toxic Mn(III/IV) oxides, which can further participate in the transformation of other heavy metals and organic pollutants, playing a crucial role in environmental remediation. This study aims to conduct a bibliometric analysis of research papers on bacterial Mn(II) oxidation using CiteSpace, and to explore the research hotspots and developmental trends within this field between 2008 and 2023. A series of visualized knowledge map analyses were conducted with 469 screened SCI research papers regarding annual publication quantity, author groups and their countries and regions, journal categories, publishing institutions, and keywords. China, the USA, and Japan published the most significant number of research papers on the research of bacterial Mn(II) oxidation. Research hotspots of bacterial Mn(II) oxidation mainly focused on the species and distributions of Mn(II)-oxidizing bacteria, the influencing factors of Mn(II) oxidation, the mechanisms of Mn(II) oxidation, and their applications in environment. This bibliometric analysis provides a comprehensive visualized knowledge map to quickly understand the current advancements, research hotspots, and academic frontiers in bacterial Mn(II) oxidation.

The spatial distribution and source of heavy metals in soil-plant-atmosphere system in a large coal mining area

Due to the lack coupling analysis of heavy metals in soil-plant-atmosphere system, the migration and transformation of heavy metals remains unclear. In this study, 240 surface (0–20 cm) soil samples, 365 plant samples and 168 atmospheric dust samples were collected in a large coal mining area, in which selected heavy metals arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), plumbum (Pb), zinc (Zn) were analyzed, to elucidate the spatial distribution and source of heavy metals and their bioavailability. According to the results of ArcGIS mapping analysis, the distribution patterns of Cd, Cu, Pb, and Zn in the three media are similar, while the distribution characteristics of Cr and Ni are highly consistent. The factors contributing to these spatial distribution characteristics of heavy metals could be associated with human activities and the characteristics of the elements. Compared with the background value, Zn, Cr, Pb in soil, Zn, Cr, Cu in plants and Zn, Pb, Cu in atmospheric dust apparently accumulated. The geological accumulation pollution index (Igeo) shows that there exist mild to moderate levels of Cr and Cd pollution in soil, and mild to moderate levels of Cd, Cr, Pb, and Zn pollution in the air. The evaluation of potential risks of heavy metals showed moderate ecological hazards in soil and strong ecological hazards in atmospheric dust. The sources of heavy metals were assessed using principal component analysis (PCA), land use type, and spatial analysis. Anthropogenic factors are the main sources of heavy metal content. Coal-related industrial processes related to coal, sewage irrigation, the use of agricultural chemicals, and vehicle emissions may be anthropogenic sources of heavy metals.

Mineral transformation and solidification of heavy metals during co-melting of MSWI fly ash with coal fly ash.

Melting is an efficient method to turn municipal solid waste incineration (MSWI) fly ash (FA) into non-hazardous material. Coal fly ash (CFA) was selected as the silica-alumina source to carry out co-melting research with MSWI FA in this work. The effects of the temperature and the CFA content on mineral transformation and the migration characteristics of heavy metals were analyzed. The results showed that the mixtures of MSWI FA and CFA reacted at high temperatures to mainly generate Ca2Al2SiO7, Ca2SiO4, and CaAl2Si2O8 primarily and then melted and formed the amorphous-phase vitreous body when the CFA content was more than 40% and the temperature was higher than 1300 °C. During the melting process, Cd and Pb were almost volatilized, while Cr, Mn, and Ni were almost retained. Besides, the volatilization rates of Cu and Zn fluctuated with the temperature and the CFA content. Suitable treatment temperature and CFA content were conducive to the transformation of the heavy metals in the FA into stable forms, and the melting products were no longer hazardous wastes because the vitreous body could effectively encapsulate heavy metals. This study aims to help reuse the FA and CFA collaboratively and be more environmentally friendly.

Comparative investigation on composting and pyrolysis of swine manure: Heavy metals transformation, nitrogen immobilization and integrated environmental risk assessment

Harmless treatment of livestock manure has been a pressing issue due to its contamination of heavy metals and volatilization of nitrogen(N)-containing components during processing. In this paper, the chemical species transformation, leaching characteristics of Cr, Mn, Zn, Cu and conversion of amine N and inorganic N during composting and pyrolysis of swine manure (400°C, 500°C, 600°C and 700°C) were comparatively investigated, and their integrated environmental risk (η) was assessed. According to the ultimate and proximate analysis, pyrolyzed char exhibited better aromaticity and richer functional groups compared with that of compost; Cr, Mn, Zn, Cu were transformed into more stable residual states (F3+F4), and better inhibition of leaching in simulated landfill experiments was shown than compost. However, deamination and cyclocondensation reactions under higher pyrolysis temperatures resulted in N escaping in the form of NH3 and HCN. Considering various pollution coefficients, the integrated ecological risk was reduced by 59.24 % after 600°C pyrolysis, which is the optimal condition for the minimization of the risk of heavy metal toxicity and N volatilization among all the compost and pyrolysis process.

PET microplastics influenced microbial community and heavy metal speciation in heavy-metal contaminated soils

Co-occurrence of microplastics and heavy metals is common in soils. The interactions between microplastics and heavy metals have received increasing attention in recent years, particularly for revealing the mechanisms underpinning their interactions. In the present study, Polyethylene Terephthalate (PET) microplastics are added in soil, which have led to variations in soil physicochemical properties, enzymatic activity, microbial community structure, and chemical speciation of chromium (Cr), cadmium (Cd), copper (Cu), and zinc (Zn) in heavy metal contaminated soils. Soil physicochemical properties (pH, SWC and SOC) and enzymatic activities (urease and dehydrogenase) alter as result of PET addition, which promoted the transformation of heavy metal chemical speciation from bioavailable fraction to oxidizable and residual fraction. The results suggested that PET microplastics in heavy metal contaminated soil may reduce the heavy metal bioavailability by altering the properties of the soil microenvironment. The significant variations in microbial community have been observed. The relative abundance of Gp6, Gaiella, Nocardia, Herbaspirllum and Gp16 changed dramatically throughout the incubation period, which were either heavy metal tolerant or played crucial roles in modifying the chemical composition of heavy metals. This work contributes to an in-depth understanding of the effects of microplastics on the biochemical transformation of heavy metal contaminated soils, which will be valuable to the development of clean up strategy for soils with co-contaminant of microplastics and heavy metals.

Biochar derived from animal and plant facilitates synergistic transformation of heavy metals and phosphorus in sewage sludge composting

This study investigated the influence of plant-derived biochar (PB) and animal-derived biochar (AB) on behavior of heavy metals and phosphorus fractions during sewage sludge composting. PB was highly effective in reducing the bioavailability of Zn and Cu by 39% and 50%, respectively, while AB decreased the bioavailability of Pb (30%) and Cd (12%). Both biochar increased available phosphorus by over 38%. Acid extractable and bioavailable Pb in AB, and water-soluble, oxidizable and total Zn, acid extractable and oxidizable Cu in PB were positively correlated with moderately resistant organic phosphorus (MROP). Besides, in AB, Cd had strong and positive correlation with highly resistant organic phosphorus (HROP). This suggested biochar facilitated the formation of stable organometallic complexes through binding metal ions to phosphorus fractions, with notable differences based on biochar source. FT-IR showed biochar promoted humification, with PB enhancing carboxyl and polysaccharide formation, while AB encouraged quinone and aryl ether structures. These surface functional groups on the biochar likely contributed to heavy metals and phosphorus binding through chelation, adsorption, and electron shuttling.

Heavy metal tolerance and detoxification mechanism mediated by heavy metal resistance genes in compost habitat.

Heavy metal pollution from compost is one of the most concerned environmental problems, which poses a threat to the ecosystem and human health. This study aims to reveal the heavy metal tolerance and detoxification mechanism mediated by heavy metal resistance genes (HMRGs) in compost habitat through metagenomics combined with chemical speciation analysis of heavy metals. The results showed that there were 37 HMRGs corresponding to 7 common heavy metal(loid)s in composting, and they had the ability to transform heavy metals into stable or low-toxic speciation by regulating enzyme transport, redox, methylation, etc. This study summarized the heavy metal metabolism pathway mediated by HMRGs, providing a new perspective for understanding the transformation of heavy metals in the composting process.

Co-pyrolysis of municipal sludge and papermaking sludge: Pyrolysis behaviors and heavy metals immobilizations

The co-pyrolysis of municipal sludge (MS) and papermaking sludge (PS) is beneficial for improving the quality of co-pyrolysis biochar and the stability of heavy metals in it. This study investigated the synergistic effect of co-pyrolysis of MS and PS, as well as the influence of different mass ratios (1:9, 3:7, 5:5, 7:3, and 9:1; MS:PS) and pyrolysis temperatures (400–600 ℃) on the physicochemical properties of pyrolysis char, chemical speciation of heavy metals, and their potential ecological risk index. Furthermore, thermodynamic equilibrium calculations were used to study the transformation and stabilization mechanisms of heavy metals during co-pyrolysis. The results indicated that the addition of PS increased the comprehensive pyrolysis index of MS (by 2.09–16.72 times) and the specific surface area of co-pyrolysis char. The inclusion of MS effectively promoted the transformation of heavy metals from unstable fractions to stable fraction, significantly reducing the potential ecological risk index of pyrolysis biochar (by 21.93–41.58 %). The combination of thermodynamic equilibrium calculations and experimental data demonstrated that heavy metals in sludge could be stabilized through the formation of stable heavy metal compounds or their encapsulation within the lattice of inorganic components during co-pyrolysis. These findings suggest that co-pyrolysis of MS and PS is a feasible and promising approach for producing high-quality pyrolysis char with low risk of heavy metals.

Remediation of Pb-, Zn-, Cu-, and Cd-Contaminated Soil in a Lead–Zinc Mining Area by Co-Cropping Ilex cornuta and Epipremnum aureum with Illite Application

Phytoremediation is considered an effective strategy for remediation of heavy-metal-contaminated soil in mining areas. However, single-species plants cannot reach the highest potential for uptake of heavy metals due to inhibition of their growth by high concentrations of heavy metals in the soil. Therefore, this study has explored the effects of illite application and two plant species’ co-cropping on soil quality, plant growth, and heavy metal transformation in a soil–plant system. The results reveal that the addition of 1% (mass fraction) of illite significantly enhances soil pH. The co-cropping of Ilex cornuta and Epipremnum aureum is beneficial for improving the organic matter content of the soil. The contents of EDTA-extractable Pb, Zn, and Cu were significantly reduced by 29.8–32.5%, 1.85–5.72%, and 30.0–32.9%, respectively, compared to the control. The co-cropping of Ilex cornuta and Epipremnum aureum promoted enrichment effects of Epipremnum aureum on Pb and Ilex cornuta on Cd (p < 0.05). The co-cropping pattern lowered the biomass of Ilex cornuta and Epipremnum aureum; however, co-cropping of Ilex cornuta and Epipremnum aureum promoted the elimination of Pb, Zn, Cu, and Cd from the soil at 13.0–75.8%, 11.1–38.2%, 8.39–88.4%, and 27.8–72.5%, respectively. It is concluded that illite application combined with co-cropping of Ilex cornuta and Epipremnum aureum is highly effective for the elimination of Pb, Zn, Cu, and Cd from contaminated soil. This study provides a theoretical basis and pathway for the restoration of heavy-metal-contaminated soil in mining with the application of bentonite combined with phytoremediation.

Transformation of heavy metals from contaminated water to soil, fodder and animals

A serious environmental problem that threatens soil quality, agricultural productivity, and food safety is heavy metal pollution in water sources. Heavy metal pollution is the main problem in tehsil Pasrur, Sialkot, Pakistan. Present study was arranged to notice the heavy metals in water, soil, forages and buffalo milk. There are seven sites that were used for this experiment. Highest malondialdehyde (MDA) contents (3.00 ± 0.01) were noticed in barseem roots at site 7. Ascorbate Peroxidase (APX) was reached at its peak (1.93 ± 0.01) at site 7 in the fresh barseem. Maximum protein contents (0.36 ± 0.01) were observed in fresh plant samples at site 2. Site 3's buffalo milk samples had the highest Ni content (7.22 ± 0.33 ppm), while Site 3's soil samples had the lowest Cr content (8.89 ± 0.56 ppm), Site 1's plant shoots had the lowest Cr content (27.75 ± 1.98 ppm), and Site 3's water had the highest Cr content (40.07 ± 0.49 ppm). The maximum fat content (5.38 ± 2.32%) was found in the milk of the animals at site 7. The highest density (31.88 ± 6.501%), protein content (3.64 ± 0.33%), lactose content (5.54 ± 0.320%), salt content (0.66 ± 0.1673%), and freezing point (− 0.5814 ± 0.1827 °C) were also observed in the milk from animals at site 7, whereas site 5 displayed the highest water content (0.66 ± 0.1673%) and peak pH value (11.64 ± 0.09). In selected samples, the pollution load index for Ni (which ranged from 0.01 to 1.03 mg/kg) was greater than 1. Site 7 has the highest conductivity value (5.48 ± 0.48). Values for the health risk index varied from 0.000151 to 1.00010 mg/kg, suggesting that eating tainted animal feed may pose health concerns. Significant health concerns arise from metal deposition in the food chain from soil to feed, with nickel having the highest health risk index.

Effect of aluminosilicates on the release and form transformation of semi-volatile heavy metals during the combustion of hyperaccumulator plants

With the development of phytoremediation of heavy metals contaminated soils, the harmless disposal of hyperaccumulator plants has been imminent. In this study, the effects of typical aluminosilicates on the heavy metal migration during the combustion of ryegrass were compared. The addition of kaolin, montmorillonite, and γ-Al2O3 were all effective in solidifying Pb and Zn. As the combustion temperature increased, the toxicity of remaining Pb and Zn decreased. Compared to montmorillonite, kaolin was more effective in capturing Pb and Zn at 600–825 °C, and the remaining Pb and Zn were mainly in acid-soluble form. The large amount of alkali/alkaline earth metals will react with the Si/Al components to form various potassium feldspars and calcium silicates, thus inhibiting the solidification of heavy/alkali metals. However, the Si in montmorillonite was more effective in converting Pb and Zn into residual form than kaolin when faced with the competition solidification of alkali/alkaline earth metals, at which time the percentage of Pb and Zn in the residual form increased from 13.27% and 6.00% to 39.21% and 9.84%, respectively. This work provided a research perspective based on the solidification of heavy metals by Si component, which enables a more comprehensive analysis of the differences in heavy metals solidification by Si/Al-based minerals.

Endogenous iron-enriched biochar derived from steel mill wastewater sludge for tetracycline removal: Heavy metals stabilization, adsorption performance and mechanism

Steel mill wastewater sludge, as an iron-enriched solid waste, was expected to be converted into iron-enriched biochar with acceptable environmental risk by pyrolysis. The purpose of our study was to evaluate the chemical speciation transformation of heavy metals in biochar under various pyrolysis temperatures and its reutilization for tetracycline (TC) removal. The experimental data indicated that pyrolysis temperature was a key factor affecting the heavy metals speciation and bioavailability in biochar, and biochar with pyrolysis temperature at 450 °C was the most feasible for reutilization without potential risk. The endogenous iron-enriched biochar (FSB450) showed highly efficient adsorption towards TC, and its maximum adsorption capacity could reach 240.38 mg g−1, which should be attributed to its excellent mesoporous structure, abundant functional groups and endogenous iron cycling. The endogenous iron was converted to a stable iron oxide crystalline phase (Fe3O4 and MgFe2O4) by pyrolysis, which underwent a valence transition to form a coordination complex with TC by electron shuttling in the FSB450 matrix. The study provides a win-win approach for resource utilization of steel wastewater sludge and treatment of antibiotic contamination in wastewater.

Effect of organic matter on the environmental behavior of sulfur and heavy metals in mariculture sediments during the aging process

Organic matter (OM) significantly impacts the environmental behavior of sulfur and heavy metals. In this study, the effects of OM on the migration and transformation of sulfur and heavy metals in mariculture sediments were investigated. The results indicated that baiting had a strong impact on the accumulation of acid volatile sulfur (AVS) (P < 0.05) and increased the environmental risk of sulfide in sediments. The addition of bait promoted the generation of chromium (II)-reducible sulfur (CRS); however, the resistance of AVS to CRS conversion increased with increasing bait addition. The addition of bait considerably influenced Cd accumulation. The acid-soluble fractions of Cr and Cu and the oxidizable fraction of Cd were primarily affected by the bait addition (coefficient of variation>15 %). An increase in the reducible fraction promoted the conversion of AVS to CRS, which reduced the degree of sediment aging. Higher OM levels reduced the diversity and abundance of the bacterial communities. The sulfate respiration functional microbiota was particularly affected by OM.

Combination Mechanism of Soil Dissolved Organic Matter and Cu2+ in Vegetable Fields, Forests and Dry Farmland in Lujiang County

Dissolved organic matter (DOM) serves as a critical link in the migration and transformation of heavy metals at the soil–solid interface, influencing the migration behaviour and transformation processes of Cu2+ in soil. There have been studies on the combination mechanisms between DOM and Cu2+ in paddy soils. However, the adsorption/complexation and redox processes between DOM and Cu2+ in other agricultural soil types (such as dry farmland and vegetable fields) are unclear. In order to reveal the combination process of DOM with Cu in different agricultural soil types and the dynamic changes in chemical behaviour that occur, this study analysed the variability of DOM components and structure in three soils using three-dimensional fluorescence spectroscopy and X-ray photoelectron spectroscopy. In addition, the priority order of different DOM compounds in combination with Cu and the change process in relation to the Cu valence state in the soil of Lujiang County, Anhui Province, was revealed based on laboratory experiments. The results showed that the composition of soil DOM was mainly composed of humic-like and fulvic-like substances with a clear terrestrial origin and that the organic matter showed a high degree of decomposition characteristics. The results indicated that the composition of soil DOM is mainly composed of humic and fulvic acid-like substances, and they have obvious characteristics of terrestrial origin. In addition, the soil organic matter showed high decomposition characteristics. The complex stability constants (lgKM) of humic acid-like substances with Cu2+ follow the order of forest land (lgKM = 5.21), vegetable land (lgKM = 4.90), and dry farmland (lgKM = 4.88). The lgKM of fulvic acid-like substances with Cu2+ is in the order of dry farmland (lgKM = 4.51) and vegetable land (lgKM = 4.39). Humic acid-like substances in soil DOM combine preferentially with Cu2+, showing a stronger chelating affinity than fulvic acid-like substances. Cu2+ complexes mainly include hydroxyl, phenolic hydroxyl and amino functional groups are included in soil DOM, accompanied by redox reactions. In comparison to dry farmland, the soil DOM in forest and vegetable fields undergoes more intense redox reactions simultaneously with the chelation of Cu2+. Therefore, the application of organic fertilisers to vegetable and forest soils may lead to uncertainties concerning the fate of heavy metals with variable chemical valence. These results contribute to a deeper understanding of the interaction mechanisms between DOM and Cu2+ in agricultural soils.

Role of phosphate in microalgal-bacterial symbiosis system treating wastewater containing heavy metals

Phosphorus is one of the important factors to successfully establish the microalgal-bacterial symbiosis (MABS) system. The migration and transformation of phosphorus can occur in various ways, and the effects of phosphate on the MABS system facing environmental impacts like heavy metal stress are often ignored. This study investigated the roles of phosphate on the response of the MABS system to zinc ion (Zn2+). The results showed that the pollutant removal effect in the MABS system was significantly reduced, and microbial growth and activity were inhibited with the presence of Zn2+. When phosphate and Zn2+ coexisted, the inhibition effects of pollutants removal and microbial growth rate were mitigated compared to that of only with the presence of Zn2+, with the increasing rates of 28.3% for total nitrogen removal, 48.9% for chemical oxygen demand removal, 78.3% for chlorophyll-a concentration, and 13.3% for volatile suspended solids concentration. When phosphate was subsequently supplemented in the MABS system after adding Zn2+, both pollutants removal efficiency and microbial growth and activity were not recovered. Thus, the inhibition effect of Zn2+ on the MABS system was irreversible. Further analysis showed that Zn2+ preferentially combined with phosphate could form chemical precipitate, which reduced the fixation of MABS system for Zn2+ through extracellular adsorption and intracellular uptake. Under Zn2+ stress, the succession of microbial communities occurred, and Parachlorella was more tolerant to Zn2+. This study revealed the comprehensive response mechanism of the co-effects of phosphate and Zn2+ on the MABS system, and provided some insights for the MABS system treating wastewater containing heavy metals, as well as migration and transformation of heavy metals in aquatic ecosystems.