Organic amendment application affects the release behaviour, bioavailability, and speciation of heavy metals in zinc smelting slag: Insight into dissolved organic matter.

Organic amendments are commonly used in assisted phytostabilization of mine wastes by improving their physicochemical and biological properties. These amendments are susceptible to leaching and degradation, resulting in the generation of dissolved organic matter (DOM), which significantly influences the geochemical behaviour of heavy metals (HMs). However, the geochemical behaviour of HMs in metal smelting slag driven by organic amendment-derived DOM remains unclear. In this study, we investigated the impact of cow manure-derived DOM on the release behaviour, bioavailability, and speciation of HMs (Cu, Pb, Zn, and Cd) in zinc smelting slag using a multidisciplinary approach. The results showed that DOM enhanced the weathering of the slag, with a minimal impact on the slag's mineral phases, except for causing gypsum dissolution. The DOM addition resulted in a slight increase in HM release from the slag during the initial inoculation period, followed by a reduction in HM release during the later period. Furthermore, the DOM addition increased the diversity and relative abundance of the bacterial community. This, in turn, led to a decrease in the dissolved organic carbon (DOC) content and enhanced the transformation of labile DOM compounds into recalcitrant compounds. The variation in HM release during various inoculation periods can be attributed to the bacterial decomposition and transformation of DOM, which further enhanced the transformation of HM fractions. Specifically, during the later period, DOM promoted the conversion of a portion of the reducible and oxidizable fractions of Cu, Pb, and Zn into the acid-soluble and residual fractions. Moreover, it partially transformed the reducible, oxidizable, and residual fractions of Cd into the acid-soluble fraction. Overall, this study provides new insights into the geochemical behaviour of HMs in slag governed by the coupling effect of DOM and the bacterial community. These findings have implications for the use of organic amendments in assisted phytostabilization of metal smelting slag. ENVIRONMENTAL IMPLICATION: Metal smelting slag is hazardous due to its high levels of HMs, and its improper disposal has serious consequences for the ecosystem. Organic amendments are employed in assisted phytostabilization of the slag site by improving its microecological properties. However, the impact of organic amendment-derived DOM on HM migration and transformation in slag remains unclear. This study indicated that the coupling effects of DOM and microbes governed the geochemical behaviour of HMs in slag. These findings provide new insights into how organic amendments impact the geochemical behaviour of HMs in slag, contributing to the development of phytostabilization technology.

Effect of ultrasound-assisted EDTA and citric acid washing on heavy metal removal, residual heavy metal mobility, and sewage sludge quality.

We investigated the effects of ultrasound-assisted ethylenediaminetetraacetic acid (EDTA) and citric acid (CA) washing on heavy metal (HM) removal, residual HM mobility, and sewage sludge quality. EDTA and CA washing of sewage sludge successfully reduced the total concentration of HMs after one round of washing, but the mobility of residual HMs increased significantly. The eluate had a high concentration of HMs and nutrients (nitrogen, phosphorus, potassium, and total organic carbon), although the nutritional content of the sludge remained high. The three-phase ratio of the sludge after six rounds of washing by CA was closest to the ideal three-phase ratio, and the degree of influence on the physical structure of the soil after a land application was reduced, according to the fluctuation of generalized soil structure index (GSSI) and soil three-phase structure distance (STPSD) values. The results indicate that CA as an environmental-friendly washing agent can be the superior choice for sludge HM washing; single washing of sewage sludge may increase the mobility of residual HMs, so multiple washings should be considered for land application of sludge.

Effect of simulated root exudates on the distribution, bioavailability, and fractionation of potentially toxic elements (PTEs) in various particle size fractions of zinc smelting slag: Implication of direct revegetation.

Direct revegetation is a promising strategy for phytostabilization of metal smelting slag sites. Slag comes into direct contact with root exudates when slag sites undergo direct revegetation. The slag particle size fractions are considered the key factor influencing the geochemical behaviour of potentially toxic elements (PTEs). However, the effects of root exudates on the geochemical behaviours of PTEs in various slag particle size fractions remain unclear. Here, the effects of simulated root exudates of perennial ryegrass (Lolium perenne) directly revegetated at a zinc smelting slag site on the distribution, bioavailability, and fractionation of PTEs (Cu, Pb, Zn, and Cd) in various slag particle size fractions were investigated. The results showed that PTEs mainly occurred in the <1 mm slag particles; the mass loads of PTEs in the <1 mm slag particles were higher than those in the >1 mm slag particles. The bioavailability of Cu, Zn, and Cd rather than Pb in the slag increased as the particle size decreased. There was a decrease in the <0.25 and 1-2 mm slag particles and an increase in the 0.25-0.5, 0.5-1, and >2 mm slag particles in the presence of root exudates. Root exudates enhanced the transformation of acid-soluble PTEs into other more stable fractions in various slag particle size fractions. Root exudates enhanced the aggregation of slag particles associated with the migration of PTEs, causing differences in the geochemical behaviour of PTEs in various slag particle size fractions. These findings are beneficial for understanding the geochemical behaviour of PTEs in metal smelting slags undergoing direct revegetation and provide an important basis for the guidance of environmental risk management of the revegetated metal smelting slag sites.

Effects of dissolved organic matter derived from cow manure on heavy metal(loid)s and bacterial community dynamics in mercury-thallium mining waste slag.

Organic amendments in aided phytostabilization of waste slag containing high levels of heavy metal (loid)s (HMs) are an important way to control the release of HMs in situ. However, the effects of dissolved organic matter (DOM) derived from organic amendments on HMs and microbial community dynamics in waste slag are still unclear. Here, the effect of DOM derived from organic amendments (cow manure) on the geochemical behaviour of HMs and the bacterial community dynamics in mercury (Hg)-thallium (Tl) mining waste slag were investigated. The results showed that the Hg-Tl mining waste slag without the addition of DOM continuously decreased the pH and increased the EC, Eh, SO42-, Hg, and Tl levels in the leachate with increasing incubation time. The addition of DOM significantly increased the pH, EC, SO42-, and arsenic (As) levels but decreased the Eh, Hg, and Tl levels. The addition of DOM significantly increased the diversity and richness of the bacterial community. The dominant bacterial phyla (Proteobacteria, Firmicutes, Acidobacteriota, Actinobacteriota, and Bacteroidota) and genera (Bacillus, Acinetobacter, Delftia, Sphingomonas, and Enterobacter) were changed in association with increases in DOM content and incubation time. The DOM components in the leachate were humic-like substances (C1 and C2), and the DOC content and maximum fluorescence intensity (FMax) values of C1 and C2 in the leachate decreased and first increased and then decreased with increasing incubation time. The correlations between HMs and DOM and the bacterial community showed that the geochemical behaviours of HMs in Hg-Tl mining waste slag were directly influenced by DOM-mediated properties and indirectly influenced by DOM regulation of bacterial community changes. Overall, these results indicated that DOM properties associated with bacterial community changes increased As mobilization but decreased Hg and Tl mobilization from Hg-Tl mining waste slag.

Vertical distribution of nutrients, enzyme activities, microbial properties, and heavy metals in zinc smelting slag site revegetated with two herb species: Implications for direct revegetation.

Direct revegetation is an important measure to immobilize heavy metals and improve the microecological properties of metal smelting slag sites. However, the vertical distribution of nutrients, microecological properties, and heavy metals at a directly revegetated metal smelting slag site remains unclear. Here, the distribution characteristics of nutrients, enzyme activities, microbial properties, and heavy metals in the vertical profile at a zinc smelting slag site directly revegetated with two herb species (Lolium perenne and Trifolium repens) for 5 years were investigated. The results showed that the nutrient contents, enzyme activities, and microbial properties decreased with increasing slag depth after revegetation with the two herb species. The nutrient contents, enzyme activities, and microbial properties of the surface slag revegetated with Trifolium repens were better than those in the surface slag revegetated with Lolium perenne. The higher root activity in the surface slag (0-30 cm) resulted in relatively higher contents of pseudo-total and available heavy metals in the surface slag. Moreover, the contents of pseudo-total heavy metals (except for Zn) and available heavy metals in the slag revegetated with Trifolium repens were lower than those in the slag revegetated with Lolium perenne at most slag depths. Overall, the greater phytoremediation efficiency of the two herb species occurred mainly in the surface slag (0-30 cm), and the phytoremediation efficiency of Trifolium repens was higher than that of Lolium perenne. The findings are beneficial for understanding the phytoremediation efficiency of direct revegetation strategies for metal smelting slag sites.

Molecular insights into the chemodiversity of dissolved organic matter and its interactions with the microbial community in eco-engineered bauxite residue.

Dissolved organic matter (DOM) plays an important role in the biogeochemical function development of bauxite residue. Nevertheless, the DOM composition at the molecular level and its interaction with microbial community during soil formation of bauxite residue driven by eco-engineering strategies are still relatively unknown. In the present study, the DOM composition at the molecular level and its interactions with the microbial community in amended and revegetated bauxite residue were explored. The results showed that the amendment applications and revegetation enhanced the accumulation of unsaturated molecules with high values of double bond equivalent (DBE) and nominal oxidation of carbon (NOSC) and aromatic compounds with high values of modified aromaticity index (AImod) as well as the reduction of average weighted molecular mass of DOM molecules. Significant correlations between DOM molecules and the microbial community and Fe/Al oxides were found. DOM molecules were decomposed by the microbial community and sequestered onto Fe/Al oxides, which were the main driving factors that changed DOM chemodiversity in the amended and revegetated bauxite residue. These findings are beneficial for understanding the biogeochemical behaviours of DOM and providing a critical basis for the development of eco-engineering strategies towards soil formation and the sustainable revegetation of bauxite residue.

Rhizosphere activity induced mobilization of heavy metals immobilized by combined amendments in a typical lead/zinc smelter-contaminated soil.

The persistence of the stabilization effect of amendments on heavy metals (HMs) is of great concern when they are used for remediating HM-contaminated soil. Here, pot experiments were conducted to investigate the effects of two consecutive seasons of vegetable cultivation on the mobilization of HMs (Cu, Pb, Zn, and Cd) immobilized by different application ratios (0, 20, 40, and 80 g kg-1, labelled C0, C2, C4, and C8) of a combined amendments (lime: sepiolite: biochar: humic acid = 2:2:1:1). The results showed that HM bioavailability decreased with increasing application ratios of the combined amendments in control (CK) treatments. The DOC contents, HM bioavailability, and HM contents in the leaves of vegetables increased, but the pH decreased during two consecutive seasons of vegetable cultivation; however, the HM bioavailability in the C2, C4, and C8 treatments was lower than that in the C0 treatments with vegetables. Catalase, urease, alkaline phosphatase, and dehydrogenase activities in the combined amendment treatments with and without vegetables were decreased compared to those in the C0 treatments. The relative abundances of the dominant bacterial phyla in the different treatments were Actinobacteria > Proteobacteria > Chloroflexi > Acidobacteria > Gemmatimonadetes > Bacteroidetes for the first season and Proteobacteria > Actinobacteria > Chloroflexi > Acidobacteria > Bacteroidetes > Gemmatimonadetes for the second season. Correlations showed that the pH and DOM properties during two consecutive seasons of vegetable cultivation were important factors influencing HM bioavailability, enzyme activity, and bacterial community composition. The bacterial community composition shift indirectly influenced the mobilization of HMs immobilized by the combined amendments. Thus, rhizosphere activity induced the mobilization of HMs immobilized by combined amendments during two consecutive seasons of vegetable cultivation.

Soil microbial community responses to the application of a combined amendment in a historical zinc smelting area.

Farmland soils that surround a historical zinc smelting area in northwestern Guizhou, China, are characterized by high levels of heavy metal accumulation. Previous studies have mainly focused on the potential risk evaluations of heavy metals in soil and crops. However, at present, the effects of amendment applications on the bioavailability of heavy metals and on microbial community in the heavily contaminated soils of the mining region are still unclear. A pot experiment was conducted to determine the effect of applying a combined amendment (e.g. lime, sepiolite, and vermicompost) on the diversity and composition of microbial community in the contaminated soil. The results showed that the contents of DTPA- and TCLP-extractable heavy metals (e.g. Cd, Pb, and Zn) decreased and that the pH, SWC, EC, and soil available nutrient (e.g. AN, AP, and AK) contents increased after the application of the combined amendment. Furthermore, application of the combined amendment decreased the diversity of soil bacterial and fungal communities and increased the relative abundances of the dominant bacterial and fungal communities such as Proteobacteria, Bacteroidetes, and Ascomycota; however, the relative abundances of Acidobacteria and Actinobacteria decreased. Redundancy analysis (RDA) and structural equation model (SEM) analysis showed that the bioavailability of heavy metals decreased and that soil physicochemical characteristics improved and had positive or negative effects on the diversity and composition of soil microbial community.

Effect of different direct revegetation strategies on the mobility of heavy metals in artificial zinc smelting waste slag: Implications for phytoremediation.

The establishment of vegetation cover is an important strategy to reduce wind and water erosion at metal smelting waste slag sites. However, the mobility of heavy metals in waste slag-vegetation-leachate systems after the application of revegetation strategies is still unclear. Large microcosm experiments were conducted for revegetation of waste slag for 98 d using combined amendments, i.e., phosphate rock and an organic waste coming from the anaerobic digestion of pig manure (named as biogas residue), and by single- and co-planted perennial ryegrass (Lolium perenne L.) and Trifolium repens (T. repens). The results showed that the application of biogas residue slightly increased the concentrations of Zn and Cd in the leachates; however, the establishment of plants could avoid the excessive leaching of heavy metals coming from the biogas residue. The bioavailability of Cu, Zn, and Cd slightly increased, but Pb bioavailability significantly decreased regardless of single- or co-planting patterns. Additionally, the bioavailability of Cu, Zn, and Cd in the waste slag revegetated with perennial ryegrass was lower than that in T. repens under the single-planting pattern. The change in the heavy metals bioavailability under different revegetation strategies was mainly due to the root-induced change in the pH and speciation of heavy metals in the waste slag. The application of biogas residue and phosphate rock tends to the immobilization of Pb. Heavy metals mainly accumulated in the underground parts of the two herbs, and the heavy metal contents in the underground parts of perennial ryegrass were higher than those in T. repens regardless of single- or co-planting patterns. The heavy metals accumulated in T. repens were lower than those in perennial ryegrass in the single-planting pattern. The bioaccumulation and transportation factors of the two herbs were extremely low. Thus, the two herbs are potential candidates for phytostabilization of zinc smelting waste slag sites.

Study on bioleaching of heavy metals and resource potential from tannery yard sludge.

Tannery sludge that has accumulated in the natural environment of a tannery industrial zone for a long time contains large amounts of toxic heavy metal elements such as Cr, which has a serious impact on the surrounding environment. This study used indigenous acidophilic sulfur-oxidizing bacteria from local tannery wastewater treatment plants to examine the effects of bioleaching on the removal of heavy metals in local tannery sludge accumulated in the natural environment. The effect of pre-oxygenation on bioleaching was investigated, and trends of sludge dewaterability during bioleaching and changes in the total amount of heavy metals, total nitrogen (TN), and total phosphorus (TP) during bioleaching were determined. Changes in the contents of different bound forms of heavy metals in tannery yard sludge during the bioleaching process were revealed. The experimental results showed that pre-oxygenation treatment of tannery yard sludge can shorten the bioleaching period (by at least 4 days) and improve the removal efficiency of all heavy metals. To ensure sludge dewaterability, the pH of the leaching system at the end of the bioleaching must not be lower than 1.67. The main components of heavy metals were stable in the tannery yard sludge, which critically affected the final removal efficiency of all heavy metals. The dissolution process of heavy metals showed that the morphology of heavy metals changed from stable to unstable forms in the bioleaching process, further dissolving into the liquid phase to be removed. In this experiment, the removal efficiency of all heavy metals in the tannery yard sludge was higher than 88.49%, and these heavy metals had good stability in morphology (the stable forms accounted for more than 87% of the total). In addition, the TN content in the remaining sludge was 27.9 g/kg, which is much higher than fertilizer TN requirements, indicating a high potential for resource utilization. Therefore, the method of bioleaching to remove heavy metals in tannery yard sludge for reuse is worthy of in-depth study and promotion.

Migration characteristics of heavy metals in the weathering process of exposed argillaceous sandstone in a mercury-thallium mining area.

Lanmuchang mercury-thallium mine, a typical polymetallic mine is located in southwestern Guizhou, China, is the most serious and typical area resulted from multi-metal contamination (Tl, Hg, As, and Sb). After the mercury-thallium mining, a large area of surrounding rocks such as argillaceous sandstone with high contents of Tl, Hg, As, and Sb is exposed to air. Weathering caused the argillaceous sandstone to form different weathering layers, including the grey-black external layer, the brown-yellow middle layer and the gray-white inner layer, and the external layer was enriched with higher heavy metals. However, the reason of heavy metal migration and transformation in argillaceous sandstone caused by weathering is unclear. The objective of this paper was to investigate the migration, transformation and release characteristics of Tl, Hg, As, and Sb in argillaceous sandstone during the weathering. The results indicated that weathering not only promoted an acidic oxidation environment in argillaceous sandstone, but also increased its specific surface area, pore volume and hydrophilicity, which are beneficial to the permeability of oxygen and etching liquids during the process of weathering and leaching. Meanwhile, weathering led to the transformation or decomposition of hydrophilic groups, such as -OH and -CË­O in the grey-black external layer of argillaceous sandstone, resulting in the further release of heavy metals bound to these groups. The concentration of Tl, Hg, As, and Sb in the leaching solution of argillaceous sandstone represented a positive correlation with that of Fe, Ca, Mg at different levels, indicating that Tl, Hg, As, and Sb were released with the dissolution of Fe, Ca and Mg during weathering and leaching. In summary, these results indicated that weathering caused the dissolution and migration of heavy metals in the argillaceous sandstone. Tl, Hg, As, and Sb migrated from the grey-white inner layer to the grey-black external layer and partially adsorbed by free alumina (Ald), jarosite and Ca-bearing minerals, showing enrichment phenomena, partially released into the environment, causing environmental pollution.

Root-induced changes in aggregation characteristics and potentially toxic elements (PTEs) speciation in a revegetated artificial zinc smelting waste slag site.

Root-induced changes play a crucial role in influencing the fate and speciation of potentially toxic elements (PTEs) in contaminated soils, but their role in the phytostabilization of waste slag sites remain unclear. The aim of this study was to determine the effect of four phytostabilization plants, Broussonetia papyrifera, Arundo donax, Robinia pseudoacacia, and Cryptomeria fortunei, planted in a zinc smelting waste slag site for 5 years on PTEs speciation and the mineral and aggregation characteristics at the interface of the waste slag-plant system. The results showed that the presence of a higher content of oxalic acid in the rhizosphere slags of the four plant species than in the bare slag. Revegetation of the waste slag with the four plant species significantly changed the mineral composition and morphology of the waste slag. The mass percentage of large particles (1-5 mm) and small particles (0.5-1 mm, 0.25-0.5 mm, and <0.25 mm) in the rhizosphere slags decreased and increased, respectively. The PTEs (Cu, Pb, Zn, and Cd) in most of the rhizosphere slags were mainly distributed within the small particles, and the enrichment coefficients of PTEs in the large particles and small particles were less than and greater than 1, respectively. The bioavailability of the PTEs in the waste slag increased with decreasing particle size. Root-induced the transformation of acid-soluble PTEs into their reducible, oxidizable, and residual forms in the different waste slag particles weathered in the rhizosphere. These results suggested that there are root-induced changes in the aggregation characteristics and geochemical behaviours of PTEs in waste slag fractions during the phytoremediation of waste slag sites.

Effect of particulate organic matter fractions on the distribution of heavy metals with aided phytostabilization at a zinc smelting waste slag site.

Particulate organic matter (POM) significantly affects the distribution of heavy metals in contaminated soil. However, the effect of POM on the fate of heavy metals during in situ-aided phytostabilization of waste slag is unclear. The objective of this study was to investigate the distributions of heavy metals such as Cu, Pb, Zn, and Cd in the POM fractions at a zinc smelting waste slag site under in situ-aided phytostabilization after five years. The results showed that the litters and residues of four plants-Arundo donax, Broussonetia papyrifera, Cryptomeria fortunei, and Robinia pseudoacacia-decomposed to form different POM size fractions. The percentage of the 0.05-0.25 mm POM size fraction was the highest, followed by the >1 mm and 0.5-1 mm POM size fractions, and that of the 0.25-0.5 mm POM size fraction was the lowest. The masses of POM derived from the four plants were in the following order: C. fortunei > B. papyrifera > A. donax > R. pseudoacacia. The contents, enrichment coefficients, and mass loads of heavy metals such as Cu, Pb, Zn, and Cd in the POM increased with decreasing POM size, and those in the 0.05-0.25 mm POM size fraction were the highest. The mass load of heavy metals in the POM occurred in the following order: Cu > Cd > Zn > Pb. The surfaces of the POM with coarser and smaller size fractions were smoother and rougher, respectively, and the smaller POM size fractions had larger specific surface areas. The main functional groups in the different POM size fractions were -COOH, -OH, CO, CC, C-H, Si-O, and -CH3. The POM fractions played a significant role in determining the distribution of heavy metals in the revegetated waste slag. These findings have important implications for aided phytostabilization, which significantly influences the fate and speciation of heavy metals at the phytoremediation site.

Bacterial community structure and diversity responses to the direct revegetation of an artisanal zinc smelting slag after 5 years.

This comparative field study examined the responses of bacterial community structure and diversity to the revegetation of zinc (Zn) smelting waste slag with eight plant species after 5 years. The microbial community structure of waste slag with and without vegetation was evaluated using high-throughput sequencing. The physiochemical properties of Zn smelting slag after revegetation with eight plant rhizospheres for 5 years were improved compared to those of bulk slag. Revegetation significantly increased the microbial community diversity in plant rhizospheres, and at the phylum level, Proteobacteria, Acidobacteria, and Bacteroidetes were notably more abundant in rhizosphere slags than those in bulk waste slag. Additionally, revegetation increased the relative abundance of plant growth-promoting rhizobacteria such as Flavobacterium, Streptomyces, and Arthrobacter as well as symbiotic N2 fixers such as Bradyrhizobium. Three dominant native plant species (Arundo donax, Broussonetia papyrifera, and Robinia pseudoacacia) greatly increased the quality of the rhizosphere slags. Canonical correspondence analysis showed that the differences in bacterial community structure between the bulk and rhizosphere slags were explained by slag properties, i.e., pH, available copper (Cu) and lead (Pb), moisture, available nitrogen (N), phosphorus (P), and potassium (K), and organic matter (OM); however, available Zn and cadmium (Cd) contents were the slag parameters that best explained the differences between the rhizosphere communities of the eight plant species. The results suggested that revegetation plays an important role in enhancing bacterial community abundance and diversity in rhizosphere slags and that revegetation may also regulate microbiological properties and diversity mainly through changes in heavy metal bioavailability and physiochemical slag characteristics.