Study On The Passivation Effect And Mechanism Of Electrolytic Manganese Slag-biochar Composite Material On Soil Antimony And Arseni

Large-scale mining and metallurgical activities on antimony（Sb）mineral re-sources have resulted in Sb and arsenic（As）pollution in the surrounding soils,which seriously threaten the ecological environment and human health.Remediation of Sb and As multi-contaminated soils for the Sb mining area is imminent.Therefore,there is a critical need to develop cost-effective,sustainable,and stable technologies to re-mediate Sb and As multi-contaminated soils.Biochar has been widely used in environ-mental remediation and soil amendment,but due to its electronegativity,its immobili-zation effect on anionic metal（loid）s is poor in some cases.To improve the immobili-zation ability of biochar on anionic metal（loid）s,metal oxide-biochar can be prepared through chemical treatment,but the modification cost is relatively high.In recent years,the modification of biochar with metal-containing solid waste has gradually attracted attention.Electrolytic manganese residue（EMR）is the main solid waste from electro-lytic manganese production,which contains multiple metal oxides.The use of EMR for biochar modification may alter its surface functional group content and specific surface area,thereby enhancing its ability to remediate the Sb and As multi-contaminated soils.In this study,EMR,distillers grains,and pine sawdust were used as raw materials,and EMR-biochar composite（EB）was prepared by one-step pyrolysis method for the im-mobilization effect and mechanism of Sb and As multi-contaminated soils.Adsorption experiments based on different influencing factors were conducted to explore the opti-mal adsorption conditions of Sb and As in water by EB.Different adsorption models combined the FTIR,XRD,and XPS characterization were used to investigate the pos-sible adsorption mechanisms of Sb and As by EB.The immobilization effect and the underlying immobilization mechanisms of EB on Sb and As in soils were investigated by soil column leaching experiments.The effects of EB on the microbial community in Sb and As multi-contaminated soils were evaluated by the soil incubation experiment.Finally,the effect of EB on the bioavailability of Sb and As in soils and pakchoi（Bras-sica chinensis L.）growth was analyzed in combination with pot experiments.The main findings are as follows:（1）The characterization results of the physicochemical properties of the pristine biochar and EB showed that the pristine biochar and EB were alkaline,the p H gradually increased with increasing pyrolysis temperature,and the cation exchange capacity and anion exchange capacity of EB was higher than those of the pristine biochar.A large number of micro-sized particles were distributed on the surface of EB,and its surface is relatively rough compared to the pristine biochar.EB prepared at 750℃pyrolysis temperature mainly included mineral phases such as Si O₂,Fe OOH,Fe₃O₄,Mn Fe₂O₄,and Ca S.Compared with pristine biochar,chemical bonds such as Fe-O,Ca-O,and Mn-O existed on the surface of EB,and elements such as Ca,Fe,Mn,and Si were abundant.（2）The results of batch adsorption experiments showed that the adsorption effect of EB prepared at 750℃pyrolysis temperature and a mass ratio of EMR to the biomass of 3∶1（wt%）is the best when the dosages of 2.5 g L^-1（Sb（III））,7.5 g L^-1（Sb（V））,1.5g L^-1（As（III,V））,respectively.Coexisting HCO₃^-,PO₄^3-and humic acid had the great-est effect on the adsorption of Sb（III,V）and As（III,V）by EB.The adsorption of EB on Sb（V）was heterogeneous physicochemical adsorption,and the adsorption of Sb（III）and As（III,V）was mainly chemisorption.The adsorption of Sb（III,V）and As（III,V）by EB fit the Langmuir model better,and the adsorption process was a spontaneous exothermic reaction.The maximum adsorption capacities of EB for As（III）,As（V）,Sb（III）,and Sb（V）reached 40.92,40.54,34.36,and 9.42 mg g^-1,respectively.The re-sults of adsorption experiments combined with the characterization results such as XPS,FTIR,and XRD showed that EB not only adsorbed Sb（III,V）and As（III,V）by surface complexation and hydrogen bonding but also promoted the oxidation of Sb（III）and As（III）to Sb（V）and As（V）.（3）The results of soil column leaching experiments showed that the application of EMR-distillers grains biochar composite（EDB）raised soil EC and p H,and reduced the loss of water and the dissolution of dissolved organic carbon in the soil.The cumulative water retention of distillers grains biochar（DB）and EDB on soils increased by 17.93%–52.05%and 20.22%–43.69%,respectively.EDB and DB reduced the mobility of Sb and As in soil.Compared with untreated soil,the cumulative content of Sb and As in soil leachate decreased by 29.21%–55.65%and 53.51%–68.95%respectively after adding 4 wt%EDB,and the immobilization effect of EDB was significantly better than that of DB.Meanwhile,detailed characterization analysis indicated that the underlying mechanisms of Sb and As immobilization on EDB involved ligand exchange,complex-ation,hydrogen bonding,and pore filling.（4）The results of soil incubation experiments showed that the application of EDB promoted the sulfur metabolism function of soil microorganisms.EDB reduced the di versity of the soil bacterial community by affecting EC and the bioavailability of Sb a nd As（Shannon index decreased by 7.8%）and increased the relative abundance of Thi obacillus and Sulfuriferula redox-related bacteria of Sb and As.The predominant bact eria at the phyla level in the soil treated with DB and EDB were Proteobacteria,Actin obaciota,and Acidobacillus.Chemoheterotrophy and Aerobic＿chemoheterotrophy are the main functions of the microbial community.（5）The results of the pot experiment showed that EDB application significantly increased soil p H,EC,and soil organic matter content and decreased the bioavailability of Sb and As in soils.The addition of 1 wt%EDB increased the plant height（24.50%–26.56%）,fresh weight（22.24%–29.88%）,dry weight（18.20%–56.40%）,and chlo-rophyll content（14.46%–16.61%）of pakchoi.EDB significantly inhibited the uptake and distribution of Sb and As in different tissues of pakchoi.The content of Sb and As absorbed in the roots of pakchoi was higher than in the shoots.As was easier absorbed by the roots of pakchoi than Sb and was translocated from the roots to the shoots.The contents of bioavailable Sb（14.55%–71.92%）and As（6.91%–53.81%）in soil de-creased significantly with the increasing application rate of EDB.The application of EDB promoted the adsorbed and amorphous and poorly crystalline hydrous oxides of Fe and Al of Sb and As were transformed into the well-crystallized hydrous oxides of Fe and Al and residual phases in soils,reducing their mobility and phytotoxicity.