Remediation Of Cadmium And Arsenic In Water、Soil Environment By Manganese Modified Iron Sludge-based Carbon Materials

Heavy metal pollution such as cadmium（Cd）and arsenic（As）has the characteristics of persistence,high toxicity and bioaccumulation,which can greatly harm the ecological environment and human health.Biochar has attracted much attention as a low-cost remediation material with a wide range of sources,among which binary metal oxides/zero-valent iron/biochar composites show excellent performance in remediation of Cd and As contamination.It has been shown that iron-rich sludge has a high potential for resource utilization due to its high content of iron and biomass,which is able to prepare iron sludge-based carbon materials by pyrolysis.Therefore,in this study,manganese modified iron sludge-based carbon materials were prepared by impregnation pyrolysis method using iron-rich sludge as raw material and potassium permanganate as modifier.To investigate the adsorption performance and mechanism of Cd and As by adsorption experiments and adsorption model fitting,combined with a series of characterization means such as XRD and XPS of manganese modified iron sludge based carbon materials;To investigate the effects and changes in soil physicochemical properties,Cd and As deposition patterns and soil enzyme activities of manganese modified iron sludge-based carbon materials on Cd and As-contaminated soils through soil cultivation experiments,and to reveal the remediation effects and mechanisms of Mn-modified iron-peat-based materials on Cd and As in the environment by combining liquid-phase adsorption and indoor soil cultivation.The main research conclusions are as follows:（1）Preparation and properties of FMBC-7In this paper,a manganese-modified iron sludge-based carbon material（FMBC-7）with optimal adsorption performance was successfully prepared by impregnation pyrolysis at 700℃ using iron-rich sludge as raw material and 5%potassium permanganate as activating modifier.The characterization results show that FMBC-7has a rough surface with a larger pore size structure,and the iron element mainly exists in the form of Fe⁰,Fe²⁺and Fe³⁺,while the manganese element is mainly MnO₂.The heavy metal concentration test confirms that FMBC-7 can be used as a soil conditioner in gardens,pastures and arable land where no food crops are grown.（2）Adsorption performance and mechanism of FMBC-7FMBC-7 has better adsorption performance for Cd（Ⅱ）and As（Ⅲ）than FeBC-7.The adsorption of Cd（Ⅱ）and As（Ⅲ）by FMBC-7 was more in accordance with the second-order kinetic model,and the adsorption process was mainly based on the chemisorption of non-homogeneous phases.The adsorption of Cd（Ⅱ）by FMBC-7 was more consistent with the Freundlich model,while the adsorption of As（Ⅲ）was in better agreement with the Langmuir model,with a maximum adsorption capacity of95.34mg·g^-1 for Cd（Ⅱ）and 116.83mg·g^-1 for As（Ⅲ）.The initial pH of the solution can affect the adsorption effect of FMBC-7 on Cd（Ⅱ）and As（Ⅲ）.The coexisting anions PO₄^3-and NO₃^-significantly inhibited the adsorption of As（Ⅲ）by FMBC-7;the coexisting cations Ca²⁺,Na⁺and Mg²⁺inhibited the adsorption of Cd（Ⅱ）by FMBC-7.According to the results of adsorption experiments and characterization analysis,the adsorption mechanism of FMBC-7 on Cd（Ⅱ）mainly includes mineral precipitation（CdCO₃、Cd（OH）₂、Cd₂Mn₃O₈）,ion exchange interaction（-COOH、-OH）,complexation of oxygen-containing functional groups（C=O）,cation-π-bonding interaction and redox interaction(Cd²⁺-π).The adsorption mechanism for As（Ⅲ）mainly includes ion exchange interaction（-COOH、-OH）,complexation of oxygen-containing functional groups（C=O）and oxidation of As（Ⅲ）to As（V）by redox reactions.（3）Passivation effect of FMBC-7The addition of 2%FeBC-7 and FMBC-7 into the Cd-As composite contaminated soil increased the soil pH,CEC,and SOM values.Among them,the addition of FMBC-7 raised soil pH by 0.70-0.92 units;soil CEC by 21.79%-41.05%;and soil organic matter by 24.40%.In addition,the application of FeBC-7 and FMBC-7 both effectively reduced the content of soil effective state Cd with passivation efficiencies of 7.55%-20.45%and 19.26%-25.32%,respectively.At day 60,FeBC-7 application resulted in a1.2%increase in soil effective state As content.In contrast,the application of FMBC-7was effective in reducing the soil effective state As content for all incubation times（0-60 d）,with passivation efficiencies ranging from 4.96%to 18.64%.Correlation analysis showed that soil pH,CEC and SOM were significantly negatively correlated with soil available Cd content（P<0.01）,while soil pH and SOM were not significantly correlated with soil available As.FMBC-7 applied to soil induces the conversion of weak acid Cd and weak acid As to the more stable residual Cd and residual As,achieving the stabilization of Cd and As in soil.In addition,the application of FeBC-7 and FMBC-7 increased soil enzyme activities,among which FMBC-7 had a better effect,and the activities of catalase,urease and sucrase in soil increased by 55.80%,14.55%and 16.45%,respectively.