Effect Of Aging On Adsorption And Passivation Of Cd In Mining Soil On Biochar

Bayan Obo mine is a rare polymetallic symbiotic deposit in the world,and the long-term open-pit mining has led to an increasingly serious problem of heavy metal pollution in the surrounding soil.In recent years,biochar has been widely used in the field of remediation of heavy metal pollution in the soil,relying on its extraordinary adsorption capacity.However,after fresh biochar is applied to the soil environment,its physicochemical properties will change under various effects,which in turn affect its ability to absorb and passivate heavy metals.In this study,corn straw（CB）biochar and sludge biochar（SB）were pyrified at 350℃and 650℃,and artificially simulated aging was performed by three different methods（chemical oxidation,wet-dry cycles,freeze-thaw cycles）to investigate the changes in the physicochemical properties and the adsorption of heavy metals by biochar before and after aging.The best adsorption performance of CB was selected to further characterize and quantify the adsorption mechanism of various heavy metals by batch equilibrium experiments combined with characterization methods.Finally,the biochar with the best adsorption performance was applied to the heavy metal-contaminated soil in the mining area at an addition rate of 3%to study the effect of aging on biochar passivation of heavy metals in the mining area,and the following conclusions were mainly obtained:（1）The pH of biochar decreased and O content increased after artificially simulated aging in three types of aging.The specific surface area（SSA）of most of the biochar increased significantly and the ash content decreased after aging.Scanning electron microscopy showed that the surface structure of biochar became clearer after chemical oxidation,and the surface pores of biochar were damaged after wet-dry cycles and freeze-thaw cycles.The changes of surface oxygenated functional groups during the aging process varied depending on the combined effects of pyrolysis temperature,feedstock type and aging technique.Three aging treatments had little effect on the crystalline structure of SB,but the KCl peaks of CB weakened or even disappeared after aging.The above characterization tools combined with the results of adsorption experiments indicated that one of the main reasons for the changes in the adsorption of Cd²⁺by SB was the increase or decrease in the content of oxygen-containing functional groups（mainly carboxyl groups）,which enhanced or inhibited the ability to complex heavy metals.For CB,the decrease in Cd²⁺adsorption capacity is due to the decrease in ash content and thus the weakened precipitation of inorganic mineral fractions with Cd²⁺.The changes in the adsorption characteristics of biochar are the result of the combined effect of feedstock type and aging behavior.（2）Under the single-metal system,cation exchange(Q_ce)is the main mechanism of adsorption in low-temperature biochars before and after aging,with the adsorption percentage accounting for 35.03%-48.23%.Mineral precipitation(Q_pre)is the main mechanism of adsorption in CB650（38.04%-57.19%）in the single-metal system.The competitive adsorption between Pb²⁺and Cd²⁺under the binary-metal system significantly improved the proportion of the Q_pre.Aging increased the contribution of surface complexation to the adsorption of Pb²⁺and Cd²⁺in all biochars,especially in low temperature biochars,but weakened the contribution of minerals to the adsorption.In addition,the contribution of other potential mechanisams to the adsorption of Pb²⁺and Cd²⁺increased after aging of high-temperature biochar.（3）Under the effect of aging,the CaCl₂-extractable Cd²⁺concentration in the soil after adding CB650 was between 0.81-0.97 mg/kg,and the availability of Cd²⁺was reduced by19.32%-30.67%compared to CK.The addition of CB650 reduced the concentration of total Cd²⁺in the soil to some extent.In addition,the BCR result showed that the sum of the percentages of acid extractable and reducible fractions（F1+F2）of Cd²⁺in the soils added with biochar after dry-wet cycles（DCB）,freeze-thaw cycles（FCB）and natural aging（YCB）decreased by 3.72%,3.13%and 7.15%,respectively,compared with DCK,FCK and TCK.Compared to the control group,the TCLP-Cd content of the biochar added treatment under aging conditions was reduced by 0.13-0.35 mg/kg.The above results showed that cadmium-contaminated soil repaired by biochar even after aging,it could transform some of the original soil’s exbioavailable fraction into relatively stable fraction,thus reducing the toxic effect of Cd²⁺.（4）The aging effect has a certain influence on the migration of Cd²⁺in the leaching state.Compared with natural aging,freeze-thaw cycles could promote the transport of Cd²⁺in vertical level.In terms of cumulative release of Cd²⁺,the effect of freeze-thaw aging was not significant compared to natural aging,but the treatment of wet-dry cycles aging could reduce the release of Cd²⁺in the porewater.The kinetic fitting results showed that the modified Elovich equation,the double constant rate equation,and the parabolic diffusion equation were all good fits for the release of Cd²⁺under drenching conditions,but the modified Elovich equation was relatively the best to describe the migration and release of Cd²⁺under aging.