Study On The Passivation Of Heavy Metal Lead In Soil By Oyster Shell Modified Poplar Wood Biochar

The soil environment is an important foundation for food safety and environmental security.Excess heavy metals in agricultural soils can affect human health through enrichment by crops.Pb is currently one of the major soil heavy metal contaminants.In order to reduce heavy metal Pb contamination in soil,it is crucial to find a soil amendment that is less environmentally disturbing,non-polluting and has a stable remediation effect.Biochar has received much attention as a carbon-rich material with a wide range of sources,excellent adsorption performance and stable structure.Oyster shells are gradually being explored and applied in the study of acidic soil improvement and heavy metal remediation because of their chemical similarity to natural limestone and their comparable effectiveness to lime in raising soil pH and exchangeable calcium content after roasting.In this study,I prepared modified poplar wood biochar composites with different oyster shell addition ratios,investigated their adsorption mechanism and adsorption stability by adsorption experiments on lead contaminated solutions,and applied them to lead contaminated soil remediation.The surface structure and compositional changes of the materials were analysed by characterisation of physicochemical properties such as BET,SEM/EDS,XPS,pH and XRD.The effect of the modification ratio on the adsorption stability of the biochar material and the mechanism of remediation was investigated by means of lead solution adsorption experiments,combined with adsorption kinetics and isothermal adsorption experiments and SEM/EDS,XRD and XPS spectrum of biochar before and after adsorption of lead ions.Through soil heavy metal remediation experiments,the changes in the effective state Pb content and the changes in the distribution of soil Pb morphology were analyzed,and the effects of oyster shell modified biochar on the remediation of soil Pb pollution and soil fertility were investigated in conjunction with soil pH,organic matter content and soil CEC.The main research results obtained are as follows:(1)This modification method successfully resulted in a homogeneous loading of CaO onto the biochar.Characterisation analysis of the biochar showed that the CaO particles were uniformly distributed in the interlayer and pore spaces of the modified biochar material,the pyrolytic reaction of the biochar material was more adequate,and the pH and cation exchange capacity of the modified biochar were significantly increased.(2)The lead adsorption capacity and adsorption stability of the modified biochar were significantly enhanced.This study investigated the effect of different modified biochar on the total amount and stability of lead fixation in solution.The results showed that compared to BC,the adsorption capacity of the modified biochar for Pb in solution was increased by 1.44～3.00 times,with OBC10 having the highest total Pb fixation of 195.48 mg·g-1,which was better than the adsorption of the two materials(BC and OC)used singly.From the analysis of adsorption stability,it was found the higher the percentage of oyster shell powder added to the modified biochar,the higher the percentage of the stable state of fixed lead to the total fixed amount,the stable state of fixed lead of OBC100 can reach 97.35%of the total fixed amount,which is 92.14%higher than that of BC,and has the best stability for the remediation of lead contaminated solution.The characterisation of the material before and after remediation revealed that the calcium content of the modified carbon material decreased after remediation,the calcium compounds were converted from CaO to Ca(OH)2 and CaCO3,and Pb 4f energy spectrum peaks were detected in the XPS spectra,indicating that the material contained substances such as lead oxidation state and PbCO3,while the presence of Pb3(CO3)2(OH)2(aqueous leucite)form was considered.The remediation mechanism of oyster shell modified biochar for lead contaminated solutions mainly includes physical and electrostatic adsorption of its porous structure,metal ion exchange of calcium oxides,complexation and co-precipitation of oxygen-containing functional groups of biochar.(3)Improved soil lead passivation and stability with modified biochar.The effects of different modified biochar,application amounts and remediation times on the effective state Pb content and Pb morphological distribution of exogenous Pb-contaminated soil were investigated.The results showed that the ability of oyster shell modified biochar to reduce the effective state Pb in soil varied significantly,with the effective state Pb content of soil applied with OBC50 at 5%addition rate being 1.39 mg·kg-1 and that of soil applied with OBC100 at 0.09 mg·kg-1 at 30 days of remediation.At 7.5%,both OBC50 and OBC100 soils had Pb levels close to 0 mg·kg-1,which was a significant increase in passivation capacity compared to BC.Considering the impact on the environment and the cost of preparation,the application of 5%OBC50 material for 30 days achieves excellent soil remediation of heavy metal lead.Oyster shell modified biochar promotes the conversion of soil heavy metal Pb from a weak acid extracted state with high mobility to a stable residue state.At a remediation time of 30 days and a 5%addition rate,the proportion of Pb in the residue state was 18%in the soil to which OBC50 was applied,an increase of 14%compared to CK,and a reduction of 5%in the reducible state and 9%in the weak acid extracted state.The addition of oyster shell modified biochar can effectively increase the pH,organic matter content and soil CEC,enhance the buffering capacity of the soil against acid and alkaline changes and the retention capacity of heavy metals,and improve soil fertility.