Remediation Mechanisms Of A New Bridged Chlorapatite On Heavy Metals In Soil

As the development of society and industrialization,the problem of heavy metal in water and soil becoming more prominent.In order to remove heavy metals from water,stabilized chlorapatite nanoparticle has been widely used because of its good sorption on heavy metals in water and soil.In order to remediate heavy metals contaminated soil,especially lack of effective method for multi-metals in the contaminated acid soil,it is difficult to guarantee the long-term stability of heavy metals by adding traditional immobilizing agents.Therefore,in order to reuse the soil polluted by heavy metals and promote the sustainable development of China's cities,it is of great significance to develop cheap,efficient and stable bridged chlorapatite,and to carry out in-depth research on the immobilization efficiency,mechanism and engineering application of bridged chlorapatite for immobilization of multi-metals in soil.In this study,carboxymethyl cellulose(CMC)was firstly used as bridging agent to prepare a new type of bridged chlorapatite(CMC-CAP)material.Firstly,the CMC-CAP was used to remove heavy metal ions in water,and the tests including adsorption kinetics,isotherm and so on,were studied.For immobilization of multi-metals in acid soil,the most economical and effective immobilizing agent,the Ca O,was firstly selected from the commonly used alkaline materials,and then the CMC-CAP and Ca O were used to immobilize of multi-metals in acid soil.Finally,we carried out the field assessment of CMC-CAP for immobilization of heavy metal in soil.The specific objectives were:(1)We prepared a type of carboxymethyl cellulose(CMC)bridged chlorapatite(CMC-CAP,CMC:CAP molar ratio=0.0013)nanoparticles and tested the material for removal of Zn²⁺and Cd²⁺from water.CMC macromolecules were attached to CAP by bidentate bridging and hydrogen bonding,preserving the high adsorption capacity of CAP nanoparticles while allowing for easy gravity-separation of the nanoparticles.CMC-CAP showed rapid adsorption kinetics and 22.8%and 11.2%higher equilibrium uptake for Zn²⁺and Cd²⁺,respectively,than pristine CAP.An extended dual-mode isotherm model provided the best fits to the sorption isotherms,giving a maximum Langmuir sorption capacity of 141.1 mg g^-1 for Zn²⁺and 150.2mg g^-1 for Cd²⁺by CMC-CAP.The results show that the adsorption of CMC-CAP on Zn²⁺and Cd²⁺is influenced by p H,dissolved organic matter,Na⁺ion strength,and Ca²⁺ion strength.Material characterizations and surface binding analyses revealed that ion exchange,surface precipitation,and surface complexation were the removal mechanisms for the heavy metals.This study demonstrates stabilizer bridging may serve as a convenient strategy to facilitate the use of nanoparticles.(2)We investigated three common alkaline agents(Na OH,Ca O,and Mg(OH)₂)for immobilization of four heavy metals(Pb,Zn,Cu,and Cd)in a field-contaminated soil.Batch tests showed that although Na OH caused the highest p H spike in the soil,while Ca O and Mg(OH)₂(?0.1 mol OH^-(kg·soil)^-1)was able to immobilize all four metals,while Na OH failed.Column elution tests showed that amendments by Ca O and Mg(OH)₂ lowered the leachable Pb²⁺,Zn²⁺,Cu²⁺,and Cd²⁺.Sequential extraction revealed that the soil amendments converted the exchangeable fraction of the metals to the much less available forms.XRD and FTIR analyses indicated that formation of metal oxide precipitates and complexation with soil organic matter were responsible for the metals immobilization.Taken together the chemical cost,technical effectiveness,and environmental impact,Ca O is the most suitable alkaline agent for remediation of soil contaminated with heavy metals.(3)We prepared carboxymethyl-cellulose-bridged nano-chlorapatite(CMC-CAP).On the basis of the above research,we tested CMC-CAP and Ca O for simultaneous immobilization of Pb,Zn,Cu,and Cd in a field-contaminated acidic soil.Based on batch and column tests,amending the field-contaminated soil using 0.5 wt.%CMC-CAP and 0.1 wt.%Ca O was most effective in immobilizing the four metals.Sequential extraction revealed that the soil amendment converted the exchangeable fractions to the much less available Fe-Mn oxides bound and residual forms,and thus,lowered the risk levels to“low risk”for all the metals.The immobilization of the metals was facilitated through formation of stable metal(chloro)phosphates,surface complexation,and/or ion exchange reactions.Combined CMC-CAP and Ca O may serve as an effective formulation for simultaneous and long-term immobilization of multiple heavy metals in acidic soil.(4)On the basis of the above research,this study pilot-tested a new type of carboxymethyl cellulose(CMC)bridged chlorapatite microparticles(CMC-CAP)for immobilization of Pb at an abandoned chemical plant site,and monitored long-term(1 year)stability of Pb under the field conditions.CMC-CAP at a dosage of 0.5 wt.%resulted in the highest immobilization of Pb,and decreased the acid-leachable Pb in the soil from 1.452 mg L^-1 to 0.008 mg L^-1 after 1day of reaction,and further to 0.005-0.006 mg L^-1when aged for 180 and 365 days under the field conditions.The long-term aging consolidated the Pb immobilization,securing the compliance with the Chinese regulatory limit(0.010 mg L^-1 for Pb).Sequential extraction data confirmed that the CMC-CAP amendment converted the more leachable Pb fractions into the less available Pb species.The soil amendment reduced the risk assessment code from“High risk”for the untreated soil to(Low risk)after 120 days of aging.XRD and FTIR results revealed the immobilization of Pb by CMC-CAP was achieved through the formation of the highly stable pyromorphite,surface complexation,and/or ion exchange reactions.Based on the field data,the remediation cost of the CMC-CAP based technology was estimated to be$36.2 m^-3.This study provides important field data on the effectiveness of CMC-CAP for cost-effective immobilization of Pb in soil.