Study On The Influence Ragarding Transport And Transformation Of Cr(Ⅵ) By Biochar With The Effect Of Low Molecular Weight Organic Acids

The heavy metal chromium（Cr）exists in the environment mainly in hexavalent and trivalent forms,among which Cr（Ⅵ）is much more toxic than Cr（III）and is a serious hazard to human health.Biochar is used as green materials for soil remediation of heavy metals.Low-molecular-weight organic acids（LMWOAs）,a natural component in soil,have certain reducing properties,and the interaction with biochar may affect the migration and transformation behavior of Cr（Ⅵ）.Therefore,there is an urgent need to clarify the reduction and transformation behavior of biochar on Cr（Ⅵ）in the migration process under the action of LMWOAs.In this study,biochar（BC）was prepared from ramie straw as biomass,and three kinds of LMWOAs（succinic acid（SA）,malic acid（MA）and tartaric acid（TA））with different amounts ofα-hydroxyl groups were selected to investigate the mechanism of Cr（Ⅵ）adsorption and reduction by biochar under the action of LMWOAs through batch adsorption tests;the effects of LMWOAs and biochar on the transport and transformation of Cr（Ⅵ）in different physicochemical conditions were investigated by column tests to reveal the reduction behavior of biochar on the dynamic transport of Cr（Ⅵ）under the action of LMWOAs.The main findings are as follows:1.The LMWOAs containingα-hydroxy functional groups could synergistically adsorb and reduce Cr（Ⅵ）with biochar,and the more the number ofα-hydroxy functional groups,the more significant the synergistic effect was.The adsorption capacity of biochar for Cr（Ⅵ）at50 mg L^-1 was 8.31,6.71,20.24 and 24.80 mg g^-1 without LMWOAs and in the presence of SA,MA and TA,respectively;the adsorption reduction efficiency of biochar for Cr（Ⅵ）increased by 28.55%and 39.45%with MA and TA,respectively,while SA inhibited Cr（Ⅵ）adsorption was inhibited by SA.The kinetic and isothermal adsorption tests showed that the adsorption of Cr（Ⅵ）by biochar alone and by LMWOAs was mainly a chemisorption process of multimolecular layers.Combined with the characterization analysis,the phenolic hydroxyl groups contained in biochar can undergo redox reactions with Cr（Ⅵ）in the presence of no reducing LMWOAs;when MA and TA are present,biochar acts as an electron conductor to transfer electrons and induce the reduction of Cr（Ⅵ）to Cr（III）.Meanwhile,the influence of adsorption reduction admissibility conditions（biochar dosage,LMWOAs concentration,ionic strengths and coexisting anions）of LMWOAs and biochar on Cr（Ⅵ）.2.The changes of LMWOAs and BC concentrations and the number of cycles determine the transport conversion ability of Cr（Ⅵ）.Increasing the filling amount of BC and increasing the concentration of MA and TA induced the conversion of Cr（Ⅵ）to Cr（III）,which led to a decrease in the transport rate of Cr（Ⅵ）,and the generated Cr（III）was easy to form organic complexes with LMWOAs for transport.When filled with 2%BC（w/w）and MA and TA concentrations of 10 mmol L^-1,the mobilities of Cr（Ⅵ）were only 5.97%and 4.49%,respectively.The low concentration of SA reduces the electrostatic repulsion between BC and Cr（Ⅵ）and decreases the transport ability of Cr（Ⅵ）,while the high concentration competes with Cr（Ⅵ）for deposition sites,which enhances the transport ability of Cr（Ⅵ）.The transport ability of Cr（Ⅵ）was enhanced from the second cycle onwards due to the occupation of BC active sites when Cr（Ⅵ）was cycled through.3.Environmental factors affect the transport conversion of Cr（Ⅵ）by changing the retention time and electrostatic force of Cr（Ⅵ）.The increase of flow rate shortened the transport time of Cr（Ⅵ）in the column,resulting in enhanced transport ability and suppressed reduction.When the flow rate was increased from 0.25 m L min^-1 to 1 m L min^-1,the recoveries of Cr（Ⅵ）in the presence of MA and TA increased from 34.38%and 22.80%to 57.94%and46.72%,respectively,while the recoveries of Cr（III）increased from 60.06%and 68.93%to37.64%and 44.76%,respectively.The increase of ionic strength reduced the surface potential of BC,and Cr（Ⅵ）was easily deposited on BC and reduced by LMWOAs.p H 3,PO₄^3-hydrolysis existed as HPO₄^2-and H₂PO₄^-,so the transport ability of Cr（Ⅵ）under different anion conditions showed SO₄^2->PO₄^3->Cl^-,and the transport law of Cr（III）is the opposite.In addition,the mobility of total Cr did not change significantly under different environmental conditions due to the reduction of Cr（Ⅵ）and transport of Cr（III）in the presence of MA and TA.In summary,BC can mediate the reduction of Cr（Ⅵ）by LMWOAs containingα-hydroxyl groups,which leads to the conversion of Cr（Ⅵ）to Cr（III）in the transport process,and the transport transformation ability of Cr（Ⅵ）in this system depends on the concentration of LMWOAs,the filling amount of BC and the number of cycles.Meanwhile,different environmental factors also have significant effects on the transport transformation of Cr（Ⅵ）.This study aims to reveal the transport and transformation pattern of biochar on Cr（Ⅵ）to the subsurface environment in the presence of LMWOAs,and provide a theoretical basis for assessing the remediation of Cr-contaminated soil by biochar.