Impact Of Exogenous Iron On The Removal Of Cr(Ⅵ) By Biochar And Application In Wastewater Treatment

Chromium（Cr）is one of the most concerned elements among heavy metals,which is widely used in electroplating,pharmaceuticals and smelting industries.The electroplating wastewater and waste residues caused by these industries are producing contaminations in water and soil environment,as well as unignorable indirect health risk to human body.Biochar is a carbon-rich material obtained by the pyrolysis of biomass,and is gaining increasing attentions for its excellent adsorption and reduction abilities due to porous structure and abundance in surface functional groups.Iron（Fe）is a reactive metal which widely occurs in natural environment.Ferrous（Fe（II））and ferric（Fe（III））containing materials are commonly employed in the treatment of wastewaters and polluted soils due to their strong redox ability.Electroplating wastewater is often treated with Fe SO₄,however,the solid waste,residual Cr（III）and low p H of the treated wastewater are causing subsequent pollution.Exploring the influence and mechanisms of exogenous iron on Cr（VI）containing wastewater treatment by biochar,would of great importance for the development of Cr（VI）removal process.In this study,we explored the Cr（VI）removal mechanism of two kinds of biochar,i.e.maize straw derived biochar（MBC）and rice straw derived biochar（RBC）,as well as the influence of environment factor like p H,dissolved organic matter and sulfate.Furthermore,the impacts of soluble Fe（II）and Fe（III）as well as insoluble ferrihydrite and goethite were also investigated.Last but not least,combination of biochar and Fe²⁺were employed for the Cr（VI）removal in the actual electroplating wastewater.The results indicates that the higher surface charge of MBC than RBC made it easier for Cr（VI）adsorption,while MBC also contains higher amount of persist free radicals and the surface functional groups were more significantly involved in the Cr（VI）removal reaction.During the process of removing Cr（VI）,PFRs and oxygenated functional groups like-OH played the crucial role as electron donors.When p H increased,the adsorption ability of the two kinds of biochar weakened generally,while RBC was more sensitive for the changes.Due to the alkalinity of biochar,a high dosage of biochar didn’t promise good performance in Cr（VI）removal,but the benefit of enhanced biochar dosage could only be maintained under the condition that the environment p H could be kept<4.The impact of dissolved organic matter（DOM）from biochar or the co-existing sulfate was insignificant on Cr（VI）removal.Adding soluble Fe²⁺or Fe ³⁺made enhancement to Cr（VI）removal rate of biochar.The coupling effects of soluble iron and biochar brought higher removal rate than the sum of their effects individually.The coupling effects may include:（1）The hydrolysis of Fe²⁺or Fe³⁺in liquid environment inhibited the rising trend of p H value,which would produce a lowered p H range favored by Cr（VI）removal of biochar;（2）The induced changes of PFRs and organic functional groups like-OH,-C-O,-C=O provided more electron transfers for Cr（VI）reduction,or being electron shuttles for electron transfer;（3）The redox cycle of Fe²⁺/Fe³⁺brought more route for electron transfer;（4）The precipitation of Fe（III）provided more adsorption site for Cr（III）immobilization.Compared with MBC,RBC relied less on surface functional groups as electron donors after adding soluble iron,while more on the electron donor and electron shuttle effects from PFRs which was greatly increased by adding Fe²⁺or Fe³⁺.Deposition of ferrihydrite and goethite on biochar slightly decreased the Cr（VI）removal rate,while goethite brought less influence than ferrihydrite.The ferrihydrite and goethite didn’t bring higher removal rate for Cr（VI）removal of biochar due to（1）ferrihydrite and goethite can’t adjust p H environment like soluble Fe²⁺of Fe³⁺did;（2）Fe²⁺/Fe³⁺reduction cycle wasn’t generated after adding ferrihydrite and goethite,leading to less efficiency for electron transfer.According to results above,we selected the combination of Fe²⁺/MBC which shown best cooperation effect for the treatment of Cr（VI）containing electroplating wastewater.The results revealed that presence of high concentration of sulfate causing precipitation of Fe²⁺and thus passivated the surface which would lead to a lower Cr（VI）removal rate than theoretical expectation.Therefore,simply increasing the dosage of Fe²⁺could not help to break the bottleneck.By adding biochar,the coupling effect of biochar/Fe²⁺resulted in much better Cr（VI）adsorption,reduction as well as Cr（III）immobilization.However,adding too high dosage of biochar would raise the p H environment which is not beneficial for Cr（VI）adsorption and reduction of biochar,thus the dosage of biochar needs to be optimized at appropriate range.Lower p H in wastewater creates better environment for the coupling effect of biochar and Fe（II）.In practical application in wastewater treatment,conditions like p H value,biochar and Fe²⁺dosages need to be adjusted according to the initial p H,Cr（VI）concentration and other status of electroplating wastewater.