Highly Efficient Removal Of Hexavalent Chromium From Water And Soil By Polyaniline/Polyvinyl Alcohol/Sodium Alginate Composite Gel Beads

Polyaniline（PANI）can adsorb hexavalent chromium（Cr（Ⅵ））efficiently and reduce the more toxic Cr（Ⅵ）to the less toxic trivalent chromium（Cr（Ⅲ））due to its large number of amino and imine groups in its molecular structure.Therefore,PANI has been extensively studied and applied in the remediation of Cr（Ⅵ）polluted water.However,the nano-sized PANI is not easy to be recycled and easily causes secondary pollution to the environment,because of the small size and the strong aggregation effect.The strong aggregation effect also hinders the application of this adsorbent in soil remediation.In order to solve the above problems,gel materials were used as the carrier of nano PANI.In this study,a millimeter-sized PANI/PVA/SA composite（PPS）was synthesized by embedding polyaniline（PANI）into polyvinyl alcohol（PVA）/sodium alginate（SA）gel beads.And a series of characterization methods were used to test the physical and chemical properties of PPS.Additionally,the PPS was used to recover hexavalent chromium（Cr（Ⅵ））contaminated water and soil to study the remediation impacts and mechanism.The main work and findings of this study are as follows:（1）Response surface methodology was used to optimize the synthesis of PPS.And it was found that the optimal content of PVA,SA,and PANI was 3%,0.5%,and 2.5%.（2）The effect of various environmental factors on the Cr（Ⅵ）adsorption performance of PPS was investigated and the results showed that the removal rate of Cr（Ⅵ）by PPS increased with increasing dosing,decreasing pH,and increasing temperature.The adsorption process was a spontaneous,endothermic,and entropy-increasing process.（3）The kinetics and isothermal adsorption models were fitted and analyzed for the adsorption process of PPS.The results showed that the pseudo-second-order model was most suitable for the adsorption kinetics of Cr（Ⅵ）by PPS,and the chemical adsorption was the main rate-limiting step.Both R-P and Langmuir’s isothermal adsorption model can fit well with the isothermal adsorption results,and Q_m in Langmuir model showed that Langmuir’s maximum removal capacity of PPS for Cr（Ⅵ）was 83.1 mg·g^-1.（4）The reusability of PPS was investigated.The results showed that 1 mol·L^-1Na OH followed by 1 mol·L^-1 HCl had the best desorption effect.The results showed that the adsorption efficiency of Cr（Ⅵ）using the above four eluents remained 95.25%after four cycles.（5）The PPS was applied to the in situ stabilization study of soil Cr（Ⅵ）.The results showed that after remediation,the pH of the soil decreased,SOM and EC increased,and the of the total Cr,Cr（Ⅵ）contents,weak acid extracted and water-soluble Cr in the soil decreased with the increase of PPS dosage.The results of the leaching experiment showed that the immobilization of PPS gel beads could effectively reduce the leaching toxicity of Cr in soil.（6）The mechanism of removing Cr（Ⅵ）by PPS was as follows:First,there was a rapid attachment of Cr（Ⅵ）to the surface of PPS,and Cr（Ⅵ）diffused into the mesoporous material according to size selection.In this process,the protonated N in the PPS could increase the electrostatic attraction of Cr（Ⅵ）anions.Simultaneously,the oxidation of–NH–to=N–was accompanied by reduction of Cr（Ⅵ）to Cr（Ⅲ）,and the C–OH/–OH groups of SA and PVA may also act as electron-donating moieties and participated in the reduction of Cr（Ⅵ）.Finally,the Cr（Ⅲ）can be further adsorbed by complexation or chelation with imino groups and the hydroxyl groups on SA and PVA.