Optimization Of Core@Shell Structured Iron@Carbon@Polyaniline For Enhanced Cr(?) Reduction

Nano-zero-valent iron(NZVI),with a low redox potential(-0.44 V)and active chemical properties,can be used to reduce Cr(?)in acidic conditions.However,H2 would be generated and escaped from the reaction system,which lead to the huge waste of the electrons from the ZVI.In order to improve the electron utilizaiton,a trible layered core-shell structured Fe@C@PANI composite was successfully prepared.The mechanism involved in Cr(?)reduction by the ZVI of synthesized Fe@C@PANI were disclosed.Based on disclosed mechanism,the C and PANI shell layers have been optimized based on the the electronic utilization of ZVI.In this study,Fe@C@PANI composites were prepared by a combined hydrothermal-calcination-chemical polymerization method.Fe@C@PANI composites can achieve rapid removal of Cr(VI).With the initial pH=1,1.0 mg/L of Cr(VI)wastewater can be completely reduced within 5 min.A high removal capacity(508 mg/g)has also achieved for the Fe@C@PANI.The C@PANI shell layers acted as effective protection layer for ZVI against the H in the solution,increasing the electronic utilization of ZVI from 8.5%to 50.9%.PANI,a buffer layer,rapidly caputure H+ from solution,then,H+ was released slowly and diffused into the inner layer through the carbon layer.The C layer,as the diffusion channel of H+ and generated H2,improved the electronic utilization of ZVI by slowing down the reaction between ZVI and H+.The porous structure of C layer has been optimized.The sludge-based magnetic carbon(ZVI@C)was prepared by carbonization methord using activated sludge as carbon source.Ultrasonic pretreatment was employed to facilitate soluble carbon source release from activated sludge and the optimal pretreatment power optimized as 800 W.203 mg/g of Cr(?)removal capacity of ZVI@C was achieved,and the electronic utilization rate of ZVI was increased from 8.5 to 12%.Moreover,the porous structure of the sludge derived activated Carbon(SAC)can be adjusted by the carbonization conditions,heavy metals content and bonded water in the sludge.Carbonization conditions were optimized as following:900°C of carbonization temperature,10“C/min of heating rate,60 min of retention time and 2.00%of bound-water content.The specific surface area of SAC reached 1018.8 m2/g under the optimized condition,significantly higher than the reported SAC(50-200 m2/g).The soluble carbon source released from sludge by pretreatment is beneficial to the formation of core-shell structure.Therefore,the effects of molecular weight,C/Fe ratio and carbon source concentration on the electronic utilization of ZVI were further investigated.Under the conditions of glucose as carbon source,C/Fe=20 and carbon source concentration of 0.24 mol/L,the electronic utilization rate of ZVI@C synthesized by carbonization can reach 82.4%.The structure of PANI layer is optimized.The capture of H+,electron donation and the further Cr(?)removal can be synergistically controlled by the adjusting the pore sturctre,P-type conjugate structure and doping acid of the PANI.In order to enhance the pore structure of PANI,the porous PANI was synthesized by using bacteria as template.The specific surface area of the prepared porous polyaniline(Bac@PANI)reached to about 51 m2/g,which was higher the pristine PANI(27.5 m2/g).Maximum Cr(?)removal capacity of Bac@PANI reached to 835.06 mg/g.PANI with different oxidation degree was prepared to prove that the oxidation degree of PANI was proportional to the storage capacity of H+,and its maximum storage capacity of H+ was 0.278 mmol/g.It was found that the H+ release rate of PANI was negatively correlated with the amount of H+ ionized from doped acids.Doping with different acids,the equilibrium time of H+released from PANI increased from 15 to 35 min.The slow-release ability of H+can make PANI more effective in protecting ZVI in Fe@C@PANI composite,improving the electronic utilization of ZVI.