Sodium citrate and biochar synergistic improvement of nanoscale zero-valent iron composite for the removal of chromium(Ⅵ)in aqueous solutions

Sodium citrate(SC)is a widely-used food and industrial additive with the properties of com-plexation and microbial degradation.In the present study,nano-zero-valent iron reaction system(SC-nZVI@BC)was successfully established by modifying nanoscale zero-valent iron(nZVI)with SC and biochar(BC),and was employed to remove Cr(Ⅵ)from aqueous solu-tions.The nZVI,SC-nZVI and SC-nZVI@BC were characterized and compared using X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FTIR),thermogravimetric analy-ses(TGA),vibrating sample magnetometer(VSM),scanning electron microscope(SEM),X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS).The results showed that nZVI was successfully loaded on the biochar,and both the agglomeration and surface pas-sivation problems of nanoparticles were well resolved.The dosage of SC,C∶Fe,initial pH and Cr(Ⅵ)concentration demonstrated direct effects on the removal efficiency.The maximum Cr(Ⅵ)removal rate and the removal capacity within 60 min were 99.7%and 199.46 mg/g,respectively(C∶Fe was 1∶1,SC dosage was 1.12 mol.%,temperature was 25℃,pH=7,and the original concentration of Cr(Ⅵ)was 20 mg/L).The reaction confirmed to follow the pseudo-second-order reaction kinetics,and the order of the reaction rate constant k was as follows:SC-nZVI@BC>nZVI@BC>SC-nZVI>nZVI.In addition,the mechanism of Cr(Ⅵ)removal by SC-nZVI@BC mainly involved adsorption,reduction and co-precipitation,and the reduction of Cr(Ⅵ)to Cr(Ⅲ)by nano Fe0 played a vital role.Findings from the present study demon-strated that the SC-nZVI@BC exhibited excellent removal efficiency toward Cr(Ⅵ)with an improved synergistic characteristic by SC and BC.