Enhanced azo dye removal from wastewater by coupling sulfidated zero-valent iron with a chelator

Abstract

Recent research on zero-valent iron (ZVI) has evolved from a proof-of-concept stage to an application-focus stage with the goal of enhancing the applicability of ZVI for water decontamination. Sulfidation represents one of the most prominent ways to enhance the reactivity and efficiency of ZVI. In this study, sulfidated ZVI (S-ZVI) coupled with ethylenediamine tetraacetic acid (EDTA) was used to remove acid red 73 (AR 73) from wastewater and the results showed that the coupling of chelator and sulfidation significantly enhanced the removal of AR 73. The first-order kinetic constant (kobs) followed a bimodal pattern with increasing S/Fe ratio, chelator concentration, and mixing rate, but a unimodal pattern with changes in initial AR 73 concentration. Under the optimal conditions, the removal rate of AR 73 obtained in the EDTA + S-ZVI system was about 6.5 times faster than that in the ZVI system. Further analysis by gas chromatography-mass spectrometry (GC-MS) confirmed that reduction played a major role in the removal of AR 73. X-ray photoelectron spectroscopy (XPS) energy dispersive X-ray spectroscopy (EDS), and Fourier transform infrared spectrometry (FTIR) were used to investigate the surface chemical compositions of the solids before and after reaction. The results showed that the surface of freshly prepared S-ZVI was covered with FeSx, and EDTA on the surface of S-ZVI could complex with the Fe(II) to form an FeII-EDTA complex. The FeII-EDTA complex can inhibit the oxidation of Fe(II) and reduce the redox potential of the solution, which together enhance the removal of AR 73.