Spectroscopic characterization analysis on Cr(VI) removal mechanism by low-cost adsorbent of rice husk ash

Analyzing methods of Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction analysis (XRD) and X-ray fluorescence spectroscopy (XRF) were used to reveal the adsorption mechanism of Cr(VI) by low-cost adsorbent of rice husk ash. Results of FTIR showed that amide II band, Si-O-Si, and O-Si-O were important for Cr(VI) removal. SEM micrographs suggested that series of needle-shaped precipitation appeared on cell surface, and inorganic precipitation mechanism and redox mechanism might work in the test. XPS spectra showed that main elements of rice husk ash were C, N, O, P and Si, existing state of C was mainly aldoketones groups, coordination reaction existed between C functional groups and Cr(VI); N element, -NH₂ in chief, could remove Cr(VI) in the way of physical adsorption based on electrostatic interaction; Si-O might be useful in adsorption process. XRD data suggested representative characteristic adsorption band of SiO₂, and the increasing crystallinity of rice husk ash showed metallic compound formed between Cr(VI) and rice husk ash. XRF results indicated that the content of K, Na, Mg and Ca changed, and two new elements were detected, the evidence of ion exchange mechanism. The functional groups played different roles in Cr(VI) adsorption process, and inorganic precipitation mechanism, redox mechanism, surf-complexation mechanism and ion exchange mechanism were important ways in Cr(VI) removal, which could provide theoretical support in further application.