Background electrolytes and pH effects on selenate adsorption using iron-impregnated granular activated carbon and surface binding mechanisms. (March 2018)

• Record Type: Journal Article
• Authors: Zhang, Ning; Gang, Daniel Dianchen; McDonald, Louis; Lin, Lian-Shin
• Is Part Of: Chemosphere. Volume 195(2018)
• Page Start: 166
• Page End: 174
• Publication Date: 2018-03
• Subjects: Selenate adsorption -- Adsorption mechanisms -- Raman -- X-ray photoelectron analysis -- Electrolytes; Pollution -- Periodicals; Pollution -- Physiological effect -- Periodicals; Environmental sciences -- Periodicals; Atmospheric chemistry -- Periodicals; 551.511
• DOI: 10.1016/j.chemosphere.2017.11.161
• Languages: English
• ISSNs: 0045-6535

Abstract

Abstract: Iron-impregnated granular activated carbon (Fe-GAC) has been shown effective for selenite adsorptive removal from aqueous solutions, but similar effectiveness was not observed with selenate. This study examined the effects of background electrolytes and pH on selenate adsorption on to Fe-GAC, and surface bindings to elucidate the selenate adsorption mechanisms. The decrease magnitude of selenate adsorption capacity under three background electrolytes followed the order: LiCl > NaCl > KCl, as ionic strength increased from 0.01 to 0.1 M. Larger adsorption capacity differences among the three electrolytes were observed under the higher ionic strengths (0.05 and 0.1 M) than those under 0.01 M. Multiplet peak fittings of high resolution X-ray photoelectron spectra for O1s and Fe2p3/2 indicated the presence of iron (III) on adsorbent surface. pH variations during the adsorbent preparation within 3–8 in NaCl solutions did not cause appreciable changes in the iron redox state and composition. Raman spectra showed the formation of both monodentate and bidentate inner sphere complexes under pHs <7 and a mixture of outer sphere and inner sphere complexes at pH 8. These results explained the lower selenate adsorption under alkaline conditions. Mechanisms for monodentate and bidentate formations and a stable six-member ring structure were proposed. Two strategies were recommended for modifying Fe-GAC preparation procedure to enhance the selenate adsorption: (1) mixed-metal oxide coatings to increase the point of zero charge (pHzpc ); and (2) ferrous iron coating to initially reduce selenate followed by selenite adsorption. Graphical abstract: Highlights: Selenate forms inner sphere complex with Fe-GAC at pH lower than 7, and a mixture of inner and outer sphere complex at pH 8. The complex structure that formed by selenate and Fe-GAC is illustrated by a six-member-ring. Two adsorbent modification are proposed to increase the selenate adsorptive removal.