Selective adsorption of arsenic over phosphate by transition metal cross-linked chitosan

Paul Anastas, Julie Zimmerman and 6 other contributors

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    The ability of transition metal chitosan complexes (TMCs) of varying valence and charge to selectively adsorb As (III) and As(V) over their strongest adsorptive competitor, phosphate is examined. Fe(III)-chitosan, Cu(II)-chitosan, Al(III)-chitosan, Ni(II)-chitosan, and Zn(II)-chitosan are synthesized, characterized via Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy (ATR-FTIR) and X-ray Diffractometry (XRD), and their selective sorption capabilities towards arsenite and arsenate over phosphate are evaluated. It was found that the stability of the metal-chitosan complexes varied, with Al(III)- and Zn(II)-chitosan forming unstable complexes resulting in precipitation of gibbsite, and Wulfingite and zincite, respectively. Cu(II)-, Ni(II)-, and Fe(III)- chitosan formed a mixture of monodentate and bidentate complexes. The TMCs which formed the bidentate complex (Cu(II)-, Ni(II)-, and Fe(III)-) showed greater adsorption capability for arsenate in competitive systems with phosphate. Using the binary separation factor ?t/c, it can be shown that only Fe(III)-chitosan is selective for As(V) and As(III) over phosphate. Density Functional Theory (DFT) modeling and extended X-ray absorption fine structure (EXAFS) determined that Fe(III)-chitosan and Ni(II)-chitosan adsorbed As(V) and As(III) via innersphere complexation, while Cu(II)-chitosan formed mainly outer-sphere complexes with As(V) and As(III). These differences in complexation likely result in the observed differences in selective adsorption capability towards As(V) and As(III) over phosphate.