Preparation and evaluation of activated carbon-based iron-containing adsorbents for enhanced Cr(VI) removal: Mechanism study

Activated carbon-based, iron-containing adsorbents (TAC/Fe) were developed through a simple impregnating-air drying protocol for enhanced removal of Cr(VI) from aqueous solutions. The preparation conditions have been optimized using Taguchi method. Cr(VI) adsorption behaviors on TAC/Fe and native TAC under different Cr(VI) concentration, ionic strength and solution pH conditions were investigated. Both the Langmuir and Freundlich models correlated the sorption isotherms on the sorbents quite well. The adsorption showed strong dependence on solution pH. XPS and FTIR analyses were employed to investigate the sorption mechanisms. Results demonstrated that Cr(VI) adsorption onto TAC/Fe consists of four reaction steps: (1) fast protonation or deprotonation of carbon surface in the presence of solution H+ or OH−; (2) conjunction of Cr(VI) with the surface functionalities and formation of inner-sphere complexes of Cr(VI) with the Fe (hydr)oxides; (3) reduction of Cr(VI) into Cr(III), accompanied by oxidation of the carbon surface to form new oxygen-containing functional groups; and (4) ion exchange between the reduced Cr(III) and the created carboxyl and/or hydroxyl groups. The enhanced Cr(VI) removal efficiency on TAC/Fe was derived from its iron component, higher content of acidic functional groups and more negative surface.

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► The preparation conditions for iron-containing activated carbons were optimized.
► Cr(VI) removal on activated carbons was greatly enhanced after iron modification.
► Cr(VI) adsorption on the iron-containing activated carbons consists of four steps.
► XPS and FTIR spectroscopy were used to investigate the adsorption mechanisms.