Mechanisms of chromium(VI) removal from solution by zeolite and vermiculite modified with iron(II)

Abstract

Mechanisms of Cr(VI) reduction by Fe(II) modified zeolite (clinoptilolite/mordenite) and vermiculite were evaluated. Adsorbents were treated with Fe(SO₄)·7H₂O to saturate their exchange sites with Fe(II). However, this treatment decreased their CEC and pH_PZC, probably due to the dealumination process. Vermiculite (V-Fe) adsorbed more Fe(II) (21.8 mg g⁻¹) than zeolite (Z-Fe) (15.1 mg g⁻¹). Z-Fe and V-Fe were used to remove Cr(VI) from solution in a batch test to evaluate the effect of contact time and the initial concentration of Cr(VI). The Cr(VI) was 100% reduced to Cr(III) by Z-Fe and V-Fe in solution at 18 mg L⁻¹ Cr(VI) after 1 min. Considering that 3 mol of Fe(II) are required to reduce 1 mol of Cr(VI) (3Fe⁺² + Cr⁺⁶ → 3Fe⁺³ + Cr⁺³), the iron content released from Z-Fe and V-Fe was sufficient to reduce 100% of the Cr(VI) in solutions up to 46.8 mg L⁻¹ Cr(VI) and about 90% (V-Fe) and 95% (Z-Fe) at 95.3 mg L⁻¹ Cr(VI). The Fe(II), Cr(III), Cr(VI), and K⁺ contents of the adsorbents and solutions after the batch tests indicated that the K⁺ ions from the ${\mathrm{K}}_{2}{\mathrm{Cr}}_{2}{\mathrm{O}}_{7}$ solution were the main cation adsorbed by Z-Fe, while vermiculite did not absorb any of these cations. The H⁺ of the acidic solution (pH around 5) may have been adsorbed by V-Fe. The release of Fe(II) from Z-Fe and V-Fe involved cation exchange between K⁺ and H⁺ ions from solution, respectively. The reduction of Cr(VI) by Fe(II) resulted in the precipitation of Cr(III) and Fe(III) and a decrease in the pH of the solution to < 5. As acidity limits the precipitation of Cr(III) ions, they remained in solution and were not adsorbed by either adsorbent (since they prefer to adsorb K⁺ and H⁺). To avoid oxidation, Cr(III) can be removed by precipitation or the adsorption by untreated minerals.