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 and vermiculite were evaluated. Adsorbents were treated with Fe(SO4).7H2O to saturate their exchange sites with Fe(II). 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 the solution by batch test to evaluate the effect of contact time and Cr(VI) initial concentration. Cr(VI) was 100% reduced to Cr(III) by Z-Fe and V-Fe from solution with 18 mg L−1 Cr(VI) in 1 minute. Considering that 3 moles Fe(II) are required to reduce 1 mole Cr(VI) (3Fe+2 + Cr+6 → 3Fe3+ + Cr+3), the iron content released from Z-Fe and V-Fe were sufficient to reduce 100% of Cr(VI) in solution by up to 46.8 mg L−1 Cr(VI), and about 90% (V-Fe) and 95% (Z-Fe) in solution with 95.3 mg L−1 Cr(VI). The assess of the Fe(II), Cr(III), Cr(VI), and K+ contents of the adsorbents and solutions after batch tests indicated that K+ ions from K₂Cr₂O₇ 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. Therefore, the release of Fe(II) from Z-Fe and V-Fe involved cation exchange between, respectively, K+ and H+ ions from solution. The reduction of Cr(VI) by Fe(II) caused the precipitation of Cr(III) and Fe(III), and the decrease of pH of the solution to < 5. As acidity limits the precipitation of Cr(III) ions, they remained in solution and were not adsorbed by both adsorbents (since they prefer to adsorb K+ and H+). To avoid oxidation, Cr(III) can be removed by precipitation or adsorption by untreated minerals.