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

Abstract

Mechanisms of Cr(VI) reduction by Fe(II) modi ed zeolite and vermiculite were evaluated. Adsorbents were treated with Fe(SO 4 ).7H 2 O 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 → 3Fe 3+ + Cr +3 ), the iron content released from Z-Fe and V-Fe were su cient 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 K Cr O 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.

Sulfite Poses a Risk of Hexavalent Chromium Rebound in Vadose Zone: A Challenge of the Stability of CrxFe1–x(OH)3

“…This implies that despite the low release of Cr­(VI) in the solution (only 0.01 mg/L), there was indeed a conversion of Cr­(III) to Cr­(VI) on the surface of Cr­(OH) 3 . As depicted in Figure b, the proportion of Cr­(VI) on the surface of Cr 0.5 Fe 0.5 (OH) 3 is relatively lower compared to that of Cr­(OH) 3 and consistently remains below 10%, despite the significant release of Cr­(VI), which may be attributed to desorption or rereduction processes. − …”

“…The abnormal paucity of SO 4 2– is incongruous with the prevalence of REDOX reactions on the surface of Cr 0.5 Fe 0.5 (OH) 3 , but it does correspond to the decrease in the proportion of Cr(VI). The anomalous decrease in both SO 4 2– and Cr(VI) percentage might be attributed to the competitive adsorption of a significant amount of instantaneously formed CrO 4 2– and SO 4 . − Both CrO 4 2– and SO 4 2– were desorbed from the surface of Cr 0.5 Fe 0.5 (OH) 3 . Subsequently, dissolved CrO 4 2– is reduced by dissolved sulfite under the catalysis of dissolved Fe(III) to form Cr(III) ions, which elucidates the notable release of Cr(III). SO 3 · − + SO 3 · − → normalS 2 normalO 6 2 − normalS 2 normalO 6 2 − + normalH 2 …”

“…According to the ORP (oxidation reduction potential) values of sulfite at pH 3, , sulfite is thermodynamically capable of reducing Cr(VI). However, the kinetics of Cr(VI) reduction by sulfite is relatively inefficient. , Fe can act as a catalyst and accelerate this process (eqs and ). − Therefore, the excess Cr(III) released from Cr 0.5 Fe 0.5 (OH) 3 in the presence of sulfite is expected to undergo free radical oxidation to form Cr(VI), followed by reduction to Cr(III) catalyzed by iron, i.e., through REDOX-mediated dissolution. 2 Fe 3 + + HSO 3 − + normalH 2 normalO → 2 Fe 2 + + SO 4 2 − + 3 normalH + 3 Fe 2 + + HCrO 4 − + 7 normalH + → 3 Fe 3 + + Cr 3 + + 4 normalH 2 normalO …”