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

• Published in: Environmental science and pollution research international Vol. 29; no. 33; pp. 49724 - 49738
• Main Authors: Rosa, Maria Isabel Garcia, Boga, Gabriella Andrade, Cruz, Suellen Silva Vieira, Andrade, Fabio Ramos Dias de, Furquim, Sheila Aparecida Correia, Shinzato, Mirian Chieko
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-07-2022; Springer Nature B.V
• Subjects: Acidity; Adsorbents; Adsorption; Aluminum Silicates; Aquatic Pollution; Atmospheric Protection/Air Quality Control/Air Pollution; Cation exchange; Cation exchanging; Cations; Chemical precipitation; Chromium; Chromium - chemistry; Earth and Environmental Science; Ecotoxicology; Environment; Environmental Chemistry; Environmental Health; Environmental science; Ferrous Compounds - chemistry; Hydrogen; Hydrogen-Ion Concentration; Ions; Iron; Iron - chemistry; Minerals; Oxidation; Oxidation-Reduction; pH effects; Potassium; Potassium dichromate; Reduction; Research Article; Trivalent chromium; Vermiculite; Waste Water Technology; Water Management; Water Pollutants, Chemical - analysis; Water Pollution Control; Zeolites; Cation exchange; Redox; Zeolite; Chromium(VI); Iron(II); Vermiculite
• ISSN: 0944-1344; 1614-7499
• DOI: 10.1007/s11356-022-19366-w

Mechanisms of Cr(VI) reduction by Fe(II) modified zeolite (clinoptilolite/mordenite) and vermiculite were evaluated. Adsorbents were treated with Fe(SO
4
)·7H
2
O 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
K
2
Cr
2
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.