......
Per this article, "Advanced Oxidation Process Based on the Cr(III)/Cr(VI) Redox Cycle", by Alok D. Bokare and Wonyong Choi, published in Environ. Sci.
Technol., 2011, 45 (21), pp 9332–9338, DOI: 10.1021/es2 . To quote from the abstract:
"Oxidative degradation of aqueous organic pollutants, using 4-chlorophenol (4-CP) as a main model substrate, was achieved with the concurrent
H2O2-mediated transformation of Cr(III) to Cr(VI). The Fenton-like oxidation of 4-CP is initiated by the reaction between the aquo-complex of Cr(III)
and H2O2, which generates HO• along with the stepwise oxidation of Cr(III) to Cr(VI). The Cr(III)/H2O2 system is inactive in acidic condition, but
exhibits maximum oxidative capacity at neutral and near-alkaline pH. Since we previously reported that Cr(VI) can also activate H2O2 to efficiently
generate HO•, the dual role of H2O2 as an oxidant of Cr(III) and a reductant of Cr(VI) can be utilized to establish a redox cycle of
Cr(III)–Cr(VI)–Cr(III). As a result, HO• can be generated using both Cr(III)/H2O2 and Cr(VI)/H2O2 reactions, either concurrently or
sequentially. The formation of HO• was confirmed by monitoring the production of p-hydroxybenzoic acid from [benzoic acid + HO•] as a probe
reaction and by quenching the degradation of 4-CP in the presence of methanol as a HO• scavenger. The oxidation rate of 4-CP in the Cr(III)/H2O2
solution was highly influenced by pH, which is ascribed to the hydrolysis of CrIII(H2O)n into CrIII(H2O)n-m(OH)m and the subsequent condensation to
oligomers. The present study proposes that the Cr(III)/H2O2 combined with Cr(VI)/H2O2 process is a viable advanced oxidation process that operates
over a wide pH range using the reusable redox cycle of Cr(III) and Cr(VI)."
Link: http://pubs.acs.org/doi/abs/10.1021/es2021704
.....
Per Wiki on Cr(VI), to quote:
"Hexavalent chromium compounds are genotoxic carcinogens. Due to its structural similarity to sulfate, chromate (a typical form of chromium(VI) at
neutral pH) is transported into cells via sulfate channels.[5] Inside the cell, hexavalent chromium (chromium(VI)) is reduced first to pentavalent
chromium (chromium(V)) then to trivalent chromium (chromium(III)) without the aid of any enzymes.[5][6] The reduction occurs via direct electron
transfer primarily from ascorbate and some nonprotein thiols.[5] Vitamin C and other reducing agents combine with chromate to give chromium(III)
products inside the cell.[5] The resultant chromium(III) forms stable complexes with nucleic acids and proteins.[5] This causes strand breaks and
Cr-DNA adducts which are responsible for mutagenic damage.[5] According to Shi et al., the DNA can also be damaged by hydroxyl radicals produced
during reoxidation of pentavalent chromium by hydrogen peroxide molecules present in the cell, which can cause double-strand breakage.[6]"
"For drinking water the United States Environmental Protection Agency (EPA) does not have a Maximum Contaminant Level (MCL) for hexavalent chromium.
California has finalized a Public Health Goal of 0.02 parts per billion (ppb or micrograms per liter)[12] and established a MCL of 10 ppb.[13]"
"Workers who are exposed to hexavalent chromium are at increased risk of developing lung cancer, asthma, or damage to the nasal epithelia and skin.[2]
Within the European Union, the use of hexavalent chromium in electronic equipment is largely prohibited by the Restriction of Hazardous Substances
Directive."
Link: https://en.wikipedia.org/wiki/Hexavalent_chromium
[Edited on 30-9-2017 by AJKOER] |
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"Problem solved", I thought until someone here claimed I didn’t know what I was talking about.
