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Author: Subject: Is a Stainless Steel Electrode Safe for Disinfecting Water?
AJKOER
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[*] posted on 30-9-2017 at 12:58
Is a Stainless Steel Electrode Safe for Disinfecting Water?


First some facts:

1. Stainless Steel: Per Wikipedia (https://en.wikipedia.org/wiki/Stainless_steel):

"In metallurgy, stainless steel, also known as inox steel or inox from French inoxydable (inoxidizable), is a steel alloy with a minimum of 10.5%[1] chromium content by mass."

2. 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

3. Electrolysis of water capable of forming H2 and O2 could create some in situ H2O2. More so if the water was acidic (pH under 4.8), then electrolysis, in the presence of O2, could create •HO2 (from e- + H+ + O2), which further self reacts forming O2 and H2O2.

My point is that patents suggesting the use of stainless electrodes for disinfecting and citing the advantage of Fenton reactions forming hydroxyl radicals should also disclose the possible creation of Cr(VI) via a Fenton-type mechanism based on content of chromium metal in the stainless steel. While the amount of Cr(VI) creation may be small, unfortunately, the recommended exposure limits (cited below) are really small also.

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

Bottom line, use of stainless steel electrodes in water purification is probably a bad idea, especially if the pH is acidic.

However, there are patents using stainless steel to disinfect drinking water, for example, "Electro ionic water disinfection apparatus", US 20030164308. To quote from:

"Water, both in drinking and wastewater applications, contains microorganisms. Various water treatment systems are provided for disinfecting water to a level suitable for human and animal consumption."

"The electrodes are formed of iron, stainless steel, carbon or copper and connected to a power supply voltage in the range of 20 to 100 volts and establishing a current in the range of 1 to 6 amperes. Disinfection results from either metal ions impacting microbial cells or through the generation of hydrogen peroxide, hydroxyl radicals and hypochlorous acid. When the electrodes are copper, toxic metal contamination limits are established through proper design of the flow cell. An ultrasonic transducer is connected to the electrodes and enhances hydroxyl radical generation."

Link: https://www.google.com/patents/US20030164308

No mention or possible recognition of a hexavalent chromium issue!

[Edited on 30-9-2017 by AJKOER]
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clearly_not_atara
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[*] posted on 30-9-2017 at 13:04


Patents often include in the claimed scope a number of "possible methods" which are not actually practical, to ensure that modifications do not take advantage of this as a "loophole". The authors did not necessarily intend for anyone to use stainless steel electrodes, they just wanted to claim that anyone who does so while following the rest of their method is in fact violating their patent.
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AJKOER
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[*] posted on 30-9-2017 at 13:23


If purchasing a low end unit that reputedly electronically purifies water, where the internal workings are not visible (a black box in essence), how would one know for sure that the electrodes are not stainless steel?
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The problem, in my opinion, is public education. The US federal government appears to be unwilling to even establish guidelines on Cr(VI), which may be a possible case of deference to industries polluting for years with Cr(VI) (not too encouraging).
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As acidic conditions, or a low pH zone, could lead to •HO2 formation, what if there was oxygen mixing occurring in otherwise near neutral conditions pre-electrolysis?

In other words, stirring or solution turbulence (from agitation or even sonolysis) during the electrolysis of water could increase H2O2 formation ([EDIT]: apparently yes, adding external air/oxygen also is benefical, see preview available at https://link.springer.com/article/10.1023/A%3A1017588221369?... ). This may lead, with stainless steel electrodes, to increased Cr(VI) formation.

[Edited on 1-10-2017 by AJKOER]
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