The Influence of Water Chemistry on H2 Production and Uptake during Anaerobic Iron Corrosion
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Iron corrosion is the most important economic and aesthetic problem facing utilities. In the water distribution system, problems caused by iron corrosion include "red water", scale buildup, and pipe failures. It is necessary to improve our mechanistic understanding of anaerobic iron corrosion in order to better address these concerns. Experiments were conducted to investigate the effect of soluble constituents (Fe2+, PO43-, and NH4+) on H2 evolution during anaerobic iron corrosion. At pH 7.0 when sulfide was absent, variable Fe2+ did not have much influence on H2 release rates, whereas PO43- and NH4+ promoted H2 evolution. If present, soluble sulfide controlled H2 release rates in the solutions with Fe2+ or PO43-; however, NH4+ and S2- combined to inhibit H2 release. A simplistic empirical model was developed that fit data on corrosion rates from previous researchers studying effects of sulfate-reducing bacteria (SRB) on iron corrosion. As a whole, the experimental data and the model results support the notion that water quality controls iron corrosion rates in the presence of SRB. The practical relevance of previous research is somewhat in doubt given the atypical levels of nutrients used in relation to those actually present in water and wastewater. A second phase of research was aimed at exploring the equilibrium and kinetic aspects of iron corrosion in the presence of phosphate. The hypothesis that anaerobic iron corrosion is influenced by low pressure H2 (<1 atm) buildup was examined. At pH 2.75 and pH 7.0 in the presence of 100 mg/L P-PO43-, variations in H2 release were measured under different circumstances. Addition of PO43- formed a protective film, possibly vivianite Fe3(PO4)2, on the iron surface that eventually stopped H2 release. However, results were consistent with the idea that corrosion is an irreversible process that is relatively insensitive to low level H2 (<1 atm). Possible alternative explanations were provided to reconcile the past research data that purportedly demonstrated that removal of H2 increased corrosion rates. A reaction that caused "decay" of H2 in the presence of high phosphate was discovered that can not be readily explained.
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