Co-PI: Anne Camper (Montana State University and the Center for Biofilm Engineering)
Funding agency: AWWA Research Foundation
Background
Many water utilities, especially those in larger and older U.S. cities, are struggling to meet the 15 m g/L lead action level ( AL ) of the 1991 Lead and Copper Rule. Orthophosphate, sometimes in combination with polyphosphate, is the most commonly used corrosion control chemical and has proven to be effective for lead corrosion problems. Unfortunately, phosphate is an important nutrient and its addition to water can stimulate the growth of microorganisms in some systems. Stannous chloride (SnCl2) is a relatively new corrosion inhibitor in that it has only recently been approved for use in potable water distribution systems. According to the manufacturer, the chemical is currently being used by only one water utility, for control of lead corrosion. The chemical also has been used as an antioxidant and preservative in food products ( e.g. , canned asparagus) with a maximum permissible concentration of 0.0035% of SnCl2 in the finished food. Unfortunately, little is known about the mechanism and effectiveness of SnCl2 for lead corrosion control in water distribution systems and to our knowledge, there is no information available in the peer-reviewed literature.
When added to water containing dissolved oxygen or chlorine or both, stannous ion (Sn2+) will likely be oxidized to the stannic or Sn(IV) form because the standard electrode potential for the Sn4+ /Sn2+ redox couple is +0.1539 V. One possibility is that the Sn4+ combines with Pb2+ at the anode to form a low solubility precipitate such as lead stannate (PbSnO3). Another possibility is that Sn2+ provides cathodic protection by forming a film at the cathode comprised of stannous or stannic oxide/hydroxide. Although there are no known human health effects from ingesting drinking water containing sub-mg/L concentrations of SnCl2 , it has been reported in the literature that the chemical is toxic to Escherichia coli. Experiments in our laboratory have shown that SnCl2 decreased heterotrophic plate counts relative to an untreated control in a pipe loop system. This suggests another possibility, that SnCl2 reduces microbial acitivity and subsequent microbially-influenced corrosion.
Research Plan and Progress to Date
The main goal of this research is to elucidate the mechanism or mechanisms by which SnCl2 decreases the corrosion of lead and the corresponding release of Pb into the water supply. The objectives of the research are as follows: (1) improve our understanding of Sn chemistry in drinking water, (2) investigate the toxicity of Sn to both planktonic and biofilm bacteria, and (3) determine the mode of action of SnCl2 in controlling corrosion of lead. The research involves a combination of laboratory experiments and analysis of pipe samples collected from water distribution systems. Batch experiments are being performed to investigate the kinetics of oxidation of Sn2+ to Sn4+ and to study the toxic effects of Sn on planktonic bacteria. In addition, continuous-flow reactors will be operated to investigate the toxicity of Sn to surface-associated or “biofilm” bacteria (the dominant type in water distribution systems) and to investigate the effectiveness and mode of action of SnCl2 in controlling corrosion of lead.
Significant findings to date include: (1) Pseudomonas aeruginosa PAO1 and Escherichia coli were inactivated by SnCl2, with the effectiveness dependent on the dose (mass of SnCl2 per mass of cells) and (2) SnCl2 addition consumes free chlorine and monochloramine but the effect is likely not to be significant at the low SnCl2 doses used in practice (< 0.3 mg/L). We are currently investigating whether SnCl2 is toxic to ammonia oxidizing bacteria.
This ongoing research project is the first detailed investigation of the chemistry of Sn2+ in drinking water and its role as a corrosion inhibitor. In light of recent highly publicized lead corrosion problems in Washington , D.C. , research on lead corrosion control strategies is of utmost importance. This research will improve our understanding of SnCl2 as a corrosion inhibitor and provide guidance for water utilities struggling to meet the lead AL that also are concerned about the possible detrimental effects of phosphate on microbiological water quality.
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