Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Engineering Science

Program

Chemical and Biochemical Engineering

Collaborative Specialization

Environment and Sustainability

Supervisor

Herrera, Jose E.

Abstract

Lead service lines and fittings were installed in Canada's drinking water networks until 1986 and currently pose severe health risks. As pipes corrode over decades of use, lead is released into drinking water resulting in dangerous exposure to elevated lead levels. Sodium silicates can be used for corrosion control of lead, however a basic understanding of how they work is lacking. This thesis describes the results of experiments conducted to determine how silicates interact with lead and aluminum in drinking water, using microscopic and spectroscopic techniques. The results show the presence of silicates inhibited Pb2+ carbonates from oxidizing into less soluble Pb4+ oxides. Furthermore, aluminum interacts with silicates resulting in allophane formation on the lead surface and extending into the bulk. Allophane did not protect against lead dissolution. Silicates may not be adequate as a corrosion control option in drinking water systems rich in solid lead carbonate (Pb2+) phases.

Summary for Lay Audience

Exposure to lead from drinking water can pose serious health concerns. Lead was used for drinking water service lines and fittings in Canada until 1986; the majority are still in place today and have corroded over time with use. Health Canada has set the maximum allowable concentration for lead in drinking water at 5 ppb, as research has shown exposure to even very small amounts of lead can have dire consequences for infant and child development. Corrosion control strategies are needed to reduce human exposure and provide safe drinking water to Canadians. Current methods of corrosion control for lead include pH adjustment and orthophosphate treatment. Lower lead release rates are observed at more basic pH values, resulting in lower dissolved lead concentrations. When water is treated with orthophosphate, a lead-phosphate layer is formed on the pipe wall that prevents lead dissolution. Unfortunately, both methods of corrosion control include drawbacks such as build up of calcium along the pipe wall and increased environmental risk.

Sodium silicate treatment is a third method of corrosion control for lead. Many studies have investigated the effectiveness of silicate treatment; however, few have focused on determining how it works. This thesis examines how silicates interact with lead compounds and the effects on dissolved lead levels. We found that in the presence of lead carbonate in chlorinated drinking water silicates stopped lead from oxidizing to insoluble lead oxides, resulting in higher dissolved lead levels. We also investigated the role of aluminum, as aluminum is often found in drinking water due to use of alum in the water treatment process. We found that in the presence of aluminum, silicates will precipitate out of water and form an aluminosilicate layer. In the short term, this layer does not protect against lead dissolution or result in lower dissolved lead levels. As a result of findings from this research, we do not suggest implementing sodium silicates as a corrosion control option for lead in lead carbonate-rich systems. Future long-term studies on dissolved lead levels during the formation of the aluminosilicate layer in lead oxide rich systems are suggested.

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