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Arsenic Accumulation and Release Studies Using a Cast Iron Pipe Section from a Drinking Water Distribution System
Citation:
LYTLE, D. A., J. Liggett, AND J. Conover. Arsenic Accumulation and Release Studies Using a Cast Iron Pipe Section from a Drinking Water Distribution System. Presented at AWWA WQTC, Nov 2011, Phoenix, AZ, November 13 - 17, 2011.
Impact/Purpose:
To inform the public.
Description:
The tendency of iron solid surfaces to adsorb arsenic and other ions is well known and has become the basis for several drinking water treatment approaches that remove these contaminants. It is reasonable to assume that iron-based solids, such as corrosion deposits present in drinking water distribution systems, have similar adsorptive properties and could, therefore, concentrate arsenic and other trace contaminants in the distribution system. Recent work has looked at the accumulation of arsenic and other contaminants in the distribution system. For example, in one study 67 solids samples from 15 drinking water utilities were examined. The arsenic content of these solids ranged from 10 to 13,650 μg As/g solid (as high as 1.37% by weight) and the major element of most solids was iron. The obvious concern is that accumulated contaminants can be released back into the water at elevated levels and affect consumers. Unfortunately, little is known about how water chemistry impacts the accumulation and release of contaminants from distribution system materials. The objective of this work is to examine the uptake of a model contaminant, arsenic, onto an old cast iron pipe section with time. The impact of water chemistry on its release back to water was also examined. Arsenic accumulation and release experiments were conducted using a section of cast iron pipe (approximately 90 years old) removed from the drinking water distribution system of the City of Cincinnati, Ohio. The pipe was approximately 15 cm (5.9 inches) in height and 10.2 cm (4 inches) in diameter. Approximately 1.3 centimeters (0.5 inches) of the original effective inside diameter was lost to corrosion deposits. Ten liters of deionized water were chemically adjusted to achieve desired water quality goals: pH= 7.5, saturated dissolved oxygen, 25 mg C/L DIC, 10 mg/L chloride and sulfate, 1 mg Cl2/L, and 75 µg/L arsenic (V), and mixed. The test water was then pumped through plastic tubing into the pipe section (standing vertically) from the bottom of the pipe at a rate of 350 mL/min. A pump recirculated water from a plexiglass water reservoir into the pipe section through the influent plumbing. Once the water went through the pipe section, it exited through the effluent tubing and into a chiller to keep the water at the desired temperature. Water was recirculated through the pipes for 48 hours before being replaced with new water, during which time a computer automated dual titration system was used to maintain the target pH within 0.1 units. Iron, arsenic and other water chemistry variables were monitored following each 48-hour period as well as at 24 and 72 hours. Tests for free chlorine and for total metals analysis was taken every day for effluent water and every other day for influent water. Temperature, pH, DO, total and ferrous iron, and free chlorine were measured immediately following collection. Results at the time of abstract submission showed that the short section of iron pipe had a seemingly large capacity to remove arsenic from the water. After nearly 225 days of operation, approximately 35,000 µg (35 mg) of arsenic had accumulated on the pipe section surface. The rate of accumulation increased linearly with time at a rate of approximately 153 µg/day and the rate showed no sign of decreasing after 225 days of operation. If the result (to date) were extrapolated to a mile long pipe, 370 grams of arsenic would accumulate. One pH upset (increase) occurred resulting in an immediate release of arsenic back to the water at a concentration of 210 µg/L. The presentation will include updated data such as the impact of additional water quality upsets on arsenic release. Lastly, previously unpublished case studies involving the release of arsenic and other contaminants from the distribution systems of water utilities will be briefly summarized.