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Accumulation and release of arsenic from an old cast iron drinking water main
Citation:
Lytle, D., M. Tang, AND J. Botkins. Accumulation and release of arsenic from an old cast iron drinking water main. 2021 One Water Technical Conference, Cincinnati, July 26, 2021.
Impact/Purpose:
Arsenic (As) is naturally present in many groundwaters and can gradually adsorb to and accumulate in iron pipe scale. Iron pipes are widely present in water distribution and there are increasing concerns over undetected mobilization and release of As from aged iron pipe scale into drinking water. This work showed that old cast iron scale constantly took up As from water and concentrated As in iron pipe scale, which later leached into water at concerning high levels. Short-term addition of orthophosphate to drinking water and changes in source water As levels promoted As release from iron scale; long-term addition did not exacerbate As release. This work has important health implications for water systems with orthophosphate treatment.
Description:
Arsenic (As) is naturally present in drinking water sources in the forms of arsenite (As[III]) and arsenate (As[V]), with As[V] the dominant form found in water distribution systems. Arsenic can pose human related health threats if consumed drinking water has an excess of total 0.010 mg/L As. Trace levels of As in finished water can gradually accumulate in corroding iron pipe scales, containing iron oxide and oxyhydroxide minerals which have a high affinity for As. The concentrated As in scale can be mobilized into the finished water at elevated levels during disruptive events such as water treatment and source changes. Many water utilities use or may consider using orthophosphate treatment to minimize lead and copper levels. Due to the similarity in chemical structure, orthophosphate (PO4) is well known to compete with As(V) for accumulation sites on iron surfaces and they share similar accumulation mechanisms and isotherms with iron oxyhydroxide and oxides. This is the first study to examine the long-term As mobility for water systems with a corroding iron pipe. Additionally, we also investigated the impact of PO4 on accumulation and release of As on iron scale. A simulated drinking water distribution system was developed and comprised of a 10-liter water reservoir, one 100-year old 5.25” section of cast iron pipe (5” ID) that was harvested from a local drinking water distribution system, and plastic-based fittings and connections. The simulated drinking water in the reservoir had a chemistry of pH 7.6, 25 mg C/L dissolved inorganic carbon, 10 mg/L chloride, 10 mg/L sulfate, and 1 mg/L free chlorine residual at ~23ºC temperature. Different initial As(V) (0, 75, and 180 µg/L as As) and initial PO4 (0, and 3 mg/L as PO4) concentrations were added to the simulated drinking water intermittently over the course of 7.5 years. The water in the reservoir was replaced with freshly made water three times a week, and was analyzed daily during weekdays (i.e., Monday to Friday) for elemental analyses (total As, iron (Fe), and phosphate (P)). Our study showed that iron pipe scale had relatively large capacity for As accumulation. The corroding cast Fe pipe section scale in this simulated drinking water system accumulated 27% of the total 600 mg As added over the course of 7.5 years under the given test water conditions. The As accumulation was kinetically fast when the simulated drinking water was added with an initial As level of 75 µg/L, and the As levels in water decreased almost linearly to 55 µg/L within 48 hours resulting in an accumulation rate of 0.27/hr (R2=0.80, p < 0.001). The higher initial As concentration of 180 µg/L (vs 75 µg/L) led to higher As accumulation rate at 0.25 mg/day (vs 0.08-0.11 mg/day). After As in drinking water was accumulated to and concentrated in the cast Fe pipe scale surfaces, the accumulated As was released into water over a combined period of 2.5 years, sometimes at elevated levels up to 44 µg/L exceeding the EPA regulation standard of 10 ug/L. Orthophosphate addition did not seem to influence the total Fe release and Fe scale stability. However, addition of 3 mg/L PO4 released the pre-accumulated As in the iron pipe scale minerals that gradually decreased over time, while PO4 was incorporated into the scale; the released As was only 3.4-7.3% of accumulated PO4 on a molar basis, and most accumulated-PO4 remained tightly bound to certain Fe scale sites. Due to the potential impact of PO4 on As mobility, proactive sampling, monitoring, and communication measures should be taken, when water utilities have As problems or consider PO4 corrosion control treatment.