Accumulation and Release of Arsenic from Cast Iron: Impact of Initial Arsenic and Orthophosphate Concentrations.

Various iron oxyhydroxide and oxide minerals commonly found in old cast iron pipe scale were shown to exhibit high and similar affinity for arsenate [As(V)] and orthophosphate (PO) via adsorption, co-precipitation, and other factors. PO is a common drinking water corrosion inhibitor. This 7.5-year study examined the accumulation and release of As from an old cast iron pipe scale by changing initial As(V) (0, 75, or 180 µg/L as As) and initial PO (0 or 3 mg/L as PO) levels in the simulated drinking water. The results showed that sites within the iron scale accumulated As with a large capacity and concentrated 27% of the total amount As in water into the scale during the 7.5-year study. When no PO was added, the As accumulation followed a linear regression model with an accumulation rate of 0.27/hr (R = 0.80, p < 0.001), and higher initial As level of 180 µg/L (vs 75 µg/L) resulted in 2.3-3 times larger As accumulation rate at 0.25 mg/day (vs 0.084-0.11 mg/day). As much as 44 µg/L As was released back to water following the changes in the initial As and PO concentrations in water. Addition of 3 mg/L PO caused a rapid increase in As release from iron scale that gradually dropped off with time while PO was incorporated into the scale and most PO remained tightly bound to certain iron scale sites. Proactive measures such as sampling for As in the distribution systems following PO corrosion control treatment changes would help identify exposure risks.