LEAD UPTAKE BY IRON CORROSION SCALES: INFLUENCE OF MINERALOGY

Lead (Pb) in tap water (released from pipes, solder, and brass plumbing) remains an important risk to human health. High concentrations of Pb have recently been found to accumulate in iron (Fe) corrosion scales of household plumbing, but the interaction is not well understood. The objective of this study is to gain fundamental understanding of how Pb from upstream sources interacts with the Fe corrosion scales, and to evaluate the impact of this interaction on Pb levels in drinking water at the tap.

We found in field studies that point-of-entry sediment filters that accumulate Fe oxy-hydroxides become effective agents for capturing Pb in water, and therefore might serve as laboratory analogs for the capture of Pb by Fe corrosion products. A model system of simulated Fe-bearing corrosion scales in premise plumbing was developed. Modified Cincinnati tap water was pumped from a large water reservoir to 2 kitchen faucets. Placed ahead of the faucets were a pair of 1-micron sediment filters impregnated with 5 different Fe minerals, which are representative of actual Fe corrosion scale solids. These were 1) feroxyhyte (FO); 2) ferrihydrite (FI); 3) goethite (GO); 4) lepidocrocite (LE); and 5) magnetite (MA).

Iron filters were exposed to known concentrations of Pb (40 µg/L) in a continuous flow experiment using about 400 liters of water for each faucet (about 6 hours). Water quality was monitored and a mass (or material) balance performed on Pb to evaluate the Fe filters’ effectiveness. X-ray diffraction, X-ray fluorescence (XRF), inductively coupled plasma-atomic emission spectrometry, and scanning electron microscopy-energy dispersive X-ray analysis were employed to characterize the Pb-Fe complex and assess the distribution of Pb in the Fe-impregnated filters.

Based on XRF data, filter efficiency was evaluated using normalized ratios of Pb, phosphorous, copper and, zinc sorption to Fe. This revealed very different behavior for the minerals. Lead sorption to the filter was highest with LE, followed by FO, FI, GO, and MA. Phosphorous was most strongly associated with LE, followed by GO, FI, FO, and MA. Copper and zinc were most sorbed by FO and LE, but did not bind to the other three. Under the conditions of the experiment LE seems to be the most effective scavenger for Pb, but further investigation is underway.