Controlling Lead Release To Drinking Water: Impacts Of Iron Oxides, Complexing Species, Orthophosphate, And Lead Pipe Replacement

Abstract

This work explored various approaches to controlling lead exposure via drinking water: orthophosphate treatment, limiting complexing or adsorbing species in distributed water, and lead service line replacement (LSLR). As determined by residential sampling, full LSLR reduced water lead levels by 35 – 87% within one month, while partial LSLR was associated with a greater frequency of elevated lead even after six months (27% of 1st draw samples > 15 µg L-1 vs. 13% pre-replacement). Upstream iron corrosion was a significant factor in these observations: compared with lined distribution mains, lead levels 6 mo. post-replacement were higher by factors of 1.9 (full LSLR) and 4.8 (partial LSLR) at sites supplied by unlined (corroded) iron pipe. Upstream iron corrosion also accompanied an average increase in lead release of 96 µg L-1 from a model system. This effect may explain particularly elevated lead levels after partial LSLR and it may be attributed to adsorptive and electrochemical phenomena. Size-exclusion chromatography (SEC) data were consistent with mobilization of lead via adsorption to suspended iron (oxyhydr)oxides: colloidal/nanosize iron and lead were strongly correlated in point-of-use samples (R2average = 0.96). Galvanic corrosion of lead by iron oxide minerals also increased lead release by a factor of 9.0 compared with uniform corrosion in the absence of iron. Orthophosphate immobilized lead oxidized in galvanic cells as insoluble hydroxypyromorphite (Pb5(PO4)3OH) and in field studies, increasing the orthophosphate concentration from 0.5 to 1.0 mg PO4 L-1 reduced lead release by 38% within 8 months. Humic acid, by contrast, mobilized lead oxidized in galvanic cells, increasing lead release by a factor of 9.3 compared with an organic-free electrolyte. This was attributed largely to complexation: SEC data were consistent with formation of a soluble lead-humate species. Polyphosphates may also increase lead solubility via complexation, and SEC may aid in understanding this phenomenon in the field. In a water system dosing polyphosphate, lead was present in a soluble SEC fraction strongly associated with phosphorus. In field samples collected from a separate system—where polyphosphate reverted almost entirely to orthophosphate—lead was instead present in a colloidal fraction associated with iron and natural organic matter.