Stability of Lead-Containing Precipitates: Implications for Lead Transport in Pipes and Soils
Lead consumption can pose severe health risks to the public. To reduce lead exposure risks, phosphate addition is widely adopted to immobilize lead via the formation of lead phosphates with low solubility, especially pyromorphites such as hydroxylpyromorphite. However, the transport behavior of lead phosphate particles in response to their surrounding environment is not well understood. The overall objective of this dissertation research is to identify the key water chemistry parameters and mechanisms controlling the aggregation and deposition of lead phosphate particles in engineered and natural systems. The first study investigates lead phosphate formation and aggregation under conditions typical of drinking water supply systems. High aqueous PO₄/Pb molar ratios (> 1) enhanced the colloidal stability of the lead phosphate particles due to phosphate adsorption, whereas the presence of divalent cations (Ca²⁺ and Mg²⁺) promoted the aggregation of lead phosphate particles at pH 7 via charge neutralization. However, such promoted aggregation was effectively countered by the presence of natural organic matter (NOM), which imparted steric repulsion. The second study investigates the deposition of lead phosphate particles under conditions representative of soil environments through column experiments. Clean bed filtration models failed to predict particle deposition since both ripening and straining occurred at ionic strengths ≥ 12.5 mM. The influences of the P/Pb ratio and the presence of Ca²⁺ and NOM on the aggregation and deposition behavior were consistent with that observed in the prior study. The third study investigates the influence of the molecular weight and chemistry of various types of NOM (two river NOM extracts, and soil and coal humic acid extracts) on the aggregation behavior of lead phosphate particles. All types of NOM induced disaggregation and steric stabilization of the particles in the presence of Na⁺ and low Ca²⁺ concentrations. However, for the soil and coal humic acid extracts, a threshold was observed where bridging flocculation (rather than steric stabilization) occurred at NOM concentrations ≥ 10 mg/L and Ca²⁺ concentrations ≥ 3 mM. Overall, this research demonstrates the importance of myriad mechanisms that must be considered to predict lead particle transport and exposure risks in applications of phosphate for lead remediation.