Natural organic matter flocculation behavior controls lead phosphate particle aggregation by mono- and divalent...

Phosphate addition is commonly applied to remediate lead contaminated sites via the formation of lead phosphate particles with low solubility. However, the effects of natural organic matter (NOM) with different properties, as well as the contributions of specific interactions (particle-particle, particle-NOM, and NOM-NOM) in enhanced stabilization or flocculation of the particles, are not currently well understood. This study investigates the influence of two aquatic NOM and two soil or coal humic acid (HA) extracts on the aggregation behavior of lead phosphate particles and explores the controlling mechanisms. All types of NOM induced disaggregation and steric stabilization of the particles in the presence of Na⁺ (100 mM) or low (1 mM) Ca²⁺ concentrations, as well as at low NOM concentrations (1 mg_C/L). However, for the soil and coal HA, a threshold at NOM concentrations of 10 mg_C/L and high (3 mM) Ca²⁺ concentrations was observed where bridging flocculation (rather than steric stabilization) occurred. In situ attenuated total reflectance - Fourier transform infrared characterization confirmed adsorption of the soil and coal humic acid extracts (10 mg_C/L) onto the surface of the lead phosphate particles in 3 mM Ca²⁺, whereas dynamic and static light scattering demonstrated extensive HA flocculation that dominated the overall scattered light intensities. These results imply that the accelerated aggregation was induced by a combination of HA adsorption and bridging flocculation by Ca²⁺. Overall, this research demonstrates that the type of NOM is critical to predict the colloidal stability of lead phosphate particles. Aquatic NOM stabilized the particles under all conditions evaluated, but soil or coal HA with higher molecular weight and aromaticity showed highly variable stabilization or flocculation behavior depending on the HA and Ca²⁺ concentrations available to adsorb to the particles and participate in bridging. These results provide new mechanistic insights on particle stabilization or destabilization by NOM.