Impact of phosphate sorption on water-treatment residual surface characteristics: Prelude to reuse

Phosphate amended water-treatment residual (WTR) may hold promise as a soil amendment. This work examines the hydrophilicity and subsequent implications for mobility of WTR as a function of phosphate adsorption, pH, and dewatering. The utility of an extended Derjaguin-Landau-Verwey-Overbeek (DLVOEX) model for predicting changes in WTR colloidal hydrophilicity, as a function of phosphate adsorption, is also examined. An increase in phosphate adsorption, below monolayer coverage, resulted in an increased hydrophilicity, as indicated by a decreased mean WTR aggregate diameter. Phosphate adsorption beyond monolayer coverage resulted in an increase in mean WTR aggregate diameter, suggesting a decrease in particle hydrophilicity. This change in WTR hydrophilicity may be a result of phosphate bridging and/or a reduction in the solvency of water as a function of ionic strength. Undried WTR exhibited thermodynamic surface characteristics similar to that of polyacrylamide, a common water-treatment polymer. Dewatering of WTR, in drying beds, resulted in a transformation to a more hydrophilic material. Mobility of phosphate-amended WTR, in agricultural soils, will depend on the quantity of phosphate added, extent of dewatering, and equilibrium pH of the soil-WTR mixture. The DLVOEX model predicted that increases in WTR hydrophilicity caused by adsorption of phosphate onto WTR, will result in large energy barriers and may prohibit WTR adhesion in the primary minimum onto common soil minerals.