SURFACE COMPLEXATION MODELING OF PHOSPHATE ADSORPTION BY WATER-TREATMENT RESIDUAL

Use of water treatment plant residuals (WTR), as a soil amendment is a promising alternative to landfill disposal. Unfortunately, WTR has a propensity to bind with phosphate, which is an important plant nutrien t. Phosphate may be added to WTR prior to soil application. This type of pretreatment may convert WTR from a phosphate consumer to a phospha te supplier. The binding of phosphate to WTR is typically attributed t o surface complexation with metal oxides. However, attenuated total re flectance Fourier transform infrared (ATR-FTIR) data and phosphate-WTR adsorption equilibrium data indicate that phosphate also binds to a c ationic polyelectrolyte that is added during water treatment processes . Using the FITEQL optimization program, equilibrium constants and tot al number of surface sites were determined for the polymer. Results fr om the FITEQL optimization were used to model binding of phosphate by cationic polymer. Binding of phosphate by hydrous ferric oxide was mod eled using a diffuse double layer model, which included surface precip itation (MICROQL). The model was validated through the use of phosphat e equilibrium partitioning data at pH values of 6 and 8. The model pre dicted that a significant fraction of phosphate adsorbed onto WTR is a ssociated with the cationic polymer.