Impact of phosphate on the surficial properties of a ferric hydroxide matrix: Linkage between surface complexation and colloid stability

Date of Completion

January 1997


Engineering, Chemical|Chemistry, Agricultural|Engineering, Environmental




Phosphate amended water treatment residual (WTR) may be a beneficial soil amendment. This work focuses on three issues regarding the addition of phosphate to WTR: binding of phosphate by WTR; change in stability of WTR as a function of phosphate binding; and, impact of electrolytes on the solvency of water (which is applied to a WTR system).^ ATR-FTIR data and phosphate-WTR pH adsorption edges confirmed that phosphate binds to oxides and a cationic polyelectrolyte added during water treatment processes. The polymer appears to form multidentate complexes with phosphate. Using the FITEQL optimization program, equilibrium constants and total number of surface sites were determined for the polymer. The binding of phosphate by WTR was modeled by combining the results of the FITEQL optimization with a ferric hydroxide diffuse double layer model that included surface precipitation.^ The stability of colloidal WTR as a function of phosphate adsorption, pH and dewatering was investigated. Phosphate adsorption, below monolayer coverage, resulted in partial repeptization of WTR aggregates as indicated by decreased mean aggregate diameter. Phosphate adsorption beyond monolayer coverage resulted in an increase in mean aggregate diameter suggesting a decrease in free energy of interaction between WTR aggregates. WTR dewatering resulted in transformation to a more hydrophilic material. DLVO$\sb{\rm EX}$ modeling suggested that amending WTR with phosphate will inhibit adhesion in the primary minimum on dolomite and silicon dioxide, i.e. increase its propensity to move through typical agricultural soils.^ The impact of aqueous electrolytes on van der Waals and Lewis acid/base energies of water were quantified using hydrophobic and hydrophilic substrates. Comparable increases in solid-water interfacial energy on polytetrafluoroethylene (PTFE) and sodium montmorillonite coated with organic matter systems were noted. Change in water/PTFE interfacial energy, as determined by contact angle measurements, was greater than that predicted by screening of the nondispersion component of the Hamaker constant. Increases in interfacial energy, as a function of salt concentration, were greater with increasing substrate hydrophilicity. All substrates exhibited similar trends in interfacial energy as a function of ionic strength. Similarity in the results suggests that aqueous electrolytes may change water structure within the interfacial region. ^