The
Al-WTR used in this study had a pHpzc of 8.5 (Yang et al., 2006). At
pH below the pHpzc, the surface would be positively charged.
Therefore, at low pH (with abundant positive sites), phosphate
adsorption will be facilitated by electrostatic and chemical attraction
onto the positively charged surface, but as the pH rises towards
and above the pHpzc, the surface becomes predominantly negatively
charged due to competitive adsorption of OH and phosphate
adsorption decreases. A maximum adsorption capacity of
31.9 mg P g1 was obtained at pH 4, about three times the value
obtained at pH 9. Several industrial by-products including slags,
shale, fly ash and bottom ash have been tested for their P removal
capacities. Their removal capacities evaluated from batch studies
ranged from 0.31 mg P g1 to 44.2 mg P g1 (Westholm, 2006).