Equilibrium and kinetic analysis of phosphorus adsorption from aqueous solution using waste alum sludge

Excess phosphorus (P) in wastewaters promotes eutrophication in receiving waterways. A cost-effective method such as use of novel low-cost adsorbents for its adsorptive removal would significantly reduce such impacts. Using batch experiments, the intrinsic dynamics of P adsorption by waste alum sludge (an inevitable by-product of drinking water treatment plants) was examined. Different models of adsorption were used to describe equilibrium and kinetic data, calculate rate constants and determine the adsorption capacity. Results indicate that the intraparticle rate constant increased from 0.0075 mg g-1 min-1 at 5 mg L-1 to 0.1795 mg g-1 min-1 at 60 mg L-1 indicating that more phosphate is adsorbed per gram.min at higher P concentration. Further analyses indicate involvement of film and particle diffusion mechanisms as rate controlling steps at lower and higher concentrations respectively. Mass transfer coefficient obtained ranged from 1.7 × 10-6 to 1.8 × 10-8 indicating a rapid transportation of phosphate molecules onto the alum sludge. These results further demonstrates that alum sludge – hitherto thought of as undesirable waste, can be used as novel adsorbent for P removal from wastewater through various applications, thus offsetting a portion of the disposal costs while at the same time improving water quality in sensitive watersheds.