Removal of phosphate from aqueous solutions using calcined metal hydroxides sludge waste generated from electrocoagulation

Removal of heavy metals by electrocoagulation generates substantial amount of electrocoagulated metal hydroxides sludge (EMHS). This paper reports calcined EMHS as an effective adsorbent for removal of PO43- from aqueous solutions. Cumulative % removal Of PO43- is investigated in a batch adsorber with different initial adsorbate concentrations, adsorbent dose, pH of the solution and adsorption temperature. Adsorption of PO43- is due to ligand exchange between the OH group on oxide surface and PO43- in the aqueous medium with formation of inner surface complex. Removal of PO43- is found to be high at lower pH values and higher temperatures indicating strong positively charged metal oxide surface at acidic pH (< pH(zpc)) and the endothermic nature of adsorption. The experimental results preferably fitting the Langmuir isotherm suggest monolayer coverage of adsorbed molecules. Pseudo-second-order kinetic model provides the best fitting to experimental results at different adsorbent dose and adsorption temperature. The magnitude of the activation energy (10.33 kJ/mol) is calculated based on pseudo-second-order rate constants from Arrhenius equation indicates removal of PO43- corresponds to activated chemisorption. (c) 2006 Elsevier B.V. All rights reserved.