Temperature effect on the mechanism of phosphate anions sorption by b-MnO2

Phosphate anions excess in stored water reservoirs are known to stimulate algal growth and scaling in vessels used for cooling purposes in industries. Several techniques such as ion exchange, adsorption on activated carbon, chemical precipitation and reverse osmosis have been used for its removal. The use of inorganic ion exchangers like oxide/hydroxides has been preferred recently due to their stability at high temperature, to ionizing radiations and high selectivities for specific anion and cations. In the present investigation phosphate sorption on (beta)-MnO2 is studied as function of concentration (1.07^–11.85×10^−1 mmol L^−1) and temperature (293–313 K) at three different pH values of 3, 5 and 7. The sorption of phosphate is found to decrease with the increase in temperature in the pH range 3–7. Freundlich equation derived from the Law of mass action equation was found applicable to the sorption data. Effect of background electrolyte concentration and FTIR studies showed that mechanism of uptake process was the formation of outer-sphere complexes at all the temperatures under investigation. The thermodynamic parameters such as (delta)H◦, (delta)S◦ and (delta)G◦ values along with isosteric heats of adsorption were calculated, which showed that the solid preferred the multiply charged phosphate at low temperature and pH.