Adsorption of phosphate from aqueous solution on to pyrolyzed drinking water treatment residuals: Statistical process optimization, equilibrium, and kinetic analysis

Abstract

While the unique properties of drinking water treatment residuals (DWTRs) make it possible to reuse them as a beneficial adsorbent for phosphate removal, the dispersed impurities on the surface or in the pores of raw DWTRs can hinder phosphate adsorption capabilities. Hence, in this study, a pyrolysis process (200-800 degrees C for 2-8h) was employed to remove such impurities. Testing confirmed that the pyrolyzed drinking water treatment residuals (PDRs) proved to be powerful adsorbent for the removal of phosphate from aqueous solutions. When pyrolysis temperature was increased up to 300 degrees C, the phosphate adsorption capacity was enhanced due to the removal of impurities on the DWTRs surface, and the maximum phosphate adsorption capacity increased from 14.67 to 31.74mgg(-1), as indicated by the Freundlich model. However, a significant decrease of phosphate adsorption capacity was observed with further increase in temperature. Although structural and chemical properties of PDRs were found to be similar to raw DWTRs, the surface area and pore diameter became worse for adsorption due to the melting behavior of aluminum particles during pyrolysis. Subsequently, a statistical optimization was carried out using response surface methodology to optimize the pyrolysis conditions. As results, a phosphate adsorption capacity of 35.60mgg(-1) was recorded using the statistically optimized-PDRs (SO-PDRs). The results of phosphate adsorption equilibrium, isosteric heat of adsorption, and kinetic analysis at different temperatures tend to suggest that the main mechanisms of phosphate adsorption on to SO-PDRs are endothermic and chemisorption reactions; the surfaces are energetically heterogeneous for adsorbing phosphate. (c) 2016 American Institute of Chemical Engineers Environ Prog, 35: 1035-1046, 2016