Screening of commercial sorbents for the removal of phosphates from water and modeling by molecular simulation

Abstract

Eight commercial sorbents of different origin and nature were studied in the present work for the removal of phosphate from water using synthetic solutions and a wastewater from an anodizing company. The materials included activated carbons, bone char, catalytic carbon, natural silica, natural zeolite, a manganese(II) oxide composite and iron(III) hydroxide. These materials were characterized with different analytical techniques such as nitrogen adsorption isotherms at −196 °C, FT-IR spectroscopy, SEM/EDX analysis and X-ray diffraction. The adsorption studies were performed in batch systems. Iron(III) hydroxide was found the best sorbent, showing a maximum adsorption capacity of 193.75 mg/g at pH 7 in contrast with natural zeolite and silica, which registered very low adsorption values (2.92 and 4.17 mg/g, respectively). According to molecular simulation studies, the adsorption of phosphates from water on iron(III) hydroxide allowed the formation of the complex ≡FePO4H2, with a Gibbs free energy of −21.38 kcal/mol, showing that it is possible to recover the phosphates and reuse them later. The adsorbed amount of phosphates on iron(III) hydroxide using synthetic solutions and the industrial wastewater was similar thus suggesting that iron(III) hydroxide is a selective sorbent for the removal of phosphates.