Sorption mechanism(s) of orthophosphate onto Ca(OH)₂ pretreated bentonite

Bentonite was chemically pretreated with Ca(OH)₂ to enhance orthophosphate phosphorus (OPP) sorption capacity (q_e). The pretreatment resulted in an increase of q_e from about 0.3 mg P per g to about 8 mg P per g at pH 7 and 25 °C, and of OPP concentration 100 mg P per L. The effects of solution pH, OPP concentration, sorbent dosage, and temperature on OPP sorption onto pretreated bentonite were investigated. The variation of initial pH (4-9) did not affect q_e, however, re-adjustment of the final pH showed that the variation of the final pH had a significant positive effect on q_e. The sorption kinetics showed a low rate, reaching half of q_e in the first 4 h and equilibrium after 96 h. The calculated Langmuir q_max was 11.68 mg P per g. Thermodynamic analysis suggests that the sorption process is spontaneous and endothermic. Desorption of OPP was high in solutions of 0.1 M HCl and 0.01 EDTA-Na₂ and reached 83% and 98%, respectively. We conclude that the predominant sorption mechanism for OPP uptake is inner-sphere surface complexation (ligand exchange). Other less important mechanisms such as surface precipitation and replacement of P⁵⁺ for Si⁴⁺ on the tetrahedral sites of montmorillonite may operate during the OPP uptake process by pretreated bentonite with Ca(OH)₂.