Enhanced phosphate removal from aqueous solution by ferric-modified laterites: Equilibrium, kinetics and thermodynamic studies

The phosphate removal performances of a series of ferric-modified laterites (ML) were tested and compared with raw laterite (RL) in this study. After the modification with 0.5 M FeCl3 solution, the resulting adsorbent ML-C exhibited 90.12% of phosphate removal, which was 37.47% higher than that of RL under the same experimental condition. This may be attributed to the significant increase of BET surface area and total pore volume for ML-C, arising from the formation of akaganeite. The effects of contact time, initial phosphate concentration, temperature, pH, and co-existing ions on the adsorption capacity of ML-C were investigated in detail. The equilibrium data of ML-C were fitted better by the Freundlich model than the Langmuir model, suggesting the heterogeneity of the adsorbent surface. The maximum adsorption capacity was estimated to be 31.53 mg P/g at 25 °C, which decreased with increasing temperatures. The negative change in free energy (ΔG°) and in enthalpy (ΔH°) indicated that the adsorption was a spontaneous and exothermic process. The phosphate adsorption kinetics was better described by the pseudo-second-order model, which indicated the adsorption process was chemisorption. Phosphate removal capacities decreased with increasing pH. The presence of 0.001 M and 0.01 M completive ions, e.g. Cl−, NO3-, SO42-and HCO3-, had neglectable effects on the phosphate adsorption. In the reusability study, the adsorbent showed no significant loss in their adsorption performance after four adsorption-desorption cycles, indicating that ML-C was able to be utilized as a potential cost-effective phosphate adsorbent for practical applications.