Adsorption kinetics of phosphate from aqueous solutions by waste iron sludge

The kinetics of phosphate adsorbed on waste iron sludge was investigated in this study. Static batch adsorption experiments were performed at different initial phosphate concentrations, initial pH and temperature of solution. The initial adsorption rate increased with increasing the phosphate concentration. A shorter time, however, was required to reach adsorption equilibrium at low phosphate concentration due to external surface adsorption. Although the efficacy of phosphate adsorbed on waste iron sludge depended significantly on pH of solution, the adsorption kinetics was relatively independent on pH. Increasing solution temperature contributed to accelerating reaction rate of adsorption and raising the uptake of phosphate adsorbed on waste iron sludge. The kinetic process of phosphate adsorbed on waste iron sludge could be well described by both pseudo second-order and Elovich kinetic models, indicating chemisorption occurred. The simulation of intra-particle and Boyd model demonstrated that the intra-particle diffusion occurred, but the liquid film diffusion was the rate-limiting step. The of apparent activation energy of adsorption value (33.40 kJ·mol-1) also indicated that chemical reaction was not the only rate-limiting step and diffusion was involved within the whole adsorption process. Results of these findings suggested that the intra-particle diffusion and chemisorption occurred in the entire adsorption process, but the mass transfer in the liquid film was the dominant rate-limiting step.