Flocculation Optimization of Orthophosphate with FeCl3 and Alginate Using the Box-Behnken Response Surface Methodology

A coagulation-flocculation process was employed to remove orthophosphate (P-i) in aqueous media using a ferric chloride (FeCl3) and alginate flocculant system. Jar tests were conducted, and the response surface methodology (RSM) was used to optimize the P-i removal variables. The Box-Behnken design was used to evaluate the effects and interactions of four independent variables: pH, FeCl3 dose, alginate dose, and settling time. The RSM analysis showed that the experimental data followed a quadratic polynomial model with optimum conditions at pH 4.6, [FeCl3] = 12.5 mg.L-1, [alginate] = 7.0 mg.L-1, and a 37 min settling time. Optimum conditions led to a P-i removal of 99.6% according to the RSM optimization, in good.agreement with experimental removal (99.7 +/- 0.7%), at an initial concentration of 10.0 mg P-i/L. The isotherm adsorption data at the optimized conditions were analyzed by the pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models and several isotherms models (Langmuir, Freundlich, and Sips). The PFO kinetic model and Langmuir isotherm model yielded the best fit to the isotherm results. The maximum adsorption capacity of the flocculant system was 83.6 mg.g(-1). The flocculation process followed electrostatic charge neutralization and an ion-binding adsorption mechanisms.