Response surface modeling and optimization of composite nanofiltration modified membranes

The experimental design and response surface methodology (RSM) have been used to develop predictive models for simulation and optimization of nanofiltration modified membranes by UV-initiated graft polymerization technique. The objective is to prepare optimum membrane with high nanofiltration performance and low fouling. The factors considered for experimental design were the UV-irradiation time, UV-intensity and the concentration of the monomer N-vinyl-2-pyrrolidone (NVP) in aqueous grafting solution. The dip method has been followed for membrane grafting employing 365 nm wavelength UV-lamp. The significant factors were optimized using a central composite design of orthogonal type. The nanofiltration membrane performance has been studied using humic acid model solution 15 mg/L at a pH value 7. Pure water permeation flux, permeate flux when using humic acid feed solution, rejection factor and irreversible membrane fouling parameters have been determined. The nanofiltration performance index and the recoverable flux ratio have been considered as responses. The quadratic models between each response and the independent parameters were developed and the response surface models were tested with analysis of variance (ANOVA). By applying the desirability function approach maximal output responses have been predicted and confirmed experimentally. The obtained optimal point was located in the valid region and the experimental confirmation tests were conducted showing a good accordance between the predicted optimal points and the experimental ones. The optimum operating conditions determined were a monomer concentration of 5.13 g/L, UV-intensity of 16.57 × 104 mW/m2 and UV-irradiation time of 37.9 s. Under these optimal conditions maximum nanofiltration performance index, 84.88 L/m2 h and recoverable flux ratio 88.14% have been achieved. These values are the highest compared to all experimental data carried out within the overall region of experimentation. The optimum NVP UV-grafted membrane exhibits low humic acid fouling tendency than the unmodified membrane.