An integrated OVAT-RSM design to gaps-filling in the study of phosphate sorption process onto cationic modified bottle gourd shell

The conventional approach to researching the phosphate sorption process using a modified bottle gourd sorbent (the quaternary ammonium Lagenaria vulgaris shell), regardless of a large number of conducted experiments, indicated certain shortcomings in the prediction of the optimal process conditions. To fill the gaps in previous research, a design study involving conventional (comparative kinetic, thermodynamic and equilibrium) and "One-Variable-At-a-Time" (OVAT) experimental approaches integrated with "Design of Experiments" (DoE) methodology was carried out. The integrated experimental design is implemented in the phosphate sorption process optimization to evaluate significant factors (optimal sorption conditions), and their influence on the response (sorption efficiency), and to predict the maximum sorbent capacity in accordance with the sorption mechanism. The critical factors of the sorption process were determined through the OVAT method. The evaluation of factors' interactive effects was realized using a central composite design (CCD) method within response surface methodology (RSM). A statistical software (JMP Pro16) was applied for planning the experiments , data processing and statistical analysis of the obtained results, which randomly generated a CCD matrix with 4 factors (initial concentration-C o , pH value, temperature-T, time-t) on three levels (-1, 0, +1). The derived second-order polynomial model of phosphate sorption proved to be highly significant (p<0.0001). A statistically significant factor interaction is between the pH value and the initial concentration of phosphate ions. The experimental maximum sorption capacity (17.48 mg/g), very close to the predicted value (17.58 mg/g), indicated the capability and applicability of the regression model. The model finds a potential application in the optimization of the phosphate-contaminated wastewater treatment processes using the environmentally friendly cationic bottle gourd shell.