INVESTIGATION OF FACTORS AFFECTING REMOVAL OF NICKEL BY PRE-TREATED WALNUT SHELLS USING FACTORIAL DESIGN AND UNIVARIATE STUDIES

The main objective of this study was to identify the operating conditions which influence Ni(II) adsorption onto pre-treated walnut shell. For this purpose, in the first step, a series of experiments were planned according to a full 24 factorial experimental design. The factors investigated here were solution pH, adsorbent dose, the initial concentration of nickel, and temperature. The results predicted using the factorial regression model showed high values of regression coefficient (R²=99.55%), indicating good agreement with the experimental data. It was observed that the main effect of all the factors, besides the interaction of pH and metal concentration, were significant within a 95% confidence level, while for the conditions tested in this study, the pH of the solution was the most influential parameter in the removal of nickel. In the second stage, univariate studies were conducted to validate the results obtained in the first part and also to find the conditions that would give the highest removal of nickel from aqueous solutions. The univariate experimental results provided evidence supporting the sorption data obtained in the factorial design phase of this study. Under the optimal conditions obtained, the nickel ion adsorption equilibrium could be satisfactorily described by the Langmuir isotherm model. The maximum pre-treated walnut shell adsorption capacity for Ni(II) was 8.57 mg/g. The kinetics of Ni(II) adsorption onto pre-treated walnut shell followed the pseudo second-order model.