Basic red 2 and methyl violet adsorption by date pits: adsorbent characterization, optimization by RSM and CCD, equilibrium and kinetic studies

Abstract

The potential of raw date pits as a natural, widely available and low-cost agricultural waste has been studied in order to adsorb cationic dyes from an aqueous solution. Date pits were characterized by FTIR, SEM, BET, and XRD analysis. To optimize removal of two industrial dyes, basic red 2 (BR2) and methyl violet (MV), from aqueous solution using date pits, response surface methodology (RSM) is employed. Tests were carried out as per central composite design (CCD) with four input parameters namely contact time, temperature, initial concentration of adsorbate, and pH. Second-order polynomial model better fits experimental data for BR2 and MV and optimum values were then determined. In the optimum conditions, kinetic study was conducted and the pseudo-second-order model was found the best fitted model compared to pseudo-first-order model. Moreover, it was shown that intraparticle diffusion was not the sole controlling step and could be associated with other transfer resistance. On other hand, equilibrium isotherms were obtained for BR2 and MV and their maximum adsorption capacities were 92 and 136 mg g⁻¹ respectively. Two-parameter isotherm models like Langmuir, Temkin, Freundlich, Dubinin–Radushkevich, and Halsay were investigated to fit equilibrium data. Three error functions of residual root mean square error, chi-square statistic, and average relative error were used to comfort us in the selected models, which were actually Dubinin–Radushkevich and Langmuir for BR2 and Frendlich, Temkin, and Halsay for MV.