Valorization of Crataegus azarolus stones for the removal of textile anionic dye by central composite rotatable design using cubic model: optimization, isotherm, and kinetic studies.

In this study, the central composite rotatable design (CCRD) was used in the optimization of the operating parameters for the removal of the direct blue 86 (DB86), an anionic dye, because of its hazardous impact on human health and aquatic environment. In addition, DB86 is a recalcitrant and non-biodegradable dye whose presence considerably inhibits photosynthesis. Its removal in aqueous medium was achieved by biosorption onto the novel biosorbent Crataegus azarolus stones (CAS). The parameters like the solution pH, biosorbent dose, initial DB86 concentration, and temperature were studied in the ranges 2-6, 0.8-4 g L-1, 20-100 mg L-1, and 10-50 °C, respectively. The significance of the experimental parameters and their interactions was investigated by the Student's t test and p values with 5% error limits using JMP 11.0.0 software. The regression analysis of the experimental data obtained from 31 batch runs provides a cubic model. The optimum conditions obtained for the maximum DB86 elimination from the synthetic solution were found to be pH 2, biosorbent dose of 4 g L-1, initial DB86 concentration of 20 mg L-1, and temperature of 10 °C, leading to a theoretical maximum removal of 123%. The experimental data were analyzed by the Langmuir, Freundlich, and Temkin equilibrium models. The Langmuir isotherm gave the best fit with a maximum biosorption capacity of 24.02 mg g-1. The results of the kinetic study revealed that the biosorption kinetic of DB86 follows a pseudo-second-order model. All results confirmed that CAS are an efficient, economic, and ecological alternative for the treatment of industrial wastewaters loaded with anionic dyes.