RSM-CCD directed modeling and optimization of a low-cost adsorbent based on sodium dodecyl sulfate for the selective removal of malachite green and methylene blue dyes: Kinetics, isotherm, and thermodynamics analysis

Sodium dodecyl sulfate (SDS) is widely used as a surface modifier to improve the sorption capacity of sorbents due to its high surface area, non-toxicity, biodegradability, and strong affinity for various impurities. The present works aims at investigating the efficiency of hydrogel adsorbent prepared from sodium dodecyl sulfate for treating malachite green (MG) and methylene blue (MB) dyes. An RSM-based central composite design was utilized to optimize the percentage swelling of the four key synthesis parameters: initiator concentration, monomer concentration, crosslinker concentration, and solvent. The optimal process conditions are as follows: 12.5 mM L- 1 initiator, 306 mM L- 1 monomer, 23 mM L- 1 crosslinker, and 14 mM L- 1. Additionally, the mechanism of their formation was investigated by various physical methods, viz., FTIR, XRD, Raman, H1-NMR, SEM-EDS, TGA, and BET. The applicability of the adsorbent was examined in detail as a function of time, pH, adsorbent dose, initial dye concentration, and temperature. The optimum conditions are as follows: pH=7, adsorbent amount = 0.1 g/L, and initial dye concentration = 100 mg/L, contact time = 120 min at ambient temperature. The hydrogel exhibits maximum sorption capacities of 137.74 mg g- 1 for MG and 373.13 mg g- 1 for MB, respectively. The mechanism of dye adsorption has been established, and the kinetic study suggests that the adsorption follows a pseudo-second-order reaction. Furthermore, the adsorption isotherms at different equilibrium conditions fit well with the Langmuir adsorption model, indicating a chemisorption process. Thermodynamic analyses justified the spontaneous and exothermic nature of dye adsorption, primarily through electrostatic interactions. The adsorption-desorption study of the resulting hydrogel exhibited excellent adsorption efficiency of 42.07% for MG, and 63.35% for MB, and desorption efficiency of MG was lowered from 55.3 % to 22.63 % and MB from 35.57 % to 18.8 % even after 5 consecutive cycles. Consequently, this sorbent provides a versatile and sustainable solution for treating dyes waste water.