Water remediation using low cost adsorbent walnut shell for removal of malachite green: Equilibrium, kinetics, thermodynamic and regeneration studies

Batch adsorption experiment of malachite green (MG) was studied with walnut shell (WS). Adsorption of MG onto WS was confirmed by FTIR analysis. Particle size, dosage, effect of dye concentration, pH, temperature and ionic strength were investigated. The optimized conditions for adsorption process in this study was carried out using WS of dosage 0.03 g/20 mL dye, at room temperature, ambient pH and agitation rate of 250 rpm for 2 h. The kinetics of the adsorption process was studied using four models: Lagergren 1st order, pseudo 2nd order, Weber–Morris intraparticle diffusion and the Boyd models. Kinetics data is best fitted with pseudo 2nd order. Weber–Morris model showed that intraparticle diffusion may be present, but is not the rate-limiting step while Boyd model suggested that film diffusion may be the controlling mechanism. Four isotherm models namely the Langmuir, Freundlich, Redlich–Peterson and Sips models were used for describing the adsorption process. The inclusion of non-linear isotherm models together with four error functions (ARE, EABS, χ2 and MSPD) suggested the Langmuir model best described the adsorption process. The Langmuir isotherm predicted the maximum monolayer adsorption capacity of 90.8 mg g−1. Thermodynamic studies showed that adsorption system is spontaneous and endothermic in nature. Regeneration of WS was investigated using three different solvent solutions and the results showed 0.1 M NaOH was able to regenerate and improve the adsorption capability of WS. Based on all the data in this study, WS is a potential low-cost material for the removal of MG.