Functionalized agro-waste for the sustainable remediation of malachite green from wastewater

Herein, a novel eco-friendly spent coffee grounds (SCG) impregnated guar gum adsorbents (SGH-x) were fabricated by using borax as the cross-linking agent. Several influences as borax dosage, initial dye concentration, pH, contact time, and temperature on the adsorption properties of SGH-x for Malachite Green (MG) were investigated. At a borax dosage of 0.9 wt%, the SGH-x possess maximum adsorption capacity of 921.84 mg/g towards MG and water swelling ratio of 2075 % owing to the presence of three-dimensional network structure and abundant hydroxyl groups. At pH= 10, the removal of MG by SGH-x is nearly 100 %. An increase of adsorption capacities with the molecular weight of cationic dyes followed the sequence as: Rhodamine B (RhB, 479.0 Da), Crystal violet (CV, 407.98 Da), and Basic red 9 (BR9, 323.82 Da) can be explained as the synergy between the pore filling and charge effect. Kinetic and isothermal studies revealed that the adsorption process of MG adhered to pseudo-second-order kinetic model and Freundlich isothermal model, suggesting a chemical adsorption on heterogeneous surfaces. Additionally, regeneration experiments showed SGH-x maintain high adsorption capacity (above 80 mg/g) of MG after three cycles. This study provided a feasible route to prepare biogradable and cost-effective adsorbents for water purification.