Selective and effective adsorption of malachite green and methylene blue on a non-toxic, biodegradable, and reusable fenugreek galactomannan gum coupled MnO2 mesoporous hydrogel

Abstract

A new FG-g-AM-cl-MBA-MnO2 hydrogel has been synthesised via a free radical cross-linking reaction using fenugreek gum containing macromolecular galactomannan units, acrylamide, N, N′-methylenebisacrylamide, potassium persulfate, and MnO2 for removal of methylene blue and malachite green dyes from water. Surface morphology of synthesised hydrogel was porous and rough as determined by FESEM. The presence of functional groups and cross-linking mechanism were validated by FTIR spectroscopy. TGA revealed that FG-g-AM-cl-MBA-MnO2 hydrogel is thermally stable up to 700 °C and mass ratio of MnO2 was found to be 25.9%. BET technique gave surface area as 5.59 m2/g with a pore radius of 3.654 nm. The swelling ratio of FG-g-AM-cl-MBA-MnO2 hydrogel was 1862% at 313 K. To evaluate the influence of adsorbent dose, contact time, temperature, pH, and dye concentration on adsorption behaviour of hydrogel several batch experiments were conducted. The hydrogel preferentially removes MG (98.5%) and MB (96.3%) effectively following Langmuir model with 526.31 and 483.09 mg/g adsorption capacity, respectively. The adsorption process followed pseudo-second-order kinetics and thermodynamics study revealed that process is endothermic and spontaneous. The hydrogel retained more than 85% adsorption capacity even after ten cycles of dye adsorption-desorption process and thus, serves as a proficient, reusable, and eco-friendly adsorbent for wastewater remediation.