Adsorptive removal of the cationic dye malachite green from wastewater with an anionic hydrogel

Contamination of water with various hazardous chemicals is increasing due to the rapid growth of industries, which has devastating effects on all living organisms. Thus, the decontamination of water has become crucial. This work reports the removal of a toxic cationic dye, malachite green (MG), from water using a bulk anionic hydrogel synthesized specifically for this purpose. The hydrogel was fabricated through the free radical polymerization of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and acrylamide (AAm) employing N,N'-methylenebisacrylamide (MBA) as the crosslinking agent. FTIR spectroscopic analysis confirmed the formation of the p(AMPS-co-AAm) hydrogel. Structural characterization using SEM revealed a well-interconnected porous network, supporting efficient dye uptake. Swelling analysis demonstrated a maximum equilibrium swelling of 2009.49% and a water content of 95.35%, with water transport following a non-Fickian transport mechanism. Adsorptive removal of the MG dye was investigated under varying conditions of hydrogel dosage, initial dye concentration, contact duration, and solution pH. A maximum adsorption capacity of 930.31 mg g^-1 was recorded at 400 ppm, while 99.5% removal of the dye was achieved at 180 ppm. The adsorption data were best fitted to the Langmuir isotherm, which confirmed monolayer adsorption, while kinetic analysis aligned with the pseudo-first-order (PFO) kinetic model, indicating a physisorption-governed rate mechanism. The adsorption process was found to be strongly pH dependent, with a maximum uptake of 302.33 mg g^-1 and an efficiency of 98.99% achieved at pH 11, attributed to the enhanced electrostatic interactions between the deprotonated anionic hydrogel surface and the cationic MG molecules. The synthesized p(AMPS-co-AAm) hydrogel exhibited an ultrahigh adsorption capacity for malachite green, demonstrating its potential as an efficient adsorbent for wastewater treatment. Collectively, these results establish the bulk p(AMPS-co-AAm) hydrogel as a highly effective and scalable adsorbent for the removal of MG and other cationic contaminants from wastewater.