Highly selective removal of cationic dye using a novel synthesized polyacrylic polyacrylamide phosphate (PAA@PAm@P) hydrogel

In this study, a novel phosphate-functionalized hydrogel adsorbent, poly(acrylic acid-g-polyacrylamide) phosphate (PAA@PAm@P), was successfully synthesized for considerable adsorption of methylene blue (MB)-dye from aquatic solutions. The base hydrogel, poly(acrylic acid-g-polyacrylamide) (PAA@PAm), was prepared via free-radical copolymerization of acrylic acid and acrylamide, followed by mechanical homogenization to obtain hydrogel particles. Surface modification was subsequently achieved through phosphorylation using trisodium phosphate at 180 °C for 4.0 h, yielding the functionalized micron-sized PAA@PAm@P hydrogel. The fabricated materials were characterized using different analytical techniques, such as SEM, FTIR, TGA, and EDS, to confirm successful structural modification and functional groups incorporation. Batch adsorption experiments were conducted to investigate the influence of operational parameters including contact time, adsorbent dose, initial pH, initial MB-dye concentrations, NaCl concentrations, and temperature. The modified micron-sized PAA@PAm@P hydrogel exhibited rapid adsorption kinetics, reaching equilibrium within 15.0 min, which is twice as fast as the unmodified PAA@PAm (30.0 min). The material also demonstrated exceptional swelling behavior with a maximum swelling ratio of 5590% within 3 min, significantly higher than that of PAA@PAm (1712% after 7.0 min). Experimental results indicated that kinetic data were best described by pseudo-second-order (R$^2$ = 0.9978) and intra-particle diffusion mechanisms (R$^2$ = 0.971) while isotherm data were best fitted well with the Freundlich isotherm model with a remarkably high maximum adsorption capacity of 1000 mg g$^{-1}$ for MB-dye. Furthermore, micron-sized PAA@PAm@P hydrogel showed excellent regeneration capability, maintaining high adsorption efficiency over five consecutive adsorption–desorption cycles. Comparative analysis with recently reported hydrogel adsorbents confirmed the superior adsorption performance of the developed material. These findings demonstrate that the phosphate-functionalized PAA@PAm@P hydrogel is a highly efficient, reusable, and environmentally sustainable adsorbent for rapid removal of cationic dyes from contaminated water.