Development of an original PAm@P hydrogel for high-performance cationic dye removal from wastewater

Abstract In this study, an original and simple method was employed to modify polyacrylamide (PAm) with sodium phosphate (Na₃PO₄), yielding a sodium phosphate-modified polyacrylamide hydrogel (PAm@P) with promising swelling and adsorption properties compared to pristine, unmodified polyacrylamide. The PAm@P hydrogel was characterized using various advanced techniques to confirm successful modification of PAm and the formation of the PAm@P hydrogel. Swelling experiments demonstrated that phosphate modification significantly enhanced the swelling capacity of PAm@P hydrogel (%S = 7000 at t = 60 min and pH 9.5) compared to unmodified PAm. The adsorption performance of PAm@P was evaluated using methylene blue (MB) dye. The adsorption kinetics followed a pseudo-second-order model exhibiting the best fit with the highest value for the correlation factor (R 2 ) of 0.999, comparing with R 2 values obtained for the other investigated models (0.976, 0.976, 0.993, and 0.654 for Pseudo first order, Liquid film diffusion, and intra-particle diffusion (step1 and step 2) kinetic models, respectively. These results demonstrated that the investigated adsorption process may be explained as chemisorption is the dominant mechanism. Also, the Langmuir isotherm model illustrated the R² value (0.999) with a maximum monolayer adsorption capacity (q max ) of 746.26 mg g − 1 for MB dye. While the R² values for the other investigated adsorption isotherm models were 0.254, 0.672, and 0.704 for Freundlich, Tempkin, and Flory-Higgins models, respectively. Also, the highest experimental adsorption capacity (q max = 1490 mg g –1 ) was obtained under the optimum adsorption conditions with an initial MB-dye concentration of 300 mg L –1 , PAm@P hydrogel dosage of 0.005 g, solution volume of 25 mL, pH 7, contact time of 120 min, and temperature of 25 °C. Furthermore, the PAm@P hydrogel exhibited selective adsorption and separation of the cationic dye MB in the presence of other dyes, such as Rhodamine B (RhB) and Azorubine (E122). The adsorption selectivity of the fabricated PAm@P hydrogel was quantitatively evaluated by calculating the selectivity factor (SF) where it was 2.31 for MB/Rhodamine B and 3.10 for MB/E122, confirming the preferential adsorption of MB-due over the other investigate dyes under identical experimental conditions. Moreover, the PAm@P hydrogel exhibited good regeneration and reusability performance over three consecutive adsorption–desorption cycles. The adsorption efficiency decreased gradually from 96.56% in the first cycle to 62.63% after the third cycle, indicating that approximately 64.9% of the initial adsorption efficiency was retained. Finally, the application of the prepared PAm@P hydrogel to real wastewater samples demonstrated its practical feasibility and highlighted its potential as an efficient adsorbent material for industrial wastewater treatment applications. This confirms that the investigated hydrogel is economical, promising, and can significantly reduce wastewater pollutants.