Synthesis and characterization of gamma irradiated magnetic biochar derived from rice straw and grafting with (acrylic acid-co-acrylamide) hydrogel for removing methylene blue dye

In this study, acid-activated biochar derived from rice straw was combined with Fe₃O₄ nanoparticles using the co-precipitation method, followed by grafting with acrylic acid (AAc) and acrylamide (AAm) at different monomer ratios of 70:30 (RS4), 50:50 (RS5), and 30:70 (RS6) by ⁶⁰Co gamma irradiation at a dose of 15 kGy. The aim was to develop magnetic copolymer hydrogel adsorbents with enhanced swelling properties and capacity for methylene blue (MB) dye removal. The synthesized adsorbents were thoroughly characterized using FTIR, Raman spectroscopy, XRD, SEM–EDX, TEM, VSM, BET analysis, zeta potential measurements, and particle size distribution (PSD). The maximum adsorption capacities (qmax) were 467.31 mg/g (RS4), 478.82 mg/g (RS5), and 271.73 mg/g (RS6) under optimal conditions of 180 min contact time, a temperature of 25°C, an adsorbent dosage NN of 25 mg, pH 7, and a 100 mg/L initial dye concentration. Kinetic data was best fitted by the nonlinear pseudo-second order (PSO) model, indicating that adsorption was primarily governed by the availability of active sites. The Freundlich isotherm provided a better fit for RS4 and RS5, suggesting heterogeneous multilayer adsorption, whereas RS6 followed the Langmuir isotherm, reflecting a more uniform surface. Thermodynamic parameters (ΔH, ΔS, and ΔG) confirmed that the adsorption process was spontaneous and endothermic. The findings demonstrate that tuning the AAc/AAm ratio during γ-irradiation is an effective method for optimizing the structural and adsorption properties of magnetic biochar hydrogel composites for efficient dye removal.