Equilibrium modeling of the adsorptive eradication of Safranin-O dye from aqueous solutions using the oxalic acid-treated peanut husk as an upcycled agro-waste.

In this study, oxalic acid-treated peanut husk (ATPH) was prepared and evaluated as a low-cost biosorbent for the removal of Safranin-O dye from aqueous solutions. Batch adsorption experiments were performed to investigate the effects of pH, adsorbent dosage, contact time, temperature, and agitation speed. The maximum dye removal of 31.25 mg g-1 was achieved under optimized conditions (pH 4, adsorbent dose 0.8 g, contact time 70 min, agitation speed 175-200 rpm, and temperature 313 K). Kinetic studies revealed that the adsorption process followed the pseudo-second-order model. The process is spontaneous (negative ΔG°) and endothermic (positive ΔH° = +1.822 kJ mol-1) and is governed by a weak, reversible electrostatic physisorption mechanism rather than chemisorption. Non-linear regression indicated that the Langmuir model fitted the best (R 2 = 0.9932, χ 2 = 1.24, and MAPE = 4.8%). Film diffusion was identified as the rate-limiting step using Weber-Morris and Boyd analyses. Among the desorbing agents, HCl was the most effective, achieving 80% desorption in the first cycle. ATPH exhibited excellent stability with only 2.3-5.7% solid mass loss per cycle and retained 92-98% of its original adsorption capacity after three HCl regeneration cycles. Oxalic acid treatment significantly enhances the adsorption performance of peanut husks, providing a promising, low-cost, and sustainable strategy for the removal of cationic pollutants from wastewater.