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

Abstract

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 (R2 = 0.9932, χ2 = 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.