Bio-composite-mediated chromium (VI) removal from aqueous solutions: nonlinear isotherms, kinetics, and thermodynamic study

• Pseudo-first-order model described adsorption kinetics ( R² = 0.925). • FTIR confirmed C=O, C–O, and –OH groups on composite surface. • Temkin model ( R² = 0.999) best fit the adsorption isotherm data. • Adsorption was endothermic with ΔH = +0.0854 kJ/mol. • Non-spontaneous process confirmed by positive ΔG values. The rapid growth of industrialization and urbanization has led to significant water contamination by toxic heavy metals, notably hexavalent chromium (Cr(VI)). This study investigates the potential of an eco-friendly bio-composite adsorbent synthesized from agricultural waste for the efficient removal of Cr(VI) from aqueous solutions. Characterization using Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) confirmed the presence of key functional groups (C=O, C–O, –OH) and a favorable surface morphology, respectively. Batch adsorption experiments were conducted by varying Cr(VI) concentration (20–100 mg/L), contact time (15–95 min), pH (2–10), and adsorbent dosage (0.1–1.3 g). A removal efficiency of 90 ± 0.64 % was achieved using 0.7 g of adsorbent at 20 mg/L Cr(VI), pH 2, and 45 min of contact time, resulting in a maximum adsorption capacity (Qm) of 18.944 mg/g. The nonlinear Temkin model (RMSE= 0.238, χ² = 0.009, R² = 0.999) provided the best fit to the adsorption data, while the pseudo-first-order kinetic model (RMSE= 0.173, χ² = 0.02946, R² = 0.925) most accurately described the adsorption kinetics. Thermodynamic analysis indicated that a positive value of ΔH° (+0.0854 kJ/mol) suggests that the adsorption of Cr(VI) ions onto the adsorbent is endothermic. This implies that increasing the temperature enhances adsorption efficiency by providing the energy required for interactions. A negative entropy value (ΔS° = –0.27 J/mol ○ K) indicates decreased randomness at the solid–solution interface, while ΔG < 0 across the entire temperature range signifies that the adsorption process is spontaneous. Overall, the bio-composite exhibited high adsorption potential and presents a sustainable approach for the removal of Cr(VI) from wastewater.