Adsorption of Methyl Red Using Agricultural Waste-Derived Adsorbents: A Case Study with Argan Nut Shells

The effluents from textile and tannery industries contain high levels of synthetic dyes such as Methyl Red (MR), which represent a serious threat to aquatic ecosystems and human health due to their toxicity and persistence. To search for cost-effective and sustainable adsorbents derived from agricultural residues is therefore an urgent environmental priority. In this study, Argan Nut Shells (ANS), an abundant byproduct of argan oil production, were investigated as a low-cost adsorbent for MR removal. Adsorption performance was evaluated under different conditions, and results showed that 80 mg of ANS removed 53.5% of MR from 20 mL of solution. Although this efficiency appears modest, it remains competitive when compared with other natural adsorbents such as rice husk and coconut shells, particularly considering the economic and ecological value of valorizing argan residues. Kinetic modeling demonstrated that the adsorption process followed a pseudo-second-order model, with intraparticle diffusion not being the sole rate-limiting step, while equilibrium was reached after 80 minutes of contact. The Freundlich model, indicating a heterogeneous adsorption surface, best described the adsorption isotherms. Thermodynamic parameters revealed that the adsorption of methyl red onto ANS was spontaneous (ΔG° < 0) and exothermic (ΔH° < 0). The point of zero charge (pHPZC) value was determined to be 3.68. Fourier-Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy-Dispersive X-Ray Spectroscopy (EDS) analyses confirmed surface modification after MR adsorption and provided insights into the interaction mechanism.