From adsorbent to electrode modifier: chemically modified <i>Ayous</i> sawdust for methylene blue removal and electroanalysis

Synthetic dyes, widely used in textile industries, pose serious environmental threats due to their toxicity and persistence in water systems. Addressing this issue with cost-effective, sustainable adsorbents is vital, and <i>Ayous</i> sawdust, an abundant wood waste in Central Africa, presents a promising biosorbent alternative. This study explores the chemical modification of <i>Ayous</i> sawdust (<i>Triplochiton scleroxylon</i>) to enhance its adsorption properties for methylene blue removal and its application in electrochemical sensing. Methylene blue was used as a model compound because of its good adsorption by lignocellulosic matrix and its well-known electrochemical behaviour. The sawdust was subjected to alkaline treatment followed by grafting with mercaptopropyltriethoxysilane (MPTMS) to introduce thiol functional groups. Characterisation by FTIR, NMR spectroscopies, and elemental analysis confirmed successful functionalisation, with a significant increase in sulphur content for the modified material. Adsorption studies showed that alkaline-treated sawdust exhibited a higher removal efficiency (75.14 mg.g⁻¹) than pristine sawdust (66.30 mg.g⁻¹), while thiolated sawdust, despite its strong affinity for methylene blue, displayed lower adsorption capacity (45.56 mg.g⁻¹) due to the reduced accessibility of active sites linked to its high hydrophobicity, which limits its practical application as shown on proposed adsorption mechanisms. Electrochemical application demonstrated that sawdust-modified carbon paste electrodes enhanced electron transfer kinetics, with alkaline-treated sawdust providing the most stable and sensitive detection (LOD = 40.1 pmol.L⁻¹) than the native sawdust (LOD = 47.2 pmol.L⁻¹). The sensors showed good performance in real water samples, with minimal signal degradation in the presence of competing ions. These findings highlight the potential of alkaline-treated sawdust as an efficient and cost-effective adsorbent and electrochemical sensor modifier for methylene blue detection.