Adsorptive Removal of Synthetic Textile Dyes Using Raw Almond Shells: A Sustainable Approach to Wastewater Treatment

The adsorption capacity of raw almond shells (RASs) was evaluated for the removal of two synthetic textile dyes, Bemacron Blue RS 01 (BB-RS01) and Bemacid Marine N-5R (BM-N5R), from aqueous solutions. Prior to adsorption tests, RAS was characterized using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM-EDX), Brunauer–Emmett–Teller (BET) surface analysis, thermogravimetric/differential thermal analysis (TGA/DTA), and point of zero charge (pHpzc) determination to assess its physicochemical properties relevant to adsorption. Batch adsorption experiments were then conducted under varying operational parameters, including contact time, dye concentration, biosorbent dosage, particle size, pH, and temperature. Rapid dye uptake occurred within the first 30 minutes, reaching equilibrium after two hours. Maximum dye removal efficiencies of 90% for BB-RS01 and 80% for BM-N5R were achieved at acidic pH (2–4) and low initial concentrations (15 mg.L^-1). Increasing biosorbent dosage and decreasing particle size enhanced adsorption efficiency. The adsorption process exhibited endothermic behavior, with optimal performance at 35 °C.
Kinetic data were best fit by a pseudo-second-order model, suggesting chemisorption as the controlling mechanism. Equilibrium data followed the Langmuir model for BB-RS01 (R² = 0.995) and the Freundlich model for BM-N5R (R² = 0.850), indicating monolayer and heterogeneous multilayer adsorption, respectively. The maximum adsorption capacities were 208.33 mg.g^-1 for BB-RS01 and 243.90 mg.g^-1 for BM-N5R. Regeneration studies showed stable adsorption performance for up to four reuse cycles.
These findings confirm that RAS is a promising, sustainable, and low-cost biosorbent for the treatment of dye-contaminated wastewater, supporting circular economy and green chemistry principles.