Efficient adsorption of cationic and anionic dyes using agricultural waste–based activated carbon

This study investigates the adsorption of cationic (methylene blue, MB) and anionic (methyl orange, MO) dyes onto activated carbon synthesized from almond seed shells (ASS-AC). The adsorbent was produced via carbonization and chemical activation and characterized using FTIR, XRD, SEM, and BET analyses. FTIR results revealed abundant oxygen-containing functional groups that enhanced electrostatic attraction and hydrogen bonding with dye molecules. XRD analysis indicated a predominantly amorphous carbon structure, while BET analysis showed a high surface area of 520 m²/g with dominant microporosity. Batch adsorption experiments evaluated the effects of contact time, initial dye concentration, pH, adsorbent dosage, and temperature. Adsorption kinetics followed a pseudo-second-order model (R² > 0.99), suggesting chemisorption as the rate-limiting step. Thermodynamic parameters (ΔG° < 0, ΔH° > 0) confirmed that the adsorption process was spontaneous and endothermic. Equilibrium studies showed that MB adsorption was best described by the Langmuir model with a maximum adsorption capacity of 162 mg/g, while MO adsorption followed the Freundlich model, indicating heterogeneous multilayer adsorption. Overall, ASS-AC demonstrates strong potential as a sustainable, low-cost adsorbent for efficient removal of both cationic and anionic dyes from wastewater.