Natural volcanic tuff from Türkiye as an efficient adsorbent for methylene blue: Kinetic, isotherm, and RSM analysis

Abstract Water pollution caused by synthetic dyes, particularly cationic species such as methylene blue (MB), represents a significant environmental concern because of their toxic nature, long‐term stability, and resistance to biological degradation. This study focuses on the use of natural volcanic tuff from the Sakarya–Hendek region (Türkiye) as a low‐cost natural adsorbent for the removal of MB from aqueous solutions. The novelty of this study lies in evaluating natural Sakarya–Hendek volcanic tuff as an adsorbent for MB removal, distinct from previous studies primarily focused on clinoptilolite and other natural zeolitic adsorbents. The material was analyzed using XRD, FTIR, BET, XRF, FE‐SEM–EDX, and zeta potential measurements. Batch adsorption studies were performed at the natural pH (8–9) without pH adjustment, and the effects of adsorbent dosage, initial dye concentration, and contact time were optimized through response surface methodology (RSM). The optimized conditions were an initial MB concentration of 56.1 mg/L, an adsorbent dosage of 0.45 g, and a solution volume of 126.1 mL, achieving approximately 100% MB removal. Kinetic analysis showed that MB adsorption followed a pseudo‐second‐order model, while the equilibrium data were best described by the Langmuir isotherm, yielding a maximum adsorption capacity (qmax) of 42.8 mg/g. Thermodynamic analysis indicated that the adsorption process was spontaneous (ΔG° = −37.1 J/mol at 297.25 K) and endothermic (ΔH° = 9.4 kJ/mol), with a positive entropy change (ΔS° = 31.9 J/(mol·K)). These results suggest that volcanic tuff has considerable potential as an efficient, environmentally friendly, and low‐cost alternative to conventional adsorbents for dye‐contaminated wastewater treatment.