Synergistic Ball Milling and KOH Modification of Wheat Straw Biochar for Methylene Blue Adsorption

Methylene blue (MB), a widely used industrial dye, is a significant environmental pollutant due to its inherent toxicity, carcinogenicity, and poor biodegradability. In this study, we developed a cost-effective approach to synthesize ball milling and KOH co-modified wheat straw biochar (KOH-BM-biochar) for the efficient adsorption of methylene blue. The adsorption performance was systematically investigated as a function of the solution pH, temperature, and initial MB concentration. Compared to unmodified biochar, the KOH-BM-biochar demonstrated a significantly enhanced performance. Under optimal conditions (pH 8.0), it achieved a removal efficiency of 95.0% within 180 min and exhibited a maximum adsorption capacity of 300.66 mg·g −1 . Kinetic analysis revealed that the adsorption process was well-described by the pseudo-second-order model, indicating that chemisorption is the primary rate-limiting step. Furthermore, thermodynamic studies confirmed the process to be both spontaneous and endothermic in nature. Mechanistic investigations suggest that MB adsorption is governed by a combination of hydrogen bonding, π–π stacking, electrostatic attraction, and pore filling. In continuous-flow column experiments, the breakthrough curves were accurately fitted by both the Thomas and Yoon–Nelson models, validating the adsorbent's practical applicability. Moreover, the adsorbent demonstrated good reusability, maintaining a removal efficiency of 74.80% after five regeneration cycles. Therefore, these findings highlight that the synergistic modification via KOH activation and ball milling produces a highly effective biochar adsorbent, positioning it as a promising, low-cost material for dye removal in environmental remediation.