Competitive adsorption of direct red 23 and reactive turquoise blue Q-G125 onto peanut shell biochar for textile effluent treatment

Textile effluents commonly contain mixtures of dyes, making competitive adsorption studies necessary to evaluate adsorbent performance under conditions closer to real wastewater treatment. In this study, nine adsorbents were prepared from five agricultural, agro-industrial, and forestry residues, and peanut shell biochar (BPS) was selected for detailed evaluation of Direct Red 23 (DR23) and Reactive Turquoise Blue Q-G125 (RTB Q-G125). In single-component systems, BPS reached adsorption capacities of 6.14 ± 0.14 mg·g−1 for DR23 and 6.69 ± 0.10 mg·g−1 for RTB Q-G125. In the binary system, equilibrium capacities decreased to 4.30 ± 0.25 mg·g−1 at 508 nm and 4.64 ± 0.13 mg·g−1 at 666 nm, indicating competitive adsorption. Model selection based on goodness of fit and parameter reliability indicated the pseudo-second-order model for DR23 and the binary system and the Elovich model for RTB Q-G125, while the Vermeulen and Boyd analyses showed contributions from both external mass transfer and intraparticle diffusion. Freundlich best described the DR23 equilibrium data and Sips best described RTB Q-G125; for the binary system, the Jain-Snoeyink model provided the most reliable mechanistic interpretation, supporting competition for common or partially overlapping sites and preferential adsorption of DR23. When applied to real textile effluent, BPS reduced more than 80% of visible absorbance in the raw effluent and up to 64% in the treated effluent. These results support BPS as a simply prepared, waste-derived adsorbent for color attenuation and polishing of textile wastewater under competitive conditions.