Dye-Dependent Competitive Adsorption of Congo Red and Reactive Blue 19 by CTAB-Modified Wood Biochar

Anionic dyes with similar charge characteristics may exhibit substantially different adsorption behavior when competing for the same modified adsorbent surface. In this study, cetyltrimethylammonium bromide (CTAB)-modified wood biochar (CWBC) was evaluated for the single and competitive adsorption of Congo red (CR) and Reactive Blue 19 (RB19), two structurally distinct anionic dyes. CTAB modification markedly enhanced the adsorption of both dyes relative to unmodified wood biochar, while the equilibrium adsorption capacity was considerably higher for CR than for RB19. Competitive adsorption experiments conducted at a fixed nominal total dye concentration of 100 mg L−1 showed preferential adsorption of CR over RB19 across the investigated mixing ratios. At a 1:1 CR:RB19 mass ratio, the adsorption capacities of CR and RB19 were approximately 12.81 ± 0.46 and 1.43 ± 0.31 mg g−1, respectively, corresponding to an approximately nine-fold difference. Physicochemical characterization indicated that CTAB treatment altered the surface and interfacial characteristics of the biochar, including its pH-dependent surface charge. Overall, the results demonstrate that CR and RB19 can exhibit markedly different competitive adsorption behavior on CWBC despite their common anionic character.