High-capacity and reusable nickel borate adsorbent for sulfonated anionic dye removal: comparative adsorption,...

The study conducted a comparative investigation into the performance of a newly developed nickel borate (Sr₂Ni(BO₃)₂, NiB) adsorbent for the removal of two anionic dyes, Trypan Blue and Ponceau S, from aqueous solutions. The adsorption experiments assessed various parameters, including contact time, pH, adsorbent dosage, agitation speed, temperature, initial dye concentration, and regeneration cycles. High removal efficiencies for both dyes were typically observed under conditions approaching neutral pH, at an agitation speed of 250 rpm and a temperature of 25 °C. In contact-time experiments, the removal efficiencies reached 93.72% for Trypan Blue and 90.54% for Ponceau S within 60 min. Regarding pH studies, Trypan Blue removal was 91.55% at pH 7.50, while Ponceau S removal was 89.72%. Isotherm calculations, adjusted for actual initial concentrations, indicated that at an initial dye concentration of 100 mg L^-1, the highest experimental adsorption capacities were 463.08 mg g^-1 for Trypan Blue and 463.85 mg g^-1 for Ponceau S, with removal rates exceeding 92% for both dyes. The equilibrium data were further evaluated using the non-linear forms of the Langmuir, Freundlich, Temkin, Dubinin-Radushkevich, Sips, and Redlich-Peterson isotherm models. Because a clear saturation plateau was not reached within the tested concentration range, model-derived maximum capacity parameters were interpreted cautiously as apparent estimates rather than definitive maximum adsorption capacities. In the kinetic evaluation, the adsorption data were assessed using non-linear kinetic fitting and error-function-based model validation. The results indicated that the adsorption process was better interpreted as a multi-step mechanism involving rapid surface attachment, surface rearrangement, and diffusion-related contributions rather than as a single rate-controlling process. Thermodynamic analysis confirmed the spontaneous, exothermic nature of the adsorption process. In regeneration experiments, thermal treatment outperformed HCl regeneration, maintaining removal efficiencies of 72.5% for Trypan Blue and 68.0% for Ponceau S by the ninth cycle. Overall, these findings demonstrate that NiB is a high-capacity, multi-mechanism, and reusable inorganic adsorbent candidate for the removal of sulfonated anionic dyes.