Kinetics and dispersion characteristics of transition metal oxide-modified calcined black soil nanocomposites for efficient removal of cationic and anionic dyes

Calcined black soil (CBS)-supported transition metal oxide nanocomposites were developed as dispersion-controlled adsorbents for the efficient removal of organic dyes from aqueous media. The novelty of this work lies in the utilization of naturally abundant CBS as a low-cost support matrix for the fabrication of ZnO/CBS, NiO/CBS, and ZnO/NiO/CBS nanocomposites via a simple co-precipitation method, together with a systematic investigation of the influence of nanoparticle dispersion on adsorption performance. The synthesized materials were characterized using XRD, FT-IR, UV-visible diffuse reflectance spectroscopy, SEM-EDS, BET, TEM, and SAED analyses. Among the prepared nanocomposites, ZnO/NiO/CBS exhibited the smallest crystallite size (11.0 nm), the highest specific surface area, and improved nanoparticle dispersion. Adsorption performance was evaluated under various operating conditions, including pH, initial dye concentration, adsorbent dosage, and contact time. The ZnO/NiO/CBS nanocomposite demonstrated superior adsorption efficiency, achieving maximum dye removals of 96.9 ± 1.9% for crystal violet and 91.7 ± 1.8% for alizarin red at pH 7, an initial dye concentration of 60 ppm, an adsorbent dosage of 0.05 g, and a contact time of 60 min. Adsorption equilibrium was best described by the Langmuir and Dubinin–Radushkevich isotherm models, indicating predominantly monolayer physical adsorption, while adsorption kinetics followed a pseudo-second-order model (R2 > 0.99). Thermodynamic analysis confirmed that the adsorption process was spontaneous and exothermic. Furthermore, the synthesized nanocomposites exhibited satisfactory reusability over three regeneration cycles. These results highlight the potential of CBS-supported transition metal oxide nanocomposites as low-cost, reusable, and environmentally sustainable adsorbents for wastewater treatment applications.