Ternary removal of Zn, Ni, and Mn from metal industry wastewater using soybean hulls as adsorbents

Abstract

With the growth of the metalworking industry, effective control of wastewater with phosphate has become a global concern. This study took advantage of the abundant supply of natural soybean hulls as an adsorbent for the direct treatment of wastewater, aiming to remove Ni, Zn, and Mn from real wastewater produced during the phosphating stage of the metalworking industry to address this issue. Soybean hulls presented a specific surface area of 0.31 m² g⁻¹, average diameter of 0.2705 mm, and a pH value for PCZ of 6.43 at 25 °C. Real wastewater was acidic (pH 3.68) with COD of 1270 mg L⁻¹ and highly concentrated in Ni, Mn, and Zn (343.45 mg L⁻¹, 818.6 mg L⁻¹, and 953.85 mg L⁻¹, respectively). It was observed that the process depended on the adsorbent dosage, which can be linked to the low surface area of the material. The optimized pH value was found to be the natural pH of the effluent, which varied between 3 and 4. The average removal rates were 24.5% for Ni, 28.6% for Zn, and 16.5% for Mn, corresponding to the respective removal of 84.15, 135.07, and 272.80 mg L⁻¹ in a ternary system. The maximum adsorption capacities were observed at 50 °C, estimated as 3.125 mg g⁻¹ for Ni, 14.128 mg g⁻¹ for Zn, and 7.8 mg g⁻¹ for Mn. When evaluating the process kinetics, it was observed that adsorption capacity increased significantly during the initial 60 min, followed by a slower rate until saturation. The pseudo-first-order model provided the best fit for Ni adsorption, while Zn and Mn demonstrated the best fit with the pseudo-second-order model. This trend possibly occurred due to the different initial concentrations of each metal, which has shown to be a key factor in mass-driven adsorption mechanisms. Thus, using raw soybean hulls can be considered a viable alternative for coupling adsorption as a low-cost step to other treatment methods for metalworking wastewater.