Bio-adsorption of heavy metals from aqueous solution using the ZnO-modified date pits

The bio-adsorption of heavy metals (including Cu²⁺, Ni²⁺, and Zn²⁺) in aqueous solution and also in an industry wastewater using the ZnO-modified date pits (MDP) as the bio-adsorbent are investigated. The fresh and used bio-adsorbents were characterized by FT-IR, SEM, BET, and XRD. The bio-adsorption parameters (including the pH of solution, the particle size of MDP, the shaking speed, the initial concentration of heavy metals, the dosing of MDP, the adsorption time, and the adsorption temperature) were screened and the data were used to optimize the bio-adsorption process and to study the bio-adsorption isotherms, kinetics, and thermodynamics. Two adsorption models (Langmuir isotherm model and Freundlich isotherm model) and three kinetic models (pseudo-first-order model, pseudo-second-order model, and intra-particle diffusion model) were applied to model the experimental data. Results show that the maximum adsorption amount of Cu²⁺, Ni²⁺, and Zn²⁺ on a complete monolayer of MDP are 82.4, 71.9, and 66.3 mg g^-1, which are over 4 times of those of date pits-based bio-adsorbents reported in literature. The bio-adsorption of heavy metals on MDP is spontaneous and exothermic, and is regulated by chemical adsorption on the homogeneous and heterogeneous adsorption sites of MDP surface. This work demonstrates an effective modification protocol for improved bio-adsorption performance of the date pits-based bio-adsorbent, which is cheap and originally from a waste.