The mechanism by which direct zinc oxide adsorbs heavy metals in wastewater treatment is mainly based on its unique surface chemical properties and physical structure. As a typical N-type semiconductor material, direct zinc oxide possesses a large number of oxygen and zinc vacancies in its crystal structure. These defect sites endow its surface with unsaturated coordination active centers, becoming key sites for the adsorption of heavy metal ions. When heavy metal ions (such as copper, zinc, and lead) in wastewater come into contact with the zinc oxide surface, the ions bind to the surface charged sites through electrostatic attraction, forming the initial physical adsorption. This process is significantly affected by the solution pH. Within a suitable pH range, the protonation and deprotonation of hydroxyl groups (-OH) on the zinc oxide surface can regulate the surface charge distribution, enhancing the selective adsorption of specific heavy metal ions.
Chemical adsorption is the core mechanism for the removal of heavy metals by direct zinc oxide. The zinc and oxygen atoms on the zinc oxide surface exhibit high reactivity due to their unsaturated coordination, allowing them to undergo coordination exchange or covalent bonding with heavy metal ions. For example, under acidic conditions, zinc ions on the zinc oxide surface may partially dissolve, releasing coordination sites to form surface complexes with copper ions (Cu²⁺) in wastewater. In neutral or alkaline environments, surface hydroxyl groups can form stable inner-layer complexes with lead ions (Pb²⁺) through shared oxygen atoms. This chemisorption is not only highly potent but also selective, preferentially removing highly toxic heavy metal ions.
Ion exchange further enhances the adsorption capacity of direct zinc oxide. Zinc ions (Zn²⁺) in the zinc oxide crystal structure can exchange with homovalent or heterovalent heavy metal ions in wastewater, such as replacing and fixing cadmium ions (Cd²⁺) in the solution within the crystal lattice. This process is influenced by ionic radius, charge number, and solution ionic strength; generally, ions with similar radii and charges have higher exchange efficiency. Furthermore, a hydrated layer may form on the surface of zinc oxide due to the adsorption of water molecules. Hydrogen ions (H⁺) or hydrated hydrogen ions (H₃O⁺) within this layer can participate in ion exchange, indirectly promoting the removal of heavy metal ions.
Surface precipitation is another important pathway for direct zinc oxide to adsorb heavy metals. When the concentration of heavy metal ions in wastewater is high, the surface of zinc oxide may act as a heterogeneous nucleation substrate, inducing the precipitation of heavy metal hydroxides or sulfides. For example, in sulfur-containing wastewater environments, zinc ions on the zinc oxide surface can react with sulfur ions (S²⁻) to form zinc sulfide (ZnS) precipitate, while simultaneously adsorbing and co-precipitating surrounding copper ions to form mixed metal sulfides. This surface precipitation not only expands the adsorption capacity of zinc oxide but also reduces the risk of secondary release of heavy metals through the encapsulation effect of the precipitate.
Environmental factors have a significant regulatory effect on the adsorption performance of direct zinc oxide. Increased temperature usually accelerates the ion diffusion rate and improves adsorption kinetic efficiency, but excessively high temperatures may lead to the desorption of already adsorbed heavy metal ions. Coexisting substances in solution (such as humic acid and inorganic salts) may inhibit the removal of heavy metals through competitive adsorption or complexation. For example, the carboxyl and phenolic hydroxyl groups in humic acid can bind to sites on the zinc oxide surface, reducing the adsorption of lead ions. Furthermore, the physical properties of zinc oxide, such as particle size, morphology, and crystallinity, also affect its adsorption performance. Nanoscale zinc oxide, due to its large specific surface area and numerous surface defects, typically exhibits higher adsorption capacity and rate.
Regeneration and recycling of direct zinc oxide are crucial for reducing treatment costs. Saturated zinc oxide can be regenerated through methods such as acid washing, alkali washing, or chemical reduction. For instance, dilute hydrochloric acid can be used to dissolve adsorbed heavy metal ions, restoring the surface activity of zinc oxide. Regenerated zinc oxide can be reused in wastewater treatment, but after multiple cycles, structural damage may lead to a decline in adsorption performance. Optimizing regeneration conditions or introducing doping modifications (such as introducing iron or manganese) are necessary to maintain its long-term stability.
Direct zinc oxide achieves highly efficient removal of heavy metal ions in wastewater treatment through the synergistic effects of multiple mechanisms, including physical adsorption, chemical adsorption, ion exchange, and surface precipitation. Its adsorption performance is influenced by multiple factors, including surface chemistry, crystal structure, environmental conditions, and coexisting substances. By regulating these factors, its application effect can be optimized. Future research can focus on developing highly selective and stable zinc oxide-based composite materials, as well as exploring low-cost and low-energy regeneration technologies to promote its large-scale application in industrial wastewater treatment.
