Titanium dioxide nanoparticles have emerged as a promising material for the reduction and removal of hexavalent chromium (Cr(VI)) from contaminated water sources. Cr(VI) is a highly toxic and mobile heavy metal pollutant commonly found in industrial effluents, particularly from electroplating, tanning, and textile manufacturing. Its removal is critical due to its carcinogenic and mutagenic effects. TiO₂ nanoparticles offer a dual mechanism of Cr(VI) elimination: photocatalytic reduction under UV light and surface adsorption. Additionally, modifications such as doping with nitrogen or other elements enhance their efficiency under visible light, making them more practical for real-world applications.
The photocatalytic reduction of Cr(VI) using TiO₂ nanoparticles occurs through a light-induced redox process. When TiO₂ is irradiated with UV light, electrons in the valence band are excited to the conduction band, generating electron-hole pairs. The photogenerated electrons reduce Cr(VI) to the less toxic and less soluble trivalent chromium (Cr(III)), while the holes oxidize water or other electron donors. The overall reaction can be summarized as:
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
The efficiency of this process depends on several factors, including light intensity, TiO₂ crystal phase (anatase, rutile, or mixed phases), particle size, and surface area. Anatase-phase TiO₂ generally exhibits higher photocatalytic activity due to its favorable bandgap and charge carrier dynamics. Nanoparticles with high surface area provide more active sites for Cr(VI) adsorption and subsequent reduction.
Surface adsorption is another critical mechanism for Cr(VI) removal. TiO₂ nanoparticles possess hydroxyl groups on their surface, which can adsorb Cr(VI) oxyanions (e.g., CrO₄²⁻, HCrO₄⁻) through electrostatic interactions or ligand exchange. The adsorption capacity is strongly pH-dependent, as pH influences the surface charge of TiO₂ and the speciation of Cr(VI). At low pH (pH < 4), the TiO₂ surface is positively charged, favoring the adsorption of negatively charged Cr(VI) species. As pH increases, the surface becomes more negatively charged, reducing adsorption efficiency. Optimal Cr(VI) removal typically occurs in the pH range of 2–4.
To extend the photocatalytic activity of TiO₂ into the visible light spectrum, dopants such as nitrogen, sulfur, or transition metals are incorporated into the TiO₂ lattice. Nitrogen doping introduces mid-gap states that narrow the bandgap, allowing visible light absorption. For example, nitrogen-doped TiO₂ nanoparticles have demonstrated enhanced Cr(VI) reduction under solar irradiation compared to undoped TiO₂. The presence of dopants can also reduce electron-hole recombination, improving overall photocatalytic efficiency.
The reusability of TiO₂ nanoparticles is a key consideration for practical applications. Studies have shown that TiO₂ can be reused for multiple cycles of Cr(VI) reduction with minimal loss of activity, provided the nanoparticles are properly washed and regenerated. However, the accumulation of Cr(III) on the TiO₂ surface may gradually decrease performance over time. To mitigate this, post-treatment methods such as alkaline washing can be employed to dissolve Cr(III) precipitates and restore photocatalytic activity.
The toxicity of byproducts, particularly Cr(III), must also be addressed. While Cr(III) is significantly less toxic than Cr(VI), its long-term environmental impact requires careful management. Cr(III) tends to form insoluble hydroxides or oxides at neutral to alkaline pH, facilitating its removal via precipitation or filtration. In some cases, Cr(III) can be further recovered or stabilized to prevent re-oxidation to Cr(VI).
In electroplating wastewater treatment, TiO₂ nanoparticles have been tested both in slurry reactors and immobilized on supports such as glass beads or membranes. Slurry systems offer high surface area and efficient mass transfer but require post-treatment separation of nanoparticles. Immobilized systems simplify recovery and reuse but may suffer from reduced activity due to limited light penetration and surface accessibility. Pilot-scale studies have demonstrated Cr(VI) removal efficiencies exceeding 90% under optimized conditions.
Comparatively, zerovalent iron (ZVI) nanoparticles are another widely studied material for Cr(VI) removal. ZVI reduces Cr(VI) through direct electron transfer, forming Cr(III) and iron oxides or hydroxides. While ZVI operates effectively under a broader pH range and does not require light activation, it generates large amounts of iron sludge, complicating disposal and increasing operational costs. TiO₂, in contrast, produces minimal secondary waste and can harness solar energy, making it more sustainable for long-term use.
The choice between TiO₂ and ZVI depends on specific application requirements. TiO₂ is advantageous in scenarios where sunlight can be utilized, and minimal sludge production is desired. ZVI may be preferred for rapid treatment in highly acidic conditions where light availability is limited. Hybrid systems combining both materials have also been explored to leverage their complementary strengths.
In summary, TiO₂ nanoparticles provide a versatile and effective solution for Cr(VI) reduction and removal, combining photocatalytic and adsorptive mechanisms. Doping strategies enhance their performance under visible light, while pH optimization and regeneration protocols ensure practical applicability. Despite the challenges of byproduct management and light dependence, TiO₂-based systems offer a sustainable alternative to conventional methods, particularly in industrial wastewater treatment. Further advancements in material design and reactor engineering will continue to improve their efficiency and scalability for environmental remediation.