Photocatalysis for Heavy Metal Treatment: A Review

Environmental and human health are threatened by anthropogenic heavy metal discharge into watersheds. Traditional processes have many limitations, such as low efficiency, high cost, and by-products. Photocatalysis, an emerging advanced catalytic oxidation technology, uses light energy as the only source of energy. It is a clean new technology that can be widely used in the treatment of organic pollutants in water. Given the excellent adaptability of photocatalysis in environmental remediation, it can be used for the treatment of heavy metals. In this comprehensive review, the existing reported works in relevant areas are summarized and discussed. Moreover, recommendations for future work are provided.

Fabrication of copper phthalocyanine/reduced graphene oxide nanocomposites for efficient photocatalytic reduction of hexavalent chromium

Copper(II) phthalocyanine (CuPc) and non-peripheral octamethyl-substituted copper(II) phthalocyanine (N-CuMe₂Pc) were combined with reduced graphene oxide (rGO) via a precipitation method to form CuPc/rGO and N-CuMe₂Pc/rGO nanocomposites, respectively. CuPc nanorods are distributed on rGO, and N-CuMe₂Pc exists as nanorods and nanoparticles on rGO. The Cr(VI) removal ratio of N-CuMe₂Pc/rGO exposed in simulated sunlight is 99.0% with a fast photocatalytic reaction rate of 0.0320 min^-1, which is approximately 1.5 times faster than that of CuPc/rGO (0.0215 min^-1) and far surpasses that of pristine phthalocyanine and rGO. As an electron acceptor, rGO can suppress the recombination of photo-induced electron-hole pairs and also can provide a large surface area for Cr(VI) removal, both of which are beneficial to the reducing capacity of the nanocomposites. The higher removal efficiency of N-CuMe₂Pc/rGO compared with that of CuPc/rGO is attributed to the higher specific surface area, higher light harvesting, higher conductivity and more negative lowest unoccupied molecular orbital level of N-CuMe₂Pc/rGO. The N-CuMe₂Pc/rGO nanocomposite shows excellent photochemical recyclability which is essential for application in wastewater treatment.

Excellent photoreduction performance of Cr(vi) over (WO4)2−-doped metal organic framework materials

Negatively charged (WO₄)²⁻ anions doped into 2D layered metal organic frameworks bismuth terephthalate provides a valuable strategy for excellent photoreduction of Cr(vi).

Dual phototransformation of the pollutants methyl orange and Cr (VI) using phthalocyanine-cobalt ferrite based magnetic nanocomposites

Bifunctional nanocomposites based on zinc phthalocyanines and glutathione capped CoFe2O4 magnetic nanoparticles (GSH-CoFe2O4 MNPs) are applied in a binary system wherein simultaneous photooxidation of methyl orange (MO) and photoreduction of Cr (VI) are conducted. The photoactivity of two zinc Pcs with different functional moieties are compared based on their interactions with GSH-CoFe2O4 MNPs. Conjugation of the Pcs to the GSH-CoFe2O4 MNPs not only enhanced their singlet oxygen production but also their photocatalytic activity in both photooxidation and photoreduction experiments. Using electron paramagnetic resonance (EPR) spectroscopy, the Pc-MNP conjugates reported herein were found to exhibit superparamagnetic behaviour, giving the advantage of easy separation using an external magnetic field post application, an attractive attribute for heterogeneous catalysis. The catalysts reported herein are therefore good candidates as catalysts for real life water purification analyses as they facilitate the treatment of both organic and inorganic water pollutants.

Mechanisms of removal of heavy metals and arsenic from water by TiO2-heterogeneous photocatalysis

This article is an overview of recent work performed in our laboratory on TiO2 heterogeneous photocatalysis of aqueous systems containing toxic forms of chromium, arsenic, lead, uranium and mercury. The cases of chromium and arsenic are treated in profundity. Photocatalytic treatments can convert the ionic species into their metallic solid form and deposit them over the semiconductor surface, or transform them into less toxic soluble species. When a transformation to the zerovalent state is possible, this allows the recovery of the metal from the waters, with an important economical return. Three types of mechanisms can be considered, all of them taking place through successive thermodynamically allowed monoelectronic electron transfer steps: (a) direct reduction by photogenerated electrons; (b) indirect reduction by intermediates generated by hole or hydroxyl radical oxidation of electron donors (reducing radicals); (c) oxidative removal by holes or hydroxyl radicals. Fundamentals of oxidative or reductive heterogeneous photocatalysis are described, with special emphasis on the role of organic donors present in the medium acting as synergists. This indicates the possibility of simultaneous treatment of pollutants of different chemical structure and properties present in the same medium.