Photocatalysis for arsenic removal from water: considerations for solar photocatalytic reactors

TiO2/Fe2O3 and Fe2O3/TiO2 heterojunction nanocomposites applied to As(III) decontamination

Arsenic contamination in water, particularly in its toxic form As(III), is a significant environmental issue in Brazil and globally. To address this, simple oxides of Fe2O3 and TiO2, along with heterojunction structures TiO2/Fe2O3 and Fe2O3/TiO2, were synthesized using an adapted Pechini method for the decontamination of As(III) via heterogeneous photocatalysis. TiO2 exhibited only the anatase phase, while Fe2O3 showed only the hematite phase (α-Fe2O3). The Fe2O3/TiO2 structure displayed both the hematite and anatase phases, whereas the TiO2/Fe2O3 heterojunction exhibited the anatase, rutile, hematite, and maghemite (γ-Fe2O3) phases. The materials displayed micro/mesoporous characteristics, with surface areas ranging from 20 to 45 m2 g-1, and band gap energies in the range of 2.1 to 3 eV. Hematite was the most effective adsorbent for arsenic. Under UV light irradiation, photolysis achieved 87% oxidation of As(III) in 20 min. The decontamination efficiencies achieved were 63%, 88%, 88%, and 99% for Fe2O3, TiO2, Fe2O3/TiO2, and TiO2/Fe2O3, respectively. Catalyst reuse tests demonstrated excellent stability, with all catalysts maintaining over 80% decontamination efficiency after four cycles. These results highlight the promising potential of TiO2/Fe2O3 heterojunctions for efficient and sustainable As(III) decontamination from contaminated water.

Solar-based technologies for removing potentially toxic metals from water sources: a review

Technological advances have led to a proportional increase in the deposition of contaminants across various environmental compartments, including water sources. Heavy metals, also known as potentially toxic metals, are of particular concern due to their significant harmful impacts on environmental and human health. Among the available methods for mitigating the threat of these metals in water, solar radiation-based technologies stand out for their cleanliness, cost-effectiveness, and efficiency in removing or reducing the toxicity of heavy metals. The performance and productivity of these methods in removing heavy metals such as arsenic (As), chromium (Cr), mercury (Hg), and uranium (U) from water still need to be comprehensively synthesized. Thus, this work aims to address that gap. The performance, potential, and challenges of real-world applications of conventional solar stills (CSS), membrane-based solar stills, and solar heterogeneous photocatalysis are concisely summarized and critically reviewed. CSS and membrane-based stills are highly effective (efficacy > 98%) in removing and capturing heavy metals from water. However, structural and functional improvements are needed to enhance productivity (especially for CSS) and usability in real-world environmental remediation and drinking water supply scenarios. Solar heterogeneous photocatalysis is highly effective in removing and/or converting As, Cr, Hg, and U into their non-toxic or less toxic forms, which subsequent processes can easily remove. Further research is necessary to evaluate the safety of photocatalytic materials, their integration into scalable solar reactors, and their usability in real-world environmental remediation applications.

Photocatalytic treatment of diverse contaminants of potential concern in oil sands process-affected water

Oil sands process-affected water (OSPW), generated by surface mining in Canada's oil sands, require treatment of environmentally persistent dissolved organic compounds before release to the watershed. Conventional chemical and mechanical treatments have not proved suitable for treating the large quantities of stored OSPW, and the biological recalcitrance of some dissolved organics may not be adequately addressed by conventional passive treatment systems. Previous work has evaluated photocatalytic treatment as a passive advanced oxidation process (P-AOP) for OSPW remediation. This work expands upon this prior research to further characterize the effects of water chemistry on the treatment rate and detoxification threshold. Under artificial sunlight, buoyant photocatalysts (BPCs) detoxified all OSPW samples within 1 week of treatment time with simultaneous treatment of polycyclic aromatic hydrocarbons, naphthenic acid fraction components (NAFCs), and un-ionized ammonia. Overall, these results further demonstrate passive photocatalysis as an effective method for treatment of OSPW contaminants of potential concern (COPCs).

Efficient wide-spectrum one-dimensional MWO4 (M = Mn, Co, and Cd) photocatalysts: Synthesis, characterization and density functional theory study

Broadening the absorption region to near-infrared (NIR) light is critical for the photocatalysis due to the larger proportion and stronger penetration of NIR light in solar energy. In the present paper, one-dimensional (1D) MWO4 (M = Mn, Co, and Cd) materials synthesized by electrospinning technique, were studied by combining the density functional theory (DFT) with experiment results, which possessed the enhanced light absorption capability within the range of 200-2000 nm. It was proved that in the ultraviolet-visible (UV-Vis) region, the absorption bands of CoWO4 and MnWO4 samples were attributed to the metal-to-metal charge transfer mechanism, while the absorption of CdWO4 sample may be referable to the ligand-to-metal charge transfer mechanism. In the near-infrared (NIR) region, the absorption of CoWO4 and MnWO4 primarily originated from the d-d orbital transitions of Mn2+ and Co2+. The photocatalytic experimental results showed that the degradation rates for bisphenol A (BPA) over CoWO4, MnWO4, and CdWO4 photocatalysts under UV-Vis/NIR light irradiation for 140 min/12 h were 78.8 %/75.9 %, 23.8 %/21.3 %, 12.8 %/8.7 %, respectively. This research offers the novel insights into the precise construction of tungstate catalytic systems and contributes to the advancement of UV-Vis-NIR full spectrum photocatalytic technology, and lays a foundation for a cleaner and more environmental-friendly future.

Comparative Efficiencies for Phenol Degradation on Solar Heterogeneous Photocatalytic Reactors: Flat Plate and Compound Parabolic Collector

Phenol is a recalcitrant anthropogenic compound whose presence has been reported in both wastewater and drinking water; human exposure to phenolic substances can lead to health problems. The degradation of phenol (measured as COD decrease) through solar heterogeneous photocatalysis with immobilized TiO2 was performed in two different reactors: a flat-plate reactor (FPR) and a compound parabolic collector (CPC). A 23 full factorial experimental design was followed. The variables were the presence of TiO2, H2O2 addition, and the type of reactor. Data were fitted to the pseudo-first-order reaction-rate-kinetics model. The rate constant for photocatalytic phenol degradation with 1 mM of H2O2 was 6.6 × 10−3 min−1 for the FPR and 5.9 × 10−3 min−1 in the CPC. The calculated figures of merit were analyzed with a MANCOVA, with UV fluence as a covariate. An ANCOVA showed that the type of reactor, H2O2 addition, or fluence had no statistically significant effect on the results, but there was for the presence of TiO2. According to the MANCOVA, fluence and TiO2 presence were significant (p < 0.05). The CPC was on average 17.4% more efficient than the FPR when it came to collector area per order (ACO) by heterogeneous photocatalysis and 1 mM H2O2 addition.