Welman Curi Elias

Heterogeneous photocatalysis has been studied for the treatment of recalcitrant compounds for half a century, but one of the severe limitations for practical oxidation treatment continues to be the inhibitory effect of salt found in real water matrices. The negative impact of Na+, K+, Cl–, and other ionic species is rarely quantified, as there has been no systematic way to discuss salt effects, making the evaluation of photocatalysis performance across studies difficult and imprecise. In analyzing 50 years of literature for this review, we identified >1,200 publications on heterogeneous photocatalysis in different water types, such as seawater, brackish water, and industrial brines. Just 84 studies reported degradation rates in deionized and salt-containing water, with nearly all involving TiO2 as the photocatalyst composition and charged dye compounds as the model contaminant class. To analyze the kinetic data extracted from these studies, we introduce the photocatalysis salt-effect (PSE) metric as calculated through the relative salt-effectiveness evaluation (RSEE) method, quantifying the percent change in degradation rate when the water matrix contains a salt species. We identified an unexpected correlation between PSE and ionic strength (IS), irrespective of the contaminant, at solution pH values less than the TiO2 point-of-zero-charge (PZC), which is the plausible result of salt-induced particle aggregation at high ionic strengths during photocatalyst testing. While most published results showed salt inhibition of photocatalysis (−100% < PSE < −5%), there is evidence for salt promotion (PSE > +5%) and salt tolerance (−5% < PSE < +5%), which challenges the notion that salinity is always detrimental to photocatalytic activity. These findings highlight the opportunity to study fully and systematically the relationships among contaminant structure, photocatalyst PZC, solution pH, IS, and salt type. The PSE-based framework would be critical in understanding the effects of more complex water matrices and in ultimately developing salt-tolerant and salt-promoted heterogeneous photocatalysis for practical water treatment.

Nitrate pollution of water, primarily due to widespread fertilizer use, is not only toxic to humans, but also leads to eutrophication. Catalytic destruction of nitrate using catalysts such as InPd has been extensively studied because of their high catalytic activity. However, their application in the treatment of real water remains largely underexplored still, as it is commonly assumed that cosolutes like chloride inhibit activity through competitive adsorption or irreversible poisoning. In this study, we found that chloride unexpectedly promotes NO3– reduction over InPd and InRh catalysts. NaCl concentrations (up to 2 M) had a volcano-shaped trend in InPd/C catalyst activity and increased InRh/C activity by 4-fold. In aqua surface-enhanced Raman spectroscopy using CO as a probe molecule showed that chloride has an electronic effect on catalysis, weakening the N–O bond of the crucial NO* reaction intermediate. Challenged with a simulated brine (ionic strength of 2.68 M) with 5000 mg-NO3–/L (1130 mg-N/L), both catalysts showed comparably high reduction activity. InRh/C was more selective to ammonium than InPd/C (90% vs 16% at 25% NO3– conversion). This study demonstrates how salt species can alter intermediate-catalyst interactions to surprisingly improve activity, which widens the prospects of hydrogenation catalysis for treating real water.

Boron nitride (BN) is a commercially available photocatalyst with demonstrated activity for degrading per- and polyfluoroalkyl substances (PFAS), but reactor-scale performance has not been reported. Here, we evaluate BN photocatalysis (BN + UV) in a 16 L recirculating reactor equipped with low-pressure mercury lamps emitting vacuum ultraviolet (VUV, 185 nm) and UV–C (254 nm) radiation. Using perfluorooctanoic acid (PFOA) as a model compound at mg/L concentrations, BN + UV achieved >99.8% removal within 1 h and 88% defluorination after 7 h, outperforming direct photolysis (UV-only; 92% removal, 30% defluorination) and TiO₂ photocatalysis (TiO₂ + UV; 63% removal, 10% defluorination) under identical conditions. A reactor-scale kinetic model yielded an intrinsic illuminated-zone rate constant (k_A) of 110 h^–1 for BN + UV, 11-fold greater than photolysis and 24-fold greater than TiO₂ + UV. BN + UV correspondingly presented the lowest electrical energy per order removed (E_EO = 1.82 kWh m^–3) and per order defluorinated (E_EOD = 6.68 kWh m^–3) based on lamp power. Furthermore, BN + UV degraded a 13-compound PFAS mixture in simulated semiconductor fabrication wastewater at μg/L levels. Long-chain compounds were completely removed within 24 h, while perfluorobutanesulfonic acid (PFBS) persisted, consistent with the known recalcitrance of short-chain sulfonates. These results demonstrate the energy efficiency of BN photocatalysis for destructive PFAS treatment in complex industrial matrices.

Highly ordered TiO₂ nanostructures, known as nanotube arrays (NTAs), exhibit potential in various energy, chemical sensing, and biomedical applications. Owing to its simplicity and high degree of control, titanium anodization serves as the prevailing NTA synthesis method. However, the practicality of this approach is marred by sluggish and inconsistent growth rates, on the order of 10 nm min⁻¹. Growth rates strongly depend on the electrolyte conductivity, yet most reports neglect to consider this property as a measured and controllable parameter. Here, we have systematically determined a broad set of conditions (at 60 V applied potential, elevated temperatures) that allow researchers to fabricate NTAs quickly and simply. By modulating conductivity through variation of bulk electrolyte temperature and the controlled addition of several hydroxy acid species, we achieve consistent accelerated growth up to 10 times faster than traditional methods. We find that regulating the solution conductivity within a desired region (e. g., ∼800–1000 μS cm⁻¹) enabled the fabrication of double‐sided NTA layers of around 10 μm and 90 μm NTA in 10 and 180 min, respectively.