Samuel D. Snow

The use of advanced oxidation processes (AOPs) for micropollutant removal in wastewater is challenging, given the scavenging effects of dissolved organic matter (DOM). Chlorine photolysis (UV/Cl) offers unique advantages over conventional AOPs, with wavelength- and pH-dependent generation of reactive species, while being largely compatible with existing disinfection dosing infrastructure. Here, the UV/Cl treatment of carbamazepine (CBZ) was optimized as a test case by varying UV wavelength and solution pH in the presence of humic acid and various types of wastewater DOM isolates (colloidal, hydrophobic, and transphilic). CBZ degradation rates, competitive quenching experiments, and electron paramagnetic resonance spectroscopy revealed important wavelength- and pH-dependent trade-offs in the dominant oxidants. At pH 3, CBZ degradation was hindered by DOM due to scavenging of nonselective radicals. At pH 8, ozone formation minimized DOM interference and improved CBZ removal. Shifting to longer wavelengths caused changes parallel to raising the pH. Three-dimensional excitation–emission matrix fluorescence analysis revealed preferential reactivity with fulvic- and humic-like fluorophores during treatment. Microtoxicity bioassays indicated a slight increase in acute toxicity after UV/Cl exposure, and LC–MS identified hydroxylated, epoxidized, and chlorinated transformation products. Results highlight the potential for strategic optimization of UV/Cl to mitigate DOM interference in water treatment.

Water scarcity is a growing global challenge. Developing energy-efficient and sustainable desalination methods is crucial to addressing the global water crisis. In recent years, hydrogel-based desalination has emerged as a promising approach due to the excellent water absorption capacity and stimuli-responsive properties of hydrogels. However, conventional dewatering methods for recovering water from hydrogels often suffer from low efficiency and poor stability. To address these challenges, a novel approach using ammonia gas to dewater poly(acrylic acid-co-acrylamide) hydrogels was investigated, achieving significantly higher dewatering rates (up to 110 g_H2O/g_hydrogel/h), among the highest reported, with water production rates ranging from 600 to 1140 L_H2O/kg_hydrogel/day and an average salt rejection of around 60% for both synthetic and real seawater water. The process also demonstrated excellent stability, maintaining a consistent performance over 100 cycles. This technology shows great potential for desalinating brackish water for agricultural irrigation or integration with reverse osmosis systems for seawater desalination, significantly reducing energy consumption and capital costs. This innovative dewatering method could enable practical applications of hydrogel-based desalination.

The efficient harnessing of solar power for water treatment via photocatalytic processes has long been constrained by the challenge of understanding and optimizing the interactions at the photocatalyst surface, particularly in the presence of nontarget cosolutes. The adsorption of these cosolutes, such as natural organic matter, onto photocatalysts can inhibit the degradation of pollutants, drastically decreasing the photocatalytic efficiency. In the present work, computational methods are employed to predict the inhibitory action of a suite of small organic molecules during TiO₂ photocatalytic degradation of para-chlorobenzoic acid (pCBA). Specifically, tryptophan, coniferyl alcohol, succinic acid, gallic acid, and trimesic acid were selected as interfering agents against pCBA to observe the resulting competitive reaction kinetics via bulk and surface phase reactions according to Langmuir-Hinshelwood adsorption dynamics. Experiments revealed that trimesic and gallic acids were most competitive with pCBA, followed by succinic acid. Density functional theory (DFT) and machine learning interatomic potentials (MLIPs) were used to investigate the molecular basis of these interactions. The computational findings showed that while the type of functional group did not directly predict adsorption affinity, the spatial arrangement and electronic interactions of these groups significantly influenced adsorption dynamics and corresponding inhibitory behavior. Notably, MLIPs, derived by fine-tuning models pretrained on a vastly larger dataset, enabled the exploration of adsorption behaviors over substantially longer periods than typically possible with conventional ab initio molecular dynamics, enhancing the depth of understanding of the dynamic interaction processes. Our study thus provides a pivotal foundation for advancing photocatalytic technology in environmental applications by demonstrating the critical role of molecular-level interactions in shaping photocatalytic outcomes.

Photocatalytic advanced oxidation processes (AOPs) promise a chemical-free route to energy-efficient degradation of waterborne micropollutants if long-standing mass transfer and light management issues can be overcome. Herein, we developed a dual-porous photocatalytic system consisting of a mesoporous (i.e., 2-50 nm pores) TiO₂ (P25) photocatalyst supported on macroporous (i.e., >50 nm pores) fused quartz fibers (P25/QF). Our reusable photocatalytic AOP reduces chemical consumption and exhibits excellent energy efficiency, demonstrated by degrading various pharmaceutical compounds (acetaminophen, sulfamethoxazole, and carbamazepine) in natural waters with electrical energy per order (E_EO) values of 4.07, 0.96, and 1.35 kWh/m³, respectively. Compared to the conventional H₂O₂/UVC AOP, our photocatalytic AOP can treat water without chemical additives while reducing energy consumption by over 2800%. We examine these improvements based on mass transport and optical (UVA and UVC) transmittance and demonstrate that the enhancements scale with increasing flow rate.

In the face of significant challenges to practical applications of photocatalysis for water treatment, recent reports revealed a potential route to overcome a problem posed by dissolved organic matter (DOM). These studies showed that inhibition of photocatalytic processes by DOM is driven largely by competition for active surface sites on TiO2 or other catalysts, and controlling the type of DOM present in solution could significantly mitigate DOM fouling. Whether or not control of solution parameters could achieve the same preventative action is not known. Here, a series of DOM isolates, including humic acid (HA) and transphilic (TPI), hydrophobic (HPO), or colloidal fractions of organic matter from a membrane bioreactor mixed liquor supernatant, were tested for inhibitory activity under a range of pH values (3, 5, 7, and 9) and ionic compositions (NaCl, CaCl2, and Al2(SO4)3 with ionic strengths (IS) ranging from 0 to 3 M). The resulting TiO2-DOM agglomerates were monitored for size and zeta- potential. Inhibitory profiles were generated using para-chlorobenzoic acid (pCBA) as probe with varying concentrations of inhibitory DOM for each solution condition to discern the extent of surface-phase quenching of radicals. Manipulation of pH clearly impacted inhibition, and the effect varied by DOM type; for example, interference occurred at all pHs for HA, at neutral or basic pHs for TPI, and only at pH 7 for HPO. Particle sizes did not correlate with inhibitory action of DOM. Increases in ionic strength induced growth of TiO2 and TiO2-DOM agglomerates, but again, particle sizes did not correlate to inhibition by DOM. The changes to IS, regardless of ion type, were not affected by the presence of TPI or HPO. Since particle stability did not correlate directly with photocatalytic activity, we suggest that surface-based quenching reactions arise from site-specific adsorption rather than generalized particle destabilization and aggregation.

Fullerene's low water solubility was a serious challenge to researchers aiming to harness their excellent photochemical properties for aqueous applications. Cationic functionalization of the fullerene cage provided the most effective approach to increase water solubility, but common synthesis practices inadvertently complicated the photochemistry of these systems by introducing iodide as a counterion. This problem was overlooked until recent work noted a potentiation effect which occurred when photosensitizers were used to inactivate microorganisms with added potassium iodide. In this work, several photochemical pathways were explored to determine the extent and underlying mechanisms of iodide's interference in the photosensitization of singlet oxygen by cationic fulleropyrrolidinium ions and rose bengal. Triplet excited state sensitizer lifetimes were measured via laser flash photolysis to probe the role of I^- in triplet sensitizer quenching. Singlet oxygen production rates were compared across sensitizers in the presence or absence of I^-, SO₄^2-, and other anions. 3,5-Dimethyl-1H-pyrazole was employed as a chemical probe for iodine radical species, such as I·, but none were observed in the photochemical systems. Molecular iodine and triiodide, however, were found in significant quantities when photosensitizers were irradiated in the presence of I^- and O₂. The formation of I₂ in these photochemical systems calls into question the interpretations of prior studies that used I^- as a counterion for photosensitizer materials. As an example, MS2 bacteriophages were inactivated here by cationic fullerenes with and without I^- present, showing that I^- moderately accelerated the MS2 deactivation, likely by producing I₂. Production of I₂ did not appear to be directly correlated with estimates of ¹O₂ concentration, suggesting that the relevant photochemical pathways are more complex than direct reactions between ¹O₂ and I^- in the bulk solution. On the basis of the results here, iodine photochemistry may be underappreciated and misunderstood in other environmental systems.

After decades of UV disinfection practice and numerous studies on the potential for pathogens to undergo dark or photo-repair after UV exposure, recent advances in UV light emitting diode (LED) technologies prompt renewed attention to bacterial reactivation and regrowth processes after UV exposure. The aspect of photorepair conditions warrants particular attention, because even studies on conventional mercury vapor lamps have not sufficiently characterized these parameters. Wastewater encounters a wide range of environmental conditions upon discharge (e.g., solar irradiation and dissolved organics) which may affect repair processes and ultimately lead to overestimations of pathogen removal. Escherichia coli was used here to investigate the impacts of changing reactivation conditions after UV₂₅₄ and UV₂₇₈ irradiation. UV₂₅₄ and UV₂₇₈ doses of 13.75 ± 0.4 mJ cm⁻² and 28.3 ± 0.8 mJ cm⁻² were required to induce a 3.0 log inactivation of E. coli, respectively. Specifically, photoreactivation conditions were varied across dissolved organic matter (DOM) content and photoreactivation wavelengths and intensities. Photoreactivation achieved higher log recoveries than dark repair, ranging from 0.8 to 1.8 log differences, but a secondary disinfection effect occurred under UVA irradiation. During photoreactivation, humic acid inhibited the initial repair of UV₂₇₈-dosed E. coli, but culture media enhanced recovery for both dosage wavelengths. Photoreactivation profiles under UV₃₉₅, UV₃₆₅, and visible light depended on both fluence and time, with more regrowth observed upon exposure to visible light and the least under 365 nm. The susceptibility of E. coli to UVA was increased by prior exposure to UVC.

Dissolved organic matter (DOM) poses a serious challenge to applied photocatalysis. Membranes may offer a promising synergistic opportunity to enable efficient photocatalysis in the presence of DOM. Membrane bioreactor (MBR) effluent from a municipal treatment plant was studied to elucidate the effects of filtration and organic matter composition on photocatalysis. Effluent samples were collected from MBR units during routine operation and before/after chemical cleaning. Additional DOM samples from the bulk supernatant were separated into colloidal, hydrophobic and transphilic fractions, providing a novel examination of the inhibition potential of DOM. These DOM fractions and the effluent organic matter (EfOM) samples were then characterized utilizing three-dimensional excitation–emission matrix (3DEEM) fluorescence spectroscopy and assayed for their potential to inhibit TiO₂-mediated photocatalytic degradation of a probe compound, para-chlorobenzoic acid (pCBA). The colloidal fraction of DOM was found to exert the strongest inhibition, followed by the transphilic, then the hydrophobic fractions; at 5 mgC L⁻¹, these fractions reduced the photodegradation rates by approximately 75%, 27%, and 17%, respectively. Of the effluent samples, EfOM from the recently-cleaned membrane caused the greatest inhibition of photocatalysis (∼100% reduction at 0.5 to 2.0 mgC L⁻¹), whereas the effluent from the fouled membrane provided the least inhibition (∼33% reduction at 2.0 mgC L⁻¹). The 3DEEM analysis predicted inhibitory action of both DOM and EfOM, based on total fluorescence volumes. Results here demonstrate the prospective utility of combining membrane technologies with photocatalytic processes.