Applicability of light sources and the inner filter effect in UV/acetylacetone and UV/H2O2 processes

Unravelling structural features of small molecules for photochemical transformation of environmental contaminants

Small molecules, including natural metabolites, organic matter decomposition products, and engineered oxidation byproducts, are widespread in aquatic environment. However, the limited understanding of the photochemical interactions of these small molecules with water pollutants hampers the development of effective environmental protection strategies. This study explores the structural features governing the photochemical transformation of toxic oxyanions by α- and β-dicarbonyl compounds. By integrating experimental observations with quantum chemical calculations, a robust correlation network was constructed. The correlation network reveals that the reactivity of small organic molecules with oxyanions could be quantitively predicted by their intrinsic properties, such as electronic transition energy, bond dissociation energy, molecular softness, molecular orbital gap, atomic charge, and molecular surface local ionization energy. This network maps the relationship between the molecular architecture of chemicals and their photochemical behaviors. This perspective offers fresh insights into the photochemical behaviors of small molecules in diverse environmental and chemical contexts and are helpful for developing advanced water treatment strategies toward a sustainable future.

What is the role of nitrate/nitrite in trace organic contaminants degradation and transformation during UV-based advanced oxidation processes?

The effectiveness of UV-based advanced oxidation processes (UV-AOPs) in degrading trace organic contaminants (TrOCs) can be significantly influenced by the ubiquitous presence of nitrate (NO3-) and nitrite (NO2-) in water and wastewater. Indeed, NO3-/NO2- can play multiple roles of NO3-/NO2- in UV-AOPs, leading to complexities and conflicting results observed in existing research. They can inhibit the degradation of TrOCs by scavenging reactive species and/or competitively absorbing UV light. Conversely, they can also enhance the elimination of TrOCs by generating additional •OH and reactive nitrogen species (RNS). Furthermore, the presence of NO3-/NO2- during UV-AOP treatment can affect the transformation pathways of TrOCs, potentially resulting in the nitration/nitrosation of TrOCs. The resulting nitro(so)-products are generally more toxic than the parent TrOCs and may become precursors of nitrogenous disinfection byproducts (N-DBPs) upon chlorination. Particularly, since the impact of NO3-/NO2- in UV-AOPs is largely due to the generation of RNS from NO3-/NO2- including NO•, NO2•, and peroxynitrite (ONOO-/ONOOH), this review covers the generation, properties, and detection methods of these RNS. From kinetic, mechanistic, and toxicologic perspectives, future research needs are proposed to advance the understanding of how NO3-/NO2- can be exploited to improve the performance of UV-AOPs treating TrOCs. This critical review provides a comprehensive framework outlining the multifaceted impact of NO3-/NO2- in UV-AOPs, contributing insights for basic research and practical applications of UV-AOPs containing NO3-/NO2-.

The role of ozone combined with UVC/H2O2 process for the tertiary treatment of a real slaughterhouse wastewater

The main goal of this work is to evaluate the usage of ozone (O₃) as a pre-treatment or simultaneously combined with UVC/H₂O₂ process for the polishing stage treatment of real bio-treated slaughterhouse wastewater. Two different treatment strategies were tested: i) pre-ozonation of the wastewater followed by an UVC/H₂O₂ process (two-step treatment); ii) simultaneous application of O₃/UVC/H₂O₂ combined process (one-step treatment). For the two-step strategy, the pre-treatment with 30 mg O₃/min for 10 min reduces significantly total suspended solids (TSS), turbidity and colour, reducing light filtering effects and increasing the efficiency of the following UVC/H₂O₂ process. In turn, the one-step treatment strategy (O₃/UVC/H₂O₂) allows a more efficient use of injected O₃ by reducing the amount of O₃ required (from 273 to 189 mg O₃/L_effluent) to achieve similar mineralization levels. The real bio-treated slaughterhouse wastewater treated by O₃/UVC/H₂O₂ process achieved final colour values of 20 Pt/Co, TSS of 35 mg/L and COD of 61 mg O₂/L, allowing its direct discharge into water compartments according to European Council Directive 91/271/EEC.

Key structural features that determine the selectivity of UV/acetylacetone for the degradation of aromatic pollutants when compared to UV/H2O2

Acetylacetone (AA) has proven to be a potent photo-activator for the decolorization of dyes. However, there is very limited information on the quantitative structure-activity relationship (QSAR) and the mechanisms of dye degradation by UV/AA. Herein, the photolysis of 65 aromatic compounds (dyes and dye precursors) was investigated at three pH values (4.0, 6.0, 9.0) by UV/AA and UV/H₂O₂. The obtained pseudo-first-order photodegradation rate constants (k₁) were processed using statistical analysis. The correlation between the k₁ values and the number of photons absorbed by AA, together with the observed pH effect, suggested that the protonated enol structure of AA plays a crucial role in the photodecolorization of dyes. According to quantum chemical computation, photo-induced direct electron transfer between the excited state of AA and the dye was the main mechanism in the UV/AA process. QSAR models demonstrated that the molecular size and stability were the key factors that determined the efficiency of UV/H₂O₂ for dye degradation. Statistically, the UV/AA process was target-selective and suffered less from the inner filter effect, which made it more effective than the UV/H₂O₂ process for dye degradation. The selectivity of the UV/AA process was mainly embodied in the substituent effects: dyes with hydroxyl groups in conjugated systems decomposed faster than those with nitro-substitution or ortho-substituted sulfonate groups. The results can be used for the selection of appropriate photochemical approaches for the treatment of dye-contaminated water.

Performance of UV/acetylacetone process for saline dye wastewater treatment: Kinetics and mechanism

Futility of traditional advanced oxidation processes (AOPs) in saline wastewater treatment has stimulated the quest for novel "halotolerant" chemical oxidation technology. Acetylacetone (AA) has proven to be a potent photo-activator in the degradation of dyes, but the applicability of UV/AA for saline wastewater treatment needs to be verified. In this study, degradation of crystal violet (CV) was investigated in the UV/AA system in the presence of various concentrations of exogenic Cl^- or Br^-. The results reveal that degradation, mineralization and even accumulation of adsorbable organic halides (AOX) were not significantly affected by the addition of Cl^- or Br^-. Rates of CV degradation were enhanced by elevating either AA dosage or solution acidity. An apparent kinetic rate equation was developed as r = -d[CV]/dt = k[CV]^a[AA]^b = (7.34 × 10^-4 mM^1-(a+b) min^-1) × [CV]^a=0.16 [AA]^b=0.97. In terms of results of radical quenching experiments, direct electron/energy transfer is considered as the major reaction mechanism, while either singlet oxygen or triplet state (³(AA)*) might be involved. Based on identification of degradation byproducts, a possible degradation pathway of CV in the UV/AA system is proposed.

Role of complexation in the photochemical reduction of chromate by acetylacetone

Organic ligands can alter the redox behavior of metal species through the generation of metal-ligand complexes. Photo-induced complexation between ligands and metals is an important, but under-appreciated, aspect of process. Acetylacetone (AA) is a good chelating agent due to keto-enol tautomerization. In the presence of AA, photoreduction of Cr(VI) is accelerated; however, it is unclear exactly how complexation is involved in UV/AA mediated Cr(VI) reduction. On the basis of spectral and kinetic analyses, this study shows that the formation of {Cr(VI)-AA}* complexes is the main mechanism of Cr(VI) reduction by UV/AA. Evidence for this includes (1) the formation rate constant of Cr(III)-AA complexes in the UV system was 2-3 orders of magnitude greater than that in the thermal system; (2) there was a linear relationship between the photons absorbed by AA and the reduction rate constants of Cr(VI); and (3) the reaction appeared initially zero-order in Cr(VI) and turned to first-order as the pool of available Cr(VI) ran out. The results presented here are not only important for the better understanding of the complexation effects in the reduction of Cr(VI), but also crucial for the possible application of the UV/AA process in many other scenarios.

Visible light activation of persulfate by magnetic hydrochar for bacterial inactivation: Efficiency, recyclability and mechanisms

The development of "green" water disinfection technology utilizing solar energy is highly desired but remains challenging. In this study, sulfate radical (•SO₄^-)-mediated bacterial inactivation was first attempted by using Fe₃O₄-based magnetic hydrochar (MHC) as a recyclable catalyst for persulfate (PS) activation under visible light (VL) irradiation. Complete treatment of 8.0 log E. coli cells was reached within 40 min in VL/PS/MHC system, compared with that of only 2.0 log-reduction was obtained in the PS/MHC system under the same conditions. The system was applicable in wide range of pH (3.0-9.0), and increasing dissolved O₂ could further promote the efficiency. A three-route mechanism was proposed, in which the PS activation by ≡Fe(II) of Fe₃O₄ and photo-generated electron captured by PS were the major processes. The bacterial cell lesion process was found to be triggered directly via •SO₄^-, which caused the damage of outer membrane, followed by up-regulation of intracellular ROSs and destroy of chromosomal DNA, finally leading to irreversible cell death. Moreover, the VL/PS/MHC system is also effective to inactivate versatile pathogenic bacteria including P. aeruginosa and S. aureus. As a proof-of-concept, our study provides meaningful information to advance the areas of "green" water disinfection technology which can be realized by recyclable photocatalytic systems using solar energy.

Enhanced Photooxidation of Hydroquinone by Acetylacetone, a Novel Photosensitizer and Electron Shuttle

Quinones are important electron shuttles as well as micropollutants in the nature. Acetylacetone (AA) is a newly recognized electron shuttle in aqueous media exposed to UV irradiation. Herein, we studied the interactions between AA and hydroquinone (QH₂) under steady-state and transient photochemical conditions to clarify the possible reactions and consequences if QH₂ and AA coexist in a solution. Steady-state experimental results demonstrate that the interactions between AA and QH₂ were strongly affected by dissolved oxygen. In O₂-rich solutions, the phototransformation of QH₂ was AA-independent. Both QH₂ and AA utilize O₂ as the electron acceptor, but in O₂-insufficient solutions, AA became an important electron acceptor for the oxidation of QH₂. In all cases, the coexistence of AA increased the phototransformation of QH₂, whereas the decomposition of AA in O₂-saturated and oversaturated solutions was inhibited by the presence of QH₂. The underlying mechanisms were investigated by a combination of laser flash photolysis (LFP) and reduction potential analysis. The LFP results show that the excited AA serves as a better electron shuttle than QH₂. As a consequence, AA might regulate the redox cycling of quinones, leading to significant effects on many processes, ranging from photosynthesis and respiration to photodegradation.

A novel combined process for efficient removal of Se(VI) from sulfate-rich water: Sulfite/UV/Fe(III) coagulation

The efficient removal of Se(VI) from sulfate-rich water is challenging as most reported processes last for hours to days. In this study, a combined sulfite/UV/Fe(III) coagulation process was proposed for efficient Se(VI) removal from sulfate-rich water within a short time (∼1 h). In the presence of sulfate (1000 mg L^-1), over 99% of Se(VI) (initially at 10 mg L^-1) could be reduced by sulfite (5.0 mM) with a UV dose of 16 J cm^-2 (within 20 min) into Se(IV) as the sole observed product. An alkaline pH (>9) was required for the reduction process, which was naturally obtained with the addition of sulfite. Scavenging experiments with N₂O and NO₃^- both indicated that hydrated electrons (e_aq^-) were responsible for Se(VI) reduction by sulfite/UV. The presence of chloride, sulfate, phosphate, and carbonate (up to 10 mM) showed negligible influence on Se(VI) reduction, whereas nitrate and humic acid inhibited Se(VI) reduction to different extents depending on their concentrations. By Fe(III) coagulation, Se(IV) in the co-presence of sulfite and sulfate was efficiently removed at an OH^-/Fe molar ratio of 1.8-2.8. The removal of Se(IV) by Fe(III) coagulation responded insignificantly to chloride, nitrate, or sulfate (up to 10 mM), whereas it was adversely affected at high levels of carbonate (10 mM) and phosphate (1 mM). The combined sulfite/UV/Fe(III) coagulation process was validated for the efficient removal of Se(VI) from synthetic sulfate-rich solution, simulated wastewater, and authentic smelting wastewater (in 1.1 h). The introduced sulfite underwent minor consumption during UV irradiation and was almost (∼90%) removed after coagulation.

Wavelength-dependent chlorine photolysis and subsequent radical production using UV-LEDs as light sources

UV-LEDs are considered as the most promising UV light sources, because it has the potential to replace conventional UV lamps in some water treatment applications in the foreseeable future. In this study, UV-LEDs at four wavelengths in the UV-C or near UV-C range (i.e., 257.7, 268, 282.3, and 301.2 nm) were used to investigate the wavelength-dependency on chlorine photolysis and its subsequent radical formation. The fluence-based photodecay rates of hypochlorous acid (HOCl) and hypochlorite (OCl^-) were monotonically correlated to their molar absorption coefficients and quantum yields, and the chlorine photodecay rates were much more significantly affected by molar absorption coefficients (β  = 0.949) than quantum yields (β  = 0.055). An empirical model that incorporated the chlorine photodecay rate constants, quantum yields, and molar absorption coefficients of HOCl and OCl^- was established, validated and then used to predict the chlorine photodecay rate at any wavelength (257.7-301.2 nm) and pH (5-10). The modelling results suggested that the maximum fluence-based rate constant (1.46 × 10^-4 m² J^-1) was obtained at 289.7 nm and pH 9.95. The wavelength dependency was larger at alkaline pH than at acidic pH, and the pH dependency was the largest at the longest wavelength. The formation of hydroxyl radicals (HO·) and reactive chlorine species (RCS) decreased with increasing wavelength at pH 6, and increased with increasing wavelength at pH 7. More HO· was formed at pH 6 than pH 7, but RCS showed the opposite pH-dependency. The findings in this study provide the fundamental information in selecting UV-LEDs with specific wavelength for enhancing/optimizing chlorine photodecay and/or its radical generation at different pHs in real-world applications.

Feasibility of the UV/AA process as a pretreatment approach for bioremediation of dye-laden wastewater

Biodegradability and toxicity are two important indexes in considering the feasibility of a chemical process for environmental remediation. The acetylacetone (AA) mediated photochemical process has been proven as an efficient approach for dye decolorization. Both AA and its photochemical degradation products had a high bioavailability. However, the biocompatibility and ecotoxicology of the UV/AA treated solutions are unclear yet. In the present work, we evaluated the biocompatibility and toxicity of the UV/AA treated solutions at both biochemical and organismal levels. The biodegradability of the treated solution was evaluated with the ratio of 5-d biological oxygen demand (BOD₅) to chemical oxygen demand (COD) and a 28-d activated sludge assay (Zahn-Wellens tests). The UV/AA process significantly improved the biodegradability of the tested dye solutions. Toxicity was assessed with responses of microorganisms (microbes in activated sludge and Daphnia magna) and plants (bok choy, rice seed, and Arabidopsis thaliana) to the treated solutions, which showed that the toxicity of the UV/AA treated solutions was lower or comparable to that of the UV/H₂O₂ counterparts. The results are helpful for us to determine whether the UV/AA process is applicable to certain wastewaters and how the UV/AA process could be effectively combined into a sequential chemical-biological water treatment.

Scattering and absorption differ drastically in their inner filter effects on fluorescence, resonance synchronous, and polarized resonance synchronous spectroscopic measurements

Reported herein is the finding that photon scattering and absorption differ drastically in inducing the sample IFE in SSF, RS2, and the PRS2 spectra measurements.