Energy efficiency for the removal of non-polar pollutants during ultraviolet irradiation,visible light photocatalysis and ozonation of a wastewater effluent

This study aims to assess the removal of a set of non-polar pollutants in biologically treated wastewater using ozonation, ultraviolet (UV 254 nm low pressure mercury lamp) and visible light (Xe-arc lamp) irradiation as well as visible light photocatalysis using Ce-doped TiO₂. The compounds tracked include UV filters, synthetic musks, herbicides, insecticides, antiseptics and polyaromatic hydrocarbons. Raw wastewater and treated samples were analyzed using stir-bar sorptive extraction coupled with comprehensive two-dimensional gas chromatography (SBSE–CG × GC–TOF–MS). Ozone treatment could remove most pollutants with a global efficiency of over 95% for 209 μM ozone dosage. UV irradiation reduced the total concentration of the sixteen pollutants tested by an average of 63% with high removal of the sunscreen 2-ethylhexyl trans-4-methoxycinnamate (EHMC), the synthetic musk 7-acetyl-1,1,3,4,4,6-hexamethyltetrahydronaphthalene (tonalide, AHTN) and several herbicides. Visible light Ce–TiO₂ photocatalysis reached ∼70% overall removal with particularly high efficiency for synthetic musks. In terms of power usage efficiency expressed as nmol kJ⁻¹, the results showed that ozonation was by far the most efficient process, ten-fold over Xe/Ce–TiO₂ visible light photocatalysis, the latter being in turn considerably more efficient than UV irradiation. In all cases the efficiency decreased along the treatments due to the lower reaction rate at lower pollutant concentration. The use of photocatalysis greatly improved the efficiency of visible light irradiation. The collector area per order decreased from 9.14 ± 5.11 m² m⁻³ order⁻¹ for visible light irradiation to 0.16 ± 0.03 m² m⁻³ order⁻¹ for Ce–TiO₂ photocatalysis. The toxicity of treated wastewater was assessed using the green alga Pseudokirchneriella subcapitata. Ozonation reduced the toxicity of treated wastewater, while UV irradiation and visible light photocatalysis limited by 20–25% the algal growth due to the accumulation of reaction by-products. Three transformation products were identified and tracked along the treatments.     

Inactivation/reactivation of antibiotic-resistant bacteria by a novel UVA/LED/TiO2 system

In this study, an effective photocatalytic disinfection system was established using the newly emerged high power UVA/LED lamp. Crystallizing dish coated with TiO₂ was prepared by 32-times impregnation-drying processes, and served as the supporting container for water samples. This study focused on the application of this UVA/LED system for the photocatalytic disinfection of selected antibiotic-resistant bacteria, Escherichia coli ATCC 700891. The disinfection performances were studied under various light intensities and illumination modes. Results show that higher light intensity could reach more significant inactivation of E. coli ATCC 700891. With the same UV dose, log-removal of antibiotic-resistant bacteria decreased with circle time in the studied range, while increased with duty circle. A “residual disinfecting effect” was found in the following dark period for bacteria collected at different phases of photocatalytic process. Residual disinfecting effect was found not significant for bacteria with 30 min periodic illumination. While residual disinfecting effect could kill almost all bacteria after 90 min UV periodic illumination within the following 240 min dark period.     

Photooxidation of the antidepressant drug Fluoxetine (Prozac®) in aqueous media by hybrid catalytic/ozonation processes

This article examines the oxidative disposal of Prozac^® (also known as Fluoxetine, FXT) through several oxidative processes with and without UV irradiation: for example, TiO₂ alone, O₃ alone, and the hybrid methods comprised of O₃ + H₂O₂ (PEROXONE process), TiO₂ + O₃ and TiO₂ + O₃ + H₂O₂ at the laboratory scale. Results show a strong pH dependence of the adsorption of FXT on TiO₂ and the crucial role of adsorption in the whole degradation process. Photolysis of FXT is remarkable only under alkaline pH. The heterogeneous photoassisted process removes 0.11 mM FXT (initial concentration) within ca. 60 min with a concomitant 50% mineralization at pH 11 (TiO₂ loading, 0.050 g L⁻¹). The presence of H₂O₂ enhances the mineralization further to >70%. UV/ozonation leads to the elimination of FXT to a greater extent than does UV/TiO₂: i.e., 100% elimination of FXT is achieved by UV/O₃ in the first 10 min of reaction and almost 97% mineralization is attained under UV irradiation in the presence of H₂O₂. The hybrid configuration UV + TiO₂ + O₃ + H₂O₂ enhances removal of dissolved organic carbon (DOC) in ca. 30 min leaving, however, an important inorganic carbon (IC) content. In all cases, the presence of H₂O₂ improves the elimination of DOC, but not without a detrimental effect on the biodegradability of FXT owing to the low organic carbon content in the final treated effluent, together with significant levels of inorganic byproducts remaining. The photoassisted TiO₂/O₃ hybrid method may prove to be an efficient combination to enhance wastewater treatment of recalcitrant drug pollutants in aquatic environments.     

UV-induced photochemical heterogeneity of dissolved and attached organic matter associated with cyanobacterial blooms in a eutrophic freshwater lake

Cyanobacterial blooms represent a significant ecological and human health problem worldwide. In aquatic environments, cyanobacterial blooms are actually surrounded by dissolved organic matter (DOM) and attached organic matter (AOM) that bind with algal cells. In this study, DOM and AOM fractionated from blooming cyanobacteria in a eutrophic freshwater lake (Lake Taihu, China) were irradiated with a polychromatic UV lamp, and the photochemical heterogeneity was investigated using fluorescence excitation–emission matrix (EEM)-parallel factor (PARAFAC) analysis and synchronous fluorescence (SF)-two dimensional correlation spectroscopy (2DCOS). It was shown that a 6-day UV irradiation caused more pronounced mineralization for DOM than AOM (59.7% vs. 41.9%). The EEM-PARAFAC analysis identified one tyrosine-, one humic-, and two tryptophan-like components in both DOM and AOM, and high component photodegradation rates were observed for DOM versus AOM (k > 0.554 vs. <0.519). Moreover, SF-2DCOS found that the photodegradation of organic matters followed the sequence of tyrosine-like > humic-like > tryptophan-like substances. Humic-like substances promoted the indirect photochemical reactions, and were responsible for the higher photochemical rate for DOM. The lower photodegradation of AOM benefited the integrality of cells in cyanobacterial blooms against the negative impact of UV irradiation. Therefore, the photochemical behavior of organic matter was related to the adaptation of enhanced-duration cyanobacterial blooms in aquatic environments.     

Photocatalytic destruction of anionic SAS by ozone and oxygen

The article has investigated photocatalytic destruction of anionic SAS—sodium alkylbenzene sulfate (ABS) in the aqueous medium by ozone on TiO₂ Degussa P-25 at UV radiation by a DB-15 low-pressure mercury-quartz lamp. The paper has determined the parameters of photocatalytic ozonization securing complete ABS destruction (100% in terms of organic carbon). We have assessed advantages of photocatalytic ozonization of ABS solutions compared with photocatalytic oxidation by oxygen and O₃/UV treatment.     

A photoelectrocatalytic process that disinfects water contaminated with Mycobacterium kansasii and Mycobacterium avium

Nontuberculous mycobacteria are resistant to conventional water treatment; indeed, they have been recovered from a wide variety of environmental sources. Here, we applied the photoelectrocatalytic technique using a Ti/TiO₂–Ag photoanode to inactivate mycobacteria. For a mycobacteria population of 5 × 10⁸ CFU mL⁻¹, we achieved 99.9 and 99.8% inactivation of Mycobacterium kansasii and Mycobacterium avium with rate constant of 6.2 × 10⁻³ and 4.2 × 10⁻³ min⁻¹, respectively, after 240 min. We compared the proposed method with the photolytic and photocatalytic methods. Using a mycobacteria population of 7.5 × 10⁴ CFU mL⁻¹, the proposed Ti/TiO₂–Ag photoanode elicited total mycobacteria inactivation within 3 min of treatment; the presence of Ag nanoparticles in the electrode provided 1.5 larger degradation rate constant as compared with the Ti/TiO₂ anode (1.75 × 10⁻² for M. kansassi and 1.98 × 10⁻² for M. avium). We monitored the degradation of the metabolites released during cellular lysis by TOC removal, sugar release, chromatography, and mass spectrometry measurements; photoelectrocatalysis and Ti/TiO₂–Ag photoanodes furnished the best results.     

Oxidative and photochemical processes for the removal of galaxolide and tonalide from wastewater

Synthetic musks have been reported in wastewaters at concentrations as high as tens of micrograms per litre. The two most significant polycyclic musk fragrance compounds are 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta(g)-2-benzopyran (HHCB, trade name galaxolide^®) and 7-acetyl-1,1,3,4,4,6-hexamethyltetrahydronaphthalene (AHTN, trade name tonalide^®). We report the result of several irradiation and advanced oxidation processes carried out on samples of the effluent of a wastewater treatment plant located in Alcalá de Henares, Madrid. Wastewater samples were pre-ozonated and spiked with 500 ng/L of tonalide or galaxolide in order to obtain final concentrations in the same order as the raw effluent. The treatments assayed were ozonation with and without the addition of hydrogen peroxide (O₃, O₃/H₂O₂), ultraviolet (254 nm low pressure mercury lamp) and xenon-arc visible light irradiation alone and in combination with ozone (UV, O₃/UV, Xe, O₃/Xe) and visible light photocatalytic oxidation using a Ce-doped titanium dioxide photocatalyst performed under continuous oxygen or ozone gas bubbling (O₂/Xe/Ce-TiO₂, O₃/Xe/Ce-TiO₂). In all cases, samples taken at different contact times up to 15 min were analyzed. An analytical method based on stir bar sorptive extraction (SBSE), followed by comprehensive two-dimensional gas chromatography (SBSE-GC × GC-TOF-MS), was used for the automatic searching and evaluation of the synthetic musks and other nonpolar or semipolar contaminants in the wastewater samples. In all cases tonalide was more easily removed than galaxolide. The best results for the latter (more than 75% removal after 5 min on stream) were obtained from ozonation (O₃) and visible light photocatalytic ozonation (O₃/Xe/Ce-TiO₂). A significant removal of both pollutants (∼60% after 15 min) was also obtained during visible light photocatalysis (O₂/Xe/Ce-TiO₂). UV radiation was able to deplete tonalide (+90%) after 15 min but only reduced the concentration of galaxolide to about half of its initial concentration. The toxicity of treated samples decreased for O₃/UV and O₃/Ce-TiO₂, but increased during irradiation processes UV, Xe and Xe/Ce-TiO₂. Ozone treatments tend to decrease toxicity up to a certain dosage, from which point the presence of toxic transformation products has adverse effects on aquatic microorganisms.     

Coupling membrane separation and photocatalytic oxidation processes for the degradation of pharmaceutical pollutants

The coupling of membrane separation and photocatalytic oxidation has been studied for the removal of pharmaceutical pollutants. The retention properties of two different membranes (nanofiltration and reverse osmosis) were assessed. Comparable selectivity on the separation of pharmaceuticals were observed for both membranes, obtaining a permeate stream with concentrations of each pharmaceutical below 0.5 mg L⁻¹ and a rejected flux highly concentrated (in the range of 16–25 mg L⁻¹ and 18–32 mg L⁻¹ of each pharmaceutical for NF-90 and BW-30 membranes, respectively), when an initial stream of six pharmaceuticals was feeding to the membrane system (10 mg L⁻¹ of each pharmaceutical). The abatement of concentrated pharmaceuticals of the rejected stream was evaluated by means of heterogeneous photocatalytic oxidation using TiO₂ and Fe₂O₃/SBA-15 in presence of hydrogen peroxide as photo-Fenton system. Both photocatalytic treatments showed remarkable removals of pharmaceutical compounds, achieving values between 80 and 100%. The nicotine was the most refractory pollutant of all the studied pharmaceuticals. Photo-Fenton treatment seems to be more effective than TiO₂ photocatalysis, as high mineralization degree and increased nicotine removal were attested. This work can be considered an interesting approach of coupling membrane separation and heterogeneous photocatalytic technologies for the successful abatement of pharmaceutical compounds in effluents of wastewater treatment plants.     

Multi-wavelength spectroscopic and chromatography study on the photocatalytic oxidation of natural organic matter

The effect of TiO₂ photocatalytic oxidation on the natural organic matter (NOM) properties of two Australian surface waters were quantified using UV-vis spectroscopy, high performance size exclusion chromatography (HPSEC) with a multi-wavelength UV detector, liquid chromatography with organic carbon detector (LC-OCD), and trihalomethane formation potential (THMFP) analyses. Both the UV absorbance at wavelengths greater than 250 nm and dissolved organic carbon (DOC) content decreased significantly with treatment, although complete mineralization of NOM could not be achieved. Multi-wavelength UV detection of HPSEC analysis was shown to be useful to display further changes to NOM composition and molecular weight profiles because the organic molecules was transformed into compounds that absorb weakly at the typical detection wavelength of 250-260 nm. The multi-wavelength HPSEC results also revealed that photocatalytic oxidation yields by-products with a low aromaticity and low molecular weight. The LC-OCD chromatograms indicated that low molecular acids and neutral compounds remained after photocatalytic oxidation. Those groups of compounds did not seem to contribute significantly to the formation of trihalomethanes.     

Effects of water parameters on the degradation of microcystin-LR under visible light-activated TiO2 photocatalyst

A study was performed to determine the effect of pH, alkalinity, natural organic matter (NOM) and dissolved oxygen in the performance of nitrogen and fluorine doped TiO₂ (NF-TiO₂) for the degradation of hepatotoxin microcystin-LR (MC-LR) in synthetic and natural water under visible light irradiation. The initial degradation rate of MC-LR was fastest under acidic conditions (3.50 ± 0.02 × 10⁻³ μM min⁻¹ at pH 3.0) and decreased to 2.29 ± 0.07 × 10⁻³ and 0.54 ± 0.02 × 10⁻³ μM min⁻¹ at pH 5.7 and 7.1, respectively. Attractive forces between the opposite charged MC-LR and NF-TiO₂ are likely responsible for the enhancement in the photocatalytic decomposition of MC-LR resulting from increased interfacial adsorption. For carbonate buffered solutions, the photocatalytic activity of NF-TiO₂ was reduced when increasing the carbonate concentration up to 150 mg CaCO₃ L⁻¹. The scavenging of radical species by the bicarbonate ion at pH 7.1 is discussed. In the presence of NOM, the degradation rates decreased as pH and initial concentration of the NOM increased. The inhibition was higher with fulvic acid than humic acid under alkaline conditions. Oxygenated solution yields higher NF-TiO₂ photocatalytic degradation of MC-LR compared to nitrogen sparged solution at pH 5.7. The involvement of specific reactive oxygen species implicated in the photodegradation is proposed. Finally, no significant degradation is observed with various natural waters spiked with MC-LR under visible light (λ > 420 nm) but high removal was achieved with simulated solar light. This study provides a better understanding of the interactions and photocatalytic processes initiated by NF-TiO₂ under visible and solar light. The results indicate solar photocatalytic oxidation is a promising technology for the treatment of water contaminated with cyanotoxins.     

Anatoxin-a degradation by Advanced Oxidation Processes: Vacuum-UV at 172 nm, photolysis using medium pressure UV and UV/H2O2

Two Advanced Oxidation Processes, namely vacuum-ultraviolet (VUV) photolysis at 172 nm and ultraviolet/hydrogen peroxide (UV/H₂O₂) were investigated for the degradation of anatoxin-a in aqueous solutions. Solutions of anatoxin-a-fumarate were treated with VUV light at 172 nm with a UV dose of 200 mJ/cm², where fumaric acid served as a reference compound for a competition kinetics analysis. The second-order rate constant for the reaction between anatoxin-a and the hydroxyl radical was found to be (5.2 ± 0.3) × 10⁹ M⁻¹ s⁻¹ and was independent of pH, temperature, and initial concentration of anatoxin-a. The direct photolysis of anatoxin-a using a medium pressure (MP) UV lamp was also investigated, in which case a UV dose of 1285 mJ/cm² was required to degrade anatoxin-a by 88% and 50% at concentrations of 0.6 mg/L and 1.8 mg/L of toxin, respectively. Treatment of anatoxin-a with a low pressure (LP) UV lamp in the presence of 30 mg/L of H₂O₂ was examined, where it was found that more than 70% of toxin could be degraded at a UV dose of 200 mJ/cm². The degradation arises from the oxidation of the toxin by hydroxyl radicals. The addition of H₂O₂ clearly enhanced the degradation of anatoxin-a, up to a concentration of 40 mg/L, after which addition of more H₂O₂ had little effect on the degradation kinetics of anatoxin-a. The effect of background constituents in the water on the degradation of anatoxin-a was also investigated using natural and synthetically produced model waters.     

The effect of UV/H2O2 treatment on disinfection by-product formation potential under simulated distribution system conditions

Advanced oxidation with ultraviolet light and hydrogen peroxide (UV/H₂O₂) produces hydroxyl radicals that have the potential to degrade a wide-range of organic micro-pollutants in water. Yet, when this technology is used to reduce target contaminants, natural organic matter can be altered. This study evaluated disinfection by-product (DBP) precursor formation for UV/H₂O₂ while reducing trace organic contaminants in natural water (>90% for target pharmaceuticals, pesticides and taste and odor producing compounds and 80% atrazine degradation). A year-long UV/H₂O₂ pilot study was conducted to evaluate DBP precursor formation with varying water quality. The UV pilot reactors were operated to consistently achieve 80% atrazine degradation, allowing comparison of low pressure (LP) and medium pressure (MP) lamp technologies for DBP precursor formation. Two process waters of differing quality were used as pilot influent, i.e., before and after granular activated carbon adsorption. DBP precursors increased under most of the conditions studied. Regulated trihalomethane formation potential increased through the UV/H₂O₂ reactors from 20 to 118%, depending on temperature and water quality. When Post-GAC water served as reactor influent, less DBPs were produced in comparison to conventionally treated water. Haloacetic acid (HAA5) increased when conventionally treated water served as UV/H₂O₂ pilot influent, but only increased slightly (MP lamp) when GAC treated water served as pilot influent. No difference in 3-day simulated distribution system DBP concentration was observed between LP and MP UV reactors when 80% atrazine degradation was targeted.     

Photocatalytic degradation of 2,4-dinitrophenol (DNP) by multi-walled carbon nanotubes (MWCNTs)/TiO2 composite in aqueous solution under solar irradiation

Nanosized multi-walled carbon nanotubes (MWCNTs)/TiO₂ composite and neat TiO₂ photocatalysts were synthesized by sol-gel technique using tetrabutyl titanate as a precursor. The as prepared photocatalysts were characterized using XRD, SEM, FTIR and UV-vis spectra. The samples were evaluated for their photocatalytic activity towards the degradation of 2,4-dinitrophenol (DNP) under solar irradiation. The results indicated that the addition of an appropriate amount of MWCNTs could remarkably improve the photocatalytic activity of TiO₂. An optimal MWCNTs:TiO₂ ratio of 0.05% (w/w) was found to achieve the maximum rate of DNP degradation. The effects of pH, irradiation time, catalyst concentration, DNP concentration, etc. on the photocatalytic activity were studied and the results obtained were fitted to the Langmuir-Hinshelwood model to study the degradation kinetics. The optimal conditions were an initial DNP concentration of 38.8 mg/L at pH 6.0 with catalyst concentration of 8 g/L under solar irradiation for 150 min with good recyclisation of catalyst. The degree of photocatalytic degradation of DNP increased with an increase in temperature. The MWCNTs/TiO₂ composite was found to be very effective in the decolorization and COD reduction of real wastewater from DNP manufacturing. Thus, this study showed the feasible and potential use of MWCNTs/TiO₂ composite in degradation of various toxic organic contaminants and industrial effluents.     

Treatment of a sanitary landfill leachate using combined solar photo-Fenton and biological immobilized biomass reactor at a pilot scale

A solar photo-Fenton process combined with a biological nitrification and denitrification system is proposed for the decontamination of a landfill leachate in a pilot plant using photocatalytic (4.16 m² of Compound Parabolic Collectors - CPCs) and biological systems (immobilized biomass reactor). The optimum iron concentration for the photo-Fenton reaction of the leachate is 60 mg Fe²⁺ L⁻¹. The organic carbon degradation follows a first-order reaction kinetics (k = 0.020 L kJ_UV⁻¹, r₀ = 12.5 mg kJ_UV⁻¹) with a H₂O₂ consumption rate of 3.0 mmol H₂O₂ kJ_UV⁻¹. Complete removal of ammonium, nitrates and nitrites of the photo-pre-treated leachate was achieved by biological denitrification and nitrification, after previous neutralization/sedimentation of iron sludge (40 mL of iron sludge per liter of photo-treated leachate after 3 h of sedimentation). The optimum C/N ratio obtained for the denitrification reaction was 2.8 mg CH₃OH per mg N-NO₃⁻, consuming 7.9 g/8.2 mL of commercial methanol per liter of leachate. The maximum nitrification rate obtained was 68 mg N-NH₄⁺ per day, consuming 33 mmol (1.3 g) of NaOH per liter during nitrification and 27.5 mmol of H₂SO₄ per liter during denitrification. The optimal phototreatment energy estimated to reach a biodegradable effluent, considering Zahn-Wellens, respirometry and biological oxidation tests, at pilot plant scale, is 29.2 kJ_UV L⁻¹ (3.3 h of photo-Fenton at a constant solar UV power of 30 W m⁻²), consuming 90 mM of H₂O₂ when used in excess, which means almost 57% mineralization of the leachate, 57% reduction of polyphenols concentration and 86% reduction of aromatic content.     

Interpretation of the variability of levels of regional background aerosols in the Western Mediterranean

Results on interpretation of the variability of regional background PM levels in the Western Mediterranean basin (WMB) are presented. Mean PM levels recorded at Montseny, MSY (North-Eastern Spain) in the 2002-2007 period reached 17, 13 and 11 µg/m³ of PM₁₀, PM_2.5 and PM₁, respectively.The daily evolution of PM levels is regulated by the breeze circulation (mountain and sea breezes). PM levels are lower at the rural sites at night owing to the nocturnal drainage flows and to the lowering of the mixing layer height below the MSY high. These nocturnal low levels allowed us to estimate the continental background PM levels. At midday, the atmospheric pollutants accumulated in the pre-coastal depression are transported upwards by the breeze, increasing PM levels.Maximum PM₁₀ levels were recorded in summer, and February-March and November, and minimum values in the rest of the year coinciding with the highest frequency of Atlantic advection. PM peak episodes attributed to Saharan dust outbreaks were recorded in summer and February-March. In addition, anticyclonic situations (February-March and November) may impact in elevated rural areas by increasing hourly levels of PM₁ up to 75 µg/m³. This scenario induces the stagnation of pollutants in the pre-coastal depression. Solar radiation activates mountain winds, transporting polluted air masses from the valleys to elevated areas resulting in an increase of fine PM levels in areas outside the boundary layer.A significant decrease in PM annual means (40% and 34% for the entire monitoring period, 7 µgPM₁₀/m³ and 5 µgPM_2.5/m³) was recorded at MSY between 2002 and 2007. There appears to be no single cause behind these trends. This could partially be ascribed to the varying frequency and intensity of Saharan dust episodes, but also to large-scale meteorological processes or cycles, and/or to local or meso-scale processes such as nearby anthropogenic emission sources.     

Visible-light-induced photocatalysis of low-level methyl-tertiary butyl ether (MTBE) and trichloroethylene (TCE) using element-doped titanium dioxide

While the photocatalytic degradation of various volatile organic compounds in conjunction with UV light has been widely reported, visible-light-induced photocatalytic degradation of low-levels of the pollutants MTBE and TCE, which have been linked to potential adverse health effects, is rarely reported. The present study examined whether visible-light-activated S- or N-doped TiO₂ photocatalytic technology can be used to control indoor concentrations of MTBE and TCE. This study consists of the characterization of the doped TiO₂ powders, as well as an investigation of their photocatalytic activities. In regards to both powders, a shift of the absorbance spectrum towards the visible light region was observed. An activity test suggested that these photocatalysts exhibited reasonably high degradation efficiencies towards MTBE and TCE under visible light irradiation. The degradation efficiencies of MTBE and TCE by S- and N-doped photocatalysts exceeded 75 and 80%, respectively, at input concentrations (IC) of 0.1 ppm. Degradation efficiency was dependent on both IC and relative humidity. TCE could enhance the degradation efficiency of MTBE even under visible-light irradiation. The estimated mineralization efficiencies (MEs) were comparable to those of previous studies conducted with UV/TiO₂ systems. Similar to the relative degradation efficiencies, the ME of TCE was higher in comparison to that of MTBE. The CO production measured during the photocatalytic processes represented a negligible addition to indoor CO levels. These results suggest that visible-light-activated S- and N-doped TiO₂ photocatalysts may prove a useful tool in the effort to improve indoor air quality.     

Comparison of low- and medium-pressure ultraviolet lamps: Photoreactivation of Escherichia coli and total coliforms in secondary effluents of municipal wastewater treatment plants

It has been reported that Medium-Pressure (MP) ultraviolet (UV) lamps have an advantage over low-pressure (LP) lamps for water disinfection in terms of the photoreactivation of pure cultured bacteria. However, few studies have investigated the behavior of microorganisms in wastewater. Hence, in this study, the degree of photoreactivation, after UV exposure using both LP and MP lamps, in municipal wastewater samples was examined under a variety of conditions. Pure cultured Escherichia coli was also used to provide a comparison with previous studies.E. coli was found to undergo photoreactivation after both LP and MP exposure. The Colony Forming Ability (CFA) ratios were 0.60 and 0.32, and the percentage of photoreactivation was 50% and 20%, respectively, for LP and MP lamps with a germicidal UV dose of 5 mJ/cm². However, the advantage of the MP lamp was diminished for larger UV doses, since no photoreactivation was detected when the UV dose was 15 mJ/cm² for either LP or MP lamps. The microorganisms present in wastewater showed similar results to those of E. coli, however, no significant difference was found between the use of either a LP or a MP lamp. Also, when a UV dose of 40 mJ/cm² was applied, the percentage photoreactivation was less than 1%, no matter which type of lamp was used. From this work, it is concluded that the selection of the type of UV lamp for wastewater treatment plants, as regards photoreactivation of total coliforms, is not critical as long as the applied germicidal UV dose is greater than 40 mJ/cm².     

Degradation of bromoxynil and trifluralin in natural water by direct photolysis and UV plus H2O2 advanced oxidation process

The degradation of two pesticides, bromoxynil and trifluralin, was investigated in ultrapure and natural water solutions under ultraviolet (UV) light and a combination of UV and hydrogen peroxide (H₂O₂). The effect of pH on the photooxidation of the pesticides was also studied. The results indicated that under direct photolysis with monochromatic light at 253.7 nm and different conditions, the photochemical rates followed first-order kinetics, with fluence-based rate constants ranging from 9.15 × 10⁻⁴ to 6.37 × 10⁻³ cm² mJ⁻¹ and 7.63 × 10⁻³ to 1.47 × 10⁻² cm² mJ⁻¹ for bromoxynil and trifluralin, respectively. Quantum yields, in the range of 0.08-0.25 for bromoxynil and 0.12-0.72 for trifluralin, were observed in experiments using ultrapure water. The study also found that the UV/H₂O₂ process enhanced the oxidation rate in comparison to direct photolysis. A 90% degradation with UV dose of 333 and 188 mJ cm⁻² was achieved for bromoxynil and trifluralin, respectively, in natural water, in presence of 8.8 × 10⁻⁴ M H₂O₂. To assess the aquatic toxicity, the Microtox^® 81.9% screening test protocol was used before and after treatment. The test results indicated a decrease in the acute toxicity of the samples after treatment for both pesticides.     

A comparison of several nanoscale photocatalysts in the degradation of a common pollutant using LEDs and conventional UV light

A comparative study on the photocatalytic activities of four different catalysts, P-25 TiO₂, TiO₂ nanofibers, tin-doped TiO₂ nanofibers under UV light irradiation at 350 nm, and coumarin (C-343) coated TiO₂ nanofibers at 436 nm light emitting diodes (LED) is reported. Catalysts performance has been compared based on their reflectance spectrum and activity. A common water contaminant 4-chlorophenol was used as a substrate to compare the activity of the different catalysts under both direct and dye sensitized conditions. Results indicated that amongst the four different catalysts the activity of P-25 was the highest. However the activity of C-343 coated TiO₂ nanofibers in the LED (436 nm) based reactor was competitive. Identification of reaction intermediates implied that the reaction pathways under UV (band gap) and visible (dye sensitized) irradiation were different. Nonetheless, ring opening took place in all reactions with both maleic and dihydroxymaleic have been identified as intermediates. The study indicates that ordered arrays of TiO₂ irradiated by panels of arrays of low cost high intensity LEDs might be used for the design of reactors. The near monochromaticity, long life, and operation under direct currents are advantages of using LEDs.     

Treating landfill leachate using passive aeration trickling filters; effects of leachate characteristics and temperature on rates and process dynamics

Biological ammoniacal-nitrogen (NH₄⁺-N) and organic carbon (TOC) treatment was investigated in replicated mesoscale attached microbial film trickling filters, treating strong and weak strength landfill leachates in batch mode at temperatures of 3, 10, 15 and 30 °C. Comparing leachates, rates of NH₄⁺-N reduction (0.126-0.159 g m^− 2 d^− 1) were predominantly unaffected by leachate characteristics; there were significant differences in TOC rates (0.072-0.194 g m^− 2 d^− 1) but no trend relating to leachate strength. Rates of total oxidised nitrogen (TON) accumulation (0.012-0.144 g m^− 2 d^− 1) were slower for strong leachates. Comparing temperatures, treatment rates varied between 0.029-0.319 g NH₄⁺-N m^− 2 d^− 1 and 0.033-0.251 g C m^− 2 d^− 1 generally increasing with rising temperatures; rates at 3 °C were 9 and 13% of those at 30 °C for NH₄⁺-N and TOC respectively. For the weak leachates (NH₄⁺-N < 140 mg l^− 1) complete oxidation of NH₄⁺-N was achieved. For the strong leachates (NH₄⁺-N 883-1150 mg l^− 1) a biphasic treatment response resulted in NH₄⁺-N removal efficiencies of between 68 and 88% and for one leachate no direct transformation of NH₄⁺-N to TON in bulk leachate. The temporal decoupling of NH₄⁺-N oxidation and TON accumulation in this leachate could not be fully explained by denitrification, volatilisation or anammox, suggesting temporary storage of N within the treatment system. This study demonstrates that passive aeration trickling filters can treat well-buffered high NH₄⁺-N strength landfill leachates under a range of temperatures and that leachate strength has no effect on initial NH₄⁺-N treatment rates. Whether this approach is a practicable option depends on a range of site specific factors.