Dichloroacetic acid degradation employing hydrogen peroxide and UV radiation

Uncovering the impact of agricultural activities and urbanization on rivers from the Piracicaba, Capivari, and Jundiaí basin in São Paulo, Brazil: A survey of pesticides, hormones, pharmaceuticals, industrial chemicals, and PFAS

Rivers in Southeast Brazil are essential as sources of drinking water, energy production, irrigation, and industrial processes. The Piracicaba, Capivari, and Jundiaí rivers basin, known as the PCJ basin, comprises major cities, industrial hubs, and large agricultural areas, which have impacted the water quality in the region. Emerging contaminants such as pesticides, hormones, pharmaceuticals, industrial chemicals, and per- and polyfluoroalkyl substances (PFAS) are likely to be released into the rivers in the PCJ basin; however, the current Brazilian legislation does not require monitoring of most of these chemicals. Thus, the extent of emerging contaminants pollution and their risks to aquatic and human life in the basin are largely unknown. In this study, we investigated the occurrence of several pesticides, hormones, pharmaceuticals, and personal care products in 15 sampling points across the PCJ basin, while industrial chemicals and PFAS were assessed in 11 sampling points. The results show that agriculture and industrial activities are indeed causing the pollution of most rivers. Multivariate analysis indicates that some sampling points, such as Jundiaí, Capivari, and Piracicaba rivers, are largely impacted by pesticides used in agriculture. In addition, to the best of our knowledge, this is the first study reporting the presence of PFAS in rivers in São Paulo, the most populous state in Brazil. Four out of eight species of PFAS assessed in our study were detected in at least 5 sampling points at concentrations ranging from 2.0 to 50.0 ng L-1. The preliminary risk assessment indicates that various pesticides, caffeine, industrial chemicals, and PFAS were present at concentrations that could threaten aquatic life. Notably, risk quotients of 414, 340, and 178 were obtained for diuron, atrazine, and imidacloprid, respectively, in the Jundiaí River. Our study suggests that establishing a comprehensive monitoring program is needed to ensure the protection of aquatic life and human health.

Degradation of organic compounds in hypersaline wastewater concentrate by a supercritical oxidation approach

Hypersaline wastewater is a typical industrial wastewater produced by iron and steel metallurgy, food material processing and other industries. Aiming at a waste liquid produced by mechanical vapour recompression evaporation and concentration in Tianjin coastal industrial zone, an environment-friendly supercritical water oxidation technology was used to efficiently remove the high-content organic matter in the hypersaline wastewater concentrate (HWC). A comparison of the degradation effects of various oxidants in the supercritical state showed that hydrogen peroxide (H₂O₂) could be used as a suitable agent for processing the HWC. The reaction parameters were systematically optimised by single-factor experiment and response surface design. The degradation mechanism and reaction characteristics were analyzed using gas chromatography mass spectrometry. Solid residues were characterised by field emission scanning electron microscope. The results indicated that when the dosage of hydrogen peroxide was 6.39%, the reaction temperature was 380°C, the reaction time was about 90 min and the optimal total organic carbon removal rate was 96.22%. Furthermore, it was found that hydroxyl radicals produced by hydrogen peroxide initiated the bond breaking and ring-opening reactions in organic matter, which eventually degraded organic matter into water and carbon dioxide.

Alkali-catalyzed hydrothermal oxidation treatment of triclosan in soil: Mechanism, degradation pathway and toxicity evaluation

The continuous accumulation of chlorinated organic pollutants in soil poses a potential threat to ecosystems and human health alike. Alkali-catalyzed hydrothermal oxidation (HTO) can successfully remove chlorinated organic pollutants from water, but it is rarely applied to soil remediation. In this work, we assessed this technique to degrade and detoxify triclosan (TCS) in soil and we determined the underlying mechanisms. The results showed a dechlorination efficiency of TCS (100 mg per kg soil) of 49.03 % after 120 min reaction (H₂O₂/soil ratio 25 mL·g^-1, reaction temperature 180 °C in presence of 1 g·L^-1 NaOH). It was found that soil organic constituents (humic acid, HA) and inorganic minerals (SiO₂, Al₂O₃, and CaCO₃) suppressed the dechlorination degradation of TCS, with HA having the strongest inhibitory effect. During alkali-catalyzed HTO, the TCS molecules were effectively destroyed and humic acid-like or fulvic acid-like organics with oxygen functional groups were generated. Fluorescence spectroscopy analysis showed that hydroxyl radicals (OH) were the dominant reactive species of TCS degradation in soil. On the basis of the Fukui function and the degradation intermediates, two degradation pathways were proposed. One started with cleavage of the ether bond between the benzene rings of TCS, followed by dechlorination and the opening of benzene via oxidation. The other pathway started with direct hydroxylation of the benzene rings of TCS, after which they were opened and dechlorinated through oxidation. Analysis of the soil structure before and after treatment revealed that the soil surface changed from rough to smooth without affecting soil surface elements. Finally, biotoxicity tests proved that alkali-catalyzed HTO effectively reduced the toxicity of TCS-contaminated soil. This study suggests that alkali-catalyzed hydrothermal oxidation provides an environmentally friendly approach for the treatment of soil contaminated with chlorinated organics such as TCS.

Treating disinfection byproducts with UV or solar irradiation and in UV advanced oxidation processes: A review

This review focuses on the degradation kinetics and mechanisms of disinfection byproducts (DBPs) under UV and solar irradiation and in UV-based advanced oxidation processes (AOPs). A total of 59 such compounds are discussed. The processes evaluated are low pressure, medium pressure and vacuum UV irradiation, solar irradiation together with UV/hydrogen peroxide, UV/persulfate and UV/chlorine AOPs. Under UV and solar irradiation, the photodegradation rates of N-nitrosamines are much higher than those of halogenated DBPs. Among halogenated DBPs, those containing iodine are photodegraded more rapidly than those containing bromine or chlorine. This is due to differences in their bond energies (E_N-N < E_C-I < E_C-Br < E_C-Cl). Molar absorption coefficients at 254 nm and energy gaps can be used to predict the photodegradation rates of DBPs under low pressure UV irradiation. But many DBPs of interest cannot be degraded to half their original concentration with less than a 500 mJ cm^-2 dose of low pressure UV light. HO^• generally contributes to less than 30% of the degradation of DBPs except iodo-DBPs in UV/H₂O₂ AOPs. Reaction mechanisms under UV irradiation and in HO^•-mediated oxidation are also summarized. N-N bond cleavage initiates their direct UV photolysis of N-nitrosamines as C-X cleavage does among halogenated compounds. HO^• generally initiates degradation via single electron transfer, addition and hydrogen abstraction pathways. Information on the reaction rate constants of SO₄^•- and halogen radicals with DBPs is rather limited, and little information is available about their reaction pathways. Overall, this review provides improved understanding of UV, solar and AOPs.

Generation of hydroxyl radicals during photodegradation of chloroacetic acids by 254 nm ultraviolet: A special degradation process revealed by a holistic radical determination methodology

Upon ultraviolet (UV) irradiation, aqueous contaminants may undergo direct and/or indirect photolysis. Direct photolysis refers to transformation of contaminants by UV photon, and indirect photolysis refers to degradation of contaminants by UV-induced reactive species in the presence of photosensitizers. Because hydroxyl radical (•OH) was unexpectedly observed during chloroacetic acids photolysis without using photosensitizer, a question arises regarding whether direct photolysis-induced indirect photolysis (DPIP) was present and how it originated and evolved along the process. To answer these questions, this study employed multiple different yet complementary •OH detection approaches (i.e., probe, scavenger, electron paramagnetic resonance, and hydroxylation products) to prove the presence and role of •OH. Given that hydrogen peroxide (H₂O₂) was produced only in oxygenated water but not in deoxygenated water, we revealed that •OH was mainly generated by reduced oxygen. Meanwhile, several photolysis products like formate, glycolic acid, and glyoxylic acid were able to yield H₂O₂ too, suggesting that they can all trigger formation of •OH under 254 nm UV. In addition to evidences of DPIP phenomenon, this study is also novel in demonstrating a holistic methodology to prove and identify the presence and sources of radicals, which might help enhance understandings of UV processes.

Photolysis and photocatalysis of haloacetic acids in water: A review of kinetics, influencing factors, products, pathways, and mechanisms

Haloacetic acids (HAAs) are a group of pollutants ubiquitous in natural environment and anthropogenic systems, and therefore in need of control. Photolysis and photocatalysis techniques via ultraviolet (UV)-based technologies have held promise for decades in degrading organic molecules in water, but their capacities in removing HAAs remain to be explored. To better understand the trends in the existing literature and to identify the knowledge gaps that may merit further exploration, this review compares the HAAs photodegradation kinetics, influencing factors, reaction products, pathways, and mechanisms for a variety of UV technologies. The selected UV processes are classified into three types: UV-only photolysis, photooxidation, and photoreduction. Overall, although trends vary significantly depending upon many factors, the photo-susceptibility of HAAs always increases with rising molecular weight of substituted halogen atom(s), with those chlorinated HAAs being the most refractory species. Notably, while many processes proved hydroxyl radical (OH) as the forcing driver, the patterns of kinetics among HAAs were not consistent among processes, suggesting that OH was not the only driver. Compared to earlier studies focusing on specific technologies to treat numerous contaminants through a material perspective, this review commits to understanding the commonalities and differences among multiple UV-based technologies in treating only one group of compound mainly via a chemistry viewpoint.

Role of various factors affecting the photochemical treatment of N-nitrosamines related to CO2 capture

Post-combustion CO₂ capture using amine solvents is the most feasible method of reducing anthropogenic CO₂ emissions, which are the largest contributor to global warming. The formation of carcinogenic N-nitrosamines (i.e. by-products) can hinder the industrial application of this technology. In this study, the effects of direct UV photolysis (N-nitrosamine concentration and amines) and advanced oxidation processes (UV/H₂O₂ and UV/O₃) on the three specific N-nitrosamines that are commonplace in amine-based CO₂ capture (i.e. N-nitrosodiethylamine (NDEA), N-nitrosodiethanolamine (NDELA), and N-nitrosomorpholine (NMOR)) were examined. A significant decrease in the photodegradation rate constants was observed for NDEA (1.02 × 10⁰ to 2.94 × 10^-1 min^-1), NDELA (1.52 × 10⁰ to 3.32 × 10^-1 min^-1), and NMOR (1.93 × 10⁰ to 2.20 × 10^-1 min^-1) as their concentrations increased within 1-50 mg/L. This is the first report of a significant increase in the degradation rate constants of N-nitrosamine with an increase in amine concentrations (i.e. monoethanolamine, diethanolamine, and morpholine) within 10-200 mM. The photodegradation rate constants increased as the molar ratio of H₂O₂ to N-nitrosamine increased to 20, but then decreased at molar ratios beyond this. O₃ had a negligible effect on the photodegradation of N-nitrosamines.

Influence of UV irradiation on the toxicity of chlorinated water to mammalian cells: Toxicity drivers, toxicity changes and toxicity surrogates

UV irradiation was reported to be able to degrade some kinds of DBPs, yet its influence on the toxicity of chlorinated water to mammalian cells remains unknown. This study systematically investigated the influence of low-pressure UV irradiation on the DBPs and toxicity of chlorinated drinking water (DW) and reclaimed water (RW). The apparent first-order rate constant (k_obs) of degradation kinetics of known DBPs increased with the increased Br substitutions. Haloacetonitriles were identified as toxicity drivers among the detected DBPs, which even contributed more to the toxicity after UV irradiation, mainly due to the refractory bromochloroacetonitrile (BCAN) and dichloroacetonitrile (dCAN). Both total organic halogen, cytotoxicity and genotoxicity were significantly removed under UV irradiation, with the removal rate of 22.9%-41.7% for cytotoxicity and a higher rate of 33.1%-55.5% for genotoxicity under 2400 mJ/cm² irradiation. UV irradiation significantly decreased the UV₂₅₄, SUVA₂₅₄ and fluorescence intensity (FLU) of chlorinated water. Results from high performance size exclusion chromatography revealed that chlorinated DW mainly contained high molecular weight (MW) compounds (>1000 Da) while chlorinated RW mainly contained lower MW compounds (100-500 Da). Chromophores and fluorophores in compounds of 100-500 Da increased in chlorinated DW while decreased in chlorinated RW under UV irradiation. Both the removal of UV₂₅₄, SUVA₂₅₄, FLU, MW-based UV₂₅₄ (>1000 Da) and MW-based FLU (each fractions) were significantly correlated (p < 0.05) with the removal of toxicity under UV irradiation. The UV₂₅₄ of chlorinated water was recommended as the optimal surrogate for toxicity removal.

Application of different advanced oxidation processes for the removal of chloroacetic acids using a planar falling film reactor

Advanced oxidation processes (AOPs) are considered as an effective and promising method for the degradation and mineralization of aqueous recalcitrant organic pollutants. In this study, application of ozonation and various types of AOPs including photocatalysis, Fenton alone and their combinations were investigated and compared for the degradation and mineralization of chloroacetic acids (CAAs) in aqueous solutions, using a planar falling film reactor. CAAs are widely available in water treated by chlorination processes and are resistance against ozonation in the darkness. The results of the present work showed that the plain ozonation was inefficient method for the destruction of the CAAs as only about 2% degradation was observed after 90 min treatment. However, the best results were achieved by ozone in combinations with other oxidation processes. Furthermore, a synergistic effect on the removal rate was observed when these processes were exposed to the UVA light. Among the examined processes, combination of photo-Fenton with ozonation was found to be the fastest one for CAAs degradation. The effects of different parameters such as initial concentration of Fe²⁺, H₂O₂ and CAAs in photo-Fenton combined with ozonation were investigated. The optimum ratio of 0.12 of Fe²⁺/H₂O₂ concentration was found to give the best result for CAAs degradation. The degree of CAAs mineralization, measured by the total organic carbon removal, as well as the effect of falling liquid film flow rate on the removal of CAAs were also studied and discussed.

Photolysis of chloral hydrate in water with 254 nm ultraviolet: Kinetics, influencing factors, mechanisms, and products

Chloral hydrate (CH) is a common disinfection by-product found in treated water, and its effective control is important to human health. This study evaluated the effects of some environmental factors (e.g., pH, CH dosage, typical ions) and operational variables (e.g., lamp power, irradiation time) on CH photolysis efficiency via low-pressure mercury lamp-induced ultraviolet (LPUV) at 254 nm. The results demonstrated that the photolysis rate increased significantly with increasing pH from 7.0 to 10.5 and lamp power from 6 to 12 W. Meanwhile, the presence of nitrate, iodide, or free chlorine facilitated CH photolysis, whereas the existence of natural organic matter hindered the process. Together, these factors may help explain varying CH photolysis in different types of waters: seawater > ultrapure water > tap water > lake water. In addition, the initial CH dosage also played an important role, with higher CH being degraded more slowly. Mechanistically, although no catalyst or oxidant was added, CH photolysis was to some extent inhibited by a hydroxyl radical quencher, tert-butyl alcohol, suggesting that indirect photolysis was also responsible for CH loss. In terms of reaction products, the CH photolysis yielded primarily chloride ions and carbon dioxide, thus supporting mineralization as the key pathway. The results may help better understand the control of CH in water using UV.

A facile and green pretreatment method for nonionic total organic halogen (NTOX) analysis in water - Step II. Using photolysis to convert NTOX completely into halides

Adsorbable organic halogen (AOX) is a parameter conventionally used to indicate the sum of organic halogenated disinfection byproducts (DBPs), which are formed from the reactions of disinfectants with dissolved organic matter, bromide and iodide in water. To overcome the issues of the AOX analytical method, we proposed a new facile and green pretreatment method to enable the analysis of nonionic total organic halogen (NTOX) via the following three steps: 1) separation of NTOX and halides with electrodialysis, 2) conversion of NTOX with ultraviolet (UV) photolysis, and 3) analysis of halides with ion chromatography. To verify this proposal, we mainly evaluated the efficiency of vacuum ultraviolet (VUV) coupled with UV photolysis (VUV-UV) in converting NTOX into halides. Results showed that by applying VUV irradiation for 60 min and UV irradiation at pH 10-11 for another 30 min, over 85.5% of each halide from 20 representative small molecular weight DBPs (each at 100 μg-X/L level) was recovered. The purpose of UV photolysis under alkaline conditions was to reduce oxyhalides (such as bromate and iodate) formed in the VUV process back to halides. With the aid of electrospray ionization-triple quadrupole mass spectrometry, we captured the whole pictures of high molecular weight polar DBPs in a chlorinated drinking water before and after VUV-UV, through which averagely 96.4% of dehalogenation with the VUV-UV treatment was observed. An illustrative comparison of the conventional AOX method and the proposed NTOX method indicates that although the detected NTOX was lower (by 2.3-30.6%) than AOX, the results of the two methods were highly correlated (R² > 0.97). All these hence verified the photolysis as a mature yet novel tool for sample pretreatment in environmental analytical chemistry.

3D Flower-like β-MnO2/Reduced Graphene Oxide Nanocomposites for Catalytic Ozonation of Dichloroacetic Acid

Considering the potential use of manganese oxide based nanocomposite in catalytic ozonation of water contaminant, we report unique three-dimensional (3D) nanoarchitectures composed of β-MnO₂ and reduced graphene oxide (RGO) for catalytic ozonation of dichloroacetic acid (DCAA) from drinking water. The catalytic results show that the 3D β-MnO₂/RGO nanocomposites (FMOG) can be used as efficient and stable ozonation catalysts to eliminate DCAA from water. The probable mechanism of catalytic ozonation was also proposed by detecting intermediates using gas chromatography-mass spectrometry. This result likely paves a facile avenue and initiates new opportunities for the exploration of heterogeneous catalysts for the removal of disinfection by-products from drinking water.

Kinetics and degradation pathways of photolytic and photocatalytic oxidation of the anthelmintic drug praziquantel

In this study, an anthelmintic drug, praziquantel(PZQ), was degraded using the direct photolysis, photocatalysis, and oxidation processes including UV radiation, TiO₂ film, and hydrogen peroxide. The photolytic degradation with predominant wavelengths of 185/254nm (UV-C) proved to be more efficient with a half-life of 3.13min compared to the radiation of 365nm (UV-A) where the degradation did not occur. In order to enhance the rate of PZQ photolytic degradation, H₂O₂ was added, which resulted in two to three times higher degradation rates. In the photocatalytic degradation, TiO₂ film was used as catalyst. The degradation was ten times faster in the photocatalytic experiments where UV-C light (k=0.2390min^-1) was used than in those with UV-A (k=0.0201min^-1). Comparing the results from all performed experiments it can be concluded that the UV-C/TiO₂/H₂O₂ process yielded the highest degradation rate and complete degradation of PZQ was obtained in less than 7min. The degradation of PZQ followed the first order kinetics in all the experiments. The photo degradation was inhibited in the presence of methanol. The degradation pathways and the structural formulae of five degradation products (m/z 273, 269, 189, 147, 132) were proposed based on the liquid chromatography tandem mass spectrometry analysis.

Souza SMA. Photochemical UVC/H2O2 oxidation system as an effective method for the decolourisation of bio-treated textile wastewaters: towards onsite water reuse

A photochemical UVC/H₂O₂ oxidation system was applied for the decolourisation of two real textile wastewaters collected after biological oxidation from two different textile wastewater treatment plants.

Modeling of degradation kinetic and toxicity evaluation of herbicides mixtures in water using the UV/H2O2 process

The UV/H2O2 process was applied to the treatment of different mixtures of herbicides in water. Glyphosate, the herbicide most used in the world, was mixed with other hormonal herbicides with residual activity as 2,4-D and dicamba. The main goals of the study were to develop a kinetic model for interpreting the simultaneous oxidation of two mixtures (glyphosate plus 2,4-D and glyphosate plus dicamba). The model is based on a complete reaction mechanism, which comprises hydrogen peroxide photolysis and decomposition of both herbicides in each mixture studied. It takes into account the effect of non-uniform distribution of the local rate of absorbed photons. Good agreement of experimental data and the model is achieved in spite of differences in the reactivity between glyphosate and 2,4-D (or dicamba). Toxicity assays (employing Vibrio fischeri) were also performed, indicating that the toxicity of the mixture of glyphosate and 2,4-D was significantly reduced after the treatment.

Simplified reaction kinetics, models and experiments for glyphosate degradation in water by the UV/H2O2process

This work reports the degradation kinetics of glyphosate in water employing the UV/H₂O₂process. The kinetic model was experimentally validated. Results compare the kinetics of the herbicide alone and a commercial formulation.

Effectiveness evaluation of glyphosate oxidation employing the H(2)O(2)/UVC process: toxicity assays with Vibrio fischeri and Rhinella arenarum tadpoles

The H(2)O(2)/UVC process was applied to the photodegradation of a commercial formulation of glyphosate in water. Two organisms (Vibrio fischeri bacteria and Rhinella arenarum tadpoles) were used to investigate the toxicity of glyphosate in samples M(1,) M(2), and M(3) following different photodegradation reaction times (120, 240 and 360 min, respectively) that had differing amounts of residual H(2)O(2). Subsamples of M(1), M(2), and M(3) were then used to create samples M(1,E), M(2,E) and M(3,E) in which the H(2)O(2) had been removed. Acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activities were measured in tadpoles to determine possible sub-lethal effects. In V. fischeri, M(1,E), which was collected early in the photodegradation process, caused 52% inhibition, while M(3,E), which was collected at the end of the photodegradation process, caused only 17% inhibition. Survival of tadpoles was 100% in samples M(2), M(3), and in M(1,E), M(2,E) and M(3,E). The lowest percentages of enzymatic inhibition were observed in samples without removal of H(2)O(2): 13.96% (AChE) and 16% (BChE) for M(2), and 24.12% (AChE) and 13.83% (BChE) for M(3). These results show the efficiency of the H(2)O(2)/UVC process in reducing the toxicity of water or wastewater polluted by commercial formulations of glyphosate. According to the ecotoxicity assays, the conditions corresponding to M(2) (11 ± 1 mg a.e. L(-1) glyphosate and 11 ± 1 mg L(-1) H(2)O(2)) could be used as a final point for glyphosate treatment with the H(2)O(2)/UV process.

Factors affecting ultraviolet irradiation/hydrogen peroxide (UV/H2O2) degradation of mixed N-nitrosamines in water

Disinfection by-products (DBPs) are a great challenge to our drinking water security. Particularly, nitrosamines (NAms), as emerging DBPs, are potently carcinogenic, mutagenic, and teratogenic, and have increasingly attained public attention. This study was to evaluate the performance of the NAms degradation by the ultraviolet (UV) irradiation (253.7 nm) in the presence of hydrogen peroxide (H(2)O(2)). In the UV/H(2)O(2) system, hydroxyl radicals (OH), a type of nonselective and powerful oxidant, was produced to attack the molecules of NAms. Factors affecting the treatment efficiency, including the H(2)O(2) dosage, initial NAms concentration, UV irradiation intensity, initial solution pH, and inorganic anions present in water, were evaluated. All the NAms degradation exhibited a pseudo-first-order kinetics pattern. Within 60 min, 0.1 mg/L of any NAms could be almost decomposed except NDPhA that required 120 min for complete removal, at 25 μmol/L H(2)O(2) and at initial pH 7. Results demonstrate that the UV/H(2)O(2) treatment is a viable option to control NAms in water.

Synergistic effect of the combination of immobilized TiO2, UVA and ozone on the decomposition of dichloroacetic acid

The performance of a heterogeneous photocatalytic ozonation system (TiO(2)/UVA/O(3)) was evaluated on the degradation and mineralization of dichloroacetic acid as a contaminant in aqueous solutions by means of a planar reactor. The commercial product "Pilkington Active™ glass" was used as the immobilized TiO(2) photocatalyst and it was irradiated by near UV light in this study. The synergistic interaction between ozone and the photoactivated TiO(2) surface was discussed and highlighted. Furthermore, the influences of initial concentration and temperature on the degradation rate of dichloroacetic acid and the ozone consumption level during the oxidation process were investigated. The concentrations of dichloroacetic acid and chloride anions produced during degradation were measured using ion chromatography. The mineralization of dichloroacetic acid was evaluated by Total Organic Carbon (TOC) measurements. The degradation of dichloroacetic acid by photocatalytic ozonation showed good agreement with the kinetics of first-order reactions with respect to dichloroacetic acid.

Glyphosate degradation in water employing the H2O2/UVC process

Glyphosate is the organophosphate herbicide most widely used in the world. Any form of spill or discharge, even if unintentional, can be transferred to the water due to its high solubility. The combination of hydrogen peroxide and UV radiation could be a suitable option to decrease glyphosate concentration to acceptable limits. In this work, the effects of initial pH, hydrogen peroxide initial concentration, and incident radiation in glyphosate degradation were studied. The experimental device was a cylinder irradiated with two tubular, germicidal lamps. Conversion of glyphosate increases significantly from pH = 3-7. From this value on, the increase becomes much less noticeable. The reaction rate depends on the initial herbicide concentration and has an optimum plateau of a hydrogen peroxide to glyphosate molar concentration ratio between 7 and 19. The expected non linear dependence on the irradiation rate was observed. The identification of critical reaction intermediaries, and the quantification of the main end products were possible and it led to propose a plausible degradation path. The achieved quantification of the mineralization extent is a positive indicator for the possible application of a rather simple technology for an in situ solution for some of the problems derived from the intensive use of glyphosate.

Comparison of H2O2/UV and heterogeneous photocatalytic processes for the degradation of dichloroacetic acid in water

A comparative study between two advanced oxidation technologies for pollutant degradation has been made. With the use of dichloroacetic acid (DCA) as the model pollutant, the reactions with hydrogen peroxide and UV radiation (H2O2/UV, 253.7 nm) and photocatalysis with titanium dioxide (TiO2/UV, 300-400 nm) are analyzed. Three criteria have been selected to compare the performances of both processes: (i) the percentage conversion of DCA and TOC (total organic carbon) at a fixed reaction time; (ii) the quantum efficiency, employing the true radiation absorption rates for both activated species (H2O2 and TiO2); (iii) the specific energy consumption to degrade 50% of the initial TOC. The optimal molar concentration ratio of H2O2/DCA and the optimal catalyst concentration have been employed in the experiments. The results indicate that, under the optimal operating conditions, the H2O2/UV process exhibits, by a large difference, the best performance taking into account the above-mentioned criteria. Nevertheless, both systems show similar values of specific energy consumption when a thinner reactor is employed. These results cannot be safely extrapolated to other contexts if (i) other compounds of different structure are degraded and (ii) a different catalyst is used. Moreover, they were obtained under optimized conditions, and typical, real-life situations may render quite different results due to the robustness of the titanium dioxide operation. They should serve as an indication that, under the studied conditions, a much-improved catalyst performance must be achieved to parallel, with a heterogeneous process, a yield similar to the one obtained with the homogeneous system.