Mixtures of chelating agents to enhance photo-Fenton process at natural pH: Influence of wastewater matrix on micropollutant removal and bacterial inactivation

Three organic fertilizers (EDTA (Ethylenedinitrilotetraacetic acid), EDDS (Ethylenediamine-N, N'-disuccinic acid) and DTPA (Diethylene triamine pentaacetic acid)) were tested as Fe-complexes in photo-Fenton process at natural pH for micropollutants (MPs) abatement and simultaneous E.coli inactivation. Less stable Fe-complexes show high iron precipitation, stopping MPs degradation. On the contrary, stable Fe-complexes imply low kinetic rates for MPs removal. To solve these inconveniences, three mixtures of organic fertilizers were also tested, trying to improve the kinetic rates of micropollutants oxidation and overcome iron precipitation. Three different pollutants (propranolol (PROP), acetamiprid (ACMP) and sulfamethoxazole (SMX)) were used as the target compounds. As the iron release is, in part, linked to the hardness of water, two water matrices from two different secondary wastewaters (Membrane Bioreactor (MBR) and Conventional Activated Sludge (CAS)) were tested. The best performance in micropollutant degradation and E.coli inactivation was achieved with the combination of EDDS + EDTA, accomplishing a good equilibrium between iron precipitation and rate of MPs removal. For instance, total removal of propranolol was achieved at 45 min in MBR, while it was only 85.7% in CAS, being an improvement of the process comparing with that obtained using single organic fertilizers. At the end of the treatment, 2.1 log-inactivation for E.coli was reached in CAS. The differences observed between both wastewaters were related to CAS' higher DOC, turbidity, and hardness. Finally, from the physicochemical characterization conducted, including Biochemical Oxygen Demand at 5 days and phytotoxicity, it is possible to highlight the suitability of these treated effluents for its reuse in irrigation, as long as in CAS matrix the final values of E. coli are within the legal limit.

Synergies, radiation and kinetics in photo-Fenton process with UVA-LEDs

The photo-Fenton process, with UV-A LED (λ = 380-390, 390-400 and 380-400 nm) has demonstrated to be effective in the abatement of a target micropollutant, such as diphenhydramine hydrochloride (DPH). Different concentrations of iron (Fe²⁺) and H₂O₂ were tested and monitored, and the best results in DPH removal were obtained for the highest concentrations of both iron (II) and H₂O₂ (10 mg Fe²⁺/L - 150 mg H₂O₂/L). The evolution of iron and peroxide concentration was also monitored. Kinetic studies showed that dark Fenton process prevails at the beginning of the experiment, when Fe²⁺ concentration is higher. However, after these initial moments, the prevailing process is photo-Fenton and, in addition, wavelength radiation plays an important role. Concerning the effect of radiation, four LEDs (4.2 W total power) were used, emitting radiation in the wavelength range between 380-390 or 390-400 nm. Similar results were obtained in both cases in DPH removal by photo-Fenton (30 min for total elimination). However, a synergistic effect was observed when two LEDs of 380-390 nm and two LEDs of 390-400 nm were used. Total power was the same (4.2 W) in each experimental condition, but the increase in the wavelength range to 20 nm (380-400 nm) produces an increase in the rate of DPH removal, achieving its total elimination at 15 min. This fact, with the use of a simple radiation model, reveals the important role that radiation plays in the photo-Fenton process. Finally, the formed intermediates were determined and some reaction pathways were proposed.

Kinetic study of colored species formation during paracetamol removal from water in a semicontinuous ozonation contactor

Paracetamol aqueous solutions, when ozonized, acquired a strong red coloration depending on the applied ozone dose and the initial pH of the aqueous solution. Then, this color loses intensity and turns to yellow. Color formation is favored when operating at initial pH₀ = 12.0 and ozone flow-rate 4.2 mg/min. A mechanism describing color formation was proposed, being the main pathway involved an initial paracetamol hydroxylation to yield 3-hydroxyacetaminophen followed by the formation of 2-amino-5-hydroxyacetofenone. Then, these compounds are degraded to colored oxidation by-products. A model describing color evolution was also proposed, considering first-order kinetics for both color formation and degradation. The corresponding kinetic constant values were determined to be k_f = 0.01 (1/min) and k_d = 0.03 pH -0.055 (1/min), respectively. A relationship between aromaticity loss and color changes during the reaction has been estimated considering the parameter α=k_A/k_f, being α = 1.62 pH + 3.5 and the first-order rate constant for aromaticity loss given by k_A = 0.0162 pH + 0.035 (1/min).

Priority pesticides abatement by advanced water technologies: The case of acetamiprid removal by ozonation

With the aim of exploring treatment alternatives for priority insecticide acetamiprid (ACMP) abatement, the removal of this compound from water by ozonation was studied for the first time, paying special attention to the kinetic, mechanistic and toxicological aspects of the process. The second order rate constants of reactions between ACMP and both molecular ozone (O₃) and hydroxyl radicals (OH) were determined to be 0.25M^-1s^-1 and 2.1·10⁹M^-1s^-1, respectively. On the basis of kinetic results, the degradation of ACMP during ozonation could be well-explained by the reactivity of this pesticide with OH. HPLC/MS analysis of the ozonated ACMP showed ACMP-N-desmethyl, 6-chloronicotinic acid, N'cyano-N-methyl acetamidine and N'-cyano acetamidine as the major transformation products (TPs), all of them formed through amine α carbon oxidation in combination with hydrolysis. Microtox bioassays revealed an increase in the toxicity of the medium during ACMP ozonation process, followed by a decrease to relatively low values. These changes could be attributed to the synergistic effects between TPs as well as to the presence of toxic intermediate aldehydes. Even though adopting strategies to further promote ozone decomposition to hydroxyl radicals appears to be essential, ozonation can be an effective treatment process for ACMP removal and associated toxicity abatement.

Exploring ozonation as treatment alternative for methiocarb and formed transformation products abatement

Despite the high toxicity and resistance to conventional water treatments exhibited by methiocarb (MC), there are no reports regarding the degradation of this priority pesticide by means of alternative purification technologies. In this work, the removal of MC by means of ozonation was studied for the first time, employing a multi-reactor methodology and neutral pH conditions. The second-order rate constants of MC reaction with molecular ozone (O₃) and formed hydroxyl radicals (OH·) were determined to be 1.7·10⁶ and 8.2·10⁹ M^-1 s^-1, respectively. During degradation experiments, direct ozone reaction was observed to effectively remove MC, but not its formed intermediates, whereas OH· could oxidize all species. The major identified TPs were methiocarb sulfoxide (MCX), methiocarb sulfoxide phenol (MCXP) and methiocarb sulfone phenol (MCNP), all of them formed through MC oxidation by O₃ or OH· in combination with hydrolysis. A toxicity assessment evidenced a strong dependence on MCX concentration, even at very low values. Despite the OH· capability to degrade MC and its main metabolites, the relative resistance of TPs towards ozone attack enlarged the oxidant dosage (2.5 mg O₃/mg DOC) necessary to achieve a relatively low toxicity of the medium. Even though ozonation could be a suitable technique for MC removal from water compartments, strategies aimed to further promote the indirect contribution of hydroxyl radicals during this process should be explored.