Oxidation and mineralization rates of harmful organic chemicals in hydroxyl radical induced reactions.

In most of advanced oxidation processes (AOPs) used to destroy harmful organic chemicals in water/wastewater hydroxyl radical (•OH) reactions oxidize (increasing the oxygen/carbon ratio in the molecules) and mineralize (transforming them to inorganic molecules, H2O, CO2, etc.) these contaminants. In this paper, we used the radiolysis of water to produce •OH and characterised the rate of oxidation and mineralization by the dose dependences of the Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) content values. Analysis of the dose dependences for 34 harmful organic compounds showed large differences in the oxidation and mineralization rates and these parameters are characteristic to the given group of chemicals. E.g., the rate of oxidation is relatively low for fluoroquinolone antibiotics; it is high for ß-blocker medicines. Mineralization rates are low for both fluoroquinolones and ß-blockers. The one-electron-oxidant •OH in most cases induces two - four-electron-oxidations. Most of the degradation takes place gradually, through several stable molecule intermediates. However, based on the results it is likely, that some part of the oxidation and mineralization takes place parallel. The organic radicals formed in •OH reactions react with several O2 molecules and release several inorganic fragments during the radical life cycle.

Wastewater Characterization: Chemical Oxygen Demand or Total Organic Carbon Content Measurement?

The long time (2 h) required for measurement, expensive chemicals (Ag2SO4), and toxic reagents (K2Cr2O7, HgSO4) limit the application of the standard method for measuring the oxygen equivalent of organic content in wastewater (chemical oxygen demand, COD). In recent years, the COD has increasingly been replaced by the total organic carbon (TOC) parameter. Since the limit values of the pollution levels are usually given in terms of the COD, efforts are being made to find the correlation between these parameters. Several papers have published correlation analyses of COD and TOC for industrial and municipal wastewater, but the relationship has not been discussed for individual chemicals. Here, this relationship was investigated using 70 contaminants (laboratory chemicals, pharmaceuticals, and pesticides). The calculated COD values, in most cases, agreed, within ~10%, with the experimental ones; for tetracyclines and some chloroaromatic molecules, the measured values were 20-50% lower than the calculated values. The COD/TOC ratios were between 2 and 3: for macrolides, they were ~3; for fluoroquinolones and tetracyclines, they were ~2. The molecular structure dependence of the ratio necessitates the establishing of the correlation on an individual basis. In advanced oxidation processes (AOPs), the ratio changes during degradation, limiting the application of TOC instead of COD.

Elimination of oxacillin, its toxicity and antibacterial activity by using ionizing radiation.

The chemical changes caused by electron beam and γ irradiations and the biochemical characteristics of degradation products of a frequently used antibiotic oxacillin were investigated and compared with those of cloxacillin by applying pulse radiolysis, chemical and biochemical oxygen demand, total organic carbon content, oxygen uptake rate, toxicity and antibacterial activity measurements. Oxacillin was found to be non-toxic, but poorly biodegradable by the mixed microbial population of the activated sludge of a wastewater treatment plant. Therefore, it can significantly contribute to the spread of ß-lactam antibiotic resistant bacteria. However, the products formed by γ-irradiation were more easily biodegradable as they were utilized as nutrient source by the microbes of the activated sludge and the products did not show antibacterial activity. During irradiation treatment of aerated aqueous solutions mainly hydroxyl radicals induce the elimination of antimicrobial activity by making alterations at the bicyclic ß-lactam part of these antibiotics. Since the ß-lactam part is the same in oxacillin and cloxacillin, the biochemical characteristics of products of the two antibiotics are similar. The attack of hydrated electron takes place on the carbonyl groups. When the irradiation is made under anoxic conditions these reactions may also contribute considerably to alterations at the ß-lactam part and thereby to the loss of antibacterial activity.

Antibiotics in a wastewater matrix at environmentally relevant concentrations affect coexisting resistant/sensitive bacterial cultures with profound impact on advanced oxidation treatment.

Antibiotic resistance containment strategies at wastewater treatment plants need to be supported by a firm knowledge on the behavior of resistant bacteria within a diverse microbial population in the presence of trace amount of antibiotics. In this study via investigating the population dynamics of resistant/sensitive Staphylococcus aureus co-cultures in several model wastewater matrix systems, valuable insights were obtained into the effect of trace amount of antibiotics (piperacillin and erythromycin) on bacteria, and into the suitability of advanced oxidation treatment (electron beam irradiation) as a remediation measure. It appears that environmentally relevant concentration levels of the antibiotic present in a wastewater matrix leads to a shift in the population in favor of the sensitive subtype, presumably on account of triggering protective biochemical processes in the resistant mutant, which confer no selective advantage since the sensitive strain remains unaffected in this concentration range. The impact of these conditions on the population dynamics can be diminished by using advanced oxidation treatment, considering that degradation products from the wastewater matrix constituents (such as humic acid) might also have an effect. Furthermore, it became also apparent that the presence of trace amount of antibiotics while triggers biological processes in the resistant subtype, concomitantly makes the bacteria more sensitive towards the attack of free radicals during advanced oxidation treatment. The behavior of resistant bacteria under environmental conditions at the cellular and population level clearly merits more attention.

Applicability evaluation of advanced processes for elimination of neurophysiological activity of antidepressant fluoxetine.

Presence of the antidepressant fluoxetine in different water bodies has raised significant concerns due to its detrimental effects on non-targeted organisms, especially on fish. When seeking for an appropriate technology able to remove fluoxetine residue from a complex water matrix, special attention needs to be paid to the elimination of the neurophysiological activity that eventually lies behind the noxious effects of the parent compound. Our aim was to probe the applicability of advanced oxidation techniques for this purpose using in situ generated free radical system based on OH-initiated peroxyl radical-mediated processes. By performing product analysis experiments along with quantum chemical calculations, the most probable reaction paths were analyzed including aromatic hydroxylation, defluorination, O-dealkylation and C-dealkylation. The candidates for neurophysiological activity were further investigated by molecular docking. The hydroxylated derivatives are well accommodated in the binding pocket of the corresponding protein, suggesting that these compounds may retain the activity of the parent compound. From a worst-case perspective, we suggest that prolonged treatment needs to be applied to further transform hydroxylated derivatives.

Degradation of organic molecules in advanced oxidation processes: relation between chemical structure and degradability.

Hydroxyl radical induced degradation of maleic acid, fumaric acid and 20 aromatic molecules was investigated in air saturated aqueous solutions. Hydroxyl radicals were generated by an advanced oxidation process (AOP), water radiolysis. Oxidation was followed by chemical oxygen demand (COD) and total organic carbon content (TOC) measurements. Up to ∼30-50% decrease of COD the dose dependence was linear. By the ratio of the decrease of COD and the amount of reactive radiolysis intermediates introduced into the solution the oxidation efficiencies were calculated. Efficiencies around 0.5-1 (O2 molecule built in products/OH) found for most of the compounds show that the one-electron-oxidant OH induces 2-4 electron oxidations. The high oxidation rates were explained by OH addition to unsaturated bonds and subsequent reactions of dissolved O2 with organic radicals. In amino substituted molecules or in Acid Red 1 azo dye, O2 cannot compete efficiently with unimolecular transformation of organic radicals and the efficiency is lower (0.2-0.5).

Elimination of diclofenac from water using irradiation technology.

The effluents of wastewater treatment plants, usually directly emitted to the environment, often contain the anti-inflammatory drug diclofenac (DCF). The paper investigates DCF elimination using irradiation technology. Hydroxyl radical and hydrated electron reactive intermediates resulting from water radiolysis effectively degrade DCF and strongly reduce the toxicity of the solutions. ()OH attaches to one of the rings of DCF, and hydroxylated molecules, 2,6-dichloroaniline and quinoid type compounds are the products. Hydrated electron adds to the chlorine atom containing ring, in the reaction quinoid type compounds and 4-chloroacridine form. At a 0.1 mM DCF concentration, a ∼1 kGy absorbed dose is needed for the degradation of DCF molecules, but for mineralization of the products (in presence of O2) an order of magnitude higher dose is required. For irradiation of wastewater after biological treatment a ∼1 kGy dose is suggested. At this dose DCF and other drugs or metabolites present at µg L(-1) level are eliminated together with microorganism deactivation.