Hydrogen Peroxide Formation during Ozonation of Olefins and Phenol: Mechanistic Insights from Oxygen Isotope Signatures.

Mitigation of undesired byproducts from ozonation of dissolved organic matter (DOM) such as aldehydes and ketones is currently hampered by limited knowledge of their precursors and formation pathways. Here, the stable oxygen isotope composition of H2O2 formed simultaneously with these byproducts was studied to determine if it can reveal this missing information. A newly developed procedure, which quantitatively transforms H2O2 to O2 for subsequent 18O/16O ratio analysis, was used to determine the δ18O of H2O2 generated from ozonated model compounds (olefins and phenol, pH 3-8). A constant enrichment of 18O in H2O2 with a δ18O value of ∼59‰ implies that 16O-16O bonds are cleaved preferentially in the intermediate Criegee ozonide, which is commonly formed from olefins. H2O2 from the ozonation of acrylic acid and phenol at pH 7 resulted in lower 18O enrichment (δ18O = 47-49‰). For acrylic acid, enhancement of one of the two pathways followed by a carbonyl-H2O2 equilibrium was responsible for the smaller δ18O of H2O2. During phenol ozonation at pH 7, various competing reactions leading to H2O2 via an intermediate ozone adduct are hypothesized to cause lower δ18O in H2O2. These insights provide a first step toward supporting pH-dependent H2O2 precursor elucidation in DOM.

A Tale of Two Treatments: The Multiple Barrier Approach to Removing Chemical Contaminants During Potable Water Reuse.

In response to water scarcity and an increased recognition of the risks associated with the presence of chemical contaminants, environmental engineers have developed advanced water treatment systems that are capable of converting municipal wastewater effluent into drinking water. This practice, which is referred to as potable water reuse, typically relies upon reverse osmosis (RO) treatment followed by exposure to ultraviolet (UV) light and addition of hydrogen peroxide (H2O2). These two treatment processes individually are capable of controlling many of the chemical and microbial contaminants in wastewater; however, a few chemicals may still be present after treatment at concentrations that affect water quality. Low-molecular weight (<200 Da), uncharged compounds represent the greatest challenge for RO treatment. For potable water reuse systems, compounds of greatest concern include oxidation products formed during treatment (e.g., N-nitrosodimethylamine, halogenated disinfection byproducts) and compounds present in wastewater effluent (e.g., odorous compounds, organic solvents). Although the concentrations of most of these compounds decrease to levels where they no longer compromise water quality after they encounter the second treatment barrier (i.e., UV/H2O2), low-molecular weight compounds that are resistant to direct photolysis and exhibit low reactivity with hydroxyl radical (·OH) may persist. While attempts to identify the compounds that pass through both barriers have accounted for approximately half of the dissolved organic carbon remaining after treatment, it is unlikely that a significant fraction of the remaining unknowns will ever be identified with current analytical techniques. Nonetheless, the toxicity-weighted concentration of certain known compounds (e.g., disinfection byproducts) is typically lower in RO-UV/H2O2 treated water than conventional drinking water. To avoid the expense associated with managing the concentrate produced by RO, environmental engineers have begun to employ alternative treatment barriers. The use of alternatives such as nanofiltration, ozonation followed by biological filtration, or activated carbon filtration avoids the problems associated with the production and disposal of RO concentrate, but they may allow a larger number of chemical contaminants to pass through the treatment process. In addition to the transformation products and solvents that pose risks in the RO-UV/H2O2 system, these alternative barriers are challenged by larger, polar compounds that are not amenable to oxidation, such as perfluoroalkyl acids and phosphate-containing flame retardants. To fully protect consumers who rely upon potable water reuse systems, new policies are needed to prevent chemicals that are difficult to remove during advanced treatment from entering the sewer system. By using knowledge about the composition of municipal wastewater and the mechanisms through which contaminants are removed during treatment, it should be possible to safely reuse municipal wastewater effluent as a drinking water source.

Kinetic and mechanistic investigations of the oxidation of tramadol by ferrate and ozone.

The kinetics and oxidation products (OPs) of tramadol (TRA), an opioid, were investigated for its oxidation with ferrate (Fe(VI)) and ozone (O(3)). The kinetics could be explained by the speciation of the tertiary amine moiety of TRA, with apparent second-order rate constants of 7.4 (±0.4) M(-1) s(-1) (Fe(VI)) and 4.2 (±0.3) × 10(4) M(-1) s(-1) (O(3)) at pH 8.0, respectively. In total, six OPs of TRA were identified for both oxidants using Qq-LIT-MS, LTQ-FT-MS, GC-MS, and moiety-specific chemical reactions. In excess of oxidants, these OPs can be further transformed to unidentified OPs. Kinetics and OP identification confirmed that the lone electron pair of the amine-N is the predominant site of oxidant attack. An oxygen transfer mechanism can explain the formation of N-oxide-TRA, while a one-electron transfer may result in the formation of N-centered radical cation intermediates, which could lead to the observed N-dealkylation, and to the identified formamide and aldehyde derivatives via several intermediate steps. The proposed radical intermediate mechanism is favored for Fe(VI) leading predominantly to N-desmethyl-TRA (ca. 40%), whereas the proposed oxygen transfer prevails for O(3) attack resulting in N-oxide-TRA as the main OP (ca. 90%).

Degradation rates of benzotriazoles and benzothiazoles under UV-C irradiation and the advanced oxidation process UV/H2O2.

Benzotriazoles (BTs) and benzothiazoles (BTHs) are extensively used chemicals found in a wide range of household and industrial products. They are chemically stable and are therefore ubiquitous in the aquatic environment. The present study focuses on the potential of ultraviolet (UV) irradiation, alone or in combination with hydrogen peroxide (H2O2), to remove BTs and BTHs from contaminated waters. Six compounds, three out of each chemical class, were investigated using a low-pressure mercury lamp (main emission at 254 nm) as the radiation source. Initially, the direct phototransformation kinetics and quantum yield in dilute aqueous solution was studied over the pH range of 4-12. All BTs and BTHs, except for benzothiazole, exhibited pH-dependent direct phototransformation rate constants and quantum yields in accordance to their acid-base speciation (7.1 < pKa < 8.9). The direct phototransformation quantum yields (9.0 × 10(-4)-3.0 × 10(-2) mol einstein(-1)), as well as the photon fluence-based rate constants (1.2-48 m(2) einstein(-1)) were quite low. This suggests that UV irradiation alone is not an efficient method to remove BTs and BTHs from impacted waters. The second-order rate constants for the reaction of selected BTs and BTHs with the hydroxyl radical were also determined, and found to fall in the range of 5.1-10.8 × 10(9) M(-1) s(-1), which is typical for aromatic contaminants. Finally, the removal of BTs and BTHs was measured in wastewater and river water during application of UV irradiation or the advanced oxidation process UV/H2O2. The latter process provided an efficient removal, mostly due to the effect of the hydroxyl radical, that was comparable to other aromatic aquatic contaminants, in terms of energy requirement or treatment costs.