WaterSubject

Second-order rate constants of the direct ozone reactions (kO3,M) and the indirect OH radical reactions (kOH,M) for nine chemicals on the US EPA’s Drinking Water Contaminant Candidate List (CCL) were studied during the ozonation and ozone/hydrogen peroxide advanced oxidation process (O3/H2O2 AOP) using batch reactors. Except for the thiocarbamate herbicides (molinate and EPTC), all other CCL chemicals (linuron, diuron, prometon, RDX, 2,4-dinitrotoluene, 2,6-dinitrotoluene and nitrobenzene) show low reactivity toward ozone. The general magnitude of ozone reactivity of the CCL chemicals can be explained by their structures and the electrophilic nature of ozone reactions. The CCL chemicals (except RDX) are highly reactive toward OH radicals as demonstrated by their high kOH,M values. Ozonation at low pH, which involves mainly the direct ozone reaction, is only efficient for the removal of the thiocarbamates. Ozonation at high pH and O3/H2O2 AOP will be highly efficient for the treatment of all chemicals in this study except RDX, which shows the lowest OH radical reactivity. Removal of a contaminant does not mean complete mineralization and reaction byproducts may be a problem if they are recalcitrant and are likely to cause health concerns.

The ozonation and ozone/hydrogen peroxide advanced oxidation process (O3/H2O2 AOP) are recognized to be effective treatment processes to achieve degradation of contaminants in drinking water. The knowledge of chemical reaction kinetics is essential for predicting oxidation efficiencies of pollutants in these processes. During ozonation and O3/H2O2 AOP, a pollutant (M) reacts both with O3 (the direct reactions) and OH radicals (the indirect reactions) through second-order kinetics. Prediction of the extent of degradation through these two pathways relies on knowing the intrinsic second-order rate constants for reactions of the pollutant with ozone (kO3,M) and OH radicals (kOH,M). Although a large amount of kinetic data are available on the reaction rate constants of OH radicals and ozone with chemicals in aqueous solutions, there is still a dearth of data especially for the emerging pollutants such as those on the US EPA’s Drinking Water Contaminant Candidate List (CCL).
The objectives of the research were to determine the direct and indirect rate constants (kO3,M and kOH,M) and the treatability for nine chemicals on the CCL by ozonation and the O3/H2O2 AOP. The pollutants studied include thiocarbamate, triazine and urea herbicides, RDX and substituted benzene compounds. Results were compared with literature data and a parallel study using UV/H2O2 AOP. The reactivity of the chemicals was correlated to their structures and reaction mechanisms.

Most chemicals (>99% purity) were purchased from Chem Service Inc. (West Chester, PA, USA). RDX was from AccuStandard Inc. (CT, USA) sold as solution in acetonitrile (ACN). All chemicals were used as received without further purification. Stock solutions of the CCL chemicals were prepared in dichloromethane (DCM), except for RDX which was in ACN. Aqueous reaction solutions were prepared by dispensing an appropriate amount of the stock solution into a dry flask, and then blowing off the solvent by a gentle flow of nitrogen gas before the phosphate-buffered water was added. Reaction solutions were stirred overnight for complete chemical dissolution. All other stock solutions were prepared in Milli-Q nanopure water.

Aqueous ozone stock solutions were prepared by continuously bubbling ozone produced by an oxygen-feed (oxygen >99%) ozonator (OSMONICS OREC^TM V Series, Phoenix, AZ, USA) into Milli-Q water chilled in an ice-water bath (Bader and Hoigné, 1981). The concentration of ozone stock solution was about 35 mg L-1. pH of all experiments were adjusted by adding orthophosphate buffers (H3PO4/KH2PO4/Na2HPO4) so that the total phosphate concentration in the final solution was 0.02 M.
All experiments were performed in pH-buffered Milli-Q nanopure water at room temperature (22 °C). Experimental setups and procedures are similar to those describe.
The direct rate constants of diuron, linuron and RDX with ozone (kO3,M) were measured in a 1-L dispenser using solute consumption method under pseudo-first-order conditions. Initial concentrations of the CCL chemicals were about 0.5-1 ?M, and the applied ozone doses ranged from 1.8 to 4.4 ppm (37.5-91.7 ?M). Hence, the ozone concentrations were in an approximate range of 37×to 183×excess. Furthermore, experiments were performed under acidic conditions (pH 2.4) and with 10 mM tertiary butanol to minimize ozone decomposition and any OH radical reactions with the CCL chemicals. The reaction conditions were such that the ozone concentration remained relatively constant (usually 8% decrease). The rate constants for the direct reaction were determined by monitoring chemical decay over time according to the following equation:

Nine chemicals on the US EPA’s Contaminant Candidate List were studied for their reactivity with ozone and OH radicals during the ozonation and O3/H2O2 AOP process. Second-order reaction rate constants with ozone and OH radicals (kO3,M and kOH,M) were either measured or estimated from structure/activity relationships with regard to the reaction mechanisms. The urea herbicides (linuron and diuron), triazine (prometon), RDX and substituted benzene compounds (nitrobenzene, 2,4-dinitrotoluene and 2,6-dinitrotoluene) show low reactivity toward ozone, whereas the thiocarbamates (molinate and EPTC) are highly reactive toward O3 due to the O3 reactive S atom. All chemicals (except RDX) demonstrate high reactivity toward OH radicals.

In the water treatment application, only the thiocarbamate herbicides can be efficiently treated by ozonation at low pH, which involves mainly the direct ozone reactions. Ozonation at high pH and O3/H2O2 AOP will be highly efficient for the removal of the thiocarbamates, ureas, prometon and substituted benzenes. However, product studies show that most chemicals are unlikely to be mineralized and the reaction byproducts may be a problem if they are likely to cause health concerns. RDX is more resistant to ozonation and O3/H2O2 AOP than any other CCLs in this study.