Urs von Gunten

The Challenge of Micropollutants in Aquatic Systems

The increasing worldwide contamination of freshwater systems with thousands of industrial and natural chemical compounds is one of the key environmental problems facing humanity. Although most of these compounds are present at low concentrations, many of them raise considerable toxicological concerns, particularly when present as components of complex mixtures. Here we review three scientific challenges in addressing water-quality problems caused by such micropollutants. First, tools to assess the impact of these pollutants on aquatic life and human health must be further developed and refined. Second, cost-effective and appropriate remediation and water-treatment technologies must be explored and implemented. Third, usage and disposal strategies, coupled with the search for environmentally more benign products and processes, should aim to minimize introduction of critical pollutants into the aquatic environment.

Ozonation of drinking water: Part I. Oxidation kinetics and product formation

The oxidation of organic and inorganic compounds during ozonation can occur via ozone or OH radicals or a combination thereof. The oxidation pathway is determined by the ratio of ozone and OH radical concentrations and the corresponding kinetics. A huge database with several hundred rate constants for ozone and a few thousand rate constants for OH radicals is available. Ozone is an electrophile with a high selectivity. The second-order rate constants for oxidation by ozone vary over 10 orders of magnitude, between < 0.1 M(-1)s(-1) and about 7 x 10(9) M(-1)s(-1). The reactions of ozone with drinking-water relevant inorganic compounds are typically fast and occur by an oxygen atom transfer reaction. Organic micropollutants are oxidized with ozone selectively. Ozone reacts mainly with double bonds, activated aromatic systems and non-protonated amines. In general, electron-donating groups enhance the oxidation by ozone whereas electron-withdrawing groups reduce the reaction rates. Furthermore, the kinetics of direct ozone reactions depend strongly on the speciation (acid-base, metal complexation). The reaction of OH radicals with the majority of inorganic and organic compounds is nearly diffusion-controlled. The degree of oxidation by ozone and OH radicals is given by the corresponding kinetics. Product formation from the ozonation of organic micropollutants in aqueous systems has only been established for a few compounds. It is discussed for olefines, amines and aromatic compounds.

Ozonation of drinking water: Part II. Disinfection and by-product formation in presence of bromide, iodide or chlorine

Ozone is an excellent disinfectant and can even be used to inactivate microorganisms such as protozoa which are very resistant to conventional disinfectants. Proper rate constants for the inactivation of microorganisms are only available for six species (E. coli, Bacillus subtilis spores, Rotavirus, Giardia lamblia cysts, Giardia muris cysts, Cryptosporidium parvum oocysts). The apparent activation energy for the inactivation of bacteria is in the same order as most chemical reactions (35–50 kJ mol � 1 ), whereas it is much higher for the inactivation of protozoa (80 kJ mol � 1 ). This requires significantly higher ozone exposures at low temperatures to get a similar inactivation for protozoa. Even for the inactivation of resistant microorganisms, OH radicals only play a minor role. Numerous organic and inorganic ozonation disinfection/oxidation by-products have been identified. The by-product of main concern is bromate, which is formed in bromide-containingwaters. A low drinkingwater standard of 10 m gl � 1 has been set for bromate. Therefore, disinfection and oxidation processes have to be evaluated to fulfil these criteria. In certain cases, when bromide concentrations are above about 50m gl � 1 , it may be necessary to use control measures to lower bromate formation (loweringof pH, ammonia addition). Iodate is the main by-product formed duringozonation of iodidecontainingwaters. The reactions involved are direct ozone oxidations. Iodate is considered non-problematic because it is transformed back to iodide endogenically. Chloride cannot be oxidized during ozonation processes under drinking water conditions. Chlorate is only formed if a preoxidation by chlorine and/or chlorine dioxide has occured. r 2002 Elsevier Science Ltd. All rights reserved.

Elimination of Organic Micropollutants in a Municipal Wastewater Treatment Plant Upgraded with a Full-Scale Post-Ozonation Followed by Sand Filtration

The removal efficiency for 220 micropollutants was studied at the scale of a municipal wastewater treatment plant (WWTP) upgraded with post-ozonation followed by sand filtration. During post-ozonation, compounds with activated aromatic moieties, amine functions, or double bonds such as sulfamethoxazole, diclofenac, or carbamazepine with second-order rate constants for the reaction with ozone >10(4) M(-1) s(-1) at pH 7 (fast-reacting) were eliminated to concentrations below the detection limit for an ozone dose of 0.47 g O3 g(-1) dissolved organic carbon (DOC). Compounds more resistant to oxidation by ozone such as atenolol and benzotriazole were increasingly eliminated with increasing ozone doses, resulting in >85% removal for a medium ozone dose (approximately 0.6 g O3 g(-1) DOC). Only a few micropollutants such as some X-ray contrast media and triazine herbicides with second-order rate constants <10(2) M(-1) s(-1) (slowly reacting) persisted to a large extent. With a medium ozone dose, only 11 micropollutants of 55 detected in the secondary effluent were found at >100 ng L(-1). The combination of reaction kinetics and reactor hydraulics, based on laboratory-and full-scale data, enabled a quantification of the results by model calculations. This conceptual approach allows a direct upscaling from laboratory- to full-scale systems and can be applied to other similar systems. The carcinogenic by-products N-nitrosodimethylamine (NDMA) (< or =14 ng L(-1)) and bromate (<10 microg L(-1)) were produced during ozonation, however their concentrations were below or in the range of the drinking water standards. Furthermore, it could be demonstrated that biological sand filtration is an efficient additional barrier for the elimination of biodegradable compounds formed during ozonation such as NDMA. The energy requirement for the additional post-ozonation step is about 0.035 kWh m(-3), which corresponds to 12% of a typical medium-sized nutrient removal plant (5 g DOC m(-3)).

Hydroxyl Radical/Ozone Ratios During Ozonation Processes. I. The Rct Concept

Abstract The ozonation of model systems and several natural waters was examined in bench-scale batch experiments. In addition to measuring the concentration of ozone (O3), the rate of depletion of an in situ hydroxyl radical probe compound was monitored, thus providing information on the transient steady-state concentration of hydroxyl radicals (√OH). A new parameter, Rct , representing the ratio of the √OH-exposure to the O3-exposure was calculated as a function of reaction time. For most waters tested, including pH-buffered model systems and natural waters, Rct was a constant value for the majority of the reaction. Therefore, Rct corresponds to the ratio of the √OH concentration to the O3 concentration in a given water (i.e. Rct = [√OH]/[O3]). For a given water source, the degradation of a micropollutant (e.g. atrazine) via O3 and √OH reaction pathways can be predicted by the O3 reaction kinetics and Rct .

Bromate Formation During Ozonation Of Bromide-Containing Waters - Interaction Of Ozone And Hydroxyl Radical Reactions

Kinetic simulations have been tested by laboratory experiments to evaluate the major factors controlling bromate formation during ozonation of waters containing bromide. In the presence of an organic scavenger for OH radicals, bromate formation can be accurately predicted by the molecular ozone mechanism using published reaction rate data, even for waters containing ammonium In the absence of scavengers, OH radical reactions contribute significantly to bromate formation. Carbonate radicals, produced by the oxidation of bicarbonate with OH radicals, oxidize the intermediate hypobromite to bromite, which is further oxidized by ozone to bromate. During drinking water ozonation, molecular ozone controls both the initial oxidation of bromide and the final oxidation of bromite. OH radical reactions contribute to the oxidation of the intermediate oxybromine species. Bromate formation in advanced oxidation processes can be explained by a synergism of ozone and OH radicals.

Chlorination of natural organic matter: kinetics of chlorination and of THM formation

The kinetics of the formation of trihalomethanes (THMs) and of chlorine consumption for the chlorination of natural organic matter with an excess of chlorine (50 microM > [Cl2]o >210 microM) was investigated. THM precursors could be divided into a fast and a slowly reacting fraction. Long term chlorine demand and the formation of THM could be described by second order kinetics. Rate constants were between 0.01 and 0.03 M(-1) s(-1) in the pH range 7-9 for surface waters and humic materials extracted from surface waters. A groundwater gave a higher rate constant of 0.124 M(-1) s(-1). Resorcinol-type structures were tested with respect to kinetics and yield of THM formation. They could possibly be responsible for the fast reacting THM precursors. which represent 15-30% of the THM precursors of natural waters. Additional classes of compounds that might contribute to the initial THM formation include readily enolizable compounds such as beta-diketones and beta-ketoacids. Experiments with phenol showed that slowly reacting THM precursors may consist of phenolic compounds. The influence of pretreatments (UV/visible irradiation, ozone and chlorine dioxide) on chlorine demand and THM formation from NOM was also studied: UV/visible irradiation does not alter THM formation but leads to a higher chlorine demand. Preoxidation with ozone leads to a lower THM formation with an unaltered chlorine demand and preoxidation with chlorine dioxide reduces THM formation and the chlorine demand.

Oxidative transformation of micropollutants during municipal wastewater treatment: Comparison of kinetic aspects of selective (chlorine, chlorine dioxide, ferrateVI, and ozone) and non-selective oxidants (hydroxyl radical)

Chemical oxidation processes have been widely applied to water treatment and may serve as a tool to minimize the release of micropollutants (e.g. pharmaceuticals and endocrine disruptors) from municipal wastewater effluents into the aquatic environment. The potential of several oxidants for the transformation of selected micropollutants such as atenolol, carbamazepine, 17 alpha-ethinylestradiol (EE2), ibuprofen, and sulfamethoxazole was assessed and compared. The oxidants include chlorine, chlorine dioxide, ferrate(VI), and ozone as selective oxidants versus hydroxyl radicals as non-selective oxidant. Second-order rate constants (k) for the reaction of each oxidant show that the selective oxidants react only with some electron-rich organic moieties (ERMs), such as phenols, anilines, olefins, and deprotonated-amines. In contrast, hydroxyl radicals show a nearly diffusion-controlled reactivity with almost all organic moieties (k>or=10(9)M(-1) s(-1)). Due to a competition for oxidants between a target micropollutant and wastewater matrix (i.e. effluent organic matter, EfOM), a higher reaction rate with a target micropollutant does not necessarily translate into more efficient transformation. For example, transformation efficiencies of EE2, a phenolic micropollutant, in a selected wastewater effluent at pH 8 varied only within a factor of 7 among the selective oxidants, even though the corresponding k for the reaction of each selective oxidant with EE2 varied over four orders of magnitude. In addition, for the selective oxidants, the competition disappears rapidly after the ERMs present in EfOM are consumed. In contrast, for hydroxyl radicals, the competition remains practically the same during the entire oxidation. Therefore, for a given oxidant dose, the selective oxidants were more efficient than hydroxyl radicals for transforming ERMs-containing micropollutants, while hydroxyl radicals are capable of transforming micropollutants even without ERMs. Besides EfOM, ammonia, nitrite, and bromide were found to affect the micropollutant transformation efficiency during chlorine or ozone treatment.

Effect of Dissolved Organic Matter on the Transformation of Contaminants Induced by Excited Triplet States and the Hydroxyl Radical

Dissolved organic matter (DOM) has recently been shown to reduce the transformation rate of various aqueous organic contaminants submitted to oxidation by excited triplet states, apparently by inhibiting the transformation of oxidation intermediates. The main goals of the present study were to evaluate in more detail the effect of concentration and type of DOM on the triplet-induced transformation rate of four selected organic compounds and to check for an analogous inhibition effect in the case of oxidation induced by hydroxyl radical. A marked inhibition by DOM of triplet-induced oxidation was observed for N,N-dimethylaniline (DMA) and the two antibiotics sulfamethoxazole (SMX) and trimethoprim (TRI), with DOM of terrestrial origin being a more effective inhibitor than DOM of aquatic origin. The results are important to understand the role of DOM both as a photosensitizer and as an inhibitor for the triplet-induced transformation of aquatic contaminants. In contrast, no DOM-induced reduction in second-order rate constant could be observed in competition kinetics experiments for the reaction of hydroxyl radical with a series of 15 organic compounds, covering several classes of aromatic contaminants, indicating that Suwannee River fulvic acid (SRFA) used as reference DOM does not affect this reaction mechanism.

Hydroxyl Radical/Ozone Ratios During Ozonation Processes. II. The Effect of Temperature, pH, Alkalinity, and DOM Properties

Abstract The influence of temperature, pH, alkalinity, and type and concentration of the dissolved organic matter (DOM) on the rate of ozone (O3) decomposition, O3-exposure, •OH-exposure and the ratio Rct of the concentrations of •OH and O3 has been studied. For a standardized single ozone dose of 1 mg/L in all experiments, considerable variations in O3-exposure and •OH-exposure were found. This has important implications for water treatment plants regarding the efficiency of oxidation and disinfection by O3. In oligotrophic surface waters and groundwaters, minimal calibration experiments are needed to model and control the ozonation process, whereas in eutrophic surface waters more frequent measurements of O3 kinetics and Rct values are required to evaluate seasonal variations.