The hydroxyl radical (HO*) is a strong oxidant that reacts with electron-rich sites of organic compounds and initiates complex chain mechanisms. In order to help understand the reaction mechanisms, a rule-based model was previously developed to predict the reaction pathways. For a kinetic model, there is a need to develop a rate constant estimator that predicts the rate constants for a variety of organic compounds. In this study, a group contribution method (GCM) is developed to predict the aqueous phase HO* rate constants for the following reaction mechanisms: (1) H-atom abstraction, (2) HO* addition to alkenes, (3) HO* addition to aromatic compounds, and (4) HO* interaction with sulfur (S)-, nitrogen (N)-, or phosphorus (P)-atom-containing compounds. The GCM hypothesizes that an observed experimental rate constant for a given organic compound is the combined rate of all elementary reactions involving HO*, which can be estimated using the Arrhenius activation energy, E(a), and temperature. Each E(a) for those elementary reactions can be comprised of two parts: (1) a base part that includes a reactive bond in each reaction mechanism and (2) contributions from its neighboring functional groups. The GCM includes 66 group rate constants and 80 group contribution factors, which characterize each HO* reaction mechanism with steric effects of the chemical structure groups and impacts of the neighboring functional groups, respectively. Literature-reported experimental HO* rate constants for 310 and 124 compounds were used for calibration and prediction, respectively. The genetic algorithms were used to determine the group rate constants and group contribution factors. The group contribution factors for H-atom abstraction and HO* addition to the aromatic compounds were found to linearly correlate with the Taft constants, sigma*, and electrophilic substituent parameters, sigma+, respectively. The best calibrations for 83% (257 rate constants) and predictions for 62% (77 rate constants) of the rate constants were within 0.5-2 times the experimental values. This accuracy may be acceptable for model predictions of the advanced oxidation processes (AOPs) performance, depending on how sensitive the model is to the rate constants.

The broadband UV irradiation of 1.1 mM trichloroethene (TCE) aqueous solution in the presence of 10.4 mM H2O2 resulted in formic, oxalic, dichloroacetic (DCA), and monochloroacetic (MCA) acids, as organic byproducts. The organic chlorine was converted completely to chloride ion as a final product. TCE and its degradation products were completely mineralized in 30 min, under a volume-averaged UV-C irradiant power of 35.7 W/L from a 1 kW medium-pressure mercury vapor arc lamp. TCE degraded primarily through hydroxyl radical-induced reactions and onlyto a low extentthrough direct UV photolysis and chlorine atom-induced chain reactions. The experimental patterns of TCE, H2O2, and detected reaction products combined with the literature information on radical reactions in the aqueous phase were used to postulate a degradation mechanism and to develop a kinetic model to predict the TCE decay, formation and degradation of byproducts, and pH and oxygen profiles. The agreement between the model calculations and the experimental data is satisfactory.

The radical reaction mechanism that is involved in advanced oxidation processes is complex. An increasing number of trace contaminants and stringent drinking water standards call for a rule-based model to provide insight to the mechanism of the processes. A model was developed to predict the pathway of contaminant degradation and byproduct formation during advanced oxidation. The model builds chemical molecules as graph objects, which enables mathematic abstraction of chemicals and preserves chemistry information. The model algorithm enumerates all possible reaction pathways according to the elementary reactions (built as reaction rules) established from experimental observation. The method can predict minor pathways that could lead to toxic byproducts so that measures can be taken to ensure drinking water treatment safety. The method can be of great assistance to water treatment engineers and chemists who appreciate the mechanism of treatment processes.

Direct UV photolysis of trichloroethylene (TCE) in dilute aqueous solution generated chloride ions as a major end product and several reaction intermediates, such as formic acid, di- and monochloroacetic acids, glyoxylic acid, and, to a lesser extent, mono- and dichloroacetylene, formaldehyde, dichloroacetaldehyde, and oxalic acid. Under prolonged irradiation, these byproducts underwent photolysis, and a high degree of mineralization (approximately 95%) was achieved. TCE decays through the following major pathways: (1) TCE + h nu --> ClCH=C*Cl + Cl*; (2) TCE (H2O) + h nu --> ClCH(OH)-CHCl2; (3) TCE + h nu --> HC[triple bond]CCl + Cl2; (4) TCE + h nu --> ClC[triple bond]CCl + HCl; (5) TCE + Cl* --> Cl2HC-C*Cl2. A kinetic model was developed to simulate the destruction of TCE and the formation and fate of byproducts in aqueous solution under irradiation with polychromatic light. By fitting the experimental data, the quantum yields for the four photolysis steps were predicted as phi(1) = 0.13, phi(2) = 0.1, phi(3) = 0.032, and phi(4) = 0.092, respectively. The reaction mechanism proposed for the photodegradation of TCE accounts for all intermediates that were detected. The agreement between the computed and experimental patterns of TCE and reaction products is satisfactory given the complexity of the reaction mechanism and the lack of photolytic kinetic parameters that are provided in the literature.

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The issues of water supply and management will become more and more critical as the global population increases. In order to meet future demands, water supply systems must be developed to maximize the use of locally available water. It is also important to minimize the impact of water system developments on the environment. In this study, the overall environmental impacts were compared for water importation, reclamation and seawater desalination to address the water scarcity in areas where local supplies are not sufficient. The city of Scottsdale, Arizona was chosen for this study. Life Cycle Assessment (LCA) was performed and it suggests that seawater desalination has the highest impact whereas reclamation shows a relatively lower impact. However, Importation and reclamation systems have comparable results for several damage categories. The impacts of facility operations are significantly higher than the construction phase even when the life-span of infrastructure reduces from 50 year to 10 year. Due to the high impacts associated with the energy use during plant operations, different energy mixes were analyzed for their capabilities to lower the environmental burden.

Rain water harvesting (RWH) has gained popularity as a way of supplementing water supplies for various purposes, including drinking, sanitation and irrigation. This paper presents a methodology of life cycle cost assessment (LCCA) of a unit RWH system (hereafter RWH system) for toilet flushing in an industrial site. The life cycle cost and net present value benefits (NPVB) were estimated for the RWH system and compared with those of a conventional system. For the current system design, the analysis of the life cycle cost of the RWH system indicates negative NPVB for all plausible service lives up to 55 years, mainly because of the initial infrastructure investment costs, operation and maintenance (O&M) costs, and pumping costs for the system. However, sensitivity analysis concluded that an alternative design with no pump, low O&M costs (5%) and 1% tank refill volume may be economically viable given 7 years of service life. The sensitivity analysis also revealed that higher hypothetical water prices ($5/m3) may lead to positive NPVB after only 5 years of service. Full cost pricing for rainwater harvesting is important for the promotion of sustainable practices and life cycle based system design is critical to make RWH systems economically attractive.

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