Photochemical and bacterial transformations of disinfection by-product precursors in water.

In situ grab sampling from source waters and water extraction from source materials are common methods for determining disinfection by-product (DBP) formation potential (FP) of water samples or reactivity of dissolved organic matter (DOM) in forming DBPs during chlorination. However, DOM, as the main DBP precursor, collected using these techniques may not represent the DOM reacting with disinfectants due to biogeochemical alterations during water conveyance to drinking water treatment facilities. In this study, we exposed leachates from fresh litter and associated decomposed duff to natural sunlight or K-12 for 14 d and evaluated the changes, if any, on the propensity to form trihalomethane (THM), haloacetonitrile (HAN), and chloral hydrate (CHD) during chlorination. Sunlight treatment did not significantly change dissolved organic carbon (DOC) concentration but caused a 24 to 43% decrease in the specific ultraviolet absorbance (SUVA) at 254 nm, indicating that UV-active chromophores were transformed or degraded. There were significant increases ( < 0.05) in specific HAN formation potential (HAN-FP) and specific CHD formation potential (CHD-FP) (i.e., HAN and CHD formation potentials per unit carbon), but no change in specific THM formation potential (THM-FP) after sunlight exposure. In contrast, bacterial treatment did not show any significant effect on SUVA, specific chlorine demand, or any specific DBP-FPs, although bacterial colony counts suggested DOM in leachates was utilized for bacterial growth. Results of this study confirmed that the reactivity of DOM in forming DBPs could be different after biogeochemical processes compared with its source materials. For this study, photochemical reactions had a greater effect on DBP-FPs than did microbial degradation.

Photoelectrochemical degradation of Methylene Blue with beta-PbO2 electrodes driven by visible light irradiation.

Beta-PbO2 electrodes were prepared by electro-deposition and characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and linear sweep voltammetry. We confirmed pure (beta-PbO2 crystals were on the electrode and it had a high oxygen evolution potential. The photoactivity and photoelectrochemical (PEC) properties of the beta-PbO2 electrode were investigated under visible light irradiation (lambda > 420 nm) for the decolorization of Methylene Blue. Pseudo first-order kinetics parameter (K(app)) for dye decolorization using the beta-PbO2 electrode achieved 6.71 x 10(-4) min(-1) under visible light irradiation, which indicated its excellent visible light-induced photoactivity. The K(app) of the PEC process was as much as 1.41 x 10(-3) min(-1) and was 1.71 times that of visible light irradiation or electrolysis even in the presence of the beta-PbO2 electrode. A significant synergetic effect was observed in the PEC system. We also employed TiO2 modified beta-PbO2 electrodes in this test, which revealed that the TiO2 immobilized on the beta-PbO2 electrode inhibited the visible light-induced PEC efficiency despite the amount of TiO2 used for electrode preparation. The beta-PbO2 electrode was also superior to the dimensionally stable anode (Ti/Ru(0.3)Ti(0.7)O2) in visible light-induced photoactivity and PEC efficiency.

Reactivity of litter leachates from California oak woodlands in the formation of disinfection by-products.

Litter materials from forested watersheds can be a significant source of dissolved organic matter (DOM) to surface waters that can contribute to the formation of carcinogenic disinfection by-products (DBPs) during drinking-water chlorination. This study characterized the reactivity of DOM from litter leachates of representative vegetation in oak woodlands, a major plant community in the Foothill Region of California. Leachates from fresh and decomposed litter (duff) from two oak species, pine, and annual grasses were collected for an entire rainy season to evaluate their reactivity to form DBPs on chlorination. Relationships among specific ultraviolet absorbance (SΔUVA), fluorescence index (FI), specific differential ultraviolet absorbance (SΔUVA), specific chlorine demand (SCD), and the dissolved organic carbon:dissolved organic nitrogen (DOC:DON) ratio to the specific DBP formation potential (SDBP-FP) were examined. The DOM derived from litter materials had considerable reactivity in forming trihalomethanes (THMs) (1.80-3.49 mmol mol), haloacetic acid (HAAs) (1.62-2.76 mmol mol(-1)), haloacetonitriles (HANs) (0.12-0.37 mmol mol(-1)), and chloral hydrate (CHD) (0.16-0.28 mmol mol). These values are comparable to other identified watershed sources of DBP precursors reported for the California Delta, such as wetlands and organic soils. Vegetation type and litter decomposition stage (fresh litter versus 1-5 yr-old duff) were key factors that determined characteristics of DOM and their reactivity to form DBPs. Pine litter had significantly lower specific THM formation potential compared with oak and grass, and decomposed duff had a greater DON content, which is a precursor of HANs and other nitrogenous DBPs. The SΔUVA and SDBP-FP were temporally variable and dependent on vegetation type, degree of decomposition, and environmental conditions. Among the optical properties of DOM, SΔUVA was the only parameter that was consistently correlated with SDBP-FP.

Effective photocatalytic disinfection of E. coli K-12 using AgBr-Ag-Bi2WO6 nanojunction system irradiated by visible light: the role of diffusing hydroxyl radicals.

Urgent development of effective and low-cost disinfecting technologies is needed to address the problems caused by an outbreak of harmful microorganisms. In this work, we report an effective photocatalytic disinfection of E. coli K-12 by using a AgBr-Ag-Bi(2)WO(6) nanojunction system as a catalyst under visible light (lambda >or= 400 nm) irradiation. The visible-light-driven (VLD) AgBr-Ag-Bi(2)WO(6) nanojunction could completely inactivate 5 x 10(7) cfu mL(-1) E. coli K-12 within 15 min, which was superior to other VLD photocatalysts such as Bi(2)WO(6) superstructure, Ag-Bi(2)WO(6) and AgBr-Ag-TiO(2) composite. Moreover, the photochemical mechanism of bactericidal action for the AgBr-Ag-Bi(2)WO(6) nanojunction was investigated by using different scavengers. It was found that the diffusing hydroxyl radicals generated both by the oxidative pathway and the reductive pathway play an important role in the photocatalytic disinfection. Moreover, direct contact between the AgBr-Ag-Bi(2)WO(6) nanojunction and bacterial cells was not necessary for the photocatalytic disinfection of E. coli K-12. Finally, the photocatalytic destruction of the bacterial cells was directly observed by TEM images and further confirmed by the determination of potassium ion (K(+)) leakage from the killed bacteria. This work provides a potential effective VLD photocatalyst to disinfect the bacterial cells, even to destruct the biofilm that can provide shelter and substratum for microorganisms and resist to disinfection.

Zn:In(OH)ySz solid solution nanoplates: synthesis, characterization, and photocatalytic mechanism.

Zn:In(OH)ySz solid solution nanoplates (Zn:In(OH)ySz-SSNs) with uniform nanoparticle size were synthesized through a simple sodium dodecyl sulfate (SDS)-assisted hydrothermal process. To achieve better photoabsorption in the visible light (VL) region and suitable redox potentials of the Zn:In(OH)ySz solid solution (Zn:In(OH)ySz-SS), the substitution of S(2-) for OH was carried out by adjusting the concentration of thiourea and SDS in the synthesis solution, while the doping of Zn2+ was realized by adjusting Zn2+ concentration. In addition, the morphology and crystallinity of Zn:In(OH)ySz-SSs were also controlled by the concentration of SDS. Using Rhodamine B (RhB) as a target pollutant the photocatalytic performance of these Zn:In(OH)ySz-SSs with different components, diameter sizes, and morphologies was investigated. Remarkably, Zn:In(OH)ySz-SSNs prepared with atomic ratio of Zn2+ and In3+ of 0.6, 45 mmol L(-1) thiourea, and 26 mmol L(-1) SDS, have the highest visible-light-driven (VLD) photocatalytic activity, exceeding 95% for the degradation of RhB after 60 min. The investigation of photocatalylic mechanism further indicates that the holes, superoxide radical (*O2(-)) and surficial hydroxyl radical (*OHs) are the major reactive species for the photocatalytic reactions. More importantly, for the first time, a simple and versatile strategy is developed to confirm the fact that direct contact between the Zn:In(OH)ySz-SS and RhB is the prerequisite for the photocatalytic degradation of RhB. Therefore, we report not only the preparation of a novel and effective VL-driven photocatalyst, but also provide mechanistic insight into semiconductor photocatalysis.

Trihalomethane, haloacetonitrile, and chloral hydrate formation potentials of organic carbon fractions from sub-tropical forest soils.

Forest landscapes represent the major land-cover type for the watersheds of the East River, which is the source of water for 40 million people in South China. Forest soils with high levels of organic carbon are a potential terrestrial source of dissolved organic carbon (DOC) into the East River. DOC is of great concern, since it can form carcinogenic disinfection byproducts (DBPs) during drinking water treatment. In this study, soils from three altitudes (200, 450 and 900 m) in the Xiangtou Mountain Nature Reserve in South China, representing soils from evergreen moon forest, transitional evergreen broadleaf forest, and evergreen broadleaf forest, respectively, were evaluated for their potential contributions of DBP precursors into the East River. The water extractable organic carbon (WEOC) in three forest soils was physically and chemically fractionated into particulate organic carbon (1.2-0.45 microm), colloidal organic carbon (0.45-0.22 microm), and dissolved organic carbon (DOC) (<0.22 microm), hydrophobic acid (HPOA), transphilic acid and hydrophilic acid and were analysed for the formation potentials of trihalomethanes (THMs), haloacetonitriles (HANs), and chloral hydrate (CHD). Also, soils were incubated at 15, 25 and 35 degrees C for 14d in darkness to examine the impact of temperature effects on the availability and characteristics of WEOC. The extraction study showed that the amount of WEOC was proportional to soil organic carbon content, of which about 1% was water extractable. Regardless of soil type, DOC and HPOA were the most reactive fractions in forming THMs, CHD, and HANs. Production of DOC and HPOA in WEOC increased over 14 d incubation as incubation temperature increased, but the temperature did not alter the distribution of physical and chemical fractions and their reactivity in DBP formation. Results suggest higher inputs of DOC and DBP precursors from forest watersheds into source water may result in a warmer environment.