Adsorption of natural organic matter onto carbonaceous surfaces: atomic force microscopy study

The microscopic structure of carbonaceous surfaces exposed to natural organic matter (NOM) under aqueous conditions has been explored using atomic force microscopy (AFM). Dismal Swamp Water was used as the NOM source, while highly ordered pyrolytic graphite (HOPG) served as a surrogate for the graphene sheets that characterize the surface of many carbonaceous materials in aquatic environments. Under acidic conditions, the HOPG surface was covered with a densely packed monolayer of NOM molecules. In some cases, aggregates of well-defined, individual NOM molecules were observed that exhibited a degree of registry with respect to the HOPG substrate. This suggests that adsorbate-substrate interactions play a role in moderating the structure of the adsorbate layer. As the pH increased, the concentration of adsorbed NOM decreased systematically because of increasingly repulsive interactions between adsorbates. Increasing the ionic strength produced a modest increase in the concentration of adsorbed NOM. Ca2+ ions exerted a more pronounced influence on both the surface coverage of adsorbed NOM molecules and the size of individual adsorbates because of the effects of intermolecular complexation. In contrast to the spherical structures observed by AFM under aqueous conditions, adsorbed NOM formed a mixture of "ringlike" assemblies and larger aggregates upon drying.     

Influence of natural organic matter source on copper speciation as demonstrated by Cu binding to fish gills, by ion selective electrode, and by DGT gel sampler

Rainbow trout (Oncorhynchus mykiss, 2 g) were exposed to 0-5 microM total copper in ion-poor water for 3 h in the presence or absence of 10 mg C/L of qualitatively different natural organic matter (NOM) derived from water spanning a large gradient in hydrologic residence time. Accumulation of Cu by trout gills was compared to Cu speciation determined by ion selective electrode (ISE) and by diffusive gradients in thin films (DGT) gel sampler technology. The presence of NOM decreased Cu uptake by trout gills as well as Cu concentrations determined by ISE and DGT. Furthermore, the source of NOM influenced Cu binding by trout gills with high-color, allochthonous NOM decreasing Cu accumulation by the gills more than low-color autochthonous NOM. The pattern of Cu binding to the NOM measured by Cu ISE and by Cu accumulation by DGT samplers was similar to the fish gill results. A simple Cu-gill binding model required an NOM Cu-binding factor (F) that depended on NOM quality to account for observed Cu accumulation by trout gills; values of Fvaried by a factor of 2. Thus, NOM metal-binding quality, as well as NOM quantity, are both important when assessing the bioavailability of metals such as Cu to aquatic organisms.     

Fecal indicator bacteria transport and deposition in saturated and unsaturated porous media

Beach sediment and sand are recognized as nonpoint fecal indicator bacteria (FIB) sources capable of causing water quality and health risks for beach-goers. A comprehensive understanding of the key factors and mechanisms governing the migration and exchange of FIB between beach water column and sediment is desired to better predict FIB concentration variations and assess the associated risk. The transport and retention behavior of two model FIB Enterococcus faecalis (E. faecalis) and Escherichia coli (E. coli) was examined using packed-bed columns in both saturated and unsaturated porous media to evaluate FIB migration potentials at conditions simulating the coastal aquatic environment. Additionally, complementary cell characterization techniques were conducted to better understand the migration behaviors of both FIB strains observed in the column experiments. The mobility of the gram-positive species E. faecalis was much more sensitive to solution chemistry and column saturation level than that of the gram-negative species E. coli. Interaction energy calculations suggest that E. faecalis retention was largely governed by the combination of DLVO (Derjaguin-Landau-Verwey-Overbeek) and non-DLVO (most likely hydrophobic and/or polymer bridging) interactions in saturated porous media, while the combination of DLVO and steric interactions controlled the deposition of E. coli cells. The measured surface properties of the two FIB strains supported the distinct bacteria transport behaviors and the differences of the identified mechanisms for each strain. As a result, E. faecalis showed the least affinity to sand in freshwater and appeared to be irreversibly attached in primary energy minima at elevated salt conditions; whereas the retained E. coli cells were reversibly attached and mostly associated with the secondary energy minima at both freshwater and seawater conditions. In unsaturated porous media, E. faecalis cells seemed to prefer to attachment at air/water interface rather than sand surface, while E. coli showed a similar affinity to the two interfaces. It was proposed that the different surface characteristics of the two FIB strains resulted in the distinct transport and retention behavior in porous media. These results highlight the need for FIB management to consider variations in transport behavior between model FIB when assessing water quality and associated risks.     

Role of Pseudomonas aeruginosa biofilm in the initial adhesion, growth and detachment of Escherichia coli in porous media

This study systematically investigated the impact of Pseudomonas aeruginosa biofilm on the initial adhesion, growth, and detachment of indicator bacteria Escherichia coli JM109 in porous media. Two P. aeruginosa strains, the mucoid PD0300 and wide type PA01 with different extracellular polymeric substance (EPS) composition and secretion capability, were used to grow biofilm in packed beds. Results from the column breakthrough curves and retained JM109 profiles show that the amount and composition of P. aeruginosa biofilm EPS have a profound impact on the deposition and retention of E. coli in porous media. PAO1 biofilm coating improved E. coli retention in the column, whereas PDO300 biofilm coating had only a small impact on E. coli removal. Biofilm surface hydrophobicity and polymeric interactions between the biofilm and E. coli cell surfaces were found to play important roles in controlling the distribution of E. coli along the columns. After initial attachment, E. coli bacteria were able to survive and grow at similar growth rates in columns coated with either PAO1 or PDO300 biofilms with a relatively low nutrient supply. Biofilm detachment was the major mechanism that introduced E. coli bacteria to the bulk fluid long after the contamination event when E. coli cells became an integral part of the biofilm. Findings of this study suggest that biofilm plays a significant role in controlling the initial attachment, growth, and survival of bacteria in porous media, and that the interaction between bacteria and biofilm surfaces should be considered when predicting bacterial and pathogen migration in the environment.     

Quantifying the adhesion and interaction forces between Pseudomonas aeruginosa and natural organic matter

Atomic force microscopy (AFM) was used to characterize interactions between natural organic matter (NOM), and glass or bacteria. Poly(methacrylic acid) (PMA), soil humic Acid (SHA), and Suwannee River humic Acid (SRHA), were adsorbed to silica AFM probes. Adhesion forces (Fadh) for the interaction of organic-probes and glass slides correlated with organic molecular weight (MW), but not with radius of the organic aggregate (R), charge density (Q), or zeta potential (zeta). Two Pseudomonas aeruginosa strains with different lipopolysaccharides (LPS) were chosen: PAO1 (A+B+), whose LPS have common antigen (A-band) + O-antigen (B-band); and mutant AK1401 (A+B-). Fadh between bacteria and organics correlated with organic MW, R, and Q, but not zeta. PAO1 had lower Fadh with silica than NOM, which was attributed to negative charges from the B-band polymers causing electrostatic repulsion. AK1401 adhered stronger to silica than to the organics, perhaps because the absence of the B-band exposed underlying positively charged proteins. DLVO calculations could not explain the differences in the two bacteria or predict qualitative or quantitative trends in interaction forces in these systems. Molecular-level information from AFM studies can bring us closer to understanding the complex nature of bacterial-NOM interactions.     

Evidence for the aquatic binding of arsenate by natural organic matter-suspended Fe(III)

Dialysis experiments with arsenate and three different NOM samples amended with Fe(lll) showed evidence confirming the formation of aquatic arsenate-Fe(Ill)-NOM associations. A linear relationship was observed between the amount of complexed arsenate and the Fe(lll) content of the NOM. The dialysis results were consistent with complex formation through ferric iron cations acting as bridges between the negatively charged arsenate and NOM functional groups and/or a more colloidal association, in which the arsenate is bound by suspended Fe(lll)-NOM colloids. Sequential filtration experiments confirmed that a significant proportion of the iron present at all Fe/C ratios used in the dialysis experiments was colloidal in nature. These colloids may include larger NOM species that are coagulated by the presence of chelated Fe(lll) and/or NOM-stabilized ferric (oxy)hydroxide colloids, and thus, the solution-phase arsenate-Fe(Ill)-NOM associations are at least partially colloidal in nature.     

Commercial titanium dioxide nanoparticles in both natural and synthetic water: comprehensive multidimensional testing and prediction of aggregation behavior

Engineered nanoparticles (ENPs) from industrial applications and consumer products are already being released into the environment. Their distribution within the environment is, among other factors, determined by the dispersion state and aggregation behavior of the nanoparticles and, in turn, directly affects the exposure of aquatic organisms to EPNs. The aggregation behavior (or colloidal stability) of these particles is controlled by the water chemistry and, to a large extent, by the surface chemistry of the particles. This paper presents results from extensive colloidal stability tests on commercially relevant titanium dioxide nanoparticles (Evonik P25) in well-controlled synthetic waters covering a wide range of pH values and water chemistries, and also in standard synthetic (EPA) waters and natural waters. The results demonstrate in detail the dependency of TiO(2) aggregation on the ionic strength of the solution, the presence of relevant monovalent and divalent ions, the presence and copresence of natural organic matter (NOM), and of course the pH of the solution. Specific interactions of both NOM and divalent ions with the TiO(2) surfaces modify the chemistry of these surfaces resulting in unexpected behavior. Results from matrix testing in well-controlled batch systems allow predictions to be made on the behavior in the broader natural environment. Our study provides the basis for a testing scheme and data treatment technique to extrapolate and eventually predict nanoparticle behavior in a wide variety of natural waters.     

Reduction of lead oxide (PbO2) by iodide and formation of iodoform in the PbO2/I(-)/NOM system

Lead oxide (PbO2) can be an important form of lead mineral scale occurring in some water distribution systems. It is believed to be formed by the oxidation of lead-containing plumbing materials by free chlorine. Its reactivity in water, however, has not been well studied. Iodide is also found in source drinking waters, albeit at low concentrations. Consideration of thermodynamics suggests that iodide can be oxidized by PbO2. In this investigation, iodide ion was used as a probe compound to study the reduction of PbO2 and the formation of iodoform, which has been predicted to be a carcinogen, in the presence of natural organic matter (NOM). The reduction of PbO2 by iodide can be expressed as PbO2 + 31(-) + 4H+ --> Pb(2+) + I3(-) + 2H2O, and the reaction kinetics has been determined in this study. In the presence of NOM, I3- reacts with NOM to form iodoform and its concentration is proportional to the NOM concentration. Our results indicate that PbO2 is a very powerful oxidant and can possibly serve as an oxidant reservoir for the formation of iodinated disinfection byproduct through a novel reaction pathway.     

Elucidating differences in the sorption properties of 10 humic and fulvic acids for polar and nonpolar organic chemicals

In this work we present a dataset of more than 1000 natural organic matter (NOM)/air partition coefficients covering polar and nonpolar organic compounds measured in 10 different humic and fulvic acids (HAs/FAs) from terrestrial and aquatic origins. Differences of more than 1 order of magnitude in the sorption coefficients of a given compound measured in HAs and FAs from different origins were found. The terrestrial HAs exhibited substantially higher sorption coefficients compared to aquatic HAs and FAs. The difference between any two types of NOM is mainly reflected by a constant shift in the partition coefficients that applies to all compounds in the same way. This indicates that it is the number of available sorption sites per mass of sorbent rather than the types of intermolecular interactions between the sorbate and the sorbent that governs the major differences between the sorption properties of various types of NOM. The experimental partition coefficients measured in all HAs and FAs were successfully described by polyparameter linear free energy relationships (pp-LFERs) that explicitly account for van der Waals as well as H-donor/acceptor interactions between the sorbate and the sorbent. These pp-LFER equations provide for the first time a tool that allows including the variability of the sorption properties of NOM in environmental fate models.     

Efficacy of chemical treatments in eliminating Salmonella and Escherichia coli O157:H7 on scarified and polished alfalfa seeds

Alfalfa seeds are sometimes subjected to a scarification treatment to enhance water uptake, which results in more rapid and uniform germination during sprout production. It has been hypothesized that this mechanical abrasion treatment diminishes the efficacy of chemical treatments used to kill or remove pathogenic bacteria from seeds. A study was done to compare the effectiveness of chlorine (20,000 ppm), H2O, (8%), Ca(OH)2 (1%), Ca(OH)2 (1%) plus Tween 80 (1%), and Ca(OH)2 (1%) plus Span 20 (1%) treatments in killing Salmonella and Escherichia coli O157:H7 inoculated onto control, scarified, and polished alfalfa seeds obtained from two suppliers. The influence of the presence of organic material in the inoculum carrier on the efficacy of sanitizers was investigated. Overall, treatment with 1% Ca(OH)2 was the most effective in reducing populations of the pathogens. Reduction in populations of pathogens on seeds obtained from supplier I indicate that chemical treatments are less efficacious in eliminating the pathogens on scarified seeds compared to control seeds. However, the effectiveness of chemical treatment in removing Salmonella and E. coli O157:H7 from seeds obtained from supplier 2 was not markedly affected by scarification or polishing. The presence of organic material in the inoculum carrier did not have a marked influence on the efficacy of chemicals in reducing populations of test pathogens. Additional lots of control, scarified, and polished alfalfa seeds of additional varieties need to be tested before conclusions can be drawn concerning the impact of mechanical abrasion on the efficacy of chemical treatment in removing or killing Salmonella and E. coli O157:H7.     

Bromide oxidation and formation of dihaloacetic acids in chloraminated water

A comprehensive reaction model was developed that incorporates the effect of bromide on monochloramine loss and formation of bromine and chlorine containing dihaloacetic acids (DHAAs) in the presence of natural organic matter (NOM). Reaction pathways accounted for the oxidation of bromide to active bromine (Br(l)) species, catalyzed monochloramine autodecomposition, NOM oxidation, and halogen incorporation into DHAAs. The reaction scheme incorporates a simplified reaction pathway describing the formation and termination of Br(l). In the absence of NOM, the model adequately predicted bromide catalyzed monochloramine autodecomposition. The Br(l) reaction rate coefficients are 4 orders of magnitude greater than HOCl for the same NOM sources under chloramination conditions. Surprisingly, the rate of NOM oxidation by Br(l) was faster than bromide catalyzed monochloramine autodecomposition by Br(l) so that the latter reactions could largely be ignored in the presence of NOM. Incorporation of bromine and chlorine into DHAAs was proportional to the amount of NOM oxidized by each halogen and modeled using simple bromine (alpha(Br)) and chlorine (alpha(Cl)) incorporation coefficients. Both coefficients were found to be independent of each other and alpha(Br) was one-half the value of alpha(Cl). This indicates that chlorine incorporates itself into DHAA precursors more effectivelythan bromine. Model predictions compared well with DHAA measurements in the presence of increasing bromide concentrations and is attributable to the increased rate of NOM oxidation, which is rate limited by the oxidation of bromide ion in chloraminated systems.     

Relationship between strength of organic sorbate interactions in NOM and hydration effect on sorption

According to a recent conceptual model for hydration-assisted sorption of organic compounds in natural organic matter (NOM), certain polar moieties of dry NOM are unavailable for compound sorption due to strong intra- and intermolecular NOM interactions. Water molecules solvate these moieties creating new sorption sites at solvated contacts. It is expected that the greater a compound's ability to undergo specific interactions with NOM, the greater will be the hydration-assisted sorption effect, because penetration of compounds into solvated contacts must involve competition with water at the solvated contact. To test this model, we compare the hydration effect on sorption kinetics and equilibrium for 4 compounds with differing abilities to undergo specific interactions with NOM. Sorption measured on Pahokee peat in aqueous systems was fast compared with n-hexadecane (dry) systems. No concentration effect on attainment of sorption equilibrium was observed. m-Nitrophenol exhibited the greatest hydration-assisted sorption effect, benzyl alcohol showed an intermediate effect and acetophenone and nitrobenzene showed no hydration-assisted sorption, on an activity scale. The extent of hydration-assisted sorption effect correlates with compound ability to undergo specific interactions. These results support the conceptual model and demonstrate the importance of polar NOM noncovalent links in organizing the NOM phase and in controlling the hydration effect on sorption of organic compounds.     

Surface roughness of stainless steel influences attachment and detachment of Escherichia coli O157

Determining the influence of surface roughness on Escherichia coli O157 attachment to and detachment from stainless steel (SS) is important for controlling this foodborne pathogen. The aim of this study was to evaluate the interactions of six E. coli strains (four O157:H7, one O157:H12, and one O1:H7) with SS type 304 finishes of various surface roughness: 2B (unpolished surface), 4 (common food grade SS), and 8 (polished smooth surface). In attachment assays (exposure to cell suspensions with periodic swirling), bacteria were enumerated by epifluorescence microscopy, and in detachment assays a blotting technique and atomic force microscopy (AFM) were used. Attachment data suggest that E. coli attach in greater numbers to significantly smoother SS8; however, detachment assays and AFM data suggest cells are more easily removed from this finish. Conversely, attachment to SS2B was lower, and AFM data suggest that E. coli O157 may adhere more strongly to this finish. Attachment and detachment data for SS4 was variable, suggesting complex attachment mechanisms to this type of SS. SS4 is the most common material used in food processing facilities. The data from this study indicate that bacterial interactions with SS4 are complex and less easily predicted than those with SS of other finishes, including 2B and 8. These differences in bacterial attachment may be of concern to the food industry and warrant further investigation.     

Radiosensitization of Escherichia coli and Salmonella Typhi in ground beef

The radiosensitization of two pathogenic bacteria, Escherichia coli and Salmonella Typhi, was evaluated in the presence of thyme and its principal essential oil constituents (carvacrol and thymol) in ground beef. Ground beef was inoculated with E. coli or Salmonella Typhi (10(5) CFU/g), and each compound was added separately at various concentrations (0 to 3.5%, wt/wt). The antimicrobial potential of carvacrol, thymol, and thyme was evaluated in unirradiated meat by determining the MIC in percentage (wt/wt) after 24 h of storage at 4 +/- 1 degree C. Results showed a MIC of 0.88 +/- 0.12%, 1.14 +/- 0.05%, and 2.33 +/- 0.32% for E. coli in the presence of carvacrol, thymol, and thyme, respectively. MICs of 1.15 +/- 0.02%, 1.60 +/- 0.01%, and 2.75 +/- 0.17% were observed for Salmonella Typhi in the presence of the same compounds, respectively. The best antimicrobial compound (i.e., carvacrol) was selected and added to the sterilized ground beef along with ascorbic acid (0.5%, wt/wt) and tetrasodium pyrophosphate (0.1%, wt/wt). Meat samples (10 g) were packed in air and then irradiated in a 60Co irradiator at doses of 0 to 0.7 kGy for the determination of E. coli radiation D10 and 0 to 2.25 kGy for the determination of Salmonella Typhi radiation D10. Addition of carvacrol increased the relative sensitivity of both bacteria 2.2 times. The radiation D10 was reduced from 0.126 +/- 0.0039 to 0.057 +/- 0.0015 kGy for E. coli and from 0.519 +/- 0.0308 to 0.235 +/- 0.0158 kGy for Salmonella Typhi. The addition of tetrasodium pyrophosphate did not affect significantly (P > 0.05) the radiosensitization of either bacterium. However, the presence of ascorbic acid in the media reduced significantly (P < or = 0.05) the radiosensitivity of both bacteria. An additive effect of carvacrol addition and packaging under modified atmosphere conditions (60% O2-30% CO2-10% N2) was also observed on bacterial radiosensitization at 4 degrees C. Compared with the control packed under air, modified atmosphere packaging conditions in the presence of carvacrol and tetrasodium pyrophosphate improved the relative sensitivity of E. coli by 2.7 times and Salmonella Typhi by 9.9 times.     

Evidence for internalization of Escherichia coli into the aerial parts of maize via the root system

Escherichia coli introduced into the hydroponic growing medium of maize plants was detected 48 h later in the shoot. Decapitation of root tips or severing of the plant root system at the root-shoot junction enhanced bacterial internalization. The density of the bacteria in shoots of plants with damaged roots or removed root systems was 27.8 and 23.9 times higher than that in plants with intact roots, respectively. The concentration of viable cells in the hydroponic solution decreased over time from 9.3 x 10(6) CFU/ml at the time of inoculation to 8.5 x 10(1) CFU/ml 4 days thereafter. The number of E. coli cells associated with the roots also decreased with time, but a significant decline appeared only at 4 days postinoculation. At the time of sampling for E. coli presence in the shoot, 10(2) CFU/ml was present in the nutrient solution and 8 x 10(3) CFU/g was associated with the roots. The present study is the first to demonstrate internalization of E. coli via the root in a monocotyledonous plant.     

Electrolyzed oxidizing anode water as a sanitizer for use in abattoirs

The effectiveness of electrolyzed oxidizing anode (EOA) water (oxidation-reduction potential, 1,120 mV; pH 2.0) as a sanitizer for use in abattoirs was compared with the iodophor (IOD) Mikroklene (25 ppm), a sanitizer approved for use by regulatory authorities in Canada and the United States. A total of 240 swab (100 cm2) samples were obtained from 4 sites on the kill floor and 16 sites in the secondary processing areas, during two visits within a 4-week period to each of three meat packing plants, processing < or =50 animals per week. Swabs were obtained 12 h after the application of IOD and EOA and were analyzed for the presence of total aerobic bacteria, total coliforms, and total Escherichia coli. Total aerobic bacteria (log CFU/ 100 cm2) recovered from the 20 sample sites were lower (P < 0.0001) in EOA as compared with IOD (2.94 +/- 0.12 versus 3.75 +/- 0.12, respectively). Plant A was 1.5 times more likely (P < 0.0001) to have a sampling site positive for the presence of coliforms and E. coli than plants B and C. There was no difference (P > 0.05) between treatment IOD or EOA in the likelihood of obtaining a positive sample for the presence of total coliforms or E. coli among the three plants. When the kill floor and secondary processing areas are compared, the likelihood of obtaining a sample positive for coliforms or E. coli was similar (P > or = 0.05). Results indicate that EOA was more effective than IOD in reducing populations of total aerobic bacteria on equipment surfaces in the three meat packing plants studied. Because the likelihood of obtaining a positive sample for coliforms or E. coli in EOA as compared with IOD was similar, EOA may be a suitable alternative or complement to IOD as a sanitizer in small- to medium-sized abattoirs. Additional research is required to further evaluate the effectiveness of EOA to sanitize processing equipment on the basis of subsequent isolation of aerobes, coliforms, and E. coli from meat products.     

Antibacterial activity of shrimp chitosan against Escherichia coli

The effects of cell age, reaction temperature, pH value, and salts on the inhibitory activity of shrimp chitosan (98% deacetylated) against Escherichia coli were investigated. The age of a bacterial culture affected its susceptibility to chitosan, with cells in the late exponential phase being most sensitive to chitosan. Higher temperature (25 and 37 degrees C) and acidic pH increased the bactericidal effects of chitosan. Sodium ions (100 mM Na+) might complex with chitosan and accordingly reduce chitosan's activity against E. coli. Divalent cations at concentrations of 10 and 25 mM reduced the antibacterial activity of chitosan, in the order of Ba2+ > Ca2+ > Mg2+. Chitosan also caused leakage of glucose and lactate dehydrogenase from E. coli cells. These data support the hypothesis that the mechanism of chitosan antibacterial action involves a cross-linkage between the polycations of chitosan and the anions on the bacterial surface that changes the membrane permeability.     

Ag沉积N、S掺杂改性P25材料对大肠杆菌的抑菌性研究

为扩展纳米TiO_2光谱响应范围,提高其可见光催化抗菌性能,本文以商业纳米TiO_2为原料,采用Ag化学沉积,N、S非金属元素掺杂制备改性P25抑菌材料(Ag-P25、Ag-N-P25、Ag-S-P25),研究这三种材料在可见光下对大肠杆菌的抑菌效果。研究表明,三种抑菌材料对大肠杆菌均有良好的抑制效果,其抑菌率随光照时间延长而不断上升。三种材料抑菌效果的强弱顺序为:Ag-S-P25、Ag-N-P25、Ag-P25。当光照时间为100min时,三者对大肠杆菌的抑菌率均达100%,与纯P25的抑菌率(79%)相对比,提高了21%。通过扫描电镜观察和提取菌体质粒DNA的电泳检测表明,三种材料对大肠杆菌菌体造成破坏,引起DNA泄漏,导致菌体死亡。通过研究讨论改性P25材料对大肠杆菌的灭活机理,可为具有抗菌功能的新型保鲜材料的研究开发提供理论和实验依据。     

The release of lead from the reduction of lead oxide (PbO2) by natural organic matter

PbO2 has been identified as an important scale in some distribution systems that historically use lead service lines and free chlorine for maintaining a disinfectant residual. The stability of this highly insoluble scale with respect to its reductive dissolution may play an important role in lead release into drinking water. In this study, we investigated the release of lead from a commercially available PbO2 in the presence of natural organic matter (NOM) using a hydrophobic acid extracted from the Iowa River. Experiments were conducted using synthetic solutions with different NOM concentrations, solution pH, and NOM samples with different levels of prechlorination. It was found that release of lead from PbO2 occurred both in solutions with and without NOM, and the extent of lead release increased with increasing NOM concentration and decreasing pH value. Furthermore, the released lead was Pb(II) and not particulate PbO2 conclusively showing that reductive dissolution occurred. Prechlorination of NOM reduced the rate of lead release. Our results indicate that PbO2 can be reduced both by water and NOM. Characterization of final solid phases by scanning electron microscopy and X-ray photoelectron spectroscopy are also presented.     

Temperature dependence of the reaction between the hydroxyl radical and organic matter

The temperature-dependent bimolecular rate constants for the reaction of the hydroxyl radical (HO(?)) with organic matter (OM) (k(OM-HO(?))) have been measured for three natural organic matter (NOM) isolates and three bulk effluent organic matter (EfOM) samples using electron pulse radiolysis and thiocyanate competition kinetics. The range of values for the room temperature k(OM-HO(?)) was 1.21-9.37 × 10(8) M(C)(-1)s(-1), with NOM isolates generally reacting slower than EfOM samples. The NOM isolates had an average apparent activation energy of 19.8 kJ mol(-1), while the EfOM samples had an average value slightly lower (14.3 kJ mol(-1)), although one NOM isolate (Elliot Soil Humic Acid, 29.9 kJ mol(-1)) was a factor of 2 times greater than other samples studied. These apparent activation energies are the first determined for OM and HO(?), and the Arrhenius plots obtained for NOM isolates (lowest R(2) > 0.993) suggest that no significant structural changes are occurring over the temperature range 8-41 °C. In contrast, the greater scatter (lowest R(2) > 0.903) observed for the EfOM samples suggests that some structural changes may be occurring. These results provide a deeper fundamental understanding of the reaction between OM and HO(?) and will be useful in quantifying HO(?) reactions in natural and engineered systems.