N-functionalized carbon nanotubes as a source and precursor of N-nitrosodimethylamine: implications for environmental fate, transport, and toxicity

Hazardous byproducts may be generated during the environmental processing of engineered nanomaterials. Here, we explore the ability of carbon nanotubes with nitrogen-containing surface groups (N-CNTs) to generate N-nitrosodimethylamine (NDMA) during chemical disinfection. Unexpectedly, we observed that commercial N-CNTs with amine, amide, or N-containing polymer (PABS) surface groups are a source of NDMA. As-received powders can leach up to 50 ng of NDMA per mg of N-CNT in aqueous suspension; presumably NDMA originates as a residue from N-CNT manufacturing. Furthermore, reaction of N-CNTs with free chlorine, monochloramine, and ozone generated byproduct NDMA at yields comparable to those reported for natural organic matter. Chlorination also altered N-CNT surface chemistry, with X-ray photoelectron spectroscopy indicating addition of Cl, loss of N, and an increase in surface O. Although these changes can increase N-CNT suspension stability, they do not enhance their acute toxicity in E. coli bioassays above that observed for as-received powders. Notably, however, dechlorination of reacted N-CNTs with sulfite completely suppresses N-CNT toxicity. Collectively, our work demonstrates that N-CNTs are both a source and precursor of NDMA, a probable human carcinogen, while chemical disinfection can produce CNTs exhibiting surface chemistry and environmental behavior distinct from that of native (i.e., as-received) materials.

Characterization of elemental and structural composition of corrosion scales and deposits formed in drinking water distribution systems

Corrosion scales and deposits formed within drinking water distribution systems (DWDSs) have the potential to retain inorganic contaminants. The objective of this study was to characterize the elemental and structural composition of extracted pipe solids and hydraulically-mobile deposits originating from representative DWDSs. Goethite (alpha-FeOOH), magnetite (Fe(3)O(4)) and siderite (FeCO(3)) were the primary crystalline phases identified in most of the selected samples. Among the major constituent elements of the deposits, iron was most prevalent followed, in the order of decreasing prevalence, by sulfur, organic carbon, calcium, inorganic carbon, phosphorus, manganese, magnesium, aluminum and zinc. The cumulative occurrence profiles of iron, sulfur, calcium and phosphorus for pipe specimens and flushed solids were similar. Comparison of relative occurrences of these elements indicates that hydraulic disturbances may have relatively less impact on the release of manganese, aluminum and zinc, but more impact on the release of organic carbon, inorganic carbon, and magnesium.

Reductive dissolution of lead dioxide (PbO2) in acidic bromide solution

Reductive dissolution of lead dioxide (PbO(2)(s)) has been attributed to a major route leading to elevated lead concentrations in drinking water. However, surface processes involved in this heterogeneous reaction have not been elucidated. In this study, the kinetics and mechanism of reductive dissolution of PbO(2)(s) in acidic bromide solutions were investigated to reveal the detailed surface reactions. The reduction of PbO(2) by bromide can be expressed as PbO(2)(s) + 2Br(-) + 4H(+) --> Pb(2+) + Br(2) + 2H(2)O. The reaction kinetics was found to be proportional to the concentration of PbO(2)(s), and the reaction orders were 1.08 and 1.77 with respect to bromide and proton concentration, respectively. The observed kinetic data can be explained by the following reaction mechanism: adsorption of bromide on the PbO(2)(s) surface to form a precursor surface complex identical with Pb(IV)Br, and two separate one-electron transfers from adsorbed Br(-) to structural Pb(IV), followed by the release of Pb(2+) and Br(2) into water. The adsorption of bromide ion on the PbO(2) surface and the first one-electron transfer reaction were found to be important in regulating the overall rate of the reaction. It is expected that a similar reaction scheme can be applied to other reductive ions.

Reduction of lead oxide (PbO2) and release of Pb(II) in mixtures of natural organic matter, free chlorine and monochloramine

The primary focus of this paper is to elucidate the influence of mixtures of natural organic matter (NOM) and free chlorine and NOM and monochloramine on the reduction of PbO2 in drinking water. Parallel experiments using PbO2 particles of two different sizes (approximately 20 and approximately 200 nm) were conducted to explore the effects of particle size on this process. In the absence of NOM, reduction of PbO2 was observed in monochloramine solutions but not in free chlorine solutions. In the presence of NOM, significant Pb(II) formation was observed in disinfectant-free solutions. The release of Pb(II) was suppressed by the additional presence of free chlorine until the point in time when free chlorine was exhausted. Monochloramine also repressed Pb(II) formation in the presence of NOM but not as significantly as free chlorine. The presence of NOM and monochloramine does not necessarily act additively or synergistically due to complex interactions including reduction of PbO2 by NOM, monochloramine mediated reduction of PbO2, and oxidation of NOM by monochloramine. Higher surface area-normalized Pb(ll) formation was found in experiments using larger PbO2 particles. The high reactivity generally associated with nanoparticles was not observed in our study.

Release of Pb(II) from monochloramine-mediated reduction of lead oxide (PbO2)

A contributing factor causing the sudden release of excessive lead into drinking water is believed to involve the change in redox conditions occurring when monochloramine (NH2Cl) replaces free chlorine as a disinfectant. Studies suggest that NH2Cl cannot effectively oxidize Pb(II) to form PbO2, a Pb(IV) mineral scale formed from the oxidation of metallic lead and Pb(II) species by free chlorine. Unexpectedly, we observed that NH2Cl is actually capable of reducing PbO2 to form Pb(II). We systematically investigated this reaction by varying important water chemistry factors such as solution pH, total carbonate concentration, and the Cl/N molar ratio to control chloramine speciation and its rate of decomposition via a complex set of autodecomposition reactions. The amount of Pb(II) formed was found to be proportional to the amount of NH2Cl that autodecomposed regardless of the rate of this reaction. This implies thatthe rate of Pb(II) release is proportional to the absolute rate of NH2CI decomposition. It is proposed that the species responsible for the reduction of PbO2 is likely a reactive intermediate produced during the decay of NH2Cl. This finding is the first to report that NH2Cl can act as a reductant.

The influence of the pre-oxidation of natural organic matter on the formation of N-nitrosodimethylamine (NDMA)

NDMA is a recently recognized disinfection byproduct that can be formed by a reaction of monochloramine with natural organic matter (NOM). This study was undertaken to examine the influence of various preoxidation strategies (including prechlorination) on the subsequent formation of NDMA and to determine how this is correlated to the subsequent loss in specific UV absorbance (SUVA) that preoxidation causes. Batch experiments were conducted using surface-water-derived NOM exposed to various oxidants that included free chlorine, permanganate, hydrogen peroxide, and ozone. Photochemical oxidation was also studied by exposing the water to simulated sunlight The amount of NDMA formed after monochloramine was added or formed in situ, in the case when free chlorine was the preoxidant, was significantly reduced by these treatments. The reduction was proportional to the reduction in SUVA that also occurred as a consequence of these treatments indicating that SUVA may be a good surrogate for NDMA precursor content. Furthermore, the change in NDMA formation per unit change in SUVA was a constant that did not depend on the nature of the oxidant

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.

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.

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.

Formation of N-nitrosodimethylamine (NDMA) from humic substances in natural water

N-nitrosodimethylamine (NDMA)formation in chloraminated Iowa River water (IRW) is primarily attributed to reactions with natural organic matter (NOM) generally classified as humic substances. Experiments were conducted to determine the contribution of various NOM humic fractions to the NDMA formation potential (NDMA FP) in this drinking water source. NOM was concentrated by reverse osmosis (RO) and humic fractions were obtained by a series of resin elution procedures. Mass balances showed that nearly 90% of the NDMA formation potential could be recovered in the NOM concentrate and in water reconstituted using additions of the various humic fractions. Generally, the hydrophilic fractions tended to form more NDMA than hydrophobic fractions, and basic fractions tend to form more NDMA than acid fractions when normalized to a carbon basis. Overall, the hydrophobic acid fraction was the dominant source of NDMA when both formation efficiency and water composition were considered. The amount of NDMA formed in a sample was found to correlate linearly with an oxidation-induced decrease in specific UV absorbance (SUVA) value at 272 nm. This is consistent with a mechanism in which precursors are formed as the direct consequence of the oxidation of NOM. The NDMA FP estimated using the slope of this relationship and the initial SUVA value compared closely to the value obtained by measuring the NDMA formed in solutions dosed with excess concentrations of monochloramine that presumably exhaust all potential precursor sources. However, the NOMA FP could not be correlated to the SUVA value of the individual humic fractions indicating that the relationship of the NDMA FP to SUVA value is probably a water-specific parameter dependent on the exact composition of humic fractions. It is hypothesized that either specific NDMA precursors are distributed among the various humic fractions or that the humic material itself represents a "generic" nonspecific precursor source that requires some degree of oxidation to eventually produce NDMA. The nonmonotonic behavior of NOM fluorescence spectra during chloramination and lack of correlation between NOM fluorescence characteristics and NDMA formation limited the usage of fluorescence spectra into probing NDMA formation.

Modeling the formation of N-nitrosodimethylamine (NDMA) from the reaction of natural organic matter (NOM) with monochloramine

This paper presents mechanistic studies on the formation of NDMA, a newly identified chloramination disinfection byproduct, from reactions of monochloramine with natural organic matter. A kinetic model was developed to validate proposed reactions and to predict NDMA formation in chloraminated water during the time frame of 1-5 days. This involved incorporating NDMA formation reactions into an established comprehensive model describing the oxidation of humic-type natural organic matter by monochloramine. A rate-limiting step involving the oxidation of NOM is theorized to control the rate of NDMA formation which is assumed to be proportional to the rate of NOM oxidized by monochloramine. The applicability of the model to describe NDMA formation in the presence of three NOM sources over a wide range in water quality (i.e., pH, DOC, and ammonia concentrations) was evaluated. Results show that with accurate measurement of monochloramine demand for a specific supply, NDMA formation could be modeled over an extended range of experimental conditions by considering a single NOM source-specific value of thetaNDMA, a stoichiometric coefficient relating the amount of NDMA produced to the amount of NOM oxidized, and several kinetic parameters describing NOM oxidation. Furthermore, the oxidation of NOM is the rate-limiting step governing NDMA formation. This suggests that NDMA formation over a 1-5 day time frame may be estimated from information on the chloramine or free chlorine demand of the NOM and the source-specific linear relationship between this demand and NDMA formation. Although the proposed model has not yet been validated for shorter time periods that may better characterize the residence time in some distribution systems, the improved understanding of the important reactions governing NDMA formation and the resulting model should benefit the water treatment industry as a tool in developing strategies that minimize NDMA formation.

Modeling dichloroacetic acid formation from the reaction of monochloramine with natural organic matter

A kinetic model was developed to predict dichloroacetic acid (DCAA) formation in chloraminated systems. Equations describing DCAA formation were incorporated into an established comprehensive monochloramine-natural organic matter (NOM) reaction model. DCAA formation was theorized to be proportional to the amount of NOM oxidized by monochloramine and described by a single dimensionless DCAA formation coefficient, theta(DCAA) (M(DCAA)/M(DOC(ox)). The applicability of the model to describe DCAA formation in the presence of six different NOM sources was evaluated. DCAA formation could be described by considering a single NOM source-specific value for theta(DCAA) over a wide range of experimental conditions (i.e., pH, NOM, free ammonia, and monochloramine concentrations). DCAA formation appears to be directly proportional to the amount of active chlorine (monochloramine and free chlorine) that reacted with the NOM under these experimental conditions. Values of theta(DCAA) for all six NOM sources, determined by nonlinear regression analysis, varied from 6.51 x 10(-3) to 1.15 x 10(-2) and were linearly correlated with specific ultraviolet absorbance at 280 nm (SUVA(280)). The ability to model monochloramine loss and DCAA formation in the presence of NOM provides insight into disinfection by-product (DBP) formation pathways under chloramination conditions. The subsequent model and correlations to SUVA has the potential to aid the water treatment industry as a tool in developing strategies that minimize DBP formation while maintaining the microbial integrity of the water distribution system.

Modeling monochloramine loss in the presence of natural organic matter

A comprehensive model describing monochloramine loss in the presence of natural organic matter (NOM) is presented. The model incorporates simultaneous monochloramine autodecomposition and reaction pathways resulting in NOM oxidation. These competing pathways were resolved numerically using an iterative process evaluating hypothesized reactions describing NOM oxidation by monochloramine under various experimental conditions. The reaction of monochloramine with NOM was described as biphasic using four NOM specific reaction parameters. NOM pathway 1 involves a direct reaction of monochloramine with NOM (k(doc1) = 1.05 x 10(4)-3.45 x 10(4) M(-1) h(-1)). NOM pathway 2 is slower in terms of monochloramine loss and attributable to free chlorine (HOCl) derived from monochloramine hydrolysis (k(doc2) = 5.72 x 10(5)-6.98 x 10(5) M(-1) h(-1)), which accounted for the majority of monochloramine loss. Also, the free chlorine reactive site fraction in the NOM structure was found to correlate to specific ultraviolet absorbance at 280 nm (SUVA280). Modeling monochloramine loss allowed for insight into disinfectant reaction pathways involving NOM oxidation. This knowledge is of value in assessing monochloramine stability in distribution systems and reaction pathways leading to disinfection by-product (DBP) formation.

N-nitrosodimethylamine formation by free-chlorine-enhanced nitrosation of dimethylamine

The formation of N-nitrosodimethylamine (NDMA) by the nitrosation of dimethylamine (DMA) is greatly enhanced by the presence of free chlorine (HOCl). The effect of HOCl appears at first to be contrary because HOCl rapidly oxidizes nitrite and hence should reduce NDMA formation from a mechanism involving classical nitrosation. The enhanced nitrosation by the presence of HOCl is, however, consistent with a mechanism that involves the formation of a highly reactive nitrosating intermediate such as dinitrogen tetroxide (N2O4) formed during the oxidation of nitrite to nitrate. This mechanism is quite unlike another recently proposed NDMA formation pathway involving the rate-limiting oxidation of DMA directly by monochloramine. NDMA formation by the proposed HOCl-enhanced nitrosation pathway is inhibited by the presence of ammonia and occurs very quickly, only during the short period during which nitrite oxidation occurs. The general importance of this NDMA formation mechanism in actual drinking water appears to be limited by the amount of DMA and nitrite typically present. The mechanism described here, however, suggests the potential involvement of other nitrogen redox reactions that may produce reactive intermediates leading to the indirect and incidental formation of NDMA in the presence of appropriate organic nitrogen precursor.

Mechanistic studies of N-nitrosodimethylamine (NDMA) formation in chlorinated drinking water

Studies were conducted to investigate the hypothesis that N-nitrosodimethylamine (NDMA) is a potential disinfection by-product specifically produced during chlorination or chloramination. Experiments were conducted using dimethylamine (DMA) as a model precursor. NDMA was formed by the reaction of DMA with free chlorine in the presence of ammonia and also with monochloramine. We proposed a mechanism for NDMA formation in chlorinated or chloraminated water, which does not require nitrite as in N-nitrosation. The critical NDMA formation reactions consist of (i) the formation of monochloramine by combination of free chlorine with ammonia, (ii) the formation of 1,1-dimethylhydrazine (UDMH) intermediate from the reaction of DMA with monochloramine followed by, (iii) the oxidation of UDMH by monochloramine to NDMA, and (iv) the reversible chlorine transfer reaction between free chlorine/monochloramine and DMA which is parallel with (i) and (ii). A kinetic model was also developed to validate the proposed mechanism.

Modeling the kinetics of ferrous iron oxidation by monochloramine

The maintenance of disinfectants in distribution systems is necessary to ensure drinking water safety. Reactions with oxidizable species can however lead to undesirable disinfectant losses. Previous work has shown that the presence of Fe(II) can cause monochloramine loss in distribution system waters. This paper further examines these reactions and presents a reaction mechanism and kinetic model. The mechanism includes both aqueous-phase reactions and surface-catalyzed reactions involving the iron oxide product. In addition, it considers competitive reactions involving the amidogen radical that lead to a nonelementary stoichiometry. Using the method of initial rates, the aqueous-phase reactions were found to have first-order dependencies on Fe(II), NH2Cl, and OH- and a rate coefficient (kNH2Cl,soln) of 3.10 (+/-0.560) x 10(9) M(-2) min(-1). The surface-mediated reactions were modeled by assuming the formation of two surface species: >FeOFe+ and >FeOFeOH. Using numerical techniques, combined rate coefficients for the surface-mediated processes were determined to be 0.56 M(-3) min(-1) and 3.5 x 10(-18) M(-4) min(-1), respectively. The model was then used to examine monochloramine and Fe(II) stability under conditions similar to those observed in distribution systems. Our findings suggest the potential utility of monochloramine as an oxidant for Fe(III) removal in drinking water treatment.

Iron oxide surface-catalyzed oxidation of ferrous iron by monochloramine: implications of oxide type and carbonate on reactivity

The maintenance of monochloramine residuals in drinking water distribution systems is one technique often used to minimize microbial outbreaks and thereby maintain the safety of the water. Reactions between oxidizable species and monochloramine can however lead to undesirable losses in the disinfectant residual. Previous work has illustrated that the Fe(II) present within distribution systems is one type of oxidizable species that can exert a monochloramine demand. This paper extends this prior work by examining the kinetics of the reactions between Fe(II) and monochloramine in the presence of a variety of iron oxide surfaces. The identity of the iron oxide plays a significant role in the rate of these reactions. Surface area-normalized initial rate coefficients (k(init)) obtained in the presence of each oxide at pH approximately 6.9 exhibit the following trend in catalytic activity: magnetite > goethite > hematite approximately = lepidocrocite > ferrihydrite. The differences in the activity of these oxides are hypothesized to result from variations in the amount of Fe(II) sorbed to each of the oxides and to dissimilarities in the surface site densities of the oxides. The implications of carbonate on Fe(II) sorption to iron oxides are also examined. Comparing Fe(II) sorption isotherms for goethite obtained under differential carbonate concentrations, it is apparent that as the carbonate concentration (C(T,CO3)) increased from 0 to 11.7 mM that the Fe(II) sorption edge (50% sorption) shifts from a pH of approximately 5.8 to a pH of 7.8. This shift is hypothesized to be the result of the formation of aqueous and surface carbonate-Fe(II) complexes and to competition between carbonate and Fe(II) for surface sites. The implications of these changes are then discussed in light of the variable oxide studies.

Formation of N-nitrosodimethylamine (NDMA) from reaction of monochloramine: a new disinfection by-product

Studies have been conducted specifically to investigate the hypothesis that N-nitrosodimethylamine (NDMA) can be produced by reactions involving monochloramine. Experiments were conducted using dimethylamine (DMA) as a model precursor. NDMA was formed from the reaction between DMA and monochloramine indicating that it should be considered a potential disinfection by-product. The formation of NDMA increased with increased monochloramine concentration and showed maximum in yield when DMA was varied at fixed monochloramine concentrations. The mass spectra of the NDMA formed from DMA and 15N isotope labeled monochloramine (15NH2Cl) showed that the source of one of the nitrogen atoms in the nitroso group in NDMA was from monochloramine. Addition of 0.05 and 0.5 mM of preformed monochloramine to a secondarily treated wastewater at pH 7.2 also resulted in the formation of 3.6 and 111 ng/L of NDMA, respectively, showing that this is indeed an environmentally relevant NDMA formation pathway. The proposed NDMA formation mechanism consists of (i) the formation of 1,1-dimethylhydrazine (UDMH) intermediate from the reaction of DMA with monochloramine followed by, (ii) the oxidation of UDMH by monochloramine to NDMA, and (iii) the reversible chlorine transfer reaction between monochloramine and DMA which is parallel to (i). We conclude that reactions involving monochloramine in addition to classical nitrosation reactions are potentially important pathways for NDMA formation.

Monochloramine loss in the presence of humic acid

Free chlorine has been used extensively as a primary and secondary disinfectant for potable water. Where it is difficult to maintain a free chlorine residual or when disinfection by-products (DBPs) are of concern, monochloramine has been used to provide a stable disinfectant residual in distributions systems. Reactions of disinfectants, free chlorine or monochloramine, with natural organic matter (NOM) consequently result in the formation of DBPs such as trihalomethanes and haloacetic acids. However, few studies have focused on the fate and kinetics of monochloramine loss in the presence of reactive constituents such as NOM. Monochloramine is inherently unstable and decays even without reactive constituents present via a mechanism known as autodecomposition. Therefore, to predict monochloramine concentrations in the presence of NOM is clearly associated with the ability to adequately model autodecomposition. This study presents the results of a semi-mechanisiic model capable of predicting the loss of monochloramine in the presence of humic material in the pH range of 6.55-8.33. The model accounts for both fast and a slow monochloramine demand to explain the loss of monochloramine over the pH range of this study. The formation of dichloroacetic acid was also predicted due to the ability of the model to differentiate monochloramine reaction pathways in the presence NOM. The results shown here demonstrate the ability of a semi-mechanistic model to predict monochloramine residuals and DBP formation in the presence of humic material.

A kinetic model of N-nitrosodimethylamine (NDMA) formation during water chlorination/chloramination

Experiments were conducted to investigate the hypothesis that N-nitrosodimethylamine (NDMA) is a potential disinfection by-product. NDMA was formed by the reaction of dimethylamine (DMA) with monochloramine and also with free chlorine in the presence of ammonia. We proposed a mechanism for NDMA formation which does not require the presence of nitrite as in N-nitrosation. The critical NDMA formation reactions consist of i) the formation of monochloramine by combination of free chlorine with ammonia, ii) the formation of 1,1-dimethylhydrazine (UDMH) intermediate from the reaction of DMA with monochloramine followed by, iii) the oxidation of UDMH by monochloramine to NDMA, and iv) the reversible chlorine transfer reaction between free chlorine/monochloramine and DMA which is parallel with i) and ii). A kinetic model was developed to validate the proposed mechanism.

Monochloramine decay in model and distribution system waters

Chloramines have long been used to provide a disinfecting residual in distribution systems where it is difficult to maintain a free chlorine residual or where disinfection by-product (DBP) formation is of concern. While chloramines are generally considered less reactive than free chlorine, they are inherently unstable even in the absence of reactive substances. These reactions, often referred to as "auto-decomposition", always occur and hence define the maximum stability of monochloramine in water. The effect of additional reactive material must be measured relative to this basic loss process. A thorough understanding of the auto-decomposition reactions is fundamental to the development of mechanisms that account for reactions with additional substances and to the ultimate formation of DBPs. A kinetic model describing auto-decomposition was recently developed. This model is based on studies of isolated individual reactions and on observations of the reactive ammonia-chlorine system as a whole. The work presented here validates and extends this model for use in waters typical of those encountered in distribution systems and under realistic chloramination conditions. The effect of carbonate and temperature on auto-decomposition is discussed. The influence of bromide and nitrite at representative monochloramine concentrations is also examined, and additional reactions to account for their influence on monochloramine decay are presented to demonstrate the ability of the model to incorporate inorganic demand pathways that occur parallel to auto-decomposition.

Chemistry and microbiology of permeable reactive barriers for in situ groundwater clean up

Permeable reactive barriers (PRBs) are receiving a great deal of attention as an innovative, cost-effective technology for in situ clean up of groundwater contamination. A wide variety of materials are being proposed for use in PRBs, including zero-valent metals (e.g., iron metal), humic materials, oxides, surfactant-modified zeolites (SMZs), and oxygen- and nitrate-releasing compounds. PRB materials remove dissolved groundwater contaminants by immobilization within the barrier or transformation to less harmful products. The primary removal processes include: (1) sorption and precipitation, (2) chemical reaction, and (3) biologically mediated reactions. This article presents an overview of the mechanisms and factors controlling these individual processes and discusses the implications for the feasibility and long-term effectiveness of PRB technologies.

Dissolution of 226Radium from pipe-scale deposits in a public water supply

This study was undertaken to determine if dissolution of 226Radium from pipe-scale deposits contributes to enhanced waterborne 226Radium concentrations at the point of use. Water samples were collected from residential water customers of a small rural Iowa town. Sites were evenly divided between new and old water main connections. Daily samples were collected from the point-of-entry water. Point-of-use 226Radium concentrations ranged from 0.4 to 12.9 pCi L-1 (0.01 to 0.5 Bq L-1). The mean 226 Radium concentration for homes connected to old water mains was significantly higher than the mean 226Radium concentration of homes connected to new water mains, mean(standard deviation) equal 8.3(1.1) and 5.3(0.8) pCi L-1 [0.3(1.1) and 0.2(0.8) Bq L-1], respectively. 226Radium concentrations of the point-of-entry water ranged from 5.0 pCi L-1 to 10.3 pCi L-1 (0.2 Bq L-1 to 0.4 Bq L-1). This study indicates considerable variability of 226Radium exposure from drinking water among residents of the same water supply and has implications for regulatory compliance and exposure assessment in epidemiologic studies.

Radium-bearing pipe scale deposits: implications for national waterborne radon sampling methods

A point-of-use waterborne radon-222 (222Rn) survey of a small Iowa town was performed to determine the cause of unnaturally high waterborne 222Rn concentrations in the municipality. The source of the elevated 222Rn concentrations was a newly discovered reservoir of waterborne 222Rn originating from distribution-system radium-226 (226Ra) adsorbed internal pipe scale deposits. Because the proposed national drinking water regulations for 222Rn require sampling at the origin of the distribution system rather than at the point of use, the proposed scheme for collection of water samples may not represent actual consumer waterborne 222Rn exposure in all cases.