Richard L. Valentine

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.

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.

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.

Hydrogen peroxide decomposition and quinoline degradation in the presence of aquifer material

Hydrogen peroxide is used as a source of oxygen for enhanced bioremediation of contaminated subsurface environments and as an oxidant in engineered systems. While a number of aspects of hydrogen peroxide chemistry are well understood, the importance and relationship between hydrogen peroxide decomposition and contaminant degradation in the presence of subsurface materials is not clear. We report on batch and column studies examining this relationship using quinoline and a sandy aquifer material. The rate of hydrogen peroxide decomposition followed a simple first order relationship, but the loss of quinoline was much more complex than anticipated. The stoichiometric efficiency (i.e. the amount of quinoline degraded for a given loss in hydrogen peroxide) increased dramatically with decreasing concentration of aquifer material. Surface scavenging of reactive intermediates is believed to cause this unexpected behavior. This hypothesis was supported by increased quinoline removal after treatment to remove or inactivate catalytic sites. A model is proposed capable of predicting the extent of quinoline degradation as a function of the aquifer material solids concentration. The findings show that batch and column data for this system and perhaps others involving reactive intermediates must be interpreted with caution. Our results suggest that phosphate addition to retard hydrogen peroxide decomposition could increase contaminant degradation, previously attributed to only biological processes. It may also be possible to enhance chemical degradation in the subsurface and in engineered systems by addition of amendments to modify the surfaces and control reaction pathways.

Chemical and microbiological assessment of pendimethalin-contaminated soil after treatment with Fenton's reagent

This study assessed chemical effects and microbial response after Fentons treatment of pendimethalin contaminated soils. The efficiency of the rapid chemical transformation of pendimethalin varied from 25% to greater than 90%. The highest efficiency was associated with a soil having comparatively low organic matter and low acid neutralizing capacity. This is consistent with the role of organic matter as a free radical scavenger and the optimum formation of free radicals at low pH. Potential heterotrophic activity, as measured by glucose mineralization, decreased with increasing pendimethalin concentration, but this inhibitory effect was removed after Fentons treatment. Treatment also released BOD, COD, TOC, and nitrate into solution. The organic matter released into solution was biodegradable and served as a substrate for subsequent microbial growth. Analysis of the microbial population growing in the Fentons treated soil leachates showed an overall decrease in (culturable) heterotrophic diversity, but an increase in the concentration of Pseudomonas species. These results suggest that Fentons treatment of pendimethalin contaminated soil created favorable conditions for microorganisms desirable for bioremediation.

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.

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.

Oxidation behavior of aqueous contaminants in the presence of hydrogen peroxide and filter media

Abstract Hydrogen peroxide has been used as an oxidant to degrade contaminants in solutions and soils. A poor understanding of the numerous variables that are involved makes it difficult to determine dominant contaminant removal mechanisms. Our primary objective was to examine the relationship between contaminant (quinoline and nitrobenzene) degradation rate and the rate of hydrogen peroxide decomposition on filter media. Both batch and continuous flow column experiments were conducted. In general, the rate of contaminant degradation was proportional to the rate of hydrogen peroxide decomposition, but the mass of contaminant removed depended on the amount of hydrogen peroxide decomposed, filter medium concentration, and filter medium characteristics. For increasing filter medium concentration and equivalent loss of hydrogen peroxide, the mass of contaminant degraded was found to decrease. In addition, acid-hydroxylamine treatment of the selected filter medium, to examine the role of reducible metal oxide coatings, resulted in greater contaminant removals than the parent material despite a slower hydrogen peroxide decomposition rate. The observed hydrogen peroxide decomposition and contaminant oxidation results are consistent with a reaction scheme whose central elements include: (1) a rate limiting filter medium surface catalyzed reaction initiating hydrogen peroxide decomposition with the formation of a reactive intermediate, (2) a competing reaction of the intermediate with the filter medium surface, and (3) reaction of the same intermediate with the aqueous organic contaminant. Loss of quinoline and nitrobenzene is most likely a solution phase reaction because sorption of these compounds was small over the pH range 7–8 and oxidation efficiency did not increase with increasing filter medium concentration, which would be expected if the reactions were occurring on the surface. Finally, enhanced oxidation of quinoline and nitrobenzene on the treated material is explained by more efficient use of the reactive intermediates for contaminant oxidation due to a reduction in the number of scavenging sites associated with reducible metal oxide coatings.

A spectrophotometric study of the formation of an unidentified monochloramine decomposition product

Abstract This paper reports on a study of the formation of an unidentified product(s) of the slow decomposition of monochloramine in organic free aqueous solutions at pH values and Cl/N ratios of importance in the chloramination disinfection of drinking water. Chloramine and total oxidant concentrations determined spectrophotometrically in these solutions became significantly greater with time than those determined by a titrimetric method due to the absorbance of the unidentified product(s). The u.v. spectra of the product(s) was calculated from the difference between measured and predicted spectra and was similar to that obtained in a chloramine free solution resulting from the rapid decomposition of dichloramine at high pH. No spectrophotometric evidence could be found for the formation of significant concentrations of nitrite and/or nitrate. Relative concentration changes of the unidentified product(s) as measured by its calculated absorbance at 243 nm showed that the product(s) accumulates with time and therefore is not likely to be an intermediate in the formation of nitrogen gas. Both increased pH and phosphate buffer increased its formation rate. A formation mechanism involving the decomposition of dichloramine is suggested. Findings suggest that the age/history of chloramine solutions could be an important variable in toxicological studies of chloramines and their reaction products depending on the health effects of the unidentified product(s).

Dichloramine decomposition in the presence of excess ammonia

Abstract The decomposition of aqueous dichloramine in the presence of excess ammonia was studied to verify a proposed decomposition mechanism. Experiments were conducted under conditions in which the three significant reactions were isolated from others that could potentially complicate interpretation of results. The decomposition of dichloramine, producing primarily nitrogen gas and monochloramine, was initiated by mixing solutions of dichloramine and monochloramine under varying experimental conditions of pH, phosphate concentration, and initial dichloramine and monochloramine concentration. The chloramine concentrations were then monitored titrimetrically with time. Rate constants characterizing the reactions were determined using nonlinear least squares regression analysis and the reaction stoichiometry determined by comparing dichloramine loss to monochloramine formed. Phosphate did not catalyze the decomposition which suggests that the mechanism does not involve general base catalysis. A mechanism including a direct reaction of monochloramine was indicated based on both kinetic and stoichiometric considerations. The experimental results obtained at a high initial ratio of dichloramine to monochloramine could be reasonably predicted with the proposed mechanism and one set of rate constants. However, the constants were somewhat dependent on the initial dichloramine ratio. This discrepancy may be due to the existence of another reaction(s) not included in the proposed mechanism.