Sediment-water system

A stxidy of Direct Red 28 [573-58-0] (3) (Cl 22120) in a sediment-water system indicated that the amount of recovered benzidine [92-87-5] (4) accounted for only 2—5% of lost dye (256). 

The fate study of Disperse Blue 79 (262) in anaerobic sediment—water systems shows the following degradation products ... 

E. J. Weber, Studies ofBenfidine Based Dyes in Sediment-Water Systems, U.S. EPA Environmental Research Laboratory, Athens, Ga., Jan. 1990. 

Walker WW, Cripe CR, Pritchard PH, et al. 1988. Biological and abiotic degradation of xenobiotic compounds in in vitro estuarine water and sediment/water systems. Chemosphere 17 2255-2270. 

Metwally M E-S, NL Wolfe (1990) Hydrolysis of chlorostilbene oxide. II. Modelling of hydrolysis in aquifer samples and in sediment-water systems. Environ Toxicol Chem 9 963-973. 

A sediment-water system was used to study the partition and the degradation of C-labeled 4-nitrophenol and 3,4-dichloroaniline (Heim et al. 1994). The results clearly illustrated the importance of water-to-sediment partitioning, and that a substantial fraction of the substrates existed in the form of nonextractable residues. 

Heim K, 1 Schuphan, B Schmidt (1994) Behaviour of [ " C]-4-nitrophenol and [ " C]-3,4-dichloroaniline in lab sediment-water systems. 1. Metabolic fate and partitioning of radioactivity. Environ Toxicol Chem 13 879-888. 

In summary, the bioavailability and observed toxicity of synthetic pyrethroids in sediment-water systems is influenced by a number of physicochemical factors, including the quantity and type of organic and inorganic matter in sediment and in water, as well as by temperature. The use of equilibrium partitioning calculations can be a useful tool for estimating the dissolved and potentially bioavailable fraction of pyrethroids. 

For example, chloroanilines and polychlorinated biphenyl congeners have been shown to alter by microbially-mediated reductive dehalogenation in sediment/water systems, yielding less chlorinated congeners [38,48,52,68,105, 116,119,369-371]. 

To elucidate the fate of these compounds at sediment-water interfaces, sediment/water mixtures (Lake Macatawa, Holland, MI) were spiked with DCB and incubated at 20 °C for 12 months under anaerobic conditions [72]. Dehalogenation of DCB to benzidine appeared to take place through a transient intermediate, 3-monochlorobenzidine (Fig. 27), which was observed in time-course analyses of the sediment/water mixtures. No metabolites were observed in autoclaved samples, suggesting that dehalogenation of DCB in anaerobic sediment/water systems was mediated by microbial activity. The product of dehalogenation (benzidine, Fig. 27) is more toxic to humans than the parent compound, DCB. From sediment/water distribution experiments, DCB showed greater affinity for the sediment phase than its non-chlorinated derivative,... 

Biological. Using the experimentally determined first-order biotic and abiotic rate constants of chlorpyrifos in estuarine water and sediment/water systems, the estimated biodegradation half-lives were 3.5-41 and 11.9-51.4 d, respectively (Walker et al, 1988). 

In a laboratory sediment-water system incubated under anaerobic conditions, the half-life of TCDD was 500 to 600 d (Ward and Matsumura, 1978). 

Gambrell, R.P., Taylor, B.A., Reddy, K.S., and Patrick, W.H., Jr. Fate of selected toxic compounds nnder controlled redox potential and pH conditions in soil and sediment-water systems, U.S. EPA Report 600/3-83-018, 1984. 

Jafvert, C.T. and Wolfe, N.L. Degradation of selected halogenated ethanes in anoxic sediment-water systems. Environ. Toxicol C/je/n,.6(ll) 827-837. 1987. 

Weber EJ, Wolfe NL. 1986. Kinetic studies of aromatic azo compounds in anaerobic sediment/water systems abstract. In 191st National meeting American Chemical Society Division of Environmental Chemistry 26 239-40. 

Wershaw RL (1986) A new model for humic materials and their interactions with hydrophobic organic chemicals in soil-water or in sediment-water systems. J Contam Hydrol 1 29-45 Whitehouse BG (1984) The effect of temperature and salinity on the aqueous solubility of polynuclear aromatic hydrocarbons. Mar Chem 14 319-332 Wolters A, Linnemann V, Herbst M, Klein M, Schaffer A, Vereecken H (2003) Pesticide volatihzation from soil Lysimeter measurements versus predictions of European registration models. J Environ Qual 32 1183-1193... 

Though this system is perhaps an extreme example of slow sorption kinetics, it illustrates that the assumption of rapid equilibrium between the sediment and aqueous phases is questionable. The importance of such an observation to the investigation of hydrolysis kinetics in sediment/water systems must be emphasized. Certainly, any model of hydrolysis kinetics in sediment/water systems must include explicit expressions for the kinetics of the sorption/desorption process. 

This model, in light of the discussion above, is clearly not representative of all of the kinetic processes which are occurring in sediment/water systems containing hydrophobic pesticides. However, it does include at least the more labile fraction of the sorbed pesticide in the overall kinetic model. Complications due to the inadequacy of this representation will be illustrated and discussed below. 

In a second class of experiments, detailed studies of disappearance kinetics in sediment-water systems were performed. A series of centrifuge tubes was assembled containing identical concentrations of parent compound and identical sediment-to-water ratios. 

The third class of experiments was similar to the second, except that the centrifugation step was omitted and only the total parent compound concentration in the sediment-water systems was determined as a function of time. 

Neutral Hydrolysis Studies. Investigations of neutral (pH-independent) hydrolysis kinetics in sediment/water systems were conducted for three organophosphorothioate insecticides (chlorpyrifos, diazinon and Ronnel), 4-(p-chlorophenoxy)butyl bromide, benzyl chloride, and hexachlorocyclopentadiene. 

The data from a representative study of the disappearance of chlorpyrifos from an EPA-14 sediment/water system (p=0.20, fraction sorbed = 0.94) is illustrated in Figure 3. Comparison with Figure 1 shows that once sorptive equilibrium is achieved (t>14,000 minutes) the disappearance rate is first order for both the water and sediment phases. Also, the aqueous disappearance rate constant calculated from the slope of the linear portion of the natural log aqueous concentration versus time plot is 0.5 0.2 x 10 min, which is similar to the values measured in sediment-free EPA-14 supernatant (Table II). A plot summarizing two experiments using EPA-23 sediment is shown in Figure 4. The value of calculated from the... 

Data from these studies were analyzed by a computer using equations 8 based on our simple kinetic model for the sediment/water systems (eqn. 7). The computer program (23) uses concentrations of chlorpyrifos in the water and sediment phases and product concentrations (obtained by difference) as a... 

Figure 3. Chlorpyrifos disappearance from an EPA-14 sediment/ water system, P= 0.20, t = 25 °C.

Diazinon and Ronnel. The conclusion that neutral hydrolysis of sorbed chlorpyrifos is characterized by a first-order rate constant similar to the aqueous phase value is strengthened and made more general by the results for diazinon, 0,0-diethyl 0-(2-iso-propyl-4-methyl-6-pyrimidyl) phosphorothioate, and Ronnel, 0,0-dimethyl 0-(2,4,5-trichlorophenyl) phosphorothioate (10). The results for the pH independent hydrolysis at 35°C for these compounds in an EPA-26 sediment/water system (p=0.040) are summarized in Table IV. Because the aqueous (distilled) values of k for diazinon and Ronnel are similar in magnitude to the value for chlorpyrifos, and because these values were shown by the chlorpyrifos study to be slow compared to sorption/desorption kinetics, computer calculations of were not deemed necessary and were not made for these data. 

Disappearance Rate Constants (min ) in an EPA 26 Sediment/Water System for Diazinon and Ronnel at 35°C ... 

Thus, for chlorpyrifos, diazinon, Ronnel (and by extension, other organophosphorothioate pesticides), neutral hydrolysis proceeds at similar rates in both the aqueous and sediment phases of sediment/water systems. 

Several features of the PCBB experiments are different than those for chlorpyrifos. The hydrolysis reaction proceeds via a different mechanism. The rate enhancements observed for chlorpyrifos in natural waters and the aqueous phases of the sediment/water systems (as compared to sterile distilled water) are not observed for PCBB. The values of kj and k calculated for PCBB are slower than those for chlorpyrifos anS similar in magnitude to the hydrolysis rates. 

Benzyl chloride hydrolysis proceeds via a third mechanism (Sj.1). Results of studies of benzyl chloride hydrolysis ( 1) in distilled water and EPA-13 and EPA-2 sediment/water systems are summarized in Table V. Results for this compound include only overall first-order disappearance rate constants, but the data clearly show that the hydrolysis rate is independent of the fraction sorbed to sediment. Thus, the conclusion is again made that neutral hydrolysis proceeds via similar rate constants in both the aqueous and sediment-sorbed phases. 

Table V. Hydrolysis of Benzyl Chloride in Sediment/Water Systems at 25°C...

Table VI. Hydrolysis of Hexachlorocyctopentadiene in EPA-13 Sediment/Water Systems at 30°C...

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