Sediment-water systems kinetics

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. 

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. 

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. 

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... 

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. 

Alkaline Hydrolysis Studies. Alkaline catalyzed hydrolysis kinetics in sediment/water systems have been investigated for chlorpyrifos and the methyl and n-octyl esters of 2,4-dichlorophenoxyacetic acid (2,4-D). 

Chlorpyrifos. As was the case for the neutral hydrolysis studies, the most detailed kinetic investigations of alkaline hydrolysis kinetics in sediment/water systems have been conducted using chlorpyrifos (10). As can be seen from Figure 2, alkaline hydrolysis of chlorpyrifos is not second-order, so the value selected for k cannot be calculated from the pH and a second-order rate constant. Nevertheless, since aqueous kinetics at alkaline pH s for chlorpyrifos was always pseudo-first order, careful pH measurements and Figure 2 can be used to select accurate values for k at any pH. 

Studies of the disappearance of the octyl ester at pH 9.8 in sediment/water systems aged 3 days prior to pH adjustment are summarized in Figure 8. For the systems with p=0.013 and 0.005 (fractions sorbed =. 978 and. 945) the rate is pseudo first order, but the rate constant is 10 times smaller than the aqueous value (1.6x10 min ) at this pH. As was suggested for chlorpyrifos, this k value may be characteristic of the actual value of k. At p=0.001, (fraction sorbed = 0.78), the disappearance kinetics is not first order, but shows rapid disappearance of the aqueous ester, followed by disappearance of the sorbed ester at a rate similar to the studies with higher sediment to water ratios. 

Kinetic studies in sediment/water systems with Direct Red 2, Acid Black 92, Acid Red 4, Acid Red 18, and Direct Yellow 1 lead to linear and biphasic plots of dye loss over time. For all but Direct Yellow I, dye loss was usually preceded by a lag or adaptation phase. Acid Black 92 and Direct Red 2 were transformed completely in less than 24 and 48 hours, respectively, but Acid Yellow 151 and Direct Yellow 1 showed half-lives of greater than 2 years. The rapid initial drop in concentration of all dyes observed, with the exception of Acid Red 18, was presumed to be due to sorption. Tests to determine the effect of pH on... 

Park, S.S., Erstfeld, K.M. (1997) A numerical kinetic model for bioaccumulation of organic chemicals in sediment-water system. Chemosphere 34, 419 -27. 

Weber, E. J., and N. L. Wolfe (1987), Kinetic Studies of the Reduction of Aromatic Azo Compounds in Anaerobic Sediment/Water Systems, Environ. Toxicol. Chem. 6, 911 919. 

Because many organic chemicals are nonionic and have low water solubilities, they will exist primarily in the sorbed state in soil- and sediment-water systems. The sorption of nonionic chemical occurs through hydrophobic sorption or partitioning to the organic matter associated with the soil or sediment (Karickhoff, 1980 Chiou et al., 1983). Furthermore, because desorption kinetics may be slow relative to hydrolysis kinetics, to accurately predict the fate of hydrolyzable chemicals in soil-and sediment-water systems an understanding of hydrolysis kinetics in the sorbed... 

Figure 3.10. Reduction kinetics of a series of 4-alkyl substituted nitrobenzenes in an anaerobic sediment-water system. From Sanders and Wolfe (1985). Reprinted by permission of the American Chemical Society.

Another HPLC method was described in a series of experiments analyzing the kinetics and mechanisms of [14C] niclosamide degradation in sediment and water systems [71]. 

First, determination of hydrolysis kinetics for each compound in sediment-free distilled, buffered distilled or natural water systems were measured. Using sterile techniques, concentrations of the parent compounds were determined as a... 

Applying the foregoing thermodynamic and kinetic information to manganese behavior in natural water systems is considerably limited because the manganese system exemplifies the difficulties discussed earlier. On the thermodynamic side, the kinds of oxide phases in natural waters may not correspond to those for which equilibrium data are available. Also, cation exchange reactions are probably important (21). On the kinetic side, the role of catalysis by various mineral surfaces in suspension or in sediments is not really known. Of considerable importance may be microbial catalysis of the oxidation or reduction processes, as described by Ehrlich (7). With respect to the real systems, relatively... 

Our understanding of natural water systems has, until recently, been seriously limited by a lack of kinetic information on critical reactions in water, in sediments, and at interfaces. Earlier in atmospheric chemistiy (Seinfeld, 1986) and more recently in aquatic chemistry (Brezonik, 1993), a considerable growth of information on rates and mechanisms for reactions central to environmental chemistry has taken place. As a result, we are now better able to assess the characteristic time scales of chemical reactions in the environment and compare these with, for example, residence times of water in a system of interest. Schematically, as shown here, for chemical vs. fluid time scales... 

Natural waters obtain their equilibrium composition through a variety of chemical reactions and physicochemical processes. In this chapter we consider principles and applications of two alternative models for natural water systems thermodynamic models and kinetic models. Thermodynamic, or equilibrium, models for natural waters have been developed more extensively than kinetic models. They are simpler in that they require less information, but they are nevertheless powerful when applied within their proper limits. Equilibrium models for aquatic systems receive the greater attention in this book. However, kinetic interpretations are needed in description of natural waters when the assumptions of equilibrium models no longer apply. Because rates of different chemical reactions in water and sediments can differ enormously, kinetic and equilibrium are often needed in the same system. 

For many systems it is known that there exist regions or environments in which the time-invariant condition closely approaches equilibrium. The concept of local equilibrium is important in examining complex systems. Local equilibrium conditions are expected to develop, for example, for kinetically rapid species and phases at sediment-water interfaces in fresh, estuarine, and marine environments. In contrast, other local environments, such as the photosyn-thetically active surface regions of nearly all lakes and ocean waters and the biologically active regions of soil-water systems, are clearly far removed from total system equilibrium. 

Although the reductive cleavage of the azo linkage of aromatic azo compounds in anaerobic biological systems is well documented (Walker, 1970 Chung et al., 1978), relatively few studies on the fate of these compounds in natural aquatic ecosystems have been reported. Studies in this area have focused primarily on the reduction of aromatic azo compounds in anaerobic sediment-water slurries. Typically, the reduction kinetics of aromatic azo compounds in these systems are fast, with half-lives ranging from several minutes to several days. A plot of concentration versus time for reduction of azobenzene and formation of aniline in a pond sediment is illustrated in Figure 3.4 (Weber and Wolfe, 1987). 

---