Control of pH during denitrification using an encapsulated phosphate buffer
Introduction
Numerous physico-chemical and biological reactions of interest in the environmental field, such as the neutralization of industrial waste streams, precipitation of metals, microbially-mediated degradation, and disinfection with chlorine, are dependent on pH. The efficiency of these processes depends on the control of pH within an optimum range. pH control can easily be accomplished using automatic pH control dispensers. The basic operation of these devices involves dispensing measured amounts of an acid or a base to maintain the pH within a desired operating range. However, these devices may be difficult to use in relatively inaccessible locations, such as during the in situ bioremediation of contaminated groundwater. Control of pH in groundwater during in situ bioremediation is important because microbial activity can result in pH changes. For example, depending on the intrinsic buffering capacity of the soil and groundwater, denitrification activity can cause the pH of groundwater to rise beyond optimum ranges.
A promising means for controlling pH in inaccessible locations is through the use of encapsulated buffers. Encapsulation technology is widely used in the agriculture (Chamberlain and Symes, 1993), pharmaceutical (Rubinstein, 1987), food (Greenblatt et al., 1993), and biomedical fields (Lim, 1984). In encapsulation, materials of interest are contained within a wall, and the physical properties of the wall determine the mechanism and controlling factors for the transport of the compounds from the bulk solution or within the capsule through the wall (Deasy, 1984). Encapsulation technology has also found a number of applications in the environmental field. Enzymes or whole bacterial cells have been encapsulated to enhance the destruction of compounds in reactors (Siahpush et al., 1992; Prakash and Chang, 1995) and to inoculate soils and sediments (Lin et al., 1993). Lin et al. (1991)encapsulated whole cells, adsorbents, and nutrients in Ca-alginate and demonstrated that the system was successful in enhancing the biodegradation of pentachlorophenol in simulated soil extracts and sands. Using encapsulated silicone oils within nylon membranes cross linked with polyethylenimine, Poncelet et al. (1993)significantly increased the oxygen transfer efficiency and oxygen reservoir in a bioreactor. Vesper et al. (1994)used sodium percarbonate as a source of oxygen encapsulated in polyvinylidene chloride to support aerobic biodegradation.
This study involves the use of encapsulated buffers for pH control in a biological system. Although this concept has been used in medical applications (Park et al., 1984; Lehman, 1995), a review of the literature reveals that this has not been applied in environmental systems. In this study, bioremediation within a denitrification framework was used as a model system. Nitrate has been injected into groundwater contaminated with petroleum hydrocarbons to provide an electron acceptor to stimulate the in situ biodegradation of compounds (Bradley et al., 1992; Hutchins et al., 1993). The addition of electron donors (such as ethanol and acetic acid) has also been performed to stimulate in situ denitrification of contaminated aquifers (Hamon and Fustec, 1991). Since the reduction of nitrate to nitrogen gas may increase the pH of a system, denitrification is an ideal scenario for this study.
The purpose of this research was to demonstrate an alternate method for pH control using a microencapsulated phosphate buffer. Batch experiments were performed to characterize the distribution and percent core content by mass of the microcapsules and to evaluate the kinetics of release of the microcapsule core into solution. A stochastic model incorporating leakage and dissolution of the microcapsule wall as a function of pH was developed and verified with batch experiments. A mixed culture of heterotrophic denitrifying microorganisms was grown in a bioreactor using ethanol as the organic substrate and nitrate as the electron acceptor. pH control using the microcapsules was demonstrated in several batch experiments with the suspended-growth culture.
Section snippets
Characterization of the microcapsules
The microcapsules used in this study had an acidic phosphate core (KH2PO4) coated with the polymer Eudragit™ S100 (Coatings Place, Verona, WI). The polymer dissolves in solutions that have a pH greater than 7.0. To characterize the microcapsules, batch experiments were performed to determine the percent potassium content in each microcapsule and the dissolution rate of the microcapsule wall as a function of various pH values. The potassium content gives a quantitative measure of the amount of KH
Modeling kinetics of microcapsule core release
Two mechanisms were assumed to control the kinetics of release of the microcapsule core: leakage of the core through the microcapsule wall and rupture of the microcapsule wall caused by dissolution of the wall. Leakage of the core through the microcapsule wall causes a slow increase in the bulk liquid concentration of the core contents, while dissolution of the wall and subsequent rupture causes an instantaneous increase. For a single microcapsule in solution, the bulk solution concentration
Characterization of the microcapsules
Fig. 1(a) and (b) shows the frequency distribution for the mass and potassium content of individual microcapsules, respectively. The mean of the mass of the microcapsules was 0.73 mg and the microcapsules contain an average of 20% potassium by mass. Knowing the potassium content of the microcapsules and that the microcapsule core contains KH2PO4, the mass of the microcapsule wall was determined by,where Mcaps is the mass of each microcapsule; and MW refers to the
Conclusions
The masses of individual microcapsules and percent phosphate content (and hence the mass of the core) of each microcapsule were obtained to evaluate the mass of the microcapsule wall. The wall of the microcapsule was an average of 30.3% of the total microcapsule mass and was in excellent agreement with the manufacturer's specifications of 30% coating. A stochastic model using Monte Carlo methods was developed assuming two mechanisms to describe the kinetics of release of the microcapsule core
Acknowledgements
This material is based upon work supported in part by the University of South Carolina Venture Fund. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the funding agency. Mention of any specific trade name does not constitute endorsement of the product by the authors or the sponsors.
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