Environ. Sci. Technol. 1997, 31, 1607-1614
Effects of Chlorophenols and Nitrophenols on the Kinetics of Propionate Degradation in Sulfate-Reducing Anaerobic Systems VIKAS UBEROI† AND S A N J O Y K . B H A T T A C H A R Y A * ,‡ Department of Environmental Sciences, Cook College, Rutgers University, New Brunswick, New Jersey 08903, and Civil and Environmental Engineering Department, Tulane University, New Orleans, Louisiala 70118
The ability of sulfate-reducing bacteria (SRB) to effectively degrade several substituted and non-substituted aromatic and aliphatic compounds offers a promising alternative for the treatment of sulfate-containing industrial wastewaters and landfill leachates. For the design of sulfate-reducing anaerobic treatment systems for such wastewaters, it is important to determine the effects of toxicants on the kinetics of sulfate-reducing and methanogenic reactions. Batch kinetic experiments were conducted in the presence and absence of selected organic toxicants with a sulfatereducing propionate enrichment culture. Fourteen chlorophenols and four nitrophenols were selected for this study. Toxicity due to dichlorophenols and trichlorophenols to both propionate and acetate degradation was dependent on the substitution position of chlorine atoms on the benzene ring. Among the dichlorophenols, 2,3-dichlorophenol and 2,6-dichlorophenol were the least toxic and 3,5-dichlorophenol was the most toxic. Among the trichlorophenols, 2,3,5-trichlorophenol and 2,4,5-trichlorophenol were more toxic as compared to 2,3,6-trichlorophenol and 2,4,6-trichlorophenol. Toxicity due to the mononitrophenols was also dependent on the substitution position showing 4-nitrophenol as the least toxic. All of the selected nitrophenols were more toxic to acetate utilization by methanogens than propionate utilization by the SRB.
Introduction Sulfate-reducing anaerobic enrichment cultures have been shown to effectively degrade several substituted and nonsubstituted aromatic and aliphatic compounds (1-9). In some, the presence of sulfate has been shown to be inhibitory to dehalogenation of chlorinated compounds (10, 11). Overall, sulfate-reducing systems offer a promising alternative for the treatment of sulfate-containing landfill leachates and wastewaters from pharmaceutical and paper industries (1214). Unfortunately, these wastewaters could be subject to high concentrations of phenolic compounds, leading to process failure due to toxicity. For the design of treatment systems for these wastewaters, it is important to know the effects of substituted phenols on the kinetics of the system. It has been reported that, in a sulfate-reducing system, both SRB and non-SRB (fermenters, acetogens and methanogens) can be present and can degrade substrates such as molecular * Corresponding author: phone: (504) 862-3255; fax: (504) 8628941; e-mail:
[email protected]. † Rutgers University. ‡ Tulane University.
S0013-936X(96)00223-4 CCC: $14.00
1997 American Chemical Society
hydrogen and short-chain fatty acids either sequentially or simultaneously (13-21). These reports also show that, in sulfate-reducing anaerobic systems, sulfate is mainly reduced during utilization of propionate, ethanol, lactate, and benzoate and that the acetate formed is mainly converted to methane. Under non-sulfate-reducing anaerobic conditions, methanogenesis has been reported to be the rate-limiting step, which determines the efficiency of the system in the presence and the absence of chlorinated hydrocarbons (22-24). Blum and Speece (25) reported the IC50 values of several chloro- and nitro-substituted organic compounds for acetate-utilizing methanogens and found that the toxicity due to these toxicants was governed by the extent and the position of substitution on the benzene ring. Little information is available in the literature on the kinetics of sulfate-reducing systems under toxic conditions. The objective of this investigation was to determine the toxic effects of the selected chlorophenols and nitrophenols on the kinetics of propionate degradation in sulfate-reducing systems. The effects of the toxicants on the degradation of acetate, formed as an intermediate of propionate degradation, were also studied. Propionate was selected as the feed substrate because it is an important intermediate in anaerobic degradation of many complex wastes and could account for a significant amount of sulfate reduction in anaerobic environments (26-29). Propionate is converted to acetate, which is utilized by the acetoclastic methanogens. Therefore, in a sulfate-fed anaerobic system, which produces propionate as a significant intermediate, the effects of toxicants on propionate and acetate utilization could govern the process efficiency. Chlorophenols and nitrophenols were selected for this study because they are among the most important and industrially versatile organic compounds and are widely used as raw materials or intermediates in the manufacture of explosives, pharmaceuticals, insecticides, pesticides, herbicides, fungicides, pigments, dye, wood preservatives, and rubber chemicals (30, 31). Some of the chlorophenols are not industrially important. However, their formation as intermediates of dechlorination of higher chlorinated phenols makes them environmentally important (32, 33). The organic compounds selected were as follows: 2-chlorophenol (2-CP), 3-chlorophenol (3-CP), 4-chlorophenol (4-CP), 2,3-dichlorophenol (2,3-DCP), 2,4-dichlorophenol (2,4-DCP), 2,5dichlorophenol (2,5-DCP), 2,6-dichlorophenol (2,6-DCP), 3,4dichlorophenol (3,4-DCP), 3,5-dichlorophenol (3,5-DCP), 2,3,5-trichlorophenol (2,3,5-TCP), 2,3,6-trichlorophenol (2,3,6TCP), 2,4,5-trichlorophenol (2,4,5-TCP), 2,4,6-trichlorophenol (2,4,6-TCP), pentachlorophenol (PCP), 2-nitrophenol (2-NP), 3-nitrophenol (3-NP), 4-nitrophenol (4-NP), and 2,4-dinitrophenol (2,4-DNP).
Materials and Methods Propionate enrichment culture was developed in a completely mixed reactor with a continuous feed containing 1325 mg/L propionate and 1000 mg/L sulfate. Volume of the reactor was 2 L with 1.5-L culture volume, and the hydraulic retention time was maintained at 13 days. Concentrated inorganic basal nutrients were added manually (once in 2 days) with a 10-mL syringe to maintain the following concentrations (in mg/L) of the constituents in the reactor: NH4Cl, 1200; MgCl2, 500; KCl, 400; CaCl2‚2H2O, 25; (NH4)2HPO4, 80; FeCl2‚4H2O, 40; CoCl2‚6H2O, 2.5; KI, 2.5; MnCl2‚4H2O, 0.5; NH4VO3, 0.5; ZnCl2, 0.5; H3BO3, 0.5; NiCl2‚6H2O, 0.5. The reactor was maintained in a quasi-steady-state conditions for more than 3 months before using the culture for this research. In a previous study with this culture under similar conditions, it
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TABLE 1. Steady-State Conditions in the Continuous Reactor over a Period of 100 Daysa parameter
steady-state value
design retention time design influent feed propionate effluent propionate effluent acetate design influent sulfate effluent sulfate effluent dissolved sulfide effluent pH effluent total VSS
13 days 1325 mg/L
