The effect of kaolinite on oxalate (bio) degradation at 25 degrees C, and possible implications for adsorption isotherm measurements

Organic compounds derived from agricultural, industrial and domestic waste frequently contaminate soil and ground water. Any attempt to estimate the e ffect of these organic acids on water quality must include the various inte ractions between the organic material and the minerals of the host rock and soil. In the present paper, we study the effect of kaolinite surfaces on t he degradation rate of oxalate in the presence of microorganisms. The goals are to show that the rate of microbial degradation of organic acids is enh anced in the presence of mineral surface and to evaluate the relative impor tance of oxalate consumption by microorganism activity vs. (a-biotic) surfa ce catalyzed chemical degradation. We examine both the effect of presence o f kaolinite on the rate of oxalate degradation using a flow-through reactor , and oxalate adsorption on the kaolinite surface using a batch reactor. Ex periments were conducted at 25 degreesC and 50 degreesC and pH 3. The measu rement of the adsorption of organic anions on mineral surfaces is based on change in the organic anion concentration in solution. However, part of thi s change in concentration may result from degradation of the organic anions , which may be enhanced by microorganism activity. The change of oxalate co ncentration on the kaolinite surface as a function of oxalate concentration in solution is described by a general adsorption isotherm: C-s = 8 x 10(-7) 20C(Ox)(0.4)/1 + 20C(Ox)(0.4) The decomposition rates in the flow-through experiments ranged from 2 x 10( -11) to 2 x 10(-10) mol s(-1), and depended on the amount of kaolinite pres ent in the cell, Adding an antibiotic mixture to the input solution or incr easing the temperature to 50 degreesC halted the oxalate decomposition. The refore, we conclude that oxalate decomposition at 25 degreesC and pH 3 is c ontrolled by microorganism activity and not by chemical (a-biotic) processe s. By adding the same antibiotic mixture to the adsorption experiments, we show that the measurement of oxalate adsorption on the kaolinite surface is not affected by oxalate biodegradation. (C) 2001 Elsevier Science B.V. All rights reserved.