ROLE OF IONIZATION STATE IN BACTERIAL UPTAKE AND SOIL SORPTION OF AGROCHEMICALS

Authors: Sims, Gerald; HULTGREN, RYAN - UNIV OF ILLINOIS; CUPPLES, ALLISON - UNIV OF ILLINOIS; HUDSON, ROBERT - UNIV OF ILLINOIS

Submitted to: International Conference on Groundwater Quality
Publication Type: Proceedings
Publication Acceptance Date: 10/25/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Degradation by microorganisms is an important way in which residual herbicides are removed from the environment. In order to predict the time required for herbicide degradation to occur, it is necessary to understand the key steps in the degradation process. When herbicides are degraded by bacteria, the compounds are apparently taken inside the cells where the necessary enzymes occur. Penetration of herbicides into bacterial cells may be an important step that determines how quickly the compounds are degraded. Research was conducted to determine the nature of uptake of a series of herbicides and model compounds into a variety of bacterial cells. The results showed that for ionizable compounds (those that can develop a positive or negative charge), the compounds are taken up preferentially in the neutral form. As a result, uptake was highly dependent upon the pH of the environment surrounding the bacteria. This effect varied with different organisms and classes of compounds. When degradation of a compound was inducible (enzymes produced or turned on after exposure to the compound), uptake was much faster and less affected by pH after the organisms were exposed to the compound. These results will be useful in developing models to predict uptake and subsequent degradation of herbicides by bacteria. The greatest impact of our improved ability to predict herbicide behavior in the environment will be in the development practices and management tools that provide better control of plant pests with minimal negative effects on soil, water, and air quality.

Technical Abstract: Many organic contaminants are ionizable, and thus may exhibit pH dependent soil sorption or bacterial uptake. Increased sorption may promote persistence by reducing bioavailability, whereas increased microbial uptake should reduce persistence. Ionization may thus differentially control various fate processes. These complex relationships may contribute to difficulties in predicting the net effect of matrix pH on contaminant transport. Experiments were conducted to measure soil sorption and bacterial uptake of a range of neutral, basic, and acid substrates. Uptake was measured in culture using a thick cell suspension method in buffers ranging in pH from 3-9. Sorption was measured on soils of widely ranging properties using batch isotherms. Bacterial uptake occurred preferentially with the neutral species of both acids and bases. The effect of pH on sorption was a function of sorption mechanisms. Effects of pH on uptake were less important for organisms capable of rapid degradation of the substrate.