Specialist Group on Microbial Ecology and Water Engineering - IWA

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RESEARCH NEWS FROM THE MEWE SPECIALIST GROUPBiodegradation of the herbicide propanil and its metabolite 3,4-dichloroanilineMarques R., Carvalho G., Oehmen A., Reis M.A.M.REQUIMTE/CQFB, Chemistry DepartmentFCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.Propanil is a herbicide widely used in the cultivation of rice, used to control broadleaf weeds. Sincepropanil is essentially applied to flooded rice paddies, it can contaminate surface and groundwaters. Biological mechanisms to fully degrade these compounds are not often found in nature andso propanil tends to accumulate either as the intact parent compound or as a partially transformedmetabolite, mainly 3,4-dichloroaniline (DCA). Both forms represent health and ecotoxicity hazards,contaminating both soils and waters, producing toxic effects on mammals and fish as well asaffecting the human immune system (Pothuluri et al., 1991). Concentrations up to 3600 µg.L −1(propanil) and 568 µg.L −1 (DCA) have been found in irrigation water (Primel et al., 2007), farabove the maximum permitted concentration for discharge into the aquatic environment of 0.1µg.L −1 (Pesticides Framework Directive 2009/128/EC). Biological treatment using a mixed cultureapplied in situ can be an inexpensive and sustainable solution to remove propanil and DCA fromcontaminated waters and soils. The objective of this work was to understand the conditions thatenhance the selection of propanil and DCA degrading microbial communities and improve theirperformance.Microbial enrichments were initiated and cultivated from soil contaminated with several herbicides,including propanil, as well as soil from organic rice agriculture. After this, the mixed culture wasused to inoculate a sequencing batch reactor (SBR), which was initially operated with dump feedingof propanil at a concentration of 0.5 mM added once per cycle (aeration phase of 24-96h, followedby settling and decant). Batch tests at different initial propanil concentrations were carried out toevaluate biomass acclimatization and biodegradation kinetics of propanil and its metabolites.During operation with dump feeding strategy, an accumulation of the intermediate DCA wasobserved in all of the batch tests, since maximum propanil degradation rates (7.9–12.4 mmolgVSS −1 d −1 ) were always higher than those of DCA (1.4-5.6 mmol gVSS −1 d −1 ). The batch testcarried out with the highest concentration of propanil (0.59 mM, Figure 1a) resulted in theaccumulation of DCA up to 0.53 mM. In this test, propanil was completely hydrolyzed during thefirst day, while DCA degradation occurred at a faster rate only after 3.5 days. DCA accumulationwas clearly inhibitory to the biomass. Thus, the operational mode was changed to a fed-batchstrategy, with a lower initial propanil concentration (0.15 mM in the reactor after each feed), wherethe reactor was fed twice per 24-h cycle. A new series of batch tests was carried out to characterizethe kinetics during the fed-batch operation stage, approximately 30 days after switching to thisfeeding strategy. An increase in performance was clear as compared to the dump feed stage, whereDCA no longer accumulated to inhibitory levels, even when increasing the initial propanilconcentration to 0.59 mM (Figure 1b). The increased DCA degradation kinetics could be related toa shift in the microbial population composition, and/or the fact that the DCA concentration was____________________________________________________________________________________________________________6IWA <strong>Specialist</strong> <strong>Group</strong> on Microbial Ecology and Water Engineering:Newsletter July 2011
kept below inhibitory levels, thereby leading to an increased synthesis (or prolonged activity of)propanil- and DCA-degrading enzymes. Indeed, additional batch tests carried out with DCA as thesole carbon source (0.25 and 0.59 mM), showed reduced degradation rates as compared to thoseobserved in the propanil tests. This could be due to the time required to synthesize the enzymesnecessary for DCA degradation, which could be slowly produced as DCA was gradually formed inthe tests fed with propanil.Figure 1. Propanil and DCA degradation profiles obtained from batch tests carried out during SBRoperation with a dump (a) and fed-batch (b) feeding strategy and initial propanil concentrations of0.59 mM. Adapted from Carvalho et al. (2010).________________________________________________________________________________________________________IWA <strong>Specialist</strong> <strong>Group</strong> on Microbial Ecology and Water Engineering:7Newsletter July 2011
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