Jan Mertens

Sorption kinetics and its effects on retention and leaching

Sorption of pesticides to substrates used in biopurification systems is important as it controls the systems efficiency. Ideally, pesticide sorption should occur fast so that leaching of the pesticide in the biopurification system is minimized. Although modeling of pesticide transport commonly assumes equilibrium, this may not always be true in practice. Sorption kinetics have to be taken into account. This study investigated the batch sorption kinetics of linuron, isoproturon, metalaxyl, isoxaben and lenacil on substrates commonly used in a biopurification system, i.e. cow manure, straw, willow chopping, sandy loam soil, coconut chips, garden waste compost and peat mix. The first-order sorption kinetics model was fitted to the observed pesticide concentrations versus time resulting in an estimated kinetic rate constant alpha. Sorption appeared to be fast for the pesticides linuron and isoxaben, pesticides which were classified as immobile, while less mobile pesticides displayed an overall slower sorption. However, the substrate does not seem to be the main parameter influencing the sorption kinetics. Coconut chips, which is a substrate with a high organic matter content showed slow sorption for most of the pesticides. The effect of different estimated alpha values on the breakthrough of pesticides through a biopurification system was evaluated using the HYDRUS 1D model. Significant differences in leaching behavior were observed as a result of the obtained differences in sorption kinetics.

Characterizing pesticide sorption and degradation in macro scale biopurification systems using column displacement experiments

Biopurification systems treating pesticide contaminated water are very efficient, however they operate as a black box. Processes inside the system are not yet characterized. To optimize the performance, knowledge of degradation and retention processes needs to be generated. Therefore, displacement experiments were carried out for four pesticides (isoproturon, bentazone, metalaxyl, linuron) in columns containing different organic mixtures. Bromide, isoproturon and bentazone breakthrough curves (BTCs) were well described using the convection-dispersion equation (CDE) and a first-order degradation kinetic approach. Metalaxyl and linuron BTCs were well described using the CDE model expanded with Monod-type kinetics. Freundlich sorption, first-order degradation and Monod kinetics coefficients were fitted to the BTCs. Fitted values of the distribution coefficient K(f,column) were much lower than those determined from batch experiments. Based on mobility, pesticides were ranked as: bentazone>metalaxyl-isoproturon>linuron. Based on degradability, pesticides were ranked as: linuron>metalaxyl-isoproturon>bentazone.

Characterisation of the field-saturated hydraulic conductivity on a hillslope: in situ single ring pressure infiltrometer measurements

Abstract Spatial variability of surface hydraulic properties is an important factor for infiltration and runoff processes in agricultural fields. At 120 locations on a hillslope, steady-state infiltration rates were measured at two applied pressure heads with a single-ring infiltrometer. The solution of two steady-state infiltration equations for each location (the simultaneous-equation approach, SEA) yielded 41 negative α -values, 79 positive α values and 120 positive K fs -values. The sensitivity of K fs and α to small measurement errors was estimated using a Monte-Carlo simulation (MC). Results of this MC simulation showed that the uncertainty on α is extremely high while the uncertainty on K fs is fairly small. Hence, although the pressure infiltrometer technique as applied here is useful to estimate K fs at each measurement location, it is not suited for the estimation of an α -value at each measurement location. A new procedure is proposed for the simultaneous estimation of one overall ‘field α ’ and the 79 K fs values of measurement locations having a positively calculated α using SEA. Using this field α , K fs values for the other locations with a negative α are hence determined. Finally, the spatial correlation of K fs on the hillslope is examined. Ranges of ln( K fs ) between 2.85 and 3.8xa0m were observed, respectively, for the omnidirectional case and the y direction along the hillslope μ .

Monod kinetics rather than a first-order degradation model explains atrazine fate in soil mini-columns: implications for pesticide fate modelling.

Pesticide transport models commonly assume first-order pesticide degradation kinetics for describing reactive transport in soil. This assumption was assessed in mini-column studies with associated batch degradation tests. Soil mini-columns were irrigated with atrazine in two intermittent steps of about 30 days separated by 161 days application of artificial rain water. Atrazine concentration in the effluent peaked to that of the influent concentration after initial break-through but sharply decreased while influx was sustained, suggesting a degradation lag phase. The same pattern was displayed in the second step but peak height and percentage of atrazine recovered in the effluent were lower. A Monod model with biomass decay was successfully calibrated to this data. The model was successfully evaluated against batch degradation data and mini-column experiments at lower flow rate. The study suggested that first-order degradation models may underestimate risk of pesticide leaching if the pesticide degradation potential needs amplification during degradation.

Inverse modeling of pesticide degradation and pesticide―degrading population size dynamics in a bioremediation system: Parameterizing the Monod model

Pesticide degradation models are compared which simulate the response of biofilters for treatment of pesticide-contaminated waste water to time-irregular pesticide supply in which the pesticide is used for growth and mineralized. Biofilter microcosms containing a mixture of straw, peat and soil and harboring micropopulations which uses the herbicide linuron for growth, were irrigated with linuron for 28 weeks with a stop in its supply between week 12 and 17. Matrix samples were regularly taken to assay linuron mineralization. A first-order approximation of the Monod model was used to simulate the observed mineralization data, while an inverse modeling framework combining a sensitivity analysis (Morris Sensitivity Analysis) with an inverse modeling approach (Shuffled Complex Evolution Metropolis) adopted to parameterize the model. Lag times in linuron mineralization decreased during the initial weeks of linuron irrigation but increased after supply of linuron ceased. The model well-simulated the lag time dynamics which were related to the dynamics of the predicted linuron-degrading population size in the microcosms. It was predicted that the population size decreased at a rate of 0.031 d(-1) after pesticide supply ceased to reach its initial population size after 25 weeks. We conclude that modeling pesticide degradation in biofilters should incorporate biomass dynamics in case the pesticide is used as C-source. First-order approaches without incorporating biomass dynamics could lead to underestimation of the risk of pesticide leaching.

Conventionalization of germ-free rats reverses the disability of rhein anthrone to induce laxation

This study shows that rhein anthrone has no laxative potency in germ-free rats because after intracaecal administration of a dose of 50 mg/kg the large intestine transit exceeded 240 min. The time course of the laxative potency of rhein anthrone injected intracaecally was evaluated after peroral inoculation of germ-free rats with the caecal contents of conventional rats. Large intestine transit was measured at consecutive periods, on days 0, 1, 2, 3 and 5 after peroral inoculation. It appeared that 1 day after peroral inoculation the laxative potency of rhein anthrone was already established (large intestinal transit < 10 min) and laxation remained on the following days (days 2, 3 and 5). We concluded that rhein anthrone is inactive in germ-free rats and acquires laxative potency after peroral inoculation of germ-free rats with caecal contents of conventional rats.

The use of low cost compact cameras with focus stacking functionality in entomological digitization projects

Abstract Digitization of specimen collections has become a key priority of many natural history museums. The camera systems built for this purpose are expensive, providing a barrier in institutes with limited funding, and therefore hampering progress. An assessment is made on whether a low cost compact camera with image stacking functionality can help expedite the digitization process in large museums or provide smaller institutes and amateur entomologists with the means to digitize their collections. Images of a professional setup were compared with the Olympus Stylus TG-4 Tough, a low-cost compact camera with internal focus stacking functions. Parameters considered include image quality, digitization speed, price, and ease-of-use. The compact camera’s image quality, although inferior to the professional setup, is exceptional considering its fourfold lower price point. Producing the image slices in the compact camera is a matter of seconds and when optimal image quality is less of a priority, the internal stacking function omits the need for dedicated stacking software altogether, further decreasing the cost and speeding up the process. In general, it is found that, aware of its limitations, this compact camera is capable of digitizing entomological collections with sufficient quality. As technology advances, more institutes and amateur entomologists will be able to easily and affordably catalogue their specimens.

Newly discovered populations of the Ethiopian endemic and endangered Afrixalus clarkei Largen, implications for conservation

Abstract Knowledge of the Ethiopian amphibian fauna is limited and Southwest Ethiopia remains understudied. This part of Ethiopia, where most of the country’s remaining natural forest is situated, is known to harbour the only populations of Afrixalus clarkei (Largen), an endemic banana frog, worldwide. This species is under great threat of extinction and is therefore classified as endangered on the IUCN red list. We surveyed different potential habitats for this species outside its known range and found several new populations extending its known habitat preference, and the geographical and altitudinal range of the species. We here show that Afrixalus clarkei is more common than previously thought.