Tineke De Wilde

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.

Sorption characteristics of pesticides on matrix substrates used in biopurification systems

On-farm biopurification systems were developed to remove pesticides from contaminated water generated at the farmyard. An important process in the systems efficiency is the sorption of pesticides to the substrates used in the biopurification systems. The composition and type of material present in the biobed are crucial for retention of chemicals. This study investigated the sorption of linuron, isoproturon, metalaxyl, isoxaben, bentazon and lenacil on substrates commonly used in a biopurification system, i.e. cow manure, straw, willow chopping, soil, coconut chips, garden waste compost, and peat mix. Linear, Freundlich, and Langmuir sorption isotherms were fitted to the obtained data. The best fit was obtained with the Freundlich model. More immobile pesticides (i.e. linuron and isoxaben) tended to associate with the organic substrate, while more mobile pesticides partition in the water (i.e. bentazon). According to sorption capacity, the substrates could be classified as peat mix > compost, coco chips, straw > cow manure, willow chopping > sandy loam soil. Sorption capacity was positively correlated with the organic carbon content, CaO and the cation exchange capacity. Furthermore, no significant differences in sorption could be found between technical and formulated isoproturon and bentazon. Moreover, the individual sorption coefficient K(d) was additive, which means that individual sorption coefficients can be used to calculate the sorption coefficients of a mixture of substrates. What concerns the mutual interaction of pesticides it could be observed that the sorption of linuron and metalaxyl was significantly lower in combination with isoproturon and bentazon, while the latter pesticides were not influenced by the presence of linuron and metalaxyl. As guidelines, firstly, it could be stated that using the most sorbing materials such as peat mix, might significantly increase the biopurification systems efficiency. Secondly, the treatment of very mobile pesticides, such as bentazon, should be taken with care as these will easily leach through the system. Additional chemical treatment might be necessary for these type of pesticides.

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.

Transport and degradation of metalaxyl and isoproturon in biopurification columns inoculated with pesticide-primed material.

Laboratory column displacement experiments were performed to examine whether addition of pesticide-primed material to the matrix of an on-farm biopurification system (BPS), intended to remove pesticides from agricultural waste water, positively affects the degradation of mobile pesticides in the system. Percolated column microcosms with varying types and amounts of metalaxyl and/or isoproturon-primed material or non-primed material were irrigated with water artificially contaminated with isoproturon and/or metalaxyl. Transport of isoproturon was well described using the convection dispersion equation and no dissipation was observed, even in columns inoculated with isoproturon-primed material. On the other hand, delayed dissipation of metalaxyl, i.e., after an initial lag phase, was encountered in all columns receiving metalaxyl. In all systems, dissipation could be described using the Monod model indicating that a metalaxyl degrading population grew in the systems. There was a clear correlation between the lag phase and the amount of metalaxyl-primed material added to the system, i.e., increasing amounts of added material resulted into shorter lag phases and hence more rapid initiation of growth-associated metalaxyl degradation in the system. Our observations suggest that indeed pesticide-primed material can reduce the start-up phase of degradation of mobile pesticides in a BPS and as such can increase its efficiency. However, the primed material should be chosen carefully and preferentially beforehand tested for its capacity to degrade the pesticide.

Transport and degradation of pesticides in a biopurification system under variable flux Part II: A macrocosm study

The efficiency of a biopurification system, developed to treat pesticide contaminated water, is to a large extent determined by the chemical and hydraulic load. Insight into the behaviour of pesticides under different fluxes is necessary. The behaviour of metalaxyl, bentazone, linuron, isoproturon and metamitron was studied under three different fluxes with or without the presence of pesticide-primed soil in column experiments. Due to the time-dependent sorption process, retention of the pesticides with intermediate mobility was significantly influenced by the flux. The higher the flux, the slower pesticides will be sorbed, which resulted in a lower retention. Degradation of the intermediate mobile pesticides was also submissive to variations in flux. An increase in flux, led to a decrease in retention, which in turn decreased the opportunity time for biodegradation. Finally, the presence of pesticide-primed soil was only beneficial for the degradation of metalaxyl.

The influence of small‐ and large‐scale composting on the dissipation of pesticide residues in a biopurification matrix

BACKGROUND Pesticides are efficiently retained and degraded in the organic matrix of a biopurification system. However, as this matrix mineralizes slowly over time, nutrients will start to become depleted and thus a decay in biomass will probably occur. At that moment, the efficiency of the system decreases and the matrix should be replaced. The spent matrix might still contain residues of pesticides. Hence treatment of this matrix is essential. In this study we opted to use composting or incubation as an effective and environmentally friendly treatment strategy. RESULTS Small- and large-scale composting/incubation trials were set up to treat the presence of linuron, bentazone, metalaxyl and bifenthrin in a contaminated matrix. Large-scale composting, performed in an industrial composting facility, resulted in decreased concentrations of metalaxyl, linuron and bentazone. Degradation of bifenthrin was very limited. In the small-scale incubation process, a decrease in concentration was noted for bifenthrin, metalaxyl and bentazone. A reduction in extractable pesticide concentration does not, however, always indicate degradation but could be attributed to the formation of non-extractable residues. CONCLUSION Industrial and small-scale composting/incubation reduced the concentration of some pesticides during the timeframe studied, although little reduction was obtained for the persistent pesticide bifenthrin in the industrial composting process and for linuron in barrel incubation.