Electrokinetic-assisted phytoremediation of soils polluted by organic pesticides
- Autores: Virtudes Sánchez Sánchez
- Directores de la Tesis: Luis Rodríguez Romero (dir. tes.), Francisco Javier López-Bellido Garrido (codir. tes.)
- Lectura: En la Universidad de Castilla-La Mancha ( España ) en 2020
- Idioma: español
- Tribunal Calificador de la Tesis: Ángel Fernández Mohedano (presid.), Cristina Saez Jiménez (secret.), Cynthia Alcántara Pollo (voc.)
- Programa de doctorado: Programa de Doctorado en Ingeniería Química y Ambiental por la Universidad de Castilla-La Mancha
- Materias:
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- Resumen
The presence of hazardous organic pollutants in soils, as pesticides, represent a threat to human, animal and environmental health. Among these, atrazine (selective herbicide) is the second most extensively used pesticide in the world; it is characterized by a high leaching potential, being able to remain in soils and ground waters for long times. Although European Union banned atrazine in 2004, nowadays it is still one of the pesticides more frequently found in soils and waters from different European ecosystems.
Phytoremediation is defined as the use of plants for the removal of pollutants from the environment by means of different mechanisms: uptake, accumulation, immobilization and/or degradation of pollutants. On the other hand, the electrokinetic remediation technology is a physical-chemical treatment based on the application of low intensity electric fields among electrodes adequately distributed in the soil. The electric field applied produce different physical and chemical processes (electromigration, electrophoresis, electroosmosis, electrolysis and electrokinetic heating) that can lead on the separation and extraction of the pollutants contained in the soil (metals and/or organic species). Electrokinetic-assisted phytoremediation (EK-phytoremediation) consists of the coupling of phytoremediation and electrokinetic remediation to achieve a technology able to overcome the current disadvantages of the individual technologies. This technique has already shown very promising results for metals, although, to date, it has not been deeply assessed in soils polluted by organic compounds.
In this frame, the present PhD Thesis has assessed the applicability of the EK-phytoremediation for the removal of atrazine from spiked soils using different plant species, electric field application conditions and experimental scales. Atrazine was chosen because its widespread use and for its physicochemical properties (moderately polar and partially soluble in water). Along with the selection of plant species with tolerance to atrazine and electric current, the influence of the different parameters related with the application of DC current to the soil (i.e. voltage gradient, time of application and unidirectional or polarity reversal modes) on the uptake, degradation and accumulation of atrazine by EK-phytoremediation and on the changes in soil characteristics (pH, electrical conductivity) were checked. Lastly, another important breakthrough of this research was the scaling up (from pots to mesocosms) of the EK-phytoremediation technology since it had not yet been studied.
Four experimental series were carried out in order to achieve those goals. Experimental Series 1 consisted of a phytoremediation pot test conducted in order to select plant species capable of degrading and/or accumulating atrazine at different soil doses (2, 5 and 10 mg per kg of soil); four plant species were tested: tall fescue (Festuca arundinacea), ryegrass (Lolium perenne), barley (Hordeum vulgare) and maize (Zea mays). As a result, maize and ryegrass were selected to carry out the following pot experiments of EK-phytoremediation. Experimental Series 2 used maize to study the EK-phytoremediation of atrazine at different soil concentrations (0, 5 and 10 mg kg-1) and voltage gradients (0, 2 and 4 V cm-1); the electric field was applied for 14 days at a rate of 4 h per day and switching the polarity every 2 h. Ryegrass was used in the Experimental Series 3 to conduct a kinetic experiment of EK-phytoremediation (applied to a soil polluted with 2 mg kg-1 of atrazine) using a voltage gradient of 1 V cm-1 applied for 20 days at rated of 6 and 24 h per day (with change of polarity every 2 h). Finally, the last experimental series (Experimental Series 4) consisted of the scaling up the EK-phytoremediation technology by carrying out a mesocosm scale experiment using ryegrass. Glass fiber reinforced PVC mock-ups with dimensions of 2.25 (L) x 0.49 (W) x 0.50 (H) m containing 0.386 m3 of soil were used in this experiment. Four different treatments were used, i.e. application of unidirectional DC current and with reversal polarity, and electrokinetic remediation with the same two types of electric field application. The initial dose of atrazine used in all the treatments was 2 mg kg-1, while a voltage gradient of 0.6 V cm-1 was applied continuously (in the treatments with reversal polarity, it was periodically changed each 24 h).
Results from the Experimental Series 1 showed that atrazine can be taken up from polluted soils and degraded by the four plant species tested, showing that phytoremediation is more effective than natural attenuation for the removal of atrazine from the soil, in approximately 10-36%. Maize was the plant species with the highest ability to accumulate atrazine derivatives, reaching up to 38.4% of the initial atrazine added to the soil.
The most relevant conclusion from the EK-phytoremediation pot experiments using maize and ryegrass (Experimental Series 2 and 3) was increase of the atrazine soil removal efficiency by 36.5% and 7% for maize and ryegrass, respectively, as compared to phytoremediation ones, therefore demonstrating the initial hypothesis in which this research was based. Only 6-34% of the overall atrazine removed from the soil remained in the plant tissues at the end of the experiments; this fact, along with the detection in soils and plant tissues of significant concentrations of dealkylated metabolites (deethylatrazine and deisopropylatrazine) coming from the biochemical degradation of atrazine by plants and rhizosphere microorganisms, suggested that phytodegradation and rhizodegradation in the system plant-rhizosphere were the most important mechanisms for soil decontamination. The kinetic study conducted with ryegrass (Experimental Series 3) allowed to fitted data of atrazine soil concentration vs time to a pseudofirst-order kinetic equation; as a result, atrazine half-life values of 8.2, 7.1 and 5.4 days for the treatments corresponding, respectively, to 0, 6 and 24 h day-1 of electric current application, were found. The enhancement of atrazine removal by applying DC current had the counterpart of the decrease in biomass observed for the plant species tested. In fact, it decreases in the range 9.1-32% depending on the electric voltage gradient, the time of application and the plant species. It was attributed to the changes in soil pH and the higher availability (and toxicity) of atrazine caused by the electric current application. Nevertheless, the utilization of periodical polarity changes avoided to get the extreme pH values usually reached in the electrokinetic remediation experiments.
Results from the EK-phytoremediation tests at mesocosm scales (Experimental Series 4) showed, in a clearer way than the pot experiments, that atrazine and its metabolites were effectively carried by electroosmosis (from anode to cathode) and electromigration (from cathode to anode) after applying DC current, among these two electrokinetic fluxes, electroosmosis was shown to be the most relevant at mesocosm scales. Moreover, the presence of plant roots greatly influenced the 3-D profiles of soil pH, water content and atrazine concentrations, showing the significant role of plants in the process. The results of this experimental series were compared with those of the pot tests. In spite of the difficulties to get an adequate geometric and operational similarity between setups of different scale, the main output parameters of the EKPR process (electric current, specific current charge, overall atrazine removal, specific atrazine removal efficiency, root biomass:soil weight ratio) were discussed. It was shown that, although the processes carried out are essentially the same in both scales, their extent may be quite different; it highlights the limitations of small-scale experiments to predict the results at field conditions.
