Nitrate and Pesticide Removal from Groundwater Using Different Electrochemical Reactors

Abstract

[eng] Groundwater has been traditionally considered a reliable source of freshwater. However, the low replenishment rates and the increasing contamination due to different anthropogenic activities, such as the use of pesticides and fertilizers to increase crop production, the spread of manure and urban and industrial discharge effluents have affected the water quality in these underground reservoirs around the globe. The different nature of the discharged contaminants in this natural water body adds complexity for its treatment, owing to their different chemical properties. The electrochemical processes for GW treatment have emerged as viable alternatives to the conventional methods. Moreover, the low conductivity and high hardness of these water matrices are drawbacks that must be conveniently addressed beforehand. In this Thesis, different strategies to overcome the major drawbacks have been used in order to remove model pesticides spiked from softened groundwater by electro-oxidation (EO) and/or the nitrate concentration present in the actual water matrix, which was higher than the recommended limit established by the WHO, by electroreduction (ER). Two different softening methods have been used in order to remove the alkaline-earth ions from the groundwater, avoiding the precipitation of calcium and magnesium carbonates and hydroxides on the cathode surface. This would lead to an increase of the electrical resistance of the system and hence, the treatment cost. One process was based on sodium-charged zeolites and the other consisted in forcing the precipitation of the alkaline-earth ions. The cell configuration and the materials employed as electrodes are critical in order to develop a system able to remove specific pollutants. Two pesticides, imidacloprid and terbuthylazine, were removed by either direct or indirect EO using DSA® and boron-doped diamond (BDD) anodes. In the presence of chloride in the water matrix, this species was oxidized to active chlorine, which was able to oxidize organic pollutants in the bulk of the solution increasing the pesticide removal rate. Higher mineralization degrees were achieved when the BDD anode was employed, owing to its superior oxidant power, although chlorinated oxyanions as chlorate and perchlorate were accumulated in solution. A post-treatment based on a Purolite® A532E resin has been considered to remove such by-products generated along the electrochemical treatment. Different iron-based steel materials were also used as cathodes to study the nitrate electrochemical removal by electroreduction in different water matrices. Mild steels have shown superior electrocatalytic activity for the nitrate removal with complete conversion towards ammonia while stainless steel showed poor activity achieving worse nitrate removals at analogous conditions. This reaction has been electrochemically characterized using a mild steel rotating disk electrode (RDE) through a Koutecky-Levich analysis. The number of electrons involved in nitrate ER over a mild steel cathode has been determined (n = 8) as well as the heterogeneous charge transfer kinetic constant (kh) at different cathode potentials (Ecath). The role of atom hydrogen (•H) as a reducing agent, generated on the cathode surface as a hydrogen evolution by-product, has also been studied. Simultaneous removal of a pesticide by electro-oxidation and nitrate by electroreduction has been assessed, alongside the effect of using different anode-cathode combinations. Furthermore, a mild steel rotating cylinder electrode (RCE) reactor has been employed in galvanostatic and potentiostatic conditions in order to enhance the mass transport towards a mild steel cylinder electrode. The diffusion-convection mass transport of this system has been characterized by obtaining the following Sherwood-Reynolds-Schmidt correlation: Sh = 0.7Re0.46Sc0.356 aiming at a future scale-up. The same RCE reactor system has been modeled in order to predict its behavior upon use of a suitable electrocatalytic RCE for nitrate removal. The developed mathematical model has been validated by comparing the simulations obtained by computational fluid dynamics (CFD) with the experimental trends.

[cat] L’aigua subterrània ha estat considerada tradicionalment una font fiable d’aigua potable. L’ús intensiu d’aquest recurs, en combinació amb el desenvolupament de les poblacions, ha portat a una disminució de l’accessibilitat de d’aigua subterrània i a un increment de la contaminació d’origen antropogènic de la mateixa. La varietat de contaminants presents en aquestes masses d’aigua dificulta enormement el desenvolupament de tractaments genèrics per a la seva potabilització. Els processos electroquímics per al tractament de l’aigua subterrània han emergit en els últims anys com a tecnologies prometedores, tot i que la baixa conductivitat i la duresa d’aquestes matrius aquoses en dificulta la seva aplicació directa. En aquesta Tesi s’han estudiat diverses estratègies per a l’aplicació de processos electroquímics per a l’eliminació de contaminants típics presents als aqüífers com són els pesticides i l’anió nitrat en diverses matrius aquoses. L’electrooxidació, tant directa com indirecta, ha sigut emprada per a l’eliminació dels pesticides imidacloprid i terbutilazina utilitzant ànodes d’òxid d’iridi o de ruteni i de diamant dopat amb bor. Per altra banda, l’eliminació de l’anió nitrat ha estat duta a terme mitjançant processos d’electrorreducció utilitzant diferents acers com a càtodes. Els acers amb baix contingut en carboni han demostrat una gran activitat per a l’eliminació electroquímica de nitrat en comparació amb la pobre activitat dels acers inoxidables. S’ha caracteritzat electroquímicament la reacció d’electrorreducció del nitrat sobre un elèctrode de disc rotatori d’ acer AISI 1018 mitjançant un anàlisis de Koutecky-Levich, fet que ha permès determinar el número d’electrons involucrats en la reacció (n = 8) i les constants heterogènies de transferència de càrrega (kh) a diferents potencials catòdics. També s’ha estudiat el rol de l’hidrogen atòmic (•H) format sobre la superfície catòdica com a agent reductor. S’ha utilitzat un reactor d’elèctrode de cilindre rotatori per a l’estudi de l’eliminació de nitrat tant en condicions potenciostàtiques com galvanostàtiques i se n’ha caracteritzat el transport de massa per difusió-convecció. Aquest fet ha permès establir la correlació de números adimensionals de Sherwood-Reynolds-Schmidt, obrint la porta a futurs escalats del procés. S’ha realitzat un model matemàtic per a descriure el sistema i s’ha validat el model comparant els resultats obtinguts de les simulacions realitzades per dinàmica de fluids computacional amb els resultats obtinguts experimentalment realitzant les electròlisis