Degradation of Pesticides in Groundwater by Advanced Oxidation Processes

Abstract

Safe and clean drinking water is a right of every human being on this earth. The industrial and agricultural activities have made this precious element unfit for human consumption. Major reasons for water pollution being discharge from different industries and indiscriminate use of pesticide in agricultural fields. Pesticide pollution of surface water and groundwater has been recognized as a major problem in many developing countries because of the persistence of pollutants in aquatic environments. One of the consequences of indiscriminate use of pesticide is the adverse health impact on society in general and vulnerable population like children in particular. Humans are exposed to pesticides found in environmental media (soil, water, air and food) by different routes of exposure such as inhalation, ingestion and dermal contact. Some of the well-known health effects of pesticide exposure include acute poisoning, cancer, neurological effects and reproductive and developmental effects. Conventional treatment technologies like physico-chemical, biological treatment, air stripping and carbon absorption have limitations as they merely change the phase of the pollutant. Biological methods are incapable especially where the organic pollutants are bio-recalcitrant in nature like pesticides. Thus, incapability of conventional and biological treatment methods for removal of pesticides has substantially increased the concern. Lack of knowledge, repeated and overdose of pesticides has irreversibly damaged our land, air and water bodies, thus has invited many researchers for novel treatment options. In recent past, advanced oxidation processes (AOPs) have proved their worth for the remediation of contaminated wastewaters containing non- biodegradable organic pollutants. The main mechanism of AOPs is the generation of highly reactive free radicals like hydroxyl radicals (OH•) which are effective in destroying organic chemicals because of their high reactive electrophilic behaviour. The use of advanced oxidation processes (AOPs) such as heterogeneous as well as homogeneous photocatalysis have been the widely studied in the past for the degradation of pesticides. These advanced processes are attractive treatment options as they completely degrade the pesticides without transferring them in other phases unlike other treatments like adsorption and air stripping. TiO2, now widely accepted photocatalyst, is inexpensive, stable and inert. The mechanism of the heterogeneous photo-oxidation and reduction is well established and discussed extensively in the literature. Hydroxyl radicals (•OH) generated are non-selective in nature and they can react without any other additives with a wide range of contaminants whose rate constants are usually in the order of 106 to 109 mol L-1 s-1. It makes new oxidized intermediates with lower molecular weight or carbon dioxide and water in case of complete mineralization. A large amount of research has been undertaken in recent past on advanced oxidation processes (AOP) especially heterogeneous photocatalysis for the degradation of toxic compounds. TiO2 photocatalysis, in slurry form, is gaining more importance in the area of AOP to degrade toxic compounds as it is chemically inert, stable, strong oxidizing power and relatively inexpensive. Main technical limitation of this slurry form is basically separation of catalyst from the slurry and huge cost associated behind effective separation. This is one of the major reasons challenging its commercial applications. This problem can be resolved by immobilizing the catalyst on effective support material. Many studies dedicated to TiO2 immobilization have been reported over different inert supports like glass beads, pumice stone, wood, plastics etc. In these works, different coating methods, reactor arrangements have been stressed upon from both engineering and fundamental point of view. But the durability, cost and recyclability of the immobilized system have not been effectively addressed. Moreover, the inertness of support material is also a major concern, which has not been stressed upon. The work presented in this thesis has been undertaken to study the degradation of herbicide isoproturon and insecticide imidacloprid by photocatalytic oxidation process in slurry and fixed photocatalysis using laboratory and pilot-scale reactors as well. Experiments under solar conditions demonstrated the efficiency of the photocatalytic reactors for field-scale applications. The study has been presented in five major chapters in the following text. The first chapter introduces the origin of the problem and its content, which is subsequently supported by literature review on the pesticides pollution and photocatalytic degradation in second chapter. The experimental procedures, reagents and chemicals used and analytical techniques are elaborated in chapter three. Chapter four discusses the study on photocatalytic degradation of herbicide isoproturon and insecticide imidacloprid in slurry and fixed form using laboratory and pilot-scale reactors. Heterogeneous photocatalytic degradation and mineralization of herbicide derivative Isoproturon was investigated in aqueous solutions containing titanium dioxide (Section 4.3) at laboratory scale. The degradation rate was found to be strongly dependent on catalyst concentration, initial pH, substrate concentration, intensity of the light (UV) source and area/volume ratio (A/V). The degradation rate was observed to follow first-order kinetics. The degradation was monitored by observing the change in absorption intensity in UV range and through HPLC analysis. Reduction in COD and TOC values along with the generation of ammonium further indicated the mineralization of the herbicide. An attempt was also made to identify the intermediates (degradation products) through LC-MS analysis. TiO2 loading 0.5 g L-1, pH 5.0, C0=25 mg L-1 are the optimized conditions for obtaining the better degradation rates. The COD reduction (96%) and TOC reduction (90%) along with ammonium ion generation confirmed the mineralization of the herbicide, isoproturon. The intermediates could be further degraded into different intermediate compounds as evident through HPLC and LC-MS data. The observations, clearly demonstrates that outcome of the study can be suitably utilized for removal of pesticides from water/wastewaters using this technology as pre or post treatment. Section 4.4 presents the investigation on the fixed-bed photocatalysis at lab-scale for the degradation of herbicide isoproturon. The use of cement beads for the immobilization of TiO2 has been presented and subsequently tested for the degradation of isoproturon. The immobilized system was effective in degrading and mineralizing the herbicide for continuous thirty cycles without losing its durability. Effect of operating parameters like number of catalyst coatings, bead diameter, UV intensity, calcination temperature etc., were also studied for field scale applications. Catalyst coating was characterized by SEM-EDAX for checking the durability of the catalyst. The degradation rate followed first order kinetics as measured by change in absorption intensity in UV range as well as HPLC analysis. Two rounds of TiO2 coating on inert cement beads with average diameter 1.5 cm at 25 Wm-2 calcined at 400oC were the optimized conditions for the degradation of herbicide isoproturon. More than 90% TOC and COD reduction along with ammonium ions generation (80%) confirmed the mineralization of isoproturon. Fixed bed baffled reactor (FBBR) studies under solar irradiations using the TiO2 immobilized beads confirmed 85% degradation after 6h. LC-MS studies confirmed the intermediates formation and their subsequent degradation using immobilized system.

The photocatalytic degradation of insecticide imidacloprid using shallow pond slurry reactor at lab-scale is being described in section 4.5. The degradation was studied by varying the operating conditions like catalyst dose, oxidant addition, operating pH, initial concentration, area/volume (A/V) ratio, as in the case of isoproturon (section 4.3) and optimum conditions were determined. The maximum degradation was observed with TiO2 dose of 1.0 gL-1, pH 6.5, H2O2 3.0 gL-1 with A/V 1.18 cm2 mL-1. The 86% COD reduction along with the chloride ions (Cl-1) generation confirmed the mineralization of parent compound during photocatalytic irradiations experiments. In continuation of section 4.4, the photocatalytic degradation of imidacloprid using TiO2 immobilized on suitable support material is described in section 4.6. The TiO2 immobilized cement beads were used to study degradation of imidacloprid at lab-scale batch reactor. Effect of certain parameters like number of coatings, calcination temperature, exposed area, UV intensity etc. were checked for studying the degradation of imidacloprid. Durability of support was checked by recycling the beads for 20-30 times for imidacloprid degradation. Authenticity of the lab-scale results can be verified by scaling–up the photocatalytic process to promote the feasibility of photocatalytic water treatment technology at industrial scale in near future. The studies presented in section 4.7 discussed the scale-up for both slurry and fixed-bed reactor used for the degradation of isoproturon. In scale-up slurry reactor, the study of various parameters has been done like depth of solution to be treated, flow-rate variation for constant area/volume. The parabolic trough concentrator (PTC) was used for the degradation of pesticide solution using immobilized TiO2 catalyst (cement beads) under concentrated sunlight. Parameters like effect of flow rate of pesticide solution, reusability of coated beads were studied using solar irradiations. Chapter five summarises that photocatalytic technologies (AOP) have potential for treating the water/wastewater containing biorecalcitrant compounds like pesticides. Experimental results demonstrate the efficacy of both slurry and fixed-bed photocatalysis for the degradation of pesticides. Successful scale-up studies using slurry batch reactor and fixed-bed reactor (PTC) have shown potential of photocatalytic processes to handle large volume of industrial wastewater. Even for fixed and slurry TiO2, the operational costs of these AOPs for total oxidation/degradation of bio-recalcitrant remain relatively very high as compared to biological processes. However, their use as a pre or post treatment option to enhance biodegradability of wastewater containing these bio-recalcitrant compounds can potentially be justified.