Instituto Tecnológico y de Estudios Superiores de Monterrey
Campus Monterrey
School of Engineering and Sciences
Paraquat Degradation by Electrochemical Oxidation in Surface Water:
Mathematical Modeling
A thesis presented by
Jorge Luis Valle Verduzco
Submitted to the
School of Engineering and Sciences
in partial fulfillment of the requirements for the degree of Master of Science
In
Engineering Science
Monterrey Nuevo León, June, 2021
Dedication
I dedicate this stage of my life to my parents, Maru and Jorge, who always believed in me, always motivated me, always supported me and to whom I owe everything. I will never forget who I am, a Valle Verduzco that can achieve anything.
To my sisters, Venecia, Valeria and Vianney, because each word of encouragement and support that they you said to me filled my heart and gave me more strength to continue.
To my girlfriend, Kitzia, who always supported me, motivated me to continue and improve. A stage in life, which is a success for both of us. More adventures will come, but it will be together. I do not know where I am going, but if you want, come with me. I love you.
Acknowledgements
First, I would like to thank Dr. Miguel Angel López Zavala, who was my advisor these 2 years and supported me every time I needed help. In addition to academic teaching, he taught me life lessons. Thank you.
A huge acknowledgement to the laboratory of the Water Center of Latin America and the Caribbean, Andrea, Alejandra, Juan, and Abraham, for the support they gave me during the pandemic.
I want to thank my parents and my sisters, my family who always believed in me and encouraged me.
To my girlfriend, Kitzia Rodríguez, who was with me since before I started my master's degree, she always supported me, and she was always there when I needed to talk with someone, I always found a place of tranquility in her. Thank you so much.
A huge thank you to my good friend, the engineer José Enrique Acuña Cota, who always supported me throughout the master's degree, many nights explaining any topic to me, especially equations and the chemical topic. Without a doubt, he is an excellent friend. Thank you so much.
I would like to thank to Alejandra Flores, who supported me from the beginning of my experiments, and she was helping me until the end. She was a key piece of my experimentation and support. Thank you for the talks that we had in the laboratory. Thank you so much.
I would also like to thank the committee members, Dr. Blanca Elizabeth Monárrez Cordero and Dr. Jorge Humerto García Orozco for all your advice, comments and corrections you gave me.
To my friends from the master's degree who always supported me, especially Christian Narváez, Marvin Feyt, Óscar Serrano, Ramiro Velasco, Rubén Rodríguez and David Pacheco.
To my colleagues from the research group of the Water Center, for their help and support.
To Tecnológico de Monterrey for the scholarship granted to study the Master of Science.
To thank CONACyT for the scholarship during these 2 years.
Paraquat Degradation by Electrochemical Oxidation in Surface Water:
Experimentation and Mathematical Modeling
by
Jorge Luis Valle Verduzco Abstract
Today’s society must face a series of challenges to ensure continuous survival. In addition, agriculture is one of the main activities in rural areas. In some cases, this is the only possible activity in the economy of these regions and, therefore, the only engine of growth. Pesticides are agents that protect crops from pests and diseases. Paraquat (1,1’-dimethyl-4,4’-bipyridinium dichloride) is a non-selective, contact broad- spectrum herbicide that has been widely used as an herbicide for decades. Paraquat poisoning has a high mortality rate. Several methods have been investigated to remove paraquat from aquatic environments. Currently, adsorption and degradation are the two main methods to remove/reduce paraquat. Solutions of paraquat dichloride tetrahydrate (100%, AccuStandard) were prepared with surface water (“Rodrigo Gomez” dam) with a concentration of 13 mg/L, 14 mg/L and 15 mg/L. Initial pH (8.2) of the solutions was adjusted to 3, 7 and 9. Then, electrochemical oxidation of samples of each solution was conducted at DC densities of 16.29 mA/cm2 (6.5 V), 30 mA/cm2 (12 V) and 60 mA/cm2 (24 V). The reaction times were 1, 2.5, 5, 7.5, 10, 15, 20, 25, 30, 35, 40, 50 and 60 minutes. Paraquat and their transformation products (TP) were analyzed by HPLC using an Agilent 1260 HPLC- DAD equipment (Agilent Technologies, Santa Clara, CA, USA) with a reverse phase Zorbax Eclipse XDB C-18 column with dimensions of 150 x 4.6 mm and 5 µm diameter spherical particles (Agilent Technologies, Santa Clara, CA, USA). All samples were filtered using 0.45µm polytetrafluoroethylene syringe filters before analysis. For the mathematical modeling of paraquat degradation, four scenarios were proposed. The first scenario was anodic oxidation with active electrodes with active hydroxyl radicals, the second scenario was with active chlorine species at pH < 3 with Cl2 (gas), the third scenario was with active chlorine species at 3 < pH <
8 with HClO as oxidant, and the fourth scenario was with active chlorine species at pH > 8 with ClO-. At pH 3 was the only treatment that completely degraded paraquat within 60 minutes since pH 7 and pH 9 did not show complete degradation of the herbicide. From the experiments carried out, the current density of 30 mA/cm2 at pH < 3 was the one that showed the best degradation.
The validation of the mathematical model, at a current density of 30 mA/cm2, resulted in a correlation of 94% of paraquat degradation and 95% of Cl2 production (pH < 3). The formulation of ordinary differential equations facilitated the modeling.
List of Figures
3.1. Humaya, Tamazula, and Culiacan rivers within the Culiacan basin .….….….………... 5
3.2. Paraquat molecule ……….…………7
3.3. Synthesis of the Paraquat molecule ..………...…… 8
3.4. Paraquat reduction - oxidation ………. 9
3.5. Scheme for the biodegradation of paraquat on and in soil ……….…… 11
3.6. Mass spectra obtained after 45 minutes of photocatalytic degradation ….….….…... 12
3.7. Mechanism for Paraquat degradation by photocatalysis or photolysis ……….. 13
3.8. General mechanism of direct anodic oxidation of organic compounds ………...…….. 14
3.9. Electrochemical processes considered in the cathodic zone ………. 17
3.10. Electrochemical processes considered in the anodic zone ………. 18
4.1. Electrochemical oxidation cell ………..…. 19
4.2. Paraquat dichloride tetrahydrate ……… 20
4.3. HPLC system Agilent 1260 HPLC-DAD ………..… 21
4.4. Reverse phase Zorbax Eclipse XDB C-18 column ……… 21
5.1. Identification of Paraquat in surface water at pH 3, 7 and 9 ….………. 23
5.2. Calibration curve from 2 mg/L to 10 mg/L ………..………...….. 24
5.3. Degradation of paraquat at different current density at pH 7 ….………24
5.4. Degradation of paraquat at different pH (3,7 and 9) at 30 mA/ cm2 ………. 25
5.5. Paraquat degradation at pH 3 and a current density of 30 mA/cm2 (12 Volts) ...…..… 26
5.6. Paraquat degradation over time at pH 7 ……… 26
5.7. Paraquat degradation over time at pH 9 ……….……... 27
5.8. Transformation products in the chromatogram at minute 7.5 at pH 3 ………..… 28
5.9. Paraquat degradation and transformation product at minute 40 at pH 3 ………... 28
5.10. Transformation product in the chromatogram at minute 5 at pH 7 ………... 29
5.11. Paraquat degradation and transformation product at minute 60 at pH 7 ………... 29
5.12. Degradation product in the chromatogram at minute 2.5 at pH 9 ………. 30
5.13. Paraquat degradation and transformation product at minute 60 at pH 9 ………... 30
5.14. Area of iron oxide (TP 1) at voltage 12 at different pH ……… 31
5.15. Area of transformation product 2 at voltage 12 at different pH ……… 31
5.16. Paraquat degradation and transformation products at pH 3 ………... 32
5.17. Paraquat degradation and transformation products at pH 7 ………... 32
5.18. Paraquat degradation and transformation products at pH 9 ………... 32
5.19. Staining of the samples at minute 60 ………. 33
5.20. Polytetrafluoroethylene syringe filters ordered from minute 0 to 60 at pH 3 .………... 33
5.21. Polytetrafluoroethylene syringe filters ordered from minute 0 to 60 at pH 7 and 9 ….. 33
5.22. Results obtained [simulation (lines) versus experimental data (points)] of the degradation of paraquat (pH 3) ……….. 43
5.23. Results obtained [simulation (lines) versus experimental data (points)] of the formation of Cl2 (pH 3) ………. 44
5.24. Degradation kinetics of paraquat and chlorine formation ………..……… 45
5.25. Results obtained [simulation (lines) versus experimental data (points)] of the degradation kinetics of paraquat and chlorine formation 16.29 mA/cm2 ……….………. 45
5.26. Results obtained [simulation (lines) versus experimental data (points)] of the degradation kinetics of paraquat and chlorine formation at 60 mA/cm2 ………... 46
List of Tables
3.1. Pesticides in the agricultural valley of Culiacan, Sinaloa ………...……… 6
3.2. Paraquat physicochemical properties ………..…… 8
5.1. Paraquat degradation correlation ……….... 48
5.2. Chlorine formation correlation ………...… 48
Contents
Abstract v
List of Figures vi List of Tables vii 1. Introduction 1 2. Objectives 3 2.1. Objectives of this work ……… 3
3. Literature review 4 3.1. Agriculture in Mexico ………. 4
3.1.1. Agriculture in Sinaloa ………... 4
3.1.2. Culiacan Valley ……….... 6
3.1.3. The use of the pesticides in agriculture ………...………. 7
3.2. Paraquat ………... 7
3.2.1. Health risks of paraquat ……… 8
3.2.2. Environment impact of paraquat ………. 9
3.3. Oxidation Process ………... 10
3.3.1. Advanced Oxidation Process (AOPs) ………...……... 10
3.3.2. Electrochemical Advanced Oxidation Process (EAOPs) ………. 10
3.3.3. Paraquat oxidation products ………. 11
3.4. Mechanism of oxidation reaction ………13
3.4.1. Anodic oxidation ……….. 13
3.4.2. Active chlorine species ……… 14
3.5. Mathematical model ……….. 15
3.5.1. Base model ………... 15
4. Methodology 19 4.1. Experimental device for the degradation of paraquat ………... 19
4.2. Experimental procedure ……… 20
4.3. Preparation of the model and simulation for describing the electrochemical oxidation process of paraquat ……… 22
5. Results and discussion 23 5.1. Degradation of paraquat ……… 23
5.1.1. Identification of degradation according to current density ………….………. 24
5.1.2. Paraquat degradation at different pH ……...……… 25
5.2. Effect of pH and current density on paraquat ………... 26
5.2.1. Degradation of paraquat at pH 3 and current density 30 mA/cm2 …………... 26
5.2.2. Degradation of paraquat at pH 7 and current density 30 mA/cm2 ……… 26
5.2.3. Degradation of paraquat at pH 9 and current density 30 mA/cm2 ……… 27
5.2.4. Formation of transformation products ……….. 27
5.3. Mathematical modeling and simulation of paraquat oxidation ………... 34
5.3.1. Formulation of the model and mass balance equations ……… 34
5.3.2. Simulation of paraquat oxidation ……….. 43
5.3.3. Correlation between experimental data and modeling …………..……... 47
6. Conclusion and recommendations 48
A Annex 49
References 53
Chapter 1 Introduction
Today’s society must face a series of challenges to ensure continuous survival. The population continues to grow, and the principle of sustainability ensures that the same opportunities are provided for future generations. These include water and food supplies, eradication of hunger and poverty, and protection of a healthy natural environment [35]. In addition, agriculture is one of the main activities in rural areas. In some cases, this is the only possible activity in the economy of these regions and, therefore, the only engine of growth [14]. On the other hand, this sector is the main consumer of water resources, so it has a direct impact on the use of water resources. In addition, agriculture is a source of environmental pollution, and its excessive degree will lead to over-exploitation and degradation of water resources [11] [27].
Pesticides are agents that protect crops from pests and diseases. The beneficial results of pesticides make it an important tool to maintain and improve the living standards of the global population. An average of 2 million tons of pesticides are used globally every year to combat weeds, insects, and pests. Conventional pesticide classifications based on target species include herbicides, insecticides, rodenticides, fungicides, etc. Herbicides and insecticides are the most used types of pesticides, 47.5% and 29.5% respectively, according to the worldwide consumption [52]. It is an emerging pollutant due to the lack of knowledge about its behavior in the environment and this compound is present in the aquatic environment with extremely low concentrations [17].
Paraquat (1,1’-dimethyl-4,4’-bipyridinium dichloride) is a non-selective, contact broad- spectrum herbicide that has been widely used for decades. Herbicides are sold in about 130 countries/regions and can be used to control large and small farms, rice fields and non-agricultural weeds. Gramoxone is the commercial name of paraquat. The use of paraquat has been discussed for decades in international, national, and nongovernmental organizations [20]. Paraquat poisoning has a high mortality rate. The lung is the target organ in paraquat poisoning, and respiratory failure with acute pulmonary fibrosis is the most common cause of death [53].
Several methods have been investigated to remove paraquat from aquatic environments.
Currently, adsorption and degradation are the two main methods to remove/reduce paraquat.
For adsorption, commonly materials such as activated carbon, activated bleaching earth, modified zeolite, montmorillonite and organoclay are used. On the other hand, among physical and chemical degradation methods of paraquat can be mentioned titanium dioxide, ozone, ultraviolet radiation, and various advanced oxidation processes (AOPs) [19].
CHAPTER 1. INTRODUCTION
Recently, several methods called electrochemical advanced oxidation methods (EAOP) have been proposed based on the degradation of organic pollutants, such as anodic oxidation (AO), electro-Fenton (EF) and photo electro- Fenton (PEF) [13]. The main degradation mechanism of paraquat for anodic oxidation is by hydroxyl radicals (•OH), that are generated electrochemically at the electrode.
Therefore, the aim of this study is the evaluation of pH and current density on electrochemical oxidation using stainless steel electrodes in surface water. Propose a mathematical model for describing the electrochemical oxidation process compared to experimental results using ordinary differential equations.
Chapter 2 Objectives
2.1 Objectives of this Work
General Objective:
Propose and evaluate the use of electrochemical oxidation process to degrade paraquat using stainless-steel electrodes and formulate a mathematical model to describe its related chemical and electrochemical kinetics.
Specific Objectives:
• Conduct the electrochemical oxidation of paraquat using stainless-steel electrodes.
• Evaluate the effect of pH and current density on the electrochemical oxidation of paraquat in surface water.
• Propose a mathematical model for describing the electrochemical oxidation process.
Chapter 3
Literature review
3.1 Agriculture in Mexico
Agriculture is a source of environmental pollution, and its excessive degree will lead to over- exploitation and degradation of water resources [11] [27]. Mexican agriculture is a typical example of the relevance of this sector. According to data from SIAP (Food, Agricultural and Fisheries Information Service), Mexico has a land area of 135 million hectares dedicated to agricultural activities. The area is divided into 25 million hectares of crops and 110 million hectares of pasture [50]. Agriculture in Mexico is a basic activity in the rural environment, currently, an estimated 27.5 million Mexicans live in rural areas, accounting for a quarter of the country’s population [16].
The vast area of Mexico covers a variety of climate zones. Considering rainfall, evapotranspiration, and the flow of water in and out of neighboring countries, Mexico has 451.55 billion cubic meters of renewable fresh water [10]. Agriculture is the main consumer of water, represents 76% of total consumption. In total, 63.6% of the water used for agriculture comes from surface water, while 36.4% of the water comes from groundwater [10]. The National Water Conservancy Plan for 2019-2024 identified inefficient water use as one of the problems related to water resources, especially in the agricultural sector, causing water loss of more than 40% [49].
3.1.1 Agriculture in Sinaloa
In Mexico, the main economic activity is agriculture, with an agricultural area of approximately 25 million hectares [50], mainly distributed in Sinaloa State, producing approximately 12,170 tons of food. The arable land area is about 1.2 million hectares, of which furrow irrigation accounts for 40%, rain irrigation accounts for 50%, and both account for 10%. The main crops are corn with 51% and beans with 8% of the total area respectively [32].
CHAPTER 3. LITERATURE REVIEW
Figure 3.1. Humaya, Tamazula, and Culiacan rivers within the Culiacan basin. Scale 1:250000 [32].
Through dams and aquifers, the Culiacan River provides fresh water to 1,059,617 residents and irrigates 273,475 hectares of farmland, with an estimated value of US $885,860 of the crops [32].
The main water stream is the Culiacan River, which is born at the intersection of two enormous waterways: Humaya and Tamazula, and cross the city of Culiacan, until its end in the Gulf of California (Figure 3.1), with a normal slope of 0.05% [32].
In Sinaloa State, the amount of water used for irrigation of farmland is 44 hm3/year. In the agricultural drainage infrastructure, there are an average of 55 hm3/year of water-containing fertilizers and pesticides. Many of these compounds are not degraded but have contact with water bodies in very high concentrations and may cause eutrophication processes. This phenomenon is observed in the river basin, affecting the ecosystem [32].
CHAPTER 3. LITERATURE REVIEW
3.1.2 Culiacan Valley
In terms of area and yield, one of the most important agricultural areas is the Culiacan Valley. It has approximately 333,000 hectares of agricultural land, where 217,000 hectares are highly mechanized and dedicated to growing some grains, such as corn and sorghum [50]. In the Culiacan Valley, intensive agriculture is mainly carried out, with the massive use of pesticides to avoid losses due to pests [24].
According to the inventory of pesticide containers from the “Campo Limpio” program, from November 2011 to October 2012, 263 commercial products were identified, corresponding to the record of 40,486 containers, representing a total of 246 ton of pesticides applied. Table 3.1 is the container inventory, organized from the highest to the lowest total application amount, corresponding to 10 pesticide active ingredients [24].
Table 3.1. Pesticides in the agricultural valley of Culiacan, Sinaloa
Active
ingredient Chemical class Classification Total amount
(kg year-1) Toxicity
COFEPRIS Toxicity
WHO Extraction Detection Ref.
Mancozeb Dithiocarbamate Fungicide 25,552.20 CT IV CT II SLE UV-Vis [29]
Paraquat Bipyridyl Herbicide 19,203.01 CT II CT I SLE GC/MS and HPLC [31]
[54]
Chlorotalonil Chloronitrile Fungicide 6,673.19 CT IV CT I LLE GC/MS [42]
Malathion Organophosphorade Insecticide 3,591.80 CT IV CT II LLE and
SPE HPLC [30]
Cypermethrin Pyrethroid Insecticide 2,109.24 CT III CT II MSPD GC/ECD [35]
Oxamil Carbamate Insecticide 1,782.10 CT Ib CT I LLE and
SPE HPLC [44]
Endosulfan Organochlorines Insecticide 1,535.10 CT II CT I SPME GC/MS [30]
Dichlorvos Organophosphorade Insecticide 1,275.77 CT Ib CT Ib SLE GC/MS [46]
Cupric
hydroxide Inorganic
compound Fungicide 1,239.62 CT IV CT IV SLE Carbon
analysis [25]
Naled Organophosphorade Insecticide 1,224 CT II CT I LLE and
SPE HPLC [31]
CT I (Extremely hazardous), CT Ib (Highly hazardous), CT II (Moderately hazardous), CT III (Slightly hazardous), CT IV (Slightly toxic), SLE (Solid-liquid extraction), LLE (Liquid- liquid extraction), SPE (Solid phase extraction), SPME (Solid phase microextraction), MSPD (Matrix solid phase dispersion), HPLC (High performance liquid chromatography), GC/MS (Gas chromatography–mass spectrometry), GC/ECD (Gas Chromatography Electron Capture Detector), UV-Vis (Ultraviolet-visible spectroscopy).
Based on the table 3.1, paraquat was selected to conduct this study, since the amount used in the Culiacán valley is 19,203 kg year-1. In addition, its toxicity is high and harmful to humans and biodiversity.
CHAPTER 3. LITERATURE REVIEW
3.1.3 The use of the pesticides in agriculture
Corn fertilization requires 200 kg N-1 per year and 50 kg P ha-1 per year [18]; in addition, it is important to emphasize that from 2013 to 2016, the corn area increased by 60,000 hectares, including an increase of 12×103 tons of nitrogen and 3.0×103 tons of phosphorus. In Mexico, Sinaloa State has the highest agricultural production and is almost completely intensive.
Fertilization rates in this area are 700 kg N ha-1 and 150 kg P ha-1 [32].
Persistent chemicals in pesticides cause pesticide pollution of water released from agricultural activities, urban use, and pesticide production factories. Farmer are the main users of pesticides; they use a lot of pesticides to protect and increase crop yields. In addition, the wood processing industry uses a lot of treat the raw materials with pesticides. According to the characteristics of pesticides, chemical pesticides tend to be released into the environment and become one of the sources of pollution in water [55].
Direct discharge of pesticides into groundwater is a common point source pollution, in which pesticide spraying and improper pesticide disposal cause pesticides to enter water wells.
The non-point source of pesticides comes from farmland, from runoff and erosion events, causing pesticides to gradually seep into groundwater and surface water [1].
3.2 Paraquat
It is often used in agricultural areas. It has provided farmers with a history of more than 40 years, and it is the world’s second selling agrochemical. The herbicidal properties of paraquat were discovered in 1955, and its active ingredients were introduced into the world market under the trade name Gramoxone in 1962, and exist in the form of 20% or 40% aqueous solutions [53].
Paraquat (1,1’-dimethyl-4,4’-bipyridinium dichloride) is one of the most widely used herbicides for controlling weeds in crops. It has strong herbicidal activity, easy to handle, low vapor pressure, high water solubility and high binding potential with the soil [54]. The struc ture formula of paraquat is represented in the figure 3.2
Figure 3.2. Paraquat molecule [4].
CHAPTER 3. LITERATURE REVIEW
Pyridine is treated with sodium in ammonia followed by oxidation. The resulting 4,4’- bipyridine is methylated with chloromethane to give the paraquat (Figure 3.3) [4].
Figure 3.3. Synthesis of the Paraquat molecule [4].
Some of the characteristics of paraquat are shown in table 3.2.
Table 3.2. Paraquat physicochemical properties [36]
Characteristics Value
Category Herbicide
Chemical class Bipyridyl Physical appearance Colorless, white Molecular formula C12H14N2Cl2 Molecular weight 257.16 g mol-1 Density at 20 °C 1.24 g cm-3 Melting point 180 °C Boiling point 300 °C
Solubility at 20°C 100,000 mg L-1
3.2.1 Health risks of paraquat
Paraquat molecules have properties similar to membrane receptors at the alveolar level. This explains the selective concentration of toxins in lung tissue. Generally, it prefers tissues with significant oxygen saturation (such as lung, liver, and kidney) [53].
So far, the most common route of poisoning is ingestion, because its absorption is not through the respiratory tract or the entire skin. There is no evidence that inhalation can cause fatal poisoning [21]. Once in the body, the poison will spread through the blood and be fixed in various tissues (especially the lungs), causing serious diseases and high mortality. The toxicity of paraquat is related to the production of superoxide ions [53].
Paraquat can cause damage to the lungs through all exposure routes. Progressive proliferation of fibrous connective tissue was observed in the alveoli where paraquat was selectively concentrated. The mechanism of action is generated by metabolically catalyzed single- electron oxidation/reduction reactions, leading to depletion of nicotinamide adenine dinucleotide phosphate (NADPH) and the production of oxygen free radicals. For example, unsaturated lipids that form superoxide radicals and attack cell membranes (Figure 3.4). This in turn generates lipid
CHAPTER 3. LITERATURE REVIEW
Figure 3.4. Paraquat reduction - oxidation [3]
The charge that the molecule has, together with its conjugated electronic structure, gives paraquat the property of producing free radicals that react with oxygen, forming ionic radicals of superoxide and hydrogen peroxide that regenerate bipyridyl [53].
3.2.2 Environment impacts of paraquat
Paraquat is very persistent and may be related to soil particles carried by erosion in sur face water systems [15].
Paraquat pollutes the aquatic environment of streams due to its high solubility in water. In countries that allow the use of paraquat to control aquatic plant species, fish mortality indirectly increases [48].
Based on acute oral and subacute diet, paraquat is moderately toxic to poultry. Bird reproduction studies have shown that when adult birds are exposed to more than 30 ppm of paraquat, the reproduction will be affected. Existing mammalian chronic data indicate that after 12 weeks of exposure, paraquat has a lethal effect on certain small mammals, and the content is less than 25 ppm. The results of the 96-hour acute toxicity study showed that paraquat is slightly toxic to fish [15].
The compound is modestly poisonous to birds. According to reports, the oral LD50 of Japanese quails are 970 mg kg-1. Paraquat is very poisonous to numerous amphibian living beings (counting rainbow trout, blue gill, and channel fish). At high levels, paraquat represses the photosynthesis of certain green growth in stream water. Paraquat is non-poisonous to honey bees [38].
.
CHAPTER 3. LITERATURE REVIEW
3.3 Oxidation Process
3.3.1 Advanced Oxidation Process (AOPs)
Advanced methods are needed to effectively treat groundwater and surface water contaminated by herbicides. Relevant results were obtained through an advanced oxidation process (AOP), in which •OH radicals were generated through chemical, electrochemical, photochemical and photocatalytic systems [7].
Within the framework of liquid pollution flow, AOPs can be broadly defined as aqueous phase oxidation methods, which are based on the intermediateness of highly reactive species in the mechanism that leads to the destruction of pollutants. Hydroxyl radical (•OH) is a powerful oxidant that can destroy most organic and organometallic pollutants indiscriminately until they are completely mineralized into CO2, water, and inorganic ions [51].
The application of AOP is mainly used in wastewater treatment; however, this process is also used in different field, such as groundwater treatment, soil remediation, municipal wastewater, odor removal and flavor from drinking water [51].
3.3.2 Electrochemical Advanced Oxidation Process (EAOPs)
Recently, based on the degradation of organic pollutants with in-situ electrically generated hydroxyl radicals, various advanced oxidation methods called Electrochemical Advanced Oxidation Process (EAOP) have been proposed, and have been widely used to treat various toxic and biological non-degradable organic pollutants, including pesticides, dyes and pharmaceu ticals [13].
The EAOPs include anodic oxidation (AO), in which heterogeneous •OH are generated at the anode surface, as well occurs on electro-Fenton (EF) and photoelectro-Fenton (PEF) [51].
Hydroxyl radicals are oxidized on the anode by anodic oxidation (AO) or by indirect electro- oxidation, and also the electrode can generated Fe2+ and hydrogen peroxide (H2O2) [13].
Different materials for electrodes can be used. For the cathode, the most common are graphite, carbon felt, and gas diffusion electrodes, and for the anodes the most used are platinum, sacrificial iron, and boron-doped diamond electrodes [13].
CHAPTER 3. LITERATURE REVIEW
3.3.3 Paraquat oxidation products
Until now, there is no specific literature on the degradation pathways of paraquat and its oxidation products by anodic oxidation. Microbiological and photocatalytic degradation studies report that monoquat as an intermediate product of the physicochemical degradation of paraquat, 4-carboxy- 1-methylpyridinium ion and short-chain carboxylic acid appear to be the main oxidation products [28], and mineralization, mainly in CO2 [19].
Scientific study by Ricketts entitled "Microbial biodegradation of paraquat in soil" [45], cultures of soil microorganisms were used to elucidate the degradation pathway of paraquat with a ring mark (Figure 3.5). The result of the investigation showed that the bacteria and fungi Corynebacterium fascians, Lipomyces starkeyi, Aspergillus niger, Penicillium frequentans, Fusarium sp and Pseudomonas sp were able to degrade the herbicide. Paraquat degradation was rapid, it was extensively metabolized with 50% mineralization production of carbon dioxide within three weeks [45].
Figure 3.5. Scheme for the biodegradation of paraquat on and in soil. m=micro-organism cultures; hv=sunlight (paraquat on soil) or UV light (paraquat in solution) [47]
In another research, to understand which compounds were mineralized by photocatalysis, the oxidation products of paraquat were analyzed by LC-MS and ion chromatography. The figure 3.6 shows the mass spectrum obtained after 45 minutes of photocatalytic degradation [27].
CHAPTER 3. LITERATURE REVIEW
Figure 3.6. Mass spectra obtained after 45 minutes of photocatalytic degradation with an initial paraquat concentration of 10 ppm [27]
Experiments show that during the degradation process, the initial paraquat solution contains paraquat (m/z = 93), reduced paraquat (m/z = 186), a quat (m/z = 171), and paraquat pyridine. In the mixture of (m/z = 201), a strong peak appears at m/z = 138 on the mass spectrum, which corresponds to the oxidation product 4-carboxy-1-methylpyridinium ion [27].
At different elution times, other oxidation products were detected as carboxylated anions:
8.9 minutes (acetate), 10.5 minutes (format), 19.6 minutes (succinate), 20.4 minutes (maleate) and 21.6 minutes (oxalate) by chromatography [27].
The degradation mechanism of paraquat by photocatalysis or photolysis under UV-C radiation was proposed (Figure 3.7) and the analysis were by LC-MS and ion chromatography equipment. First, the paraquat molecule is transformed into a similar molecular structure with a bipyridinium group (monoquaternary ammonium, reduced paraquat or paraquat monopyridone).
Fragmentation occurs and leads to lower molecular weight compounds. These compounds can be degraded into the final products of oxidation, especially carbon dioxide [27].
CHAPTER 3. LITERATURE REVIEW
Figure 3.7. mechanism for Paraquat degradation by photocatalysis or photolysis under UV-C radiation [27].
3.4. Mechanism of oxidation reaction
3.4.1. Anodic oxidation
The anodic oxidation, which forms •OH in situ by electrocatalysis, stands out among the available methods. The efficiency of anodic oxidation also depends on some key operating conditions, such as the nature and concentration of the supporting electrolyte, the initial concentration of organic pollutants, current density, temperature, and pH [22].
To treat wastewater containing non-biocompatible pollutants (such as phenol) to form biocompatible organic matter direct anodic oxidation is used, which can be biologically treated. In electrochemical conversion, organic compounds can be directly oxidized on the anode surface by physically adsorbed hydroxyl radicals MOx (•OH) to generate carbon dioxide as the final product.
This hydroxyl radical also produces higher oxide species (MOx+1) on the dimensionally stable anode (DSA). DSA is an inert metal, coated with precious metal oxides such as RuO2 and IrO2
[34].
CHAPTER 3. LITERATURE REVIEW
Figure 3.8 illustrates the mechanism of direct anodic oxidation. At the same time, hydroxyl radicals also enhance the production of oxygen. The evolution of oxygen is a factor limiting the efficiency of the electrochemical process. Therefore, it is recommended to use an oxidizer to increase the oxidation rate [34].
Figure 3.8. General mechanism of direct anodic oxidation of organic compounds [34]
3.4.2. Active chlorine species
The mediated oxidation with active chlorine is based on the direct oxidation of chloride ions at the anode to yield chlorine (Cl2) through equation 1.
2 𝐶𝑙−→ 𝐶𝑙2+ 2 𝑒− (1)
The oxidant diffuses into the water and structures hypochlorite and chloride in the response medium through disproportionation (Equations 2 and 3).
𝐶𝑙2+ 𝐻2𝑂 → 𝐻𝐶𝑙𝑂 + 𝐻++ 𝐶𝑙− (acidic médium) (2) 𝐶𝑙2+ 2 𝑂𝐻−⇆ 𝐶𝑙𝑂−+ 𝐶𝑙−+ 𝐻20 (alkaline medium) (3) Hypochlorous acid is deprotonated to frame hypochlorite (Equation 4).
𝐻𝐶𝑙𝑂 + 𝑂𝐻−⇆ 𝐶𝑙𝑂−+ 𝐻2𝑂 (4)
Since hypochlorite is the principle finished result, it is regularly found in the literature that chloride is straightforwardly changed over to hypochlorite (Equation 5) [51].
𝐶𝑙−+ 𝐻2𝑂 → 𝐶𝑙𝑂−+ 2 𝐻++ 2 𝑒− (5)
It ought to be noticed that the sort and grouping of the dynamic chlorine species present in arrangement is subject to its pH, for a 0.5 mol L-1 Cl- concentration, at 25 °C; under these conditions, Cl2 is the transcendent species when pH < 3, HOCl for 3 < pH < 7.5, and ClO- for pH > 7.5. If the Cl- concentration in the solution decreases, the pH interval where HClO species dominate will increase as it expands to lower pH values [12]. Since HClO (Eº = 1.49 V/SHE) and Cl2 (Eº = 1.36 V/SHE) exhibit higher redox potentials than ClO− (Eº = 0.89 V/SHE), the oxidation of organics should be faster in acidic than in alkaline media [12].
CHAPTER 3. LITERATURE REVIEW
3.5 Mathematical model
Some models have been created to describe the advancement of a mathematical model that is reliable with the processes that occur in the electrochemical treatment of wastewater containing organic pollutants.
The models for such processes were proposed by Polcaro, who built up a model that describes the electrochemical oxidation of watery squanders containing chlorophenols. The model expects that the oxidation is intervened by hydroxyl radicals and that the electrochemical cell acts like a blended tank reactor [43].
Cañizares presented a model pointed toward clarifying the processes happening in a consolidated batch oxidation-adsorption reactor, containing two anodic materials (stainless steel and activated charcoal). The model assumes that oxidation happens all the while on the anode surface (direct oxidation) and in the bulk solution (oxidation interceded by hydroxyl radicals) [5].
Comninellis built up a model where in the contamination in wastewater is measured in wording of chemical oxygen demand (COD). In this model, it is expected that electrochemical responses happen exceptionally near the electrodes, and the mass-transfer process (evaluated by a mass-transfer coefficient) is hence included. In the description of the model, two types of behavior were considered relying upon the worth of the current intensity. The important of this model is that applied current intensity, solution flow rate, and mass-transfer coefficient are known [41].
3.5.1. Base model
To obtain a useful model, many simplified assumptions should be made.
The model divides the electrochemical reactor into areas: the area near the electrode (electrochemical area) and the bulk area (chemical area). In each area, it is assumed that the concentration at all positions is the same and changes only over time. Direct reactions may occur in the electrochemical area, and mediation processes may occur in the chemical area. The kinetic equation shows that the direct oxidation rate depends on the intensity of the applied current, the mass transfer limitation and the difficulty of the compounds in the system [6].
The mass-transfer rate can be calculated from Equation 1, where Si* and Si are the concentrations (mol m-3) of component i in the electrochemical and chemical zones, respectively;
k (m s-1) is the mass-transfer coefficient; and A (m2) is the specific interfacial area between the electrochemical and chemical zones. It can be assumed that the mass transfer coefficient (k) depends only on the flow rate conditions because the concentration of the compound is very low [6].
𝑟
𝑖= 𝑘𝐴(𝑆
𝑖∗− 𝑆
𝑖)
(1)The method of dividing the electrochemical cell into several regions can simplify the mathematical complexity of the model. Therefore, the complex system of partial differential equations is reduced to a system of ordinary differential equations that is easy to solve [6].
CHAPTER 3. LITERATURE REVIEW
The rate of the electrochemical process is limited by the applied current density (I) and is given by Equation 2, where F is the Faraday constant [6].
𝑟 =
𝐼𝐹 (2)
Many processes may occur on the electrode surface; therefore, the total applied current must be shared between all these processes. Therefore, a part of the applied current intensity (𝛼𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒 ) corresponds to each process i, and the rate of each process can be calculated according to equation 3 [6].
𝑟
𝑖=
𝐼𝐹
𝛼
𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒 (3)Assuming that the difference between the battery potential and the oxidation/reduction potential (Vi) is the driving force for the electron distribution, the proportion of electrons involved in a particular electrochemical process (𝛼𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒 ) is easily related to the measurable parameters (equation 4) [6].
𝛼
𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒=
(𝛥𝑉𝑤𝑜𝑟𝑘− 𝛥𝑉𝑖)∑ (𝛥𝑉𝑖 𝑤𝑜𝑟𝑘−𝛥𝑉𝑖) (4)
As shown in Equation 5, the chemical reaction can be modeled based on the concentration of the oxidant (Sox) and the organic compound (Si), assuming a second-order rate expression. In this equation, ki is the kinetic constant of process i [6].
𝑟
𝑖= 𝑘
𝑖𝑆
𝑜𝑥𝑆
𝑖 (5)If the oxidant produced by electricity is a very active substance, it can be assumed to be a steady-state approximation, and a pseudo-first-order (equation 6) or even pseudo-zero-order (equation 7) rate expression can be proposed [6].
𝑟
𝑖= 𝑘
𝑖𝑆
𝑖 (6)𝑟
𝑖= 𝑘
𝑖 (7)Once the cell description and mass transfer and considering the kinetic considerations, the mass balance of the reaction system provides the resulting model equations. For a typical batch processing system, the following mass balance equation for the anode (equation 8) and cathodic (equation 9) electrochemical zones and for the chemical zone (equation 10) can be obtained [6].
𝑣𝑐𝑑 𝑆𝑖,𝑎
𝑑𝑡 = ∑ 𝑣𝑖𝑗 𝐼 𝐹𝛼𝑗
𝑎𝑛𝑜𝑑𝑒+ 𝑘𝐴(𝑆𝑖,𝑏− 𝑆𝑖,𝑎) (8)
𝑛
𝑗=1
𝑣𝑐𝑑 𝑆𝑖,𝑐
𝑑𝑡 = ∑ 𝑣𝑖𝑗 𝐼 𝐹𝛼𝑗
𝑐𝑎𝑡ℎ𝑜𝑑𝑒+ 𝑘𝐴(𝑆𝑖,𝑏− 𝑆𝑖,𝑐) (9)
𝑚
𝑗=1
𝑣𝑏𝑑 𝑆𝑖,𝑏
= 𝑘𝐴(𝑆𝑖,𝑎− 𝑆𝑖,𝑏) + 𝑘𝐴(𝑆𝑖,𝑐− 𝑆𝑖,𝑏) + ∑ 𝑣𝑖𝑗𝑟𝑗 (10)
𝑝
CHAPTER 3. LITERATURE REVIEW
In these equations, vij is the stoichiometric coefficient of compound i in process j, and the subscripts a, c, and b represent the anode, cathode, and bulk (chemical) zones [6].
In electrochemical treatment for organic pollutant wastewater, the main process related to the removal of pollutants involves an irreversible oxidation pathway. Therefore, the reduction process is not important, and it can be assumed that only hydrogen evolution (a) occurs in the cathode zone (Figure 3.9) [6].
Figure 3.9. Electrochemical processes considered in the cathodic zone by anodic oxidation [6].
CHAPTER 3. LITERATURE REVIEW
When using typical inactive materials, the anode only serves as an electron sink. In this case, the scheme representing the oxidation process can be explained as shown in Figure 3.10 [6].
Figure 3.10. Electrochemical processes considered in the anodic zone by anodic oxidation [6].
Chapter 4 Methodology
4.1 Experimental device for the degradation of paraquat
A glass fiber electrochemical oxidation cell, reported by Lopez Zavala and Anglés Vega (2020), was prepared with stainless steel electrodes. The cell was used to run all tests. The volume of the equipment is 365 mL. The electrode mesh consists of 14 stainless steel electrodes. Seven electrodes work as active anodes, and seven electrodes are used as cathodes. The diameter is 2mm, the effective length is 130mm, and they are separated 2mm each. The power source is a direct current (DC) power supplier Kaselco (Sill Beach, California, USA). The device provides a voltage of 0 V to 50 V and a current intensity of 0 to 10 A. In this study, a batch operation of electrochemical oxidation cells was carried out. Figure 4.1 shows the schematic representation of the experimental device [26].
Figure 4.1. Electrochemical oxidation cell [26]
CHAPTER 4. METHODOLOGY
4.2 Experimental procedure
Surface water from the dam “Rodrigo Gómez” located in Santiago, Nuevo León, México was used for all the experiments. It was stored in 20 liters containers at 4°C until its use.
Three solutions of Paraquat dichloride tetrahydrate (100%, AccuStandard) were prepared with surface water with a concentration 13 mg/L, 14 mg/L and 15 mg/L (Figure 4.2).
By using hydrochloric acid (HCl) and/or potassium hydroxide (KOH), as required, initial pH (8.2) of the solutions was adjusted to 3, 7 and 9. Then, electrochemical oxidation of samples of each solution was conducted at DC densities of 16.29 mA/cm2 (6.5 V), 30 mA/cm2 (12 V) and 60 mA/cm2 (24 V) as shown in table 4.0. The reaction times were 1, 2.5, 5, 7.5, 10, 15, 20, 25, 30, 35, 40, 50 and 60 minutes. After each reaction time, samples were collected in HPLC vials of 2 mL for analysis.
Figure 4.2. Paraquat dichloride tetrahydrate.
Paraquat and their transformation products (TP) were analyzed by a HPLC Agilent 1260 HPLC-DAD equipment (Agilent Technologies, Santa Clara, CA, USA, Figure 4.3) with a reverse phase Zorbax Eclipse XDB C-18 column (Figure 4.4) with dimensions of 150 x 4.6 mm and 5 µm diameter spherical particles (Agilent Technologies, Santa Clara, CA, USA). All samples were filtered using 0.45µm polytetrafluoroethylene syringe filters before analysis.
Paraquat and its transformation products were analyzed following the procedure reported by Wang & Liu in 2013 [23], with some modifications. The operating conditions of the system were temperature 30°C. Mobile phase consists of ammonium acetate (10 mM, pH 3.5) and acetonitrile in a 90:10 (v/v) proportion. The method was an isocratic flow. The mobile phase was conducted at a flow rate of 1.0 mL/min and detection at 260 nm, injection volume of 10 µL. The analysis was by triplicate.
CHAPTER 4. METHODOLOGY
Figure 4.3. HPLC system Agilent 1260 HPLC-DAD
Figure 4.4. Reverse phase Zorbax Eclipse XDB C-18 column
CHAPTER 4. METHODOLOGY
4.3 Preparation of the model and simulation for describing the electrochemical oxidation process of paraquat
For the mathematical modeling of paraquat degradation, kinetic procedures from different authors such as Cañizares, Polcaro and Comninellis for anodic oxidation were considered [6]
[43] [41]. The model presented involves the use of pseudo-first order kinetic equations according to Cañizares.
For the reaction mechanism, there are different oxidation diagrams in the anodic, cathodic, and chemical zone, where the zones were adapted to the degradation of paraquat.
The method of dividing the electrochemical cell into several zones can simplify the mathematical complexity of the model. Therefore, the complex system of partial differential equations is reduced to an easy-to-solve system of ordinary differential equations.
There are four possible scenarios that can occur during degradation. The first scenario is anodic oxidation with active electrodes with active hydroxyl radicals, the second scenario is with active chlorine species at pH < 3 with Cl2 (gas), the third scenario is with active chlorine species at 3 < pH < 8 with HClO as oxidant, and the fourth scenario is with active chlorine species at pH > 8 with ClO-. The modeling was programmed in Matlab.
Chapter 5
Results and Discussions
5.1 Degradation of paraquat
The solution prepared with surface water at a concentration of 10 mg/L of paraquat was analyzed to know the time in which it appears in the HPLC equipment. The paraquat standard appears with a retention time of 1.218 minutes (Figure 5.1). The same retention time was obtained at different pH such as 3, 7 and 9 at a current density of 30 mA/cm2 (12 volts). The chromatographic peak appears in the same place without having any change.
Figure 5.1. Identification of Paraquat in surface water at pH 3, 7 and 9
CHAPTER 5. RESULT S AND DISCU SSI O N S
The calibration curve was prepared with 2, 3, 4, 5, 7 and 10 mg/L paraquat (Figure 5.2). An accepted correlation (0.999) was obtained to be able to measure the concentration of the samples of different pH and current density.
Figure 5.2. Calibration curve from 2 mg/L to 10 mg/L
5.1.1. Identification of degradation according to current density
An experimental, from 0 to 60 minutes was carried out, with a solution prepared with paraquat at a concentration of 12, 13 and 14 mg/L at pH 7 with a current density of 16.29 mA/cm2 (6 volts), 30 mA/cm2 (12 volts) and 60 mA/cm2 (24 volts), to compare the different current densities and find out which was the most optimal for degradation. The current density of 16.29 mA/cm2 (6 volts) did not show adequate degradation, it was maintained at a concentration higher than 8 mg/L. The current density of 30 mA/cm2 (12 volts) showed a degradation of 40% at reaction time 60 minutes. The current density of 60 mA/cm2 (24 volts) showed a degradation of the herbicide, however, the voltage was high, the solution began to heat up to high temperatures inside the oxidation cell, so at 25 minutes reaction time the experiment was ended (Figure 5.3). In this way, density 30 mA/cm2 (12 volts) was used to degrade the herbicide at different pH.
Figure 5.3. Degradation of paraquat at different current density at pH 7.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0 5 10 15 20 25 30 35 40 45 50 55 60
C/Co
Time (min)
6 V 12 V 24 V
CHAPTER 5. RESULT S AND DISCU SSI O N S
5.1.2. Paraquat degradation at different pH
In figure 5.4, a comparison is made between the different pH 3, pH 7 and pH 9 at the current density of 30 mA/cm2 (12 volts) from 0 to 60 minutes. The optimal pH for the degradation of paraquat was pH 3 since it was shown to degrade the herbicide in 60 minutes.
In surface water conditions (pH 8.12), the main specie is ClO- at pH > 8. The other cases are HClO at 3 < pH < 8, and Cl2 until pH < 3. Since Cl2 (E° = 1.36 V vs SHE) and HClO (E° = 1.49 V vs SHE) are compared with ClO- (E° = 0.89 V vs SHE), the oxidation mediated by active chlorine species is more effective in acid than in alkaline media. Therefore, in this experiment, active chlorine species (Cl2) oxidize paraquat molecules at pH 3. The chloride ions present in the surface solution enhance the degradation activity of the active anode (stainless steel electrode).
he degradation activity of Cl2 is higher than that of active oxygen species [26].
Figure 5.4. Degradation of paraquat at different pH (3,7 and 9) at 30 mA/cm2
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0 5 10 15 20 25 30 35 40 45 50 55 60
C/Co
Time (min)
pH 3 Ph 7 pH 9
CHAPTER 5. RESULT S AND DISCU SSI O N S
5.2 Effect of pH and current density on paraquat
5.2.1. Degradation of paraquat at pH 3 and current density 30 mA/cm
2From 0 to 60 minutes, at pH 3 (Cl2), the degradation of paraquat was obtained by decreasing the concentration to 0.91 mg/L (Figure 5.5), and the current density applied was 30 mA/cm2 (12 volts), since this was the current density that showed better degradation of this molecule. Since Cl2 (E° = 1.36 V vs SHE) the oxidation mediated by active chlorine species is more effective in acid than in alkaline media. The degradation activity of Cl2 is higher than that of active oxygen species [26]. Minute 35 is the inflection point where the degradation decays to a very low concentration.
Figure 5.5. Paraquat degradation at pH 3 and a current density of 30 mA/cm2 (12 Volts)
5.2.2. Degradation of paraquat at pH 7 and current density 30 mA/cm
2From 0 to 60 minutes, at pH 7, the degradation of paraquat was obtained by decreasing the concentration to 6.25 mg/L (Figure 5.6), and the current density applied was 30 mA/cm2 (12 volts), since this was the current density that showed better degradation of this molecule. The solution at pH 7 needs more time to fully degrade the paraquat, this shows that the pH used is not optimal for experimental purposes. There are not enough active chlorine species to oxidize paraquat since the surface water initially had a pH of 8.2 and a few HCl was used to lower it to pH 7.
Figure 5.6. Paraquat degradation over time at pH 7
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
0 5 10 15 20 25 30 35 40 45 50 55 60
C/Co
Time (min)
Paraquat
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0 5 10 15 20 25 30 35 40 45 50 55 60
C/Co
Time (min)
Paraquat
CHAPTER 5. RESULT S AND DISCU SSI O N S
5.2.3. Degradation of paraquat at pH 9 and current density 30 mA/cm
2From 0 to 60 minutes, at pH 9, the degradation of paraquat was obtained by decreasing the concentration to 7.62 mg/L (Figure 5.7), and the current density that applied was 30 mA/cm2 (12 volts), since this was the density that showed better degradation of this molecule. The solution at pH 9 needs more time to fully degrade the paraquat, this shows that the pH used is not optimal for experimental purposes. In surface water conditions (pH 8.12), the main specie is ClO- at pH
> 8. Since ClO- (E° = 0.89 V vs SHE), the oxidation potential is lower than Cl2 and HClO-, so at pH 9 a longer reaction time is needed.
Figure 5.7. Paraquat degradation over time at pH 9
5.2.4. Formation of transformation products
The chromatogram shows that at minute 7.5 of the experiment (Figure 5.8), a transformation product appears in a retention time of 1.312 minutes (TP 1) and at 1.696 minutes (TP 2) in an experimental run of 4 minutes. Which, its concentration is lower than the initial one of the paraquat in the solution. According to Lopez Zavala M., and Anglés Vega D. [26], TP 1 corresponds to iron oxide; therefore, it is not exactly a byproduct of paraquat oxidation, but it is also considered a conversion product of the oxidation process because its concentration increases with the increase of reaction time [26]. TP 2 corresponds to a compound that was present in surface water, there was no reaction since its area remains stable over time.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0 5 10 15 20 25 30 35 40 45 50 55 60
C/Co
Time (min)
Paraquat
CHAPTER 5. RESULT S AND DISCU SSI O N S
Figure 5.8. Transformation products in the chromatogram at minute 7.5 at pH 3
In Figure 5.9, at minute 40 the paraquat concentration is very low compared to the transformation product, with a retention time of 1.316 minutes (TP 1). The iron oxide increased, while the paraquat decreased. The chromatographic peak at 1.696 min (TP 2) continues with the same initial area.
Figure 5.9. Paraquat degradation and transformation product at minute 40 at pH 3
CHAPTER 5. RESULT S AND DISCU SSI O N S
The chromatogram shows that at minute 5 of the experiment (Figure 5.10), a transformation product was detected in a retention time of 1.305 minutes (TP 1) and at 1.700 minutes (TP 2). Its area is lower than the initial one of the paraquat (1.217 min) in the solution due to the beginning of the iron oxide production from the oxidation of the electrodes.
Figure 5.10. Transformation product in the chromatogram at minute 5 at pH 7
In figure 5.11, at minute 60 the paraquat concentration is lower than the iron oxide, however, in the last minute of the experiment there is not enough degradation but there is a significant increase of iron oxide. While chromatographic peak at 1.691 min (TP 2), continues with the same initial concentration.
Figure 5.11. Paraquat degradation and transformation product at minute 60 at pH 7
CHAPTER 5. RESULT S AND DISCU SSI O N S
The chromatogram shows at minute 2.5 of the experiment (Figure 5.12), a degradation product was detected in a retention time of 1.310 minutes (TP 1), which is iron oxide, and at 1.704 minutes (TP 2) in an experimental run of 4 minutes. Its concentration is lower than the initial one of the paraquat in the solution due to the beginning of the iron oxide production from the oxidation of the electrodes.
Figure 5.12. Degradation product in the chromatogram at minute 2.5 at pH 9
In figure 5.13, minute 60 the paraquat concentration is lower than the iron oxide, however, in the last minute of the experiment there is not enough degradation but a significant increase in transformation product. While the TP 2 (1.693 min), continues with the same initial concentration.
Figure 5.13. Paraquat degradation and transformation product at minute 60 at pH 9
CHAPTER 5. RESULT S AND DISCU SSI O N S
The iron oxides (TP 1) correspond to the retention time of 1.308 minutes which increased as the paraquat decreased. The current density applied was 30 mA/cm2 (12 volts) at different pH (3, 7 and 9) (Figure 5.14). In the pH 3 results, as paraquat decreased in concentration, iron oxides (TP 1) began to increase. It was this pH that showed greater production of TP 1.
Figure 5.14. Area of iron oxide (TP 1) at voltage 12 at different pH
The transformation product 2 (TP 2) is not significant, since during all the time that it is experienced, it does not increase. TP 2 corresponds to a compound that was present in surface water, there was no reaction since its area remains stable over time. (Figure 5.15).
Figure 5.15. Area of transformation product 2 at voltage 12 at different pH
0 200 400 600 800 1000 1200
0 5 10 15 20 25 30 35 40 45 50 55 60
Area (mAU*s)
Time (min)
pH 3 pH 7 pH 9
0 5 10 15 20 25 30 35
0 5 10 15 20 25 30 35 40 45 50 55 60
Area (mAU*s)
Time (min)
pH 3 pH 7 pH 9
CHAPTER 5. RESULT S AND DISCU SSI O N S
Figure 5.16 represents the transformation products and the degradation of paraquat represented in area. Where at pH 3at a current density of 30 mA/cm2, it is seen considerably that the paraquat area tends to zero while the iron oxides (TP 1) increases and the TP 2 remains constant, without varying.
Figure 5.16. Paraquat degradation and transformation products at pH 3
At pH 7 and 9, as paraquat degrades, iron oxides (TP 1) increase its area; however, paraquat is not completely degraded. The TP 2 remains constant (Figure 5.17 and Figure 5.18).
Figure 5.17. Paraquat degradation and transformation products at pH 7
0 200 400 600 800 1000
0 5 10 15 20 25 30 35 40 45 50 55 60
Area (mAU*s)
Time (min)
Paraquat Iron oxides (TP 1) TP 2
0 100 200 300 400 500 600 700
0 5 10 15 20 25 30 35 40 45 50 55 60
Area (mAU*s)
Time (min)
Paraquat Iron oxides (TP 1) TP 2
CHAPTER 5. RESULT S AND DISCU SSI O N S
Figure 5.18. Paraquat degradation and transformation products at pH 9
The color of the water in the samples changed and is very colorful at minute 60 (Figure 5.19).
The pH 3, its color was brown, and the samples of pH 7 and pH 9, had an olive-green color. The coloration corresponds to the oxidation of the electrodes, which is iron oxide, and the sediments correspond to iron hydroxides.
Figure 5.19. Staining of the samples at minute 60
Once obtaining the samples from the oxidation cell, they were filtered with polytetrafluoroethylene syringe filters, and the samples stored in 2 mL vials for analysis on HPLC (Figure 5.20 and Figure 5.21) and the samples were from 0 to 60 minutes at pH 3, 7 and 9.
Figure 5.20. Polytetrafluoroethylene syringe filters ordered from minute 0 to 60 at pH 3
Figure 5.21. Polytetrafluoroethylene syringe filters ordered from minute 0 to 60 at pH 7 and 9
0 100 200 300 400 500 600 700 800 900
0 5 10 15 20 25 30 35 40 45 50 55 60
Area (mAU*s)
Time (min)
Paraquat Iron oxides (TP 1) TP 2
