Biodegradation of organic compounds is known as an effective and eco-friendly method to remove organic pollutants from the aqueous environment (Zhan et al., 2018). Various micro-organism, including bacteria and fungi, have been reported to use glyphosate as a sole carbon, nitrogen, and/or phosphorus source. Table 1.6 shows that the microorganisms responsible for glyphosate biodegradation are mainly bacteria, only little fungi strains have been reported. Among these microorganisms, most species use glyphosate as sole phosphorus source. Some exceptions use glyphosate as nitrogen or carbon source.
To assess glyphosate-degradation performance of microorganisms, it is necessary to optimize culture conditions, including culture temperature, initial pH, glyphosate concentration, inoculated biomass and incubation time (Zhan et al., 2018). The culture conditions most used for glyphosate-degrading microorganism are a temperature of 25-37℃, a pH of 6-7.5 and aerobic medium. Only Obojska et al. (2002) observed a thermophilic bacteria, Geobacillus caldoxylosilyticus T20, which could achieve more than 65% of glyphosate removal at 60℃ with initial glyphosate concentration of 1 mM. Kryuchkova et al. (2014) found a facultative anaerobic strain, Enterobacter cloacae K7, which could utilize glyphosate as sole phosphorus source and obtain 40% degradation with glyphosate initial concentration of 5 mM.
Two major degradation pathways have been identified in glyphosate-degrading microorganisms (Fig. 1.6). One pathway is glyphosate converted to stoichiometric quantities of AMPA and glyoxylate through the cleavage of C-N bond by the enzyme glyphosate oxidoreductase (Zhan et al., 2018). Glyoxylate usually enters the tricarboxylic acid cycle as a convenient energy substrate for most glyphosate-degrading bacteria (Sviridov et al., 2015). Three pathways exist for AMPA: (i) AMPA releases to the Environment (Jacob, 1988; Lerbs et al., 1990); (ii) AMPA is further metabolized to methylamine and phosphate, catalyzed by C-P lyase (Pipke et al., 1987; Jacob, 1988; Pipke and Amrhein, 1988); (iii) AMPA is first metabolized to phosphonoformaldehyde by transaminase and then transformed to phosphate and formaldehyde for further metabolism by phosphonatase (Sviridov et al., 2014).
The second degradation pathway is glyphosate metabolized to phosphate and sarcosine through the direct cleavage of the C-P bond, catalyzed by C-P lyase (Firdous et al., 2017). Sarcosine can be used as growth nutrient (carbon and nitrogen source) for microorganism and is further metabolized to glycine and formaldehyde by sarcosine-oxidase (Borggaard and Gimsing, 2008). Glycine is further metabolized by microorganism and formaldehyde enters the tetrahydrofolate-directed pathway of single-carbon transfers to produce CO2 and NH4+ (Borggaard and Gimsing, 2008). Some reports indicate that AMPA and sarcosine pathways simultaneously exist in some bacteria, such as Bacillus cereus CB4, Ochrobacterium anthropic GPK3, and Pseudomonas sp. LBr (Jacob, 1988; Fan et al., 2012; Sviridov et al., 2012). The AMPA pathways is not generally subjected to Pi (inorganic phosphorus) concentration, however, glyphosate conversion to sarcosine strongly depends on the concentrations of exogenous and endogenous Pi, which rarely occurs in natural environments (Sviridov et al., 2015) due to C-P lyase activity generally induced under phosphate starvation condition (Borggaard and Gimsing, 2008; Sviridov et al., 2015).
At least, another degradation pathway was observed in Achromobacter sp. Kg16 which utilized glyphosate as sole phosphorus source, resulting in production of acetylglyphosate (Shushkova et al., 2016). However, Achromobacter sp. Kg16 is not able to further utilize acetylglyphosate as a phosphorus source, causing its poor growth. Although the glyphosate biodegradation has been extensively studied, the precise degradation mechanism and pathways are still not known.
Most studies reported have focused on the glyphosate biodegradation by pure culture of bacteria. Little research on glyphosate biodegradation was carried on mixed culture. Hallas and Adams (1992) reported glyphosate removal from wastewater effluent discharged from an activated sludge process in lab columns and found that more than 90% of glyphosate degradation was achieved for an initial concentration of 50 mg.L-1. Nourouzi et al. (2010) reported that 99.5% of glyphosate (300 mg.L-1) was converted to AMPA and 2% of AMPA was degraded to further metabolites by mixed bacteria isolated from oil palm plantation soil. The mixed cultures are more likely able to completely degrade contaminants, compared to pure culture due to the various enzymes available in mixed culture (Barbeau et al., 1997; Nourouzi et al., 2012). Moreover, due to the high requirements of pure culture, mixed culture processes are more suitable for industrial applications.
Advanced oxidation processes (AOPs)
AOPs are promising technologies, which have been widely used for the treatment of toxic, recalcitrant organic compounds in water (Divyapriya et al., 2016), including photolysis, ozonation, Fenton, electro-oxidation, wet air oxidation (WAO) and supercritical water oxidation. The mechanism in AOPs system is to oxidize organic contaminants to CO2, H2O and inorganic ions due to the generation of hydroxyl radical (•OH), hydrogen peroxide (H2O2), and superoxide (O2∙) in the system (Malato et al., 2002; Manassero et al., 2010). The hydroxyl radical (•OH) is a non-selective, strong oxidant (2.8 V oxidation potential), which can act very fast on a wide range of organic compounds (Mota et al., 2008; Divyapriya et al., 2016). Compared to conventional treatment, AOPs can non-selective completely mineralized pollutants without chemical or biological sludge production.
AOPs could be an alternative technology to effectively treat glyphosate at a short time compared to physical (adsorption and filtration) and biological treatment. Recently, single or combined AOPs have been reported to treat glyphosate-containing wastewater, such as photolysis oxidation, Fenton oxidation, electrochemical oxidation, ozonation oxidation, and combined oxidation process. Table 1.7 summarizes some AOPs process used for glyphosate treatment.
Table 1.7 shows that photolysis-based oxidation can lead to high glyphosate removal efficiency up to 99.8% at low concentration (less than 50 mg.L-1) and the use of photocatalyst improve the photodegradation of glyphosate. TiO2 is the common used heterogeneous photocatalyst for glyphosate photocatalytic degradation because of its stability, non-toxic and low cost (Echavia et al., 2009). In order to improve the photocatalytic activity of TiO2, several attempts have been reported, such as non-metal doping (Echavia et al., 2009), metal doping (Xue et al., 2011) and metal and non-metal codoping (Lin et al., 2012). Although complete glyphosate removal has achieved (Echavia et al., 2009), while, the mineralization efficiency is not high (less than 74%). Meanwhile, the preparation processes for modified TiO2 are generally complicated, resulting in an increase of the cost. In order to decrease the cost, the combination of hydrogen processes and UV radiation (H2O2/UV) has been reported to treat glyphosate with higher concentration (up to 91.26 mg.L-1) compared to photocatalytic degradation, which is a simple and convenient process (Manassero et al., 2010; Junges et al., 2013; Vidal et al., 2015; López et al., 2018). H2O2/UV process induced a good degradation of glyphosate (>70%), but it requires a long treatment time (more than 5 h). Meanwhile, due to the high cost of electricity associated with using energy-consuming UV lamps (Echavia et al., 2009), the disposal of catalysts and difficulties to control the conditions (Zhan et al., 2018) hamper the development of these photolysis-based processes at large scale application (Tran et al., 2017).
Fenton based oxidation (Table 1.7) has been reported to be a successful technology for glyphosate treatment, which has the advantages of simple operation, no mass transfer limitation and easy implementation as a stand-alone or hybrid system and easy integration in existing water treatment processes (Chen et al., 2007; Bokare and Choi, 2014). 95.7% and 62.9% removal of total phosphate and chemical oxygen demand (COD), respectively, have been achieved by conventional Fenton process (Liao et al., 2009). However, several drawbacks exist in conventional Fenton process: the continuous loss of oxidants and iron ions, the formation of solid sludge and the high costs and risks associated with handling, transportation and storage of reagents (Zhang et al., 2019). In order to overcome these shortcomings, Fenton process is improved to form various optimized Fenton processes for glyphosate treatment, i.e. electro-Fenton (Balci et al., 2009; Lan et al., 2016) and photo-Fenton processes (Huston and Pignatello, 1999; Souza et al., 2013). Electro-Fenton process overcomes the limitations of the accumulation of iron sludge and the high costs and risks related to the handling, transportation, and storage of reagents. Photo-Fenton process can reduce iron sludge production (Zhang et al., 2019). Electro-Fenton and photo-Fenton processes have both reported to achieve complete glyphosate removal and good mineralization at low concentration (Balci et al., 2009; Souza et al., 2013). However, electro-Fenton consumes extensive anode (Aramyan, 2017; Zhang et al., 2019). Photo-Fenton process faces several challenges, such as short working life span, high energy consumption and economic costs (Aramyan, 2017; Zhang et al., 2019). Moreover, Fenton based process needs an acidic reaction condition (usual pH at 2-4). This consumes a lot of acid along with high cost due to extra electrical energy (UV lamp). Thus, Fenton-based processes are generally used in a synthetic and low concentration glyphosate wastewater rather than real wastewater from the glyphosate production (Huston and Pignatello, 1999; Balci et al., 2009; Liao et al., 2009; Souza et al., 2013).
Electrochemical oxidation is one of the cleanest technologies to effectively degrade glyphosate compared to other AOPs (Villamar-Ayala et al., 2019), with high energy efficiency and easy operations (Sirés et al., 2014). And electrochemical oxidation has been reported to treat effluents with wider glyphosate concentration ranging from 16.9 to 1000 mg.L-1, compared to other AOPs. Complete glyphosate mineralization has been achieved by electrochemical oxidation at glyphosate concentration less than 100 mg.L-1 (Kukurina et al., 2014; Rubí-Juárez et al., 2016). Even when the initial glyphosate concentration up to 1000 mg.L-1, high mineralization (91%) was also obtained by Aquino Neto and De Andrade (2009), on PuO2 and IrO2 dimensionally stable anode (DSA®). PbO2, born doped diamond (BBD) and Ti/PbO2 have been also used as anode for electrochemical oxidation of glyphosate (Kukurina et al., 2014; Rubí-Juárez et al., 2016; Farinos and Ruotolo, 2017; Tran et al., 2017). Electrochemical degradation could be affected by several parameters: pH, glyphosate initial concentration, supporting electrolyte nature and concentration, electronic composition, electrolysis and current density (Aquino Neto and de Andrade, 2009; Aquino Neto and De Andrade, 2009; Moreira et al., 2017). However, some drawbacks exist during electrochemical oxidation process: the high costs related to the electrical supply, the addition of electrolytes required due to the low conductance of wastewaters and the loss of activity and the short lifetime of electrode by fouling due to the deposition of organic compounds on the surface of electrode (Sirés et al., 2014). More research should be studied to overcome these disadvantages.
Compared to other AOPs, ozonation oxidation can effectively treat glyphosate-containing wastewater at a shortest time under low concentration. Complete glyphosate degradation and 97.5% mineralization have been obtained by Speth (1993) and Assalin et al. (2009), respectively. Both high removal efficiencies of glyphosate (>99%) and AMPA (85%) were achieved with simultaneous use of O3 and H2O2 under a short reaction time (Jönsson et al., 2013). However, it is generally applied to treat glyphosate-containing wastewater at low concentration rather than real glyphosate industrial wastewater. Furthermore, there are several drawbacks for ozonation which hinders its application into practice: (1) ozone is unstable under normal conditions; (2) due to its low solubility in water, special mixing techniques are needed; (3) ozone water treatment is much expensive due to the high service and maintenance; (4) high toxicity and chemical hazards; (5) harmful disinfection by-products maybe generate (Rice, 1996).
In addition, Barrett and McBride (2005) obtained 71% and 47% of glyphosate and AMPA removal efficiency by Manganese oxidation, respectively. Zhang et al. (2011) combined adsorption treatment and Fenton oxidation using the nano-metal/resin complexes as the adsorbent to treat the industrial wastewater containing glyphosate. They found that the maximum degradation rate of glyphosate (258 mg.L-1) was enhanced up to 60.5%. Xing et al. (2018) reported that 100% glyphosate removal and over 93% organic phosphorus removal for real glyphosate wastewater (containing 200-3000 mg.L-1 glyphosate) was achieved by catalytic wet oxidation using modified activated carbon as a catalyst in a co-current upflow fixed bed reactor, which could be a potential method for glyphosate-containing wastewater treatment.
Fig. 1.7 The possible oxidation pathway of glyphosate under different processes. Information based on (Barrett and McBride, 2005; Chen et al., 2007; Muneer and Boxall, 2008; Balci et al., 2009; Echavia et al., 2009; Manassero et al., 2010; Lan et al., 2013; Xing et al., 2018; Yang et al., 2018)
Meanwhile, Fig. 1.7 summarizes the possible oxidation pathway of glyphosate under different AOPs reported in the literature. It shows that glyphosate oxidation process generally follows two mechanisms related to the cleavage of C-P and C-N bonds attributed to hydroxyl radicals. In the first case, glyphosate is attacked by hydroxyl radicals to yield sarcosine and PO43- and to generate AMPA and glycolic acid in the second case. The two mechanisms can exist alone or together during glyphosate oxidation process. The glyphosate photo-degradation are often related to the both AMPA and sarcosine pathways, however, only sarcosine pathway during glyphosate photo-degradation is presented by Yang et al. (2018) on goethite and magnetite. This is because the formation of Fe-O-P bond in the presence of iron oxide would change the electron density distribution around the phosphorus center of glyphosate, and potentially induce the C-P bond more assailable to reactive oxygen species generated in goethite and magnetite suspension under UV irradiation (Yang et al., 2018). Besides, a few studies have proved the direct generation of glycine at high pH without the generation of sarcosine in TiO2/UV process (Muneer and Boxall, 2008; Manassero et al., 2010). The mechanism for this phenomenon is still unclear and further research is needed. The single sarcosine pathway is also been reported in the electrochemical and Manganese oxidation of glyphosate (Barrett and McBride, 2005; Lan et al., 2013), whereas, the single AMPA pathway is found in electro-Fenton and CWO process of glyphosate (Balci et al., 2009; Xing et al., 2018). Sarcosine could be further oxidized to glycine, formaldehyde or formic acid. Glycine could be transferred to methylamine, formaldehyde, and NH4+. AMPA may be further converted to formaldehyde, NH4+, NO3- and PO43-through the cleavage of C-P bond. Other small molecules organic compounds may also exist in the glyphosate oxidation processes, such as acetic acid and glycolic acid. Even though the possible oxidation pathways of glyphosate have been abundantly reported, the precise mechanisms are still unknown which is needed further studies.
In conclusion, adsorption and filtration can’t transfer glyphosate to other products nor reduce its toxicity, causing that post-treatment may be needed. The conventional biological processes is a friendly and low-cost glyphosate treatment technology, however, these processes generally require a long residence time. Furthermore, industrial wastewater emitted from glyphosate-manufacturing factory is characteristic of high COD, strong toxicity, poor biodegradability and complicated constituents, which can’t be effectively treated directly using biological treatment (Xing et al., 2017). AOPs takes advantages of degrading glyphosate with high reaction rate and efficiency at low concentration. However, the main disadvantages for AOPs are their high treatment cost caused by the high consumption of electrical energy for devices, such as UV lamps, heater, and ozonizers, and long processing times and AOPs tend to be difficult for complete chemical mineralization of glyphosate due to the formation of the oxidation intermediates during treatment. Thus, to develop a more effective, safe and affordable technology to degrade glyphosate is necessary. Recently, combining AOPs and biological processes has been a promising treatment technology for organic compounds, considering environmental and economic advantages (Mantzavinos and Kalogerakis, 2005). The toxic and/or non-biodegradable effluent is first treated by AOPs during a short time to generate easily biodegradable intermediates which can be completely degraded by a subsequent biological treatment (Azabou et al., 2010). Among different AOPs, wet air oxidation (WAO) is a very promising technology to treat organic contaminants due to its fast reaction rate and high efficiency (Chakchouk et al., 1994; Lin and Chuang, 1994; Kaçar et al., 2003; Suarez-Ojeda et al., 2005). It is reported that the compact process coupling WAO and biological treatment shows high degradation efficiency for various organic compounds or effluents, such as polyethylene glycol (Mantzavinos et al., 1997), substituted phenols (Suarez-Ojeda et al., 2007), deltamethrin (Lafi and Al-Qodah, 2006), Afyon alcaloide factory’s wastewater (Kaçar et al., 2003), olive mill wastewaters (Chakchouk et al., 1994). However, no information regards the application of the combined WAO and biological processes to treat glyphosate-containing wastewater.
Wet air oxidation
Wet air oxidation (WAO) is an attractive treatment for waste streams containing organic compounds which are too dilute to incinerate and too concentrated for biological treatment (Luck, 1999), which was first proposed by Zimmermann (1954). WAO process is defined to oxidize organic compounds into carbon dioxide and water or less toxic intermediates at elevated temperatures and pressure by using oxygen or air as oxidants (Mishra et al., 1995; Luck, 1999; Debellefontaine and Foussard, 2000). Typical conditions for WAO process are 398-573 K for temperature, 0.5-20 MPa for a total pressure, 15-120 min for residence times and 10-150 g.L-1 for the preferred COD load (Debellefontaine and Foussard, 2000; Lefèvre et al., 2011a, 2011b; Lefevre et al., 2012). The elevated temperature enhances the solubility of oxygen in aqueous solutions and the elevated pressure is to keep water in the liquid state and provide a strong driving force for gas-liquid transfer and oxygen solubility (Mishra et al., 1995). The oxygen used for WAO reactions is provided by air bubbled through the liquid phase in the reactor (Joglekar et al., 1991). WAO is one of the few processes which do not transfer contaminants from one form to another, but really make it disappear (Debellefontaine et al., 1996). Organic carbon is oxidized to small acid chains and CO2; organic nitrogen is converted to ammonia, nitrite, or elemental nitrogen; sulfur is turned to sulfuric acid or sulfates; phosphorus and chlorine are transferred to phosphate and hydrochloric acid, respectively (Mishra et al., 1995; Debellefontaine and Foussard, 2000).
WAO for single organic compounds treatment
Table 1.8 summarize available literature studies on the treatment of single organic compounds by WAO process. It depicts that WAO process could achieve high degradation efficiency (more than 80%) to treat carboxylic acids, phenolics and dyes at a short time (always less than 2 h). However, the complete mineralization cannot always be achieved by WAO process for these compounds due to the high TOC or COD remaining, which need to be further treated to meet the discharge standard. Many studies have reported that the biodegradability of effluents treated by WAO process is improved, which could be easily treated by a following biological treatment (Patterson et al., 2002; Kaçar et al., 2003; Suárez-Ojeda et al., 2008). Among these organic compounds, phenolic substances have attracted more attention due to their toxicity and frequency of industrial wastewaters. However, there is no information reported on the glyphosate degradation by WAO process.
Table of contents :
Chapter 1 Bibliographical study
1.1.1 Overview of emerging contaminants
1.1.2 Overview of glyphosate
1.2 Occurrence, behavior, and fate of glyphosate in aqueous environment
1.3 Treatment technology for glyphosate from environment
1.3.3 Biological treatment
1.3.4 Advanced oxidation processes (AOPs)
1.4 Wet air oxidation
1.4.1 WAO for single organic compounds treatment
1.4.2 WAO for real industrial wastewater treatment
1.4.3 Reaction mechanisms
1.4.4 Reaction kinetics for WAO
1.4.5 Mass transfer for WAO
1.5 Microfluidic device
1.6 Coupled advanced oxidation technology and biological treatment to treat emerging contaminants
Chapter 2 Bubble characterization and gas-liquid interfacial area in two phase gas-liquid system in bubble column at low Reynolds number and high temperature and pressure
2.2 Experimental methods
2.2.1 Experimental setup
2.2.2 Data analysis
2.3 Results and discussion
2.3.1 Effect of superficial gas velocity
2.3.2 Effect of superficial liquid velocity
2.3.3 Effect of pressure
2.3.4 Effect of temperature
2.3.5 Effect of different liquid and gas phase
2.3.6 The correlation for average bubble size and gas holdup
Chapter 3 Kinetic study of glyphosate degradation by a wet air oxidation process
3.2 Materials and methods
3.2.2 Experimental procedure
3.2.3 Analytical methods
3.3 Results and discussion
3.3.1 Glyphosate degradation
3.3.2 Kinetic modeling
3.3.3 Formation of byproducts
3.3.4 Proposed degradation pathway
Chapter 4 Oxidation of glyphosate by wet air oxidation using a microfluidic device
4.2 Materials and methods
4.2.2 Microreactor fabrication
4.2.3 Evaluation of glyphosate degradation
4.2.4 Analytical methods
4.3 Results and discussion
4.3.1 Glyphosate degradation
4.3.2 TOC and COD reduction
4.3.3 The formation of byproducts
Chapter 5 Acclimation of aerobic activated sludge to glyphosate biodegradation: experimental study and kinetics modelling
5.2 Materials and methods
5.2.2 Acclimation process of activated sludge
5.2.3 Compared experiments between acclimated and non-acclimated sludge
5.2.4 Analytical methods
5.3 Results and discussion
5.3.1 Bacteria activity validation during acclimation process
5.3.2 Removal efficiency of glyphosate and TOC during acclimation process
5.3.3 Validation of the acclimation performances compared to fresh activated sludge
5.3.4 Kinetics of glyphosate biodegradation
Chapter 6 The removal of glyphosate from wastewater by a compact process combining wet air oxidation and biological treatment
6.2 Materials and methods
6.2.2 Experimental set-up for WAO process
6.2.3 Experimental set-up for biological treatment process
6.2.4 Analytical procedures
6.3 Results and discussion
6.3.1 WAO pretreatment
6.3.2 Biological process
6.3.3 Combination of WAO and biological process
6.3.4 Identification and quantification of by-products
6.3.5 The degradation pathway of glyphosate
Chapter 7 Conclusions and Perspectives