Chlorpyrifos Removal from Aqueous Solutions by Emulsion Liquid Membrane: Stability, Extraction, and Stripping Studies

The current paper focuses on assessing key parameters affecting the extraction of Chlorpyrifos as well as emul - sion stability using the emulsion liquid membrane technology. Five parameters affecting the extraction have been studied: homogenizer speed, emulsification time, agitating time, surfactant concentration, and stripping phase con - centration taking into consideration the emulsion breaking. Experiments proved that using the resulting optimum values will maximize both extraction and stripping efficiencies (93.8% and 94.7% respectively), while minimizing the emulsion breakage (increasing the stability of emulsion) to 0.73% with no need to employ a carrier agent. A 10 min agitating time, 3% (v/v) Span 80 as a surfactant, 12700-rpm homogenizer speed, 0.25 M HCl as an internal phase concentration, and 5 min emulsification time are chosen to be the optimum parameters’ values. A study of extraction kinetics and estimation of mass transfer coefficient was also accomplished (3.89×10 -9 m/s). The conclusions of this work can be extended to the removal of other types of pesticides from water.


INTRODUCTION
Pesticides are commonly employed in agricultural industry for controlling a wide range of harmful organisms, which destroy the produce. Pesticides are undeniably beneficial to agricultural crops, even though as a result, lower levels of certain residues may continue being in harvest posing a risk of chronic toxicity and in some cases acute toxicity for humans (Osman et al. 2014). Pesticides are categorized as organophosphorus, organochlorines, chlorophenols, carbamates, and synthetic pyrethroids based on their chemical structure. (Marican and Durán-Lara 2018). Organophosphates (OPs) are esters of pentavalent phosphorous acid, displaying an extensive spectrum of toxicity in living beings. Chlorpyrifos (CP) is a category of organophosphorus pesticides (OP), that find many applications as insecticide, acaricide and termiticide (Akhtar et al. 2009, Nandhini et al. 2021). Chlorpyrifos (CP), an organophosphorus insecticide (OP), commonly consumed in public health and agriculture, has a high octanol water partition coefficient (K ow = 4.70) (Osman et al. 2014). The aqueous hydrolysis DT50 and photolysis of CP are up to 72 d and 29.6, respectively (Zhu et al. 2021). The characteristics of extensive use of CP are long-range transport potential, high lipophilicity, bioaccumulation, and extension of CP half-life leading to slow degradation and high toxicity (Nandhini et al. 2021). Concentrations of CP detected in both surface and groundwater ranged between 0.13 μg/L to 0.24 μg/L (Qurie et al. 2016). A number of conventional and physical-chemical methods, such as biotreatment, Fenton oxidation, titanium dioxide catalytic, powdered activated carbon, reverse osmosis, and filtration have been established for the elimination of organic pollutants including pesticides (Costa et al. 2018). These techniques are engrossed in pesticides dissolving in aqueous solutions (Osman et al. 2014 (Dâas et al. 2014). Liquid membrane processes have recently received a lot of attention as an advanced extraction technique, and are considered an attractive alternative treatment for the elimination of organic contaminants existing in waterbodies. Liquid membrane extraction, commonly known as emulsion liquid membrane (ELM), was presented as a substitute technique for liquid-liquid extraction by solid polymeric membrane (Jusoh et al. 2016, Muthusaravanan et al. 2019). Comparing with membrane separation technique, the ELM process allows selective solute targeted to pass through a liquid membrane, mostly with adding carrier components (Hu et al. 2020). A carrier agent is used in some liquid membrane systems to facilitate the transfer of solutes, resulting in additional costs (Shorki et al. 2020). The mechanism of ELM is based on the diffusion of solutes passing through a liquid membrane where the membrane is an organic phase (Othman et al. 2017). The ELM process entails the following basic stages: (i) emulsification, (ii) dispersion and extraction, (iii) settling, and (iv) demulsification, i.e. breaking the resulted emulsion. For the emulsification step, preparing the emulsion by emulsifying both the organic membrane phase and the internal phase. Then dispersing the prepared emulsion into the external feed phase that contains the contaminant. Afterward, a settling process by gravity takes place in order to separate the emulsion from external solution. Finally, the membrane phase is recovered by employing a demulsification process (Jusoh et al. 2016). The chief advantages of the emulsion liquid membrane technique are: (1) contaminant (solute) high rate of diffusion through the membrane, (2) owing to the small sized droplets, availability of high interfacial area for solute mass transfer at the internal water-oil interface, (3) ability to treat an assortment of compounds and elements in industrial setting at higher speeds along with a extraordinary level of effectiveness, while utilizing various solutes (contaminants) volume concentration, and (4) simultaneous performance of both extraction and stripping at external interface and the internal interface respectively in the same system (Sabry et al. 2007).
Achieving a maximum solute extraction efficiency is always the coveted target for a liquid membrane system. However, the main obstacle in the EML method is maintaining the emulsion stability, meaning the emulsion instability that causes a breakdown and releases the internal phase of the emulsion droplet. Membrane breakage (instability) has heavily affected the efficiency of the ELM system (Buddin et al 2019). There are three main phenomena that could lead to emulsion instability: swelling, coalescence, and membrane breakage or rupture (Chakraborty et al. 2010). It is usually ruled by rupturing of the emulsion followed by leaking of the internal phase, causing a significant decrease in the stripping phase volume (Ho et al., 1992). This usually propels the driving force for mass transfer; while reducing the concentration gradient which increases the external aqueous feed concentration meanwhile a lowering of the extraction efficiency occurs (Mohammed et al. 2020a). This instability could be caused by the formulation of emulsion and the emulsification condition (Djenouhat et al. 2008).
In this paper, chlorpyrifos extraction from contaminated water employing emulsion liquid membrane (ELM) was studied. The influence of emulsification speed, agitation time, surfactant concentration, emulsification time, and internal phase concentration on the stability of the prepared W/O emulsion was investigated to determine the best conditions for attaining minimal membrane breakage in the emulsion system while reaching maximum extraction efficiency.

Chemicals and equipment
Analytical reagent-grade chemicals, along with distilled water were employed in this work. Both hydrochloric acid (HCl) and sodium hydroxide are acquired from Thomas beaker, India. The liquid membrane phase consists of a surfactant and a diluent, the nonionic surfactant engaged was sorbitan monooleate, commonly recognized as Span 80, which was obtained from Merck; Darmstadt; Germany (Sigma Aldrich); while n-hexane (diluent) was obtained from Thomas beaker (India). All laboratory tests were carried out at room temperature of (20 ± 2) °C. The equipment used in this research are homogenizer (Mtops, SR 30), compact digital mixer system (Heidolph, RZR 2021), quartz cells, UV Spectrophotometer (ThermoSpectronic, USA), centrifuge (Isolab), pH meter (ATC) and a magnetic stirrer with temperature controller (Isolab).

Experimental work
The experimental structure was confined into two main sections. The first one is the ELM formulation, and determining the membrane stability via diverse operating parameters. The second section concerns investigating the ELM performance on the extraction of chlorpyrifos (CP) from simulated solution (feed phase). For the first part, a water-in-oil emulsion was formed by the addition of internal phase (HCl) dropwise into the membrane phase (Span 80 and n-hexane) while using a homogenizer (high speed mixer) for a specified time. The membrane phase was formed via dissolving a specific amount of surfactant (Span 80) in n-hexane by gently stirring via magnetic stirrer. While the internal aqueous solution was formed by taking the required amount of acid solution (HCl) in the allocated amount of distilled water. The emulsion is poured to external aqueous solution while mixing continuously, causing globule formation. Each globule is made of droplets of stripping solution encased in the membrane solution that contains the surfactant. A flow diagram of ELM process is outlined in Figure 2. Samples were taken from the mixture at certain time intervals using syringes and pH values were recorded. By the completion of each experiment, the resulting double emulsion is allowed to be naturally separated from feed solution due to gravitational force, then a demulsification process was achieved by applying centrifugal force on the emulsion to segregate the phases making up the emulsion resulting in the capability of reusing membrane solution while the contaminant  would be extracted as a concentrated solution.
Samples are filtered using syringe filters (pore diameter 0.22 µm). CP concentration in the separated external phase and in filtered samples is measured using an ultraviolet spectrophotometer (UV) corresponding to a 290 nm wavelength to evaluate the stripping and extraction capacity of Chlorpyrifos. The operating parameters with their corresponding ranges are listed in Table 1. All experiments are conducted in batch configuration, which is easier to operate and study.

ANALYSIS AND CALCULATIONS Extraction process of CP
The CP concentration in the separated external phase is measured which by ultraviolet spectrophotometer (UV). Afterwards the extraction percentage of CP was calculated from the following equation: • 100 where: C 0 -represents the initial CP concentration in the external feed phase; C -represents the CP concentration after a specific amount of time in the external feed solution.

Stripping
The resulting double emulsion was allowed to be naturally separated from feed solution due to gravity force, then a demulsification process was achieved by applying centrifugal force on the emulsion. The CP concentration extracted from internal phase was measured, and the stripping efficiency was then calculated according to the following equation: • 100 where: C f,int -represents the final CP concentration in the internal phase; C f,ext -represents the final CP concentration external phase.

Membrane leakage/breakage
When dispersing emulsion into the aqueous feed phase, the emulsion has to be stable enough to extract the CP molecules into the emulsion droplets. Usually stabilization is achieved by the surfactant added thus the degree of extraction efficiency for any solute is mainly affected by the emulsion stability. Breakdown of emulsion usually occurs after a period of time when the emulsion is considered unstable causing a gradual lowering of removal efficiency along with a significant loss of extracted species (Laguel et al. 2019). The liquid membrane stability is considered the most essential factor affecting the solute removal efficiency. Using a tracer (H + ion) with a specific concentration within the internal aqueous phase solution. Breakage is the result of tracer transfer from internal-to-external phase solutions. Any alteration in pH value (i.e., H + ions leaking) of the feed phase results from the emulsion breakage (rupture) because of the excretion of the HCl from internal towards external aqueous phase. The emulsion rupture or breakage (ɛ) signifies the percentage ratio of the internal aqueous phase volume escaped (leaked) to the external feed aqueous phase using the following general equation: where: Vs -represents the leaked volume of the stripping phase (internal phase); V i° -represents the initial volume of the internal stripping phase. Vs was computed using the following equation (Mohammed et al. 2018).
• 100 where: V b° -represents the external feed phase initial volume; pH -represents external feed phase pH after being in contact with the emulsion; pHo -represents the initial external phase pH; C H+ int -represents the initial acid concentration [H + ] of the internal phase solution.

Effect of homogenizer speed
Emulsification speed can be considered a significant parameter that influences the emulsion stability, thereby affecting the entire extraction process (Mohammed et al. 2020b). The stability of emulsion is a vital factor in ELM process; low values lead to break up of the emulsion while extremely high stability of emulsion causes the emulsion to be impossible to breakdown via mechanical method. Homogenizer speed was investigated using values stated above in Table 1. The results displayed in Figure 3 show that when the speed is increased from 3000 rpm to 12700 rpm, the emulsion stability increases as the breakage drops, which gives a minimum breakage value of 2.2% while the extraction and stripping efficiencies increase from 73.1% and 62.7% to 81.8% and 76.4% respectively. This occurs as a result of instigating smaller sized droplets when employing higher speeds where the sauter mean diameter lessened from 17.2 µm at 3000 rpm to 6.45 µm at 12700 rpm, which leads to increasing the interfacial surface area for droplets thus increases the mass transfer rate. At higher homogenizer speed (19700 rpm), a decrease in the emulsion stability occurs (breakage: 6.58%) also the efficiency of extraction decreases to 75.37%. Speed lower than 12700 rpm leads to a decrease in stability , i.e. for 3000 rpm homogenizer speed, an increase in breakage percentage to 7.05% occurs, this is because of the bigger droplet diameter, and the coalescence phenomenon that transpire in a short amount of time ( Mohammed 2007, Salman and Mohammed 2019). Consequently, a speed of 12700 rpm is nominated as the optimum homogenizer speed. Table 2 below shows the extraction percentage, emulsion breakage, and the sauter mean diameter at varying homogenizer speeds.

Effect of emulsification time
To produce a stable emulsion, it is crucial to provide appropriate time to surround the internal phase into the organic oil phase. The emulsion stability was examined within a range of 5-20 min. emulsification times as stated in Table 1. Figure 4 shows that the highest efficiency of extraction and stripping; 87.9% and 78.9% respectively; occurred at 5 min. emulsification time, in accordance it offers the highest stability along with a lowest breakage value (2.07%) and a Sauter mean diameter of 4.85 µm. When increasing emulsification time (10, 15, and 20 min.), the breakage percentage increases whereas extraction efficiency decreased. At 20 min. emulsification time, the breakage increases up to 10.27%, whereas a decrease in extraction efficiency was registered at 74.6%. This happens due to the occurrence of high internal shear, therefore leading to a massive quantity of emulsion globules by unit volume, in return, it facilitates its diffusion into the feed aqueous phase (Laki et al. 2016). Hence, the optimum emulsification time of 5 min. was considered for this study.

Effect of surfactant concentration
Surfactant plays a major part in establishing the emulsion stabiliy since it diminishes the tension between the immiscible phases. A span 80 surfactant was added as an emulsifier that serves as a shielding barrier between the external and internal phases, therefore forfending an emulsion breakage. Numerous surfactant concentrations (1, 2, 3, and 4 % (v/v)) were used to investigate the influence of surfactant concentration on the stability of emulsion. Figure 5 shows extraction, stripping percentage, and membrane breakage with different surfactant concentrations. It was analyzed that the stability rises slightly when increasing Span 80 concentration until 3% (breakage = 0.82%). With increasing the concentration of surfactant, further surfactant is being absorbed within the interphase between the internal aqueous phase and the oil membrane phase, hence improving the adsorption layer strength therefore increasing stability. In addition, the surfactant concentration increase heightens emulsion stability as a result of the interfacial tension reduction

Eff ect of internal (stripping) phase concentration
In order to improve the stability of emulsion and lower the amount of the internal solution released into the external phase, and since extraction process at the external-membrane interface is necessary; a re-extraction process is essential at the internal-membrane interface in order to remove the CP entirely. A stripping agent (HCl) was employed as the internal aqueous phase to re-extract (strip) CP. Stripping agent concentration ranged from 0.05 to 0.45 M. When raising HCl concentration accordingly, breakage percentages diminished while extraction effi ciencies improved from 4.3% and 72.5% to 0.82% and 93.3%, respectively as shown in Fig. 6. This could be attributed to the chief driving force in the emulsion liquid membrane system which is the amount of [H + ] ions variation between two aqueous phases (Sabry et al. 2007). Furthermore, when the internal phase acidity is being raised above 0.25 M, it causes a partial increase in breakage, leading to additional release of the enclosed internal constituent outwards into the external aquous phase ensuring an extraction effi ciency decline. It could be attributed to the reaction between the surfactant and HCl that occurs due to the relatively high acidity achieving a signifi cant drop in the surfactant properties which lead to a de-stabilization of emulsion (Mohammed 2007). In conclusion, a 0.25M of HCl which achieves higher extraction effi ciency along with lower breakage percentage and was selected for this work.

Eff ect of mixing time
Mixing time is a necessary factor in the ELM process. It can be specifi ed as the time required to achieve maximum extraction of CP. Mixing time, also known as contact time, is defi ned as the period of time in which the external feed phase stays in direct contact with the emulsion while continuously stirring (Benderrag et al. 2022, Ahmad et al. 2021). Figure 7 shows the ; therefore, the extracted molecules revert backward to the feed phase, hence decreasing extraction and stripping efficiencies. Therefore, a 10 min. mixing time was chosen as the optimal period to extract CP.

Evaluation of the solute (CP) extraction kinetics and estimation of mass transfer coefficient
Extraction kinetics of CP using the ELM method were investigated in accordance with the approach performed by (Kohli et al. 2019, Raji et al. 2018), using the first order rate Eq.: • 100 where: t -represents time in minutes; K obs -is the rate constant of extraction (min −1 ), which can be evaluated from the slope of the resulting line from the previous equation representing the K obs value. Because the value obtained was positive, the extraction process follows the firstorder kinetics. K obs value obtained was 0.2173 min −1 .
Total mass transfer coefficient for the ELM system is shown in the following equation (Kasaini et al. 1998): • 100 .89 × 10 −9 m/s K T = 3.89 × 10 −9 m/s (6) where: K T -represents total mass transfer coefficient (m/s); K F -represents interfacial reaction rate constant (m/s); K M -represents mass transfer coefficient of the external phase (m/s), estimated by Skelland-Lee correlation (Kohli et al. 2019) correlation given in the following equation: • 100 ) where: N -represents mixing speed (rpm); T and d -represent the mixing tank and impeller diameter, respectively (m); Vi, Ve, and Vm -denote the internal, external, and membrane phase volumes, respectively; D -represents species diffusivity (CP) in the organic membrane phase, determined using Wilke-Chang correlation (Treybal 1981) shown in Eq. (9).
In conclusion, the calculated mass transfer coefficients are:

CONCLUSIONS
The present study investigates the stability of ELM while evaluating extraction and stripping efficiencies of the emulsion for chlorpyrifos pesticide removal from aqueous wastewater. A minimum emulsion breakage of 0.73% and the highest removal percentage of 93.8% and 94.7% stripping efficiency within 10 min contact time were achieved at the optimum experimental conditions, which are: emulsification speed, 12700 rpm; emulsification time, 5 min; surfactant concentration, 3% (v/v); and internal (stripping) phase concentration, 0.25M; with no requirement of an additional carrier. With mass transfer coefficients (K M , K F , and K T ) values of 2.6×10 -4 m/s, 3.89×10 -9 m/s, and 3.89×10 -9 m/s respectively.
Generally, it can be concluded that the emulsion liquid membrane represents a productive, efficient, and suitable advanced separation method for the treatment of wastewater contaminated with pesticides. It could be a promising alternative technique to minimize environmental pollution caused by pesticides to large extent.