Guidelines for Canadian drinking water quality – Malathion: Analytical and treatment considerations

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• Analytical methods to detect malathion
• Treatment considerations

Analytical methods to detect malathion

Standardized methods available for the analysis of malathion in source and drinking water and their respective MDLs are summarized in Table 4. MDLs are dependent on the sample matrix, instrumentation and selected operating conditions and will vary between individual laboratories. These methods are subject to a variety of interferences, which are outlined in the respective references.

A number of accredited laboratories in Canada were contacted to determine MDLs and MRLs for malathion analysis and the MDLs were in the same order of magnitude as those reported in Table 4. The MRLs ranged between 0.02 to 5 μg/L for Gas Chromatography with Mass Spectrometry Detection (GC/MS) (AGAT Laboratories Ltd., 2019; ALS Environmental, 2019; CARO Analytical Services - Richmond Laboratory, 2019; Element Materials Technology Canada Inc., 2019; and SGS Environmental Services, 2019).

The MDLs or MRLs from provincial and territorial data range from 0.0001 to 15 μg/L (refer to the section on exposure).

Drinking water utilities should discuss sampling requirements with the accredited laboratory conducting the analysis to ensure that quality control procedures are met and that MRLs are low enough to ensure accurate monitoring at concentrations below the MAC. Sample processing considerations for the analysis of malathion in drinking water (for example, sample preservation, storage) can be found in the references listed in Table 4. Additionally, a non-standardized method to analyse malathion in water based on high performance liquid chromatography with tandem mass spectrometry is presented in Rocha et al. (2015).

It is important to note that quenching is critical if an oxidant is present in samples in order to reduce additional degradation of malathion. Malathion has limited stability due to hydrolysis, with decreased half-life at increased pH and temperature (Wolfe et al., 1977; EFSA, 2006). As such, cooling of the samples and rapid analysis are recommended.

Treatment considerations

Treatment technologies available to effectively decrease malathion concentrations in drinking water include activated carbon, membrane processes, oxidation and advanced oxidation processes. Published data on malathion removal in water using these technologies indicates a large range of removal efficiencies (less than 50% up to approximately 100%) (Chian et al., 1975; Roche and Prados, 1995; Kiso et al., 2000; Duirk et al., 2009; Zhang and Pagilla, 2010; Beduk et al., 2012; Chamberlain et al., 2012; Fadaei et al., 2012; Sorour and Shaalan, 2013; Jusoh et al., 2014; Li et al., 2016). At the residential scale, certified treatment devices relying on reverse osmosis (RO) or activated carbon adsorption are expected to be effective for removal of malathion.

Municipal scale

The selection of an appropriate treatment process for a specific water supply will depend on many factors, including the raw water source and its characteristics, the operational conditions of the selected treatment method and the utility's treatment goals. Bench or pilot testing is recommended to ensure the source water can be successfully treated and optimal process design is established.

When using oxidation or advanced oxidation processes (AOP) for pesticide removal in drinking water, it is important to be aware of the potential for formation of by-products due to degradation of the target compound (Ikehata and Gamal El-Din, 2006; Beduk et al., 2012; Li et al., 2019). Malathion has several degradation by-products that may form through oxidation (refer to the section on oxidation and hydrolysis) or advanced oxidation processes (refer to the section on advanced oxidation processes), including malaoxon, which is of health concern. The primary objective should be removal of the pesticide with the secondary objective being the minimization of by-product formation. In addition, water utilities should consider the potential for the formation of disinfection by-products depending on the oxidant selected and the source water quality.

Conventional treatment

Conventional filtration (chemical coagulation, clarification and rapid sand filtration) and chlorine disinfection may reduce malathion concentrations through oxidation during the disinfection step depending on the oxidant (Roche and Prados, 1995; Duirk et al., 2009; Beduk et al., 2012; Chamberlain et al., 2012). However, degradation processes like oxidation result in the formation of by-products, such as malaoxon (refer to the section on oxidation and hydrolysis).

A bench-scale study evaluated chemical coagulation and sedimentation treatment technologies for the removal of both malathion and malaoxon (Matsushita et al., 2018). The study used river water and the results showed no removal (see Table 5).

A bench-scale study was conducted to evaluate the cumulative removal of malathion through coagulation, flocculation and filtration followed by chlorination (see Table 6) (Costa et al., 2018). The first part of this study differed from the previous study with the addition of a filtration step and showed 62.21% removal of malathion. The removal increased further after chlorination and the authors noted the formation of malaoxon.

Activated carbon adsorption

Activated carbon adsorption is a widely used technology to reduce the concentration of micropollutants, including a wide range of pesticides, in drinking water (Haist-Gulde and Happel, 2012; van der Aa et al., 2012). Activated carbon can be applied in 2 ways: slurry applications using powdered activated carbon (PAC) or fixed-bed reactors with granular activated carbon (GAC) (Chowdhury et al., 2013).

Data generated through bench-scale testing to determine adsorption coefficients for pesticides are useful in predicting whether activated carbon adsorbs a particular pesticide (U.S. EPA, 2011). In general, pesticides with an adsorption capacity constant (for example, Freundlich coefficient) greater than 200 µg/g(L/µg)^1/n are considered to be amenable to removal by carbon adsorption (Speth and Adams, 1993; Speth and Miltner, 1998; U.S. EPA, 2011). However, it is important to note that the presence of natural organic matter (NOM) adds complexity to activated carbon treatment because NOM competes directly for adsorption sites or fouls the carbon by blocking pores (Chowdhury et al., 2013). Since the capacity of activated carbon can be affected by many factors, including the compound's ionic character and the solution pH, appropriate testing (for example, jar tests, rapid small-scale column tests) should be conducted to confirm removal.

Powdered activated carbon

Many pesticides have been found to strongly adsorb to PAC (Chowdhury et al., 2013). The use of PAC offers the advantage of providing virgin carbon when required (for example, during the pesticide application season) (Miltner et al., 1989). The removal efficiency of PAC depends on the PAC characteristics (type and particle size), dose, contact time, contaminant adsorbability and NOM presence (Gustafson et al., 2003; Summers et al., 2010; Haist-Gulde and Happel, 2012; Chowdhury et al., 2013).

A bench-scale study was conducted to determine the adsorption of malathion to PAC, as well as that of malaoxon (see Table 7) (Matsushita et al., 2018). With a PAC dose of 10 mg/L, similar removal efficiencies of 69% and 76% were observed for malathion and malaoxon, respectively.

Granular activated carbon

The use of GAC is an effective approach for treating organic contaminants that are regularly found in source water at concentrations of concern (Chowdhury et al., 2013). The capacity of GAC to remove pesticides by adsorption depends on the filter velocity, empty bed contact time (EBCT), the GAC characteristics (type, particle size, reactivation method), the adsorbability of the contaminant and the filter run time (Haist-Gulde and Happel, 2012). In addition, because GAC fixed-bed adsorbers are typically operated on a continuous basis, the GAC can become fouled (or preloaded) with NOM and it may be completely or partially ineffective for pesticide removal (Knappe et al., 1999; Summers et al., 2010; Haist-Gulde and Happel, 2012; Chowdhury et al., 2013).

Column experiments were conducted on 2 different GACs [palm shell activated carbon (PSAC) and coconut shell activated carbon (CSAC)] (Jusoh et al., 2014). The authors found that the malathion removal efficiency for CSAC was greater than that for PSAC (see Table 8). The authors also concluded that the adsorption capacity increased as flow rate decreased. In other words, removal efficiency increased with longer EBCT.

Membrane filtration

In general, nanofiltration (NF) and RO are effective pressure-driven membrane processes for the removal of pesticides from drinking water (Van der Bruggen and Vandecasteele, 2003; U.S. EPA, 2011). The effectiveness of NF and RO for pesticide removal is dependent on the membrane characteristics, pesticide properties, feed water composition, operating conditions and membrane fouling (Van der Bruggen and Vandecasteele, 2003; Plakas and Karabelas, 2012).

Since the main mechanism for pesticide removal using NF and RO membranes is size exclusion, the molecular weight cut-off (MWCO) of the membrane is an important characteristic. Based on the molecular weight of malathion (217 Da), membranes with a MWCO varying between 200 and 400 Da are considered appropriate for malathion. In addition to the sieving effect, retention of small pesticide molecules by larger pore size membranes can be influenced by the physicochemical interactions between the pesticide and the membrane surface (Plakas and Karabelas, 2012).

Bellona et al. (2004) presented a flow-chart using the characteristics of the pesticide in water (for example, molecular weight, log K_ow, molecular diameter) and those of the membrane (for example, MWCO, pore size) which could be used to determine the potential for removal of malathion by membrane filtration. It is important to perform appropriate testing prior to full-scale implementation with membrane and source water under the proposed operating conditions to ensure that adequate malathion removal is occurring.

Malathion removal was investigated through several bench-scale wastewater studies (see Table 9). Chian et al. (1975) used 2 different membranes and both achieved greater than 99% malathion rejection. A second bench-scale study by Kiso et al. (2000) investigated malathion removal using 4 membranes. The malathion removal using the 2 poly(vinyl alcohol)/ polyamide membranes was high (greater than 88%), whereas the removal was much lower for the membranes composed from sulfonated polyethersulfone (less than 42%). Another study had similarly high malathion removal at a trans-membrane pressure of 1 120 kPa and showed improved rejection with increased trans-membrane pressure (Zhang and Pagilla, 2010). A bench-scale study by Sorour and Shaalan (2013) showed increased rejection with increased initial malathion concentration.

Oxidation and hydrolysis

Chemical oxidation and hydrolysis are the most important degradation pathways for the organophosphorus pesticides under drinking water treatment conditions (Durik et al., 2006; Newhart, 2006). Degradation of malathion in water is pH dependent and it degrades quickly in water with pH > 7.0. The half-life range of malathion is 0.2 weeks in water at pH 8.0 compared to 21 weeks at pH 6.0 (Newhart, 2006). The studies examining degradation of malathion using various oxidants are presented in Table 10.

Common oxidation/disinfection processes showed a wide range of reactivity for malathion (Roche and Prados, 1995; Durik et al., 2009, 2010; Chamberlain et al., 2012). Bench-scale testing conducted with typical drinking water disinfection doses of chlorine (Cl₂) and ozone (O₃) have reported moderate to high removal of a low concentration of malathion (Chamberlain et al., 2012) (Table 10). The authors reported a greater than 50% removal of malathion using chlorination conducted at both pH levels of 6.6 and 8.6 and with an ozonation process at pH of 6.6. It was found that ozonation at pH 8.6 achieved a moderate removal ranging from 20% to 50%. Oxidants such as monochloramine (NH₂Cl), chlorine dioxide (ClO₂), permanganate (MnO₄), hydrogen peroxide (H₂O₂) and direct ultraviolet (UV) photolysis at 254 nm achieved less than 20% removal of malathion. Hydrolysis tests conducted at pHs 2, 7 and 12 also reported similar results (Chamberlain et al., 2012). The application of direct UV photolysis was also reported as being ineffective for the degradation of malathion by Beduk, et al. (2012) and Li et al. (2019). Direct photolysis of organophosphorus pesticides using low- and medium-pressure UV lamps was reported to be very slow with a low quantum yield, which was defined as a number of moles of the reactant being degraded per mole of photons absorbed (Wu and Linden, 2008).

The degradation efficiency of malathion is influenced by several parameters, including water matrix, ozone dose and contact time (Roche and Prados, 1995; Beduk et al., 2012). In a bench-scale ozonation test, Roche and Prados (1995) studied the effect of water alkalinity on the oxidation efficiency of eleven pesticides, including malathion. Due to the inhibiting role of carbonate species, the removal of malathion was higher in water with a low alkalinity (specific data were not provided). Ozonation tests conducted by Beduk et al. (2012) reported an increase of malathion degradation rate with an increased ozone dose and pH level of the water. A direct ozone reaction (ozonolysis) was responsible for the degradation of malathion at a low pH, while a high pH of 9.0 involved a non-selective hydroxyl radical (*OH) formation.

Duirk et al. (2009) examined the degradation of malathion in deionized water using hypochlorous acid (HOCl). The oxidation rate of malathion was rapid under the tested conditions and the oxidation efficiency strongly depends on the pH of the water. HOCl is a weak acid that dissociates to produce hypochlorite ion (OCl^-), with a dissociation constant (pK_a) of approximately 7.6 at 20°C. Chlorine species in the water shift from HOCl to hypochlorite ion (OCl^-) when the pH increased from neutral to alkaline. The study reported that OCl^- ion did not oxidize malathion to malaoxon (degradation by-product, discussed below in this section). However, it accelerated the hydrolysis of malathion. Similar experiments investigated oxidation of malathion by chloramines using deionized water and pH range from 3.0 to 9.0 (Duirk et al., 2010). The initial malathion concentration was 0.5 µM and the initial monochloramine dose was 50 µM. Auto-decomposition of monochloramine is a pH-dependent process and allows for multiple chlorinated oxidants to coexist at neutral pH [i.e., monochloramine (NH₂Cl), dichloramine (NHCl₂) and HOCl] (Valentine and Jafvert, 1992). The reaction rate of monochloramine to degrade malathion was low. Dichloramine exhibited a reaction rate 2 orders of magnitude higher than monochloramine, but 3 orders of magnitude lower than hypochlorous acid. Of the 3 chloramines, monochloramine is the preferred species for use in disinfecting drinking water because of its biocidal properties, relative stability, and because it rarely causes taste and odour problems when compared with dichloramine and trichloramine (Health Canada, 2020c). The authors reported that a 56% degradation of malathion was due mostly to the oxidation by dichloramine, when oxidation was conducted at a pH of 6.5. Above pH 8.0, alkaline hydrolysis was the primary degradation pathway for malathion, achieving 93% degradation (Duirk et al., 2010).

Organophosphorus pesticides contain a phosphorous/sulphur bond (P=S) that is highly reactive and easily degraded by oxidation, producing oxons having phosphorous/oxygen (P=O) bonds as a primary degradation by-product (Magara et al., 1994; Kamel et al., 2009; Beduk et al., 2012). Malaoxon is more persistent than malathion and has a degradation kinetic lower than its parent molecule (Magara et al., 1992; Durik et al., 2010; Beduk et al., 2012; Li et al., 2019). Li et al. (2019) reported that ratios of degradation rate constant of malathion to degradation rate constant of malaoxon (k_malathion/k_malaoxon) ranged from 4.3 to 5.6 for several oxidation and AOPs reactions. The degradation rate constants of 6.5 x 10⁴ and 1.4 x 10⁴ cm² mJ^-1 were measured for malathion and malaoxon, respectively, in UV oxidation process; while for UV/H₂O₂ oxidation (discussed in the section on advanced oxidation processes) the degradation rates were 133.6 x 10⁴ and 23.9 x 10⁴ cm² mJ^-1 for malathion and malaoxon, respectively. Additionally, a study conducted by Aizawa and Magara (1992) (as cited in Magara et al., 1994) reported that 2 other degradation by-products, ethyl chloromaleic acid and ethyl maleate, formed during chlorination of malathion. Newhart (2006) also reported on several degradation by-products resulting from hydrolysis of malathion in alkaline aerobic conditions such as malathion alpha and beta monoacids, diethyl fumarate, diethyl thiomalate, O,O-dimethylphosphorodithioic acid, diethylthiomalate and O,O-dimethylphosphorothionic acid. No treatment information was provided in the study.

Beduk et al. (2012) investigated malathion degradation by ozonation and the formation of malaoxon. While the malathion concentration of 200 µg/L was completely removed, malaoxon at a concentration of 12 µg/L was formed at an ozone dose of 1.5 mg/L and pH 9.5. Increasing the ozone dose to 2 and 2.5 mg/L caused the malaoxon formation to drop to 8 and 7 µg/L, respectively. The authors concluded that even high ozone doses were not efficient for complete removal of malaoxon. Duirk et al. (2010) reported that malaoxon was highly stable in the presence of chloramine at a pH of 8.5.

Advanced oxidation processes

AOPs use chemical reactions to form hydroxyl radicals that are used to oxidize chemical compounds, such as pesticides (Crittenden et al., 2012). Several different advanced oxidation processes have been investigated for malathion degradation, including UV/hydrogen peroxide (H₂O₂), O₃/UV and O₃/H₂O₂/UV (see Table 11).

In laboratory tests, the presence of carbonate and sulphate ions was found to negatively impact the degradation of malathion when UV/H₂O₂ was used, with carbonate having the most impact (Fadaei, et al., 2012). The authors reported that malathion degradation was highest in distilled water, followed by tap water and then river water. This observed difference in malathion degradation was due to hydroxyl scavenger property of bicarbonate and sulphate ions and the presence of organic carbon in natural waters. An increase of pH and hydrogen peroxide concentration increased the degradation rate for malathion.

Beduk et al. (2012) investigated the degradation of malathion and subsequent formation of malaoxon in aqueous solution using photocatalytic ozonation (O₃/UV and O₃//UV/ H₂O₂). Efficient removal of both malathion and the formed malaoxon was found for O₃/H₂O₂/UV after 10- and 30-minutes' reaction time, respectively.

A bench-scale study by Roche and Prados (1995) achieved a greater than 99% degradation of malathion for all applied doses of O₃ with H₂O₂ added at a constant ratio of 0.4 g H₂O₂/g O₃. The results in Table 11 indicate that approximately 100% degradation of malathion was achieved with 1.0 mg O₃/L and an addition of 0.4 mg H₂O₂/L, as compared to the process with ozone alone requiring 4.0 mg O₃/L. A similar study by Li et al. (2019), showed a much higher reaction rate (2 orders of magnitude) for UV/H₂O₂ oxidation as compared to direct UV photolysis. The degradation reaction by direct UV photolysis involved a photon adsorption, while the UV/H₂O₂ reaction involved formation of hydroxyl radical. Li et al. (2019) also evaluated the formation of by-products. The study reported that each AOP was found to form their own respective grouping of degradation by-products. Based on total organic carbon (TOC) analysis, low mineralization was achieved for malathion under the studied processes. Malathion was converted to degradation by-products rather than being mineralized to CO₂ and water.

Prior to full-scale application, appropriate pilot-scale or bench-scale testing would need to be conducted evaluating malathion removal as well as the degradation products.

Combined technologies

As discussed in the oxidation and hydrolysis section, formation of by-products such as malaoxon may occur through processes like chlorination. A bench-study by Li et al. (2016) investigated both the removal of malathion and the resulting formation of malaoxon. The authors illustrated that the removal efficiency by coagulation and a combination of coagulation and PAC was better for malathion (5% and 38%, respectively) than malaoxon (2% and 24%, respectively). The authors then examined the impacts of various pre-chlorination doses on overall malathion removal throughout the treatment process by investigating the gross removal of both malathion and malaoxon after the various stages. A treatment train consisting of pre-chlorination, PAC-assisted coagulation-sedimentation-filtration and post chlorination was used with varying doses of pre-chlorination (0 to 3 mg/L) (see Table 12). The best total gross removal of both malathion and malaoxon was for the scenario in which no pre-chlorination occurred. Without pre-chlorination, malathion was not oxidized to the less well-removed malaoxon, resulting in overall better gross removal. As the pre-chlorination dose increased, malaoxon formed, causing the overall removal to decline.

Residential scale

In cases where malathion removal is desired at the household level, for example, when a household obtains its drinking water from a private well, a residential drinking water treatment unit may be an option for decreasing malathion concentrations in drinking water. Before a treatment unit is installed, the water should be tested to determine the general water chemistry and malathion concentration in the source water.

To verify that a treatment unit is effective, water entering and leaving the treatment unit should be sampled periodically and submitted to an accredited laboratory for analysis. Units can lose removal capacity through use and time and need to be maintained and/or replaced. Consumers should verify the expected longevity of the components in the treatment unit according to the manufacturer's recommendations and service it when required. Systems classified as residential scale may have a rated capacity to treat volumes greater than that needed for a single residence, and thus, may also be used in small systems.

Health Canada does not recommend specific brands of drinking water treatment units, but it strongly recommends that consumers use units that have been certified by an accredited certification body as meeting the appropriate NSF International Standard/American National Standard (NSF/ANSI) for drinking water treatment units. The purpose of these standards is to establish minimum requirements for the materials, design and construction of drinking water treatment units that can be tested by a third party. This ensures that materials in the unit do not leach contaminants into the drinking water (that is, material safety). In addition, the standards include performance requirements that specify the removal that must be achieved for specific contaminants (for example, reduction claim) that may be present in water supplies. Certification organizations (that is, third party) provide assurance that a product conforms to applicable standards and must be accredited by the Standards Council of Canada. Accredited organizations in Canada include:

• Groupe CSA
• NSF International
• Water Quality Association
• UL LLC
• Bureau de normalisation du Québec (available in French only)
• International Association of Plumbing and Mechanical Officials and
• Truesdail Laboratories, Inc

An up-to-date list of accredited certification organizations can be obtained from the Standards Council of Canada.

The drinking water treatment technologies that are expected to be effective for malathion removal at the residential-scale include adsorption and RO. Currently, malathion is not included in the performance requirements of NSF/ANSI standards. However, consumers can use a treatment unit that is certified to the standards for RO or adsorption to ensure that the material safety has been tested. These standards are NSF/ANSI Standard 53 Drinking Water Treatment Units - Health Effects and NSF/ANSI Standard 58 Reverse Osmosis Drinking Water Treatment Systems (NSF/ANSI, 2020 a, 2020b). In addition, systems or units that have been certified for the removal of pesticides (for example, atrazine) are more likely to be effective for the removal of malathion.

Water that has been treated using RO may be corrosive to internal plumbing components. Therefore, these units should be installed only at the point of use. As large quantities of influent water are needed to obtain the required volume of treated water, these units are generally not practical for point-of-entry installation.