Sustainable Water Monitoring via Analytical Techniques and Protocols Applied in the Assessment of Organochlorine Pesticides

: Water contamination with organochlorine pesticides (OCPs) is strongly linked to agricultural practices, and it still represents an environmental issue, despite the OCPs bans in many countries and despite the reported sustainable remediation technologies for their removal. Considering the environmental persistence of OCPs, the imbalances produced in the ecosystem, and the bioaccumulation tendency in living organisms through the food chain, the monitoring of OCPs and of their metabolites has crucial importance. The accuracy of the results obtained is strongly connected to the selection of reliable and accurate analytical procedures, especially considering the multitude of challenges related to OCP quantification. The purpose of this paper is to present an overview of the analytical techniques and protocols reported for OCP assessment in water, and to offer scientists a presentation of the current state of the literature on this subject. Nevertheless, it must be considered that each method has advantages and disadvantages, and, in most cases, the protocols reported in the literature must be adapted and improved. In addition, the levels of OCPs identified in surface water, groundwater, and rainwater have been reviewed. This review paper is directly connected to sustainability practices, since environmental sustainability is related to the responsibility to conserve natural resources and to prevent pollution, and for scientists, these objectives are fulfilled by conducting chemical analyses to track and quantify pollutants, as part of environmental studies.


Introduction
Organochlorine pesticides (OCPs) represent a class of persistent organic pollutants (POPs) that are highly toxic and resistant to degradation, which have a tendency to bioaccumulate in the environment.During World War II, these compounds were used to control disease-carrying insects.At the end of the war, OCPs were adapted for the control of pests in agriculture and public health, and were regarded as "miracle" pesticides due to their high efficacy and low cost [1].Nevertheless, the discovery of pesticidal residues in various sections of the environment has prompted serious concerns regarding their continued use, which outweigh the overall benefits derived from their use.Even though several have been banned in many countries since the 1970s due to their slow degradation rate, they are still being detected in environmental samples worldwide.More than half of all insecticides used globally come from Asia.India occupies the third place in pesticide usage in Asia [2] after China and Turkey.
In addition to environmental-related issues, concerns about their chronic effects have also emerged from both animal and human studies, which have associated many OCPs with different types of cancer [3][4][5].
In addition to environmental-related issues, concerns about their chronic effects have also emerged from both animal and human studies, which have associated many OCPs with different types of cancer [3][4][5].
Organochlorine pesticides can contaminate freshwater resources, including lakes and rivers, through several routes.These include agricultural runoff, the discharge of wastewater from industrial plants, leakage from storage sites, rainfall, and atmospheric deposition.Aquatic ecosystems frequently serve as the ultimate repository for these compounds, representing the terminal link in their accumulation chain [6].
Nowadays, human contact with pesticides is more frequent than that with any other pollutants, and considering their known high toxicity, monitoring and quantification in different samples is becoming increasingly important to avoid environmental pollution and health impacts.Considering the complexity of sample matrices, the protocols for the sampling, conditioning, extraction, enrichment, and quantification of target compounds are continuously being improved and developed.Moreover, the analyte in environmental samples is rarely found at trace levels; therefore, sample preparation is the most time-consuming and challenging step.Furthermore, lowering the detection limit when using conventional techniques is possible by adopting pre-concentration methods or certain sample preparation protocols.
Even if OCPs have been restricted for a very long time and the use of those that are allowed under certain conditions is strictly monitored, they are still being found in the environment and represent a current problem.
This review paper structures the scientific information related to analytical techniques and protocols used for the extraction and quantification of OCPs from different water sources.This information was gathered from a multitude of studies on the concentrations of OCPs and their metabolites in surface water, groundwater, and rainwater.

Legislative Aspects-Brief Presentation
POPs, and thus OCPs, are subject to a multitude of stringent regulations (Figure 3).This arises from the fact that they exert long-lasting impacts on both the environment and human health.The global regulation of POPs is governed by several international agreements, including the Stockholm Convention ("POPs Convention") and the Aarhus Protocol ("POPs Protocol").Within the European Union, these two initiatives have been translated into the European Regulation (EU) 2019/1021 ("POPs Regulation") [7].The objective of this regulatory framework is to minimize and to potentially eliminate the release of these substances, and to manage waste containing or contaminated by them.
In force since 2004, the Stockholm Convention on Persistent Organic Pollutants (POPs) calls for the reduction or elimination of the release of these chemicals, both at the national and global levels.Parties to the Convention commit to the non-production and non-use of the chemicals listed in its annexes; these chemicals are subject to regular updates to reflect the latest scientific developments.As of today, 185 countries have ratified the Convention and 34 POPs have been listed under it, with 17 pesticides, 15 industrial chemicals, and 7 unintentional by-products included [8].Furthermore, EU POP Regulation 2019/1021 is intended to protect the environment and human health and therefore bans or restricts the production and/or use of POPs in the European Union [7].
Despite the ban on their production in the European Union, some of the remaining stocks were still utilized following the adoption of the Stockholm Convention, contributing to a historical contamination of agricultural soils, groundwater, and surface waters by these substances.Persistence in the environment is a characteristic of these substances, as reflected in the acronym POPs.Moreover, it has been shown that OCPs are resistant to environmental degradation processes [11].3), DDTs (Table 4), Drins (Table 5), Chls (Table 6), Sulphs and other OCPs (Table 7).Groundwater: OCPs (Table 8).Rainwater: OCPs (Table 9)).

Legislative Aspects-Brief Presentation
POPs, and thus OCPs, are subject to a multitude of stringent regulations (Figure 3).This arises from the fact that they exert long-lasting impacts on both the environment and human health.The global regulation of POPs is governed by several international agreements, including the Stockholm Convention ("POPs Convention") and the Aarhus Protocol ("POPs Protocol").Within the European Union, these two initiatives have been translated into the European Regulation (EU) 2019/1021 ("POPs Regulation") [7].The objective of this regulatory framework is to minimize and to potentially eliminate the release of these substances, and to manage waste containing or contaminated by them.
In force since 2004, the Stockholm Convention on Persistent Organic Pollutants (POPs) calls for the reduction or elimination of the release of these chemicals, both at the national and global levels.Parties to the Convention commit to the non-production and non-use of the chemicals listed in its annexes; these chemicals are subject to regular updates to reflect the latest scientific developments.As of today, 185 countries have ratified the Convention and 34 POPs have been listed under it, with 17 pesticides, 15 industrial chemicals, and 7 unintentional by-products included [8].Furthermore, EU POP Regulation 2019/1021 is intended to protect the environment and human health and therefore bans or restricts the production and/or use of POPs in the European Union [7].
Despite the ban on their production in the European Union, some of the remaining stocks were still utilized following the adoption of the Stockholm Convention, contributing to a historical contamination of agricultural soils, groundwater, and surface waters by these substances.Persistence in the environment is a characteristic of these substances, as reflected in the acronym POPs.Moreover, it has been shown that OCPs are resistant to environmental degradation processes [11].
Concerning the OCP and metabolite levels in water, there are guideline values set by the WHO to protect human health and maximum acceptable levels/standards set by different countries.In the European Union, environmental quality standards for inland waters, as well as other surface waters, to protect the aquatic environment against pollution are regulated by Directive 2008/105/EC [12] and those for drinking water intended for human consumption are regulated by Directive (EU) 2020/2184 [13].

Main Characteristics of OCPs and Their Metabolites
Organochlorine pesticides (OCPs) consist of a diverse array of chemical compounds, including herbicides, insecticides, and fungicides.Each of these compounds possesses unique physicochemical properties that contribute to their efficacy in controlling various pests [14].OCPs are characterized by three key properties: low polarity, low aqueous solubility, and high lipophilicity [15].Compared with other OCPs, HCHs (especially lindane) are more soluble (Table 1), which represents a serious inconvenience for the environment, especially since it is still used in developing countries.Their persistence in the environment varies from moderate to high, as can be observed in Table 1.Data from Table 1 were collected from information provided by the following resources: [6,[16][17][18][19][20][21].Concerning the OCP and metabolite levels in water, there are guideline values set by the WHO to protect human health and maximum acceptable levels/standards set by different countries.In the European Union, environmental quality standards for inland waters, as well as other surface waters, to protect the aquatic environment against pollution are regulated by Directive 2008/105/EC [12] and those for drinking water intended for human consumption are regulated by Directive (EU) 2020/2184 [13].

Main Characteristics of OCPs and Their Metabolites
Organochlorine pesticides (OCPs) consist of a diverse array of chemical compounds, including herbicides, insecticides, and fungicides.Each of these compounds possesses unique physicochemical properties that contribute to their efficacy in controlling various pests [14].OCPs are characterized by three key properties: low polarity, low aqueous solubility, and high lipophilicity [15].Compared with other OCPs, HCHs (especially lindane) are more soluble (Table 1), which represents a serious inconvenience for the environment, especially since it is still used in developing countries.Their persistence in the environment varies from moderate to high, as can be observed in Table 1.Data from Table 1 were collected from information provided by the following resources: [6,[16][17][18][19][20][21].
The persistence of OCPs in the environment is evaluated based on their half-life degradation time (DT 50 ), which represents the time needed for initial concentration in a specific medium to be reduced to half (Figure 4) [22].HCHs and DDTs are characterized by high persistence in the environment, where they may persist for decades (Table 1).The presence and persistence of OCPs in the environment are related to their structures and physical properties, and also to conditions associated with the location where they are found.For example, aerobic biodegradation may occur faster than the anaerobic process.Also, OCP decomposition is strongly influenced by ultraviolet light, which is valued in photocatalytic degradation strategies [23].
Although OCPs are very stable, some molecular alterations are possible in certain circumstances, and the resultant metabolites are either as toxic and persistent as the parent compound, or, fortunately, less so.
Albeit the degradation of lindane, a gamma enantiomer of hexachlorocyclohexane, may occur either under aerobic or anaerobic conditions, it occurs frequently in aerobic conditions mediated by bacterial strains [24,25].A metabolite of lindane obtained under aerobic degradation is γ-pentachlorocyclohexene [24].Lindane's microbial degradation is intensively investigated because it could be employed in remediation strategies.
DDT's metabolites are DDD and DDE (Figure 5), but it has been found that DDT is metabolized mainly into DDE and that is a possible explanation for the higher DDE levels compared with DDT, reported in some cases [26].According to Wei and his team [27], DDT is converted to DDD under anaerobic conditions; meanwhile, aerobic conditions enable the transformation of DDT into DDE.This behavior could be utilized to gather information on the pollution source using DDD/DDE or (DDE + DDD)/ΣDDTs ratios.The presence and persistence of OCPs in the environment are related to their structures and physical properties, and also to conditions associated with the location where they are found.For example, aerobic biodegradation may occur faster than the anaerobic process.Also, OCP decomposition is strongly influenced by ultraviolet light, which is valued in photocatalytic degradation strategies [23].
Although OCPs are very stable, some molecular alterations are possible in certain circumstances, and the resultant metabolites are either as toxic and persistent as the parent compound, or, fortunately, less so.
Albeit the degradation of lindane, a gamma enantiomer of hexachlorocyclohexane, may occur either under aerobic or anaerobic conditions, it occurs frequently in aerobic conditions mediated by bacterial strains [24,25].A metabolite of lindane obtained under aerobic degradation is γ-pentachlorocyclohexene [24].Lindane's microbial degradation is intensively investigated because it could be employed in remediation strategies.
DDT's metabolites are DDD and DDE (Figure 5), but it has been found that DDT is metabolized mainly into DDE and that is a possible explanation for the higher DDE levels compared with DDT, reported in some cases [26].According to Wei and his team [27], DDT is converted to DDD under anaerobic conditions; meanwhile, aerobic conditions enable the transformation of DDT into DDE.This behavior could be utilized to gather information on the pollution source using DDD/DDE or (DDE + DDD)/ΣDDTs ratios.It has been shown that DDT degradation is influenced by pH, while hydrolysis is increased in alkaline waters [28].
In environmental studies, there are also frequently monitored levels of OCPs and metabolites.For instance, Sibali and co-workers [29] assessed the levels of DDT and its metabolites in surface water from the Jukskei River in South Africa, and the low levels of DDT metabolites indicated recent contamination with DDT.If high levels of DDD and DDE had been found, this would have clearly indicated past DDT application practices and the detected metabolites would have resulted from DDT.
In environmental studies, there are also frequently monitored levels of OCPs and metabolites.For instance, Sibali and co-workers [29] assessed the levels of DDT and its metabolites in surface water from the Jukskei River in South Africa, and the low levels of DDT metabolites indicated recent contamination with DDT.If high levels of DDD and DDE had been found, this would have clearly indicated past DDT application practices and the detected metabolites would have resulted from DDT.

Analytical Methods and Protocols Used for OCP Assessment
OCP analysis is very challenging because there are many aspects that may interfere with the analysis: extraction and cleanup issues, sample type, the level of pesticide in the sample, etc.In addition, the analysis is complicated when there are chemical changes in the pesticide caused by hydrolysis, under UV light or sunlight, or in the case of complex matrices.
Nevertheless, the analysis of OCPs from different samples follows several steps, as suggested in Figure 6 [31].
with the analysis: extraction and cleanup issues, sample type, the level of pesticide in the sample, etc.In addition, the analysis is complicated when there are chemical changes in the pesticide caused by hydrolysis, under UV light or sunlight, or in the case of complex matrices.
Nevertheless, the analysis of OCPs from different samples follows several steps, as suggested in Figure 6 [31].

Detection
As OCP analysis is both expensive and time-consuming, it is highly recommended to perform a preliminary evaluation with a cheaper test before the quantitative analysis.Nowadays, there are rapid pesticide tests for different OCPs from various matrices.Also, there are reported screening or color tests for common groups of pesticides, including OCPs [32].

Extraction
The most important and laborious step in OCP analysis is represented by extraction, because the association between the proper extraction technique and suitable solvents assures the efficient extraction of the OCPs and the correctness of the obtained results.In addition, the more complex the sample matrix, the more complicated and laborious the extraction protocols are.
The extraction procedure is performed because it enriches the analyte level and lowers the interferences, and it is selected based on the type of pesticide and the nature of the sample to be analyzed [33].Furthermore, the effectiveness of extraction is influenced by solvent polarity, dispersion coefficient, hydrogen bonding, and the solubility of OCPs in certain solvents [34].According to the literature [35], the extraction procedure should be characterized by recoveries of at least 80% (lower values are synonymous to erroneous results) to require a small amount of organic solvent and minimum cleanup before quantification.
The most frequently used methods reported for OCP extraction are presented in the following paragraphs.

•
Liquid-liquid extraction (LLE) is widely used for OCP extraction from aqueous matrices.It is considered as a simple method and it involves the dissolution of analytes in two different immiscible liquids (mainly water and organic solvents, such as ethyl acetate, dichloromethane, and petroleum ether) (Table 2).LLE is

Detection
As OCP analysis is both expensive and time-consuming, it is highly recommended to perform a preliminary evaluation with a cheaper test before the quantitative analysis.Nowadays, there are rapid pesticide tests for different OCPs from various matrices.Also, there are reported screening or color tests for common groups of pesticides, including OCPs [32].

Extraction
The most important and laborious step in OCP analysis is represented by extraction, because the association between the proper extraction technique and suitable solvents assures the efficient extraction of the OCPs and the correctness of the obtained results.In addition, the more complex the sample matrix, the more complicated and laborious the extraction protocols are.
The extraction procedure is performed because it enriches the analyte level and lowers the interferences, and it is selected based on the type of pesticide and the nature of the sample to be analyzed [33].Furthermore, the effectiveness of extraction is influenced by solvent polarity, dispersion coefficient, hydrogen bonding, and the solubility of OCPs in certain solvents [34].According to the literature [35], the extraction procedure should be characterized by recoveries of at least 80% (lower values are synonymous to erroneous results) to require a small amount of organic solvent and minimum cleanup before quantification.
The most frequently used methods reported for OCP extraction are presented in the following paragraphs.

•
Liquid-liquid extraction (LLE) is widely used for OCP extraction from aqueous matrices.It is considered as a simple method and it involves the dissolution of analytes in two different immiscible liquids (mainly water and organic solvents, such as ethyl acetate, dichloromethane, and petroleum ether) (Table 2).LLE is time-consuming and it also has other disadvantages; one that it is worth mentioning is the large volume of solvents that are associated with environmental issues and, worse, with carcinogenic effects on humans [35], and low enrichment of the analytes [33].
LLE with ethyl acetate on water samples collected from the Kabul River near an abandoned pesticide factory provided extracts that were quantified by GC with an electron capture detector and evidenced an alarming presence of p,p'-DDT [36].Furthermore, LLE extraction with petroleum ether of OCPs from surface and groundwater samples of the Shaying River (China), followed by analysis by GC with a micro-cell electron capture detector (µECD), reported total OCP levels of 21.0 to 61.4 ng•L −1 for groundwater and 12.3 to 77.5 ng•L −1 for surface water [37].Another team [38] used dichloromethane for the LLE of DDT and its metabolites from ditches and ponds near a closed-down factory in Bangladesh, followed by GC-ECD analysis.The values ranged from 590 to 3010 ng•L −1 .In addition, Fatoki and Awofolu [39] used light petroleum, hexane, and dichloromethane for LLE extraction, but from all the extracting solvents, dichloromethane gave the best results.For instance, the mean percentage recoveries of OCPs with dichloromethane ranged between 90.09 ± 8.03 and 102.95 ± 2.84%, the recoveries using light petroleum were between 68.18 ± 13.80 and 96.02 ± 4.90, whilst the recoveries using hexane were between 64.49 ± 3.04 and 98.92 ± 3.55.
A modification of LLE is solvent microextraction (SME), which involves the partitioning of the analyte between the aqueous phase and a very small volume of organic solvent.This extraction procedure combined with a GC technique, was used by de Jager and Andrews [40] to detect organochlorine from water samples.The proposed protocol was completed in less than 9 min and allowed the extraction and preconcentration of pesticides into a 2 µL drop of hexane.Via this procedure, OCP concentrations down to 0.25 ng•mL −1 were detectable.
Other modifications of LLE, namely liquid-liquid extraction with low-temperature partition (LLE-LTP), were presented by Mesquita and his team [41] for the assessment of OCPs in water samples.In the LLE-LTP procedure, a water sample and acetonitrile are mixed and placed in a freezer at −20 • C for 1 h for phase separation.When the temperature is lowered, partitioning occurs between water and acetonitrile and results in the extraction of OCPs in the organic phase.

•
Dispersive liquid-liquid microextraction (DLLME) requires injecting an appropriate mixture of extraction solvent and disperser solvent rapidly into an aqueous sample, resulting in a cloudy solution.DLLME is a low-cost, rapid, and easy-to-operate procedure with high recovery and it is also environmentally friendly, if we consider that very low volumes of solvents are used.This method is used to extract hydrophobic compounds.In addition to OCP extraction, this method has proven its utility for organophosphorus pesticides, herbicides, and polycyclic aromatic hydrocarbons [42].
The DLLME procedure was also adopted by Jorfi and his team [43] to extract OCPs (chlordane, dieldrin, DDT, heptachlor, lindane, and endrin) from the water of a water treatment plant in Ahvaz City, Iran.They used 10 µL of tetrachloroethylene (extraction solvent) and 1000 µL of acetone (disperser solvent).
Eighteen OCPs were extracted from water samples with an optimized DLLME procedure using 10 µL of tetrachloroethylene (extraction solvent) and 1 mL of acetone (disperser solvent), and afterward, 2 µL of the extractant was subjected to GC-MS analysis.The detection limits of the method proposed were in the range of 1-25 ng•L −1 ; the method was suitable for assessing ultra trace levels of OCPs in dirty and clean water samples [44].
Furthermore, another team [45] used DLLME coupled with GC-ECD for the extraction and quantification of 14 OCPs.The extraction protocol, which lasted for less than 5 min, consisted of using a mixture composed of 13.5 µL of carbon disulfide (extraction solvent) and 0.50 mL of acetone (dispenser solvent), which was injected into the sample.It was found that the enrichment factors were between 647 and 923 at room temperature and the OCP recoveries at two spiking levels (2.00 µg•L −1 and 10 µg•L −1 ) were 88.0-111.0%and 95.8-104.1%,respectively.
A modification of DLLME was reported by Tsai and Huang [46].In this case, a smaller volume of disperser solvent than that used for DLLME was used; hence, the name of the method is dispersive liquid-liquid microextraction with little solvent consumption (DLLME-LSC).DLLME-LSC presents good repeatability and high sensitivity, and it could be used for the investigation of OCPs from different types of water.The authors [46] used 13 µL of solvent mixture (tetrachloroethylene:tert-butyl methyl ether = 4:6, v/v) to extract five OCPs from clean water.

•
Vortex-assisted liquid-liquid microextraction (VALLME) was developed by Ozcan [47] for OCP quantification from aqueous samples.This optimized method used bromoform (50 µL) for extraction, followed by vortex extraction for 2 min at 3000 rpm with no NaCl addition for ionic strength adjustment, centrifugation for 5 min at 4000 rpm, and a 5 mL water sample.The mean recoveries for the OCPs assessed were between 71% and 104%.The performance of this extraction procedure was compared with that of LLE, and the conclusion was that recovery values were comparable (75-105%).This extraction method is suitable for the qualitative and quantitative assessment of OCPs from aqueous matrices, and it is easy to use and rapid.

•
Solid-phase extraction (SPE) involves the use of disks or columns able to retain analytes, which are then released with small volumes of solvents.This is a great advantage over LLE.The conditioning of cartridges containing octadecyl groups chemically bonded to silica is performed using different solvents, such as methanol [48], hexane [49], acetonitrile [50], ethyl acetate [14], or solvent mixtures [51].
A common disadvantage of both LLE and SPE is that volatile analytes may be lost during the evaporation process [35].
Table 2 summarizes the SPE protocols with experimental details, instrumentation, and all other details.
The extraction of OCPs from shallow groundwater samples from the Taihu Lake region in China was performed by an SPE procedure using C 18 cartridges that were washed with 5 mL of ethyl acetate, conditioned with 5 mL of methanol, and then washed with distilled water.Water samples were passed through the cartridges and the elution of OCPs was performed with 6 mL of ethyl acetate.After performing drying and concentrating the extract, the OCP residue levels were determined by GC-µECD [52].
Chen and his team [53] adopted a similar SPE protocol to extract OCPs from the surface waters of Shanghai, China.Unlike the above-mentioned study, 5 mL of methanol and 5 mL of dichloromethane were used for elution, and the OCPs were quantified via the GC-MS technique in the EI mode.
The extraction of OCPs from surface water in Greece [54] was also performed by SPE, but the elution of pesticides was performed with 10 mL of hexane and afterward, the detection was performed by GC equipped with 63 Ni-ECD.The recoveries for OCPs for three concentration levels (0.04 µg•L −1 ; 0.2 µg•L −1 ; and 0.4 µg•L −1 ) were between 28.65% and 201.34%, 38.12% and 177.32%, and 49.67% and 144.59%, respectively.The OCP concentrations obtained were higher than the EU target levels.
For OCP determination from water samples, a modification of the SPE protocol was proposed [55], the so-called magnetic-SPE, which involves the use of Fe 3 O 4 magnetic nanoparticles coated with oleic acid and then quantification by GC-MS.The recoveries of the proposed method were in the range 44% to108% for three fortification levels.
Another extraction procedure for OCPs from water [29] is activated carbon extraction (ACE), which could be employed in monitoring DDT and its metabolites in water.The activation of carbon is performed using methanol and double-distilled water.Among the advantages of ACE, the low cost of obtaining activated carbon (coconut shell, coal, and wood) and good recoveries ranging from 75% to 84% (compared with SPE, 56% to 70%) are worth mentioning.

•
Solid-phase microextraction (SPME) was developed during the 1990s and presents superiority over the above-mentioned extraction procedures, as it is solvent-free, with lower detection limits, sensitivity, and good reproducibility, and it has proven its efficiency when coupled with GC techniques [33][34][35].Moreover, SPME proved its utility for OCP extraction from aqueous samples.For instance, Jackson and Andrews [56] reported that the SPME of OCPs, separation (micro-bore 0.1 mm capillary column), and measurement (GC) from river water samples took less than 10 min.
The catalytic degradation of DDT in water by dehydrochlorination mediated by Pdbased nanoparticles and the monitoring of by-products were efficiently investigated by the SPME-GC-MS method, which provided high recovery (over 88.75%) and low detection limits (0.03 µg•L −1 ) [57].
For the analysis of OCPs from the Aries River in Romania, Miclean and co-workers [58] adopted, for extraction, SPE, followed by headspace solid-phase microextraction (HS-SPME).The latter involves the exposure of the fiber in the headspace above the sample.Afterward, the quantification performed by GC with ECD showed values between 144.5 ng•L −1 and 316.4 ng•L −1 for γ-HCH, DDT, and its metabolites.

•
Magnetic solid-phase extraction (MSPE) is another method based on the adsorption of the analyte of interest on a magnetic adsorbent, the advantage being that magnetic particles with nano size dimensions have a large specific surface area and consequently, a higher extraction capacity.
Nodeh and co-workers [59] synthesized new graphene-based silica-coated magnetic nanoparticles (Fe 3 O 4 @SiO 2 -G) used for the pre-concentration of OCPs from aqueous samples.This adsorbent has a large surface area, great adsorption capacity, and the presence of aromatic rings in its composition (which are able to interfere through π-π stacking with aromatic rings of OCPs), making it suitable for the extraction of benzene-based species.The investigation of this adsorbent's properties revealed excellent recovery values (80.8-106.3%)at pH 6.5.Furthermore, Zhou and his team [60] used magnetic polyamidoamine dendrimers to adsorb OCPs from water samples.
A brief presentation of the extraction protocols is presented in Figure 7; meanwhile, experimental details are provided in Table 2. Also, advantages and drawbacks for several extraction procedures can be found in Figure 8.

Removal of Interfering Species-Cleanup
The cleanup of the extract is a procedure that occurs before the quantification of samples with a complex matrix.In the case of water samples, the cleanup procedure is not required for relatively clean samples (groundwater and drinking water).A column of Florisil, silica gel, alumina, or charcoal [6,61,65,72,80] may be used for this step, if needed.In addition, it is possible that several OCPs are found in the sample; therefore, a crude separation into subcategories could be performed by using various types of column chromatography [83].
Huang and co-workers [72] investigated the presence of OCPs in groundwater from the Zigui karst area in China.After the LLE procedure, a neutral alumina-silica gel column (v/v, 1:2) was used for cleanup, followed by OCP elution with a mixture of dichloromethane/n-hexane (2:3) and concentrated to 0.2 mL prior to GC analysis.The results showed that 24 OCPs were detected in spring water (300-32,200 ng•L −1 ) and river water (318-2250 ng•L −1 ).The same extraction procedure for OCPs from water collected

Removal of Interfering Species-Cleanup
The cleanup of the extract is a procedure that occurs before the quantification of samples with a complex matrix.In the case of water samples, the cleanup procedure is not required for relatively clean samples (groundwater and drinking water).A column of Florisil, silica gel, alumina, or charcoal [6,61,65,72,80] may be used for this step, if needed.In addition, it is possible that several OCPs are found in the sample; therefore, a crude separation into subcategories could be performed by using various types of column chromatography [83].
Huang and co-workers [72] investigated the presence of OCPs in groundwater from the Zigui karst area in China.After the LLE procedure, a neutral alumina-silica gel column (v/v, 1:2) was used for cleanup, followed by OCP elution with a mixture of dichloromethane/n-hexane (2:3) and concentrated to 0.2 mL prior to GC analysis.The results showed that 24 OCPs were detected in spring water (300-32,200 ng•L −1 ) and river water (318-2250 ng•L −1 ).The same extraction procedure for OCPs from water collected from the Mediterranean Sea and the River Nile of Rosetta was used by El-Alfy and his team [64].The cleanup procedure was performed by passing the extract through a Florisil column and, afterward, elution was performed with diethyl ether.The OCP levels were below the detection limit of the method used.This outcome was related to the moment when the water was sampled, namely in the summer, when OCPs may hydrolyze more easily and volatilize.Another team [65] studied the presence of OCPs in drinking water from a cocoa Farm in Ghana.For cleanup, the team used silica cartridges through which concentrated OCPs were passed.For elution, they used dichloromethane.The detected OCPs were lindane, α-endosulfan, endosulfan sulfate, dieldrin, and p,p'-DDT.

Preconcentration or Enrichment
The preconcentration of a sample is usually required before cleanup and it involves solvent removal.In the case of OCPs, this process is performed by evaporation or filtration under pressure through a membrane.
Mirzaei and Rakh [84] reported a preconcentration method for OCPs from water samples.The method consists of joining of SPE to DLLME based on a solidification of floating organic drop (DLLME-SFO), which provided an ultra-enrichment factor (8280-28,221) for nine OCPs.In addition, the matrices of water samples did not influence the performance of the SPE-DLLME-SFO method; the recoveries were between 72% and 112%.

Detection and Quantification Methods of OCPs and Theirmetabolites
In the case of pesticides, the methods used are categorized as chromatographic, spectrophotometric, electrochemical, biological, and radiochemical, with the latter being rarely used.
OCPs are usually determined by gas chromatographic techniques (GC) using electron capture detectors [61,63,69].These techniques are accurate, but time-consuming and expensive.
In today's studies, for the determination of OCPs, researchers predominantly use fused silica capillary columns with lengths of 25 to 50 m, an internal diameter of0.15 to 0.25 mm, and a film thickness higher than 0.15 mm to prevent on-column degradation [85].
El-Gawad [86] reported the validation of an optimized method for the determination of 18 OCPs by gas chromatography operating with a quadrupole mass detector (GC-QMS).The validation was applied to the OCPs quantification in freshwater from the Ismailia Canal in Egypt.
Ali et al. [87] developed a GC method preceded by SPE for the assessment of various OCPs in water from the Hindon River in India.The extraction recoveries for the investigated OCPs were between 89.5% and 98.1%.The optimization of the GC conditions was conducted by using different columns, gas flow rate variations, and temperatures.
In addition to the so-called traditional methods for OCP quantification, there are other methods that involve the use of different sensors.For instance, an electrochemical sensor based on Fe 3 O 4 nanostructures decorated with indium tin oxide has proven specificity for OCPs based on the ability of chlorine atoms from OCPs to interact with Fe 3 O 4 nanoparticles [88].Another sensor, based on gold nanoparticles, was reported [89] due to its ability to detect endosulfan and catalyze the decomposition of endosulfan into non-toxic products.
The detection of chlordane, heptachlor, lindane, and mirex in marine water can be performed by cyclodextrin-promoted fluorescence modulation, which is characterized by low micromolar detection limits, selectivity, and the possibility to be used for different water samples with different salinity degrees [90].In addition, this method can be implemented in the manufacturing of portable detection devices.
Surface-enhanced Raman spectroscopy (SERS) is a valuable method for OCP detection, but there is still a need to amplify the signal of OCPs.Many strategies to overcome this issue were reviewed by Moldovan and her team [91].Furthermore, Li and co-workers [92] reported a SERS procedure for the detection of 4,4 ′ -DDT, α-endosulfan, and chlordane in farmland, river, and fishpond water with recoveries between 90.20% and 109.4%.
The detection of lindane in an aqueous environment by using a polymer-modified electrode was achieved by Noori et al. [93].The measurement time was as short as 20 s and the stability of the electrode for repeated use stretched for over a week.All these are excellent attributes for field measurements.

Confirmatory Techniques
To obtain a reliable result with a given technique, the analyst should use an additional technique to confirm the results, because in some cases, the interfering compounds may generate false-positive results.In the case of OCPs, the primary technique is GC equipped with ECD, and the confirmatory technique is mainly GC-MS.
For example, Gao and his colleagues [48] detected lindane, p,p'-DDT, and heptachlor oxide in surface water from China by GC equipped with µECD.Peak confirmation was achieved by GC coupled with an MS detector in SIM.The obtained results indicated that, from the investigated compounds, lindane was more frequently detected, more precisely in 83.9% of samples, with a mean concentration of 31.3 ng•L −1 .It was followed by p,p'-DDT, which was detected at 63.1% from samples with a mean value of 14.6 ng•L −1 .
Mahmoud and co-workers [38] monitored the levels of DDT and its metabolites in water collected from regions that are close to an abandoned DDT factory.The extraction of OCPs was performed via an LLE procedure; meanwhile, quantification was performed by GC with ECD and confirmed by GC-MS.The recovery values were 83% to 110% and the DDTs detected in water ranged between 0.59 and 3.01 µg•L −1 .The levels of p,p'-DDT, which were higher than those of o,p'-DDT, indicated pollution with technical-grade DDT (65-80% p,p'-DDT) from the factory.
An analysis of the literature on the levels of OCPs and their metabolites found in surface waters, groundwaters, and rainwaters is summarized in Tables 3-7 (OCPs were sorted into categories of HCHs, DDTs, Drins, Chls, Sulphs, and other OCPs), Table 8, and Table 9, respectively.The presence of reported OCPs is associated with past practices, accidental pollution, or continued fraudulent/illegal use.
Furthermore, high levels of DDT and its metabolites were identified in ditch and pond water from Bangladesh, as follows: p,p'-DDT was in the range of 190-1540 ng•L −1 , p,p'-DDE was in the range of 200-650 ng•L −1 , and p,p'-DDD was in the range of 210-830 ng•L −1 .This shows the effect of pollution caused by the random spread of pesticides after a DDT factory closure [38].Even higher DDTs concentrations (1200-3250 ng•L −1 ) were detected in the Jukskei River, South Africa, and sewage and waste dump sites around the river were suspected as possible sources [29].
Concerning the levels of Drins, those in water samples from the River Ganges, India, stand out as the average dieldrin level was 1670 ng•L −1 [70].Wide ranges of variations in the aldrin concentrations were reported for the Küçuk Menderes River (17-1790 ng•L −1 ) [63] and the Kusadasi Dilek National Park (ND-2180 ng•L −1 ) [16].
The highest average levels of HCHs, DDT, and its metabolites in groundwater were found also in India (Table 8).There have been studies that demonstrate the relationship between the consumption of OCP-contaminated groundwater and the occurrence of type 2 diabetes mellitus [71].
The literature survey evidenced the presence of OCPs, mainly HCHs and DDTs, in rainwater from India (Table 9).

Figure 3 .
Figure 3. OCPs listed under the Stockholm Convention.The letter(s) in brackets indicate in which annex(es) the POP is listed (Annex A (elimination) = Parties must take measures to eliminate the production and use of the chemicals listed under Annex A. Annex B (restriction) = Parties must take measures to restrict the production and use of the chemicals listed under Annex B in light of any applicable acceptable purposes and/or specific exemptions listed in the Annex.Annex C (unintentional production) = Parties must take measures to reduce the unintentional release of chemicals listed under Annex C with the goal of continuing minimization and, where feasible, ultimate elimination.).

Figure 3 .
Figure 3. OCPs listed under the Stockholm Convention.The letter(s) in brackets indicate in which annex(es) the POP is listed (Annex A (elimination) = Parties must take measures to eliminate the production and use of the chemicals listed under Annex A. Annex B (restriction) = Parties must take measures to restrict the production and use of the chemicals listed under Annex B in light of any applicable acceptable purposes and/or specific exemptions listed in the Annex.Annex C (unintentional production) = Parties must take measures to reduce the unintentional release of chemicals listed under Annex C with the goal of continuing minimization and, where feasible, ultimate elimination.).

Figure 6 .
Figure 6.Diagram of the steps of OCP analysis.

Figure 6 .
Figure 6.Diagram of the steps of OCP analysis.

Figure 8 .
Figure 8.A brief presentation of the advantages and disadvantages of the main extraction procedures.

Figure 8 .
Figure 8.A brief presentation of the advantages and disadvantages of the main extraction procedures.

Table 1 .
Main characteristics of OCPs and metabolites.

Table 1 .
Main characteristics of OCPs and metabolites.

Table 2 .
OCP extraction protocols and instrumentation.

Table 2 .
OCP extraction protocols and instrumentation.