Occurrence, distribution, fate and quantitation of phthalates in soils: a review

ABSTRACT Phthalates are ubiquitous contaminants occurring in all environmental compartments worldwide. In soils, it occurs at higher levels compared to other organic contaminants. Consequently, humans are exposed to phthalates constantly via air, dust, food and water. This paper reviews phthalates occurrence in soils of agricultural, industrial and urban areas. Diethyl hexyl phthalate and di-n-butyl phthalate are frequently detected in these areas, and their combined concentrations contribute about 60–80% to total phthalate concentrations. Microbial degradation is the primary process determining the fate of phthalates in soil. Gas, and liquid chromatographic techniques coupled to mass spectrometry techniques are generally used for phthalates determination. Reliable quantitation warrants stringent quality control measures as phthalates are present in laboratory air, solvents, and other labwares. Suggestions to minimise background contamination are outlined.


Introduction
Since the introduction of phthalate esters in 1920, they have been used on a large scale as solvents in industries, additives in plastics, construction materials, textiles, cosmetics, and some pharmaceutical products [1,2].The addition of PAEs to plastics imparts flexibility, improves low-temperature performance and flame retardance; hence, they are used as additives in many consumer goods.Availability of polyvinyl chloride (PVC) on a commercial level in 1931 and the production of diethyl hexyl phthalate (DEHP) in 1933 started the rapid growth of PVC industries [3].The global production of PAEs is around 6 million tons per year.China has become the largest consumer, accounting for 42% of the world's consumption [4][5][6].Depending on the application, PAEs in plastics may range up to 50% [7].As PAEs are used as additives, they do not bond covalently to polymers; instead, they distribute themselves between the macromolecules.As a result, PAEs are released slowly from the products.PAEs are semivolatile; hence, they find their way to atmospheric air.Substantial quantities of PAEs may leach out into water and soil during the lifecycle of products containing PAEs and solid wastes.In addition, domestic wastewater contains a significant quantity of PAEs released from personal care products, dishwashers, floor cleaners, cloths etc.In addition, some diffuse sources such as agrochemicals, building materials, emissions from waste disposal sites, etc., also contribute PAEs to the environment.Thus, large scale production and wide application of PAEs is the primary reason for the contamination of the environment [8][9][10].
Several epidemiological and toxicological studies have demonstrated the toxic effects of long-term exposures to PAEs [11,12].Exposure to PAEs has been associated with malformation of the male reproductive tract [13][14][15], damage to the liver and kidneys, and some types of cancers [16,17].Biomonitoring studies on humans have shown that PAEs alter androgen-responsive brain development [18].PAEs are well known endocrine-disrupting compounds and has a detrimental effect on children's growth [11].DEHP and butyl benzyl phthalate (BBzP) are classified as probable carcinogens in humans [19].The non-carcinogenic risk of PAEs follows the order DEHP>din-butyl phthalate (DnBP)>diethyl phthalate (DEP)>BBzP [20].Soils contaminated by PAEs has a significant impact on flora and fauna.PAEs are linked to testicular atrophy, decreased fertility, lower foetal weight, and antiandrogenic activity in rodents.Exposure to DEHP and its metabolites in rodents has caused foetal death and abnormalities [8,9].However, the International Agency for Research on Cancer (IARC) has modified its classification of DEHP from 'possibly carcinogens (Group 2B)' to 'non-classifiable' category (Group 3) [21].
Soil is considered as the sink/source of PAEs; hence, various studies were devoted to soil contamination of PAEs during the last decade [31][32][33].Few reviews have been published, indicating higher levels of PAEs in soil and their negative impact on the environment and public health [4,34].This updated review focuses on the occurrence of PAEs in soil, their distribution in agricultural, urban, and industrial soils, environmental fate, and analytical workflow for the quantification of PAEs.Various classical and advanced techniques with their merits and demerits for the extraction and cleanup of PAEs from soil samples are presented.As PAEs are ubiquitous in the laboratory environment, the analysis of samples with lower concentrations is challenging.Thus, suggestions to control the background contamination problems in PAEs analysis are outlined.

Physical properties of phthalates
Phthalates are colourless or yellowish oily liquids, odourless, and are soluble in oil and organic solvents.PAEs have a melting point in the range of −70°C to −4°C (baring a few PAEs such as DMP, di-undecyl phthalate (DUP), butyl 2-ethyl hexyl phthalate (BOP), and di-cyclohexyl phthalate (DcHP) and boiling point in the range of 195°C to 523°C.The most common PAEs and their physicochemical properties are given in Table S1 (in supplementary file).

Occurrence of PAEs in soil
PAEs are released into the environment due to a variety of anthropological activities such as release from plastics, construction and demolition activities, vehicular movement, textiles, improper waste disposal, personal care products, etc [35].Thus, PAEs are referred to as non-persistent but persistently present in the environment [36,37].Since they are always present in the environmental media, humans are exposed to them directly (ingestion) or indirectly (inhalation).Given the current rate of PAEs consumption and the environmental release, PAEs are considered 'unmanageable pollutants' in the environment, and their accretion causes wide distribution in soils.

Agricultural soil
Plastic films used for mulching, fertilisers, wastewater irrigation, sludge amendment, etc., contribute to PAEs in agricultural soils [38,39].Atmospheric deposition of particulate containing PAEs and their longer half-lives results in the accumulation of PAEs in soil.DnBP and DEHP were the most abundant PAEs in the vegetable production fields and orchards of China.The total concentration of six PAEs in the soil samples of the suburban area of Xianyang city ranged between 128.60 and 10,288.42µg/kg [40].This is the highest concentration of PAEs in agricultural soils reported from China.Another study in soils of farmland, vegetable, orchard, and wasteland reported PAEs in the range of 0.05 to 10.4 mg/kg, with DEHP and DnBP being the most abundant phthalates [41].Several other studies have also reported a higher concentration of PAEs in the agricultural soils of China.PAEs concentrations in Shandong province varied from 756-1590 µg/kg [42], and in Yangtze river delta, PAEs concentrations ranged from 5.42 to 1580 ng/g [29].In another study, fifteen PAEs in agricultural soil samples ranged between 75-6369 µg/kg [39].Seven PAEs in soils of plastic greenhouse were detected in the range of 60-80%.The higher concentration of DEHP and DnBP in plastic greenhouses is probably due to the volatilisation of DnBP in air and the sinking of DEHP to soil [43].Table S2 (in supplementary material) provides PAEs detected, their concentration range, with probable sources in agricultural soils of several countries.
The application of sewage sludge to agricultural soil provides plants with essential micro and macronutrients.Apart from imparting these nutrients, they also enrich the soil with other organic contaminants, which subsequently find their way into the food chain [44].Several studies have demonstrated the application of sewage sludge increases the level of PAEs and other organic contaminants in soil [45,46].Sludge samples collected from wastewater treatment plants reported the mean concentration of PAEs in the range of 0.004-97.4mg/kg, with DEHP and DnBP as predominant PAEs [44].Sludge amendments of soils result in higher daily intake of PAEs via food and soil ingestion in humans up to 16.4 µg/kg bw/day.Elevated levels of DEHP in soils after applying sewage sludge has been reported [20,47,48].Figure 1 shows the mean concentration of six priority PAEs in agricultural soils (µg/kg) in China and several other countries [31,42,[49][50][51][52][53][54][55].
The sorption of PAEs in the soil is influenced by soil organic matter (SOM), pH, and soil type.The interaction SOM with PAEs may happen via three ways viz. 1) SOM and PAEs in the soil can compete for the adsorption sites, 2) SOM can bind with PAEs and increase their water solubility, and 3) SOM can form complex with PAEs and get adsorbed on soil leading to adsorption of PAEs [56].The aromaticity of SOM directly correlates with PAE affinity because the molecules with more aromatic groups tend to absorb more PAEs due to π-π interaction [57,58].However, this is not always the case as other types of interactions such as hydrogen bonding and peptide groups present in natural organic matter also show affinity towards PAEs in soil [59].Soil pH is negatively correlated with soil PAEs, as alkaline soils neutralises carboxyl and hydroxyl groups, thereby reducing the accumulation of SOM on soil agglomerates [60].

Urban soils
Urban soils receive PAEs from various point and non-point sources; hence, wide variations in concentration are noticed.The application of plastic films on the surface of products has been drastically increased from 1.33 to 2.36 million tons from 2000 to 2016.This has been linked to the elevated level of PAEs in the urban soils [42,61].Soils of different urban areas such as residential, industrial, commercial, and roadside areas show different distribution patterns of PAEs, as different sources impact these.For e.g. in the city of Xi'an, China, the highest mean concentration of DnBP, DEHP, and Σ 6 PAEs were found in traffic areas; DEP and DnBP in industrial areas; DMP in residential areas; and DnOP in diverse commercial and traffic areas [62].The most abundant PAEs in urban soils were DEHP and DnBP, albeit wide variation in the observed concentrations were noticed, i.e. 43-69% and 25-45%, respectively.Similarly, PAEs in urban park soils in Tianjin varied from 0.001 to 0.1249 mg/kg, whereas in road dust samples, Σ 6 PAEs ranged between 0.39 to 7.54 mg/kg [61].The concentration of DnBP in the People's park soil, China was 0.0827 mg/kg, which was higher than farmland soil samples (0.070 mg/ kg) of Tianjin province of China [41].Roadside soil samples of non-industrialised areas reported PAEs in the range of 2.84-51.05mg/kg, and the distribution followed the order DEHP>BBzP>DEP>DnBP>DnOP>DMP [63].These observations indicate that elevated PAEs in urban soils are due to human activities such as liberal use of plastic items, traffic emissions, personal care products, and improper waste disposal.The important source of PAEs in roadside soil samples is due to vehicular emission [63].Figure 2 represents the distribution of PAEs in urban soils of China [62][63][64][65][66][67][68][69].

Industrial sites
According to the Global E-Waste Monitor 2017, India generates 3 million tons (MT) of E-waste per year annually, and by 2021 it may reach 5 MT [70].The top three countries that generate E-waste are China, the United States, and India.E-waste or waste electric and electronic equipment (WEEE) is a combination of plastic and various other toxic chemicals such as heavy metals, polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), PAEs, brominated/novel flame retardants (BFR/NFR), etc.These get easily released into the surrounding environment during the the handling, recycling, or improper disposal.Illegal dismantling and recycling of E-waste is the major contributor to soil PAEs in developing countries [71].Soils of e-waste recycling areas in four metropolitan cities indicated PAEs contamination was an order of magnitude higher in the recycling areas, i.e. than in dumpsites, i.e. 31-3143 ng/g, and 74-442 ng/g, respectively [72].However, these concentrations ranges were far less than that of similar E-waste recycling areas of China [41].For e.g.concentration of Σ 6 PAEs in soils close to electronic manufacturing units in China was in the range of 8.63-171.64 mg/kg, and their concentration followed the order DEHP>DnBP>DEP>DnOP>DMP>BBzP [63].Another E-waste dismantling site in China, viz.
Guangdong, reported Σ 6 PAEs concentration in the range of 11.8-17.9mg/kg with DEHP and DnBP as the major PAEs [73].E-waste dust generated from recycling areas of China showed Σ 9 PAEs in the range of 170-5300 mg/kg [74].These studies indicate that e-waste handling sites are one of the major sources of phthalates release.Compared to China, only a few studies reported PAEs contamination in Indian soils [23,25,29].The occurrence of phthalates in the sediments of Gomti river (Σ 5 PAEs range 0-364.14µg/kg), surface water samples from Cochin estuary (Σ 6 PAEs range ND-27.993µg/L) and seawater samples from Mumbai (Σ 14 PAEs range 1.2-25.1 µg/L) were few studies reported from India [75][76][77].Sediment samples from Cochin estuary reported higher concentrations PAEs, and concentrations of DEHP and BBzP have exceeded the permissible risk level (PEL), indicating higher risk to benthic biota [78].
Single-use plastics (SUP) have been identified as the major contributor to urban plastic pollution in India [79].Since the onset of Covid 19 pandemic, SUPs have been widely used in various personal protection equipment (PPEs) and packaging.As per the reports of the Central Pollution Control Board (CPCB), the quantum of plastic waste (PW) generated in India was about 3.3 MT in 2018-19 [79], and it is not clear as to how COVID-19 pandemic has increased PW.Hence, monitoring of PAEs in soils is important to assess the level and human exposure.

Degradation of PAEs in soil
Esters are responsive to hydrolysis but at a slower rate [80].The degradation of PAEs by microorganisms is considered the primary pathway in soils.Microbial degradation can occur under aerobic or anaerobic conditions, and hydrolysis of diester leads to the corresponding monoester and alcohol.Under aerobic conditions, the monoester could be further degraded to phthalic acid by enzymes.It follows either 3, 5, or 4, 5 dihydroxy phthalates path to give procatechuate, which further follows an ortho pathway to give pyruvate and oxaloacetate or a meta pathway to give acetate, succinate, and carbon dioxide (CO 2 ).
In contrast, under the anaerobic condition, the monoester is degraded to phthalic acid, further degraded to benzoic acid, and later to CO 2 and water via catechol as an intermediate [81,82].Another pathway for the degradation of phthalates under anaerobic conditions is converting phthalic acid first to benzoic acid by decarboxylation, which further undergoes a β-oxidation to give adipic acid.The intermediates involved during this process include 1-cyclohexene 1-carboxylic acid and 2-hydroxy cyclohexane carboxylic acid [83][84][85].Studies have reported that PAEs with shorter alkyl chain such as DMP, DEP, DnBP, BBzP, etc. can be easily degraded and mineralised; however, PAEs with longer or bulky alkyl groups such as DnOP, DEHP are less susceptible to degradation due to their high hydrophobicity [86].Fig. S1 gives the degradation pathway for PAEs in soil (in the supplementary material).
The degradation efficiency of PAEs by microorganisms varies with the type of strains and environmental conditions.Also, soil pore water is an extensive site for the degradation of PAEs by microorganisms.PAEs concentration in soil pore water is associated with the bioavailability of PAEs, which depends on temperature, pH, soil porosity, and SOM [6].The addition of root exudates changes the soil pH, speeds up the dissolution of metallic cations, and breaks the linkage between soil and SOM [87].In the case of long-chain PAEs such as DnOP and DEHP, the alkyl side chain inhibits hydrolysis by steric hindrance [82].Details of bacterial strains capable of degrading PAEs in soil and their removal efficiency are listed in Table S3 (in supplementary material).
Biosurfactants can enhance the biodegradation process by increasing substrate bioavailability for microorganisms or reducing surface and interfacial tension [88].Biosurfactants increase the solubility of hydrophobic PAEs, thereby enhancing their degradation.For example, Gordonia species (sp.)Dop 5 can efficiently degrade PAEs with shorter or longer alkyl groups with the same efficiency, which is probably due to the release of extracellular biosurfactant that enhances the solubility of PAEs [89].However, surfactants also influence the degradation rate due to differences in toxicities [90].Generally, non-anionic surfactants such as Tween 80, Brij 35, Triton x-100 are less toxic than anionic surfactants (e.g.sodium dodecyl sulphate), enhancing PAEs degradation in their early stage.Solvent tolerant Bacillus subtilis strain 3C3 can degrade di-n-propyl phthalate (DnPrP), DnBP, BBzP very rapidly after the addition of Tween-80 at concentrations 1.25 to 125 times above their critical micelle concentration [91].Strain YC-RL4 can degrade a mixture of phthalates in five days with a degradation rate of over 90%, whereas natural degradation is relatively slow.During DEHP degradation by strain YC-RL4, the intermediate obtained were monoethyl hexyl phthalate (MEHP), phthalic acid, and benzoic acid [86]; however, using strain Cupriavidus Oxalaticus E3, BOP was obtained as an intermediate [92].

Analytical workflow
Quantification of PAEs in the environmental samples is the first step in monitoring their distribution in the environment.Accurate analysis of PAEs requires proper planning and execution of analysis.Generally, soil analysis of PAEs includes three main steps: sampling, sample preparation and analysis.From sample collection to quantification, several steps are involved in developing an analytical method.However, sample preparation is the most laborious and time-consuming part.Hence, it is considered as the major source of inaccuracy in the overall analytical workflow.Several studies are available which describe the methods for the analysis of PAEs with varying levels of challenges.

Sampling, transportation and storage
Generally, the top 10 cm of soil strata samples are collected, and the number of samples to be collected depends upon the variability of PAEs concentration within the location [40,69].Surface soil samples can be collected using hand auger/stainless steel shovel/ scoop, and wide-mouth glass bottles/aluminium foil envelopes can be used as sample containers.Depending on the purpose, samples may be grab, composite, or incremental samples.Samples must be preserved at 4°C by keeping them in an ice-box and transported to the laboratory.Care must be taken to avoid analyte loss and crosscontamination during sampling, transportation, storage, and processing [41].

Extraction techniques
Quantitative extraction of PAEs from the solid matrix is a prerequisite.Several extraction techniques are available with the slightly polar solvent used alone or as solvent mixtures.
The choice of extraction technique often depends on the complexity of the soil matrix besides the availability of equipment.This section provides an overview of commonly used extraction techniques, their merits, and their demerits.

Soxhlet extraction (SE)
The historical legacy of SE is the most important advantage of this method because all the newly developed extraction techniques for solid samples are compared with SE.Industrial and contract laboratories also compare their results based on the extraction efficiency of SE.In SE, the sample is repeatedly brought in contact with the fresh portion of extractant, which facilitates the displacement of transfer equilibrium.Another important advantage is that the extracts need not be filtered or centrifuged, and the sample throughput can be increased by performing several simultaneous extractions.SE generally results in higher extraction efficiency for different PAEs in soil samples [93].Recovery rates generally range between 77-121% [39,40,63,75].A modified version of soxhlet viz.automated SE is widely used, which has the advantages of less solvent and time consumption.Commercial automated soxhlet devices (Soxtec® System HT, Soxtherm, etc.) perform the extraction with similar efficiency but with a significant saving of time and solvent.To perform the two extraction steps i.e. boiling and rinsing, the device uses a combination of reflux boiling and extraction, followed by recovery of the solvent.Exchange from one step to another is achieved by switching the lever.The soxtec counterpart B811, which is software controlled, can perform the same step as soxtec device and can also be used as a conventional soxhlet technique [94,95].Recoveries of PAEs using automated SE were in the range of 90-100% [96].
SE is a very simple method, and a large number of samples can be extracted as compared to the latest extraction methods.However, this technique has certain drawbacks, including the extraction time that may vary from 12 to 24 hours and consumption of more solvents which are not only expensive but can cause environmental contamination.

Ultrasonic extraction (UE)
UE is a robust method for extracting organic contaminants from solid matrices because it takes advantage of the cavitation phenomenon occurring when the sample is in contact with the extraction solvent [97].The application of ultrasound forms the cavitation bubble throughout the solvent, which then collapses, causing changes in temperature and pressure, thereby increasing the rate of mass transfer of analytes to the solvent.Studies for the extraction of PAEs from soil matrices has shown a recovery in the range of 75-117% [29,98].Determination of PAEs from greenhouse soil samples have shown recovery range of 81-113% [59].Monitoring of ΣPAEs in soil samples has shown a recovery rate of 70-118% [99].UE with 0.01 M hydrochloric acid (HCl) was reported to yield better recoveries of PAEs metabolites such as mono butyl phthalate (MBP) (35-105%) and MEHP (34.5-95.6%)[100].This method allows the extraction of unstable analytes, which degrade when using organic solvents.Ultrasonication increases the polarity of the system, thereby increasing the extraction efficiency [101].The demerit of this technique includes wave attenuation in dispersed phase and a decrease in soundwave amplitude with distance.

Microwave assisted extraction (MAE)
MAE is a technique that uses microwave energy to heat solvents in contact with the sample to partition the analytes from the sample matrix into the solvent.Ganzler and Salgo [102,103], first reported microwave energy to extract organic compounds from contaminated soil.Proper choice of extraction solvent is important for the method to work, as the solvent dielectric property is important for microwave absorption.Often MAE is considered as a green extraction method owing to its lower energy and solvent consumption.The moisture content of the sample matrix is the main target of microwave heating during MAE.As water evaporates, the intercellular pressure increases, increasing the leaching of compounds [104].However, this positive effect of moisture has not been reported for PAEs.MAE consumes less solvent, and the results obtained are more reproducible than traditional methods.Sinha et al. and Liang et al. [105,106] have used MAE to determine PAEs in the soils.Σ 18 PAEs in soil samples have shown recovery in the range of 67-122% using the MAE technique [107].The efficiency of MAE can be very poor if the target analyte is volatile [108,109].When analyte recovery is low, large cooling or venting time is needed after extraction [109,110].

Accelerated solvent extraction (ASE)
ASE is a rapid technique for extracting PAEs from soil samples performed with minimum solvent, high temperature and pressure.Hence, this makes it a more effective method to extract analytes from soil samples with good recovery rates [111][112][113].At higher temperatures, mass transfer and solubility of analyte are improved, thereby reducing the strength of the sample-analyte bond, solvent viscosity, and surface tension [114].For the estimation of Σ 11 PAEs in soil samples using ASE, recoveries were reported in the range of 73-102% [115].A novel extraction method using water as the solvent to extract 12 PAEs with 84-89% recovery rates for DEHP has been reported [116].Xia et al. [117] reported recoveries in the range of 85-109% for the urban soils of China.The recoveries of PAEs ranged between 47-114% when using methanol, ethanol, and hexane for extraction.Mono-PAEs showed recovery in the range of 26-121% when extracted using methanol and ethanol [118].Notwithstanding these advantages, analyte degradation at higher temperatures and solvent-dependent extraction efficiency may result in poor recoveries [119].

Supercritical fluid extraction (SFE)
SFE is another green extraction method to classical organic solvent extraction due to its ease of separation and no requirement of organic solvent.SFE is a technique in which supercritical fluid (SCF) is added to the sample matrix, and extraction is performed based on solubility differences.This technique considerably reduces sample preparation time due to excellent mass transfer properties.SFE has higher selectivity because solvation power can be adjusted by changing temperature and pressure [120].The commonly used solvent in SFE is CO 2 because it prevents the extracts from destruction by providing a nonoxidising atmosphere in extractions [121].A study of PAEs in sediment has shown that SFE is a promising technique for extracting PAEs from solid matrices [122].Bautista et al. [123] reported 100% recovery for di-iso-butyl phthalate (DiBP) and di-iso-octyl phthalate (DiOP) in soil samples.SCFs such as nitrous oxide (N 2 O) produce comparable extraction efficiencies as that of CO 2 (i.e.84-92% and 82-99% respectively for N 2 O and CO 2 ) [124].However, due to the high cost of SCF systems and complex configuration, SFE is not very common and lacks acceptability for PAEs/organic contaminants extraction.

Microextraction techniques
Liquid-phase microextraction (LPME) techniques are relatively new and emerging techniques and are environmentally friendly.The different variants of LPME are single drop liquid-phase microextraction (SD-LPME), hollow fibre liquid-phase microextraction (HF-LPME), and dispersive liquid-liquid microextraction (DLLME).These procedures are costeffective and have no sample carryover issues.SD-LPME decreases the volume of organic solvent to a great extent, but extensive stirring may cause the droplet to break up.The HF-LPME approach stabilises the extraction droplet by placing it in a hollow fibre, but this method requires a longer extraction time.DLLME is a simple, efficient, and fast approach that has been used to determine a variety of compounds in different matrices.During recent years, several modifications have been introduced, viz.ultrasonic-assisted DLLME, vortex assisted DLLME, microwave-assisted DLLME, and air-assisted liquid-liquid microextraction (AALLME).The target is to eliminate the dispersive solvent [125].A combination of DLLME with other techniques have been investigated for the determination of organic contaminants in solid samples.For example, SFE combined with DLLME has been used for the extraction of PAHs and organophosphorus pesticides in soil and sediment samples [126,127], which extends the application of DLLME to solid samples.It holds great potential for the analysis of trace organic compounds in solid matrices.Studies are available indicating the use of DLLME, AALLME, SD-LPME, and HF-LPME techniques to extract PAEs from water samples [128][129][130].However, no study has been reported for the extraction of PAEs from soil samples; however, it can be used as a cleanup method after carrying out extraction by other methods.

Cleanup techniques
Cleanup of the sample extract is very crucial as co-extractants and interferants need to be removed.Cleanup capacity is another important aspect that must be considered while choosing the cleanup technique.The choice of cleanup technique depends on the interfering components such as co-eluting compounds, high boiling compounds, other organic compounds etc., in sample extract and to what degree the cleanup is required [131].

Column chromatography
The main principle involved in column chromatography is adsorption of solute from sample extract onto the stationary phase and selective elution of the analytes.Column stationary phases commonly used is alumina, silica, florisil, and activated carbon.Alumina is present in three pH range for cleanup.Each type of alumina has its strength in the removal of interferants and limitations.Basic alumina is used to separate the basic and neutral compounds; neutral alumina separates aldehydes, ketones, esters, lactones, etc., whereas acidic alumina is used to separate strong acids, acidic pigments, and lipids [132-134].However, certain factors like the dimension of the column, the particle size of the adsorbent, the nature of the solvent used, and the flow rate impact the efficiency of this cleanup technique.Sulphur and its compounds may cause problems and become a challenge for instrumental analysis, epically soil samples rich in OM [135].The reactivity of sulphur-containing compounds becomes a concern when analysing at a higher temperature [136].Treatment of extract with activated copper or tetrabutylammoniumsulfite (TBA) is helpful to remove elemental sulphur [137].

Solid phase extraction (SPE)
SPE technique became popular in late 1990s and it is the most commonly used cleanup technique for emerging contaminants, including PAEs.SPE reduces the risk of background contamination and simplifies the overall analytical workflow.In SPE, the sample is in contact with the solid phase, and the analyte is selectively sorbed on the solid phase.
The principle of SPE involves the partition of compounds between two phases.The analyte must have a greater affinity for solid-phase than the sample matrix.Traditional stationary phases like silica, alumina, or fluorisil are common in SPE.Bonded phase and a variety of polymeric phases are used currently.Among bonded phase, C 8 , C 18 , hydrophilic-lipophilic balanced (HLB) are classic sorbents for SPE.However, the drawback of these sorbents is low selectivity for different kinds of analytes [138].SPE cleanup of soil extracts with C 18 sorbent for PAEs determination with dispersive SPE and C 18 sorbent for cleanup, for PAEs in soil provided 73% analyte recovery [139].Different SPE sorbents such as C 18 , silica and HLB showed average recovery of 85.4%, 69.8% and 82.4%, respectively [140].Poly (dopamine)-coated magnetic nanoparticle (Fe 3 O 4 @pDA) has been used as adsorbents in magnetic dispersive SPE for determination of Σ 10 PAEs with the mean recovery range of 70-120% [141].Other studies have also reported the use of SPE cleanup technique [142].Several studies have utilised nanomaterial as sorbents because of their excellent extraction efficiency.Graphene oxide is one such material characterised by a large surface area and excellent sorption properties for aromatic compounds due to π-π interaction [143][144][145].The combination of nanoparticles with the magnetic field has also been used extensively because the separation of components is rapid by the adsorbents employing an external magnetic field.Fe 3 O 4 @MIL-100 and Fe 3 O 4 @SiO 2 @polythiophene have shown good repeatability, improved efficiency, and chemical stability.Combining these two nanomaterials improves the extraction efficiency of water-soluble PAEs [146,147].

Gel permeation chromatography (GPC)
GPC is a size exclusion cleanup technique that uses organic solvents and hydrophobic gels to separate larger macromolecules from the analyte.This technique is a promising one for removing high boiling compounds.Several studies are available which have utilised GPC cleanup for water, sediments, sludge and soil samples [31,148].

Analysis by chromatographic techniques
Studies have shown that (Table S4, in supplementary material), PAEs can be separated, determined, and quantified by Gas chromatography (GC) and high performance liquid chromatography (HPLC).GC coupled with detectors such as flame-ionisation detector (FID), electron capture detector (ECD), mass spectrometer (MS), are used for PAEs quantification.However, MS is preferred due to its specificity and low detection limit.

Gas chromatography technique
PAEs can be analysed by gas chromatography (GC) either in split/splitless mode.In splitless mode, more samples could be introduced into the column so that better LODs can be obtained.Carrier gas plays a vital role in separating the components from the mixture.Hence selection of carrier gas and its flow rate influence the chromatographic efficiency and retention time.A glass column (2 m × 3 mm) packed with 1.5% OV-17 and 1.95% QF-1 on chromosorb WHP (100/120 mesh) has been used for the separation of PAEs from sewage sludge samples [149].Comparison of different columns such as HP-5 MS, DB-17 MS, and DB-WAX indicated that HP-5 MS gave the best chromatographic efficiency for separating PAEs [140].However, column selection criteria depend on analysis time, selectivity, resolution, detection limit, column inertness, phase robustness, and thermal stability of stationary phase.A longer column generally improves the separation of analytes and increases resolution; however, it increases retention time and peak broadening.
Detectors such as FID, ECD, MS, etc. are generally used to determine PAEs.ECD is a highly sensitive detector and is commonly used for halogenated and unsaturated compounds.Studies have shown good recovery of PAEs using ECD [150,151].Another GC detection system that is widely used is FID.It provides high sensitivity to a wide range of compounds and reliable routine operation.Problems that appear in FID are few and can be solved easily.MS will be the most suitable and selective detector for the analysis of PAEs.Various types of MS analysers like single quadrupole, triple quadrupole, and ion trap are available.Tandem MS can be used with the high specificity of PAEs at the trace level.High-resolution mass spectrometry (HRMS) and accurate mass measurements by employing orbitrap and time of flight (TOF) mass analysers have gained more popularity for the determination of PAEs because of their high sensitivity and ability to provide more structural information [105].

High-performance liquid chromatography (HPLC)
Several methods are published for the determination of PAEs using HPLC [40,152,153].Separation of PAEs can be achieved by using C 8 , C 18 analytical columns.The separation occurs based on the chemical and physical interaction of the sample with the mobile and stationary phases.Solvents such as acetonitrile or methanol and water are used as a mobile phase, generally buffered with ammonium acetate or acetic acid.For the better separation of peaks, the column is thermostated at 40°C.For resolving isomeric mixtures like DiNP and DiBP, ultra-high performance liquid chromatography (UHPLC) can be used, and resolution can be achieved by optimising mobile phases and gradient eluting conditions [115].The advantages of UHPLC includes small column dimensions, reduced particle size (< 2 µm), fast mobile phase change over, best peak resolution, and reduced run-time per injection, thus making it more versatile for the separation of PAEs from different matrices.Detectors that can be used for the detection of PAEs are ultraviolet (UV) or MS.With tandem MS, electrospray ionisation (ESI) with multiple reaction monitoring (MRM) mode can be used for the quantitative analysis of PAEs [154][155][156].

Quality control and quality assurance
The major problem that arises during PAEs analysis is the high level of background contamination in the procedural blanks.As PAEs are commonly present in building materials, laboratory wares, instrumental parts and containers, the laboratory air and dust are the major source of contamination [36].This may often lead to false-positive results or overestimated values [157].To overcome the background contamination, it is necessary to have a separate workspace with a purified air filter such as highefficiency particulate air (HEPA) filters, ultra-low penetration air (ULPA) filters to analyse PAEs.

Approaches useful to minimise the background concentration
Zhou et al. [42] found that PAEs contamination is mainly due to its presence in the laboratory air.Exposure of the extracted sample to laboratory air for five hours led to DnBP and DEHP concentrations above 200 µg/kg and 300 µg/kg, respectively.The use of cosmetics by analysts results in the elevated level of PAEs in the sample; hence, analysts should refrain from using cosmetics during analysis [158].For sampling, all materials used should be made of glass or stainless steel, including sample containers.Washing glassware with hot water, rinsing it with deionised water and acetone, and baking glasswares at 450°C for 20 to 48 hours and covering it with aluminium foil can reduce the PAEs contamination by about 95% [159,160].The gloves used should also be free from PAEs.Pipette tips, vial caps, syringes, filters are potential sources of PAEs contamination.Use of polytetrafluoroethylene (PTFE) or fluoropolymer based items instead of plastic ones and glass SPE cartridges are recommended.Guo and Kannan [157] have tested PAE contamination in pipette tips and found traces of PAEs in the range of 0.001-0.07ng/tip.Solvent contamination of PAEs is another issue, and it mainly originates due to their storage in PVC containers or the use of PVC during the production of these solvents.Solvents can be purified by adding pre-backed aluminium oxide and desorption with ethyl acetate.This procedure was reported to remove PAEs to the extent of 77% to 119% [161].

Quantification
Internal standards are useful for quantifying PAEs because several steps are involved between sampling and instrumental analysis.The analyte loss is more likely during extraction, cleanup, and preconcentration.Internal standards, which are structurally similar to analytes, can be used to compensate for analyte loss.The use of isotopically labelled standards as internal standards is recommended as they are similar to analytes.The internal standard utilised for the quantification of PAEs includes deuterated DnBP-D 4 , DEHP-D 4 , di-benzyl phthalate (DBzP)-D 4 , DEP-D 4 , di-iso-phenyl phthalate, di-n phenyl phthalate, etc [36,51,111].Analysis of procedural blanks and spiked samples between every ten samples would help the analyst to check contamination.Method detection limits reported are generally in the range of few tens of µg/kg for GC-MS to hundreds of µg/kg for GC and HPLC based methods.GC and HPLC tandem MS methods are useful for ultratrace analysis with LOD in the range of 1-10 µg/kg.More details on analytical methods for PAEs determination and their merits including detection limits are given in Table S4 (in supplementary material).

Recovery studies
Recovery of analytes of interest can be studied by the spiking method, and the recovery rate should be within the acceptable range, i.e. 80-120%.Several analytical methods are developed to study PAEs in environmental matrices, and the results obtained have shown excellent recoveries.
The recovery of six PAEs has ranged from 90%-110% by using sodium sulphate, alumina, and neutral silica gel (2:6:12) for column cleanup [113].Similar results were obtained in agricultural soil samples using the similar column cleanup method [36,40].Investigation of PAEs using C 18 adsorbent and florisil cleanup has given excellent recoveries in the range of 94-114.6%[27,139].
The use of different extraction solvents has been evaluated to obtain better recovery of PAEs.Estimation of PAEs in soil of vegetable greenhouses showed recovery in the range of 76-93% by using acetone/petroleum ether as an extraction solvent [98].Other solvents used include dichloromethane (DCM), acetonitrile, acetone, methanol, and water.Water is a good solvent for the extraction of phthalate monoesters due to its weak acidic properties.Maintaining extraction at two pH units above the pKa values improves extraction efficiency [100].Novel approaches such as bubble-in-drop single drop microextraction (BID-SDME) using chloroform improved the extraction of low molecular weight (LMW) PAEs, whereas high molecular weight (HMW) PAEs were extracted with dodecane.Since the LMW PAEs are sterically less hindered than HMW PAEs, the ester functional group of the former interacts more with chloroform [161].Carbon tetrachloride was successfully utilised to determine DMP, DEP, DnBP, and DEHP using solvent-vapour-assisted LLME with recoveries in the range of 80-100% [162].

Conclusions
The present review demonstrates that phthalate contamination of soil is ubiquitous and elevated concentrations were observed in agricultural, industrial, and urban areas.Among industrial areas, e-waste/WEEE industrial sites showed the highest phthalate contamination.The use of plastics in mulching, wastewater irrigation, sludge amendment contributes to PAEs in agricultural areas.It appears that the soils of China are more contaminated with PAEs compared to other countries.This may be related to the ongoing industrial and economic development in China.Among different PAEs, BBzP, DnBP, DEHP and DnOP are detected mostly in soils of many countries.The occurrence of DEHP and DnOP at elevated levels is a cause of concern as high molecular weight PAEs are suspected anti-androgens and implicated with several other adverse health effects.Mismanaged plastic waste is one of the major sources of PAEs in developing countries.Therefore, relevant policy and regulatory measures should be implemented for the environmentally sound management of municipal solid waste and plastic waste.Reliable quantitation methods are essential for the estimation of human exposure and risk assessment.The present review summarises various approaches for soil samples extraction, cleanup and analytical instrumental techniques.The authors hope this information would be useful for generating representative data on PAEs in various matrices.Studies should focus on human biomonitoring for comprehensive human exposure assessment, as it provides integrated exposure assessment.Developing countries in Asia, especially China and India, need to focus on epidemiological studies based on biomonitoring.

Figure 1 .
Figure 1.Mean concentration of six priority PAEs in Agricultural Soils (µg/kg) in China and other countries.

Figure 2 .
Figure 2. Distribution of PAEs in soils of urban areas of China.