1 Introduction

Benzene, toluene, ethylene benzene, and xylene, also known as BTEX, are some of the most common groundwater petroleum contaminants. The source of these contaminates could be from refineries discharge as they are an inevitable raw material for various plastic and polymer industries and as solvents in rubbers, paints, lubricants, detergents, drugs, and dye industries, or from underground oil storage tank leakages [1,2,3]. Generally, BTEX is released into the environment from a variety of sources. The three principal BTEX sources to the environment are pyrogenic, petrogenic, and processed goods [4]. When organic material is subjected to high temperatures during pyrolysis, pyrogenic BTEX is created. Both the thermal cracking of petroleum and the combustion of coal are pyrolysis processes. BTEX makes up approximately 61.1–94.8% of the VOCs released by incomplete combustion of coal or wood [4, 5]. The second source of BTEX compounds is building materials or products that have undergone chemical processing. BTX compounds represent 7%, 19%, and 13%, respectively, from the most significant VOC species in architectural or furnishing coatings [4, 6]. The term “petrogenic” describes the BTEX generated when crude oil matures or goes through similar processes. One of the most crucial sources of energy for industrial and agricultural production is oil. The demand for oil energy is rising as a result of the quick development of modern industry. There is an increase in the freight volume of oil during recent decades; this trend will continue until new energy completely substitutes oil. As a result, BTEX environmental pollution from oil transportation is still a major issue. The leakage of oil is another problem that cannot be disregarded.

According to the Agency for Toxic Substances and Disease Registry, of the US Department of Health and Human Services, these volatile organic compounds (VOCs) are designated as priority contaminants [7]. Their presence is undesirable due to their high toxicity and is considered to be causing many major internal organ malfunctioning and diseases in living things including humans [8, 9]. Therefore, decreasing the concentration and removal of BTEX from water and wastewater is a critical need [10]. Table 1 presents the physical and chemical properties of BTEX compounds.

Table 1 Physical and chemical properties of BTEX compounds [23]
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Various efforts have been dedicated in removing BTEX from wastewater using different chemical, physical, and biological methods such as chemical oxidation [11], chemical precipitation (i.e. lime and limestone) [12], adsorption [13], sand filtration [11], membrane-based technology [14], de-oiling, cyclone separation [15], photo-catalysis degradation [16], electro-dialysis [11], biodegradation [17], and trickling filters [11]. These methods vary in efficiency, associated limitations and restrictions, and their environmental impact. For instance, large scaled setup of membrane-based technology including ultrafiltration, reverse osmosis, and electrodialysis is economically not feasible and requires sophisticated experimental setup to operate, whereas methods supported by biological microorganisms like bacteria, fungi, and algae are impractical in large scales. The threat of secondary pollution existing together with the chemical precipitation methods for BTEX removal is huge and it discourages the implementation of that method [7,8,9, 18,19,20]. In recent years, adsorption technology has become the most promising and suitable technique applied for purifying water from organic contaminates owing to its high removal efficiency and capacity, low operation energy and cost, ease in operation, and availability of various types of material employed for this purpose [21, 22].

This review aims to critically analyse the recent available literature related to removal of BTEX from aqueous solution using adsorption technology. The purpose is to highlight the classification of adsorbents applied, the factors, and operation parameters affecting the adsorption efficiency and performance. To the best of our knowledge, there is no comprehensive review published recently focused on the adsorptive removal of BTEX compounds from aqueous solutions.

2 BTEX measurement methods

Different methods and techniques are available for BTEX measurements such as gas chromatography (GC), high-performance liquid chromatography (HPLC), ultraviolet–visible (UV–Vis) detector, and total organic carbon (TOC) analyser. Each technique has different principles and works under specific conditions. GC is commonly used technique for determination of BTEX concentration and it has different models, for instance GC equipped with a mass spectrometer GC–MS, Master DANI GC, GC equipped with photoionisation detector (GC-PID), and GC equipped with a flame ionisation detector (FID). In some cases, two methods are used simultaneously or combined to determine the BTEX concentration after treatment, for example the removal of BTEX solutions was measured by HPLC followed by UV–Vis detector as was reported in literature [24, 25]. In another work, TOC was employed along with head space GC–MS/HS [26].

3 Techniques for BTEX removal and recovery from aqueous solutions

Producing toxic pollutants results in detrimental effects on both health of living things and the environment including the water, soil, and air. Additionally, the modern world’s industries and society have greatly increased pollution. Several efforts have been dedicated for water treatment field specifically removal and recovery of BTEX from wastewater including traditional and newly developed techniques. Some were promising while others are still in the early stages of development. Table 2 shows the advantages and disadvantages of each method.

Table 2 The properties and challenges of common BTEX removal technologies
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Remediation of BTEX from aqueous phase can be done either in the form of degradation or in the form of recovery. Owing to its effectiveness, low cost, and green process of BTEX removal, biodegradation gradually attracted the researchers’ attention and considered as an innovative technology for the removal of BTEX by disintegration of the toxic material into products that are safe and non-toxic with the help of microbes. Another great advantage of this process is that the removal efficiency can be greatly enhanced by varying and optimising the experimental conditions such as the neurites, temperature, and pH. Khodaei et al. reported the BTEX degradation performance of a novel-isolated strain named Pseudomonas sp. BTEX-30 in water phase [4, 27, 28]. Another remediation technique based on plant assistance is phytoremediation in which the low molecular weight possessing BTEX diffuse through root into the cellular membrane and plant tissues where the toxins are degraded [29]. Pollutants are absorbed through the plant’s stomata and cuticular wax where the composition and the amount of these cells can significantly affect the amount of BTEX absorbed. Shores et al. reported that perennial ryegrass and foxtail barley can absorb BTEX&N from produced-water spills [30]. Advanced oxidation processes (AOPs) are a group of treatments aim to destroy harmful organic contaminates existing in the tested matrix that led in highly reactive hydroxyl radicals (OH). One of the common AOPs for transforming organic pollutants into safe products like CO2, H2O, and mineral acids is photocatalysis. It is suitable for treating a variety of organic contaminants and is affordable, safe, non-selective, and compatible as it uses sunlight or ultraviolet (UV). Carbon nanotubes (CNTs) and semiconductor such as titanium dioxide (TiO2), zinc oxide (ZnO), and tin dioxide (SnO2) are the most widely photocatalysts used for photocatalytic degradation of toxic organic contaminants like BTEX [31].

For the recovery of BTEX compounds from aqueous solution, mainly three techniques, absorption, membrane technology, and adsorption, are used. Absorption is the process of choosing the appropriate solvents to absorb any soluble VOCs that can be transferred to the liquid phase [32]. To facilitate the transfer of pollutants, the absorption tower offers the necessary liquid–vapour contact area. However, the absorbing liquid needs to be treated in order to prevent secondary pollution. Polydimethylsiloxane (PDMS) is considered one of the best VOC absorbers owing to its simplicity to be moulded and it can keep its shape after curing. Furthermore, it has a high affinity for a variety of non-polar molecules, including aromatic hydrocarbons, which makes it a perfect material for a variety of uses [4, 33]. Membrane separation technology for produced water treatment has received a lot of attention due to its wide range of applications, portable operation system, and environmental friendliness [34, 35]. A common technique for removing organic molecules from water is membrane pervaporation, in which organic molecules diffuse through the membrane instead of water by changing their state from liquid to gas [34, 36]. The difference in the relative volatilities of the feed solution components allows for the separation of oil and water using membrane pervaporation [34, 37]. Membrane pervaporation, however, may not be effective for the removal of some frequently found fatty acids, such as hexanoic acid and octanoic acid, which have boiling points above 200 ℃; therefore, this technique is only effective for the separation of volatile organics with low boiling points such as BTEX. Other conventional membrane separation processes relied heavily on the mechanism of size exclusion to remove organic molecules, which frequently required a significant amount of water to pass through the membrane. The trade-off between water flux and organic rejection efficiency does, however, exist for almost all membranes due to the hydrophobic nature of most membrane materials, and the decline in water flux brought on by membrane fouling or scaling is another significant challenge for membrane separation technology [34, 38, 39].

4 Adsorption

One of the most effective and economical technologies, both on a laboratory and an industrial scale, is the adsorptive removal process. In general, adsorbent parameters like pore size, adsorption capacity, and low toxicity can be used to assess the adsorbent’s quality [40]. Among them, porosity is considered as the key factor for the effectiveness of adsorption which has paved the way for the design and development of various adsorbents. Primary adsorbents which are effectively used for the adsorptive removal of BTEX from solutions include carbon structures [41], modified zeolites [42], polymeric resins [43], and clays and its modifications [44].

Generally, the adsorption capacity of an adsorbent for BTEX can be measured as:

$${q}_{e}= \frac{{(C}_{o}-{C}_{e }){V}_{o}}{W}$$
(1)

where qe is the adsorption capacity of adsorbent (mg/g); Co is the initial concentration of BTEX (mg/L); Ce is the equilibrium concentration of BTEX in solution (mg/L); Vo is the volume of the initial solution (mL); and W is the weight of the adsorbent (g).

4.1 Factors impacting the adsorption process

4.1.1 Nature of the adsorbent

The nature of sorbent and its microstructure determine its adsorption efficiency. The properties, and hence the performance and application of the materials, are directly related to material’s synthesis (preparation) and its internal microstructure. Chemical components and physical structures are the key factors in adsorption such as specific surface area, pore diameter, particle size, and micro-structure [45]. Each type of adsorbent material possesses its own characteristics and specialties in the adsorption process. For instance, carbonaceous sorbents are well known by their excellent performance due to the complex heterogeneous nature of carbon. Even though sorbents like activated carbon (AC), organically modified carbon (OMC), and carbon nanotubes (CNTs) including single walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) are having carbon as the basic structure, their physical and chemical properties are different. Owing to its micro-pore structure and presence of different oxygenated functional groups, AC showed superior capacity to adsorb BTEX from aqueous solutions [46]. Due to its large specific surface area, uniform pore widths in the 2–10-nm range, pore diameters ranging from 2 to 50 nm, high porosity, the existence various functional groups, and good thermal and chemical stability, OMC is also proved to be an effective sorbent [47, 48].

CNTs have gained considerable interest as a novel form of adsorbent for the removal of different organic contaminants from massive quantities of wastewater because of their extremely porous and hollow structure, huge specific surface areas, surface functional groups, and hydrophobic surfaces [49]. Additionally, the excellent removal performance of CNTs compared to other sorbent is attributed to the several sites where adsorption of BTEX may take place like intratube cavities or endohedral (a in Fig. 1A), interstitial channels (b in Fig. 1A), external grooves (c in Fig. 1A), and external surfaces (d in Fig. 1A) [50]. Unlike carbon materials, clay materials have layered/sheet structure. The electrostatic interaction between the exchangeable cations and the layers/sheet is weak interaction. This structure along with OH groups exist in the clay surface provides many special features such as ion exchange capability, sorption capacity, and swelling behaviour which has a great impact to the environment by serving as a natural adsorbent of pollutants by taking up cations and/or anions through either ion adsorption or exchange [51]. Polymeric resins, which have become very effective adsorbents, offer a high degree of selectivity as compared to activated carbons because of differences in hydrophobicity, pore size, and surface area [52]. Number of polymeric resin possess a torus shape with an exterior that is hydrophilic and an interior that is hydrophobic, which allows them to form host–guest complexes with various chemical molecules, particularly aromatics [53].

Fig. 1
figure 1

(A) A bundle of CNTs. (B) Zeolite microstructure [56]

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Zeolites are composed similarly to clay minerals. Both substances are particularly alumino-silicates, but their crystalline structures are different [54]. Aluminium, silicon, and oxygen are found in the regular framework of zeolites, which are three-dimensional, microporous, crystalline solids with well-defined structures (Fig. 1B). Free channels range in diameter from 0.3 to 3 nm in the structure, giving zeolite their “molecular sieve” characteristics and sorption behaviours. In addition, the existence of cations and water in their pores possess the ion exchange capabilities of zeolites facilitating surface modification, hence enhancing its adsorption efficiency [55]. In general, the quality of the adsorbent may typically be assessed using a number of adsorbent properties, including adsorption capacity, pore size, environmental friendliness, and surface area. High surface area helps to boost the adsorption capacity of adsorbents. However, the adsorbents’ functional groups have a significant impact on their adsorption characteristics and can be introduced by using chemical or physical modification of sorbents.

4.1.2 Impact of temperature

Temperature plays a crucial role in the pathway of adsorption of various species by adsorbents thereby affecting its adsorption efficiency and capacity. The effective analysis of effect of temperature on adsorption process gives an idea whether the process is exothermic or endothermic in nature. According to the study conducted by Konggidinata et al., ordered mesoporous carbon (OMC) used as an adsorbent for BTEX compounds showed positive influence on the adsorption process as temperature increased [47]. They varied temperature and studied adsorption at three different temperatures 25 ℃, 45 ℃, and 65 ℃ as shown in Fig. 2a. Their thermodynamic parameters calculated proved that physisorption is the dominant adsorption mechanism for BTEX onto OMC. In another study, Nourmoradi et al. used montmorillonite (Mt) modified with polyethylene glycol (PEG-Mt) as an adsorbent for BTEX removal [57]. They studied the effect of lower temperature starting from 10 to 40 ℃ on the adsorption capacity as shown in Fig. 2b. The free energy change (∆G) for all BTEX compounds by PEG-Mt was found to be negative which indicated the feasibility of the sorption process. The ∆G value increased at higher temperature indicating that the sorption is favourable at higher temperature.

Fig. 2
figure 2

Effect of temperature on the following: (a) adsorption capacity of BTEX onto OMC [47], (b) adsorption capacity of BTEX onto PEG-Mt [57], (c) removal efficiency of BTEX onto OBW-NaOH-CA [58], (d) removal efficiency of BTEX onto nZVI [59], (e) removal efficiency of BTEX onto HZSM-5 nanozeolite [60], (f) removal efficiency of BTEX onto entrapped nZVI [61]

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In another report, Arshadi et al. investigated the adsorption behaviour of modified ostrich bone wastes (OBW) with citric acid (OBW-NaOH-CA) as bioadsorbent at different temperatures 15 ℃, 24 ℃, 40 ℃, and 80 ℃ as shown in Fig. 2c [58]. The adsorption efficiency was increased by the increase in temperature may be due to the endothermic nature of the adsorption of organic molecules to the active sites of the bone sample. The enthalpy change (∆H) is found to be positive indicating the endothermic nature of the process. Mahmoud et al. reported the use of zero valent iron (nZVI) nanoparticles for the removal of BTEX from aqueous solution at different temperatures [59]. The increase in adsorption performance with temperature while fixing the remaining factors showed that the process is better at higher temperature as visible in Fig. 2d.

On the other hand, increasing the temperature had a negative influence on the adsorption efficiency when mesoporous HZSM-5 nanozeolite, hydrothermally prepared from a type of coal fly ash, was utilised as sorbent as shown in Fig. 2e [60]. This was in agreement with study reported by El-Shafei et al. where the BTEX removal efficiency of entrapped nZVI in alginate polymer was studied at different initial temperatures as illustrated in Fig. 2f [61]. This reduction indicates that the adsorption process onto HZSM-5 nanozeolite is spontaneous and thermodynamically favourable at lower temperature but it changes to non-spontaneous mode at higher temperature.

4.1.3 Impact of pH

The solution pH involved in the adsorption process is one of the important factors affecting the adsorption efficiency of the adsorbent. This phenomenon can be explained by the functional groups (positively or negatively charges) present on the surface of adsorbent used in adsorption study. BTEX compounds are characteristic of functional group that are negatively charge. In case the adsorbent with a functional group contains positive charge, the adsorption is favourable in acidic media as excess of H+ exist and the attraction between the adsorbate and adsorbent will increase which will enhance the adsorption capacity [62,63,64]. On the other hand, adsorption efficiency will decrease when adsorbents with negativity charge are used in acidic media. H+ ions in acidic media will be attracted by the adsorbent functional group causing less active site to be available for the adsorbate to interact with the adsorbent [65]. In some cases, pH of solution has negligible effects in adsorption study when some adsorbents are used and this indicates high stability in wide range of pH [66].

Aghdam et al. reported the effect of pH solution on removal capacity of benzene using paper mill sludge-based activated carbon as sorbent, and had negligible effect of the removal capacity of the remaining organic compounds (TEX) as observed in Fig. 3a [46]. However, varying pH showed insignificant effect on the removal capacity of BTEX when oxidised multi-walled carbon nanotubes (MWCNTs) and PEG-Mt were used as sorbents as shown in Fig. 3b and Fig. 3c, respectively [49, 57], which indicates high stability of these materials as BTEX adsorbents in wide range of solution pH. However, the removal efficiency of BTX by oxidised activated carbon (O-AC) decreased as a function of pH ranging from 5 to 9 as presented in Fig. 3d. This suggests that there are strong possibilities for chemisorption to happen during the adsorption process [67].

Fig. 3
figure 3

Effect of pH on the following: (a) adsorption capacity of BTEX onto paper mill sludge-based AC [46], (b) adsorption capacity of TEX onto MWCNTs [49], (c) adsorption capacity of BTEX onto PEG-Mt [57], (d) removal efficiency of BTX onto O-AC [67], (e) removal efficiency of BTEX onto entrapped nZVI [61], (f) removal efficiency of BTEX onto smectite organo-clay [68]

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The pH dependence of BTEX removal efficiency employing smectite organo-clay as sorbent is reported by Carvalho et al. [68]. They investigated the adsorption behaviour at three different pHs, 4, 6, and 9, and found that the removal efficiency was maximum at pH 9 for all BTEX compounds, and the removal efficiencies ranged from 70 to 90% as presented in Fig. 3e [68]. The case was different when El-Shafei and his team used nanozero valent iron nZVI in Ca–alginate as sorbent and examined the effect of pH on the removal efficiency. Their study showed that the percent removal reached the maximum at pH 7 and started to decrease onward as presented in Fig. 3f [61]. This implies that the sorbent is efficient at neutral environment.

4.1.4 Impact of initial concentration of BTEX

Adsorption being a mass transfer phenomenon is highly influenced by the initial concentration of pollutants since it can influence the flow of pollutants to the adsorbent site. Increasing the initial concentration of BTEX causes increase in adsorption capacity, while keeping all other parameters constant. This behaviour can be attributed to the increase of Van der Waal’s forces that drives the adsorbate to the active sites of the adsorbents at high concentration, which dominates the mass transfer resistance among the solution and solid phase. On the other hand, at high concentration, the active sites on the surface of adsorbents become saturated and this reduces the removal efficiency of BTEX when the initial concentration increases [58, 60, 69, 70]. Lu et al. reported the effect of varying BTEX initial concentration ranging from 5 to 125 ppm on the adsorption capacity by scrap tyre while fixing initial pH at 7 and scrap tyre concentration 50 g/L for 24 h [71]. As shown in Fig. 4a, the removal capacity of microbe immobilised waste tyre increased with increasing the initial concentration of the BTEX. In another report, Bandura et al. investigated the potential role of synthetic zeolite Na-P1 obtained from fly ash in BTEX removal [72]. They found that increasing the initial concentration of BTEX enhanced the removal capacity and decrease the adsorption efficiency as presented in Fig. 4b and c. Nanozeolite prepared from coal fly ash was also used for adsorbing BTEX and it showed similar results of decreasing the adsorption efficiency with the increase in initial concentration as illustrated in Fig. 4d [60].

Fig. 4
figure 4

Effect on BTEX initial concentration on the following: (a) adsorption capacity of BTEX onto scrap tyre [71], (b, c) adsorption capacity and removal efficiency of BTEX onto Na-P1 [72], and (d) removal efficiency of BTEX onto HZSM-5 nanozeolite [60]

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4.1.5 Impact of contact time

The effect of contact time, i.e. the time for which the adsorbent is in contact with the adsorbate, also plays a significant role in the process of adsorption and equilibrium. This investigation has practical relevance as it encourages smaller designed-system volumes that promise high effectiveness and economy. The studies show that the removal efficiency and the contact time have a proportional relation until the system reaches equilibrium. The rate of the removal capacity at the early stages of adsorption process is very rapid and when the time elapses, the rate starts to decrease until it reaches equilibrium. This phenomenon is attributed to the high concentration gradient between the adsorbate in solution and the adsorbent surfaces, the variety of functional groups existing at the adsorbent surface, and the more availability of vacant active sites on the adsorbent surface at initial times and during the adsorption process, the number of available active sites decreases [46, 47, 57, 58]. In addition, BTEX component shows different kinetic performances as a result of their differences in boiling point, hydrophobicity, molecular weight, and water solubility [46, 73]. Figure 5a shows the effect of contact time on the BTEX removal efficiency using paper mill sludge-based activated carbon as an adsorbent, synthesised by chemical activation and pyrolysis, using 40 mg/L and 1000 mg/L as initial concentrations of BTEX and adsorbent, respectively, as reported by Aghdam et al. [46]. The removal capacity increased with increase in contact time and the equilibrium was reached within 270 min approximately. In another work, equilibrium was obtained in shorter time within 20 min when Moringa oleifera seed cake (MOSC) was used as sorbent in different contact times varying from 10 to 50 min with initial concentration 1 mg/L of the BTEX solution and pH at 6 as observed in Fig. 5b [74]. Generally, it can be understood that as time elapses, the adsorption performance is improved until it reached equilibrium. Reaching equilibrium varies from system to another depending on different experimental conditions and nature of the material. In some cases, after reaching equilibrium, the material starts to desorb; therefore, it is highly important to understand the equilibrium adsorption time and the adsorption kinetics of the system under study.

Fig. 5
figure 5

Effect of contact time on the following: (a) adsorption capacity of BTEX onto paper mill sludge-based AC [46], (b) removal efficiency of BTEX onto MOSC [74]

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4.1.6 Impact of adsorbent dosage

The adsorbent dosage also has significant effect on the efficiency of the adsorption process and on the adsorbent’s capacity for specific dosage under operating conditions. The availability of sorption sites at the adsorbent’s surface will increase as the dosage of adsorbent is increased, which generally results in an increase in the percentage of BTEX removal [60, 70]. This was demonstrated in the study of Anjum et al. where changing the dosage of oxidised activated carbon (O-AC) from 0.01 to 0.1 g at 50-mL volume for 40 min and pH 7 improved the removal percentage from 65 to 95% [67]. In the study of BTEX removal using paper mill sludge-based AC, the dosage varied from 0.25 to 1.5 g/L, the adsorption capacity was increased from 2 to 9.8 mg/g [46]. Similar results were obtained using copper-modified zeolite 4A and nZVI in Ca–alginate as sorbents [25, 61].

However, some studies have also reported contradicting results to the above discussed trends, as the adsorbent dosage increases, the adsorption capacity decreases. Yang et al. reported the use of Beta cyclodextrin (β-CD) modified poly (BMA-Co-CD) for the adsorptive removal of BTEX from solution [52]. Initially, the adsorption capacity increased with increasing the β-CD upto 30.95 µmol/g on poly (BMA-Co-CD) followed by a decrease in adsorption capacity from 30.95 to 76.12 µmol/g. This may be due to the complete occupancy of the pores of the poly (BMA-Co-CD) by β-CD, thereby reducing active sites for the adsorption of organic molecules. In another study using modified ostrich bone waste (OBW-NaOH-CA), the removal percentage of BTEX increased as the dosage increased from 0.10 to 0.75 g. However, after 0.75 g, there was no improvement in removal efficiency as the dosage was increased to 1.25 g. This may be due to the fact that at lower concentrations, the dispersion of adsorbent is better and the number of active sites available for the adsorbate to attach with will be high [58]. As the concentration increases, the number of adsorbent sites with higher active energy decreases, thereby decreasing the removal efficiency.

5 Various adsorbents used in the removal of BTEX

5.1 Carbon base adsorbents

Among all the suggested sorbent materials, carbon-based sorbents are the most preferred for removing contaminants from wastewater. Adsorption on carbonic sorbents, in particular, has proven to be a highly successful substitute for more expensive treatment approaches adopted for removing a wide range of organic pollutants from wastewaters [75, 76]. For instance, carbon-based nanomaterials such as carbon nanotubes (CNTs) and activated carbon (AC) pose huge surface areas and well-developed porosity which make them excellent sorbents in removing aromatic pollutants [77]. However, there are a number of drawbacks to carbon-based sorbents, including the fact that it is relatively costly, low selectivity, and has a difficulty with regeneration [75, 76]. Table 3 summarises carbon-based sorbents used for BTEX removal.

Table 3 Summary of various carbon base adsorbents used for BTEX removal from aqueous medium
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Different interactions control the adsorption mechanism of BTEX on carbon-based adsorbents including electrostatic interaction, hydrogen interaction, and π-π electron-donor–acceptor interaction [40]. A schematic representation of each interaction is shown in Fig. 6. Generally, these interactions are mostly influenced by the existence of functional groups on the adsorbent surface and the aqueous environment such as solution pH. Figure 6a shows π-π interaction between the aromatic double bond of BTEX compounds and the hexagonal network structure of the carbon which is the most dominant interaction existing in the adsorption process of BTEX on carbonaceous adsorbent as reported in many studies [2, 49, 78,79,80,81]. Adsorption mechanism is controlled by electrostatic interaction when the presence of surface functional groups and pH variation is significant [67, 81]. The occurrence of oxygen-containing functional groups serves as electron-donors and ring structures serve as electron-acceptors as illustrated in Fig. 6b. Dipole–dipole interactions result from interactions between atoms with low electron density and atoms with high electron density. Hydrogen bonding is assumed to occur when these electron-poor and electron-rich locations are closer together as presented in Fig. 6c.

Fig. 6
figure 6

Schematic interactions of BTEX on carbonaceous adsorbent represented by red dashed line: (a) π-π interaction, (b) electrostatic interaction, (c) hydrogen interaction

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5.2 Clay base adsorbents

The clays are typically referred to as the minerals that form the colloid fraction (< 2 µm) of soils, sediments, rocks, and water. They may be composed of both fine-grained clay minerals and crystals of other minerals with clay-sized grains such as metal oxides, quartz, and carbonate. Owing to the potential exchangeable ions exits on their surface, clays play a significant function in the environment by serving as a natural absorber of pollutants by taking up cations and/or anions through either ion adsorption or exchange [75, 76]. Clays can be categorised into a number of different groups, including kaolinite, sepiolite, smectites (montmorillonite, saponite), vermiculite, mica (illite), serpentine, and pyrophyllite (talc). Recently, there has been an increasing attention in using raw clay material as they pose many features as promising adsorbents such as low cost, high sorption capacity to absorb organic and inorganic molecules, and layered nature that act as a template to get attached to for the adsorbates and counter ions [57, 82, 83]. Numerous studies have shown that clay minerals have promising BTEX removal properties. A summary of clay-based adsorbents reported for BTEX removal is presented in Table 4.

Table 4 Summary of various clay-based adsorbents used for BTEX removal from aqueous medium
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The adsorption mechanism of BTEX on clay adsorbents can be attributed to the interaction between radicals in organo-clay and non-ionic polar organic compounds. The source of radicals in clay materials is aluminium, silanol groups, silicon, and organic radicals [68]. Polarities of the clay adsorbent and various non-ionic aromatic organic compounds can be used to explain affinity interactions between them [24]. The presence of π electrons in an aromatic ring causes the polarity of BTEX compound. Schematic representation of the mechanism of adsorption of organic compound on clay surface is represented in Fig. 7.

Fig. 7
figure 7

Proposed adsorption mechanism of BTEX compounds on clay adsorbents

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5.3 Polymeric resin-based adsorbents

Polymeric resins have a high degree of selectivity from activated carbons owing to their differences in hydrophobicity, pore size, and surface area. These resins have also evolved into highly effective adsorbents. In comparison to activated carbon, polymeric resins showed a high level of selectivity together with an economical and reliable performance [10]. The porous resins have undergone years of research to become effective adsorbents for the treatment of industrial effluents as well as the recovery of adsorbed organic chemical products for later reuse [84,85,86]. Chemical components and physical structures are the key factors in adsorption such as specific surface area, pore diameter, and particle size [45]. In general, a high surface area helps to boost the adsorption capacity of adsorbents. However, the adsorbents’ functional groups have a significant impact on their adsorption characteristics. Number of polymeric resin possess a torus shape with an exterior that is hydrophilic and an interior that is hydrophobic, which allows them to form host–guest complexes with various chemical molecules, particularly aromatics [52, 53].

Makhathini et al. studied the adsorptive capacity of polystyrene resins for the removal of BTEX compounds [43]. The study showed that at 25 °C, resin was found to adsorb 98% of benzene, 88% of toluene, 59% of ethylbenzene, 84% m-; p-xylene, and 90% o-xylene at an initial concentration of 14.47 mg/L. In another study, Omondi et al. used a customised hydrogel made from acrylic acid to remove benzene and some metals simultaneously from industrial effluents with an adsorption capacity of 2.81 mg/g for benzene [87]. In this study, the adsorption process of BTEX on polymeric adsorbent was explained with the assistance of cationic polymerisation reaction. Initially, the reaction between the polymer and the protic acid resulted a cationic polymer chain followed by the attraction between the cationic polymer and the aromatic ring as shown in Fig. 8. Table 5 shows various polymeric resins reported for BTEX adsorption from aqueous medium.

Fig. 8
figure 8

Reaction mechanism profile of benzene adsorption onto PBA hydrogel active sites: (a) carbocation formation; (b) adsorption process of benzene. Adapted from [87]

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Table 5 Summary of various polymeric resins used for BTEX removal from aqueous medium
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5.4 Zeolite

Zeolite is the general name for a group of natural and synthetic minerals composed of three-dimensional aluminosilicates with AlO4 and SiO4 in its structure. The presence of exchangeable cations like Na+, K+, Ca2+, and Mg2+ initiates the cation exchange property in zeolite [88]. Zeolites have received a lot of recent attention in several industrial adsorption and catalytic processes due to their microporous nature and great thermal and chemical stability. Also, zeolites have the ability to modify their structure and physio-chemical properties [55]. Different characteristics including their excellent thermal stability, rapid kinetics, high organic contaminant selectivity, high specific capacity, high ion-exchange capacity, large cage like empty spaces on the surface and most significantly, their low cost, make them mainly suitable as adsorbents for water treatment [89,90,91]. The cation-exchange properties of zeolites together with its high surface areas enhances their ability to adsorb organic molecules [92]. The presence of low concentration of surfactants in zeolites facilitates the formation of a monolayer of hydrophobic surface by cation exchange, which further make them suitable to attach with organic molecules [93]. Recently, Sobczyk et al. demonstrated fly ash-based zeolitic composite (ZCC) material for the removal of BTEX from aqueous solutions [94]. Different mechanisms (represented in Fig. 9) including (1) partitioning, i.e. entrapment of the given BTEX molecule into an organic, carbonaceous residue present in the examined composites, (2) the partially microporous nature and the shape of the pores of the materials favours the diffusion of BTEX into the cages of zeolite and the pores of the carbonaceous component, (3) the pH of the BTEX solution, and (4) presence of carbonyl (C = O) and carboxyl (-COOH) which favours polar organic attraction, were proposed for the removal of BTEX by as prepared zeolites. In another study, natural zeolite was applied as a permeable barrier for the migration of the BTEX plume for a bench-scale tank model [95]. The results showed that the zeolite barrier reduced the BTEX concentration up to 90% of the initial value up to 120 h. Table 6 summarises different zeolites reported for the adsorption of BTEX from aqueous medium.

Fig. 9
figure 9

The expected mechanism of the BTEX removal by the Na-X ZCC material. Adapted from [94]

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Table 6 Summary of various zeolites used for BTEX removal from aqueous medium
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5.5 Industrial and biomass wastes

In recent years, wastewater remediation focused on natural and biobased materials has attracted many researchers towards developing adsorptive materials for water treatment from industrial and agriculture wastes [96]. Organic wastes like biochars and biomass and microalgae have the capability to bound organic molecules into its microporous domains [97]. The removal process is influenced by the existence of chemical groups and indeed it boosts BTEX’s affinity, along with intramolecular and/or intermolecular interactions in the adsorbent/adsorbate system. Recently, Katnic et al. reported the use of sterilised plum pomace biochar for the effective removal of BTEX from waste water. The plum pomace was initially pyrolysed, followed by gamma radiation exposure made the biochar sterilised and it improved the adsorption property there by the BTEX removal efficiency improved up to 96% [98]. Table 7 summarises industrial and biomass waste-based adsorbents for BTEX removal from aqueous solution.

Table 7 Summary of various adsorbents from industrial wastes and biomass for BTEX removal from aqueous solution
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5.6 Others

Other than conventional materials used for adsorption process, adsorbents were developed for BTEX removal that were not extensively investigated by researches. For instance, Aivalioti et al. studied BTEX removal behaviour of both natural and modified diatomite [121]. They investigated the effect of thermal, chemical, and both thermal and chemical modifications on the removal efficiency of diatomite. They succeed to prove that the diatomite sample treated chemically with HCl being the most effective in BTEX removal with removal capacities of 0.0006 (B), 0.00062 (T), 0.0008 (E), 0.0018 (m,p-X), and 0.0008 (o-X) mmol/g [121]. In another study, the performance of metal-oxide nanoparticle for the removal of BTEX was done by Bartilotti et al. [26]. They proposed for the first time the use of vermiculite modified by magnetite (Fe3O4) nanoparticles to remove BTEX from aqueous solution. The outcome of their investigation was very promising as their proposed adsorbent was able to remove 85% of BTEX [26]. In another work, Li et al. synthesised thiol-functionalised covalent organic framework (COF-S-SH) for BTEX removal [122]. The prepared adsorbent performed well with highest adsorption capacities of 150.2–255.8 mg/g for BTEX and the removal efficiencies of 63.6, 82.1, 94.6, and 95.3% for benzene, toluene, ethyl benzene, and m-xylene, respectively [122]. On the other hand, the Zr-based metal organic framework, UiO-66, was prepared and investigated for BTEX removal by Amador et al. [123]. The results of their study were potential with removal capacities of 34.1 (B), 58.5 (T), 51.3 (E), and 147.1 (X) mg/g [123]. S. Kim et al. used co-condensation technique to designed octyl-functionalised and surfactant-containing mesoporous silica nanoparticle (MSN) and studied its BTEX adsorption ability. Their results demonstrated that the prepared adsorbent poses effective removal of BTEX with maximum capacities of 0.23 (B), 0.92 (T), 2.79 (E), and 2.24 (p-X) mmol/g [124]. Table 8 summarises some non-conventional adsorbents studied for the removal of BTEX from aqueous medium.

Table 8 List of various adsorbents used for BTEX removal from aqueous solution
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6 Future perspectives

  1. 1.

    The toxicity of the adsorbents used for water treatment is always a matter of concern. Its risk assessment study including its effect on the environment needs to be assessed properly. Designing new green materials with improved sorption capacity towards BTEX compounds is an area to be explored in detail.

  2. 2.

    Reducing the volatility of the BTEX, thereby enabling the detailed quantitative analysis of the compounds in the environment is a challenging task. Effective methods need to be developed to reduce the volatility of BTEX compounds.

  3. 3.

    Various surface modifications like functionalisation using suitable groups improve target specific adsorption. However, excessive presence of functional groups hampers adsorption process. There are little techniques available to quantitatively characterise the extend of functionalisation.

  4. 4.

    The exact interaction between various adsorbents and BTEX compounds is still not scientifically proven. With further assistance from various computational tools, a systematic method needs to be developed to understand the extend of interaction during the adsorption processes.

7 Conclusions

The adverse effects of BTEX compounds present in the environment towards the living organisms are a major concern. Effective and economic methods to remove hazardous contaminants from waste water are a major challenge and absorption is a globally approved unit operation for this issue.

When compared with the pros and cons of various technologies like catalytic degradation, biodegradation, membrane technology, and absorption for the removal of BTEX, adsorption proved to be more economic and environmental friendly approach. By adsorption, surface active materials especially organic molecules like benzene, toluene, ethyl benzene, and xylene can be effectively removed through interphase transfer. However, majority of the reported works focussed on batch mode and hence the long-term application of this method is still unknown. Design and development of novel adsorptive materials which addresses current limitations including insufficient pore size and reduced adsorption capacity, by introducing suitable functionalities onto the adsorbent surface to improve interaction with the contaminants, is highly recommended.

Different types of adsorbents like CNTs, zeolites, clays and its modifications, biowastes, and polymer resins have been used to remove BTEX from solutions. The main interactions occurring between the BTEX and the adsorbents include hydrophobic interactions, π-π interaction, and hydrogen bonding. Various factors including nature of adsorbent, temperature, and pH of solution play a significant role in optimising the adsorption capacity of adsorbents. The exact interaction between the adsorbent and adsorbate needs to be identified in order to explain the behaviour of the system and also for the future applications. Currently, GC–MS is extensively used to quantitatively and qualitatively characterise BTEX compounds due to its high sensitivity and resolution. The volatility of BTEX compounds affects the efficiency and accuracy of the measured values and thus methods to decease the volatility of these compounds in various atmosphere is a main area to explore. The toxicity of the adsorbents used is also need to be addressed. Therefore, effective strategies to develop designed systems like hybrid materials or processes to efficiently remove organic contaminants should be the focus of future research. More efforts need to be done to protect the environment and human health keeping rooted to using ecologically sustainable methods.