2.7. Remediation of PAHs contaminated environment
2.7.1. Physical methods
sensitive target analytes where distillation fails and is among the oldest and most widely considered methods. This method has been considered for the removal and quantification of PAHs in contaminated water by environmental protection agency (EPA) having EPA number of 6440B (American Public Health Association et al., 2005). Generally, for the extraction or removal of PAHs from aqueous medium methylene chloride or a combination of methylene chloride and hexane is utilized. If the quantification of PAHs is needed, the extract is further concentrated and analyzed on gas chromatographic coupled with mass spectroscopy (MS) detector (American Public Health Association et al., 2005). However, there are still much concern regarding the usage of LLE for PAHs removal as it usually contains a high volume of toxic, flammable, volatile and sometimes chlorinated organic solvent.
2.7.1.2. Liquid-phase microextraction (LPME)
The rising emphasis on the clean environmentally remediation processes has subjected LLE to incessant denunciation owing to the high utilization of hazardous, combustible, volatile organic solvents and cost-ineffectiveness (Robles-Molina et al., 2013) which has led to the miniaturization of the LLE process to the liquid-phase microextraction process (LPME) (Ratola et al., 2008). whereby only a few µL of solvent is required to remove the analytes from aqueous solution rather than traditional LLE method that utilizes several mL of the solvents (Sarafraz-Yazdi and Amiri, 2010). They have further categorized into three broad types: single-drop microextraction (SDME) (Wu et al., 2008) hollow-fiber microextraction (HF-LPME) and dispersive liquid-liquid microextraction (DLLME) (Sarafraz-Yazdi and Amiri, 2010; Shi and Lee, 2010).
2.7.1.3. Filtration method
Filtration method is a process that removes solutes from fluids through a medium (filter) by permitting the fluid to pass and retains the solutes. It could either be mechanical, physical or biological. Membrane separation is the common technique used by the filtration process for PAHs removal. Considering the low size of the filtrate (PAHs), microfiltration has not really been considered for PAHs removal as the pore size of microfiltration membranes (MF) are much larger than the particle size of PAHs (Smol and Włodarczyk- Makuła, 2016). Thus, reverse osmosis (RO), forward osmosis (FO), nanofiltration (NF) and ultrafiltration (UF) are the most common processes used for PAHs removal.
2.7.1.3.1. Reverse osmosis (RO)
Reverse osmosis (RO) is a process that is power-driven by an external hydrostatic pressure that push the solution in against direction of natural osmosis through the membrane. There are various reports on the utilization of different membranes for PAHs removal from contaminated water. In the municipal wastewater, PAHs concentration was reported a removal efficiency of 81–86% using the commercially available SEPA CF-NP membrane. Also, for PAHs removal from the contaminated leachates, nylon membrane (ADF) was used as RO membrane. Nevertheless, many of the highly efficient membranes cannot be used for the RO process due to its high-pressure demand (2 MPa) leading to high energy demand (Smol et al., 2014).
2.7.1.3.2. Ultrafiltration
Ultrafiltration (UF) technique utilizes an external hydrostatic pressure that thrusts the sample through a semipermeable membrane that removes PAHs from the solution. It is commonly used to remove the particles with sizes > 10-20 nm that led to the disinfection of contaminated water by removing pollen, algae, fungi, bacteria, viruses, and organic solutes.
It also requires lesser pressure (0.1-0.2 MPa) to operate in comparison to RO. For PAHs and phthalates, retention coefficients ranged between 50.0–99.9% (Dudziak et al., 2004) whereas in the presence of humic acid, around 97% of anthracene was removal was achieved (Smol and WłodarczykMakuła, 2016; Yoon et al., 2004).
2.7.1.3.3. Nanofiltration
Nanofiltration (NF) is an another pressure-driven membrane filtration processes that retains some of the useful minerals in the water and uses smaller energy (Shon et al., 2013).
NF are capable for selective removal of target solutes (small uncharged solutes) from the complex samples because its pore size is very small (∼ 1 nm) (Shon et al., 2013). NF can be used for PAHs and pesticide removal from contaminated drinking water due to the strong hydrophobic interactions between these compounds and NF membrane (Sanches et al., 2011). However, the membrane fouling along with the pressure driven processes that require more energy and special apparatus requirements are the major issues related with RO, NF and UF (Mohammad et al., 2015).
2.7.1.3.4. Forward osmosis (FO)
For addressing the fouling and energy extensive nature of RO, NF, and UF, forward osmosis is recently explored as a potential filtration process that is relatively more advantages to the other pressure-driven methods with less energy demand and less prone to fouling along with a high water reclamation (Akinpelu et al., 2019). FO involves the natural osmotic pressure gradient to propel aqueous sample (low osmotic pressure) across the semipermeable membrane to the draw solution (high osmotic pressure) side (Zhao et al., 2012). Li et al., (2017) reported the removal of PAHs from landfill leachate by using FO technique. However, FO method is not commercially and practically viable as the process is extremely slow to match up with the present industrial need. This has led the researchers to
look for better membranes such as carbon nano-tubes (CNTs) or CNT-based materials (Das et al., 2014). In general, CNTs as nanofilters can be functionalized to improve its performance and making it more selective (Ong et al., 2010). Taking these into consideration, the application of CNTs in the filtration process could be useful for PAHs removal from contaminated wastewater with some better improvisations.
2.7.1.4. Adsorption
Adsorption is an another physical phenomenon in which the adhesion of the particles (pollutants) takes place onto surfaces or interfaces of the solid adsorbent through the interaction with contaminated water (Ali et al., 2012; Saleh 2018). It has commonly been used for PAHs removal of PAHs because it is very fast, simple to use and high recovery (Ma et al., 2010). Materials such as porous organoclay composite, matrix- immobilized organoclay, polyvinylidene fluoride, biochar and bamboo charcoal are considered to be the potential adsorbents for PAHs (Ma et al., 2010). However, for recovering high molecular weight (HMW)-PAHs is still a challenge thereby creating a need for further advancement (Ma et al., 2010).
2.7.1.4.1. Alkyl bonded silica sorbent
Bonding of alkyl chain (C1 - C30) to silica gel base material leads to the formation of alkyl-bonded silica adsorbent e.g., octadecyl (C18) (Spivakov et al., 2006). This alkyl bonded silica creates a hydrophobic phase leading to a stronger affinity for the hydrophobic compounds. Limam and Driss, (2013) reported the HMW-PAHs from the aqueous solution using a C18 cartridge in addition with 10% organic modifier. Li et al., (2007) examined the effect of alkyl chain length on PAH removal and have reported that with the increase in the number of carbon atoms, increases the distribution coefficients of PAHs in the porous layer of polymeric C18 (Ma et al., 2010).
2.7.1.4.2. Copolymer sorbents
Copolymer sorbents (CS) comprise of two monomers module in balanced proportion for instance, hydrophilic-lipophilic balanced polymers (HLB) e.g., lipophilic divinyl benzene vinyl pyrrolidone and hydrophilic N-vinyl pyrrolidone. Simultaneously, the hydrophilic component of CS helps in high mass flow of solution and lipophilic part deals with the reverse phase adsorption of PAHs in the flowing solution (Song et al., 2012). He et al., (2014) utilized the magnetic nanoparticles for organochlorine pesticides and triazine herbicides removal from contaminated water samples.
2.7.1.4.3. Magnetic sorbents
Recently, the magnetic solid-phase based sorbents is a new technique for the removal and concentration of target compound from a large volume of the sample (Gao and Chen, 2013). The magnetic or magnetizable adsorbents adsorb target analytes in the crude sample matrix and then the adsorbent with the adsorbate on it is further recovered from the solution by apt magnetic separator for its quantification analysis (Andrade-Eiroa et al., 2016; Zhang et al., 2011). Hayatsu, (1992) prepared cotton containing covalently linked copper phthalocyanine trisulphonate as magnetic SPE to remove PAHs from the complex sample matrix. Wang et al, (2013) demonstrated that microsphere-confined graphene adsorbent successfully extracted five PAHs from contaminated wastewater. However, the magnetic adsorbents are thermally thermally unstable during desorption procedures at high temperatures highly sensitive to pH (pH < 4.0) driving Fe (III) to form chelates with the target analytes such as sulfonamides (Andrade-Eiroa et al., 2016).