The effect of temperature on hydrocarbon adsorption by diphenyldichlorosilane coated zeolite and its application in permeable reactive barriers in cold regions
Introduction
Worldwide there are many reports of serious groundwater pollution resulting from accidental spills or improper disposal, including in cold regions (Bargagli, 2008). In attempts to clean-up groundwater and restore contaminated sites to near pristine conditions, scientists started to develop technologies such as the pump-and-treat method in the 1980's (Thiruvenkatachari et al., 2008). In the last decade, permeable reactive barriers (PRBs), filled with reactive materials to intercept and decontaminate groundwater plumes have become one of the most promising remediation technologies (Obiri-Nyarko et al., 2014). Compared to traditional pump-and-treat systems, PRBs generally have a longer serve life and lower operational cost (Bortone et al., 2013a). In Antarctica, similar practices were also conducted to treat petroleum-contaminated and heavy-metal-laden sites (Mumford et al., 2013) and at this stage, the major areas of research to improve PRB performance include improving the longevity of the reactive materials used in these systems and their adsorption capacity towards various contaminants (Bortone et al., 2013b, Erto et al., 2014, Santonastaso et al., 2015, Snape et al., 2001). Materials that rely solely on chemical or physical processes have a limited operational life as once the materials are saturated, they will no longer perform as required (Obiri-Nyarko et al., 2014). The combination of bioremediation and physicochemical adsorption is a more promising and viable method (Freidman et al., 2016). As long as microbes are present and stimulated, the system will operate (Thiruvenkatachari et al., 2008). Previous tests have demonstrated that indigenous microorganisms present in soil and water can migrate and thrive, on and between adsorbent materials in PRBs and degrade the hydrocarbons as they migrate through the system (De Jesus et al., 2015).
To enhance the bioremediation process, improving the reactive materials in PRBs is essential (Scherer et al., 2000). Among the available options, modified zeolites may present better characteristics than granular activated carbon (GAC) despite of its high adsorption capacity (Bortone et al., 2013b, Erto et al., 2017). Contaminants are not bonded as tightly onto modified zeolite surfaces, and so are more easily utilised by the microbes present (Mumford et al., 2015, Zhang et al., 1991). Zeolites have a rigid three dimensional structure of aluminosilicate and a highly microporous surface and a net negative charge that is balanced by electrostatically held cations (Torabian et al., 2010). Thanks to its unique structural characteristics and surface components, natural zeolite is usually used to capture target cations through ion exchange, with no applications in anions and organics adsorption (Mahabadi et al., 2007, Widiastuti et al., 2008). Research has focused on the development of modified zeolites with higher hydrocarbon adsorption capacities compared to natural zeolite (Wang and Peng, 2010). One of the most frequent methods is the use of cationic surfactants that are bound to the external zeolite surface via electrostatic forces. Examples include hexadecyltrimethylammonium bromide (HDTMA) or cetylpyridinium bromide (CPB) (Wang and Peng, 2010). This treatment reduces or reverses the charge of zeolite and thus leads to anions or non-polar organics being captured on the surfactant coating (Wang and Peng, 2010). This process has been extensively investigated and presented good outcomes compared with natural zeolite in removing soluble hydrocarbons from groundwater with a summary of the results presented in Table 1 (Bowman, 2003, Ghiaci et al., 2004, Kuleyin, 2007, Li et al., 2000, Michael Ranck et al., 2005). Despite this promising performance, cationic surfactants like HDTMA or CPB still face many challenges, such as the loss of the surface coating and the disintegration of the zeolite particle itself (Michael Ranck et al., 2005). Therefore, alternative techniques are needed. Here chlorosilane is selected as it can be covalently bound to the surface of zeolite thereby improving the material stability while still enhancing its adsorption capacity.
Chlorosilane is a reactive chemical compound with up to three nonpolar aliphatic or aromatic functional groups which can be covalently grafted to the silanol groups on a mineral adsorbent surface (Huttenloch et al., 2001). This treatment provides a stable chemical linkage between organosilane and the substrate instead of the weaker electrostatic interactions of cationic surfactants, which greatly reduces the loss of hydrophobic coating during its use (Northcott et al., 2010a). Previous research has shown that octadecyltrichlorosilane modified natural zeolite possesses high capacity in removing dissolved o-xylene and naphthalene from water (Northcott et al., 2010a) and the octylmethyldichlorosilane coated synthetic zeolite possesses a similar toluene adsorption capacity to GAC (Arora et al., 2011, Song et al., 2005). In addition to the use of zeolites as base materials, there are studies regarding the grafting of chlorosilane onto GAC to enhance 2,4-dichlorophenol removal, by improving its hydrophobicity in permeable reactive barriers (Yang et al., 2010). Though chlorosilane is less likely to be leached off from the surface of the substrate, one of the most significant challenges is finding the most efficient organosilane to enhance sorption (Huttenloch et al., 2001). In previous work, chlorosilane with phenyl headgroups have been shown to have better affinity for aromatic compounds compared with others (Huttenloch et al., 2001), and therefore can be viewed as potential coating material to enhance hydrocarbon capture.
The basic silanization process (Huttenloch et al., 2001) for surface modification on zeolite is shown in Fig. 1. In the presence of pyridine and elevated temperatures, the chlorine on silicon compounds has a high affinity to the hydrogen of hydroxyl groups on zeolite surface. This results in the combination of H+ and Cl− of the two compounds and as such the formation of a stable SiOSiC bond, leading to chlorosilane covalently grafted to the zeolite surface. The more hydroxyl groups on the zeolite surface, the more organosilane molecules with phenyl groups that may be attached, leading to a better adsorption capacity towards organic contaminants.
Besides maintaining high adsorption capacity, regeneration is another important issue for the longevity of PRBs (Obiri-Nyarko et al., 2014). In large scale tests, air sparging is commonly applied to remove adsorbed hydrocarbon from the sorbents (Michael Ranck et al., 2005, Soto et al., 2011), and previous researchers (Northcott et al., 2010a) have shown that water or organic solvent can be used to regenerate the materials in batch tests and that regenerated modified zeolite can be reused for hydrocarbon sorption at least 3 times. Normally the organic solvent selected for regeneration is methanol (Northcott et al., 2010a), as it is the simplest alcohol and most hydrocarbons may be dissolved in it. However as this material is to be used for environmental remediation purposes, it is important to minimise waste, therefore, it is worthwhile undertaking regeneration tests in water as well.
In this study, diphenyldichlorosilane (DPDSCI) modification was selected based on the results of previous work (Huttenloch et al., 2001). Beside the general surface characterization and normal temperature adsorption tests, low temperature adsorption tests and regeneration experiments were also performed to determine the capacity and longevity in the cold conditions. Furthermore, different isotherms and thermodynamics of adsorption were calculated to analyse the specific adsorption process of DPDSCI modified zeolite.
Section snippets
Raw materials
The raw zeolite material used was natural clinoptilolite zeolite (Castle Mountain Zeolite, Quirindi, N.S.W., Australia). It was sieved with an 8 × 16 US mesh sieve (2.36–0.85 mm) and measured to have a BET surface area of 18.01 m2/g (Freidman et al., 2016). Energy Dispersive Spectroscopy (EDS) Microanalysis of raw zeolite indicated that it had the following composition (% atomic weight): O-66.99; Al-6.99; Si-24.85; Fe-0.16; Na-0.8; K-0.11; Ca-0.21; N-0.00; Mg-0.02; S-0.00; P-0.01 (Freidman et al.,
Modelling
Modelling of the adsorption experiments was conducted using the Langmuir, Freundlich and linear isotherms. These isotherms are most commonly used in batch experiments to describe a system of solute molecules adsorbed onto material surface (Arora et al., 2011).
Thermogravimetric analysis
The results of the thermogravimetric analysis for the modified and unamended zeolite are presented in Fig. 2. This figure shows the weight loss curves (TG) and the corresponding differential curves (DTG) of the materials, which indicates the endothermic and exothermic reactions during the heating of the samples to 900 °C. As shown, the curves of modified zeolite are different to that of raw zeolite with different mass change steps and peak temperatures. The change in untreated zeolite as best
Conclusions
According to the results from TGA and FT-IR, the chlorosilane, diphenyldichlorosilane was successfully coated on to the surface of a natural zeolite without change to the internal porous structure. Batch adsorption tests showed a good capacity in adsorbing toluene by DPDSCI coated zeolite, compared with the raw material. The adsorption study demonstrated that the Langmuir isotherm fitted the adsorption best, indicating its monolayer and homogeneous character. The thermodynamics calculation
Acknowledgement
This study was supported by the Particulate Fluids Processing Centre, a Special Research Centre of the Australia Research Council. The Australian zeolite was supplied by the Australian Antarctic Division. Thanks to University of Melbourne and Chinese Scholarship Council for scholarship support.
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