Produced water (water generated during recovery of petroleum) contains large amounts of various monoaromatic hydrocarbons, such as benzene, toluene, ethylbenzene, and xylenes (BTEX). In this study, a natural zeolite (i. e., clinoptilolite) was modified by cationic surfactants. The effectiveness of the surfactant-modified zeolite (SMZ) was evaluated for removal of BTEX from water. The results of these investigations show that SMZ removes BTEX from produced water up to a compound-specific capacity, and that SMZ can be regenerated via air sparging without loss of sorption capacity.
Keywords: aromatic hydrocarbons, zeolite, cationic surfactant.
In recent years, contamination generated by petroleum compounds has raised concern all over the world. Among the different hydrocarbons present in petroleum, monoaromatic hydrocarbons, including benzene, toluene, ethylbenzene and xylene isomers (BTEX), are a very important category of water contaminants. These volatile compounds are very hazardous because of their fast migration into soil and water bodies and their acute and chronic toxicities when inhaled or ingested, especially benzene which is a known carcinogenic molecule . With recent progresses in health sciences and detection techniques, strict concentration limits are proposed for these compounds in water. According to the World Health Organizations (WHO) drinking water standards, the allowable concentration for benzene in drinking water is 10 ppb, i. e., 10 μg/L .
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The main sources of BTEX compounds in water and groundwater are effluents entering the environment from petrochemicals, petroleum refineries and related industries such as paint or glue factories. In addition, continuous fuel leakage from underground gasoline storage tanks in urban areas with old and cracked storage tanks is another source. Accidents in the transportation of petroleum fuels and the fracture of old oil pipes are the other major causes of the release of BTEX compounds into the environment [3- 5].
Nowadays, there is growing interest in the use of natural inorganic adsorbents for treatment of water and wastewater. Zeolite molecular sieves are a well known aluminosilicate family of inorganic adsorbents. , 
Zeolites are a well-established technology used in a range of processes and industries. Zeolites are not new materials - they have been investigated for over two and a half centuries, with stilbite and natrolite both being identified in the 1750s. Industrial applications include catalysis in the petroleum industry, various uses in agriculture, horticulture, gas separations, domestic water treatment and nuclear waste processing. The value of zeolite catalysis to petroleum cracking is well in excess of $200 billion. About 50 naturally occurring zeolites have been identified; over 150 synthetic zeolites have been prepared and characterized; and further thousands of combinations of framework and composition are available. Zeolites have long been used in the nuclear industry owing to their properties as ion exchangers. The planned siting of the United States' first deep geologic radioactive waste repository at Yucca Mountain in Nevada, where design philosophy called for both engineered and natural barriers to inhibit the transport of any potentially leaking radionuclides, was influenced considerably by the local abundance of the natural zeolites mordenite and clinoptilolite, both of which have large cationic exchange capacities. 
Zeolites are crystalline aluminosilicates, compositionally similar to clay minerals, but differing in their well-defined three-dimensional nano- and micro-porous structure. Aluminium, silicon, and oxygen are arranged in a regular structure of [SiO4] - and [AlO4]- tetrahedral units that form a framework with small pores (also called tunnels, channels or cavities) of about 0.1-2 nm diameter running through the material (fig. 1).
Fig. 1. A typical zeolite structure.
Due to the high selectivity of different zeolites, they have been used for the removal of various cations from contaminated water and wastewater. Furthermore, natural clinoptilolite can be used as a starting material for the synthesis of value added synthetic zeolites. In our previous works, natural clinoptilolite-rich tuffs were converted into zeolite P using hydrothermal processes]. Recently, a simple, fast and cheap single step procedure was also developed by our group to synthesize submicron zeolite LTA using a natural clinoptilolite-rich tuff. Surface modification of zeolites can lead to suitable adsorbents for anionic and molecular contaminants as well [9, 10].
Surfactant modified zeolites (SMZ) adsorb the major categories of water contaminants including anions, cations, organics, and pathogens (fig. 2) . These unique multipurpose characteristics make them applicable for the treatment of a variety of contaminated waters. SMZ can be used in combination with other techniques, such as chemical reduction by zero valent iron (ZVI) and biological degradation, to provide more effective decontamination of polluted water. However, for successful application of SMZ to environmental remediation, some techno-economical challenges such as the physicochemical durability of SMZ should be addressed to improve the long term chemical and physical stability of SMZ. Research has shown that the HDTMA surfactant layer of modified zeolite slowly washes off under continued leaching, although it is not readily displaced by aqueous cations and is resistant to biological and chemical degradation. Greater physicochemical stability of SMZ may be achieved by using other surfactants, zeolites from different sources and by heat treatment of the parent and modified zeolites .
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Figure 2. Schematic model of the partitioning of a typical monoaromatic hydrocarbon.
2. Experimental part
The materials that were used are:
Clinoptilolite (zeolitic tuff).
Hexadecyltrimethylammonium bromide (HDTMA-Br) (Fluka).
1. X-ray DRON CoKα X-ray difractometer.
2. Optima 2100 DV (Perkin Elmer) inductive coupled plasma spectrometer.
3. For gas chromatography analysis, it has been used a Carlo-Erba chromatograph apparatus, model 2450, with column of 3m, Chromosorb W sorbent (60-80 mesh) and dodecylphthalate- bentone filling.
2.3. Adsorbent preparation
The clinoptilolite samples were dried and activated in an oven for 24 hours, at 2000C, to remove any probable volatile organic materials. To prepare the surface modified zeolites, 4g of zeolite were contacted with 100 ml of HDTMA-Br solution (30 mmol/liter). The recipient was gently stirred for 12 hours, using a magnetic stirrer. The surface modified samples were then filtered and washed several times with distilled water until no foam was present when shaking the supernatant. The surface modified zeolite sample was the dried in an oven, at 1000C, for 24 hours.
To prepare the stock solutions of aromatic hydrocarbons, 1 ml of benzene and 1 ml of toluene were added into 50 ml volumetric flasks. The solutions were diluted with ethanol to 50 ml. The stock solutions were mixed by shaking the flask. The initial concentration was calculated. The standard solutions were prepared by spiking 1 ml of each stock solution into 100 ml volumetric flasks, and then diluted to 100 ml with distilled water.
2.4. Aromatic hydrocarbons adsorption tests
Prior to the experiments, the flasks were washed with acetone and distilled water and then dried at 1500C for one hour. For the adsorption tests, 0.2g of surface modified zeolite (SMZ) was exposed to 20 ml of the prepared aromatic hydrocarbon standard solutions. The mixtures were gently stirred for 24 hours at room temperature. The suspensions were then filtered and 5 ml of each supernatant solution were subjected to gas chromatography (GC) to determine the aromatic hydrocarbons concentration.
3. Results and discussions
After the elemental analysis of the clinoptilolite sample, the following results were obtained and listed below (table 1):
Table 1. Elemental analysis of the clinoptilolite sample.
Element concentration, ppm (mg/kg)
Recalculated conc. (% w/w)
The XRD pattern (fig. 3) and data (tabel 2) showed that the main lines appeared at a relatively high intensity at similar d-spaces as the reference clinoptilolite. The lines at 8.98, 3.96, and 2.79 with relative intensities higher than 40% have been observed in test samples and reference clinoptilolites. It can be concluded that clinoptilolite (i. e., HEU structure) was the major component of the zeolite used in this study.
Figure 3. XRD spectra of the clinoptilolite sample.
Table 2. XRD data of the clinoptilolite sample.
By analyzing the hydrocarbons concentration, before and after treatment with the impregnated zeolite, there could be observed a total retention of such hydrocarbons inside of the zeolite structures, this conclusion being valid both for benzene and for toluene (fig. 4). If the initial concentration of hydrocarbons was 200 ppm, after treatment with the impregnated zeolites, the final concentration was less than 10 ppm, practically undetectable for the gas chromatograph. Both hydrocarbons show similar retention time (2.54 for benzene, and 2.24 for toluene, respectively).
Figure 4. GC analysis of the adsorbed monoaromatics on SMZ.
The results of the GC analyses show that the SMZ was a very efficient adsorbent of the monoaromatic hydrocarbons present in the water samples, showing that it can be used to treat waters that are contaminated with aromatic hydrocarbons that appear during petroleum recovery and refining.