oil adsorption tests

The adsorption capacities were evaluated using several methods such as (i) classical standard (i) Westinghouse method, (ii) Total organic carbon (TOC) analyzer, (iii) gas chromatography (GC), and (iv) ultraviolet-visible spectroscopy (UV-Vis). Selection of analysis technique depends on the type of hydrocarbon model (octane- n-C8H18 95

%, decane- n-C11H24, 99 %, dodecane- n-C12H26, 99 %, toluene- C6H5-CH3, 99.5 % and kerosene)

Feedstock solution preparations and adsorption tests

Typical hydrocarbons present in produced/spilled oil-contaminated water include ali-phatic, alicyclic, and aromatic compounds. Accordingly, several hydrocarbon models were used for the adsorbents examination. It is worth mentioning that each hydrocar-bon model was treated and analyzed with a suitable technique. Table 5 presents a sum-mary of the performed experiments.

32

Table 5: Summary of the performed batch experiments Method/oil Carbon con.

3.4.2.1 Adsorption protocol of kerosene - water solution

The commercially available kerosene (EU number: 649-423-00-8, MOL Co.) was fur-ther purified. The obtained kerosene cut contained only alkanes from C10 to C16, and it was used for the preparation of kerosene-water mixtures. The kerosene cut had a boil-ing point range of 174–287 °C, with a density of 0.800 g/mL. The solutions were prepared in a glass flask by adding 175 µL kerosene to 250 mL distilled water, result-ing in a kerosene solution with a carbon concentration of 560 mg/L. The model solu-tions were mixed for 10 min using magnetic stirrer followed by adding the adsorbent in an amount of 100 mg. Then the solutions were kept under continuous mixing for an additional 30 min at room temperature. The adsorbent was separated from the solution with S1 porous glass filter having a pore size of 100–160 μm. The filtered water solu-tion was taken for the extracsolu-tion step, to prepare the sample for the determinasolu-tion of

1 Higher volume used due to extra extraction step needed before GC analysis

33

hydrocarbon content by gas chromatographic (GC) method according to Hungarian Standard MSZ 1484-7 (MSZ, 2009).

3.4.3.1 Adsorption of pure hydrocarbon - water solutions

The stock hydrocarbon-water solution was prepared by using as model hydrocarbon;

undecane, octane, toluene, and dodecane in range of 450-550 mg C/L. Distilled water and surfactants (sodium salt of secondary-alkane-sulfonate (SAS) or sodium-lauryl-ether-sulfate (SLES) solution was prepared. About 5 m % surfactant calculated for the hydrocarbon content was added into the hydrocarbon-water mixture to stabilize the emulsion. The solutions were kept closed and shaken at a speed of 600 rpm for 1 h.

Then, the solutions were kept under sonication for 5 min before use.

TOC and UV-Vis analysis were carried out at fixed hydrocarbon-water emulsion vol-ume (100 mL) containing hydrocarbons, while GC analysis was carried out with 250 ml. Adsorbents were added at optimum dosage, whereas all experiments were con-ducted at room temperature. The model solutions were mixed for 10 min using mag-netic stirrer followed by adding the adsorbent in an amount of 100 mg.

The dosage of zeolite-based adsorbent was varied from 0.03 to 1 g to find the optimal value for maximum removal of hydrocarbon from water. Regarding carbon-based ad-sorbent, only 10 mg dosage of MWCNTs adsorbent was used to examine the MWCNTs and µEMWCNTs adsorption capacities. The samples were agitated using a mechanical shaker at a fixed agitation speed of 300 rpm.

Instruments and protocols used for the analysis of hydrocarbon concen-tration measurements in water

The following analytical techniques were used to determine the hydrocarbon removal efficiency of the adsorbents from hydrocarbon-water mixtures:

 Classical Westinghouse method of absorbability;

 Total organic carbon analyzer (TOC);

 UV-visible spectroscopy (UV-Vis); and

 Gas chromatography (GC).

34

3.4.4.1 Classical Westinghouse method of adsorption

Naturally, primary attempts to investigate the hydrocarbon adsorption from the water had employed the simplest classical method. The classical Westinghouse method of absorbability developed by Muir and co-workers (Muir and Bajda, 2016) was used for this purpose. This test based on oil recovery/ water uptake at room temperature (25±2

oC) to indicate the performance of sorbents. The adsorption test consisted of adding drops of hydrocarbon compounds to the dry zeolite sample with a known weight (1.5 g) until the point of maximum saturation was reached. The moment when the next drop of hydrocarbon compound flowed over the sorbent (i.e., the liquid was not absorbed) was considered to be the end of the experiment. As the deep cavities are filled, mole-cules have to diffuse through a large number of intercrystalline pores to find suitable empty cavities. Once the cavities are filled, then molecules being adsorbed in cavities near the outer surface. It is to be noted that, in this test, pure hydrocarbons were used instead of the hydrocarbon-water model. The sorption percentage was determined by comparing the weights of the samples before and after the sorption of hydrocarbon compounds.

3.4.4.2 Protocol for samples analysis via total organic carbon analyzer (TOC) The hydrocarbon concentrations were measured by using a combustion type TOC an-alyzer (Elementar, model Vario TOC SELECT; detection range of 4 mg/L to 40 000 mg/L). Initially, all the glassware was washed with 2 % nitric acid and ethanol to re-move all the impurities and to avoid any further adsorption of dust or particles from the air. Three samples from the treated solution were taken for the initial concentration measurements, and average values are reported here. For final concentration measure-ment, 40 mL samples were taken at each time interval for TOC measurement. At each test, the system was calibrated using an appropriate IC (inorganic carbon) and TC (total carbon) standard solutions. For quality control, blank samples, as well as TC and IC samples, were also tested. All the experiments were performed in triplicate, and the average values are reported. The measured values were all within the range of ± 3%.

3.4.4.3 Protocol for samples analysis via UV-Vis spectrophotometric

Toluene was selected as a model hydrocarbon for the UV-Visible spectroscopic studies since this technique is suitable for the identification of aromatic hydrocarbons in water.

35

The ultraviolet-visible spectrophotometry (UV-Vis) using a deuterium lamp can be used for the determination of the aromatic hydrocarbon content of water samples. Ni-colet Evolution 500–UV-Visible double beam spectrometer (Thermo Electron Co.) with 1 cm quartz cuvette was used with measuring a range of 230-400 nm wavelength and equipped with single photomultiplier to detect the aromatic hydrocarbon content of the waters. The preparation of the model toluene-water mixture was the same as given earlier. The surfactant was used in 5% to stabilize the hydrocarbon-water emul-sion. After 30 min of running the adsorption test, the adsorbent was separated by fil-tration, and the organic phase (aromatic hydrocarbon) was extracted from the filtrate by cyclohexane (99.99%, Reanal Ltd.), and the extract was dried over sodium sulfate.

Quartz cuvette was used for spectrometric investigation of the obtained sample.

3.4.4.4 Protocol for samples analysis via gas chromatography (GC)

The liquid samples were analyzed by Agilent, GC 7890A type Gas Chromatograph with a J&W HP-5 type capillary column (30 m x 0.320 mm, 0.25 µm film thickness).

Flame-ionization detector (FID) was used for the analysis. As mentioned earlier, the determination of hydrocarbon content by gas chromatographic (GC) method was car-ried out according to Hungarian Standard MSZ 1484-7 (MSZ, 2009).

It is worthy of mentioning that the samples needed some treatment steps before per-forming GC analysis. Therefore, before the extraction with hexane, 1-chloro-octade-cane standard was added to the sample to determine the recovery efficiency of the hydrocarbons. The kerosene-water solution was extracted two times, with 15 mL hex-ane. The collected hexane fraction was dried with Na₂SO₄ powder. The blank solution was prepared in the same method without adding adsorbent. Kerosene standard was prepared by dissolving 175 µL kerosene in 25 mL hexane to determine the manipula-tion efficiency of the kerosene–water sample. An internal/injecmanipula-tion standard was added to the 25 mL hexane solution, and an aliquot of 2 µL was injected into the gas chro-matograph. An injection standard, 1,4-dichlorobenzene, was used for control of the GC analysis.

In the next two chapters, the results obtained from characterization and adsorption tests of carbon nanotube-based adsorbents and zeolite-based adsorbents will be presented.

36

The results and discussion will be given as per the type of adsorbents (i) carbon nano-tube-based adsorbents and (ii) zeolite-based adsorbents. The final conclusions will be presented in Chapter 8.

37

Chapter 4: Results of MWCNTs based