Wastewater Treatment Using Activated Carbon Produced from Oil Shale

In recent years, many researchers have expressed interest in wastewater treatment using activated carbon pro - duced from cheap raw materials. In this work, an activated carbo-aluminosilicate (ACS) – supported zero-valent iron (ZVI) composite was produced from Um AL-Rasa oil shale mine and examined to eliminate Chromium (VI) from contaminated water. Activation of raw oil shale fine particles (< 212 μm) was chemically performed using 95 and 5% wt of H 2 SO 4 and HNO 3, respectively, as activating agents. The activated material was further treated with caustic soda, named ACS, and modified with fine zero-valent iron particles < 212 μm), called ZVI/ACS composite. Kaolin was added to the composite with the ratio: (50 % wt. light kaolin: 50 % wt. ACS), named as ZVI/ACS/K. The XRD analysis for both composites confirmed iron dispersion at 45°. Adsorption experiments were carried out using the two adsorbents ZVI/ACS & ZVI/ACS/K under different values of pH, and adsorbent dosage. The results indicated that the reduction of Chromium was maximum under the 3 pH value and 2.0 gm amount of ZVI/ACS/K. Furthermore, it was found the removal rate was enhanced by 17% and 24.7% when ZVI/ACS & ZVI/ACS/K adsorbents were used, respectively, compared to that when only ACS adsorbent was used alone. Finally, the de - pendency of Chromium removal on its initial concentration by ZVI/ACS/K adsorbent was also investigated at two different temperatures of 27° and 50°. The results indicated a decrease in the removal rate of the Chromium as the concentration increased at 27°; however, the removal rate previously enhanced at 50° at all initial concentrations.


INTRODUCTION
In recent years, Jordan has suffered from limited water resources and may not be able to meet its freshwater demands shortly. Therefore, securing freshwater resources in terms of quality and quantity is becoming more and more important. One of the main sources that cause water contamination with metals (including Chromium, and Copper) is the industry and exploitation of oil shale.
Nowadays, elimination of pollutants from contaminated water by activated carbon is widely used; it is proven to be a successful method for  Yahya et al. (2020) proposed cobalt ferritesupported activated carbon (CF-AC) for the elimination of Lead and Chromium ions from contaminated water. They concluded that the ecologically friendly alternative adsorbent created from CF-AC can be employed effectively to remove Cr and Pb(II) ions from tannery wastewater. ZVI was found to be excellent at removing pollutants from wastewater, such as toxic metal ions (Ullah et al., 2020., He et al., 2020). Extensive research work was conducted on removing metals from wastewater using ZVI stabilized on carbon material, (Qu et al., 2017;Zhou et al., 2014;Congbin et al., 2021).
In recent years, Jordan has suffered from limited water resources and may not be able to meet its freshwater demands shortly. Therefore, securing freshwater resources in terms of quality and quantity is becoming more and more important. One of the main sources that cause water contamination with heavy metals is the exploitation of oil shale, which is one of the activities that take place in Jordan. Oil shale utilization technologies include mining, extraction, products, and byproducts, etc. which could pose serious risks to the surrounding environment if these issues are not addressed and treated correctly The present research focused on ZVI stabilization on an activated carbon material produced from Jordanian oil shale and examined its ability to remediate hexavalent chromium ions.

Oil shale chemical activation
As a start, 50 grams of oil shale was activated using both 95% wt. H 2 SO 4 and 5% wt. HNO 3 at 25 °C. The temperature of the product was increased rapidly to 150° indicating an exothermic reaction, as well as gas bubbling,was observed indicating that the reaction is taking place. Then, the solution was heated further to 270° on a hot plate while CO, SO 2 , carbon monoxide, carbon, sulfur dioxide, and nitrous oxide( N 2 O) were expelled out of the solution.
Once the product becomes solidified, it is cooled down to 25 °C, then washed with distilled water to remove any excess acids. The activated oil shale sample (AC) was further treated and mixed overnight with (1M ) sodium hydroxide, and then heated in a closed vessel in an oven for 24 hrs. at 160°. The sample was washed to remove excess alkaline; afterwards, the sample was dried at 70° and stored. The obtained sample is assigned as (ACS).

ZVI stabilization on ACS
Initially, 1.5 gm of chitosan powder (100,000-300,000 g/mole) was dissolved in 80 ml (2%) acetic acid to ensure a homogenous solution, then 4.5 g of ZVI particles (less than 212 micrometers) was carefully incorporated into the mixture, and then 1.5 g of ACS material was added. The mixture was then stirred and added slowly to a 450 ml Sodium hydroxide, and then stored for twelve hours at room temperature. The mixture was then decanted to remove the solid product and then deionized water was used to eliminate Sodium Hydroxide. Finally, the obtained product was dried for 24 hours at 70 °C.

ZVI stabilization on ACS/Kaolin material
The same procedure in section 2.2 was repeated replacing the material ACS with (50 % wt. light kaolin: 50 % wt. ACS), and assigned as (ZVI/ACS) for ZVI supported on ACS material, and (ZVI/ACS/K) for ZVI supported on ACS and kaolin material. A schematic of the experimental steps of the production of the two materials is shown in Figure 1.

Adsorbents optimization
To evaluate the optimum condition of Cr (VI) adsorption by the produced composites, and have a better understanding of the adsorption characteristics, the eff ects of pH, and dosage, on the removal rate were investigated as discussed in the following sections.

Optimized pH
Adsorption experiments were carried out at diff erent pH values: (10,8,6, and 3) at fi xed adsorbent mass (0.05 g), which were added to 10 ml of 50 ppm potassium dichromate solution, for both ZVI/ACS and ZVI/ACS/K materials. The pH was controlled by adding drops of HCl or NaOH using a pH meter. After placing it in an  isothermal shaker at (T = 27 °C) for 24 hrs, the solutions were fi ltered through a 0.45-micrometer fi lter paper; then, the residual Cr (VI) was estimated by a (UV) spectrophotometer.

Optimized dosage of the produced adsorbents
To optimize the dosage of the produced adsorbents, diff erent amounts of each adsorbent were used at the optimized pH and a temperature of 27 °C. The amounts used were: 0.05, 0.1, 0.2, 0.4, and 0.5 g for both ZVI/ACS and ZVI/ACS/K adsorbents.
Another batch experiment was carried out at diff erent initial concentrations of chromium (100 ppm, 200 ppm, 300 ppm ) at pH 3 and fi xed adsorbent mass of 0.2 g at two diff erent temperatures (T = 27 °C, T = 50 °C).

Characterization of adsorbents (ZVI/ACS, ZVI/ACS/K)
To determine the underlying crystal structure of the produced adsorbents and have a better understanding of the surface morphology, (XRD) analysis and (SEM) were carried out.

XRD analysis
XRD makes it possible to confi rm the crystallinity and structure of a sample. Figure 2, represents the results of XRD conducted only on ACS before the stabilization process. The presence of ZVI on ACS, and ACS/K materials was confi rmed by the XRD (Shimadzu -7000 maxima, Japan) analysis. The XRD patterns show that the ZVI/ACS and ZVI/ACS/K had additional diffraction peaks at 2 thetas of 45° corresponding to ZVI, as shown in Figures 3 and 4, respectively, which did not appear in Figure 2. The introduction of ZVI has changed the properties of the materials to be ferromagnetic that can be easily collected by a magnet.

SEM imaging
The images of (raw oil shale, AC, ACS, and ZVI/ACS/K) surfaces are obtained from a scanning electron microscope (SEM) model (Versa 3D, FEI). This analysis revealed the surface structure of the samples. SEM was carried out for the AC before and after treatment with NaOH to evaluate changes in their microstructures. In Figure 5a it may be noted that activation by acids has formed CaSO 4 (gypsum) and a variety of randomly distributed pore sizes due to the activation process. Comparing the two images, (5b and 5c) it can be inferred that a highly porous surface can be observed for the ACS due to the removal of CaSO 4 which resulted in a relatively more porous structure. In Figure 5d, the white color reveals the clay structure of the produced material and a randomly distributed pore size.

UV-visible spectrometry
Hexavalent chromium ion determination was performed using the direct method. As shown in Figure 6, the absorption band peaks at 348 nm. Another peak at 260 nm is noted, which is related to the charge transfer transition between the oxygen and the hexavalent chrome; however, this peak is not wide and maybe not be used for chromium presence (Hachair et al., 2018). At basic media (pH 10), there are two absorption bands. The fi rst one peaks at 290 nm, which corresponds to the absorption of the chromium compound as a result of charge transfer between O 2 and Cr(VI). The maximum peak is at 375 nm. The change in the wavelength from 375 to 348 nm when changing from basic to acidic conditions can be explained by the formation of the hydrogen bond as well as the split of a bond between O 2 and the Cr ion. CrO 4 -2 has higher absorbance, which means that it absorbs light at less energy than HCrO 4 or H 2 CrO 4 . (Hachiro et al., 2018).
Due to the impact of pH on Cr(VI) light absorption, two calibration curves were developed, one for basic circumstances (375 nm) at pH levels higher than 6.4 and one for acidic conditions (348 nm) at pH levels lower than Pka value (6.4). The calibration curve for acidic environments is shown in Figure 7. One can observe the linearity between 1 and 50 mg/L. Thus, the BeereLambert law can be used in this range. The linear equation is y = 0.0268x + 0.0089 with a correlation factor R 2 of 0.9988.  Figure 8 shows the calibration curve for (pH 10), as indicated the relation is linear in the range between 5 and 25 mg/L, BeereLambert law can be applicable for this range of linearity, using the equation y = 0.078x + 0.0542 with a correlation factor R 2 of 0.998.

The removal rate of Cr (VI)
The absorption rate of chromium at certain pH values is shown in Figure 9; as indicated, Cr 6+ was removed at low pH, while it decreases signifi cantly at higher values both ZVI/ACS and ZVI/ACS/K. This is due to the pH eff ect on speciation of Cr(VI), in acidic media with pH values lower than Pka (6.5) the hexavalent chrome ion in the form of HCrO 4 dominates, while under basic conditions at pH higher than Pka (6.5) the main form of Cr in solution is CrO 4 -2 (Lv et al., 2011). HCrO 4 − has less adsorption-free energy and more redox potential than CrO 4 2 ; therefore, it is favored in adsorption and easier to be reduced.   The absorption of Cr(VI) by ZVI is highly affected by protons. The concentration of H + on the particle surface increases with the decrease of the pH, this leads to the strong electrostatic attraction between Cr(VI) and the particle (Shen et al., 2020a).
The UV spectra of Cr(VI) solutions are shown in Figure 10 both before and after the adsorption with ZVI/ACS/K material at an optimum pH value of 3. These results indicate that the K 2 Cr 2 O 7 is catalyzed by ZVI/ACS/K material, and transforms into K 2 CrO 4 during the treatment. Figure 11 shows the eff ect of ZVI/ACS and ZVI/ACS/K dosages on the reduction rate of Cr(VI). It may be noticed that it increases with the adsorbent dose up to an equilibrium dose value which is: (40 g/L, and 20 g/L) for ZVI/ACS, and ZVI/ACS/K respectively. The increase in removal rate with an adsorbent dose can be explained by a high adsorbent surface beside the availability of more adsorption active sites; at a higher dose than optimum, the removal rate of adsorbent was almost constant. The reason for this behavior is due to the possible overlapping or aggregation of adsorption sites, leading to a lower total surface area. Figure 12, shows the removal rate of each adsorbent, it is noted that the introduction of ZVI on ACS & ACS/k materials has improved the removal rate from (73.9% to 90.9%) (73.9% to 98.5%), respectively, which confi rmed the enhancement of the removal due to ZVI addition and chitosan coating, as chitosan has the amine groups that have significant chelation to cationic metal ions. On the other hand, the use of kaolin to stabilize ZVI has a remarkable effect in increasing the removal rate. These findings indicate that both the kaolin and the ZVI particles on the ACS surfaces played a major role in the increased removal of Cr(VI). This is in agreement with the findings of (Xi et al., 2010).
To study the dependency of the absorption rate of Cr on its initial concentration, a Cr of 50 to 300 ppm was used. As indicated in Figure 13, the removal rate of Cr by ZVI/ACS/K decreases with the initial concentration. This is because as initial chromium concentrations increase, the adsorbent surfaces will be saturated, which agrees with the results obtained by (Noraee et al., 2019). Figure 14 illustrates that the increase in temperature from 27 to 50 °C results in enhanced reduction percentage from (27.4% to 98.5%), (24.2% to 59.0%), (19.5% to 35.5%) for the initial concentrations of 100 ppm, 200 ppm, and 300 ppm respectively. This is because of the dispersion of Cr(VI) ions and the higher kinetic energy of Cr(VI) ions heading toward the active sites of the adsorbent.

CONCLUSIONS
In this work, synthesis was developed to prepare ZVI/ACS and ZVI/ACS/K composite materials from Jordanian oil shale. The following points can be concluded. The presence of ZVI and light kaolin plays an important role in increasing the removal rate in the synthesized composite. The removal rate of hexavalent chrome by ZVI/ACS/K & ZVI/ACS is highly dependent on the pH value, with an optimum value at pH 3. The removal rate of hexavalent chrome by ZVI/ ACS/K & ZVI/ACS was found to increase with the dosage until reaching the equilibrium dose. The ZVI/ACS/K has shown a better ability to remove hexavalent chrome ions from aqueous solutions than ZVI/ACS. The removal rate was found to decrease with increasing the initial hexavalent chrome concentrations. The optimized adsorption conditions of chromium ions by ZVI/ACS/K were: (pH: 3, T = 50 °C, dose: 0.2 g, initial concentration: 100 ppm). ZVI/ACS/K was found to be an effective adsorbent for the removal of crystal violet dye from aqueous solutions. The two produced composites are ferromagnetic and could be easily collected by a magnet.