Enhanced Adsorption of Aromatic Hydrocarbon-contaminated Aquifer Using Granular Nano Zero-valent Iron

Aromatic hydrocarbons are toxic pollutants that enter into environment through various industries. These pollutants are carcinogenic and cause genetic mutations. There are various solutions, including biological methods, extraction, and electrocoagulation. This research aims to synthesize the nano zero-valent iron (nZVI) from the ferrous waste and granules of nZVI by the chemical combination of nZVI with polyvinyl alcohol (PVA). The performance of these two adsorbents was evaluated to degradation of phenol from an aqueous solution. The physical properties of the synthesized nanoparticles were determined using SEM analysis. Effect of pH, contact time, contaminant concentration, and adsorbent dosage on the removal efficiency were studied. The results showed that the maximum removal efficiency of phenol by nZVI and GnZVI was 78, 57.83 %, respectively, at the condition of pH 3, 60 minutes initial concentration of 8 ppm and adsorbent dosage of 2.5 g. The removal efficiency of phenol in acidic conditions and laboratory temperature by adsorption of nZVI is higher than GnZVI with a difference in removal efficiency of approximately 20 %. Equilibrium isotherms were analyzed by Langmuir and Freundlich equations and it was observed that these experiments followed Freundlich model.


INTRODUCTION 1
Cyclic hydrocarbons are organic compounds composed of carbon and hydrogen atoms that form rings or cyclic structures by bonding together. The simplest and first known hydrocarbon is benzene, which is highly toxic and carcinogenic. Its derivatives include phenol, which is typically soluble in aqueous media due to its use in industry. Pollution of water resources with phenol can endanger the health of humans and living organisms and pose a worrying health threat [1,2]. There are several methods for removing phenol, such as electrocoagulation [3,4] biological methods [5,6], extraction [7], electro-Fenton method [8], evaporation [9,10], precipitation [11]. one of these methods is adsorption, which uses different adsorbents. For example, in Equation (1), zerovalent iron reacts with the pollutant as an electron donor, causing a pollutant such as phenol to be reduced by receiving electrons in the form of Equation (2). * Since the preparation of the adsorbent requires a high cost of raw materials, waste that is made of this adsorbent can be used. For example, activated carbon was used to remove contaminants and it was shown that the amount of phenol adsorption depends on the surface chemistry of the adsorbents and their porosity [12]. Another study using adsorbents of almond peel and walnut charcoal reached to 91.36 and 79.17%, respectively. The removal efficiency for phenol in contaminated industrial wastewater for the natural asorbrnts of almond peel and walnut charcoal were 85.54 and 65.49%, respectively [13]. Another study in 2013 showed that adsorbents such as chemical olive stones of activated carbon also have a good ability to remove phenol [14]. Other adsorbents include chitosan beads [15], peat, fly ash, and bentonite [16,17], wheat husk [18], and sawdust [19].
In this study, the synthesis of the nano zero-valent iron (nZVI) and granular nano zero-valent iron (GnZVI) fro m the iron waste of the steel plant and the performance of two adsorbents on the degradation of phenol as an aromatic hydrocarbon from the aqueous medium were investigated. The effect of several parameters such as; pH, contact times, contaminant concentrations, and adsorbent dosages on the removal efficiency of phenol was evaluated. The SEM test was used to observe the size of nZVI.

Materials
All chemicals were purchased from the German company Merck and ferrous waste was collected from the steel plant. Also, phenol as an aromatic hydrocarbon with a density of 1.07 g/cm 3 is a colorless and moist crystalline solid that was prepared as a pollutant. The sodium borohydride (0.16 M), polyvinyl alcohol (PVA), paraffin , and sulfuric acid were used in the synthesis and granulation of nZVI. Ultrasonic bath, centrifuge, and vacuum pump were used, too.

Methods
To prepare nZVI from ferrous waste, the liquid-phase reduction method was used for synthesis, which is a chemical method using sodium borohydride as a reducing agent [20,21]. After the synthesis of nZVI, and SEM test was performed on it. The synthesis of nZVI, a granular adsorbent was made using a polymer called polyvinyl alcohol. Thus, 3 g of nZVI adsorbent with 5 g of polymer, 2 mL of sulfuric acid, and 30 mL of paraffin was converted to GnZVI after 72 hours [22,23].
After preparing the adsorbents using two Erlenmey er under constant laboratory conditions, 20 mg/L phenol was exposed to 2.5 g of the adsorbent for 60 minutes at an acidic pH of 3. Phenol concentration was measured by a spectrophotometer at wavelength of 500 nm.
To explain the equilibrium state of the ads orbed component between the solid and liquid phases, the experimental adsorption equilibrium data were studied by Freundlich isotherm models in Equations (3) and (4) and Langmuir in Equations (5) and (6).

= . (1⁄ )
(3) where n and Kf are dimensionless constants that are related to adsorption capacity and surface adsorption intensity, respectively. Freundlich parameters are calculated using the slope of the line and the width of the origin from the equation of line Log qe versus Log Ce. qe is also the amount of adsorption of phenol on the adsorbent (mg/g), Ce is the equilibrium concentration of solute in the initial solution(mg/L). qm and KL represent the adsorption capacity of a monolayer and are related to the Langmuir adsorption equation constants. The Langmuir parameters are calculated using the slope of the line and the width of the origin from the equation of the line Ce versus Ce/qe [24].

RESULTS AND DISCUSSION
Using SEM analysis, the size of the nZVI produced was approximately 55 nm Which is shown in Figure1. The nanoparticles produced are black, which indicates that they are zero-valent. Figures 2 and 3 show the results related to the removal efficiency of adsorbents of nZVI and GnZVI against different pHs with the same other laboratory and measured parameters. Phenol has a higher removal efficiency in contact with adsorbents at acidic pHs , which  can be attributed to the lack of iron hydroxide formatio n in acidic environments, which means that adsorbent surfaces have a better ability to trap phenol. Phenol degradation efficiency was evaluated at the different pHs 3, 5, 8, 9. At pH 3 phenol 78.05 showed the highest removal rate and at pH 9, 16.43% showed the lowest removal rate with nZVI, while GnZVI showed the highest removal rate at 58.19 and 9%, respectively.

Effect of contact time on the removal efficiency of phenol
Contact time is one of the influential factors in deletion. The removal efficiencies of the two adsorbents at four contact times of 10, 20, 40, and 60 minutes are shown in Figures 4 and 5. As can be seen, in the early times, nZVI and granular adsorbents were capable of removing 41.7% and 27%, respectively. However, with increasing contact time of adsorbents with phenol, the removal efficiency increases by almost 30% in 60 minutes for both adsorbents. The removal process improves over time due to more corrosion of the iron particle surface, creating cavities and increasing the surface area for adsorption.

Effect of adsorbent dosage on phenol removal efficiency
The adsorbents used in this study were prepared in the amounts of 1, 1.5, 2, 2.5 g. Figure 6 shows that the lowest amount of nZVI for phenol removal has an efficiency of 31.75% and with increasing the amount of adsorbent to 2.5 g, the removal efficiency increases to 78.05%. Gn ZVI in the lowest and highest adsorbent for phenol removal in Figure 7 shows the numbers 18.43 and 58%, respectively. In fact, with increasing the amount of adsorbents, the efficiency of the phenol removal process increases. this is due to the greater participation of particles in the process, followed by higher levels of adsorption. Figures 8 and 9 show the effect of increasing the phenol concentration from 8 to 32 ppm for the two adsorbents. The finding indicates that an increase in phenol concentration decreased the adsorption capacity. nZVI and GnZVI at concentrations of 8 ppm phenol show efficiencies of 78.05 and 57.83%, respectively. These numbers indicate that at high concentrations, the active sites of adsorption are saturated with contaminant ions, which reduces the efficiency of the process. As can be seen at a concentration of 32 ppm, the removal efficiencies for nZVI and Gnzvi adsorbents are 26.18 and 7.85%, respectively.

Evaluation of adsorption isotherms
Adsorption data were analyzed by two types of Freundlich and Langmuir isotherm models. The theoretical parameters of the models along with the regression coefficient are shown in Table 1. According to  the R 2 value of each model, it is determined that the Freundlich model has the highest value and is the best model in the adsorption of phenol by the studied nZVI adsorbent.

CONCLUSION
The results of this study show that the adsorption process with adsorbents produced from the waste is a good way to remove phenol. The removal efficiency depends on various factors such as pH, contact time, adsorbent amount, and contaminant concentration. In this study, the highest removal efficiency was obtained at acidic pH 3, contact time of 60 minutes, amount of 2.5 g of adsorbent, and concentration of 8 mg /L phenol. Under these conditions, the amount of phenol removal by nZVI and GnZVI was 78.05 and 58.19%, respectively. The findings also show that the adsorption of phenol by nZVI in all experiments is more than GnZVI, which can be attributed to the increase in contact surface and high active sites for nZVI compared to GnZVI. Results from isothermal studies showed that the correlation coefficient of the Freundlich isotherm equation in the limit is relatively high. Therefore, it can be said that phenol removal follows the Freundlich isotherm equation.