DEGRADATION OF OIL FROM SOILUSING NANO ZERO VALENT IRON.
ABSTRACT: This research reported a new method of the synthesis of nano zero valent iron. Ultrasonic wave as a novel method is used to synthesis nano particles. Particles morphology and surface composition were characterized by FESEM, BET, XRD and EDX. It was found that the morphology of nano particles changed from spherical type to plate under ultrasonic power, The XRD patterns showed the crystallinity of the NZVI prepared using ultrasonic conditions was strong. This novel method has high activity due to their nano-sized dimensions. The synthesized nano particles were then utilized as a Fenton-like catalyst for degradation of hydrocarbon contamination in soil, using ultrasound method. Degradation of oil, a clay soil was studied under the influence of ultrasound.
In the present study, optimization of this degradation was carried out with ultrasound power, temperature effects, NZVI and H2O2 concentration. From the studies, it has been observed that degradation doesn't indefinitely with an increase in ultrasound power, but instead, it reaches an optimum value and decreases with a further increase in the ultrasound power. The degradation of contaminant increased with increasing temperature, NZVI and H2O2 concentration. The result indicated that the efficiency of hydrocarbon removal by Fenton oxidation and NZVI is 98.57%.
Key word: Soil remediation, NZVI, Fenton, Ultrasonic
Soil contamination threats to human health through food and groundwater are one of the most important environmental concerns. Hydrocarbons are presented in the soil, may usually be found in urban and rural lands due to leakage of gasoline, motor oil and diesel fuel from storage tanks . Most of common methods developed for treating soil contaminated with hydrocarbons have some restrictions . For example, the using land fill can not remove all contaminants and in addition the process of soil treating is very slow ; incineration of the pollutants may produce secondary pollutants such as poly-aromatics , while some times these materials are much more toxic than the original contaminants and expensive to remove when the amounts of contaminants are large .
In biological treatment, due to resistivity of some pollutants against microorganisms and reduction of their activity indifferent climatic conditions the pollutants removal efficiency may be low  and in the thermal methods for removing volatile contaminants, high operating temperature is required which is not economical . A practical and modern alternative method to remove hydrocarbons from contaminated soil is using nanotechnology method. Nano zero valent iron particles have been used in treatment technology [1-3]. Various studies show that nano particles are very effective in removal of environmental pollutants such as chlorinated solvents (PCB1,TCE2), inorganic compound such as perchlorate and heavy metals e.g., chromium, lead, nitrate, copper, zinc [3-5].The effectiveness of NZVI may be related to their small sizes and large surface area to volume ratios.
NZVI can be easily synthesized using sodium borohydride, NaBH4, as a strong reducing agent . In order to produce NZVI with specific surface area and morphologies, various chemical and physical methods are used in literature . One of the characteristics of these particles is their tendency to aggregate and change to form a chain-like structure. It reduces stability and decreases interfacial area drastically. The stability of NZVI against aggregation can be improved by imparting electrostatic repulsion, applying noble metals, polymers, ceramic materials, dextran, oils and silicon dioxide [4-5].Another promising method that would serve this goal is to synthesize NZVI in the presence of supporting inorganic materials [7-9].
The main goal of this research is to investigate the performance of degradation of hydrocarbon contaminants in soil by using the synthesized NZVI assisted by ultrasonic wave. The effect of ultrasonic power, NZVI and H2O2 concentration, temperature on degradation efficiency are also explored in this research.
2. MATERIAL AND METHODS
2.1Chemical and equipment
FeSO4.7H2O (MW: 278.01, Analar), ethanol (absolute, Merck), H2O2 (30%, Merck), sodium hydroxide(0.1M, Merck) and hydrochloric acid (36% wt/wt, Merck) were used as received without further purification. All the required solutions were prepared using deionized and deoxygenated water. In order to generate ultrasonic waves in solution, UIP 1000hd sonication bath is used from Hielschler company.
2.2Method of Preparation of NZVI
The first step in the synthesis of nano particle is to prepare the solutions of FeSO4 and NaBH4. Solution of 2.25 gm FeSO4 (0.08M) in 100ml ethanol 30% was prepared and mixed by an agitator for5 minutes to be quite homogeneous. Ethanol was used instead of distilled water because it prevents oxidation of the particles during the preparation.
The use of ethanol 20-40% reduces the oxidation rate of the nano particles greatly. For preparation of NaBH4 solution, 1.5 gm of NaBH4 (0.4M) was dissolved in100 ml deionized water that was deoxygenated by nitrogen gas for 30 minutes.
NZVI was synthesized in 300ml flask reactor with three open necks. One of the necks was for entering the ultrasonic probe and another was for introducing nitrogen gas to purge oxygen out and to prevent oxidation of nano particles. The last of them was for entering NaBH4 solution, a peristaltic pump was used to introduce 100 mL of borohydride solution into 100 mL of ferric ion solution (a schematic diagram of experimental setup is shown in figure 1).
For adjusting the temperature at 25degC the reaction container was put within the water bath so that the solution temperature was kept constant during the reaction. By adding NaBH4 solution into the flask it can be seen that black spots within the solution was formed immediately that implies iron particles are deposited according to equation (I).
After the end of reaction time, slurry was put under ultrasonic wave for 5 minutes. The slurry was then filtered using 0.2 micron membrane filter. The black cake was rinsed with ethanol 100% for three times. Wet cake was placed into centrifuges with 3000rpm for 5 minutes to remove the remaining moisture and then the wet cake put in the vacuum container to be dried. The dried cake was kept in a refrigerator to avoid oxidization. A small amount of the sample was analyzed.
2.3 Degradation experiments
In order to investigate the degradation efficiency of NZVI, 15 grams of dried contaminated soil with 10ml crude oilis used. The sample was placed in 200 ml glass beaker and 70 ml deionized water was added, and then they were mixed for 2 minutes by a magnetic stirrer. After complete mixing, pH reached to 8 and by adding hydrochloric acid, it was adjusted at 3. Then, the synthesized NZVI was added to the mixture. In this research, ultrasonic wave was used to enhance decontamination of soil. The experiment was followed by placing 0.5 inch diameter sonic probe at the center of sample (schematic diagram of experimental setup is shown in figure2).
Due to high ultrasonic power transfer to the mixture, the temperature tended to increase therefore the sample was placed in the water bath to carefully control the temperature constant. The mixture exposed to ultrasonic power for 30 minutes and then the probe was removed and hydrogen peroxide was added to the solution. The resulting mixture was stirred by magnetic stirrer for 30 minutes to have 60 minutes total reaction time. After that, the mixture was filtered through vacuum filter. In order to find the remained hydrocarbon in remedied soil, it was washed several times by 1:1 mixture of acetone and hexane. The extracted hydrocarbons in solvent were fed to the gas chromatograph. The extracted hydrocarbon was characterized ina TR- 121033 Gas Chromatograph with flame ionization detector (GC-FID).A TRB-5 capillary column with 30 m x 0.32 mm ID and 1um film thickness was used for the separation of hydrocarbons. Typically, 0.5 uL of TPH extract was injected into the GC system.
2.4 nZVI characterization
After preparation of nano zero valent iron by ultrasonic method, their physio-chemical properties were investigated through particle size analyzing experiments, FESEM, XRD and BET. An EQUINOX 3000 diffractometer (INEL France 2010) at 45 KV and 30 mA was used for the XRD analysis. The source consisted of CuK radiation (X= 1.54 ) and the sample was scanned from 0 118 with scanning rate of 2 /min. The sample of NZVI was thoroughly washed with ethanol before the XRD analysis to remove any impurities. FESEM analysis was carried out using a HITACHI 4160 field emission scanning electron microscope (FESEM).
Particle size analysis was performed with a DT 1200 acoustic spectrometric (Dispersion technology Inc.) which utilizes the sound pulses transmitted through a particle suspension to measure the properties of suspended particles . BET surface area (SBET) was measured by N2desorption method (Nova 2200, Quantachrome). Prior to the adsorption-desorption measurements, the samples were degassed by N2flowing for 3 hours at 200degC.
3. RESULTS AND DISCUSSION
3.1 NZVI Characterization
3.1.1 NZVI particle size distribution and morphologies Figure 3 shows particle size distribution of nZVI particles. It can be found that 50% of particle 43.3 nm and 90% of particle sizes are less than 80.9 nm.
NZVI particles have PSD with a standard deviation 0.487,which is defined as the ratio of size 15.78% cumulative probability to that at 50%. In other words, this work suggests that the NZVI particles are nearly poly disperse with a majority in the nano-domain ((less than) 80 nm).
The XRD spectrum of the NZVI is shown in Figure 4. The broad peak reveals the existence of an amorphous phase of iron. The peaks at 2 of 35.8deg and 44.7deg show the existence of both iron oxide (FeO) and zero-valent iron ( ) crystalline phase. Investigations showed the nZVI particles produced by borohydride reducing agent has Fe crystalline phases for Fedeg but NZVI produced by green tea and sorghum bran extracts is amorphous in nature, possibly explaining the low intensity of diffraction lines of Fedeg in this research .
3.2 Hydrocarbon degradation experiments
3.2.1Effect of ultrasonic power
Application of ultrasonic wave in remediation field is derived from acoustic cavitation: the formation, growth and implosive collapse of bubble in solution. During the collapse, the gases within the bubble are rapidly compressed, produced hot spot with heat and pressure as high as 500K and 100atm . Cavitation phenomenon accrues presence of solid particle observed by a change in the symmetry of bubble collapse. When this phenomenon asymmetrically distributed on the particle surface. When cavitation occurs away from particles, the cavitation collapse is symmetrical and the shockwave can generate turbulence in solution . Ultrasound waves have available potential for remediation of the soil and water from organic and inorganic contaminant.
Hoffman et al. (1996) reported ultrasonic treatment is effective for the destruction of organic contaminants in water due to oxidation and high localized temperature and pressure. The breakdown of hazardous compounds into intermediate products can be relatively easy to monitor . Ultrasonic wave not only can degrade pollutants through oxidation and pyrolysis processes in soil, but also move the pollutant to the other sources such as water through soil washing which is much easier to decontaminate it [12- 15].The effect of ultrasonic power (0, 200W, 500W, 1000W) on the degradation of hydrocarbon in contaminated soil with 0.4g/Lof nZVI particles, 40degC and 30mM H2O2 is shown in Figure5.
In this experiment when the ultrasonic waves were applied, the efficiency of degradation increased by increasing sonication power. Around 500W ultrasonic power, it reaches to 98.57% and then it decreases. When cavitation occurs, the sound pressure level at a distance drops because cavitation takes power away from the field. Cavitation can reduce the effective ultrasonic power in the soil [16-18].
The % degradation of petroleum hydrocarbon is calculated by the following equation: % degradation = x 100 (1) Where C0 is the initial concentration of petroleum hydrocarbon in the soil; C is the final concentration of petroleum hydrocarbon in the treated soil.
3.2.2 Effect of nZVI concentration
The effect of nZVI concentration on degradation of hydrocarbon contaminant in the soil with 500W ultrasonic power, 40degC and 30 mM H2O2 is shown in Figure6. All the experiments were performed for a reaction time of 1h.
The results show that most of the hydrocarbon contaminates in the soil was degraded. The degradation of hydrocarbon occurs at the surface of nano particles. The transmission of oil into the nZVI solution results in the release of oil from the pores of soil into the aqueous phase where the oil will be exposed to the reactive surface of nZVI [7-9]. The degradation of hydrocarbon chain in the soils increased with increasing nZVI concentration. When the nZVI dosage increased, the concentration of hydrocarbon rapidly decreased and the degradation efficiencies increased from about 80.12% to 98.57%. The increased nano particles amount in the degradation reactions enhanced the degradation efficiency.
3.2.4Effect of H2O2concentration
After 30min of pre-treatment for the reduction of hydrocarbon contaminant with nZVI particles and ultrasonic wave, H2O2 was added at concentrations of 0, 10mM,20mM,and 30mM in solution. Hyun moon et al.(2010) studied the TOC removal efficiencies of the simultaneous and sequential nZVI/H2O2processand they found that the sequential process had higher removal efficiency . It can be explained by the fact that it is more figure, it is clear that by increasing H2O2concentration, the efficiency of degradation increased. It is explained by the fact that when H2O2concentration increased, the free radical (OHo) concentration in solution increased [15-17]. In the following the formation of free radical and propagation of degradation reaction is shown.
H2O2+Fe2+- Fe3++OH-+OH. (2) Radical propagation: OH.+RH-R.+H2O (3) R. + H2O2- ROH + OH. (4)
3.2.5Effect of Temperature
Temperature effect studied in the range of 10degC to 40degC under 500W ultrasonic power, 30mM H2O2 and 0.4g/L NZVI. A gradually better removal of oil from soil by NZVI was observed as the temperature was increased from 10degC to 40degC approximately; the similar removal efficiency at 10degC and 30degC indicates that the effect of temperature on the degradation hydrocarbon chain in soil is quiet small that shows in Figure10. It is clear that by increasing temperature the efficiency of degradation increased. effective because nano iron is added to the solution, pre-4.
Conclusions treatment with reduction reaction and ultrasonic wave, the hydrocarbon chainbreak and converted into smaller molecules that are easier to degrade as NZVI is oxidized into Fe2+ and then OH radicals are generated by H2O2.Hydrogen peroxide plays a role of oxidizingagentsin thedegradationprocess [3-5].
H2O2 concentration effect is investigated through the experiment with 0.4g/L NZVI particles, 40degC and 500W ultrasonic power. The result is shown in Figure7. From this figure, it is clear that by increasing H2O2concentration, the efficiency of degradation increased. It is explained by the fact that when H2O2concentration increased, the free radical (OHo) concentration in solution increased [15-17]. In the following the formation of free radical and propagation of degradation reaction is shown. H2O2+Fe2+- Fe3++OH-+OH. (2) Radical propagation: OH.+RH-R.+H2O (3) R. + H2O2- ROH + OH. (4)
3.2.5Effect of Temperature
Temperature effect studied in the range of 10degC to 40degC under 500W ultrasonic power, 30mM H2O2 and 0.4g/L NZVI. A gradually better removal of oil from soil by NZVI was observed as the temperature was increased from 10degC to 40degC approximately; the similar removal efficiency at 10degC and 30degC indicates that the effect of temperature on the degradation hydrocarbon chain in soil is quiet small that shows in Figure10. It is clear that by increasing temperature the efficiency of degradation increased.
The aim of this study was to investigate the reactivity of nano zero valent iron synthesized assisted ultra sonication for degradation of hydrocarbon contamination in soil. Particle size distribution, morphology and surface composition were characterized using FESEM, XRD, BET and PSD analysis. NZVI demonstrates some dispersion, with particle size ranging between 40 nm to 80 nm and BET surface area is 35.8m2/g. This work suggests that the NZVI particles are nearly poly disperse with a majority in the nano-domain ((less than)80 nm). In degradation process with increased of ultrasonic power the degradation rate was increased to a maximum around 500 W then decreased. The degradation of contaminant increased with increasing NZVI concentration from about 80.12% to 98.57%. Reactivity was found to increase with increasing temperature and H2O2 concentration but at low temperature, the rate of degradation was not changed more.
The result indicated that maximum efficiency of hydrocarbon removal by NZVI is 98.57% and the optimum conditions of degradation are (PH: 3.5, ultrasonic power: 500W, NZVI concentration: 0.4g/L, temperature: 40degC and H2O2 concentration: 30mM). Some practical application as well as suggestion for further work was mentioned such as Finding the optimum condition of nano zero valent iron synthesized, Finding effects of different parameters of hydrocarbon degradation from soil , Environmental benefits and risks of zero-valent iron nano particles for remediation.
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a Department of Chemical Engineering, Petroleum University of Technology, Abadan, Iran, b Department of Gas Engineering, Petroleum University of Technology, Ahvaz, Iran firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
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|Author:||Jameia, Mir Roozbeh; Khosravib, Mohamad Reza; Anvaripoura, Bagher|
|Date:||Dec 31, 2013|
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