Investigation of Oil Removal from Water by the Modified Nano Tin Oxide

ABSTRACT

This study investigates the performance of modified Nano tin(IV) oxide sorbent in oil pollution removal from sea water. This sorbent synthesized in two stages: synthesizing tin(IV) oxide nano particles and modifying its surface, using (3-mercaptopropyl) trimethoxysilane and grafting of N,N-dimethylacrylamide-allyl butyl ether copolymer on it. The resulting sorbent was characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, and scanning electron microscopy, then evaluated for TPH adsorption from environmental water samples. Total Petroleum Hydrocarbons (TPH) is considered as water pollution index and measured by Gas Chromatography. Batch experiments were conducted to evaluate the effect of analytical parameters of pH, contact time, and temperature. The optimum pH was 5 and contact time was 10 minutes. The capacity of the sorbent was 11.46 mg g-1. Results showed good accessibility of the active sites. The equilibrium adsorption data of TPH sorption onto grafted nano-tin(IV) oxide were analyzed using the Langmuir and Freundlich isotherm models. The adsorption data was modeled as pseudo-first-order and pseudo-second-order kinetic equations. The results show that adsorption followed by the Langmuir isotherm and the pseudo-second-order model. The sorbent removed more than 80% of the TPH from the water solution samples.
Key words: Oil removal, TPH removal from water, Nano tin oxide, Modified sorbent

1. Introduction
Oil pollution has several long-term effects on environment and economy. Cleaning oil pollutions and oil spills are very expensive, and among different methods for removing oil from water, such as physical methods, adsorption, and biological treatment, adsorption methods are widely applied. Nano-materials have interesting characteristics that make them promising sorbents in aqueous solutions. Surface modification of nano-materials by grafting of polymer chains enhances their structural properties and sorption capacity. This study focuses on removing oil pollutions from water by synthesizing an efficient nano-sorbent. Tin(IV) oxide (SnO2) nano-particles were synthesized using the co-precipitation method; after modification by 3-mercaptopropyl trimethoxysilane, they were grafted with N,N-dimethylacrylamide and allyl butyl ether. The performance of this grafted synthesized nano-sorbent was then evaluated for use in oil adsorption from water.

2. Materials and Methods
2.1. Synthesis of SnO2 nano-particles grafted with hydrophobic groups

2.1.1. Synthesis of nano-SnO2
At the first stage, SnCl2.2H2O (116.3 g) was dissolved in 250 ml distilled water to prepare 2 M SnCl2.2H2O solution. Then, 2 M NH3 solution was added gradually to the SnCl2 solution while stirring at room temperature (25°C) to reach pH = 7. The slurry was stirred for 2 h and then filtered and dried.

2.1.2. Polymer grafting
A two-step method was applied for this stage. The first step was modification of SnO2 with 3-mercaptopropyl trimethoxysilane and second step, Graft polymerization, was grafting the N,N-dimethylacrylamide (DMAA)- allyl butyl ether (ABE) copolymer onto the modified SnO2.
First step
SnO2 nano-particles were silylated using an anhydrous solution of 5% 3-mercaptopropyl trimethoxysilane in 1,4-dioxane. A mixture of SnO2 nano-particles (3 g), 1,4-dioxane (2.5 ml) and 3-mercaptopropyl trimethoxysilane (47.5 ml) was refluxed for 24 h at 100°C. The solid precipitate was filtered and washed several times with 30 ml of 1,4-dioxane and dried under vacuum in a desiccator over dry calcium chloride.
Second step
In this stage, a mixture of modified SnO2 nano-particles (3 g), ethanol (30 ml), ABE (4 ml), DMAA (1 ml) and 2,2'-azobis (2-methylpropionittrile) (0.1 g) was prepared and the mixture was refluxed in a nitrogen atmosphere for 8 h at 65°C. The grafted particles were filtered and washed with ethanol and water and then dried. This poly-grafted nano-SnO2 with (ABE-co-DMAA) is named AD-GNS.

2.2. Oil pollution index assessment
Total Petroleum Hydrocarbon (TPH) is considered as an index assessment of oil pollution in water, and this index is measured by Gas Chromatography (GC).

2.3. Batch adsorption experiments
A set of solutions (each 200 ml) containing crude oil and water were prepared and their pH values adjusted to the optimum value of 5. Then each 200 ml solution was divided to two 100 ml solutions, and one of them considered as standard solution for initial TPH assessment. Then desired dosage of AD-GNS was added to other 100 ml solution and it was shaken for 1 hr. The sorbent was filtered, and the suspensions were centrifuged at 7000 rpm for 5 min and the clear supernatant and the standard solution were analyzed using a GC-FID.

2.4. Parameters optimization for sorption and isotherm study
In this study, the effects of initial pH, contact time, and adsorbent dose were investigated to obtain the optimize condition for adsorption of oil on AD-GNS. Then adsorption isotherms, Langmuir and Freundlich isotherms, and adsorption kinetic models, the pseudo-first-order and the pseudo-second-order, are applied to investigate the mechanism of adsorption and efficiency of oil removal.

3. Results and Discussion
The AD-GNS was characterized by FTIR, TEM and SEM. FTIR of the SnO2 nano-particles confirmed the presence of Sn-O-Sn and O-H. A peak at 1091 cm-1 caused by the Si-O band in the FTIR of the modified SnO2 nano-particles confirms the modification with 3-mercaptopropyl trimethoxysilane. The presence of CH2, CH, and OH are confirmed by FTIR. The IR spectrum of the AD-GNS was compared with that of the modified SnO2nano-particles. Two additional bands appear at about 1103 and 1641 cm-1 that correspond to C-O and C=O, respectively. TEM showed spherical agglomerated micro-particles with diameters of less than 100 nm, and SEM confirmed that particle size for AD-GNS was ranged from 30 to 50 nm. The degree of oil sorption by pH value was determined using the batch equilibration technique at pH values ranging from 3 to 8. Fig. 1(a) shows the effect of pH on the sorption of oil. Evidently, adsorbate uptake depended on solution pH and the maximum adsorption was achieved at a pH of 5. The effect of contact time on the adsorption of TPH was investigated at different initial concentrations of oil and an optimized pH of 5. The AD-GNS amount was constant during this stage. As seen in Fig. 1(b), oil uptake was rapid at the start of the process in response to the huge surface area with functionalized and available active sites, but then slowed. The adsorption rate became constant as the active sites were covered with xylene and equilibrium was reached. As seen, at lower initial concentrations, adsorption was faster, and 10 minutes was usually enough for complete sorption. This reflects that active sites on the sorbent are easily accessible. The effect of adsorbent dose is also evaluated by adding different dosage of AD-GNS to solutions with constant TPH value and pH=5. The results are shown in Table. 1. For better interpretation of the results of Table. 1, Langmuir and Freundlich isotherms, can be determined, and results are listed in Table. 2. As shown in Table 2, RL =0.02, and it is in the range of 0-1, which shows highly favorable adsorption. The maximum adsorption capacity of AD-GNS for complete monolayer coverage on the surface (qmax) is 11.61 and it confirms the very high capacity of the synthesized sorbent. To analyze the data and evaluate the mechanism of adsorption and efficiency of oil removal, two adsorption kinetic models, pseudo-first-order and pseudo-second-order, are applied, and proves that the pseudo-second-order model presents the adsorption kinetics better.

4. Conclusions
A new sorbent synthesized by a two-stage method. Although the synthesis of the sorbent is simple and economical, it is time-consuming. The optimum pH for oil removal from water by the sorbent was 5, and contact time was 10 minutes. The capacity of the sorbent was 11.46 mg g-1. Results showed good accessibility of the active sites. The equilibrium adsorption data of TPH sorption onto grafted nano-tin(IV) oxide were analyzed using the Langmuir and Freundlich isotherm models. The adsorption data was modeled as pseudo-first-order and pseudo-second-order kinetic equations. The results show that adsorption followed by the Langmuir isotherm and the pseudo-second-order model. The sorbent removed more than 80% of the TPH from the real water solution samples.