Oil Spill Cleanup From Sea Water by Sorbent Materials

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Oil Spill Cleanup from Sea Water by Sorbent Materials By Ahmad Bayat, Seyed Foad Aghamiri* Ahmad Moheb, and G. Reza Vakili-Nezhaad Three sorbents were compared in order to determine their potential for oil spill cleanup. Polypropylene nonwoven web, rice hull, and bagasse with two different particle sizes were evaluated in terms of oil sorption capacities and oil recovery efficiencies. Polypropylene can sorb almost 7 to 9 times its weight from different oils. Bagasse, 18 to 45 mesh size, follows polypropylene as the second sorbent in oil spill cleanup. Bagasse, 14 to 18 mesh size, and rice hull have comparable oil sorption capacities, which are lower than those of the two former sorbents. It was found that oil viscosity plays an important role in oil sorption by sorbents. All adsorbents used in this work could remove the oil from the surface of the water preferentially.

1 Introduction Marine oil spills are the most important threat to the coastal environment and ecosystems of the sea. Furthermore, oil spills are responsible for the great loss of energy resources. They are formed mainly by occasional accidental episodes of supertankers, oil rig drilling, war, and natural events [1, 2]. For example, several hundred million gallons of oil were spilled into the sea due the demolition of oil storage tanks in Kuwait in 1991 [3]. Therefore, the ecological disasters resulting from oil spills have created a great need for cost-effective cleanup systems. In recent years, a large number of studies have been devoted to oil spill cleanup [4±6]. The methods developed for oil spill cleanup can be categorized into three main groups. The first group includes physical methods such as adsorbents, booms and skimmers [7±10], the second one comprises chemical methods such as dispersion, in-situ burning, and the use of solidifiers [11±13], and the third one considers biological methods or bioremediation [14]. Usually, a combination of all these methods should be used to achieve an effective cleanup. Booms, available in various sizes and shapes, are used to encircle and concentrate the spilled oil. Their major drawback is that much of the spilled oil sinks to the bottom, damaging the undersea life and forming tar balls, which are washed up onto the shore. Skimmers mechanically remove the encircled and concentrated spilled oil by booms from the surface of the water. However, most skimmers are inefficient and leave much of the recovered oil mixed with water, thereby making oil recycling expensive and economically impractical [15±17]. Dispersants are generally liquid chemicals which accelerate the dispersion of the oil by reducing the surface tension between the oil and water when applied to the surface of the spilled area. These chemicals are usually toxic and release volatiles to the atmosphere. Also, dispersants are costly. Therefore, their application is limited through legislation ± [*]

A. Bayat, A. Moheb, ChemicalEngineering Department, Isfahan University of Technology, Isfahan, 84154; S. F. Aghamiri ([email protected]), Chemical Engineering Department, University of Isfahan, Isfahan, 81746±73441, Iran; G. R. Vakili-Nezhaad, Chemical Engineering Department, Kashan University, Kashan, Iran.

Chem. Eng. Technol. 2005, 28, No. 12

DOI: 10.1002/ceat.200407083

considerations. In contrast to dispersants, herding agents (thickeners) can be added to an oil spill to thicken the oil. These agents increase the surface tension between the oil and water, thus reducing spreading of the spill and providing easier cleanup. Again, these chemicals are expensive and toxic. In addition, the thickened oil will sink sooner than oil which has not been treated [16, 18]. Some microorganisms have a natural ability to degrade a large number of hydrocarbons and can be found everywhere in the marine environment. Bioremediation is carried out by adding microorganisms to spilled oil or by stimulating the ones occurring naturally. It should be pointed out that, while biodegradation agents remove the saturate and some aromatic fractions of the oil, it can take much time to remove the degradable fractions, even under ideal conditions. Temperature, oxygen availability and the presence of nutrients are the factors influencing the rate of bioremediation. For instance, the lack of oxygen is usually the limiting factor for in-situ bioremediation [14, 15, 19]. Oil can be sorbed from the surface of the sea by using some suitable sorbent materials. Sorbents have a significant capacity for oil recovery from the surface of the sea, minimum harmful effects on ecosystems, and a low price. Sorbents recover the spilled oil by either adsorption or absorption mechanisms. Adsorption is the distribution of the adsorbate over the surface of the adsorbent, while absorption is the distribution of the absorbate throughout the body of the absorbent. When added to an oil spill, sorbents can change the oil from the liquid to a semisolid phase. Then, the oil will be easily recovered by the removal of the sorbent structure. Hydrophobicity and oleophilicity are the most important properties of a sorbent to be considered in oil spill cleanup. Other important factors are the retention over time, the recovery of oil from sorbents, the amount of oil sorbed per unit weight of sorbent, the reusability and the biodegradability of sorbents. Oil sorbent materials can be categorized into three major classes [20±22]: ± inorganic mineral products such as perlite, vermiculite, volcanic ash, sorbent clay and diatomite, ± organic synthetic products such as polypropylene, polyurethane foam and polyester, ± organic vegetable products such as peat moss, kenaf, straw, corncob and wood fibers. 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Full Paper The sorption capacity of organic synthetic compounds is significantly higher than that of the other groups, but a major disadvantage of these materials is that they degrade very slowly compared to mineral or vegetable products. Some sorbents that have been widely studied by researchers are milkweed, kapok, cotton fiber [23], polypropylene [24], modified expanded perlite [25], exfoliated graphite [7, 26], carbonized fir fibers [27], carbon fiber felts [28], and Sugi Bark Sorbent [29]. In this work, the performances of three different sorbents, namely, bagasse, rice hull and polypropylene nonwoven web, are compared in terms of oil sorption capacity and oil recovery efficiency. Experiments have been conducted to clean up four different oil samples from the surface of water.

Polypropylene nonwoven web was floated directly on the water surface. Since bagasse and rice hull contained small particle sizes, it was difficult to collect and remove the wetted sorbents after they had been used. Therefore, these sorbents were wrapped into a net cloth made of hydrophobic material and then floated on the water surface. It is clear that the sorption capacity might be affected by the net cloth, but material, size and weight of the net cloth were maintained constant in all experiments, so that this effect was the same. Simulated seawater according to ASTM D 1141 [30] was used. Simulated seawater is an aqueous solution of specific amounts of different salts, with a pH equal to 8.2 at room temperature.

2 Materials and Methods

3 Experiments

Four oil samples presented in Tab. 1 were used as pollutants. Light crude oil (LCO), gas oil No. 1 (GO1) and gas oil No. 2 (GO2) were obtained from Isfahan refinery, heavy crude oil (HCO) was obtained from Abadan refinery. Bagasse, rice hull and polypropylene nonwoven were used as oil sorbents. Bagasse was taken from the Karoon agricultural and industrial company as a by-product of sugar extraction from sugarcane. Since raw bagasse had a high humidity (more than 50 %) with nonuniform particle sizes, it was dried, milled and finally screened into different mesh sizes. In order to study the effect of specific surface area on the oil sorption capacity of bagasse, two mesh sizes (14 to 18 and 18 to 45) were tested. Also, rice hull was milled, screened, and one limited mesh size (10 to 14) was selected for the experiments. The other properties of the sorbents used are presented in Tab. 2.

A common dynamic sorption method was employed to determine the oil sorption capacity of the sorbents. A specified amount of oil pollutants (10, 20 and 30 g) was added to a 600-mL beaker containing 400-mL of simulated seawater [30] and placed on a shaker apparatus. A sorbent was weighed (1,000 g) and added to the system, which was shaken for 5 min at 90 cycles/min. The wetted sorbent was removed from the polluted water, drained for 2 min and then carefully weighed. The water content (quantitative) of the sorbent was determined by the ASTM D 95 distillation technique [31]. The total net amount of sorption is the difference between the weights of the initial sorbent and the drained sorbent. Finally, the net oil sorption was determined by subtracting the water content from the total net amount of sorption.

Table 1. Specification of oil samples.

Property

Unit

Density at 30 C

g/cm3

Kinematic Viscosity at 40 C

2

cm /s

Oil Sample Gas oil No. 1 (GO1)

Gas oil No. 2 (GO2)

Light crude oil (LCO)

Heavy crude oil (HCO)

0.8384

0.8311

0.8537

0.8795

0.0405

0.0363

0.0617

0.1250

Table 2. Specification of sorbent materials. Type

Bagasse

Rice hull

Polypropylene (nonwoven web)

Mesh

14±18 & 18±45

10±14

±

Source

By product of sugarcane

A milling by product

±

Thickness (mm)

±

±

5.59

±

±

12.56

±

±

593.3

Denier 2

Mass per area (g/m )

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2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Full Paper Oil recovery efficiency is considered as a measure of the hydrophobicity and oleophilicity of sorbents through the following relation: R = 100 (M0 /Mt)

(1)

where R is the oil recovery efficiency, M0 is the net oil sorption, and Mt is the total net amount of sorption. Experiments were repeated between 3 to 5 times and the standard deviations were calculated.

4 Results and Discussion The oil sorption capacities of different sorbents are presented in Tab. 3. The results represent the average oil sorption capacities (as mass of sorbed oil per unit of sorbent mass) for different pollutant-to-sorbent ratios (10, 20 and 30 g/g). Also, in this table, the standard deviations of all experiments are given in rows 3, 6, 9, and 12. The results reveal that polypropylene nonwoven has the best oil sorption capacity for different oils and can sorb 7±9 g of different oils per each gram of sorbent. The sorption capacity of 18 to 45 mesh bagasse is about 5±6 g/g. Rice hull and bagasse, 14 to 18 mesh size, have comparable oil sorption capacities of about 3±5 g/g, which are lower than those of polypropylene and 18 to 45 mesh bagasse. Interestingly, the sorption capacity of these two sorbents for heavy crude oil is higher than that for other oils. In addition, heavy crude oil is sorbed almost equally by all sorbents except by polypropylene nonwoven. The reason may be related to the high viscosity of heavy crude oil. The oil sorption within the pores and capillaries decreases as viscosity increases, but the adherence of

the oil to the sorbent increases. Compared to high-viscosity oils, low-viscosity oils are released faster from the sorbent during the drainage step, but they can be sorbed more easily into the pores and capillaries of the sorbents. An alternative explanation is that hydrophobic interaction and van der Waals forces occurring between the crude oil and wax in the natural sorbent may effect higher sorption of natural organic sorbents like bagasse and rice hull in the case of heavy crude oil in comparison with other oils. Also, the oil recovery efficiencies of different sorbents are presented in Tab. 3. The results show that the oil recovery efficiency of 18 to 45 mesh bagasse is lower than that of 14 to 18 mesh bagasse. This may be due to the lower specific surface area of the latter bagasse, which causes bagasse of this mesh to have a lower total net amount of sorption, compared to 18 to 45 mesh bagasse, and which consequently sorbs a lower amount of water. Polypropylene nonwoven and rice hull have oil recovery efficiencies comparable to bagasse of 18±45 mesh. The recovery efficiency of all sorbents for heavy crude oil is higher than that for other oils. Due to its high viscosity, this oil can occupy the sorbent pores. The oil-to-sorbent ratio (L/S) has no considerable effect on the oil sorption capacity and oil recovery ratio of the different sorbents.

5 Conclusion To determine their potential for oil spill cleanup, the performance of three sorbents was studied. The sorbents were selected from natural and synthetic categories. Bagasse and rice hull as natural materials and polypropylene nonwoven web as a synthetic sorbent were used.

Table 3. Experimental results of oil sorption capacities, recovery efficiencies and standard deviations. Oil pollutant

Sorbent

Bagasse Mesh 14±18

Bagasse Mesh 18±45

Polypropylene Nonwoven

Rice hull Mesh 10±14

Oil sorption (g/g sorbent)

3.380

5.55

8.260

3.80

Recovery efficiency (%)

100

93.13

87.7

94.6

Standard deviations

0.4

0.39

0.35

0.47


4.01

5.22

7.6

3.7

Recovery efficiency

97.75

91.51

94.27

90.29

Standard deviations

0.53

0.44

0.31

0.44


4.07

5.39

8.46

3.81

Recovery efficiency

94.23

94.53

94.54

92.99

Standard deviations

0.40

0.36

0.52

0.57


5.30

5.54

9.12

5.15

Recovery efficiency

100

96.7

95.67

96.0

Standard deviations

0.55

0.51

0.43

0.38

Results LCO

GO1

GO2

HCO




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Full Paper According to the results obtained, polypropylene nonwoven web is the best material for oil spill cleanup in terms of oil sorption capacity. Bagasse of 18 to 45 mesh has a higher oil sorption capacity than 14 to 18 mesh bagasse and rice hull. The latter two sorbents have comparable sorption capacities. Importantly, sorption capacities of all sorbents except polypropylene nonwoven web were the same for heavy crude oil. All sorbents studied in this work were capable of sorbing different oils from the water surface preferentially due to oleophilicity or hydrophobicity. A common dynamic sorption method is presented to compare the selected sorbents. This method has proved simple, rapid and efficient. It is concluded that the specific surface area and pore size distribution of the sorbent particles as well as the relatively large spaces among these particles are the important factors in the oil sorption performance of the sorbents studied. Bagasse and rice hull are agricultural wastes and pose disposal problems, but they are biodegradable and of low price. Also, bagasse and rice hull are usually burned as fuels and, having been used as oil sorbents, their heating value improves. Received: October 14, 2004 [CET 7083]

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Oil Spill Cleanup From Sea Water by Sorbent Materials

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