SORPTION BEHAVIOR OF HIBISCUS CANNABINUS L. CORE IN SIMULATED

BUNKER OIL C – SEAWATER MIXTURE

Lea C. Tan, Trina G. Napasindayao, Florinda T. BacaniChemical Engineering Department, De La Salle University-Manila, Philippines

2401 Taft Avenue, Malate, Manila, Philppines, 1004

Keywords: Kenaf; mathematical equation; quantifying sorbent capacity; oil spill; sorbent; sorption

ABSTRACT

Sorption using natural sorbents is an alternative method of oil spill treatment. This research proposed a polynomial equation that described the sorption behavior of Hibiscus cannabinus L. core in Bunker Oil C-seawater mixtures. This equation may be applied for oil concentrations of 0.001 to 0.003 mL oil/mL mixture and for a contact time of 15.00 to 120.00 min. The sorption behavior was characterized using specialized parameter define as Sorption Quantity (SQ) (g oil sorbed/g sorbent). The amount of oil sorbed by the Kenaf core was measured at different oil concentrations and contact times using mass balances. Design Experts, a computer software, was utilized in formulating the central composite design and in the statistical analysis which made use of the response surface method. Results show that the sorption behavior is best at high oil concentration and longer duration of exposure. Based on the sorption parameter, the most recommended time of contact is in the range of 65.00 to 95.00 min. The quadratic model was selected among other polynomial equations based on its R2 value of 0.9495 and p-value which is less than 0.0001 at a 0.05 confidence level.

INTRODUCTION

The availability of liquid petroleum in the form of crude oil and its refined products is a key driver in modern society, but its widespread use results in accidental and intentional releases resulting to oil spills. These oil spills have created large damages, casualties and losses on both marine and land resources. In addition, oil spills generally take about two to three years to clean up, some for even longer. Within such time periods considerable damage will have occurred at the site of the spill. Many methods have been proposed in an effort to clean up oil spills, among which involve the use of chemical dispersants, microorganisms, and bioremediation agents. However, these processes alone have not solved the oil pollution problem particularly in the Philippines. It is therefore necessary to find an alternative and immediate means to clean up oil pollution.

The advantage of sorption over conventional oil spill treatment is that it does not alter the make-up of the environment. Sorption usually employs oleophilic, synthetic sorbents. However, these fibers degrade slowly. Lignocellulosic fibers are renewable and biodegradable natural sorbents with good sorptive properties but are oleophobic and cannot be used for oil spills prior to treatment. Hibiscus cannabinus L., or Kenaf is a lignocellulosic fiber that may be used because it is oleophilic. Kenaf can grow under any wide range of weather conditions. This plant specie is native to tropical countries and is harder, stronger and more lustrous than jute based fibers. However, no prior research has been done to mathematically describe the ability of Philippine-grown Kenaf to sorb oil in an aqueous mixture.

The viability of using Philippine-grown Kenaf core as a sorbent of Bunker Oil C in seawater and the effect of contact time and oil concentration on the sorption behavior was investigated. A mathematical equation of the mass transfer of the oil into the Kenaf was generated to predict the SQ given the time and oil concentration.

THEORETICAL FRAMEWORK

Sorption

Sorption is the process where a material moves across a boundary to move from one phase to another where sorption is generally an equilibrium process and is a result of the interaction between three molecules: the sorbent, sorbate, and the solvent (LaGrega et al., 2001).

Viscosity, surface tension and wettability are certain sorbate characteristics that affect its sorptive capacity. Another important characteristic is the high surface area which will results in longer contact with the sorbate. Other factors that aid in the sorption of the sorbent include the amount of wax in the surface, the hollowed surface, and the noncollapsing lumen of the fiber. The chemical composition as well as the structure of the sorbent also affects its sorptive capacity. The structure, on the other hand, affects the ability of the liquid to penetrate the solid as well as the tendency of the sorbent to swell. Swelling occurs when the solid expands upon sorbing the liquid thus increasing the space that may be occupied by the liquid.

Response Surface Methodology

Response surface methodology (RSM) is a technique that combines statistical and mathematical means in order to develop, improve, or optimize a given process. It is also useful for processes in which the underlying mechanism is not fully understood.

In order to fit a second-order response surface, a minimum of three levels for each design variable is required. The number of distinct design points have to be greater than or equal to 1+2n+n(n+1)/2 where n is the number of levels in the experimental design (Myers and Montgomery, 1995).

Central composite design (CCD) is the most common experimental design for fitting second order models in sequential experimentation. It makes use of a two-level factorial or fraction (resolution V) along with 2n axial or star points. There are then F factorial points, 2n axial points, and nc center runs. Generally an empirical model is based on observed data and general polynomial models are linear functions with unknown coefficients which are solved using linear regression analysis. The RSM determines the number of replications needed, location of the optimum region, the most appropriate function required, the proper experimental design as well as the extent of transformation needed (Myers and Montgomery, 1995).

MATERIALS AND METHODS

Bunker Oil C was added to water with 0.5 M NaCl concentrations to prepare mixtures of the following concentrations: 0.001, 0.002 and 0.003 mL oil/mL mixture.

Kenaf core (20.00 g) was introduced into the mixtures for exposure times of 15.00, 67.5 and 120.00 min. The wet core is then distilled using 20% toluene-xylene solvent to separate the water sorbed by the Kenaf. The oil sorbed by the Kenaf was determined by mass balance and used to solve for the experimental response. The dependent variables were then plotted against time and concentration to determine their respective behavior. Central Composite Design was applied to determine the values of the independent variables for the runs while Response Surface Method was used to generate the mathematical equation and perform statistical analysis. The physical and chemical properties of the sorbent and sorbate were also correlated to the analysis of the data. The experimental set-up is shown in figure 1.

FIGURE 1. Experimental Set-up

RESULTS AND DISCUSSION

The sequential model sum of squares, lack of fit tests and model summary statistics revealed that the quadratic equation is the most appropriate polynomial equation for the sorption process. A quadratic polynomial equation was generated to determine the SQ where t is time in minutes and C is concentration in mL oil/mL mixture:

Eq. 1

at time range of: at concentration range of:

Element%

0

10

20

30

40

C 27.51 N 31.48 O 34.17 Mg 0.07 K 0.23 Nb 6.54

Analysis of variance was applied to reduce the quadratic model by determining

the respective terms that had an effect on the equation. The square of the regression coefficient for Equation 1 has a value of 0.9495. Since this is close to unity, it indicates that the generated equation has predicted values that are close to the response.

The large amount of carbon in the Kenaf (27.51%) explains why it sorbs oil instead of water since it is composed of the same organic material as shown by the lignin content and elemental percent of Carbon as shown in figure 2A with figure 2B showing the SEM result.

FIGURE 2. (A) Elemental Composition of the Kenaf Core ; (B) SEM of Kenaf Core at 10 micrometers

Sorption Quantity, SQ, shown in Figure 3, is based on the oil sorbed per gram of Kenaf core. The behavior observed follows the general sorption kinetics of organic pollutants using natural sorbents which is characterized by a rapid initial uptake and then a slow approach to equilibrium.

Sorption Rate is affected by the pore volume of the Kenaf core which dictates how much Bunker Oil C can be accommodated by vacant spaces within the Kenaf core. The BET pore volume (8.519E-04 cc/g) of the Kenaf is small; therefore not much oil can be accommodated in its interior. The high kinematic viscosity (241.9mm2/s) of Bunker Oil C also contributes to the small SQ value since its molecules are larger. This makes it more difficult to sorb and penetrate the inner portion of the Kenaf core.

(A)(B)

FIGURE 3. Sorption Quantity Interaction Profile

CONCLUSION AND RECOMMENDATION

Among the mathematical models, the quadratic model was chosen based on statistical tests. Results show that the sorption behavior is best at high oil concentration and longer duration of exposure. However, contact time was observed to have greater effect on the sorption properties compared to the oil concentration.

In the graphs generated, there is a difference in the SQ with respect to time. The common trend when all the response parameters are plotted against time is an increase in the response at the start followed by a gradual decrease until change in the response is no longer evident. The sorption of organic pollutants involves two mechanisms which results in an initial surge in the sorption which is then followed by a retarded increase. Various models have been proposed to explain this behavior.

Philippine Kenaf core is not as good a sorbent compared to activated carbon due to its low pore volume and surface area but the sorption performance of the core is the best among the other parts of the Kenaf plant. This makes the Kenaf core a good

sorbent along with the pore diameter which falls under the range of good sorbent. Its natural properties such as biodegradability and oleophilic nature make it useful as an alternative response for oil spills unlike most substances which require processing before application.

For further investigation of the sorption capacity of Kenaf core in simulated oil spill, it is suggested to apply the method of steepest ascent to optimize the response. This may be used to obtain the values of the independent variables to derive the conditions to maximize the sorption quantity. This is done by determining the principal axes of the contour lines generated in the initial experiment and then drawing a perpendicular line from it. The direction to be followed is obtained from the behavior of the contour plot as to whether it is increasing or decreasing. Other parts of Kenaf may also be used as sorbent, particularly the pitch which has a large sorption capacity. Lastly, other investigations may also be conducted on different oils used as the sorbate to determine how the different oil properties affect the sorption behavior. Properties of the sorbent (pore volume, pore size, surface area of the pore) and sorbate (viscosity, surface tension, density) which affect the sorption behavior may also be included among the independent variables in other studies to formulate a more general equation.

REFERENCE

Bedient, P.B.; Rifai, H.S.; Newell, C.J. Groundwater contamination: Transport and remediation. NJ; PTR Prentice Hall, 1994.

LaGrega, M.; Buckingham, P.; Evans, J. Hazardous waste management. New York; McGraw-Hill Co. Inc, 2001.

Lee, B.; Han, J.; Rowell, R. Lignocellulosic Fibers. 1999, 35: 423-433.

Myers, R.; Montgomery, D. Response surface methodology: process and product optimization using design of experiments. New York:Wiley Series in Probability and Statistics, 1995.

Zaveri, M. Thesis, North Carolina State University, May 2004.