Photocatalytic Degradation of 2, 2, 4 Trimethyl Pentane (Isooctane) in Aqueous Solution

Introduction

Photocatalysis has been proposed as an alternative for the treatment of organic pollutants in recent years as it is an attractive technique for the complete destruction of undesirable contami-nants (pollutants) in liquid phase by using artificial light or solar illumination (1). It is a natural phenomenon in which a substance, termed the photo catalyzer, alters the speed of a chemical reaction through the action of light. By exploiting the energy of light, photo catalyzer induce the formation of strongly oxidizing reagents which can cause the decomposition of some organic and inorganic substances present in the water (2). The advantages of photocatalysis include low opera-tion temperature, low cost and significantly low energy consumption. These factors have led to the relevant application of photocatalysis to the stage of commercialization (3). Photocatalytic processes are based on the generation of hydroxyl radicals HO (4-7).

The pollutant, isooctane is a colourless liquid with the odour of gasoline, it is also one of two chemicals used in establishing the octane rating for gasoline. The octane value is a number that

reflects the resistance of a gasoline mixture to knocking when used as fuel in an internal combustion engine. The source of isooctane is mainly from the production of petroleum products, it is also an organic pollutant that is very toxic and is easily found in the environment. Therefore, the photodegradation of 2, 2, 4 trimethyl pentane (isooctane) as the target compound in this study not only provide useful information for the treatment of isooctane but also it can be viewed as the primary method of inquiry for dealing with complicated organic pollutants.

Many examples of complete photo-oxidation of organic compounds have been reported. The photocatalytic degradation of some organics both in TiO₂ aqueous suspensions and over immobilized TiO₂ thin films have been studied (8).

In most of the above studies, Langmuir–Hinshelwood model was used to describe the degradation rate expressions in terms of the disappearance of compounds or the formation of CO₂  (9).

It has been demonstrated that catalyst dosage,
initial concentration of pollutants, effect of hydrogen peroxide, and pH of the solution are the main parameters affecting the degradation rate. It is well known that kinetic modelling of the photocatalytic process is essential to any practical applications (10). 

The objective of this study is to degrade isooc-tane using solar radiation with different catalyst such as TiO_2­, ZnO, Periwinkle shell and Snail shell ashes, to know the best catalyst for this study.

In this study, analysis was conducted to know the effects of operational parameters such as pH, initial concentration, oxidant and catalyst dose on the degradation rate. The adsorption of isooctane is further evaluated by the common isotherms (e.g., Langmuir, Freundlich isotherms) and kinetic models (e.g., Langmuir – Hinshelwood, pseudo – first – order, pseudo – second – order).

Materials and Methods

Materials

The photocatalyst TiO₂ P-25 (mixture of Anatase and Rutile form of titanium dioxide in the ratio of 70:30), ZnO grade material 99% purity (Emory), was procured from Whosco Ventures Nigeria Limited. Periwinkle shell and snail shell ashes were prepared in the laboratory. Isooctane compound was purchased from Stanvac, hydrogen peroxide, (0.1, 1M) HCl and NaOH were obtained from Stanvac, distilled water was also used.

Methods

The photocatalytic tests were conducted using the batch slurry system under light condition, it was carried out in order to evaluate the adsorption of isooctane on the surface of the four catalysts (TiO₂, ZnO, periwinkle shell and snail shell ashes), and so to obtain the most efficient catalyst needed for the study. Analysis contained fundamental parameters such as catalyst dosage, oxidant, pH, time variation and different initial concentrations of isooctane.

Determination of the best catalyst

For this purpose, 100ml of the 200mg/L solution was taken and put into four conical flasks of 250ml, 5g the optimum amount of different catalysts was added (TiO₂, ZnO, periwinkle shell and snail shell ashes) into each of the conical flasks and shaken for 2hrs in the sun with an orbital shaker. The catalysts were filtered from the sample by Whattman filter paper (0.7µm) then it was analyzed using UV – Vis spectrophotometer.

Effect of Catalyst dose

In six conical flasks, 250ml of 200mg/L
isooctane solutions was placed with various catalyst doses of 2.0 – 12.0g/L of TiO₂, and they were shaken under the sun for 1h. The samples are filtered and analyzed with UV spectrophoto-meter to obtain optimum catalyst dose.

Effect of initial concentration of Isooctane

Two hundred milliliter of 50 – 300mg/L of isooctane concentration was placed in six conical flasks. Then4.0g/L titanium dioxide (TiO₂) was added to each solution and exposed to the sun for a period of 1hr and analyzed to obtain optimum concentration.

Effect of pH

One hundred milliliter of 50mg/L (best concentration) was placed in six conical flasks whose pH was adjusted using 0.5M HCL and 0.1M NaOH solution and then measured with pH meter. The pH values were varied from 2.0 – 12.0 and the samples were shaken under the sun for 2hrs; then they were filtered and analyzed to obtain optimum pH for best degradation rate.

Effect of oxidant (H₂O₂)

One hundred milliliter of 50mg/L (best concentration) was placed in six conical flasks with catalyst dose of 4.0g/l TiO_2, at a pH of 8.0 and different volumes of hydrogen peroxide (2 – 12ml). Samples were analyzed to evaluate the effect of oxidant on photo degradation. Each of these samples was also placed on an orbital shaker, and was shaken for a period of 2 hrs, to allow equilibrium to be reached, and then it was filtered and analyzed with UV spectrophotometer.

Effect of Exposure time

Two hundred milliliter of 50mg/L (best concentration) was placed in one conical flask adjusted to a pH of 8.0 (optimum pH) and fixed catalyst dose of 4.0g/L and then it was placed on an orbital shaker. The sample was shaken under the sun for 2 hrs. With an interval of 20mins for 10ml of sample to be withdrawn with a syringe, and then it was filtered and analyzed using UV spectrophotometer.

Photocatalytic Degradation using solar radia-tion

In the analysis investigate photocatalytic degradation; TiO₂ was used as photocatalyst because it was found to be the best among the other catalysts with a percentage degradation of 57%. In the analysis using solar radiation, 200ml of varying concentration (50 - 300 mg/L) of isooctane adjusted to a pH of 8.0 were placed in

Figure 1. Percentage degradation of isooctane using different catalysts

six conical flasks, then 4.0g/l titanium dioxide (TiO₂) was added to each solution and place on an orbital shaker. All samples were shaken with a regulated speed of 175 rev/min, under the sun, to allow the penetration of light for a period of 2hrs. With an interval of 20 min for 10ml of the sample to be taken with syringe, then they were filtered with Whattman filter paper (0.7µm) and analyzed with UV spectrophotometer with a wavelength of 246nm.

Analysis

In using the UV spectrophotometer, it was switch on for at least 20mins to warm it up properly. After then, it was turn to the UV mode and the wavelength was adjusted to 246nm. A blank solution of 50% ethanol was prepared for zero checking the reading of the UV spectrophotometer. Then the filtered isooctane solution was poured into thoroughly washed glass Cuvette to at least 2/3 of its total height. One Cuvette is filled with the reference compound and the other one with the compound (ethanol) whose absorbance has to be measured at λ max. The Cuvette was placed in the Cuvette chamber inside the UV machine and the lid was closed. Then, the absorbance reading was displayed on the screen and it was recorded. To get the relationship between concentration and absorbance of the compound, a calibration curve is made. Calibration solutions are made from standard solutions with known concentration. The absorbance is plotted against known concentration of the samples. This standard curve is used to calculate the value of concentration of unknown sample from the absorbance reading. This was repeated for each of the filtered sample taken after time t and the results were recorded.

Results and Discussion

Effect of Catalyst Type

The catalyst efficiency of TiO_2, ZnO, PSA and SSA were 57%, 50%, 37% and 27.3% respectively. As a result the rate of degradation of isooctane was observed to be decreasing; this tells that UV from the sun increased the rate of degradation of isooctane. TiO₂ was the best catalyst compared to the other catalysts. In the case of the commercial and the local catalysts, among commercial catalysts (TiO₂, ZnO) the rate of degradation with using TiO₂ was higher than ZnO and among local catalyst (PSA, SSA) PSA was more effective than SSA.

Effect of Catalyst dosage

Photocatalyst (TiO₂) dosage was varied from 2.0g/l to 12g/l during reactions for studying the isooctane degradation rate at different concentrations. It was observed that degradation rate of isooctane increases with increase in catalyst dose and becomes constant above a certain level as shown in (Figure 2)and after one particular optimize dose the degradation rate of the isooctane starts to decrease. The reasons for this decrease in degradation rate are:

(i) Aggregation of TiO₂ particles at high concentrations causing a decrease in the number of surface active sites and

(ii) Decrease in opacity and light scattering of TiO₂ particles at high concentration. Therefore the optimum dose at which the highest degradation was observed was 4g/l

Figure 2. Percentage degradation of isooctane for varied catalyst dose (TiO₂)

Effect of Initial Concentration

Initial concentration was varied from 50 – 300mg/L, at constant catalyst dose of 4g/L. It was observed from the graph (Figure 3) that increasing concentration of compound solution from 50 to 300mg/L decreases the percentage of degradation rate and it was found that at 50mg/L concentration, degradation was 88.3% at 246nm and for 300mg/L compound concentration, percentage of degradation was reduced to 62.2% at 246nm. The reason behind this behavior   may be due to the increase in the extent of adsorption on the catalytic surface at reference compound concentration which reduces the catalytic activity of TiO₂. The increases in the isooctane solution concentration also decrease the path length of photon entering into the solution. Therefore 50mg/L was the optimal concentration with a percentage degradation of 88.3%.

Figure 3. Percentage of degradation of isooctane for varied initial concentration using TiO₂ catalyst dose of 4.0g/L

Effect of pH

The pH was varied from 2–12. The result shown in Figure 4 indicates that degradation increases with an increase in solution pH from 2 to a range of 8, and then led to a drastic decrease which result implies that the degradation of isooctane was favorable at alkaline conditions, and the optimum initial pH of solution is 8 (63.03%). This may be due to the fact that pH affects the state of reaction in solution and catalyst surface.

Figure 4.Effect of initial pH on photocatalytic degradation of isooctane for the Conc. = 50mg/L TiO₂ = 4.0g/L

Effect of Oxidant (H₂O₂)

Another important data from these experiments is the effect of adding   hydrogen peroxide. As it is shown in Figure 5, the addition of hydrogen peroxide has a positive effect on the degradation. The experiment was conducted by varying the hydrogen peroxide volume from 2.0 to 12 ml per 200ml of the isooctane sample. It was observed that the degradation of isooctane sample increased with increase of oxidant, showing that the use of TiO₂ by adding hydrogen peroxide molecules to generate hydroxyl radicals is more effective for this photocatalyst.

Figure 5. Percentage degradation for varing volume of oxidant (H­₂O₂) using TiO₂ Catalyst

Effect of Time Variation

Figure 6 shows the percentage of degradation of isooctane at pH 8.0, and with 50mg/L concentration for varying time using TiO₂ catalyst dose of 4.0g/L. It can be clearly seen that under light radiation with increasing the time of irradiation up to 2h, the amount of isooctane increased from 0 - 56.3%,  almost in the proportion to the irradiation time in the investigated period. This is because that with an increase in irradiation time the photons absorbed on the surface of the photocatalyst becomes greater which in turn helps in the photocatalytic process.

Figure 6. Percentage degradation of isooctane at pH 8.0 for varied time using TiO₂ catalyst dose of 4.0g/L

Kinetics and Modeling of Photocatalytic Process

The destruction rates of organics in photocatalytic oxidation have been modeled by different kinetic models. Langmuir-Hinshelwood (L-H) kinetics seems to describe many of the reactions fairly well. The rate of destruction is given by Eq.1:

   =                                                        (1)

In the ideal case, for which the L-H model is derived, C is the bulk solute concentration, k1 is the reaction rate constant, k₂ is the equilibrium adsorption constant and t represents time. The L-H reaction rate constants are useful for comparing the reaction rate under different experimental conditions. Adsorption of organic substrate onto the catalyst can be modeled by mathematical models such as Langmuir and Freundlich model.

The mathematical formulae of Langmuir and Freundlich models are shown in Eqn. (2) and (3) respectively; both equations can be rewritten into linear form as shown in Eqn. 4 and Eqn. 5 respectively.

                                                          (2)

                                                       (3)

                                              (4)

                                        (5)

Where

K_F=Freundlich constant for adsorption capacities (L/mg)

K_L=Langmuir constant (L/mg)

1/n=Freundlich exponent (surface heterogeneity)

Table 1. Linear regression analysis of kinetics of isooctane degradation

Kinetics and Modeling

Table 2 shows the linear regression analysis of the kinetics of isooctane degradation using photocatalysis. These results were used to analyze the rate of adsorption and possible adsorption mechanism of isooctane into TiO₂ catalyst using pseudo-first-order and pseudo-second-order models.  From the above Table, the first order regression range from 0.766 - 0.991 while the second order range from 0.694 - 0.974. Hence, pseudo first order gave a better regression. As the result, the photodegradation of isooctane follows the Pseudo – first order.

Langmuir – Hinshelwood Model

The results confirmed the Langmuir-Hinshelwood relationship and indicated that adsorption represents the initial step in the

Figure 7. Langmuir – Hinshelwood model for the photodegradation ofisooctane using TiO₂ catalyst

Table 2. Reaction rate, Equilibrium constants and regression for isooctane using Titanium dioxide (TiO₂)

photocatalytic process.  The k and K_LH values calculated from the slope of the straight line (r² = 0.9715) and from the ordinate, were 1.53 × 10^-3 L/mg and 3.00 mg/L.h respectively. Hence the value for adsorption is lesser than K_LH for reaction. This implies that degradation was basically result of reaction and the photocatalytic degradation of isooctane under optimized working conditions which follows satisfactorily the Langmuir – Hinshelwood model. This explanation is similar to that reported by Guettai et al 2005 (11).

Adsorption Isotherms

In order to evaluate the efficacy of the photodegradation procedure, the equilibrium adsorption of isooctane was studied as a function of equilibrium concentration. Adsorption isotherms were studied to estimate the adsorption capacity. The adsorption process of isooctane was tested with Langmuir and Freudlich isotherm models.

Figure 8. Langmuir Isotherm

Figure 9. Freundlich Isotherm

Figure 8 shows the Langmuir Isotherm. From the plot, the maximal adsorption quantity (q_m) and the Langmuir adsorption constant (K_L) were found to be 18.12 mg/m² and 9.175× 10^-3 L/mg
respectively. Also, Figure 9 shows the Freundlich Isotherm. From the plot, the Freundlich adsorption constant (K_F) was found to be 0.07353 L/mg and n to be 2.26.This explanation is similar to that reported by Alina et al 2011 (12).

The isotherm models gave good correlation coefficients with r² ≥ 0.999. The applicability of the two isotherm models implies that both monolayer adsorption (that is only a limited number of surface sites as adsorbing sites for the isooctane molecules) and heterogeneous surface conditions exist under studied experimental conditions.

Adsorption Kinetic Modeling

Several Kinetic models are used to explain the adsorption mechanism. Pseudo – first order and Pseudo – second – order kinetics allow the compu-tation of the first and second order constant, k₁ and k₂ and of linear regression correlation coefficient, r².

Figure 10. Pseudo – first – order reaction kinetics for isooctane adsorption on TiO₂ catalyst. Condition: pH 8.0, catalyst dosage 4g/L

Figure 11. Pseudo – second – order reaction kinetics for isooctane adsorption on TiO₂ catalyst. Condition: pH 8.0, catalyst dosage 4g/L