Kinetic Evaluation of Naphthalene Removal using Acid - Modified and Unmodified Bentonite Clay Mineral

: Kinetic evaluation of naphthalene onto acid – modified and unmodified bentonite clay mineral was investigated by means of the effects of concentration, contact time and pH. The amount of naphthalene adsorbed was determined spectrophotometrically. The optimum pH value and equilibrium contact time for the adsorption of naphthalene onto acid – modified and unmodified samples were found to be 6.0 and 30 minutes, respectively. The maximum adsorption capacity (q m ) for acid – modified and unmodified adsorbents was found to be 10.52 mg/g and 5.56 mg/g. These values were close with the experimental data (q e ) obtained from the adsorption of naphthalene onto acid – modified (9.0 mg/g) and unmodified (4.0 mg/g) adsorbents. The adsorption isotherm fitted well with Langmuir equation model for the acid – modified than the unmodified adsorbent. Kinetic evaluations of the adsorption showed that the process followed the pseudo – second – order equation model. The results obtained showed that modified bentonite clay mineral was reasonably effective adsorbent for the removal of naphthalene (organic contaminant), which is an important source for environmental pollutant. ©JASEM

http://dx.doi.org/10. 4314/jasem.v18i1.20 Introduction: Polycyclic aromatic hydrocarbons (PAHs) are potent atmospheric pollutants made up of chemical species with two to six fused benzene rings without heterogeneous substituent (Fetzer, 2000). Naphthalene is found in this family of PAHs. They are an important family of toxic hazardous and longpersistent environmental pollutants that have been found to be carcinogenic and mutagenic (Cabal et al., 2009). PAHs originate from natural and anthropogenic sources. Anthropogenic sources include industrial processes, crude oil refinery, incinerators, urban run-off, engine exhaust, domestic heating system, incomplete combustion of fossil fuels, coking process and smoke (Cabal et al., 2009). Natural sources are terrestrial coal deposits, forest fires and volcanic eruptions. The main sources of PAHs in surface waters are atmospheric deposition, runoff from contaminated soil and deposition from sewage discharges (Milhome et al., 2008). Most PAHs are hydrophobic with high boiling and melting points and electrochemical stability. Due to their hydrophobicity, they have low water solubility and can therefore exist and accumulate in soils or water for a long period of time (Ozer et al., 2008). The removal of these kinds of pollutants from the environment cannot be accomplished by traditional methods. It is now universally recognized that adsorption technology provides a feasible and effective method for the removal of pollutants from contaminated water resources and waste waters. Activated carbons are the most commonly used adsorbent in the adsorption process due to their high adsorption capacity, high surface area and high degree of surface reactivity; however, regeneration is difficult and expensive (Okoye and Obi, 2011). In recent years, extensive research is now focused on new, efficient, low-cost and easily obtainable natural materials such as clay materials, agricultural byproducts etc, to be used as adsorbents (Okoye and Obi, 2011;Abia et al., 2007). Clays attract attention due to the heterogeneity of their surfaces and some have been found to possess the ability to absorb ions from solutions and release them later, when the conditions change. There are about thirty different types of "pure" clays but most natural clays are mixtures of these different types, along with other weathered minerals (Ivan and Velimir, 1998). Studies have shown that natural clay is an effective adsorbent due to its efficiency, low cost and availability. The adsorption capabilities of natural clay are attributable to their high surface area and exchange capacities including the presence of negative charges on the clay mineral structure which can attract positively charged metal ions.

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OBI, C; OKOYE, IP This work is aimed at investigating the potentiality of acid -modified bentonite clay mineral in the removal of naphthalene, an organic pollutant from aqueous solution. Kinetic evaluations were explored to ascertain the mechanism of adsorption process.

MATERIALS AND METHODS
Starting material and modification process: Acid activated bentonite was obtained by following the procedure described by Mohammed et al., (2007) and Makhoukhi et al., (2009). The commercial bentonite clay (20 g) was weighed and put into a round bottom flask and 200 ml of 5 M H 2 SO 4 was added to it to form slurry. The mixture was stirred for 30 mins with a magnetic stirrer. The mixture was heated in a water bath and kept at temperature 80 o C and refluxed for 2 hrs. The mixture was left to cool, filtered and the residue was washed severally with de-ionized water to remove sulphate ions completely from the activated clay. The clay was dried for 24 hrs; hence the product is acid activated bentonite clay.
Equilibrium adsorption measurement: The batch adsorption experiments were carried out with 0.1 g of clay samples, stirred in 50 ml of 10 -60 mg/l of naphthalene solutions of at 298 K for 1 hr. The pH of the naphthalene solutions for acid -modified and unmodified were varied from 2 -12. The effect of contact time was varied from 10 -60 mins at 298 K using 50 mg/l of the naphthalene solution. In every case, the mixture was filtered and the supernatant solution collected for absorbance analysis using UV spectrophotometer at the wavelength of 219 nm.
The metal uptake by the adsorbent at equilibrium was calculated from the following expression: Where q e is the amount of organic pollutant adsorbed in mg/l, C o is the initial adsorbate concentration in mg/l, C e is the concentration after equilibrium in mg/l, V is the total volume of solution in liter and M is the mass of clay sample (adsorbent) used in g.

Effect of Initial Organic Pollutant Concentration:
The equilibrium adsorption capacities of naphthalene on acid -modified and unmodified bentonite clay minerals at constant temperature of 298 K are represented in Fig. 1. The result revealed that the adsorption capacities of both modified and unmodified increased steadily with naphthalene concentration. This can be attributed to the increase in the concentration gradient which acted as a driving force for the adsorption process. The optimum adsorption capacity was observed at the concentration of 30 mg/l for both adsorbents. However, the increase in adsorption capacity became less significant above 30 mg/l. This is probably as a result of saturation at the active sites of modified and unmodified samples. The result further showed that the adsorption capacity obtained for acid -modified sample, 10.52 mg g -1 was higher than that of unmodified sample, 5.56 mg g -1 . This could be attributed to the presence of acid sites on the modified clay sample or increase in pore size/volume.

Effect of solution pH:
The effect of pH on the removal of naphthalene by acid -modified and unmodified bentonite clay minerals from aqueous solution is represented in Fig. 2. The result showed that maximum adsorption capacity was observed at pH close to the neutral values. It was observed that the adsorption process is highly dependent on the pH of the solution. Therefore the optimum pH for the sorption process is 6.0. This could be as a result of the synergistic effect of the neutral properties of naphthalene and solvent (ethanol) used in dissolution. At higher pH values (8 -12), the adsorption capacity decreased significantly. This could be as a result of saturation of the naphthalene and solvent at the solid -liquid interface.

Effect of contact time:
The effect of contact time on the adsorption of naphthalene onto acid -modified and unmodified clay samples at 298 K is presented in Fig. 3 below: . Fig: 3 Effect of contact time on the removal of naphthalene at 298 K Maximum adsorption capacities for both acidmodified and unmodified bentonite clay samples was observed at 30 minutes, beyond which there was almost no further increase in the adsorption capacities. In order to investigate the adsorption kinetics of the adsorbate onto the adsorbents, Lagergren pseudo -first -order, pseudo -second order, Elovich and intraparticle diffusion models were applied to the experimental data. The linearized forms of the models are presented below:

OBI, C; OKOYE, IP
Where q e and q t are amounts of adsorbed at equilibrium and at various times t (mg g -1 ), k 1 and k 2 are rate constants of the Lagergren first -order and pseudo -second -order models for the adsorption process (mins -1 and g mg -1 mins -1 ), α is the initial adsorption rate (mg g -1 mins -1 ), β is the desorption constant (g mg -1 ) for Elovich equation, C is the intercept, and k p is the intraparticle diffusion rate constant (mg g -1 mins -1/2 ). The straight line plots of these models have been represented in Fig. 4 -7.

Adsorption isotherm of the organic pollutant:
The distribution between the adsorbent and adsorbate interface at equilibrium was described by the Langmuir and Freundlich equations. These two models are widely used, the former assuming that maximum adsorption occurs when the surface is covered by adsorbate and the latter being purely empirical. The linearized forms of Langmuir and Freundlich equations are commonly presented as: C e /q e = C e /q m + 1/k L q m And In q e = In k F + 1/n In Where q e is the equilibrium amount of solute adsorbed per unit weight of adsorbent in mg g -1 of clay; C e is the equilibrium organic pollutant concentration in aqueous phase in mg l -1 , q m is the q e for a complete monolayer in mg g -1 , k L is a constant related to the affinity of the binding sites and energy of adsorption in l mg -1 , k F and n are constants related to the adsorption capacity and intensity of the adsorbent respectively.
However, the equilibrium data for naphthalene over the concentration range studied at 298 K was correlated with Langmuir isotherm model by plotting C e /q e against C e . Linear plots obtained are shown below : 1.08 0.24 r 2 0.14 0.24 Pseudo -second -order k2 (g.mg -1 .min -1 ) -0.13 -1.30 qe (mg.g -1 ) 9 This implies that the sorption of naphthalene onto acid -modified and unmodified clay samples followed a monolayer formation. The result of the q m (mg g -1 ) parameter suggested that the monolayer capacity of the acid -modified was greater than unmodified.
On the other hand, a plot of In q e against In C e ( Figure  9) was linear, though over a limited region of C e which enabled determination of the constants k F and n.  Tab. 3. The numerical values of n (between 2 and 10) indicate the adsorption capacity was only slightly suppressed at lower equilibrium concentrations.

Conclusion:
The results obtained revealed that the adsorption capacities of modified and unmodified bentonite clay samples were observed to be dependent on concentration, contact time and pH. Maximum adsorption capacities of 10.52 mg g -1 and 5.56 mg g -1 were obtained for modified and unmodified bentonite clay samples respectively. The adsorption process followed a monolayer formation. The kinetics of the adsorption process followed pseudo -second -order equation. The results indicated that acid -modified bentonite clay mineral can be a promising adsorbent for the removal of naphthalene (PAH) from aqueous solutions and contaminated waste waters. Parameters