The clay and sandy soil used in this study were obtained from Ikpoba River, Benin City, Edo State, Nigeria. After collection, the materials were stored in a black polythene bag. Stones and other heavy particles were removed from the materials. A 220 µm mesh was used to remove the large non-clay fractions from the clay and a 2.5 mm mesh was used to sieve the sandy soil. Then they were dried overnight at 343˚K in a vacuum oven and stored in an air-tight container covered with a black polythene bag prior to the adsorption experiments.

The adsorbate, naphthalene was bought from a chemical laboratory in Benin City. The distilled water used for sample preparation, dilution and solution was obtained from the Department of Chemical Engineering, University of Benin, Benin City, Edo State, Nigeria.

The effect of the variation of concentration of naphthalene in the effluent solution is described by using the Langmuir, Freundlich and Tempkin Isotherm models. These isotherms are graphically represented below.

Figures 1-6 show that the equilibrium concentration and the amount of naphthalene adsorbed increased as the initial effluent concentration increased. The Langmuir, Freundlich and Tempkin Isotherm constants were calculated from the slope and intercepts of the graphs. The Freundlich Isotherm plot in Figure 3 gave the best linear plot with a correlation coefficient (R² = 0.9647) for naphthalene sorption with clay. The Langmuir isotherm plot in Figure 4 gave the best linear plot with a correlation coefficient (R² = 0.8602) for naphthalene sorption with sandy soil. The experimental data for this work best fits the Freundlich model. This result agrees with the work of [10] in which clay adsorbent was also used. The adsorption capacity Qo (mg/g) and energy of adsorption n for naphthalene were 5.734 and 0.0107, respectively.

From Figure 7, the adsorption results showed that equilibrium was achieved in 26 and 20 hrs for naphthalene in clay and sandy soil, respectively. From the equilibrium time, sandy soil was observed to reach equilibrium faster than clay for naphthalene used in this study. This can be attributed to the large pore spaces, intra-porous nature and the high permeability of sandy soil. Figure 7 also indicates that there was a steady adsorption of naphthalene by the two different adsorbent.

The effect of agitation time on the adsorption of naphthalene is shown in Figure 8. It shows that agitation time has an effect on the adsorption of naphthalene from an aqueous solution. The amount of naphthalene adsorbed increased as the agitation time increases until an equilibrium time was reached beyond which the amount of naphthalene removed becomes negligible and in the long run becomes constant. The sharp decrease in the curve for both clay and sandy soils indicates that significant adsorption is taking place at that particular time. The removal of naphthalene is rapid in the initial stages of agitation time and gradually decreases with the lapse of time until saturation. The removal curve is single, smooth and continuous indicating monolayer coverage of naphthalene on the outer surface of the adsorbent [9] .

Table 1 shows the results of the kinetic modeling of the adsorption of naphthalene onto clay and sandy soil used in this study. Comparing the values of the correlation coefficients R² of pseudo-first order and second order eq-

uations, the latter is better than the former and can be used to predict the adsorption kinetics of naphthalene on clay and sandy soil. The value of the reaction rate constant k₁ predicted for the adsorption kinetics by the pseudo-first order was found to be ambiguous. The amount of naphthalene adsorbed in the clay sample before equilibrium was 39.84 mg while in the sandy soil, the amount was 19.92 mg. Based on the amount adsorbed, given equal times before equilibrium, clay could be described as a better adsorbing agent of naphthalene than sandy soil. This property of clay over sandy soil can be attributed to its larger surface area and higher porosity than sandy soil [11] . Sand has low porosity though not as many pore spaces because the grains are very large such that in a unit of sand, the fraction of soil volume that consists of holes is a lot less than for clay. Clay has many small pore spaces in which water containing the contaminant solutes remains clinging to the clay particle surfaces. Porosity is an important consideration when evaluating the potential volume of water or amount of hydrocarbons sediments may contain. Sediments with higher porosity typically have higher hydraulic conductivity, a property of sediments that describes the ease with which water can move through pore spaces [12] .