pH: The pH value determines whether the activated carbon is acidic or basic. The acid or basic nature of an activated carbon depends on the means it was prepared, inorganic matter and chemically active groups on its surface as well as the kind of treatment applied. The pH value obtained in this present investigation revealed that MTCNS with pH of 4.42 as presented in the Table 4.1 is acidic in nature which was consistent with the result of Almond shells activated carbon (ASAC) subjected to phosphoric acid treatment having pH of 4.5 as reported by Bhatti et al. (2007). Also, this value was in agreement with the finding carried out by Cheremisinoff and Ellerbusch (1978) that the pH of either raw or modified agricultural by-products in water suspension can vary between 4 and 12, hence, it can be deduced that MTCNS is a good activated carbon material.
Conductivity: This is a measure of the ability of water to allow the passage of an electrical current, and the unit is in micromhos per centimetre (µmhos/cm) or microsiemens per centimetre (µs/cm). Conductivity can be affected by many factors which includes the presence of inorganic dissolved solids (ions that carries negative and positive charges such as Cl-, NO3-, SO42- , PO43-, Ca2+, Na+, Mg2+, Al3+, etc.); organic compounds (like oil, phenol, alcohol & sugar); and temperature (the warmer the water, the higher the conductivity). From the result obtained, it was observed that MTCNS studied has conductivity of 197 µs/cm as revealed in Table 4.1. In a similar research work, the conductivity of the phosphoric acid activated (ASAC) sample obtained by Bhatti, et al. (2007) was discovered to be 40 µs/cm.
Moisture Content: The moisture content of a sample refers to the amount of water physically bound on the sample under normal condition. The laboratory result of the moisture content was determined to be 7.73 % as shown in Table 4.1, and this was slightly higher than 7.21 % moisture content of Almond shells as reported by Erhan, et al., (2004) in their studies. The permissible limit of moisture content is 3 – 8 %; low moisture content is desired by activated carbon because its presence increases the rate of adsorption of contaminants into the microspore of the activated carbon (Inyang, et al., 2010). High moisture content allows penetration of more contaminants into the matrix of the adsorbent thus reducing working capacity of the adsorbent (Appendix A2).
Bulk Density: Bulk density is the ratio of mass of the aggregate to the volume of aggregate particles with voids between them; hence, it is used to convert quantities by mass to quantities by volume. The bulk density of activated carbon depends on several factors such as size; shape and degree of compaction of individual particles, and its data are useful to Engineers for the estimation of tank, cartridge or packing volume. Bulk density is important parameter when carbon is removed by filtration because it determines how many pounds of carbons can be contained in a filter of a given solid capacity and how much treated liquid is retained by the filter cake. The American Water Work Association has set a lower limit on bulk density at 0.25 g/ml for Granular Activated Carbon (GAC) to be of practical use (AWWA, 1991). The bulk density of prepared MTCNS (adsorbent) sample used for this work is within that limit, which is calculated to be 0.30 g/ml (Appendix A3).
Specific Gravity: This is ratio of the weight of a given volume of material (activated carbon) to the weight of an equal volume of water, indicating how much heavier (or lighter) the material is than water. The knowledge is necessary in the computation of fine particle properties like void ratio, degree of saturation, size distribution etc. The result obtained from the present study of specific gravity of MTCNS (adsorbent) was found to be 1.58 (Appendix A4), meanwhile 4.45 was obtained from activated carbon prepared from chemically treated Terminalia catappa nut shells (TTCNS) (Andal & Gohulavani, 2013).
Porosity: This is used to explain how much empty or void, space is present in a given sample. It shows the capacity of activated carbon in terms of its efficiency. Porosity of the studied MTCNS (adsorbent) was evaluated to be 39.95 % (Appendix A5). Activated carbon used in determining pore volume by Aneke and Okafor (2005) gave porosity of 21.4 %.
Ash Content: The ash content of a sample is the inorganic (non-carbon) residue left after the organic matter has been burnt off which is not chemically combined with the carbon surface; also the ash content primarily depends on the types of raw material used for the production of the activated carbon. The percentage of ash content for MTCNS (adsorbent) sample studied was found to be 2.32 % which was consistent with Romero Gonzalez, et al., (2001) reported result of 2.14 % for almond shells. The obtained value for MTCNS was favourable because the ash content serves as interference during the adsorption (Kha, et al., 2009). High ash content is not desirable and is considered as an impurity for activated carbon since it reduces the mechanical strength of carbon and affects its adsorptive capacity. The lower the ash content, the better the quality of the activated carbon. (Appendix A6).
Pore Volume: Pore volume is of importance in the facilitation of the adsorption process by providing sites and the appropriate channels to transport the adsorbate. The result obtained for MTCNS (adsorbent) was estimated to be 4.93 ml (Appendix A5). But in a similar research work carried out by Andal and Gohulavani (2013) using chemically treated Terminalia catappa nut shells (TTCNS), the pore volume was discovered to be 6.80 ml which shows that MTCNS is a good activated carbon with highly developed porous structure.
4.1.2 Fourier Transform Infrared Analysis
Figure 4.1 surmise the FTIR spectrum obtained in order to give an idea about the organic functional groups present in modified almond nut shells (MTCNS) sample that can participate in bonding with 2,6-DCP during adsorption process. The peaks emerging in the FTIR spectrum were assigned to a variety of functional groups in accordance to their respective wave numbers as stated in literatures.
Table 4.2 shows the FT-IR spectrum elucidation of modified almond nut shells (MTCNS). A sharp peak is recognized around 3777.89 cm-1 which is attributed to non-bonding (free) hydroxyl (–OH) group of water. The strong and broad absorption peak at 3394.00 cm-1 depicts that of OH bond of alcohol and carboxylic acid groups; and N-H bond of amide groups with stretched vibrations. The peak observed at 2923.13cm-1 was associated with the stretching vibrations of C-H bond of methyl, methylene and methoxy groups (Feng et al., 2008), and those peaks appearing around 1607.35 – 1734.38 cm-1 corresponded to C-H (alkane), C=C (aromatic) and C=O stretch. On the other hand, the absorption bands 1247.17 – 1442.00 cm-1 were ascribed to C-H bend, C-O (alcohol), C-N, and OH (carboxylic acid) and the one at 1046.88 cm-1 to C-O (alcohol), C-H and C=N bend (nitriles) respectively. The weak band with wave number of 604.70 cm-1 was assigned to C-H bend and C=C which are out of plane. Consequently, the FT-IR spectra indicates that hydroxyl, carboxyl, and carbonyl groups were very important (hetero-atoms) functional groups which participate in the binding of 2,6-DCP to the surface of MTCNS (adsorbent).
Effect of pH on Adsorption
pH of an aqueous solution is an essential operational parameter prevailing the adsorption process of organic chemicals or metals in solution as it not only affects the solubility of the chemical ions concentration of the counter ions on the functional groups of the adsorbent, but also influences the degree of ionization of adsorbate during reaction (Agarry et al., 2013b). The effect of variation of pH in the range of 2 -10 on the adsorption of 2,6-DCP by MTCNS (adsorbent) was studied from the data by keeping other parameters constant as presented in Table 4.3. The relations between removal percentage and pH were revealed in Fig. 4.2. It was observed that the percentage of 2,6-DCP removal increased from 92.24 % at pH 2 to 96.92 % at pH 6 which is the maximum uptake and decreased to 94.52 % at pH 10. The apparently high adsorption of 2,6-DCP at lower pH was due to high electrostatic attraction between the negatively charged 2,6-DCP molecules and positively charged adsorption sites. Increase in the pH present fewer H+ ions in the solution, consequently more negatively charged sites were made available which facilitate a decreased in 2,6-DCP removal due to electrostatic repulsion (Morlu & Bareki, 2017).
Effect of Adsorbent Dosage on Adsorption
In this study, five different dosages of MTCNS were selected ranging from 2.0 to 10.0 g, while other parameters were kept constant. The results are presented in Table 4.4 while the relationship between adsorbent dosage and removal efficiency of 2,6-DCP is shown in Fig. 4.3. It can be explain from this figure that as adsorbent dosage increases there is an increase in the removal efficiency. This kind of a trend is mostly ascribed to an increase in the biosorptive surface area and the availability of more active binding sites on the adsorbent surface (Das & Mondal, 2011).
However, significant changes in value of adsorbent dosage (from 8.0 to 10.0 g) yield little or no change in percentage adsorption of the 2,6-DCP. This revealed that the adsorption sites remain unsaturated during the adsorption reaction whereas the number of sites available for adsorption increases by increasing the adsorbent dose. Furthermore, maximum 2,6-DCP removal efficiency of 99.53 % was recorded at 8.0 g adsorbent dose of MTCNS.
4.2.3 Effect of Contact Time on Adsorption
The variation in contact time (30 – 150 minutes; 30 mins. Interval) on the adsorption of 2,6-DCP by MTCNS (adsorbent) was investigated at fixed adsorbent dose of 2 g, pH of 7.0 and initial concentration of 100 mg/l, the results are shown in Table 4.5. The effect of contact time on removal of 2,6-DCP by MTCNS as a function of time is depicted in Fig. 4.4. It can be seen that the removal efficiency of 2,6-DCP increased considerably until the optimal removal efficiency reached within about 100 minutes contact time, where a saturation adsorption has been shown. Further increase in contact time beyond this point did not show significant changes. In general, the rate of removal of adsorbate increases with an increase in contact time to a certain extent, further increase in contact time does not increase the uptake due to deposition of adsorbate on the available adsorption site on adsorbent material (Ansari & Mosayebzadeh, 2010).
Effect of Initial Concentration on Adsorption
The adsorption of 2,6-DCP onto the MTCNS (adsorbent) was studied for different concentrations ranging from 100 – 500 mg/l keeping pH 7, adsorbent dose 2.0 g and exposure time 30 minutes fixed in all the samples. The data obtained are provided in Table 4.6. The removal efficiency of 2,6-DCP was found to decrease with the increase in the initial concentration as shown graphically in Fig. 4.5. Maximum removal efficiency of 95.68 % occurred for low initial concentration which showed gradual reduction when initial concentration was raised. It could be attributed to the fixed amount of adsorbent.
With increase in 2,6-DCP concentration, the adsorption sites were occupied and attained saturation at low concentration, hence there will be no further adsorption achieved at high concentration owing to the lack of available active sites and resulted in reduced removal efficiency. Similar results have been reported in literature on the extent of removal of dyes, the initial adsorbate concentration provides an important driving force to overcome mass transfer resistance of ions between the aqueous and solid phases (Donmez & Aksu, 2002).