Preparation and characterization of adsorbents derived from bentonite and kaolin clays

The comparative study on preparation and characterization of modified bentonite and kaolin clay types is aimed at establishment of the ability of these clay types to adsorb pollutants. Its low cost, availability and recovery of clay give it added advantage of an effective adsorbent. The modified clay types were prepared by impregnation using various concentrations of either H2SO4 or NaOH in w/v ratio of 1:2. The physicochemical and adsorptive properties were investigated using standard physicochemical methods like moisture content, bulk density, pH, iodine and methylene blue numbers. FTIR, XRD and SEM analyses were also conducted on these clay types. Unmodified kaolin clay exhibited lowest moisture content (1.12%) while unmodified bentonite clay type had highest value (3.98%). The peaks at 3435 cm−1 and 3410 cm−1 assigned N–H stretching frequency were obtained for 2 M H2SO4 modified and unmodified bentonite clay types, respectively. The peaks at 912 cm−1 and 914 cm−1 that indicate the presence of Al–Al–OH deformation frequency were obtained for unmodified bentonite clay type and 2 M H2SO4 modified bentonite clay, respectively. The physical surface morphology of the clay types revealed the presence of pores with irregular shapes, while their XRD analysis showed that the clay types are crystalline. The modified clay types exhibited better potential adsorbent for removal of small metallic or gas molecules as well as large organic molecules. Adsorbents could also remove cations or anions of pollutants since their surface charge could be positive or negative depending upon pH condition.


Introduction
Clay as a natural scavenger of pollutant is important in the role of environment by taking up cations and anions either through ion exchange or adsorption or both. Thus, it contains exchangeable anions and cations on its surface such as Ca 2+ , Mg 2+ , H + , NH 4 + , Na + , SO 4 2− , Cl − and NO 3 − which can be exchanged with other ions relatively without affecting the clay mineral structure. Due to clay's large specific surface area, chemical and mechanical stability, layered structure, high cation exchange capacity (CEC) and so on (Bergaya and Lagaly 2006), clays exhibit excellent adsorbent properties. Bronsted and Lewis type of acidity in clays (El-Bayaa et al. 2009) have also enhanced to great extent the adsorption capacity of clay minerals. The Bronsted acidity arises by dissociation of water molecules of hydrated exchangeable metal cations on the surface, thereby forming H + ions on the surface. If there is net negative charge on the surface, Bronsted acidity may arise due to the substitution of Si by Al 3+ in some of the tetrahedral positions and H 3 O + cations that balanced the resultant charge. The exposure of trivalent cations mostly Al 3+ at the edges, or Al 3+ arising from rupture of Si-O-Al bonds, or through dehydroxylation of some Bronsted acid sites absorb anions, cations and nonionic and polar contaminants from natural water. The contaminants that saturate the clay surface could not easily be removed through the processes of ion exchange, coordination or ion-dipole interactions because pollutants can be held through H-bonding, Van der Waals interactions or hydrophobic bonding arising from either strong or weak interactions sometimes. The strength of the interactions is determined by various structural and other features of the clay mineral.

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195 Page 2 of 10 Till date, several adsorbents such as porous carbon, biological, agricultural and industrial waste materials have been used to remove pollutants from wastewater but have one deficiency over each other (Elwakeel et al. 2017).
The aim of this research is to prepare and modify cheap, easily recovery alternative absorbents for the removal of pollutant such as dyes, heavy metal and organic compounds from wastewater.

Geographical location of clay types
Kaolin clay was collected from the geographical location of Ijapo estate area of Akure in Ondo state, south-west region of Nigeria with longitude and latitude of 7 0 16′06.0′′N and 5 0 12′17.1′′E, respectively. While bentonite clay was collected from the geographical location of deposit of Afuze in Edo state, south-south region of Nigeria with longitude and latitude of 6 0 56′24.2′′N and 5 0 59′07.7′′E, respectively.

Preparation of the adsorbent
The collected bentonite and kaolin clay types were separately grounded into fine powder and sieved through a 0.25mm mesh size to remove all the debris and further rinsed with water and oven-dried at 105 °C until all the moisture has evaporated.
Known amounts (50 g) of bentonite or kaolin clays from the above samples were separately added to 100 ml of 2 M, H 2 SO 4 and NaOH, respectively. They were individually shaken vigorously using a uniscope SM101 shaker water bath for 24 h at 30 °C. They were separately washed with distilled water in order to remove the excess acid until effluent filtrate became neutral (pH = 7) (Abu-Elella et al. 2015) and it was placed in a watch glass and oven-dried at 80 °C for 24 h.

Nomenclature of the samples
Unmodified bentonite clay was denoted as UBC, unmodified kaolin clay as UKC, 2 M H 2 SO 4 -modified bentonite clay as 2-ABC, 2 M H 2 SO 4 -modified kaolin clay as 2-AKC, 2 M NaOH-modified bentonite clay as 2-BBC and 2 M NaOHmodified kaolin clay was denoted as 2-BKC.

Characterization of the adsorbents
The modified and unmodified clay types were characterized for moisture content, bulk density, particle density, pH and pH pzc while the chemical and mineralogical compositions were characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD) and scanning electron microscope (SEM) for functional group (Austin and Hoang 2009), structure (Ramachandran and Beaudoin 2008) and physical morphology (Mullan 2006) of the modified and unmodified clay types, respectively.
The chemical and mineralogical compositions were characterized by Fourier transform infrared (FTIR) using Perkin Elmer 1600 Series FTIR.

Moisture content determination
Moisture content of each modified and unmodified clay types was carried out using standard method (ASTM 1996;Allen 1999), and it was conducted in triplicates. The loss in weight represents the moisture content of the sample and was calculated using the formula: where Xg represents weight of sample before drying, Y g represents weight of sample after drying and % MC represents percent of moisture content.

Bulk density determination
The bulk density of the modified and unmodified clay types was determined according to the procedure described by Ahmedna et al. (1997), Huerta-Pujol et al. (2010) and was conducted in triplicates. A little vibration was applied until no particle space and constant level of clay is noticed in the cylinder and the volume was recorded. The formula used is given as:

Particle density determination
Particle density of each modified and unmodified clay types was carried out using standard method (ASTM 1996;Allen 1999), and it was conducted in triplicates and the particle density was calculated according to the formula: Bulk density g/dm 3 = Weight of clay Volume particle density = Weight of soil Volume of soil taken

Determination of pH
The pH of the modified and unmodified clay types were determined by putting 10 g of each clay types into a 100-ml beaker and 20 ml of distilled water was added and stirred at 100 rpm for 30 min differently. Each clay-type pH was measured with the pH meter INOLAB pH 7310 (ASTM 1996;Allen 1999) and was done in triplicate.

Determination of porosity test using methylene blue and iodine number
Methylene blue (MB), a basic dye, was obtained from M and B laboratory chemicals in commercial purity percent and was applied without any further purification. A stock solution of this basic dye was prepared by weighing a quantity of 1.00 g of the dye in distilled water to make 1000 mg/L. Other various concentrations were prepared using serial dilution. Various concentrations of MB were determined by using the BECH-MAN UV-visible spectrophotometer DU-640 (Chongrak et al. 1998) to measure the extent of porosity of each adsorbent. A calibration curve of optical densities against methylene blue concentrations was also obtained by using standard methylene blue solutions of known concentrations at pH value 7. This was done using 50 mg/L concentration to obtain the ʎ max of 664 nm.
The adsorption of aqueous iodine solution is considered as good adsorbate for evaluating the surface area of adsorbents such as activated carbon, agricultural wastes and mineral clays, associated with pores greater than 1 nm. Iodine number is accepted as the most fundamental parameter used to characterize absorbent performance. It gives the measure of activity level (higher number indicates higher degree of activation). The iodine number is defined as the amount of iodine adsorbed per gram of activated carbon at an equilibrium concentration measured according to the procedure established by the American Society for Testing and Material ASTM 2006. A 0.1 g of dried raw and modified kaolin and bentonite clay types were taken in dried 100-ml different conical flasks. A 5 ml of 5% HCl was added. The flasks were then swirled until the adsorbents are wetted. A 10 ml of 0.1 N iodine solutions was added to each flask and shaken properly for 4 min and filtered. A 10 ml filtrate is then titrated against standard (0.1 N) sodium thiosulphate using starch as an indicator. The concentration of iodine adsorbed by adsorbents was calculated as amount of iodine adsorbed in milligrams.
The conversion factor can be calculated as follows:

Determination of point of zero charge
Point of zero charge (PZC) is traditionally defined as the pH value where one or more components of the surface charge vanish at a specific temperature, pressure and aqueous solution composition. The PZC was determined by the pH method. It was accomplished by adding 0.1 g of the each of the adsorbents to 200 ml of 0.1 M NaCl differently to predetermine initial pH (pHi). The initial pH of NaCl solutions in different flasks to which the adsorbents were added was adjusted with 0.1 M NaOH or HCl to pH 1-12. The flasks were covered and shaken for 24 h after which the final (pHf) was measured using pH meter INOLAB pH 7310. A plot of change in pH (pHi-pHf) against the initial pH gives pHzpc where the plot crosses the horizontal axis (Ekpete and Horsfall, 2011).

Data handling
Statistical analysis such as Duncan was carried out on the data obtained using IBM SPSS 20 Package.

Fourier transform infrared (FTIR)
The surface functional group of each clay types was studied by Fourier transform infrared spectroscopy (FTIR). Each of the samples was prepared with KBr, and its tablet or pellet subjected under various characteristic chemical bonds (or stretching) was determined from which certain functional groups were derived (Socrates 1980;Austin and Hoang 2009).

X-ray diffraction (XRD)
A known quantity sample clay types were pulverized in a container and coated with tungsten carbide. Subsequently, a 7 g sample from the pulverized material was mixed with 1.4 g of organic wax and used to produce tablets for the determination of the chemical composition for each modified and unmodified clay types. These tablets were placed in Philips-P analytical PW 2400 equipment, operating with a rhodium X-ray source of 3 kW (Ramachandran and Beaudoin 2008).

Scanning electron microscope (SEM)
A scanning electron microscope (SEM) is a type of electron microscope that gives images of each clay type by scanning it with a focused beam of electrons. The electrons interact with atoms in each clay type giving various signals that can be detected and that contain information about the clay type's surface morphology and composition. The electron beam is generally scanned in a raster scan pattern, and the beam's position is mixed with the detected signal to produce an image. SEM can produce resolution better than 1 nm. Specimens can be observed in high vacuum, in low vacuum, in wet conditions (in environmental SEM) and at a wide range of elevated temperatures (Mullan 2006). This was carried out using high vacuum-based SEM SCOTECH 30000.

Result and discussion
Physicochemical characterization of modified and unmodified clay types Table 1 shows the moisture content values of 3.98% and 1.12% obtained for UBC and UKC, respectively. They are related in the order of increment to the 3% and 5% reported for kaolin and bentonite by Ramesh (2014). The 2-ABC, 2-AKC, 2-BBC and 2-BKC are found to be 3.49, 1.4, 1.5 and 1.2%, respectively. Modification of the adsorbent did not seem to cause appreciable change to the moisture content of the adsorbents when their values are compared to their respective unmodified forms. The bulk density of clay is the measurement of how dense and compact clay is. Table 1 shows the bulk density of the clay type had values of 1.42, 1.35, 1.10, 1.03, 1.80 and 1.86 g/ml obtained for UKC, UBC, 2-AKC and 2-ABC, 2-BBC and 2-BKC, respectively. The 2-BKC has the highest bulk density value of 1.86 g/ml among them all. These slightly varying values could be attributed to the variation in the organic constituents that make up the clay type and it has also reported in the literature that during the modification process involved in the production of adsorbent, the rate at which the starting material volatizes, as well as hardness, strength are dependent on their starting material's bulk densities (Pendyal et al. 1999).
Clay particle density (g/cm 3 ) is the mass of clay solids (oven dry) per unit volume of clay. Clay particle density depends on the densities of the various constituent solid and their relative abundance. The range is fairly narrow because common soil minerals but differ little in density. An average value of 2.65 g/cm 3 is often assumed. In contrast, organic soils mineral have lower particle densities since the density of organic matter is much less than that of mineral particle. From Table 1, the particle densities of the clay types are 2.50, 2.55, 2.67, 2.92, 2.16 and 2.43 g/ cm 3 for UBC, UKC, 2-ABC, 2-AKC, 2-BBC and 2-BKC, respectively.
The pH of a clay type is a measurement of the degree of acidity or alkalinity of the clay. This gives an insight on the acidic or alkaline nature of the clay. It can be observed from Table 1 that the pH of the clay types is 7.5, 7.7, 6.48, 6.41, 8.48 and 8.7 for UKC, UBC, 2-ABC 2-AKC, 2-BBC and 2-BKC, respectively. It was revealed that UKC and UBC are slightly alkaline in nature while 2-AKC and 2-ABC are acidic while 2-BBC and 2-BKC are basic in nature. The pH values for the pretreated clay samples are closer to pH 7.2 reported by Ramesh (2014).
pH point of zero charge of adsorbents prepared from these bentonite and kaolin clays was determined. The values were known by determining the point where the resulting curve cut through the pHo axis as shown in Fig. 1. The pHpzc was found to be 6.2, 8.9, 7.0, 3.7, 7.8 and 7.8 mmol/g for UBC, UKC, 2-ABC, 2-AKC, 2-BBC and 2-BKC, respectively. The influence of all the functional group present in each sample determines pHpzc, the pH at which the net surface charge on each sample is zero. At pH < pHpzc, the sample surface has a net positive charge, while at pH > pHpzc, the surface has net negative charges (Bello et al. 2016). Anion adsorption becomes to enhance at lower pH than the pHpzc while adsorption of cations is also enhanced at pH higher than pHpzc (Bello et al. 2016).
Methylene blue and iodine number were carried out to determine the adsorption capacity of these various clay types. From the results in Table 2, adsorption of methylene blue solution for UBC, 2-ABC, 2-BBC, UKC, 2-AKC and 2-BKC were found to be 0. 7, 76.58, 174.8, 175.2, 92.58 and 31.1 mg/g, respectively, while the result for iodine Comparing these results with other literature values, it was observed that these sorbents give more yields than groundnut shell, sheanut, poultry dropping and poultry waste sorbents (Itodo et al. 2010). Figure 2 shows the FTIR spectra of UKC, 2-ABC, 2-BBC, UBC, 2-AKC and 2-BKC, respectively. The spectra of the samples show the presence of several functional groups. These spectra revealed a reduction, broadening, disappearance or appearance of new peaks after the process of modification. The shifts in the spectra revealed the effect of modification. The prominent bands after modification show that the prepared adsorbents will be effective in the removal of heavy metal and dyes. The sharp bands at 3696.17 cm −1 and 3626.39 cm −1 are attributed to the N-H stretching for the UBC and 2-ABC types as it is the same as the peaks at 3697 and 3620 cm −1 reported by Georges-IVO (2005). The peaks at 3436.41 cm −1 in the 2-ABC sample and 3430.21 cm −1 in the UBC are assigned to OH stretching which is close to 3426 cm −1 and 3420 cm −1 reported by Isci et al. (2006), Aroke et al. (2013). The bands 1644.47 cm −1 and 1637.99 cm −1 are present in UBC and 2-ABC samples and   they agree to that noticed at 1150 cm −1 , 1060 cm −1 and 1030 cm −1 which were reported by Aroke et al. (2013). The peaks at 913.08 cm −1 , 916.09 cm −1 and 912.55 cm −1 are responsible for Al-Al-OH group deformation and are similar to those of 913 cm −1 and 914 cm −1 reported by Amin et al. (2010).

FTIR spectra of unmodified and modified bentonite and kaolin clay types
The sharp peaks at 3623.22 cm −1 and 3702.31 cm −1 were ascribed to the inner surface N-H stretching and inner hydroxyl group with higher amount of Al-OH in the octahedral sheet, respectively. These peaks are the same with those reported by Georges-IVO (2005) at 3697 and 3620 cm −1 , respectively. The peaks 1625.33 cm −1 are assigned to the H-OH bending of water and this closely agree to 1620-1642 cm −1 (Bhasker and Gopalakrishnarao 2010). The peaks at 1036.41 cm −1 and 1007.92 cm −1 are ascribed to the Si-O deformation bands which are closer to that reported by Georges-IVO (2005) at 1033, 1032 and 1007 cm −1 . The peaks 912-937 cm −1 were ascribed to Al-OH bending vibration of kaolin clay type and are closer to that of 914-936 cm −1 range (Bhasker and Gopalakrishnarao 2010). The peaks at 799.10 cm −1 , 757.78 cm −1 and 797.78 cm −1 were assigned to the Si-O stretching of the kaolin clay type. These peaks agreed to that reported at 796 cm −1 , 754 cm −1 and 695 cm −1 (Georges-IVO 2005) and are close to the value of bands for theoretical kaolin clay type, defined as pure kaolin clay with the chemical formula Al 2 Si 2 O 5 (OH) 4 .

XRD spectra for acid-modified and base-modified bentonite clay
Figures 3, 4, 5, 6 and 7 depict the SEM micrographs at × 500 and × 1000 magnifications for UKC, 2-AKC, UBC, 2-ABC and 2-BBC, respectively. They revealed numerous pores with irregular shapes, rough, uneven which indicated much contact area and easy pore diffusion during adsorption (Patel and sudhakar 2008). The XRD powder analysis for 2-ABC, 2-BBC and UKC, respectively, is presented in Fig. 8a-c.
There is a conspicuous peak at 2θ equal 18 o , 2θ equal 18 o and 2θ equal 22 o for the 2-ABC, 2-BBC and UKC, respectively, and showed that they are crystalline in nature.
The surface area, pore volume and pore size distribution are calculated from the initial plot of amount of N 2 gas    adsorbed at STP against the relative pressures, P/P o within the range of 0-3.2 for both 2-AKC and 2-BBC. A surface area of 233.7 m 2 /g and pore volume (0.234 cm 3 /g) obtained for 2-BBC are greater than surface area (141.4 m 2 /g) and pore volume (0.411 cm 3 /g) of 2-AKC.
These values are within the minimum range of 100-1500 m 2 /g needed for industrial application and removal of small molecules from the pore size distribution (1.324 nm) obtained for 2-AKC could be classified as microporous.
The physical observation of 2-AKC is similar to that of commercial granular active carbon (CGAC) which showed the initial fast adsorption rate. However, most average active carbon samples micropore volumes are generally between 0.2 and 0.5 cm 3 /g with surface area up to about 1000 m 2 /g (Grajek 2000). It has been reported that total pore volume in the range of 0.3-0.8 cm 3 /g of active carbon derived from olive stones is often used as precursors for high-grade active carbon (Linares-Solano et al. 1984) (Table 3).

Conclusion
This study revealed that modification of bentonite clay and kaolin clays was effective in creating well-developed pores on the surface. Various level of degree of adsorption was achieved on methylene blue and iodine molecules. The amounts of methylene blue adsorbed by these various adsorbents were strongly influenced by their adsorption capacities. The surfaces of studies samples are acidic and neutral which indicate a promising ability to remove different pollutants such as dyes, metal ions and other organic pollutants. There is further ongoing research to observe the best among all these potential adsorbents. Thus, there is high potential ability for some of these potential adsorbents to be utilized for removal of different degree of sizes of molecule either in gas, small molecules or bigger molecules which are pollutant in our environment when compared with other low-cost adsorbents.
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