Instrumental Characterization of Unmodified and HDTMA-Br Modified Kaolinite Clay: SEM-EDX, Quantachrome and TGA-DTA

© 2020 The Authors. This article is licensed under a Creative Commons Attribution 4.0 License Abstract. Kaolinite clay from Alkaleri Northeast Nigeria was pre-treated and beneficiated using physical process. The treated clay of cation exchange capacity (CEC) 9.5 meq/100 g was modified with cationic surfactant hexadecyltrimethylammonium bromide (HDTMA-Br) with amount equivalent to and doubled the CEC. The unmodified kaolinite clay (UKC) and the resultant organokaolinite clays: monolayer modified clay (MMC) and bilayer modified clay (BMC) were characterized using the following instruments: Energy Dispersive X-ray spectrometer (EDX), Scanning Electron Microscopy (SEM), Quantachrome and Thermogravimetric-Differential Thermal Analysis (TG-DTA). EDX profile analysis shows 22.41 % C, 56.17 % O & 10.56 % Al in UKC; 21.06 % C, 55.49 % O & 11.66 % Al in MMC and 18.98 % C, 54.59 % O & 12.14 % Al in BMC respectively; with Fe and K found in MMC. The SEM morphology shows UKC has high porosity and large particles, while MMC and BMC showed fine particles and darker than UKC with textural non-uniformity. TGA curve shows that UKC attains equilibrium decomposition at 997.20C with 15.32 % weight loss, MMC 997.30 C with 32.67 % weight loss and BMC 998.90 C with 37.23 % weight loss. The revealed weight loss indicates water of hydration and dehydroxylation. The DTA curves shows endothermic at 510 C, 250 C and 520 C for UKC, MMC and BMC respectively. The single point surface area were 11.9754 m/g, 3.0132 m/g and 3.8225 m/g for UKC, MMC and BMC with corresponding adsorption average pore width 355.0050 Å, 478.6275 Å and 752.8364 Å respectively. Clay materials being promising minerals when modified can achieve desired surface properties for best performance in adsorption applications.

Kaolinite clay mineral has a low-cation exchange capacity (CEC) ranges from 3-15 meq/100 g, the low values probably representative of pure kaolinite and the increase could depend on impurities and particle sizes [4,8]. The CEC of Kaolinite clay from Alkaleri as determined by ammonium acetate method is 9.5 meq/100 g [1].
Kaolin as a result of its well-packed structure, are not easily broken down and its layers are not easily separated. Most sorption activity occurs along the edges and surfaces of the structure [12]. The silicates nature that makes kaolinite effective in removal (or treatment) of contaminants has to undergo purifications. The render layered silicates miscible with polymer matrices, normally hydrophilic silicate surface has to be converted to an organophilic surface to make intercalation of polymeric chain between the silicate layers, this is done by ion-exchange reaction with cationic surfactants [15].
Organoclay refers to the natural clay minerals which have their surface modified to improve their adsorption ability, capacity and affinity for oxyanion contaminants by the replacement of the natural inorganic cations with surfactants cations such as quaternary ammonium compounds (QACs) [3,17]. The possible application of generated organoclay is the better intercalation of polymer chains between the stack of clays, which when modified will be useful in sustaining ecosystem friendly with improved properties. This method is done by modifying the clay surface in traditional ion-exchange, and organoclay which must be treated before it can be used to make a nanocomposite. Making a composite out of untreated clay would not be very effective because most of the clay would be unable to interact with the matrix [15].
Cation exchange such as Na + , Ca 2+ and Mg 2+ may occur between the silicates layers associated with water molecules. These elements are exchangeable and their exchange reaction is very important in most of the clay application. Na + or Ca 2+ or Mg 2+ by quaternary ammonium cations of the forms [(CH3)3NR] + or [(CH3)2NRR'] + at the exchangeable site of natural clays results in organoclay derivatives with organophilic properties that can act as a sorbent contaminant hydrocarbons [2,9]. The R and R' is the alkyl or aromatic hydrocarbons.
The degree of surfactant (HDTMA + ) addition is limited to the CEC of the clay being modified, where HDTMA + replaces the charge balancing cations on the surfaces [2]. The net negative charge of the natural clay minerals is balanced by alkali metal and alkali earth metals cations such as Na + , Ca 2+ and Mg 2+ [9]. The substitution of cations is presented in Equation (1) [2], while the anions are halides such as; Cl -, Br -, Iand claywhich is exchanged with the cations at surfaces.
where M + is the metal cation such as Na + , Ca 2+ and Mg 2+ .
Some common examples of monatomic/inorganic cations (metals) are alkali metal, alkaline earth metals, and transition metals while anions (halides) are chloride, bromide and iodide. Polyatomic/organic cations (metals) are ammonium, pyridinium and triethanolamine (TEA) while anions are tosyls, trifluoromethanesulfurnates and methyl sulfate. Equation (2) presents the surface action of the quaternary compounds and the clay, where clay minerals with negative charge will exchange with HDTMA + at the surface to form organoclay.
Adsorbents are usually irregular granules extruded pellets and formed spheres. The size reflects the need to pack as much surface area as possible into given volume of bed and also minimize pressure drop for flow through the bed [6]. Good adsorbents should have these qualities: large internal surface area, accessible through pores large enough to admit molecules to be adsorbed, while small pores will exclude molecules that are not desired to be adsorbed, it should not age rapidly such that it does not lose its adsorption capacity through continuous recycling and it should be mechanically strong enough to with stand bulk handling and vibration from industrial.
This research work is to instrumentally characterize unmodified and organically modified kaolinite clay from Alkaleri Local Government Area of Bauchi State, North-Eastern, Nigeria.

MATERIALS AND METHODS
The procedure used for this research work are; pretreatment and beneficiation of the raw kaolinite clay, modification of purified kaolinite clay with equivalent ratios of cationic surfactant to form organically modified kaolinite clay.

Pretreatment and beneficiation of kaolinite clay.
Kaolinite clay was obtained directly from the mining site at Alkaleri Local Government Area, Bauchi State, North-Eastern Nigeria. The clay lump was broken into particles, soaked in excess de-ionized water in a pretreated plastic container for effective dissolution and homogeneity for 48 hours. The slurry was blunged and allowed to settle for 48 hours before decantation of the excess de-ionized water and the clay residue was sun dried. This was followed by oven drying at 105 °С for 3 hours to ensure moisture free adsorbent. The dried clay, was grinded using pestle & mortar and passed through mesh sieve of 100 to obtain unmodified Alkaleri kaolinite clay (raw kaolinite clay), which was characterized and stored safe in a desiccator for further usage.

Preparation of organically modified kaolinite clay.
The characterized kaolinite clay from Alkaleri Local Government of Bauchi State, North-eastern Nigeria with 9.5 meq/100 g CEC [2] was modified with organic surfactant (HDTMA-Br), 99 % purity and 364.45 g/mol molecular weight, without any further treatment. The modification of kaolinite clay was carried out in two categories: the monolayer (CEC equivalent) and bilayer (twice CEC equivalent) modifications. The synthesis of organo-kaolinite clays was carried out by weighing equivalent CEC amount of surfactant (HDTMA-Br) to produce monolayer and twice CEC to produce bilayer, dissolved in 250 mL of de-ionized water and stirred efficiently to ensure proper dissolution of surfactant; this was then followed by addition of known amount of unmodified kaolinite clay (UKC) with cation exchange capacity 9.5 meq/100 g clay to the already dissolved surfactant. The mixtures were stirred perfectly followed by addition of deionized water to make it up to 500 mL. The mixture was then charged into a batch reactor and run at 740 rpm for 24 hours at 25 °С and 1 atmospheric pressure to achieve homogeneity [3]. After 24 hours of agitation in a batch reactor, the slurry was transferred into 800 mL beaker and allowed to settle for 24 hours, the supernatant was decanted and washed every 24 hours for four times with de-ionized water to reduce the bromide content. This was followed by drying at 25 °С for 48 hours and oven drying for 3 hours at 104 °С, which was then ground using pestle & mortar and passed through mesh sieve of size to produce monolayer modified or-gano-kaolinite clay (MMC) and bilayer modified organo-kaolinite clay (BMC) respectively.
Characterization of unmodified and modified kaolinite clays. The unmodified and modified kaolinite clays prepared were characterized using, scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDX) to determine their morphologies, porosity, textural uniformity, fissures and mineralogical compositions [14].
Quantachrome analysis was carried out to determine the surface area, pore size and pore volume of the clays [2]. Thermogravimetric analyzer/differential thermal analyzer (TGA/DTA) was carried out to examine the weight loss arising from degradation of organic substances [2,4].
Scanning electron microscopy/ Energy dispersive X-ray spectrometry. Scanning Electron Microscopy (SEM) is an electron that describes the sample by scanning a beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample surface topography, composition and properties -properties such as electrical conductivity. Scanning electron microscopy with energydispersed X-ray analysis (SEM/EDX) is commonly used for single particle study. It provides useful information on the morphology, elemental composition and particle density and also gives us a better insight about the origin of particles [11,13]. The EDX/SEM analysis was carried out using spectroscope on window SATW spectrum model. The spectrum detector was set at its maximum (X-max) to detect all elements present in the clay mineral at an elevation temperature, accelerating voltage, magnification and processing time. The adsorbent was deposited on a brass hold, followed by sputting with a thin coat of gold in a vacuum to study their morphology and surface structure. While the EDX beam were set focused into the adsorbent under study [14]. This same process was carried out for all the adsorbents to determine their mineralogical compositions and morphologies respectively.
Quantachrome analysis. Quantachrome instrument Autosorb Automated Gas Sorption System; ASAP 2020 V4.02 (V4.02 H) operating with Nitrogen as degas analyzer for adsorption analysis was used [2]. The system operated at the following conditions; thermal correction was set, warm free space was measured as well as cold free space, pressure dose and mass of adsorbent (sample). The surface area-pore, volume-pore size of sample clay and pore size the adsorption was measured by setting the width. These operating conditions include bath temperature, outgassing temperature, processing time, P/Po tolerance zero, equilibrium interval time and analysis time. This same process was carried out on each adsorbent.
Thermogravimetric analysis / Differential thermal analysis. Changes in chemical or physical properties of materials as a function of temperature in a controlled atmosphere may be determined by thermal analysis [4]. This includes changes in energy, weight of materials and enablement to determine their heating patterns. The weight loss arising from the degradation of adsorbate was examined using Netsch DTA/TGA instrument thermocouple [2]. It was carried out by preconditioning the adsorbent gas flow at room temperature for 20 minutes; which was followed by heating up to 1000 °С. The TGA/DTA trace was used to determine the weight percent at interval of 20 °С /minute until equilibrium value. This same process was carried out on all adsorbent to determine the temperature at which the organic content of the adsorbent degraded.

RESULTS AND DISCUSSION
Characterization of unmodified and modified kaolinite clays was carried out using SEM/EDX, Quantachrome and TGA/DTA. The results of findings are thus presented below: Scan electron microscopy of unmodified and modified kaolinite clays. The SEM of unmodified and modified kaolinite clays was carried out for their morphologies, porosity and textural formation as shown on plate I. The SEM electrons were selected to run at magnification to produce the images. It is always better to investigate morphologies of sorption studies of adsorbent under high magnification as it plays vital role in knowing the pressure that can allow adsorption to take place in a substance [14]. The SEM micrograph shows that UKC is highly porous, with large particle and pore sizes compared to MMC and BMC. The fissures and channels are major in UKC compared to MMC and BMC, while BMC has finer particles compared to MMC and UKC. The BMC consists of many layers and textural non-uniformity due to adequate intercalation of surface modified clay. The SEM images of unmodified kaolinite clay (UKC), monolayer modified kaolinite clay (MMC) and bilayer modi- Energy dispersive X-ray spectrometry of unmodified and modified kaolinite clays. The energy dispersive X-ray spectrometry was carried out on the adsorbents to determine their profiles (Figure 1) and elemental composition (Table 1)   Quantachrome analysis of unmodified and modified kaolinite clays. Quantachrome of both unmodified and modified clays was carried out to ascertain their surface area, pore volume and pore size by surface area analyzer using Brunauer-Emmet-Teller (BET) isotherm, Barrett-Joyner-Halenda (BJH) isotherm, Langmuir isotherm, t-plots, which was performed under these conditions; ground low temperature, nitrogen adsorption-desorption isotherms using an auto-sorb1 quantachrome instrument.
The clay surface area widely used as starting material for adsorbent varies from 10-700 m 2 /g [17]. The BET surface areas for UKC, MMC and BMC are 11.9754 m 2 /g, 3.0132 and 3.8225 m 2 /g respectively while the BJH adsorption cumulative surface areas for UKC, MMC and BMC are 11.403 m 2 /g, 3.879 m 2 /g and 7.906 m 2 /g respectively. The Langmuir surface areas for UKC, MMC and BMC are 15.6659 m 2 /g, 5.6691 m 2 /g and 7.5577 m 2 /g respectively. All surface areas for UKC was found within the range of 10-700 m 2 /g, while MMC and BMC less than 10 m 2 /g indicating effect of surface modifications. Total pore volume was calculated from the amount of nitrogen adsorbed at a relative pressure of p/po and while average pore size was calculated based on (4V/A by BET) Å and (4V/A) Å as presented in Table 2.
The micro pore area, micro pore volume and ex-ternal surface area were obtained from t-plot method [10,16].  DTA curve showed two endothermic decompositions, between 200 and 300 °С the curve is peaked at 250 °С with a corresponding derivative weight -4.5 % and between 480 and 600 °С the curve is peaked at 520 °С with a corresponding derivative weight of -0.9 %. The loss is associated with surface water loss and dehydroxylation of clay materials [2,4].
The TGA curve for BMC decomposition began at 29.43 °С exhibiting 1.7 % weight loss, the curve shows 26.7 % weight loss at 250 °C and on further calcination the curve shows 32.50 % decomposition at 570 °С until equilibrium of 998.90 °С and 37.23 % weight loss was attained.
The loss is associated with surface water loss and hydration of clay materials [2,4]. The DTA curve shows two endothermic peaks between 200 and 300 °С is peaked at 250 °С with corresponding derivative weight -5.8 % and between 400 and 600 °С the curve is peaked at 520 °С with corresponding derivative weight -1.0 %.

CONCLUSION
The SEM micrograph showed UKC with more porosity, has large particle sizes compared to MMC and BMC due to surface modification which resulted into formation of layers and textural nonuniformity. EDX show reduction in C, O and Ti in UKC and increased in their percentages in both MMC and BMC. The quantachrome analysis reveal that UKC single point surface area (11.9754 m 2 /g) falls within the range of clay surface area of 10-700 m 2 /g while MMC (3.0132 m 2 /g) and BMC (3.8225 m 2 /g) were less.