Characterising the Physicochemical Properties of Selected Geophagic Clay from the Democratic Republic of Congo (DRC) to Investigate Their Potential Applications ^†

Abstract

Clay and clay composites have been used for numerous applications around the world, for example, as construction materials, cosmetics, and absorbents. Since clay is easy to find, abundant, and sustainable, understanding its quality is crucial. This study focuses on the characterization of geophagic clay samples from various locations in the Democratic Republic of Congo (DRC) to investigate their potential uses in various sectors. Geophagic clays have different colors, morphologies, and properties. Many characterizations were carried out including X-ray diffraction and X-ray fluorescence spectroscopy. Microstructure and chemical analyses were carried out using scanning electron microscopy combined with energy dispersive spectroscopy (SEM/EDS). UV–Vis spectroscopy was also carried out to investigate reflectance. XRD revealed the presence of muscovite, kaolinite, illite, and quartz. On the other hand, XRF showed the presence of SiO₂, Al₂O₃, TiO₂, and Fe₂O₃ as major chemical compounds. A flake-like surface morphology was observed in all samples and the EDS analyses exhibited similar results to the XRF. The XRF, XRD, and EDS results were in agreement. The zeta potential was negative for all the clay samples. The properties exhibited by the selected geophagic clay were compared with the properties of various samples used for different applications. It was concluded that the selected geophagic clays demonstrated properties that could lead to their use in water and wastewater treatments and other applications, including as a sunblock (cosmetic industry) due to their mineralogical/chemical composition and UV–Vis reflectance.

1. Introduction

Around the world, there is a substantial amount of raw clay materials, which are considered to be sustainable. Clay and clay materials have numerous applications in a number of sectors, including construction, cosmetics, ceramics, and water treatment [1,2,3,4,5,6,7,8]. To be able to investigate the suitability of specific clay and clay materials for specific applications, proper characterizations are required.

There are not many studies on the characteristics of geophagic clay [9,10,11,12,13,14,15,16,17]. Geophagia is universally recognized as a deliberate habit of consuming earthy materials including clays, soils, and sediments [11]. Choquenaira-Quispe et al. [16] carried out a study on the characterization and health risks of geophagic clay. They indicated that a weekly intake of clay represented an appreciable health risk. The characterized clay mainly contained silica. Alumina, iron, sodium, magnesium, calcium, and potassium were present in smaller quantities [16]. Ekosse and Jumbam [13] also investigated the health risks of consuming clay. They concluded that people consuming clays may benefit from the possible medicinal and nutritional values of the chemical elements that they contain, even though there are possibilities of human health risks. Most studies on geophagic clay are focused on the health risk of consumers; therefore, it is important to also look at other usages of geophagic clay materials.

This study presents the characteristics of selected geophagic clay samples from the Democratic Republic of Congo (DRC) to evaluate their suitability for various applications besides being consumed by human beings. A number of properties, including surface morphology and physicochemical properties, are presented.

2. Materials and Methods

2.1. Materials

Clay samples were collected at three different locations and exhibited different morphologies and colors, as depicted in Figure 1. The colors displayed by the samples were light grey/yellow, reddish, and dark grey for BUK, LUB, and KIN, respectively. Reddish and yellowish colors were mostly associated with the presence of iron oxide. Those locations were Lubumbashi (11°39′51″ S 27°28′58″ E), Bukavu (2°30′ S 28°52′ E), and Kinshasa (4°19′30″ S 15°19′20″ E). The samples were coded as LUB, BUK, and KIN for Lubumbashi, Bukuvu, and Kinshasa, respectively.

2.2. Methods

A Rigaku, ZSX Primus II X-ray Fluorescence spectrometer was used for chemical composition, while the phases were identified using a Rigaku, Ultima IV X-ray Diffractometer. Surface morphology analysis and sample mapping were carried out using scanning electron microscopy combined with energy dispersive spectroscopy (SEM/EDS), for which a JEOL JSM-IT300 was utilized. A UV–Vis (Lambda 650S, Perkin Elmer, Shelton, CT, USA) spectrometer was used to analyze the UV–visible spectrophotometric of the clay samples. The zeta potential was measured using a Malvern Zetasizer Nano series (Malvern Panalytical, Malvern, UK). The zeta potential measurements were carried out at a neutral pH.

3. Results and Discussion

3.1. X-ray Fluorescence and X-ray Diffraction Analyses

The chemical composition using X-ray fluorescence (XRF), as depicted in

Table 1. Chemical composition (wt%) of the geophagic samples.

Component	LUB	KIN	BUK
Na2O	0.0334	1.0855	0.0821
MgO	0.554	1.0703	1.1812
Al2O3	29.8908	29.9264	30.389
SiO2	60.2544	59.3893	52.2636
P2O5	0.0411	0.0999	0.05
SO3	0.0381	0.072	0.327
Cl	0.0214	1.4382	0.0134
K2O	1.5754	2.8707	8.1733
CaO	0.0361	0.1615	0.1024
TiO2	4.0406	1.4217	1.5949
Cr2O3	0.0312	0.0161	0.0366
MnO	0.0102	0.0127	0.028
Fe2O3	3.0454	2.2351	5.4643
NiO	0.0204	0.0064	0.0155
CuO	0.0116	0.0038	0.0097
ZnO	0.0048	0.0073	0.0102
Ga2O3	0.0066	0.0063	0.0091
GeO2	0.0031	-	-
As2O3	0.0021	-	0.0027
Rb2O	0.0202	0.0199	0.0517
SrO	0.004	0.0069	0.0055
Y2O3	0.0889	-	0.0008
ZrO2	0.1235	0.062	0.0544
BaO	0.0811	0.0746	0.1312
ThO2	0.0034	-	0.0035
Nb2O5	-	0.0053	-
PbO	-	0.0079	-

, indicates that SiO₂ and Al₂O₃ are the major compounds in all three samples. K₂O was present, with the BUK sample having the highest percentage (8.1733 wt%), followed by KIN (2.8707 wt%), and then LUB (1.5754 wt%). Furthermore, the highest percentage of TiO₂ was found in the LUB sample (4.0406 wt%). This shows that the LUB sample will exhibit good photocatalytic properties [8] and can be used for the removal of pollutants in water and wastewater. The XRD results show the presence of muscovite, kaolinite, illite, and quartz (BUK), quartz, dickite, kaolinite, and montmorillonite (KIN), and quartz, kaolinite, muscovite and calcium iron aluminum oxide (LUB) (Figure 2).

3.2. Surface Morphology and Chemical Composition Mapping

The surface morphology of the three samples was analyzed and it was observed that the clay samples had similar surface morphologies as other clay materials investigated in other studies [12]. The surface morphology of all the clay samples exhibited a flake-like morphology, which could be due to the presence of amorphous materials in the samples (Figure 3). EDS mapping analysis was carried out and it was observed that Si, Al, K, and Fe were present in the analyzed spot (Figure 4). The obtained result was in agreement with the XRF and the XRD results. Furthermore, the chemical content of the examined geophagic clay samples was similar to those presented in the literature [10].

3.3. Zeta Potential and UV–Vis Spectroscopy Analyses

The zeta potential values of all three geophagic clay sample are negative, which indicate good absorbance capacity with more negative zeta potential linked to higher absorbance capacity [18]. The analysis of zeta potential is proven to be a helpful indicator of the surface charge of particles and is directly associated with their composition. The zeta potential values of the BUK, KIN, and LUB were −11.20, −23.60, and −15.00, respectively. The KIN sample had a higher absorbance capacity when compared with BUK and LUB. UV–Vis spectroscopy analyses were carried out to study the usage of clay materials in the cosmetic industry as sunblock (Figure 5). It was observed that the three samples contained TiO₂, which is used in sunscreen products, acts as a physical ultraviolet filter, and can block sunlight and reflect the ultraviolet light.

4. Conclusions

Three geophagic clay samples were collected in three different locations in the Democratic Republic of Congo (DRC). The clay samples were analyzed to assess their various potential applications. The samples mainly contained SiO₂ and Al₂O₃, and the XRF, XRD, and EDS results were in agreement. The UV–Vis results indicate that these clay materials can block and reflect light. It was concluded that the analyzed geophagic clay samples can be used in the cosmetic industry and for water/wastewater treatment. The analyzed clay samples showed the potential of being used as a substitute in cosmetic creams such as sunblock. Furthermore, those clay samples can also be used for water and wastewater treatments in various forms including ceramic filters. Further studies need to be conducted to analyze more properties and further investigate their applications.