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. 2020 Nov 13;5(46):29943–29954. doi: 10.1021/acsomega.0c04295

Geochemical and Physicochemical Characteristics of Clay Materials from Congo with Photocatalytic Activity on 4-Nitrophenol in Aqueous Solutions

Mukuna P Mubiayi 1,*, Adolph A Muleja 1, Sarre KM Nzaba 1, Bhekie B Mamba 1
PMCID: PMC7689898  PMID: 33251430

Abstract

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This study investigated the geochemical and physicochemical characteristics of natural clay collected in the Democratic Republic of Congo. The optical properties of the sample collected in Golf (GOL) were tested in the removal of 4-nitrophenol in aqueous solution. The geochemical analysis depicted that all the samples are plotted within the shale quadrant. Furthermore, the Chemical Index of Alteration (CIA) indicated that the samples are extremely weathered. The particle size distribution ranged from 0.41 to 418.6 μm, while the pore diameters for all the samples were under 100 Å. A flake-like surface morphology was observed in all the samples. SiO2, Al2O3, Fe2O3, K2O, and TiO2 were the major chemical compounds found in all the samples, while the XRD analysis showed the presence of quartz, kaolinite, magnetite, and illite. The presence of metal oxides (i.e., TiO2 and Fe2O3) indicated that these natural clays can be used for photocatalytic oxidation of pollutants. The sample collected in Katuba (KAT) displayed the higher reflectance percentages for the selected wavelengths except at 200 nm. Interestingly, the GOL sample exhibited lower energy band gaps (2.68 and 3.94 eV) necessary for photocatalysis. The untreated GOL clay sample removed 99.13% of 4-nitrophenol from aqueous solution through the photodegradation process. The usage of the untreated GOL clay could be a cost-effective solution in the removal of 4-nitrophenol in wastewater.

1. Introduction

Nowadays, it is established that the knowledge on the physicochemical properties of materials is of importance, and it enables the use of materials for various applications. Information on the properties of natural clay from various geographic locations would result in their extensive usage, which could have a positive impact on the environment.

Several researchers have studied clay samples from different locations and have provided their usages, including the construction and ceramic industry,1−4 cosmetic,5−7 and removal of pollutants such as pharmaceuticals8,9 from wastewater. Globally, hazardous chemicals including nitrophenol are released into sewages, rivers, seas, and the environment. Also, the presence of those pollutants has a detrimental effect on human beings and animals. Abdollahi and Mohammadirad10 reported that 4-nitrophenol can be found in effluents in the manufacturing of some pharmaceuticals. They further said that 4-nitrophenol can also be found in dyes, fungicides, and chemical used to darken leather. This shows that 4-nitrophenol is present in various wastewaters, and therefore, enters into the environment. The presence of 4-nitrophenol in contaminated water can expose the population to inhale and ingest the 4-nitrophenol contaminated water or dry soil (surface). For example, the personnel where the pesticide parathion is used can be exposed to 4-nitrophenol.10

Natural clays can also be combined with other materials for enhanced application in water treatment. Modified clay materials and clay composites have been utilized for the removal of 4-nitrophenol in effluents.11−16

Ozola et al.11 utilized modified organoclays and enhanced the removal efficacy of p-nitrophenol in solutions. El Ouardi et al.12 removed 99.5% of p-nitrophenol in the aqueous solution using montmorillonite clay.12 Moreover, Dos Santos et al.13 also used organoclay modified from montmorillonite clay to remove p-nitrophenol in aqueous solution. In their studies, the removal of p-nitrophenol between 79.12 and 75.67% at 25 °C was obtained. Each of the reported modified natural clays was collected from different geographical locations and presented varying physicochemical properties.11−16

Furthermore, the removal of nitrophenol has been also attempted by various researchers through adsorption and photodegradation using different materials, including aluminum metal–organic and reduced graphene composites (MIL–68(Al)/RG),17 a nanocomposite hydrous ferric oxide/aminated hyper-cross-linked polymeric adsorbent composite (HFO-802),18 nitric acid-activated fly ash (AFA),19 gold nanoparticles supported on a functionalized mesoporous carbon composite,20 biochars (rice straws),21 TiO2/natural hematite-supported bentonite (TiNHB),22 a hybrid photocatalysis/membrane,23 cobalt and copper phthalocyanine,24 TiO2, a metal-free and copper(II) porphyrins (CuPp/TiO2) composite,25 Fe-doped TiO2/SiO2 nanofibrous membrane with molecular imprinted modification,26 zinc acetyl acetonate/zeolite (ZnO/HZSM-5) nanocomposites,27 copper-anchored carbon nanotubes,28 advanced oxidation by a UV/H2O2 process.29 It is, therefore, important to investigate the removal of toxic elements using natural clays that are readily available in rural and impoverish communities where the quality of water they consume is severely compromised.

Natural clays have been used for adsorption and photocatalytic activity removal of pollutants, such as ammonium;30 cadmium and 2-chlorophenol;31 and cationic and anionic dyes.32

The characteristics of natural clay from unreported locations, such as Golf (GOL), Katuba (KAT), and Kolwezi (KOL), in the Democratic Republic of Congo warrant the need to investigate and provide scientific data based on that geographical space. Communities surrounding these mining cities consume potentially contaminated water. The usage of natural clay in the removal of pollutants in aqueous solutions will provide a cost-effective removal technique because natural clay samples can be obtained at a very low cost and in some instances free of charge.

Therefore, this study aimed to provide scientific characteristics data on the quality of clays from the Democratic Republic of Congo (DRC). This is important especially because physicochemical properties of the natural clays from the selected locations are inexistent in the open literature. In this study, the selected clay samples coded GOL, KAT, and KOL were fully characterized. In addition, the ability of untreated natural clay materials to remove 4-nitrophenol in aqueous solutions was studied through the photocatalytic process. The study also provides a starting point for more investigations on the various utilizations, i.e., wastewater treatment of the clay’s materials from the selected areas in the DRC.

2. Materials and Methods

2.1. Materials

This study was carried out using three clay samples collected from three different locations in the Democratic Republic of the Congo (DRC), namely, Golf and Katuba (in the Haut Lomami province) and Kolwezi in the Lualaba province. The latitudes of the three locations are −11.642190, −11.699710, and −10.711270, respectively, for Golf, Katuba, and Kolwezi. The three samples were coded GOL, KAT, and KOL for Golf, Katuba, and Kolwezi, respectively. The clay samples were dried at 105 °C until a constant weight was obtained, and then the samples were pulverized. The 4-nitrophenol was purchased from Sigma–Aldrich, South Africa. Figure 1 depicts the selected locations in the Democratic Republic of Congo where the clay samples were collected.33

Figure 1.

Figure 1

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Different locations where GOL, KAT, and KOL clay samples were collected.33 Reprinted with permission from Elsevier.

2.2. Methods

The density, the particle size distribution, and the pH of the clay samples were determined using a Micromeritics Accupyc 1340 gas pycnometer, a Microtrac S3500 PSD analyzer, and a Testo 206 pH meter, respectively. The specific surface area and pore volume were determined using a Micromeritics Tristar 3000 surface area and porosity analyzer. A Rigaku ZSX Primus II X-ray fluorescence spectrometer and a Rigaku Ultima IV X-ray diffractometer were used for the chemical analysis and phase identification, respectively.

The surface morphology and chemical analysis were carried out using a JEOL JSM-IT300 (scanning electron microscopy), while a Perkin–Elmer Frontier FTIR spectrometer was used for Fourier transform infrared (FTIR) spectroscopy. The weight loss and dehydration process of the clay samples were carried out using thermal analysis (TGA). The samples were heated in the thermal range from 50 to 900 °C at a rate of 10 °C/min in a nitrogen atmosphere using a Discovery TGA 550–TA system. A Lambda 650S Perkin–Elmer UV–Vis spectrometer was utilized for the UV–visible spectrophotometric analysis. UV–Vis analyses were also used to calculate the energy band gaps of the three clay samples according to the Kubelka–Munk relationship. Furthermore, based on the photoresponse of the clay samples, a preliminary analysis was conducted on the ability of the GOL clay sample in the photodegradation of 4-nitrophenol in wastewater.

A photocatalytic testing experiment for GOL clay application in wastewater treatment was devised as previously reported.34 Briefly, the degradation experiment was performed in a solar simulator (HAL–320 Asahi Spectra, Japan). A 300 W compact xenon light with apparent power of 500 VA was used as the light source. The solar simulator radiation spectrum output wavelength ranges were set from 350 to 1100 nm, and an air mass filter (1.5 global filter) was placed in front of the lamp to eliminate most of the UV radiation. The intensity distance that was the distance from the collimator lens to the sample surface was set at 37 mm. It created an effective radiated area of 50 mm2 and an irradiance of approximately 100 mW cm–2. GOL clay (0.1 g) was mixed with the 4-nitrophenol solution (80 ppm, 100 mL) and stirred in the dark for 60 min before the light was turned on. A sample was drawn from the solution using a syringe unit fitted with a 0.45 μm pore size hydrophilic PVDF membrane. The first sample was drawn after the adsorption–desorption equilibrium was considered as the initial concentration (C0). The solar simulator was prewarmed (30 min) for a stable irradiance output, and samples (a mixed solution of photocatalysts and 4-nitrophenol) were subsequently irradiated in a 250 mL quartz beaker. Samples were drawn from the solution at 45 min intervals, filtered, and analyzed using a UV–vis spectrometer. The experiments were undertaken for 225 min, and the solution was stirred throughout the experiment. The UV–vis photometric measurements of standards and samples were done at a maximum absorbance of 400 nm to obtain the concentration of 4-nitrophenol after the elapsed time interval (Ct). A calibration curve was obtained from the standards and the percentage of 4-nitrophenol degraded was calculated after eq 1:

2.2. 1

3. Results and Discussion

3.1. Density, pH, and Particle Size Distribution

The three clay samples exhibited different colors, namely, red for KOL, gray for KAT, and brown for GOL.

The density measurement of the clay samples was carried out, and the results show that the density of GOL is the lowest (2.7089 g/cm3), whereas the density values of KAT and KOL are 2.8052 and 2.8678 g/cm3, respectively. The values of the pH were acidic 5.80, 4.54, and 4.93, respectively, for GOL, KAT, and KOL clay samples. It has been reported that the skin pH values range from pH 4 to 7; nonetheless, the surface pH of natural skin is just below 5.35

In the current study, the pH of the KAT and KOL clay samples is below 5, and these can be used in the cosmetic industry. These values indicate that the three clay samples contain relatively the same type of molecules and that the clays will react similarly to the hydration. It has been reported that simple factors, such as water, exchangeable cation, and pH, can drastically affect the ability of clays to oxidize phenols to oligomers.36Figure 2 depicts the particle size distribution of the GOL, KAT, and KOL clay samples. The three samples have similar particle size distribution. The GOL sample has particle sizes ranging from 0.409 to 352 μm, whereas the KAT sample has particle sizes ranging from 0.409 to 418.6 μm. Furthermore, it was observed that the particle size of the KOL sample ranged from 0.409 to 296 μm.

Figure 2.

Figure 2

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Particle size distribution of GOL, KAT, and KOL clay samples.

3.2. Geochemical Characteristics

The three clay samples demonstrated a consistent geochemical composition. The classification scheme has been proven reliable for discriminating clastic sedimentary rocks. The geochemical characteristics of the clay samples were classified using the Heron plot.37Figure 3 depicts a plot on the SiO2/Al2O3 vs Fe2O3/K2O discrimination diagram of Herron.

Figure 3.

Figure 3

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Discrimination diagram log wt % (SiO2/Al2O3) versus log wt % (Fe2O3/K2O) using the Heron plot of the three clay samples.37

All the clay samples were plotted within the shale quadrant of the discrimination diagram. This could be further seen from the particle size distribution results, showing that the particle sizes measured in the samples demonstrated that the samples are shales. The samples investigated in the current study contained a high Al2O3 concentration (ranging from 23.0 to 33.8 wt %). This enrichment suggested the presence of clay minerals include kaolinite as well as associated clay-sized phases. This was confirmed with the X-ray diffraction results of the three clay samples.

During weathering, alkaline and alkaline-earth elements are removed in siliciclastic sediments. This process necessitates the use of Al2O3 (A), CaO and Na2O (CN), and K2O (K) to evaluate the geochemical alteration of sediments. To this end, numerous chemical indices that make use of the concentrations of these elements have been proposed. The most present of these is the Chemical Index of Alteration (CIA). The CIA is defined as [Al2O3/(Al2O3 + CaO* + Na2O + K2O)] × 100, where CaO* is Ca exclusive of carbonates and the values are in molar proportions to emphasize mineralogical relationships.38,39

Furthermore, this index is sensitive to chemical alteration of sediments and provides a quantitative estimation of secondary aluminous clay mineral abundance with respect to the primary feldspar.38 High values of CIA (ranging from c. 76 to 100) indicate intense chemical weathering occurring at the source areas, while low values (from c. 50 or less) indicate unweathered source areas. In the current study, the samples analyzed showed an average CIA value of 92, which indicated that the samples are intensely weathered sediments. Figure 4 shows the CIA values of the GOL, KOL, and KAT clay samples. Furthermore, the weathering of the samples was quantified by plotting chemical composition of the samples in the A-CN-K ternary plot of Nesbitt and Young39 (Figure 5).

Figure 4.

Figure 4

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CIA values of the GOL, KOL, and KAT clay samples. Furthermore, the weathering of the samples was quantified by plotting the chemical composition of the samples in the A–CN–K ternary plot of Nesbitt and Young38 (Figure 5).

Figure 5.

Figure 5

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Ternary diagram for the clay samples from the current study with similar age samples.40

It was observed on the diagram that the sample plot is in the neighborhood of the top apex, which is typical of clay minerals rich in Al2O3, such as kaolinite and chlorite. Furthermore, similarly, aged samples from the study of Ngueutchoua et al.40 are plotted together with studied samples (Figure 5). It can be seen that the comparison shows a high level of consistency with similar strongly weathered samples.

3.3. BET Measurements

Figure 6 depicts the BET typical nitrogen adsorption–desorption isotherms and the pore diameters for the GOL, KAT, and KOL samples. It was observed that all the clay samples are mesoporous. The findings are interesting because they imply that these three clay samples can be used in the water treatment processes. A recent study shows the application in water for mesoporous and silica-based nanomaterials.41 Other mesoporous materials for water treatment processes have also been reported.42 The average surface areas of 20.9145, 37.5422, and 39.4117 m2/g were obtained for GOL, KAT, and KOL samples, respectively. On the other hand, the average pore volume of 0.088292, 0.195899, and 0.184767 cm3/g values are respectively for GOL, KAT, and KOL samples. Furthermore, the average pore size of 178.077, 208.114, and 186.727 Å were obtained for GOL, KAT, and KOL, respectively. It was further observed that the pore diameters ranging from 29.544 to 862.318 Å, 17.963 to 932.970 Å, and 20.513 to 1475.380 Å are respectively for GOL, KAT, and KOL (Figure 6). Most of the pore diameters for all the samples are under 100 Å, which could have an impact on the water absorption of the bricks, fabricated using the selected clay.

Figure 6.

Figure 6

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BET adsorption and desorption isotherms and the pore diameter of the GOL, KAT, and KOL clay samples.

3.4. Chemical Composition and Phase Identification

The chemical composition of the three clay samples is shown in Table 1, where SiO2 is the most abundant compound in the clay samples. The presence of Al2O3 of different concentrations was observed and the concentrations were 29.8861, 33.8194, and 23.0195% respectively for KOL, KAT, and GOL. The GOL clay sample had 64.035% of SiO2, which is the highest followed by KAT with 56.5194% and KOL with 53.3235%. Furthermore, 1.5726, 2.8952, and 2.3429% of TiO2were observed for KOL, KAT, and GOL respectively. The KAT sample had the lowest concentration of K2O (1.1257%), whereas the KOL sample had the highest. On the other hand, the concentrations of Fe2O3 were 9.8978, 4.6599, and 5.9758% respectively for KOL, KAT, and KOL.

Table 1. Chemical Composition (wt %) of the Clay Samples.

component KOL KAT GOL
Na2O 0.0711 0.0327 0.0332
MgO 1.3084 0.5178 0.8732
Al2O3 29.8861 33.8194 23.0195
SiO2 53.3235 56.5194 64.035
P2O5 0.0458 0.0616 0.2298
SO3 0.0377 0.0641 0.0902
Cl 0.0245 0.0199 0.0197
K2O 3.5196 1.1257 2.3729
CaO 0.0198 0.0545 0.3617
TiO2 1.5726 2.8952 2.3429
Cr2O3 0.0396 0.0235 0.0293
Fe2O3 9.8978 4.6599 5.9758
NiO 0.0179 0.0217 0.0164
CuO 0.0075 0.0189 0.2005
Ga2O3 0.009 0.0097  
Rb2O 0.0395 0.0173 0.02
Y2O3 0.0188 0.057 0.0338
ZrO2 0.0542 0.0777 0.1248
BaO 0.1066   0.1
SrO   0.0039 0.0039
MnO     0.0526
ZnO     0.0408
PbO     0.0239
       
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The GOL sample has the highest concentration of SiO2; the KAT sample has the highest concentration of Al2O3 and TiO2, whereas the KOL sample has the highest of Fe2O3, K2O, and MgO. Madikizela et al.5 indicated that the presence of TiO2 in the clay could play a substantial role as a physical UV filter, and this is due to the ability of TiO2 to reflect UV light and block out sunlight. Therefore, the UV reflection ability of the three selected clay samples is discussed in the current study. The amounts (wt %) of TiO2 found in the samples are 1.5726, 2.8952, and 2.3429, respectively, for KOL, KAT, and GOL. The KAT clay could have higher ability to reflect UV followed by the GOL and KOL. Furthermore, the amount of TiO2 found in the clay samples had a higher concentration compared to other clay samples from different African countries.5 The main chemical components (SiO2, Al2O3, Fe2O3, K2O, TiO2, and MgO) in the clay samples are efficient oxide used as semiconductors or promoters in nanomaterial composites for environmental application, including wastewater treatment.43−46

The XRD analysis was utilized to determine the mineralogical composition of the clay sample components and to confirm the XRF results. The X-ray diffractograms are presented in Figure 7 for KAT, KOL, and GOL respectively.

Figure 7.

Figure 7

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XRD patterns of GOL, KOL, and KAT clay samples with Q = quartz, K = kaolinite, M = magnetite, and A = anatase.

All the samples contained quartz. The KAT sample also contained kaolinite and anatase (TiO2), while the KOL sample contained magnetite. In the GOL sample, it contained quartz, magnetite, kaolinite, and illite. The obtained XRD results are similar to the one found in the literature.1,47 The presence of the obtained phases is further confirmed with the FTIR analysis. It was also observed that the peaks of minor phases such as illite overlapped with those of major phases.

The XRF results were in agreement with the obtained chemical composition of the identified phases using XRD and that showed the presence of aluminum, iron, and silicon as major chemical elements. The XRD showed the presence of quartz, kaolinite, anatase, magnetite, and illite.

3.5. Surface Morphology and Functional Groups Analysis

The surface morphology of GOL, KAT, and KOL samples are shown in Figure 8. All the clay samples presented similar surface morphology. A flake-like morphology was noticed in all the three clay samples and could correspond to a part where there is a presence of an amorphous material.

Figure 8.

Figure 8

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SEM micrographs of GOL, KOL, and KAT clay samples.

The FTIR analysis assists in identifying the functional groups, which are part of the minerals in the selected materials. The identified functional groups in the three clay samples using FTIR are depicted in Figure 9.

Figure 9.

Figure 9

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FTIR spectra for the GOL, KAT, and KOL samples.

It was noticed that the FTIR spectra of all the three samples are similar. The important FTIR bands for clay materials and their assignments are in agreement with the literature.6,48 The Si–O–H stretching vibrations were observed at 681.13, 780.22, and 1006.00 cm–1. On the other hand, the Al---O–H stretching was observed at 3697.8 cm–1, while an Al---O–H inter-octahedral band was observed at 3621.39 cm–1. Furthermore, the band observed at 1628.18 cm–1 could suggest the possibility of water hydration in the clay sample (H–O–H stretching).48 The density results indicated that the three clay samples could have the same response toward hydration. The functional groups could confirm those findings. The shift in the bands could be due to the presence of other chemical elements and impurities in the clay samples. The presence of the various assigned bands linked to chemical elements confirmed the phases present in the clay samples obtained using XRD.

3.6. Thermal Analysis

TGA results are presented in Figure 10. The first mass loss that occurred at around 100 °C is mostly due to the loss of water adsorbed at the surface of the clay samples. This was in agreement with the results reported by Madikizela et al.5 The GOL sample exhibited the lowest mass loss (8.33 wt %) compared to the other two samples, namely, KAT (12.59 wt %) and KOL (12.04 wt %), implying that GOL contains fewer organic constituents. Furthermore, it has been reported that mass losses are often associated with the oxidation of organic constituents present in the materials.6 Boulingui et al.1 have also reported that there are major reactions involving loss of H2O, CO2, and/or organics in clay materials.

Figure 10.

Figure 10

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TGA thermograms of the GOL, KAT, and KOL clay.

3.7. UV Measurements

In some part of the world, including the African continent, clays are used for cosmetic purposes. Morekhure-Mphahlele et al.6 indicated that some clays in the Eastern Cape in South Africa are used for their moisturizing effect. To study the potentiality of using the three clay samples as a substitute in creams such as sunblock, the clay samples were studied in regard to investigating their abilities to protect against ultraviolet radiation in the wavelength range of 200–400 nm. This could provide information if they can be used in the cosmetic industry. Furthermore, the ability of these materials to respond to UV could indicate their ability to be used as photocatalytic materials. The assumption is more likely accurate due to the presence of TiO2 and Fe2O3, which could render natural clay photoresponse31 and is suitable for adsorption and photocatalytic activity.30−32Figure 11 presents the variation of UV reflectance percentages of the selected clay samples in the range of 200–400 nm.

Figure 11.

Figure 11

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Ultraviolet reflectance percentage variation of the selected clay samples.

The UV results in the current study are displayed in the range from 200 to 400 nm, and this is covering the entire long wave of UV-A and UV-B spectral ranges. Furthermore, the wavelength from 200 to 280 nm covers a part of the short wave UV-C spectral range.5 The reflectance percentage values of the three clay samples were observed at a specific wavelength, namely, 200, 250, 300, 350, and 400 nm and are displayed in Figure 12.

Figure 12.

Figure 12

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Variation of reflectance percentage values of the GOL, KAT, and KOL clay samples at selected wavelengths.

Reflectance values of 19.8794, 22.8894, and 21.3048% were obtained for KOL, KAT, and GOL, respectively, at 200 nm. Furthermore, the GOL sample has 15.5583, 16.8340, 16.2544, and 20.2414% reflectance at 250, 300, 350, and 400 nm, respectively. The KAT samples have 19.0457, 23.91766, 24.9590, and 32.31063% reflectance at 250, 300, 350, and 400 nm, respectively. Reflectance values 16.5750, 18.3752, 18.6058, and 21.6219% were respectively obtained at 250, 300, 350, and 400 nm for the KOL sample. The KAT sample exhibited higher reflectance percentages for the selected wavelengths except at 200 nm. Furthermore, the lowest reflectance percentage for the GOL sample is 15.5488% obtained at 320 nm; the KAT sample has 18.9396% reflectance, which is its lowest reflectance at 256 nm. The lowest for the KOL sample is 16.2131% reflectance recorded at 260 nm wavelength. It can be said that the GOL sample has the highest reflectance percentage at all the selected wavelengths. It has been reported that the presence of inorganic compounds, namely, titanium dioxide (TiO2) and zinc oxide (ZnO), in sunscreen products act as physical ultraviolet filters and have the ability to block out sunlight and reflect ultraviolet light.5 This could be seen by the presence of the TiO2 compound in all the selected clay samples. Moreover, the data also suggest that the three clay samples can be used in the UV photocatalysis. TiO2 is known to be excited from UV light leading to the generation of electrons and holes at the conduction and valence bands. Consequently, oxidizing radicals are created, and these species are essential for photodegradation in wastewater treatment.34 It can be said that the higher reflectance percentage of UV in the GOL sample compared to the other two samples could be due to the additional presence of the ZnO compound, which was not found in the other two clay samples. Further purification of the raw clay samples can enhance the blocking ability of UV of the GOL sample, which could be used for the cosmetic application. However, it is important to treat TiO2 with other materials to ensure application in the visible light.34

3.8. Photocatalytic Experiment

The energy band for the three clays, namely, GOL, KAT, and KOL was measured from the Kubelka–Munk curves as depicted in Figure 13a–c. Furthermore, the compared values of the energy band gap of the three clays are shown in Figure 13d.

Figure 13.

Figure 13

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Kulbelka–Munk curves of the two calculated energy band gaps for the GOL (a), KAT (b), and KOL (c) clay samples and the comparison (d).

The results presented in Figure 13d show that GOL clay is characterized by low energy band gaps (2.68 and 3.94 eV), which is a good photocatalytic feature for semiconductors. The energy band gaps for KAT is 2.93 and 3.85 eV, whereas the KOL displayed the energy band gaps of 2.71 and 3.44 eV. The photocatalytic activity of the untreated GOL clay was tested in the degradation of 4-nitrophenol. The photodegradation percentages were calculated using eq 1, and the results are presented in Table 2. The experimental data indicated that 99.13% of 4-nitrophenol was decomposed within 225 min of irradiation, which is closely in agreement with the literature.29 A previous study has also reported that more than 225 min is required to achieve 98% of 4-nitrophenol decomposition under the visible light irradiation.22 The results are quite interesting because they confirm that GOL clay can be used as a photocatalytic material under the current conditions in the degradation of 4-nitrophenol. Furthermore, the photolysis experiment revealed that only 2.77% of 4-nitrophenol was removed from aqueous solution using the GOL clay sample, indicating that the irradiation played a crucial role in the removal of 4-nitrophenol.

Table 2. Photodegradation Percentages of 4-Nitrophenol Using the GOL Clay Sample.

sample time (min) photodegradation %
GOL Photolysis 2.77
45 20.61
90 36.20
135 51.49
180 98.16
225 99.13
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Additional experimental data are depicted in Figure 14a,b for the UV spectra of the photoreduction reaction and UV spectra showing a decrease in the absorbance value at 400 nm and increment in 300 nm with time over 225 min. This figure shows that the characteristic absorption of 4-nitrophenol disappeared rapidly at 400 nm and implies that 4-nitrophenol has been effectively removed from the reaction solution. As the reaction proceeded, a color change was observed from the solution being colorless to orange, and then gradually disappeared with a prolonged reaction time. The results suggested that expected intermediate products in the photodegradation of 4-nitrophenol could have been present as indicated by the increment of absorption at 300 nm. The discoloration of 4-nitrophenol could be ascribed to the hydroxylation of hydroxyl radicals photogenerated in the 4-nitrophenol solution.29

Figure 14.

Figure 14

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UV spectra showing (a) the photoreduction reaction and (b) UV spectra showing a decrease in the absorbance value at 400 nm and increment at 300 nm with time over 3 h 45 min.

Based on the literature, the removal of 4-nitrophenol was attributed to the oxidation by a OH radical, which can be expressed by

3.8.

while the discoloration or disappearance rate of 4-nitrophenol can follow eq 2:

3.8. 2

where k1 represents the product of the pseudo first-order rate constant of the reaction between 4-nitrophenol and hydroxyl radicals. The pseudo first-order rate kinetic equation can be written as follows (eq 3):

3.8. 3

where k1 can be referred to as a pseudo first-order rate constant; Ct and C0 are the concentrations at time (t) and time (0) in that order.29

The kinetic graph is displayed in Figure 15, and the pseudo first-order model can explain the degradation reaction better when ln(Ct/C0) is plotted against t as displayed in the inserted table in Figure 15b.

Figure 15.

Figure 15

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Kinetic plots (a) of the concentrations (b) of the natural log of concentrations as a function for the degradation of 4-nitrophenol in aqueous solution.

The photocatalytic results obtained in this study using untreated natural GOL clay were compared with the reported data in the literature as summarized in Table 3. It is important to note that this kind of comparison is not fully accurate due to the difference in parameters, such as the source of light, time of irradiation, the concentration of the 4-nitrophenol, and the material investigated by numerous researchers.24−27 This is demonstrated in Table 3 where some authors have reported the performance of their material based on the reaction rate constant,26 while others in the form of percentage removal. However, based on the closest comparison of parameters presented in this study and those reported in the literature,24−27 it can be said that untreated natural GOL clay has demonstrated superior removal ability of 4-nitrophenol through simulated solar light irradiation.

Table 3. Comparison between the Current Results and Those in the Literature.

material/treatment procedure light source pollutant removal percentage and some parameters reference
untreated natural clay (GOL) 300 W compact xenon light 4-nitrophenol 99.13% (80 ppm) at 225 min this study
ZnO/HZSM-5 nanocomposites 30 W (UV-C) lamp (Philips). 4-nitrophenol 91% TOC (20 mgL–1) at 90 min. (27)
impinging streams photoreactor coupled with a membrane 18 W UV-C radiation (Philips Co.) 4-nitrophenol 55.6% TOC and 45% COD (30 ppm) at 180 min (23)
Fe-doped TiO2/SiO2 nanofibrous membranes 500 W xenon lamp 4-nitrophenol (10 mg/L) k (reaction rate constant) value of the target 4NP over nanofibrous photocatalysis are 0.00417, 0.00155, and 0.00263 for MI-Fe@TS, NI-Fe@TS, and Fe@TS (26)
novel copper(II) porphyrin–TiO2 photocatalysts 350 W xenon lamp under simulated solar irradiation. 4-nitrophenol 4-NP could be photodegraded much rapidly to almost zero (1 × 10–4 M) at 300 min (25)
bare TiO2 microspheres 350 W xenon lamp under simulated solar irradiation. 4-nitrophenol 35% (1 x 10–4 mol/L) at 300 min (25)
TiO2/Fe2O3-supported bentonite (TiO2/natural hematite-supported bentonite (TiNHB)) UV254nm irradiation (25 W, 18 mA) 4-nitrophenol 98% (20 mg/L) at 180 min. (22)
UV/H2O2 process A low pressure 14 W UV lamp (S2-Q-PA12, Canada R-Can Environmental Inc) 4-nitrophenol > 98% (25 ppm) in 12 min and 94% and TOC (25 ppm) in 106 min (29)
novel Co and Cu phthalocyanine under visible light 4-nitrophenol 95% for CoPc and 97% (or CuPc (0.025 M) at 1 h (24)
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4. Conclusions

The physicochemical characterization of selected clay samples from three different locations in the Democratic Republic of Congo (DRC) was investigated. The density of the three clay samples was relatively close: 2.71 g/cm3 for GOL and 2.81 and 2.87 g/cm3 for KAT and KOL, respectively. On the other hand, the particle size distribution for all the samples is ranged from 0.41 to 418.6 μm. The geochemical characteristics of the clay samples demonstrated that the samples are shales. Furthermore, the clay samples showed an average CIA of 92, which indicated that the samples are intensely weathered sediments.

The XRF results showed that SiO2 is the most abundant chemical compound followed by Al2O3 and Fe2O3. Their presence and that of TiO2 are crucial for photocatalytic activity in water treatment applications. It was observed that most of the pore diameters for all the clay samples are under 100 Å, which could have an impact on the application of the three clay samples for various purposes, including wastewater treatment. The GOL clay sample showed the highest UV reflectance percentage at all the selected wavelengths, and this could be due to the presence of a ZnO compound, which was not found in the other two clay samples. The GOL clay could be coupled with other materials and find photocatalytic activity in the UV and visible light ranges. Furthermore, preliminary analysis on the photodegradation of 4-nitrophenol using the untreated GOL clay sample showed a 99.13% degradation obtained at 225 min.

Acknowledgments

The authors would like to acknowledge the financial support from the University of South Africa.

The authors declare no competing financial interest.

References

  1. Boulingui J. E.; Nkoumbou C.; Njoya D.; Thomas F.; Yvon J. Characterization of clays from Mezafe and Mengono (Ne-Libreville, Gabon) for potential uses in fired products. Appl. Clay Sci. 2015, 115, 132–144. 10.1016/j.clay.2015.07.029. [DOI] [Google Scholar]
  2. Deliniè re R.; Aubert J. E.; Rojat F.; Gasc-Barbier M. Physical, mineralogical and mechanical characterization of ready-mixed clay plaster. Build Environ. 2014, 80, 11–17. 10.1016/j.buildenv.2014.05.012. [DOI] [Google Scholar]
  3. Masheane M.; Nthunya L.; Mubiayi M.; Thamae T.; Mhlanga S. Physico-chemical characteristics of some Lesotho’s clays and their assessment for suitability in ceramics production. Part. Sci. Technol. 2018, 36, 117–122. 10.1080/02726351.2016.1226226. [DOI] [Google Scholar]
  4. Milošević M.; Logar M. Properties and characterization of a clay raw material from Miličinica (Serbia) for use in the ceramic industry. Clay Miner. 2017, 52, 329–340. 10.1180/claymin.2017.052.3.04. [DOI] [Google Scholar]
  5. Madikizela L. M.; Nkwentsha N.; Mlunguza N. Y.; Mdluli P. S. Physicochemical characterization and In vitro evaluation of the sun protection factor of cosmetic products made from natural clay material. S. Afr. J. Chem. 2017, 70, 113–119. 10.17159/0379-4350/2017/v70a016. [DOI] [Google Scholar]
  6. Morekhure-Mphahlele R.; Focke W. W.; Grote W. Characterisation of vumba and ubumba clays used for cosmetic purposes. S. Afr. J. Sci. 2017, Volume 113, 1–5. 10.17159/sajs.2017/20160105. [DOI] [Google Scholar]
  7. Mattioli M.; Giardini L.; Roselli C.; Desideri D. Mineralogical characterization of commercial clays used in cosmetics and possible risk for health. Appl. Clay Sci. 2016, 119, 449–454. 10.1016/j.clay.2015.10.023. [DOI] [Google Scholar]
  8. Khatem R.; Miguel R. O.; Bakhti A. Use of synthetic clay for Removal of Diclofenac Anti-inflammatory. Eurasian J. Soil Sci. 2015, 4, 126–136. [Google Scholar]
  9. Ji Y.; Yang Y.; Zhou L.; Wang L.; Lu J.; Ferronato C.; Chovelon J. M. Photodegradation of sulfasalazine and its human metabolites in water by UV and UV/peroxydisulfate processes. Water Res. 2018, 133, 299–309. 10.1016/j.watres.2018.01.047. [DOI] [PubMed] [Google Scholar]
  10. Abdollahi M.; Mohammadirad A.., 2014. ″Nitrophenol, 4.″ In Encyclopedia of Toxicology (Third Edition), pp. 575–577, 10.1016/B978-0-12-386454-3.01143-X. [DOI] [Google Scholar]
  11. Ozola R.; Krauklis A.; Burlakovs J.; Klavins M.; Vincevica-Gaile Z.; Hogland W. Surfactant-modified clay sorbents for the removal of p-nitrophenol. Clays Clay Miner. 2019, 67, 132–142. 10.1007/s42860-019-00015-2. [DOI] [Google Scholar]
  12. El Ouardi M.; Laabd M.; Abou Oualid H.; Brahmi Y.; Abaamrane A.; Elouahli A.; Addi A. A.; Laknifli A. Efficient removal of p-nitrophenol from water using montmorillonite clay: insights into the adsorption mechanism, process optimization, and regeneration. Environ. Sci. Pollut. Res. 2019, 26, 19615–19631. 10.1007/s11356-019-05219-6. [DOI] [PubMed] [Google Scholar]
  13. Dos Santos A.; Viante M. F.; Pochapski D. J.; Downs A. J.; Almeida C. A. P. Enhanced removal of p-nitrophenol from aqueous media by montmorillonite clay modified with a cationic surfactant. J. Hazard. Mater. 2018, 355, 136–144. 10.1016/j.jhazmat.2018.02.041. [DOI] [PubMed] [Google Scholar]
  14. Hamidouche S.; Bouras O.; Zermane F.; Cheknane B.; Houari M.; Debord J.; Harel M.; Bollinger J. C.; Baudu M. Simultaneous sorption of 4-nitrophenol and 2-nitrophenol on a hybrid geocomposite based on surfactant-modified pillared-clay and activated carbon. Chem. Eng. J. 2015, 279, 964–972. 10.1016/j.cej.2015.05.012. [DOI] [Google Scholar]
  15. Park Y.; Frost R. L.; Ayoko G. A.; Morgan D. L. Adsorption of p-nitrophenol on organoclays. J. Therm. Anal. Calorim. 2013, 111, 41–47. 10.1007/s10973-011-2103-7. [DOI] [Google Scholar]
  16. Ely A.; Baudu M.; Kankou M. O. S. A. O.; Basly J. P. Copper and nitrophenol removal by low cost alginate/Mauritanian clay composite beads. Chem. Eng. J. 2011, 178, 168–174. 10.1016/j.cej.2011.10.040. [DOI] [Google Scholar]
  17. Wu Z.; Yuan X.; Zhong H.; Wang H.; Zeng G.; Chen X.; Wang H.; Shao J. Enhanced adsorptive removal of p-nitrophenol from water by aluminum metal–organic framework/reduced graphene oxide composite. Sci. Rep. 2016, 6, 25638. 10.1038/srep25638. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Yang W.; Yu Z.; Pan B.; Lv L.; Zhang W. Simultaneous organic/inorganic removal from water using a new nanocomposite adsorbent: a case study of p-nitrophenol and phosphate. Chem. Eng. J. 2015, 268, 399–407. 10.1016/j.cej.2015.01.051. [DOI] [Google Scholar]
  19. Zhang A.; Wang N.; Zhou J.; Jiang P.; Liu G. Heterogeneous Fenton-like catalytic removal of p-nitrophenol in water using acid-activated fly ash. J. Hazard. Mater. 2012, 201-202, 68–73. 10.1016/j.jhazmat.2011.11.033. [DOI] [PubMed] [Google Scholar]
  20. Guo P.; Tang L.; Tang J.; Zeng G.; Huang B.; Dong H.; Zhang Y.; Zhou Y.; Deng Y.; Ma L.; Tan S. Catalytic reduction–adsorption for removal of p-nitrophenol and its conversion p-aminophenol from water by gold nanoparticles supported on oxidized mesoporous carbon. J. Colloid Interface Sci. 2016, 469, 78–85. 10.1016/j.jcis.2016.01.063. [DOI] [PubMed] [Google Scholar]
  21. Chen Q.; Ma C.; Duan W.; Lang D.; Pan B. Coupling adsorption and degradation in p-nitrophenol removal by biochars. J. Cleaner Prod. 2020, 122550. 10.1016/j.jclepro.2020.122550. [DOI] [Google Scholar]
  22. Gaffour H.; Mokhtari M. Photocatalytic degradation of 4-nitrophenol using TiO2+ Fe 2O3 and TiO2/Fe2O3-supported bentonite as heterogeneous catalysts. Res. Chem. Intermed. 2016, 42, 6025–6038. 10.1007/s11164-016-2436-8. [DOI] [Google Scholar]
  23. Madadi S.; Biriaei R.; Sohrabi M.; Royaee S. J. Photodegradation of 4-nitrophenol using an impinging streams photoreactor coupled with a membrane. Chem. Eng. Process. 2016, 99, 1–9. 10.1016/j.cep.2015.10.018. [DOI] [Google Scholar]
  24. Saka E. T.; Tekintas K. Light driven photodegradation of 4-nitrophenol with novel Co and Cu phthalocyanine in aqueous media. J. Mol. Struct. 2020, 128189. 10.1016/j.molstruc.2020.128189. [DOI] [Google Scholar]
  25. Lü X. F.; Sun W. J.; Li J.; Xu W. X.; Zhang F. X. Spectroscopic investigations on the simulated solar light induced photodegradation of 4-nitrophenol by using three novel copper (II) porphyrin–TiO2 photocatalysts. Spectrochim. Acta A 2013, 111, 161–168. 10.1016/j.saa.2013.02.021. [DOI] [PubMed] [Google Scholar]
  26. Li X.; Wang J.; Li M.; Jin Y.; Gu Z.; Liu C.; Ogino K. Fe-doped TiO2/SiO2 nanofibrous membranes with surface molecular imprinted modification for selective photodegradation of 4-nitrophenol. Chin. Chem. Lett. 2018, 29, 527–530. 10.1016/j.cclet.2017.09.007. [DOI] [Google Scholar]
  27. Khatamian M.; Alaji Z. Efficient adsorption-photodegradation of 4-nitrophenol in aqueous solution by using ZnO/HZSM-5 nanocomposites. Desalination 2012, 286, 248–253. 10.1016/j.desal.2011.11.031. [DOI] [Google Scholar]
  28. Yang T.; Tang Y.; Liu L.; Gao Y.; Zhang Y. Cu-anchored CNTs for effectively catalytic reduction of 4-nitrophenol. Chem. Phys. 2020, 110738. 10.1016/j.chemphys.2020.110738. [DOI] [Google Scholar]
  29. Zhang W.; Xiao X.; An T.; Song Z.; Fu J.; Sheng G.; Cui M. Kinetics, degradation pathway and reaction mechanism of advanced oxidation of 4-nitrophenol in water by a UV/H2O2 process. J. Chem. Technol. Biotechnol. 2003, 78, 788–794. 10.1002/jctb.864. [DOI] [Google Scholar]
  30. Alshameri A.; He H.; Zhu J.; Xi Y.; Zhu R.; Ma L.; Tao Q. Adsorption of ammonium by different natural clay minerals: characterization, kinetics and adsorption isotherms. Appl. Clay Sci. 2018, 159, 83–93. 10.1016/j.clay.2017.11.007. [DOI] [Google Scholar]
  31. Hadjltaief H. B.; Sdiri A.; Ltaief W.; Da Costa P.; Galvez M. E.; Zina M. B. Efficient removal of cadmium and 2-chlorophenol in aqueous systems by natural clay: Adsorption and photo-Fenton degradation processes. C. R. Chim. 2018, 21, 253–262. 10.1016/j.crci.2017.01.009. [DOI] [Google Scholar]
  32. Chaari I.; Fakhfakh E.; Medhioub M.; Jamoussi F. Comparative study on adsorption of cationic and anionic dyes by smectite rich natural clays. J. Mol. Struct. 2019, 1179, 672–677. 10.1016/j.molstruc.2018.11.039. [DOI] [Google Scholar]
  33. Burlet C.; Vanbrabant Y.; Goethals H.; Thys T.; Dupin L. Raman spectroscopy as a tool to characterize heterogenite (CoO· OH) (Katanga Province, Democratic Republic of Congo). Spectrochim. Acta A 2011, 80, 138–147. 10.1016/j.saa.2011.03.007. [DOI] [PubMed] [Google Scholar]
  34. Muleja A. A.; Mamba B. B. Development of calcined catalytic membrane for potential photodegradation of Congo red in aqueous solution. J. Environ. Chem. 2018, 6, 4850–4863. 10.1016/j.jece.2018.07.004. [DOI] [Google Scholar]
  35. Lambers H.; Piessens S.; Bloem A.; Pronk H.; Finkel P. Natural skin surface pH is on average below 5, which is beneficial for its resident flora. Int. J. Cosmet. Sci. 2006, 28, 359–370. 10.1111/j.1467-2494.2006.00344.x. [DOI] [PubMed] [Google Scholar]
  36. Yong R. N.; Desjardins S.; Farant J. P.; Simon P. Influence of pH and exchangeable cation on oxidation of methylphenols by a montmorillonite clay. Appl. Clay Sci. 1997, 12, 93–110. 10.1016/S0169-1317(96)00043-9. [DOI] [Google Scholar]
  37. Herron M. M. Geochemical Classification of Terrigenous Sands and Shales from Core or Log Data. J. Sediment. Res. 1988, Vol. 58, 820–829. 10.1306/212F8E77-2B24-11D7-8648000102C1865D. [DOI] [Google Scholar]
  38. Nesbitt H. W.; Young G. M. Early Proterozoic Climates and Plate Motions Inferred from Major Element Chemistry of Lutites. Nature 1982, 299, 715–717. 10.1038/299715a0. [DOI] [Google Scholar]
  39. Harnois L. The CIW Index: A New Chemical Index of Weathering. Sediment. Geol. 1988, 55, 319–322. 10.1016/0037-0738(88)90137-6. [DOI] [Google Scholar]
  40. Ngueutchoua G.; Ngantchu L. D.; Youbi M.; Ngos S. III; Beyala V. K. K.; Yifomju K. P.; Tchamgoué J. C. Geochemistry of Cretaceous Mudrocks and Sandstones from Douala Sub-Basin, Kumba Area, South West Cameroon: Constraints on Provenance, Source Rock Weathering, Paleo-Oxidation Conditions and Tectonic Environment. Int. J. Geosci. 2017, 08, 393. 10.4236/ijg.2017.84021. [DOI] [Google Scholar]
  41. Min X.; Trujillo D.; Huo J.; Dong Q.; Wang Y. Amine-bridged Periodic Mesoporous Organosilica Nanomaterial for Efficient Removal of Selenate. Chem. Eng. J. 2020, 125278. 10.1016/j.cej.2020.125278. [DOI] [Google Scholar]
  42. Cooper C.; Burch R. Mesoporous materials for water treatment processes. Water Res. 1999, 33, 3689–3694. 10.1016/S0043-1354(99)00095-0. [DOI] [Google Scholar]
  43. Dontsova T. A.; Nahirniak S. V.; Astrelin I. M. Metaloxide nanomaterials and nanocomposites of ecological purpose. J. Nanomater. 2019, 2019, 1. 10.1155/2019/5942194. [DOI] [Google Scholar]
  44. Huang H.; Chen L.; Wang S.; Kang P.; Chen X.; Guo Z.; Huang X.-J. Electrochemical monitoring of persistent toxic substances using metal oxide and its composite nanomaterials: Design, preparation, and application. Trends Anal. Chem. 2019, 115636. 10.1016/j.trac.2019.115636. [DOI] [Google Scholar]
  45. Rani M.; Rachna; Shanker U. Metal oxide-chitosan based nanocomposites for efficient degradation of carcinogenic PAHs. J. Environ. Chem. Eng. 2020, 103810. 10.1016/j.jece.2020.103810. [DOI] [Google Scholar]
  46. Thy L. T. M.; Kiem N. H.; Tu T. H.; Phu L. M.; Oanh D. T. Y.; Nam H. M.; Phong M. T.; Hieu N. H. Fabrication of manganese ferrite/graphene oxide nanocomposites for removal of nickel ions, methylene blue from water. Chem. Phys. 2020, 533, 110700. 10.1016/j.chemphys.2020.110700. [DOI] [Google Scholar]
  47. Mahmoudi S.; Bennour A.; Meguebli A.; Srasra E.; Zargouni F. Characterization and traditional ceramic application of clays from the Douiret region in South Tunisia. Appl. Clay Sci. 2016, 127-128, 78–87. 10.1016/j.clay.2016.04.010. [DOI] [Google Scholar]
  48. Nayak P. S.; Singh B. K. Instrumental characterization of clay by XRF, XRD and FTIR. Bull. Mater. Sci. 2007, 30, 235–238. 10.1007/s12034-007-0042-5. [DOI] [Google Scholar]

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