Natural Clay Modiﬁed with ZnO/TiO 2 to Enhance Pollutant Removal from Water

: Raw clays, extracted from Bana, west Cameroon, were modiﬁed with semiconductors (TiO 2 and ZnO) in order to improve their depollution properties with the addition of photocatalytic properties. Cu 2+ ions were also added to the clay by ionic exchange to increase the speciﬁc surface area. This insertion of Cu was conﬁrmed by ICP-AES. The presence of TiO 2 and ZnO was conﬁrmed by the detection of anatase and wurzite, respectively, using X-ray diffraction. The composite clays showed increased speciﬁc surface areas. The adsorption property of the raw clays was evaluated on two pollutants, i.e., ﬂuorescein (FL) and p-nitrophenol (PNP). The experiments showed that the raw clays can adsorb FL but are not efﬁcient for PNP. To demonstrate the photocatalytic property given by the added semiconductors, photocatalytic experiments were performed under UVA light on PNP. These experiments showed degradation up to 90% after 8 h of exposure with the best ZnO-modiﬁed clay. The proposed treatment of raw clays seems promising to treat pollutants, especially in developing countries. Concerning the nitrogen adsorption–desorption isotherms, two different types are observed between all samples: (i) type I isotherm, with a sharp increase at low pressure followed by a plateau corresponding to microporous solid; and (ii) type IV isotherm, characterized by a broad hysteresis at high pressure (mesoporous solid). Samples containing TiO 2 have type I isotherms, and the other samples have type IV isotherms. As an example, the isotherms of Bare Clay and Clay/TiO 2 samples are plotted in Figure 3. The other isotherms are represented in Figures S1 and S2 in the Supplementary Materials.


Introduction
The population growth, intensive industrialization, and agricultural practices that occurred in recent decades have led to an increase in environmental pollution, which is now considered a global crisis [1]. This scourge has its origins in the constant improvement in the standard of living and the strong demands of consumers. In Cameroon, for example, many cotton, pharmaceutical, fertilizer, tanning, and pesticide manufacturing industries release pollutants such as dyes, pesticides, or bacteria into the environment, leading to disturbances of aquatic fauna and constituting a risk for human health [2]. Faced with this alarming situation, the global demand for water, the most vital natural resource, is increasing [3] and at the same time, the quality of freshwater sources is declining due to the presence of emerging contaminants. Most of these contaminants escape conventional wastewater treatment offered by wastewater treatment plants. The presence of these emerging pollutants in the environment is a matter of concern for most environmental agencies in developing countries [4]. This water should be treated as part of the recycling of wastewater that can be used by low-income populations for watering vegetable crops and washing cars and clothes in order to allow these populations to have a profitable and healthy economic activity.
In order to limit the arrival of these various types of refractory contaminants into the environment, effective and ecological treatment strategies have been developed, such as the use of local clays widely available in Cameroon from kaolinites, andosols, illites, and smectites [5], and globally, the use of adsorption as an efficient process to remove pollutants [6]. Clays have been the subject of different characterizations and applications [7]. For nearly three decades, many research works have been carried out on clay materials from Cameroon and their applications [8]. The search for new deposits and the characterization and valuation of clay materials are still relevant today.
Advanced oxidation processes (AOPs) have been applied in several sectors for the treatment of surface and groundwater [9,10] and for the elimination of odors and volatile organic compounds [11], as well as for water discoloration, the degradation of phytosanitary and pharmaceutical products [12], the production of molecules such as H 2 [13], and water disinfection [14]. AOPs can be used either as an oxidative pretreatment leading to easily biodegradable compounds, or as a tertiary treatment method for the removal or complete mineralization of residual pollutants [15]. This process is based on the generation of radical species able to degrade organic pollutants thanks to the use of a photocatalyst material activated by UV radiation [16]. The most-used UV-sensitive photocatalysts are TiO 2 and ZnO [17][18][19]. Different composites of photocatalysts have already been developed for pollutant removal [20][21][22][23][24].
In this work, a combination of adsorption and photocatalysis through the synthesis of mixed materials based on smectite-TiO 2 or smectite-ZnO is presented. Two types of pollutants are explored, one dye and one pesticide-type pollutant: fluorescein (FL) and p-nitrophenol (PNP), respectively. The physico-chemical properties of the pure and mixed materials are determined as well as their adsorption and photocatalytic activities. The production of mixed materials allows the use of a material already present in Cameroon and the addition of small fraction (<30%) of photocatalysts to increase the pollutant removal efficiency of the clay. The efficiency of the process and the cost can be studied and compared to other known methods.
The advantages of using semiconductor-modified clay materials for pollutant removal in water in developing countries are numerous: (i) the materials are composed primarily of natural material (the clay) directly located in the country where the pollution will be treated; (ii) the semiconductor material loading stays low (<30 wt %), reducing the cost of production; (iii) ZnO and TiO 2 are the most common semiconductor materials and can be produced with green synthesis with low use of organic reagents; (iv) the composite material presents both high adsorption capacity and photocatalytic properties, increasing its depollution properties compared to bare materials; and (v) the process for the production of the composite materials is simple.

Composition
Macroscopically, the raw clays, the Cu 2+ -modified clays, and the TiO 2 -modified clays are pale yellow. The ZnO-modified clays are slightly gray. The main compositions of the six different clay samples, determined by ICP-AES, are presented in Table 1.
The clays contain 9-21% of Si, 5-11% of Al, and 1-4% of Fe with an atomic Si/Al ratio equal to 2, consistent with a smectic composition [25]. The amount of copper increases up to 0.8% in the Cu 2+ -modified samples ( Table 1). The percentage of ZnO reaches 28.1% and 30.3% in the Clay/ZnO and Clay/ZnO/Cu 2+ samples, respectively. The percentage of TiO 2 is 28.8% and 27.6% in the Clay/TiO 2 and Clay/TiO 2 /Cu 2+ samples, respectively.
The XRD patterns of the eight samples ( Figure 1) allow us to estimate the crystallinity of the samples. The clays contain 9-21% of Si, 5-11% of Al, and 1-4% of Fe with an atomic Si/Al ratio equal to 2, consistent with a smectic composition [25]. The amount of copper increases up to 0.8% in the Cu 2+ -modified samples ( Table 1). The percentage of ZnO reaches 28.1% and 30.3% in the Clay/ZnO and Clay/ZnO/Cu 2+ samples, respectively. The percentage of TiO2 is 28.8% and 27.6% in the Clay/TiO2 and Clay/TiO2/Cu 2+ samples, respectively.
The XRD patterns of the eight samples ( Figure 1) allow us to estimate the crystallinity of the samples. The Bare Clay (♦) is mainly composed of smectite, which is a family of different clay minerals observed in Figure 1 (all the following phases are observed: augite, cristobalite, montmorillonite, illite, kaolinite, feldspar, and talc). Smectite forms an important group of the phyllosilicate family of minerals, which are distinguished by layered structures composed of polymeric sheets of SiO4 tetrahedra linked to sheets of (Al, Mg, Fe) (O,OH)6 octahedra ( Figure 2) [26][27][28]. The Bare Clay ( ) is mainly composed of smectite, which is a family of different clay minerals observed in Figure 1 (all the following phases are observed: augite, cristobalite, montmorillonite, illite, kaolinite, feldspar, and talc). Smectite forms an important group of the phyllosilicate family of minerals, which are distinguished by layered structures composed of polymeric sheets of SiO 4 tetrahedra linked to sheets of (Al, Mg, Fe) (O,OH) 6 octahedra ( Figure 2) [26][27][28]. When Cu 2+ ions are introduced to the network (Clay/Cu 2+ sample, in orange (• Figure 1), a similar XRD pattern was recorded; however, the peak around 5-6° was spr due to the Cu 2+ insertion.
The pure TiO2 sample ( Figure 1, pattern in gray (*)) is composed of anatase wi small amount of brookite (denoted A and B in Figure 1). These mixed phases were pr ously observed in aqueous sol-gel synthesis [29]. The pure ZnO sample ( Figure 1, pat in green (♠)) is made of wurtzite phase, as expected with this synthesis method [30].
The XRD results ( Figure 1) confirm the successful production of hybrid clay-ph catalytic materials. Indeed, when the clay is modified with TiO2, the corresponding X patterns (patterns in yellow (■) and mid blue (▲) in Figure 1) present the character TiO2 and clay peaks for both Clay/TiO2 and Clay/TiO2/Cu 2+ samples if the peak posit are compared to the bare samples. The XRD patterns of the ZnO-modified clays (patte in red (∞) and dark blue (♣) in Figure 1) likely present characteristic peaks of both wurt and clays. Table 2 presents the specific surface areas (SBET) of the different samples, ranging f 30 to 325 m 2 /g. The Bare Clay sample has a relatively low specific surface area (45 m which increases slightly when Cu 2+ ions are intercalated (55 m 2 /g). This increase co from the insertion of the cations in the smectite network [28]; indeed, this insertion is served in the XRD patterns ( Figure 1) with the spread of the peak around 5°. The p TiO2 sample presents an SBET value equal to 180 m 2 /g, in agreement with literature d [29]. When the clay is modified with TiO2, SBET increases to 325 and 240 m 2 /g for Clay/T and Clay/TiO2/Cu 2+ samples, respectively. This is logical, as these composite materials produced with nanospheres of TiO2, which have high specific surface area. They can enter the clay network to expand the material and thus increase its specific surface a The pure ZnO sample presents a low SBET value (30 m 2 /g). When clay is modified w ZnO, the specific surface area increases for Clay/ZnO sample (125 m 2 /g), but it stays r tively low for Clay/ZnO/Cu 2+ (50 m 2 /g). The increased surface area of the Clay/ZnO s When Cu 2+ ions are introduced to the network (Clay/Cu 2+ sample, in orange (•) in Figure 1), a similar XRD pattern was recorded; however, the peak around 5-6 • was spread due to the Cu 2+ insertion.

Texture and Morphology
The pure TiO 2 sample ( Figure 1, pattern in gray (*)) is composed of anatase with a small amount of brookite (denoted A and B in Figure 1). These mixed phases were previously observed in aqueous sol-gel synthesis [29]. The pure ZnO sample ( Figure 1, pattern in green (♠)) is made of wurtzite phase, as expected with this synthesis method [30].
The XRD results ( Figure 1) confirm the successful production of hybrid clay-photocatalytic materials. Indeed, when the clay is modified with TiO 2 , the corresponding XRD patterns (patterns in yellow ( ) and mid blue ( ) in Figure 1) present the characteristic TiO 2 and clay peaks for both Clay/TiO 2 and Clay/TiO 2 /Cu 2+ samples if the peak positions are compared to the bare samples. The XRD patterns of the ZnO-modified clays (patterns in red (∞) and dark blue (♣) in Figure 1) likely present characteristic peaks of both wurtzite and clays. Table 2 presents the specific surface areas (S BET ) of the different samples, ranging from 30 to 325 m 2 /g. The Bare Clay sample has a relatively low specific surface area (45 m 2 /g), which increases slightly when Cu 2+ ions are intercalated (55 m 2 /g). This increase comes from the insertion of the cations in the smectite network [28]; indeed, this insertion is observed in the XRD patterns ( Figure 1) with the spread of the peak around 5 • . The pure TiO 2 sample presents an S BET value equal to 180 m 2 /g, in agreement with literature data [29]. When the clay is modified with TiO 2 , S BET increases to 325 and 240 m 2 /g for Clay/TiO 2 and Clay/TiO 2 /Cu 2+ samples, respectively. This is logical, as these composite materials are produced with nanospheres of TiO 2 , which have high specific surface area. They can also enter the clay network to expand the material and thus increase its specific surface area. The pure ZnO sample presents a low S BET value (30 m 2 /g). When clay is modified with ZnO, the specific surface area increases for Clay/ZnO sample (125 m 2 /g), but it stays relatively low for Clay/ZnO/Cu 2+ (50 m 2 /g). The increased surface area of the Clay/ZnO sample could come from an insertion of some ZnO particles inside the clay network. Concerning the nitrogen adsorption-desorption isotherms, two different types are observed between all samples: (i) type I isotherm, with a sharp increase at low pressure followed by a plateau corresponding to microporous solid; and (ii) type IV isotherm, characterized by a broad hysteresis at high pressure (mesoporous solid). Samples containing TiO 2 have type I isotherms, and the other samples have type IV isotherms. As an example, the isotherms of Bare Clay and Clay/TiO 2 samples are plotted in Figure 3. The other isotherms are represented in Figures S1 and S2 in the Supplementary Materials. Concerning the nitrogen adsorption-desorption isotherms, two different types are observed between all samples: (i) type I isotherm, with a sharp increase at low pressure followed by a plateau corresponding to microporous solid; and (ii) type IV isotherm, characterized by a broad hysteresis at high pressure (mesoporous solid). Samples containing TiO2 have type I isotherms, and the other samples have type IV isotherms. As an example, the isotherms of Bare Clay and Clay/TiO2 samples are plotted in Figure 3. The other isotherms are represented in Figures S1 and S2 in the Supplementary Materials.  Figure 4 for Bare Clay, Clay/TiO2, and Clay/ZnO at two different magnifications. Bare Clay and Clay/ZnO samples have a similar aspect (Figure 4a,c), with large, granular powder, while the Clay/TiO2 powder is finely dispersed (Figure 4b). These observations are in agreement with the higher specific surface area of Clay/TiO2 and Clay/TiO2/Cu 2+ samples, characteristic of smaller hybrid particles and resulting in smaller voids between particles. This finely dispersed aspect comes from the TiO2 nanoparticles, which are very small (5-10 nm), as observed in the TEM pictures of pure TiO2 samples (Figure 5a). Contrarily, the pure ZnO sample has larger particles (Figure 5b), indicating that the composite material with clay is more similar to the Bare Clay.  Figure 4 for Bare Clay, Clay/TiO 2 , and Clay/ZnO at two different magnifications. Bare Clay and Clay/ZnO samples have a similar aspect (Figure 4a,c), with large, granular powder, while the Clay/TiO 2 powder is finely dispersed (Figure 4b). These observations are in agreement with the higher specific surface area of Clay/TiO 2 and Clay/TiO 2 /Cu 2+ samples, characteristic of smaller hybrid particles and resulting in smaller voids between particles. This finely dispersed aspect comes from the TiO 2 nanoparticles, which are very small (5-10 nm), as observed in the TEM pictures of pure TiO 2 samples (Figure 5a). Contrarily, the pure ZnO sample has larger  ICP results (Table 1) confirmed the presence of the semiconductor materials in the composite materials.

Adsorption Study
The experimental results of fluorescein adsorption are transformed with the following equation to determine the amount of FL adsorbed per g of clay (qe):   ICP results (Table 1) confirmed the presence of the semiconductor materials in the composite materials.   (Table 1) confirmed the presence of the semiconductor materials in the composite materials.

Adsorption Study
The experimental results of fluorescein adsorption are transformed with the following equation to determine the amount of FL adsorbed per g of clay (q e ): where C 0 and C e are the initial and equilibrium liquid-phase concentrations of FL (mg FL L −1 ), respectively; V is the volume of the FL solution (L); and W is the mass of clay used (g C ). q e in function of C e is represented in Figure 6a after 6 h of adsorption for Bare Clay and Clay/Cu 2+ samples. Similar curves are obtained for both samples; indeed, they have similar specific surface areas ( Table 2).
where C0 and Ce are the initial and equilibrium liquid-phase concentrations of FL (mgFL L −1 ), respectively; V is the volume of the FL solution (L); and W is the mass of clay used (gC). qe in function of Ce is represented in Figure 6a after 6 h of adsorption for Bare Clay and Clay/Cu 2+ samples. Similar curves are obtained for both samples; indeed, they have similar specific surface areas ( Table 2). In Figure 6b, the evolution of the FL concentration (C/C 0 ) with time is represented for Bare Clay and Clay/Cu 2+ with 30 mg concentrations of powder samples. After 0.5 h of the experiment, more than 75% of FL were adsorbed for both samples (Figure 6b) and the concentration did not decrease much after 6 h; thus, the equilibrium was reached.
An example of the FL UV-visible spectrum is given in Figure S4 in the Supplementary Materials.
The PNP adsorption study shows that PNP is not adsorbed on the clay. Indeed, the concentration in solution remains constant with time. The removal of this kind of pollutant requires photocatalytic properties.

Photocatalytic Activity
As observed in the previous section, the clay can adsorb some pollutants such as dye, but it is not efficient to adsorb, for instance, PNP. Therefore, the clay was modified with photocatalysts to degrade molecules that cannot be efficiently adsorbed. Adsorption experiments in the dark (to avoid an interaction with room light which can activate the photocatalytic materials) were performed on all eight samples in contact with PNP for 8 h. No change in the PNP concentration was observed, showing that none of the samples adsorb the PNP molecule.
The photocatalytic property was evaluated on the PNP degradation under UVA illumination after 8 h of exposure (Figure 7a). An example of the PNP UV-visible spectrum is given in Figure S5 in the Supplementary Materials.
The Bare Clay and Clay/Cu 2+ samples originally had no photocatalytic properties. However, such properties were attained after treatment with either TiO 2 or ZnO. PNP was degraded from 45% to 92% according to the samples. Clay/ZnO/Cu 2+ is the most efficient material, with PNP degradation of 92%. The pure TiO 2 and ZnO materials reached 100% PNP degradation, as observed in previous studies [18,19].
For the same amount of semiconductor material (TiO 2 or ZnO), the ZnO-modified clay is more efficient than the TiO 2 -modified one. As previously observed [18,31], ZnO materials have better activity than TiO 2 , due to fewer recombinations of photogenerated species. The addition of Cu 2+ ions increases the photoactivity due to an additional photo-Fenton effect that improves the PNP degradation [32]. Indeed, Cu 2+ ions can react with water when exposed to UV radiation to produce additional • OH radicals [32]. These radicals can degrade the organic molecules and thus enhance the photoactivity [32]. The equation of Cu 2+ photo-Fenton effects is the following [32]: where h is the Planck constant (6.63 × 10 −34 J.s) and ν is the light frequency (Hz).
For the two best composite materials (Clay/TiO 2 /Cu 2+ and (•) Clay/ZnO/Cu 2+ ), the evolution of the PNP degradation over time is presented in Figure 7b. The evolution of the degradation is linear; then, the degradation is first order. tocatalytic materials) were performed on all eight samples in contact with PNP for 8 h. No change in the PNP concentration was observed, showing that none of the samples adsorb the PNP molecule.
The photocatalytic property was evaluated on the PNP degradation under UVA illumination after 8 h of exposure (Figure 7a). An example of the PNP UV-visible spectrum is given in Figure S5 in the Supplementary Materials.

Presentation of the Clay
The clay material was whitish in color, sampled at 20 cm depth. The UTM coordinates of the sampling were north 5 • 06 08.3" and east 10 • 17 28.0", at an altitude of 1423 m. These coordinates corresponded to Mont Batcha, commonly called Bakotcha, in the district of Bana (West Cameroon). This region has an equatorial climate, characterized by average annual rainfall of 1300-2500 mm and a mean annual temperature of 21.23 • C [33]. The vegetation is highly anthropogenized post-forestry savannah with remains of a persisting semi-deciduous forest in areas of difficult accessibility [34]. The dominant soil types are red ferralitic soils, associated with brunified and hydromorphic soils [35]. The sampled clay was air-dried in the laboratory to a constant weight before grinding and sieving in a 160 µm diameter sieve.

Modification of Clay with Ions Cu 2+ or Interfoliar Cation Exchange
This treatment does not destroy the structure of the clay material and it allows the insertion of ions (as shown in Figure 2). We used the following reagents: copper (II) sulfate pentahydrate (>98.0% from LabChem, Gauteng, South Africa), barium sulfate (99%, pure, from Laboratoriumdiscounter), clay powder (>160 µm), and distilled water.
In order to produce a homogeneous cation exchange, 50 g of clay was mixed under stirring in 0.1 M of CuSO 4 solution for 4 h. After 2 h rest, the supernatant was poured, and the agitation was repeated with a new solution of 0.1 M of CuSO 4 . This operation was repeated twice, and excess Cu 2+ and SO 4 2− ions were washed with distilled water until the Baryum test (precipitation test) became negative. The homoionic Cu 2+ clay material was then oven-dried at 110 • C overnight.
Zinc acetate dihydrate (10.98 g) was treated with ethanol (300 mL) at 60 • C. The salt was completely dissolved in about 30 min. Oxalic acid dihydrate (12.6 g) was dissolved in ethanol (200 mL) at 60 • C for 30 min. The oxalic acid solution was added slowly, with stirring, to the hot ethanolic zinc solution, and the mixture was stirred for 90 min at 50 • C. The resulting gel was placed in an oven at 80 • C for 24 h. The product was calcined at 400 • C for 4 h. The color of the pure ZnO powder was white.
Nitric acid HNO 3 (65%, Merck) was used to acidify 250 mL of distilled water to pH 1. Then, 15 mL of TTIP was added to 15 mL of isopropanol (IsoP), and the mixture was stirred for 30 min at room temperature. The resulting solution of TTIP + IsoP mixture was added to acidified water under controlled stirring. The liquid was left under stirring for 4 h at 80 • C. The obtained sol had a clear blue color. Then, the sol was dried for 10 h under an ambient air flow to obtain a xerogel. The powders were dried at 100 • C for 1 h and a pure TiO 2 powder of yellowish-white color was obtained [32].

Clay/ZnO Materials
For the preparation of modified clays with ZnO, the procedure was similar as for pure ZnO material. However, when the oxalic acid solution was added slowly with stirring to the hot ethanolic zinc solution, 10 g of clay materials was added, and the mixture was left under stirring for 90 min at 50 • C. The resulting gel was placed in an oven at 80 • C for 24 h. The product was calcined at 400 • C for 4 h. The ZnO-modified clay powders were light gray in color.

Clay/TiO 2 Materials
For the preparation of hybrid clay/TiO 2 powders, the same protocol of preparation of pure TiO 2 powder was used with the addition of 10 g of clay material. When the mixture TTIP + IsoP was obtained, it was added to acidified water under controlled stirring and the liquid was left under stirring for 4 h at 80 • C. To the obtained sol, clear blue in color, 10 g of clayey material was added and left under stirring for 2 h. The soil was dried for 24 h under an ambient air flow. The powders were dried at 100 • C for 1 h and hybrid clay/TiO 2 powders were obtained.

Characterization of Samples
The actual composition of the bare and modified clays was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), equipped with an ICAP 6500 THERMO Scientific device (Waltham, MA, USA). The mineralization is fully described in [32]; however, we used HF instead of HNO 3 .
The crystallographic properties were observed through the X-ray diffraction (XRD) patterns recorded with a Bruker D8 Twin-Twin powder diffractometer (Bruker, Billerica, MA, USA) using Cu-Kα radiation.
The specific surface area of samples was determined by nitrogen adsorption-desorption isotherms in an ASAP 2420 multi-sampler volumetric device from Micromeritics (Norcross, GA, USA) at 77 • K.
SEM micrographs were obtained using a Jeol-JSM-6360LV microscope (Tokyo, Japan) under high vacuum at an acceleration voltage of 20 kV.
Transmission electron microscopy was performed on the LEO 922 OMEGA Energy Filter Transmission Electron Microscope (Zeiss, Oberkochen, Germany) operating at 120 kV. Sample preparation consisted of dispersing a few milligrams of each sample in water, using sonication. Then, a few drops of the supernatant were placed on a holed carbon film deposited on a copper grid (CF-1.2/1.3-2 Cu-50, C-flat™, Protochips, Morrisville, NC, USA).

Adsorption Experiments
Concerning the adsorption experiments, only the Bare Clay and Clay/Cu 2+ samples were assessed. The adsorption of two types of model pollutants, fluorescein (FL) and p-nitrophenol (PNP), was tested. For an adsorption experiment, 6 vials were prepared containing 5, 10, 15, 20, 25, and 30 mg of powder clay and 20 mL of pollutant solution in water. The samples were under continuous stirring. The remaining concentration in solution was evaluated every hour for 6 h with a Genesys 10S UV-Vis spectrophotometer (Thermo Scientific) after filtration with a syringe filter. The main absorption peaks were located at 317 and 485 nm for PNP and FL, respectively, as shown in Figures S4 and S5 in the Supplementary Materials. The initial concentration of FL was 6 × 10 −5 M and 10 −4 M for PNP.

Photocatalytic Experiments
The degradation of p-nitrophenol (PNP) was studied under UVA light (λ = 365 nm) to determine the photocatalytic activity of the synthesized material. The lamp was an Osram Sylvania, Blacklight-Bleu Lamp, F 18W/BLB-T8, considered as monochromatic at 365 nm.
Each sample was placed in a Petri dish with 20 mL of 10 −4 M of PNP solution in water. The degradation of PNP was evaluated from absorbance measurements with a Genesys 10S UV-Vis spectrophotometer (Thermo Scientific) at λ = 317 nm. Previously, adsorption tests were performed in the dark (dark tests) to show whether PNP was adsorbed on the surface of samples. A blank test, consisting of irradiating the pollutant solution for 24 h in a Petri dish without any catalyst, showed that PNP concentration under UVA illumination remained constant. The Petri dishes with catalysts and pollutants were stirred on orbital shakers and illuminated for 8 h. Aliquots of PNP were sampled at 0, 4, and 8 h. The photocatalytic degradation was equal to the total degradation of PNP, taking the catalyst adsorption (dark test) into account. Each photocatalytic measurement was triplicated to assess the reproducibility of the data. In each box, the catalyst concentration was 1 g/L.

Conclusions
In this work, natural clays were used to remove pollutants from water by adsorption and photocatalysis processes. The approach was applied on smectite-rich Cameroon clays.
The clays were preliminarily treated with Cu 2+ and then with semiconductors TiO 2 and ZnO to produce hybrid clays. The aim was to increase the depollution efficiency of these modified clayey materials by their photocatalytic properties. The protocol was controlled by XRD and ICP-AES measurements. The modified clays displayed an increase in their specific surface areas in comparison with natural clay properties. XRD confirmed the presence of crystalline TiO 2 and ZnO.
The adsorption experiments confirmed the bare clays can adsorb fluorescein, but they were not efficient on other pollutants, namely p-nitrophenol. The addition of semiconductor materials improved the degradation of the pollutants when exposed to UVA light. Photocatalytic experiments on PNP gave degradation levels of 70% to 90% after 8 h of exposition with the TiO 2 -and ZnO-modified clays, respectively.
This study emphasizes the importance of composite clays to remove pollutants via adsorption and photocatalysis processes. Such approaches offer an opportunity, especially in developing countries, to use natural clay materials with slight modifications for water purification.  Data Availability Statement: The raw/processed data required to reproduce these findings cannot be shared at this time as these data are part of an ongoing study.