Results in Surfaces and Interfaces 16 (2024) 100255Contents lists available at ScienceDirect Results in Surfaces and Interfaces journal homepage: www.elsevier.com/locate/rsurfi Development of novel composite materials based on kaolinitic clay modified with ZnO for the elimination of azo dyes by adsorption in water Pierre Ngue Song a, Julien G. Mahy a,*, Antoine Farcy a, Cédric Calberg a, Nathalie Fagel b, Stéphanie D. Lambert a a Department of Chemical Engineering – Nanomaterials, Catalysis & Electrochemistry, University of Liège, B6a, Quartier Agora, Allée du Six Août 11, 4000, Liège, Belgium b Laboratoire Argiles, Géochimie et Environnements Sédimentaires (AGEs), Department of Geology, Faculty of Sciences, University of Liège, Liège, B-4000, Belgium   A R T I C L E  I N F O   A B S T R A C T   Keywords: In this work, ZnO nanoparticles, synthesized by the sol-gel process, are immobilized on the external surface of Water treatment raw kaolinite particles and kaolinite activated by different treatments: heat treatment at 600, 700 and 800 ◦C; Surface modification treatment in a dimethyl sulfoxide (DMSO) medium; hot acid treatment (HCl, 6M) under reflux conditions or heat Adsorbent treatment at 800 ◦C followed by acid treatment. Characterization confirmed the successful immobilization of the Natural material valorization Porous materials nanocrystalline ZnO particles in the hexagonal structure of the different clay matrices. Measurement of the zeta potential showed a sudden inversion of the nature of the surface charge of certain composite materials obtained, through zeta potential values ranging from − 31 mV before doping with ZnO to +36 mV after doping. The raw kaolinite and certain composites obtained were tested in batch mode for the adsorption in aqueous solution of three anionic azo textile dyes: a monozoic (Mordant Red 19, MR19), diazoic (Direct Blue 53, DB53) and a triazoic (Direct Green 1, DG1) dye. Compared to raw kaolinite, a linear and rapid increase in the quantity of dye adsorbed is observed during the first 5 min with retention rates around 95% for the best composite materials. The adsorption efficiency strongly depends on the zeta potential of the material: the higher the latter is towards positive values, the better the adsorption capacities of the samples towards these anionic textile dyes.   1. Introduction such as light, temperature, microbial attack and oxidizing agents (Chung, 2016). Furthermore, their presence in aquatic systems, even at The different techniques and machines available make it possible to low concentrations, is very visible; they reduce light penetration and dye virtually all types of textile materials at any stage of their have a detrimental effect on photosynthesis (Selvaraj et al., 2021; manufacturing (fiber, thread, fabric or clothing). The explosion of de- Hernández-Zamora and Martínez-Jerónimo, 2019). mographics with an increasing demand for high-end products has forced This discharge also represents a potential danger of bioaccumulation the textile industry to increase production; this has also boosted the that could affect humans through transfer through the food chain economic sector in the production of cheap synthetic dyes. Thus, (Ahmad et al., 2023; Goud et al., 2020). Numerous studies carried out on annually, more than 700,000 tons of synthetic dyes are produced azo dyes have demonstrated that these chemical compounds present globally (Artifon et al., 2021; Gou et al., 2022). The category of azo dyes carcinogenic, mutagenic and teratogenic effects for humans and animals is among the most toxic (Florêncio et al., 2021; Ikram et al., 2022), and directly or indirectly through their metabolites which are carcinogenic more widespread in terms of application in the textile industries. It alone amines (Dihom et al., 2022; Ngo and Tischler, 2022). For this reason, in represents more than 50% of global production (Bera and Tank, 2021; 2000, the Danish Environmental Protection Agency (DEPA) imposed a Ravadelli et al., 2021). concentration limit of 3.1 μg/L of azo dye in drinking water to limit It is estimated that more than 10–15% of these chemical compounds cancer risks (Berkani et al., 2020). The European Commission (accord-are discharged into the effluents of textile industries without prior ing to standard IP/03/11 of January 7, 2003, having taken effect from treatment (Dihom et al., 2022; Vaiano et al., 2020). In their structure, June 30, 2004) adopted a new directive prohibiting the marketing and azo dyes are designed to be recalcitrant to environmental conditions use in the European Union of a dangerous azo chemical dye obtained by * Corresponding author. Allée du Six Août 11, 4000, Liège, Belgium. E-mail address: julien.mahy@uliege.be (J.G. Mahy).  https://doi.org/10.1016/j.rsurfi.2024.100255 Received 29 April 2024; Received in revised form 5 July 2024; Accepted 8 July 2024   Available online 11 July 20242666-8459/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255chromating and used for textiles. According to Annex XVII of the REACH specific surface area, rapid agglomeration and small particle size, which regulation, No. 1907/2006, this prohibited the use of azo dyes deemed makes recovery from the aqueous phase very difficult. Similarly, the dangerous (Brüschweiler and Merlot, 2017). release of ZnO nanoparticles into the environment could pose potential Water resources decrease as temperatures rise. In a context where health and environmental risks. Therefore, immobilization of nano-water scarcity and droughts are increasingly felt, one of the most sus- particles on an appropriate matrix would be a solution to these problems tainable solutions would be to close the water cycle within a factory (Akkari et al., 2018; Mustapha et al., 2020). (Paździor et al., 2019; Ribeiro et al., 2017). Consequently, the depol- It has been shown that supported nanoparticles generally have an lution of wastewater from textile industries contaminated by azo dyes is excellent adsorption capacity compared to unsupported metal oxide necessary for animal and human health and environmental protection, nanoparticles (Akkari et al., 2018; Mustapha et al., 2020). Clay is very but above all for the possible reuse of this unconventional water. This suitable for this purpose because of its different nanostructures, namely: depollution is a major challenge to be addressed by researchers. Several sheets as found in kaolinite and smectites, fibrous as in sepiolite and biological, physical and chemical methods have been implemented for palygorskites, or even tubular as in halloysite (Biddeci et al., 2023). It the elimination of azo dyes in aqueous solution with the aim of possible should be noted that the study of clay-semiconductor nanoarchitectures treatment of textile industrial effluents. We can cite, among others, prepared from clay materials is booming and has not yet revealed all its microbial biodegradation (Tizazu et al., 2023), advanced oxidation secrets. It has already been demonstrated that the assembly of TiO2 and processes (AOP) (Paquini et al., 2023), coagulation flocculation (Han ZnO nanoparticles with clays of different characteristics takes place et al., 2022), membrane filtration (Benkhaya et al., 2021) and adsorp- mainly on the external surfaces (Ruiz-Hitzky et al., 2019). In addition, tion (Brinza et al., 2022; Khader et al., 2023). However, most of these Németh et al. (2004) successfully generated ZnO nanoparticles in the technologies are expensive from the point of view of their imple- interfoliar space of kaolinite following an alkaline hydrolysis of the clay mentation and especially when they are applied to high flow industrial treated with a solution of zinc-cyclohexane butyrate dihydrate in DMSO. effluents. Only the adsorption technique seems to be well suited to the Until now, a very limited effort has been made to immobilize ZnO textile industry (Carmen and Daniela, 2012), given its proven effec- nanoparticles on the surface of a natural kaolinite that has previously tiveness in the elimination of organic pollutants and also for economic undergone physical or chemical activation. The scientific interest of this considerations. work is therefore to immobilize the ZnO nanoparticles, which occupy Numerous works on the adsorption, or even comparative studies of the boundary between a covalent polar semiconductor and an ionic adsorption, of anionic azo dyes in aqueous solution using natural semiconductor, on the external surface of a natural and activated kaolinitic clay as an adsorbent matrix have been carried out over the last kaolinite originating from southern Cameroon. The aim is to reverse the decade (Aragaw and Alene, 2022; Fumba et al., 2014; Paredes-Quevedo nature of the surface charge of the composite materials obtained, and to et al., 2021). However, the results obtained are insufficient because the achieve an optimal interaction between the composite materials and the quantities adsorbed at equilibrium are low (<5 mg/g). These low values anionic molecules of textile azo dyes in favor of their elimination by of adsorption at equilibrium could be explained by the fact that kao- adsorption in aqueous solution. Direct Blue 53, Direct Green 1 and linites are non-swelling clays (He et al., 2013), having a low cation ex- Mordant Red 19 will be used as model pollutants. change capacity (CEC) (3–15 meq⁄(100 g)) (Vane and Zang, 1997), low In Cameroon, the demand for colored loincloth is high, because it is porosity (Turer, 2007) and finally a small specific surface area varying associated with any cultural event (traditional dress, dress designating from 10 to 30 m2/g (Kahr and Madsen, 1995; Song et al., 2023). On the membership in a clan or tribe, wedding dress, mourning dress and other other hand, the surface charge of kaolinite is an important factor that events, etc.). Water pollution with dye is then an important problem to can also explain the poor interaction between the clay and the anionic treat. The development of a water treatment based on a material easily molecules of azo dyes in aqueous solutions. For this purpose, it is well founded in the region is of great interest in such developing countries known that natural kaolinite has a small net negative charge, which is where the access to expensive technology is not always possible. So, this attributable to the partial substitution of Si4+ ions by Al3+ or Fe3+ ions clay material can be used for the treatment of wastewater mainly loaded (Hai et al., 2015). This permanent negative charge is very beneficial for with dye and resulting from industrial or artisanal dye baths. This makes the surface modification of kaolinite by cationic surfactants, allowing it possible to contribute to economic development, viable for the treat-the adsorption of cationic pollutants (e.g., heavy metal ions and cationic ment of textile industrial effluents, while promoting sustainable devel-dyes) from wastewater. On the other hand, it reduces the affinity of opment through the valorization of this local material. The development kaolinite towards anions and thus has a negative effect about the of this clay material is of interest in the economic and social develop-adsorption of anionic pollutants by kaolinite. ment of Cameroon, because the transformation of this local raw material In short, as it has previously been demonstrated, the capacity of will make it possible to achieve foreign exchange savings, create jobs natural kaolinitic clay to adsorb cationic dyes in aqueous solution is and, consequently, improve the health and living conditions of much greater than its anionic dye adsorption capacity (Bhattacharyya populations. et al., 2014; El Hassani et al., 2023). Therefore, improving kaolinite’s adsorption of anionic azo dyes in aqueous solution remains a major 2. Materials and methods challenge to overcome. One nanomaterial considered in this respect is zinc oxide (ZnO) nanoparticles, a semiconductor metal oxide that ex- 2.1. Clay material and chemicals hibits adsorbent and photocatalytic properties (Mahy et al., 2021). Zinc oxide (ZnO) nanoparticles are considered non-toxic, and they have high The kaolinitic clay (Al2Si2O5(OH)4) used in this work comes from the resistance to chemical and optical corrosion. They also have high cata- Kribi deposit in the southern Cameroon region. Deionized water was lytic activity and stable chemical properties (Akkari et al., 2018; Mus- used as a synthesis solvent, for washing, as well as for purification of clay tapha et al., 2020). ZnO is one of the most popular piezoelectric by extraction of the clay fraction ≤2 μm. Sodium hexametaphosphate materials (Zhang et al., 2023). Indeed, its elementary cell is made up of a (Na6(PO3)6) was used as a dispersing agent during the purification of stack of positive and negative charges. It can be seen as an elementary clay by extraction of the clay fraction ≤ 2 μm. Hydrochloric acid (HCl, electric dipole with spontaneous polarization. If an external constraint is 37%) was used for chemical activation, DMSO was used as a solvent and applied, the positive and negative charges will move, thus creating a for intercalation in the interfoliar space of the kaolinite. Absolute piezoelectric polarization (Deschanvres et al., 1992). The difference in ethanol was used to wash KAO-DMSO material, zinc acetate dihydrate electronegativity between the oxygen atom and the zinc atom places (Zn(CH3COO)2.2H2O) was used as a precursor, sodium hydroxide zinc oxide on the border between a covalent polar semiconductor and an (NaOH, 0.1 M solution in deionized water) used as a precipitating agent ionic semiconductor. The disadvantages of ZnO nanoparticles are a low and finally azo textile dyes (Direct Blue 53, Direct Green 1 and Mordant 2P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255Fig. 1. Different types of textile azo dyes used in adsorption studies.  Fig. 2. Schematic diagram of the modification with ZnO material.  Red 19), whose molecular structures are illustrated in Fig. 1, were used oven under air. as received from Sigma-Aldrich. 2.4. Preparation of ZnO-kaolinite composites 2.2. Activation of clay material by thermal and chemical treatment The impregnation ratios of clay materials/zinc acetate dihydrate The procedures for washing and purification by extraction of the clay used are as follows (1:1/2; 1:1). The combination of sol-gel and co- fraction ≤2 μm, the heat treatments (600, 700 and 800 ◦C) that make it precipitation methods was used for the immobilization of nanoscale possible to obtain metakaolinites (named MK-600, MK-700 and MK- ZnO on clay materials. For this purpose, either 2.5 g or 5 g of zinc acetate 800) and finally the chemical activation by acid treatment (HCl, 6 M, dihydrate was dissolved in 55 mL of deionized water except in the 110 ◦C, named HCl) of the samples are fully described in (Song et al., specific case of nanometric ZnO immobilization on KAO-DMSO where 2023) and in Supplementary Materials. The pure kaolinite sample is the dissolution of zinc acetate dihydrate took place in 55 mL of DMSO. named KAO. This is to prevent the DMSO intercalated in the interfoliar spaces of the kaolinite from solubilizing in water. The mixture was stirred vigorously 2.3. Intercalation treatment with DMSO for 10 min at room temperature until the salt dissolved completely. 5 g of clay materials were then added to the previous solution. The mixture The kaolinite intercalated with DMSO (named KAO-DMSO) was was stirred at 85 ◦C for 1 h while 200 mL of a 0.1 M NaOH solution was prepared according to the method described by (Gardolinski et al., added dropwise using a burette so that a milky white precipitate formed. 2000). Then, a 20 g of kaolinite (clay fraction ≤2 μm, KAO sample) was The contents of the reactor are kept stirring at 85 ◦C for a further 3 h. mixed with 180 mL of DMSO and 20 mL of deionized water. The mixture After cooling to room temperature, the contents were left undisturbed was subjected to vigorous stirring at 60 ◦C for 7 days. The sample was for 12 h. After centrifugation, the solid residue was washed several times then centrifuged (10 min at 14,000 rpm) and further washed with ab- with deionized water then dried in an oven at 105 ◦C for 24 h. The solute ethanol to remove excess and residual DMSO adsorbed on the samples modified with ZnO are named with “ZnO”. The process of surface. The solid residue was subsequently dried at 85 ◦C for 24 h in an modification with ZnO is represented on Fig. 2. 3P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255Fig. 3. X-Ray diffraction (XRD) patterns of (a): KAO, KAO-ZnO, KAO-ZnO-300 ◦C, (b): KAO, KAODMSO, KAODMSO-ZnO, KAODMSO-ZnO-300 ◦C, and (c): KAO, KAOHCl, KAOHCl-ZnO, KAOHCl-ZnO-300 ◦C samples. 4P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255Fig. 3. (continued). 2.5. Material characterizations After dispersing the whole using ultrasound, the suspensions are deposited in droplets on a copper grid (Formvar/Carbon 200 Mesh Cu The quantity of elements (Al, Si, Ti and Zn) in samples was deter- from Agar Scientific). mined by inductively coupled plasma optical emission spectroscopy Zeta potential measurements aimed at evaluating the properties of (ICP-OES), using a Agilent 5100 instrument. The mineralization is fully materials in terms of surface charge were obtained using a Beckman described in (Mahy et al., 2016) except that HF was used instead of Coulter Delsa Nano C type particle analyzer instrument. In a 20 mL HNO3. capacity bottle containing 10 mg of a powder sample whose particle size The crystallographic properties as well as the different crystalline is ≤ 2 μm, 10 mL of Milli-Q water was introduced to obtain a suspension. phases observed, through the X-ray diffraction (XRD) patterns, were After dispersing the assembly using ultrasound, the necessary quantity is recorded with a Bruker D8 Twin-Twin powder diffractometer using Cu- introduced into the measurement cell after the latter has been purged Kα radiation, with a step of 0.002◦ and a scanning speed of 2◦/min. first with Milli-Q water, then the suspension itself. The specific surface area of the materials was determined by nitrogen adsorption-desorption isotherms in a Micromeritics ASAP 2420 multi- 2.6. Adsorption experiments sampler volumetric apparatus at − 196 ◦C. Prior to analysis, the sam-ples were outgassed under high vacuum (<2 μm Hg) for 5 h at ambient The azo anionic textile dyes selected for the adsorption experiments temperature, followed by 5 h at 105 ◦C, still under high vacuum. in aqueous solution are Mordant Red 19 (MR19, monoazoic), Direct Blue Scanning electron microscope (SEM) micrographs were obtained 53 (DB53, diazoic) and Direct Green 1 (DG1, triazoic). Adsorption ex-using a TESCAN Clara microscope under high vacuum at an accelerating periments in batch mode were carried out by stirring 0.1 g of material voltage of 15 keV. For sample preparation, in a 10 mL capacity bottle with 20 mL of aqueous dye solution at constant pH of 7.4 at 25 ◦C in containing 1–2 mg of a powder sample whose particle size is ≤ 2 μm, a different 100 mL brown Duran laboratory flasks. Throughout the few mL of acetone were introduced to obtain a fairly dense suspension. adsorption tests, the stirring speed remained constant at 125 rpm. At the After dispersing everything using ultrasound, the suspensions are end of the predetermined time intervals, the adsorbent was separated deposited in droplets on the glass slide prepared for this purpose. The from the solution by centrifugation. The residual dye concentrations slide is then placed in an oven under air at 60 ◦C to evaporate the were obtained by spectrophotometry using the Genesis 10S UV–vis acetone. The samples are then made conductive by applying a thin layer spectrophotometer (Thermo Scientific) at 414 nm, 602 nm and 608 nm, of gold using a metallizer and finally deposited on a sample port with an the respective absorption maximums of MR19, DG1 and DB53. Cali-adhesive carbon sheet. bration curves were made to set the concentration range of the Transmission electron microscope (TEM) micrographs were obtained experiments. using an FEI TECNAI G2-20 TWIN microscope under an accelerating To further evaluate the adsorption kinetics of dyes MR19, DB53 and voltage of 160 kV. For sample preparation, in a 10 mL capacity bottle DG1, the experimental data were analyzed with the plot of the model’s containing 1–2 mg of a powder sample whose particle size is ≤ 2 μm, a pseudo-first order and pseudo-second order kinetics using the following few mL of absolute ethanol were introduced to obtain a suspension. equations (Ho and Mckay, 1999): 5P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255Table 1 Table 3 Variation in the size of the immobilized nanometric ZnO crystallites depending Variation of the zeta potential at pH 7.2–7.5 at 25 ◦C of the nanocomposites on the nature of the matrix.  depending on the nature of the matrix and the treatment.  Samples Calcination temperature (◦C) Crystallite size D (nm) Samples Impregnation Solvent and Calcination Zeta ratio (clay synthesis temperature potential KAO-ZnO RT 38.5 material/zinc temperature (◦C) (mV) 300 38.7 acetate KAODMSO-ZnO RT 32.3 dihydrate) 300 32.6 MK600-ZnO RT 26.7 Pure ZnO – – 300 20.13 ±300 27.5 1.1 MK700-ZnO RT 28.4 KAO – – RT − 37.58 300 29.1 ± 1.6 MK800-ZnO RT 15.5 KAO-ZnO 1:1/2 H2O; 85 ◦C RT − 11.15 300 19.4 ± 0.9 KAOHCl-ZnO RT 49.2 300 − 14.82 300 48.3 ± 1.1 MK800HCl–ZnO RT 30.9 KAO-ZnO 1:1 H2O; 85 ◦C RT +16.79 300 31.2 ± 1.3 KAODMSO  DMSO; H O; RT − 26.97 RT: room temperature (18 ◦C). 260 ◦C ± 1.8 KAODMSO-ZnO 1:1/2 DMSO; 85 ◦C RT − 5.54 ±(1) 1.0 Table 2 300 − 13.23 Sample chemical composition by inductively coupled plasma-optical emission ± 1.2 ◦spectroscopy (ICP-OES).  KAODMSO-ZnO 1:1 DMSO; 85 C RT +30.10 (1) ± 1.1 Samples Ratio clay/zinc acetate Amount (in wt%) KAODMSO-ZnO 1:1 H O; 85 ◦2 C RT +19.60 dihydrate Al Si Ti Zn (2) ± 2.0 KAOHCl  HCl, 6 M; RT − 25.83 KAO – 19.50 19.70 1.10 0.01 110 ◦C ± 1.7 KAO-ZnO (1:1) 15.80 15.90 0.93 12.90 KAOHCl-ZnO 1:1/2 H2O; 85 ◦C RT − 20.45 (1:1/2) 17.10 17.00 0.99 11.40 ± 2.1 KAODMSO- (1:1) 14.90 15.00 0.45 13.90 300 − 19.97 ZnO (1:1/2) 16.50 16.90 0.60 12.80 ± 1.5 MK600-ZnO (1:1) 18.80 18.50 1.08 11.20 MK600   RT − 31.24 (1:1/2) 18.30 18.60 1.10 11.00 ± 1.9 MK700-ZnO (1:1) 18.30 18.50 1.08 10.70 MK600-ZnO 1:1/2 H2O; 85 ◦C RT +30.97 (1:1/2) 18.70 18.80 1.11 10.90 ± 2.6 MK800 – 22.20 22.50 1.33 0.02 300 +27.94 MK800-ZnO (1:1) 18.50 18.60 1.10 11.50 ± 2.4 (1:1/2) 19.00 19.10 1.09 10.80  MK600-ZnO 1:1 H O; 85 ◦2 C RT +32.02 ± 1.8 MK700   RT − 29.74 ln(qe − qt)= ln qe − k1t (1)  ± 2.3 MK700-ZnO 1:1/2 H2O; 85 ◦C RT +29.30 t 1 t ± 1.4 = 2 + (2)  300 +25.27 qt k2qe qe ± 2.7 MK700-ZnO 1:1 H2O; 85 ◦C RT +36.04 where qt (mg/g) and qe (mg/g) are the quantities of azo dyes adsorbed ± 2.1 respectively at time t (min) and at equilibrium; k1 (min− 1) and k2 (g MK800   RT − 24.49 mg− 1 min− 1) are the rate constants of the pseudo-first and pseudo- ± 2.3 ◦second order kinetics, respectively. MK800-ZnO 1:1/2 H2O; 85 C RT +29.96 ± 1.3 300 +23.75 3. Results and discussion ± 1.8 MK800-ZnO 1:1 H2O; 85 ◦C RT +34.11 3.1. Crystalline composition ± 2.4 MK800HCl  HCl, 6 M; RT − 21.50 110 ◦C ± 1.0 The crystal structure, the pure kaolinite phase (KAO) and those MK800HCl–ZnO 1:1/2 H2O; 85 ◦C RT − 23.87 amorphized at 600, 700 and 800 ◦C (MK-600, MK-700 and MK-800) as ± 1.7 well as the acid-treated samples (KAO-HCl, and MK-800-HCl) under 300 − 26.57 reflux conditions were studied and the results are presented in (Song ± 1.9 MK800HCl–ZnO 1:1 H ◦2O; 85 C RT − 22.09 et al., 2023). ± 1.5  Fig. 3 and S1 to S4 present the X-ray diffractograms of kaolinite intercalated with DMSO and those of the KAO-ZnO, KAODMSO-ZnO, 31.82◦, 34.44◦, 36.33◦, 47.66◦, 56.72◦, 62.85◦, 66.55◦MK-ZnO, KAOHCl-ZnO and MK800HCl–ZnO nanocomposites. These and 69.11◦diffractograms show the presence of ZnO nanoparticles immobilized on correspond respectively to the crystallographic planes (100), (002), the matrix whatever the composite, through the three main reflection (101), (102), (110), (103), (112) and (201) of ZnO in this crystalline peaks of ZnO having values of 2θ between 30 and 40◦ (Mahy et al., 2021, phase (Mahy et al., 2021, 2022; Chérif et al., 2023; Misra et al., 2018; 2022; Chérif et al., 2023; Misra et al., 2018; Staroń, 2023). These dif- Staroń, 2023). It was noted that the calcination of the nanocomposites ◦fractograms highlight the presence of eight reflection peaks character- (Fig. 3 and S1 to S4) at 300 C for 1 h caused an increase in the intensity istic of ZnO in the wurtzite crystalline phase with the space symmetry of the peaks. This increase in peak intensity is stronger with the group P63mc (JCPDS 36-1451). These reflection peaks recorded at 2θ = increasing calcination temperature of the kaolinite to obtain meta-kaolinite (Figs. S1–S4). This increase in the intensity of the reflection 6P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255Fig. 4. Schematic representation of the evolution of the clay’s charge density after each treatment.  Fig. 5. TEM images of KAO-ZnO (a); KAODMSO-ZnO (b) and MK800-ZnO (c).  Fig. 6. SEM-EDX picture and mapping for KAO-ZnO(1:1/2) sample.  7P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255Fig. 7. Scanning electron microscope images of (a) KAODMSO, (b) KAO-ZnO, (c) KAODMSO-ZnO, (d) KAO-ZnO300 ◦C, (e) KAODMSO-ZnO300 ◦C and (f) MK800-ZnO. which is accompanied by reorganization of the structure. Only part of Table 4 the AlO6 octahedra is preserved; the majority being transformed into Textural properties of all samples.  much more reactive tetra- and penta-coordinated units (Belver et al., Sample SBET Smeso Smicro VT Vmeso Vmicro 2002; Lenarda et al., 2007). Furthermore, the breakage or weakening of (m2/ (m2/g) (m2/g) (cm3/ (cm3/ (cm3/ the strong hydrogen bonds between adjacent layers (interfoliar spaces) g) ±5 ±5 g) g) g) 5 0 01 0 01 0 01 of the kaolinite (Song et al., 2023; Tang et al., 2017) was obtained by ± ± . ± . ± .intercalation of DMSO (a very polar organic molecule) into these spaces. KAO 20 15 5 0.09 0.08 0.01 KAO-ZnO(1:1/2) 15 10 5 0.05 0.04 0.01 To this end, the X-ray diffraction diagram (Fig. 3b) shows a complete KAO-ZnO(1:1) 13 08 5 0.03 0.02 0.01 shift in the reflection peak characteristic of kaolinite from 2θ = 12.4◦ to KAODMSO 30 25 5 0.1 0.08 0.02 2θ = 7.9◦. This is a shift in the reflection peak (001) of kaolinite from KAODMSOZnO 20 15 5 0.09 0.08 0.01 0.71 nm to 1.13 nm, thus confirming the formation of the KAO-DMSO (1)(1:1/2) complex. The increase in basal spacing of 0.41 nm is consistent with KAODMSO-ZnO 17 12 5 0.05 0.04 0.01 (1)(1:1) ordered monolayer intercalation of DMSO in kaolinite layers (Leal et al., KAODMSO-ZnO 19 14 5 0.06 0.05 0.01 2021; Mbey et al., 2013). However, it should be noted that this inter-(2)(1:1) calation is not total because the kaolinite reflection peak remains at 2θ KAOHCl 95 80 15 0.27 0.22 0.05 = 12.4◦ (Fig. 3b, KAO-DMSO). The immobilization of nanometric ZnO in KAOHCl-ZnO 65 50 15 0.20 0.18 0.02 (1:1) the interlamellar space of the kaolinite sheets would be achieved by MK600 30 25 5 0.1 0.08 0.02 expulsion of the intercalated DMSO. This is due to the strong polarity MK600-ZnO(1:1/ 18 13 5 0.05 0.04 0.01 difference between DMSO and ZnO (Marin et al., 2009; V Panasiuk et al., 2) 2014). In addition, the difference in electronegativity between the ox-MK600-ZnO(1:1) 16 11 5 0.05 0.04 0.01 ygen atom and that of zinc places ZnO at the border between a covalent MK700 30 25 5 0.1 0.08 0.02 MK700-ZnO(1:1/ 18 13 5 0.05 0.04 0.01 polar semiconductor and an ionic semiconductor (Rasmidi et al., 2021). 2) However, in the case of this kaolinite from southern Cameroon, the MK700-ZnO(1:1) 15 10 5 0.05 0.04 0.01 immobilization of nanometric ZnO in the interfoliar space occurred to a MK800 25 20 5 0.1 0.08 0.02 small extent. To this end, we observe a partial return of the reflection MK800-ZnO(1:1/ 15 10 5 0.05 0.04 0.01 2) peak characteristic of low-intensity broadband kaolinite at 2θ = 12.4◦MK800-ZnO(1:1) 13 08 5 0.03 0.02 0.01 (Fig. 3b, KAODMSO-ZnO). (Németh et al., 2004) obtained the same MK800HCl 315 175 140 0.33 0.2 0.13 result with German kaolinite from the municipality of Zettlitz. The MK800HCl–ZnO 240 140 100 0.32 0.2 0.12  presence of the reflection peak of very low intensity which remains at 2θ (1:1) = 7.9◦ (Fig. 3b, KAODMSO) after immobilization of the ZnO in the interfoliar space of this kaolinite is proof that all the DMSO intercalated peaks with temperature would be linked either to an increase in the beforehand was not expelled. This is further confirmed with its almost crystallinity of the immobilized nanometric ZnO, or to the growth of total disappearance after the calcination of the composite (KAODM-ZnO crystallites. Other authors who carried out a study on the crystallite SO-ZnO, Fig. 3b) at 300 ◦C (temperature higher than the boiling point of size and crystallinity of ZnO with increasing calcination temperature DMSO) for 1 h. We also observe the complete return of the reflection came to the same conclusions (Ashraf et al., 2015; Golsheikh et al., peak characteristic of kaolinite at 2θ = 12.4◦, of low intensity and a 2018). slightly wider band compared to that of the original kaolinite (Fig. 3b). It is known that when kaolinite is calcined above 450–500 ◦C, The absence of the reflection peak characteristic of one of the com-dehydroxylation occurs to give metakaolinite. This transformation in- pounds Al2O3, SiO2, or (Zn–Al–Si–O; Zn–Ti–Al–O; Zn–Ti–Si–O) in all volves the loss of structural water and the collapse of the crystal lattice, these diffractograms eliminate the possibility of alloy formation. Other 8P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255Table 5 Kinetic parameters for the adsorption of the 3 different dyes on kaolinitic clay and composite materials.  Adsorbent Zeta potential Dyes C0(mg/ RRmax(%) qe, exp (mg/ Pseudo-first order-model Pseudo-second order-model (mv) L) g) K1 q (mg/ R2 K (g mg− 1 2 e, cal 2 qe, cal (mg/ R(min− 1) g) min− 1) g) MK800-ZnO(1:1) þ34.11  100 99 19.85 0.0198 0.03 0.0357 2.5301 19.88 1 MK800-ZnO(1:1/2) þ29.96  100 98 19.72 0.7240 45.01 1 0.3188 19.80 1 KAODMSO-ZnO(1) þ30.10 MR19 100 99 19.81 1.2108 707.11 1 0.2400 19.92 0.9999 (1:1) KAODMSO-ZnO(2) þ19.60  100 88 17.66 0.0768 0.95 0.9956 0.2217 17.76 1 (1:1) KAO-ZnO(1:1) þ16.79  100 78 15.51 0.0741 0.72 0.9858 0.2882 15.58 1 KAODMSO-ZnO(1) ¡5.54  100 46 09.29 0.0630 0.30 0.9959 0.5457 09.32 1 (1:1/2) KAO-ZnO(1:1/2) ¡11.15  100 35 07.06 0.0850 0.32 0.8300 0.7336 07.09 1 KAO ¡37.58  100 17 3.44 0.0663 0.59 0.8782 0.2268 3.52 0.9997 Adsorbent MK800-ZnO(1:1) þ34.11  100 99 19.99 0.5678 4.79 1 1.3889 20.00 1 MK800-ZnO(1:1/2) þ29.96  100 99 19.98 0.0922 0.12 0.3348 0.4788 20.04 0.9999 KAODMSO-ZnO(1) þ30.10 DB53 100 99 19.98 0.0988 0.18 0.6838 0.3831 20.04 1 (1:1) KAODMSO-ZnO(2) þ19.60  100 92 19.96 0.0499 0.77 0.9737 0.1767 18.69 0.9999 (1:1) KAO-ZnO(1:1) þ16.79  100 83 19.91 0.0749 0.07 0.8178 0.2232 20.00 1 KAODMSO-ZnO(1) ¡5.54  100 66 13.32 0.0510 0.69 0.8781 0.1988 13.40 1 (1:1/2) KAO-ZnO(1:1/2) ¡11.15  100 43 09.45 0.1898 2.92 1 0.2037 09.56 0.9999 KAO ¡37.58  100 14 02.79 0.0494 0.82 0.9007 0.1316 02.88 0.9999 Adsorbent MK800-ZnO(1:1) þ34.11  100 97 19.35 0.8004 18.26 1 1.2679 19.38 1 MK800-ZnO(1:1/2) þ29.96  100 96 19.25 0.1000 0.47 0.7764 0.4810 19.27 1 KAODMSO-ZnO(1) þ30.10  100 95 18.99 0.0993 0.36 0.5630 0.3281 19.05 1 (1:1) KAODMSO-ZnO(2) þ19.60 DG1 100 84 16.70 0.1399 2.08 0.9579 0.1893 16.81 1 (1:1) KAO-ZnO(1:1) þ16.79  100 71 14.29 0.1540 0.68 1 0.6584 14.33 1 KAODMSO-ZnO(1) ¡5.54  100 60 11.99 0.2286 2.43 0.8682 0.2466 12.08 0.9999 (1:1/2) KAO-ZnO(1:1/2) ¡11.15  100 33 6.58 0.2163 1.44 0.9821 0.4698 6.63 1 KAO ¡37.58  100 11 2.26 0.0458 0.39 0.9328 0.3321 2.31 0.9994  authors (Misra et al., 2018) have reached the same conclusion. The XRD percentage equal to 1.10%, while zinc is present with a very low content data were used to evaluate the ZnO crystallite sizes of the nano- (0.01%). From these results we can therefore assume that quartz and composites. The Scherrer equation illustrated by relation (3) was used to aluminosilicates are predominant in this clay fraction. The Si/Al mass calculate the average diameter of these crystallites. ratio is equal to 1.01, which indicates a high kaolinite content (Khalifa k et al., 2019; Mkaouar et al., 2019). The heat treatment of this kaolinite at λD= (3)  800 ◦C caused an increase of 2.8% in the silicon and 2.7% in aluminum β Cos θ content, and only 0.23% of that of titanium. This increase in the silicon where D is the crystallite size in nanometers, k is a constant equal to 0.9, and aluminum content is essentially due to the collapse of the crystal θ is the angle at which the diffraction intensity is maximum, while β is lattice, by the transformation of part of the AlO6 octahedra into much the full width at half maximum. The average diameter of ZnO crystallites more reactive tetra and pentacoordinate units (Song et al., 2023; Belver in various nanocomposites, obtained from a clay material/zinc acetate et al., 2002). However, this increase did not affect the Si/Al mass ratio dihydrate impregnation ratio (1:1/2), is calculated using the Scherrer which remains constant due to the conservation of the material. Kenne formula and the results are presented in Table 1. Diffo et al. (Kenne Diffo et al., 2015) arrived at the same conclusion in The variation in the size of the crystallites and their growth with their study of the effect of the calcination rate of kaolin on the properties temperature is a function of the matrix on which the nanometric ZnO is of geopolymers based on metakaolin. However, whatever the nano-immobilized. Metakaolinites are ideal matrices for the immobilization of composite, we note for an impregnation ratio of (1:1/2), that the mass of ZnO whose crystallites are smaller. Zn in KAO-ZnO, KAODMSO-ZnO and MK800-ZnO is respectively 11.40%, 12.80% and 10.80%. The difference, not really being signifi-cant (<2%) from one matrix to another, suggests that whatever nano-3.2. Chemical composition composite and in the same ratio, the mass of immobilized nanometric ZnO is approximately the same. KAODMSO-ZnO had a slightly higher Zn Table 2 presents the results of the elemental analyses by ICP-OES. content (12.8%). This slightly high quantity of ZnO immobilized on These made it possible to determine the evolution and content in wt% KAODMSO is explained by the immobilization of nanometric ZnO in the of 4 essential elements (Al, Si, Ti and Zn) in the clay fraction (KAO), the interfoliar spaces of the kaolinite in addition to the surface. To this end, metakaolinite obtained at 800 ◦C and some nanocomposites (KAO-ZnO, the treatment of kaolinite with DMSO (very polar solvent) caused the KAODMSO-ZnO, MK600-ZnO, MK700-ZnO and MK800-ZnO) obtained breakage or weakening of the strong hydrogen bonds between the from the impregnation ratios of clay materials/zinc acetate dihydrate neighboring layers of the kaolinite (Song et al., 2023; Tang et al., 2017). (1:1/2) and (1:1). This was not the case with KAO and MK where the immobilization of From this analysis, the clay fraction (KAO) contains silicon and nanometric ZnO was found only on the surface. In the case of meta-aluminum at 19.70% and 19.50% respectively. Titanium has a mass kaolinites, this situation is explained by the fact that the calcination of 9P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255Fig. 8. Effect of contact time on the removal of dyes by kaolinitic clay and composite materials: with dyes (a) MR19, (b) DB53 and (c) DG1. Conditions: C0 = 100 mg/ L; V0 = 20 mL; m = 0.1 g; pH = 7.4; T = 298 K. kaolinite above 400 or even 500 ◦C, as in the case of treatment with the zeta potential of other kaolinites in the literature in the pH range DMSO, causes the breakdown of the hydrogen bonds between the 7.0–7.5 (Lu et al., 2021; Yukselen-Aksoy and Kaya, 2011). This negative adjacent kaolinite layers. This causes the collapse of the crystal lattice, charge is obtained through the contribution of two major processes, which is accompanied by a thinning of the layers due to the reduction of namely the imperfections of the crystal lattice of this kaolinite through the interfoliar space (Song et al., 2023; Ptáček et al., 2010). The increase its average degree of structural order reported by (Song et al., 2023). in the clay material/zinc acetate dihydrate impregnation ratio (1:1) These cause an isomorphic substitution inside the network. For this contributed to the increase in the Zn content to only 1.5% in KAO-ZnO, purpose, in the tetrahedral layer a Si4+ can be replaced by an Al3+ and in 1.1% in KAODMSO-ZnO and 0.7% in MK800-ZnO. This slight increase in the octahedral layer in the same way an Al3+ can be replaced by a Mg2+, the mass percentage of Zn with the impregnation ratio (1:1) simply re- Fe2+, … (Silva et al., 2017; Chen et al., 2022). In a given cell, the flects the surfaces of the different matrices having reached saturation by electropositive elements see the sum of their valences compensated by the immobilization of nanometric ZnO on them. This surface saturation the O2− ions, the replacement of the ions by ions of lower positive is quickly reached in the MK, followed by the KAO and finally by the valence results in a charge deficit which will appear in the form of a KAODMSO. negative charge carried by the network (Cuisset, 1980). However, it should be noted in this case that the sign and magnitude of the reticular 3.3. Zeta potential of samples in suspension charge are independent of the characteristics of the aqueous phase. Also, many natural colloids contain surface functional groups that can ionize. The zeta potential is established on the surface of a nanoparticle in In the case of kaolinites, these are the silanol (-Si-OH) and aluminol suspension or in solution. It is a very important interfacial property (-Al-OH) groups which can then ionize, producing negative primary making it possible to evaluate the intensity of the electrostatic repul- charges depending on the pH conditions of the medium (Ahmed, 1966). sion/attraction that exists between the different nanoparticles (Kamble On the other hand, the heat treatment at 600, 700 or even 800 ◦C of et al., 2022). It also indicates the affinity and, under certain conditions, kaolinite from southern Cameroon, having made it possible to obtain makes it possible to predict the capacity of nanoparticles to retain other MK600, MK700 and MK800, caused a significant increase in the zeta chemical species present in a solution. It therefore represents the potential despite the negative values obtained. The same is true for the effective net surface charge of nanoparticles and colloids under these treatment of kaolinite with DMSO. It was noted that this increase in zeta conditions (Henderson et al., 2008). potential becomes more and more significant as the calcination tem-Table 3 presents the variation in the zeta potential, obtained at pH perature of the kaolinite increases (from − 37.58 mV for KAO to − 24.49 7.2–7.5 at 25 ◦C, of the nanocomposites depending on the nature of the mV for MK800, Table 3), because, as mentioned above, the treatment matrix and the type of treatment undergone. It clearly appears that KAO (thermal or DMSO intercalation) causes the rupture or weakening of the (clay fraction 2 μm) has a zeta potential value equal to 37.58 mV. strong hydrogen bonds between the adjacent kaolinite layers. High ≤ −This value is in agreement with those obtained for the measurement of temperature heat treatment also contributes to this increase in this sense 10P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255Fig. 9. Electrostatic interaction between the anionic molecules of DB53 and the surface charges of MK800-ZnO. The dark dots represent ZnO nanoparticles.  by the collapse of the crystal lattice. This is accompanied by a reorga- − 20.45 mV and − 22.94 mV. ZnO is found on the border between a nization of its structure caused by the dehydroxylation of kaolinite covalent polar semiconductor and an ionic semiconductor (Rasmidi under these conditions. Furthermore, based on the authors’ previous et al., 2021); due to this atypical feature, immobilization on the surface work, hot acid treatment is necessary to accelerate the interface re- of the different matrices (KAO, KAODMSO, KAOHCl, MK600, MK700, actions of clay minerals (Jia et al., 2019). For KAO and MK800, hot acid MK800 and MK800HCl) causes the modification of the nature and of the did not greatly change the situation, despite a slight increase in the value density of the charges at the basal and side surfaces (Fig. 4). This of the zeta potential, which was more significant in KAOHCl than in modification is very intense in the case of metakaolinites and not suf-MK800HCl (from − 25.83 to − 21.50 mV, Table 3). We could have ex- ficient, at this impregnation ratio, to reverse the situation at the level of pected a different result since kaolinite’s greater resistance to acid the other matrices (KAO, KAODMSO, KAOHCl and MK800HCl), despite leaching compared to metakaolinites has been proven in other studies the fact that the quantity of immobilized nanometric ZnO, is substan-(Song et al., 2023; Belver et al., 2002). tially the same according to chemical analyses (Table 2). However, the The immobilization of the semiconductor, nanometric ZnO, on the calcination of the previous nanocomposites at 300 ◦C for 1 h resulted in surface of the kaolinite and whether the kaolinite had undergone the increased growth and crystallinity of the ZnO crystallites immobi-physical or chemical activation provided very interesting results for an lized in the metakaolinites. On the other hand, it has the opposite effect impregnation ratio of clay material/zinc acetate dihydrate (1:1/2). An of reducing the potential values of the nanocomposites by increasing the abrupt increase is observed across the positive and high values of the density of negative charges on their surfaces. recorded zeta potential, except in the case of KAO-ZnO, KAODMSO, However, for the clay material/zinc acetate dihydrate impregnation KAOHCl-ZnO and MK800HCl–ZnO where they remained negative. ratio (1:1), we note an increase in the zeta potential values compared to However, it must be noted that the increase and sudden variation in the those recorded for the clay material/zinc acetate dihydrate impregna-zeta potential after immobilization of the semiconductor depend on the tion ratio (1:1/2). As before, this variation differs from one matrix to nature of the matrix; in other words, the type of activation or treatment another. Thus, the zeta potential values of the MK600-ZnO, MK700-ZnO undergone by the kaolinite. For this purpose, the zeta potential is and MK800-ZnO nanocomposites are respectively +32.02 mV; +36.04 +30.97 mV; +29.30 mV and +29.96 mV; these values corresponding mV and +34.11 mV. Compared to the previous clay material/zinc ace-respectively to MK600-ZnO, MK700-ZnO and MK800-ZnO. The tate dihydrate impregnation ratio (1:1/2), they correspond respectively following zeta potential was found for KAODMSO-ZnO(1), whose value to an increase of: 3%, 23% and 13%. Similarly, the zeta potential values is − 5.54 mV. Finally comes those of the KAO-ZnO, KAOHCl-ZnO, of the KAO-ZnO, KAODMSO-ZnO(1) and KAODMSO-ZnO(2) nano-MK800HCl–ZnO nanocomposites with respective values of − 11.15 mV, composites are respectively +16.79 mV; +30.10 mV and +19.60 mV. 11P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255Fig. 10. Pseudo-second-order adsorption kinetics of (a) MR19, (b) DB53 and (c) DG1 on kaolinitic clay and composite materials.  Compared with the initial impregnation ratio of clay material/zinc ac- responsible for the rapid inversion of the surface charge, and by exten-etate dihydrate (1:1/2), these values are equivalent to an increase of sion the exponential increase in the zeta potential after the immobili-250%, 643% and 453% respectively. The 190% difference in the in- zation of the nanometric ZnO on these metakaolinites. This hypothesis is crease in the zeta potential value between KAODMSO-ZnO(1) and again confirmed, because the values of the zeta potentials of MK800HCl KAODMSO-ZnO(2) comes from the synthetic solvent. The KAODMSO and MK800HCl–ZnO remained in the same order of magnitude matrix was dispersed in an aqueous solution of zinc acetate dihydrate. (Table 3). This observation finds an explanation in the fact that these The water therefore caused the dissolution of part of the DMSO mole- tetra- and penta-coordinate units are much more reactive, that is to say cules intercalated in the interfoliar spaces of the kaolinite. This was not more sensitive to hot acid leaching, than the AlO6 octahedra. the case with KAODMSO-ZnO(1) or the KAODMSO matrix, it was instead dispersed in DMSO having previously dissolved the zinc acetate 3.4. Morphology of samples dihydrate. The small variation in the zeta potential, in the MK600-ZnO, MK700- 3.4.1. TEM micrographs of distributions of ZnO nanoparticles in composite ZnO and MK800-ZnO nanocomposites after the impregnation ratio materials increased to (1:1), can be explained by the saturation of the charge The TEM micrographs of certain nanocomposites at the same mag-density at level of the basal surfaces and the lateral surfaces. Chemical nifications are presented in Fig. 5. Each sample examined contains analyses also provide confirmation with the low variation in the Zn crystalline ZnO with size in the range of that determined by the Scherrer content in MK800-ZnO obtained either for an impregnation ratio (1:1/2) method from the XRD diffractograms (by measuring approximatively 20 or (1:1). All the same, it should be noted that metakaolinites (MK600, nanoparticles). Whatever the sample, ZnO aggregates are observed in an MK700 and MK800) remain ideal and appropriate matrices for immo- order of magnitude hundreds nanometers (Fig. 5a, b, c). On the other bilization of nanometric ZnO given the zeta potential values recorded at hand, we observe that these aggregates are composed of small uniform low impregnation ratios. This fundamental difference in the evolution of spheres. The two materials (clay and ZnO) can be distinguished: the the surface charge of the metakaolinites observed after the immobili- sheets in lighter material and the ZnO in the darker sphere. It is to zation of the nanometric ZnO would probably be linked to the heat ◦ remind that crystalline ZnO and clay were detected by XRD in the hybrid treatment of the kaolinite. When kaolinite is calcined above 450–500 C, materials (see section 3.1). These indicate successful coverage and dehydroxylation occurs to give metakaolinite. This transformation in- deposition of spherical ZnO nanoparticles, heterogeneously distributed volves the loss of structural water accompanied by a reorganization of its on the surfaces of the KAO, KAODMSO and MK800 (Fig. 5). These structure. Only a part of the AlO6 octahedra is preserved while the spherical ZnO nanoparticles are between 19 and 40 nm in size. As we majority is transformed into much more reactive tetra- and penta- reported for the X-ray diffractograms of the composites, we observe that coordinated units (Song et al., 2023; Belver et al., 2002). These tetra- the size of the ZnO crystallites vary from one matrix to another. and penta-coordinated units would be the determining factor, The presence of the homogeneous ZnO particles repartition on clay is 12P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255Fig. 11. Effect of initial dye concentration on the removal of (a) MR19, (b) DB53 and (c) DG1 by 5 composite materials. Conditions: V0 = 20 mL; m = 0.1 g; pH = 7.4; T = 298 K. confirmed by EDX analysis (Fig. 6, For KAO-ZnO(1:1/2) sample and under hot reflux conditions causes a significant modification of the Fig. S5 for KAODMSO-ZnO(1:1/2) and KAODMSO-ZnO(1:1)). The EDX texture as detailed in (Song et al., 2023). For this purpose, we note that analysis (Fig. 6) shows a successful coverage and deposition of ZnO the specific surface area of kaolinite increases from 20 to 95 m2/g and nanoparticles through a homogeneous distribution of Zn on the different that of MK800 from 25 to 315 m2/g. Furthermore, it should be surfaces of the KAO particles (Fig. 6). emphasized that this increase in specific surface area is accompanied by a significant development of microporosity and mesoporosity (Table 4). 3.4.2. SEM micrographs of composite materials However, the textural analysis of the composite materials provides Fig. 7a, b, c, d, e and f present the SEM micrographs of KAODMSO, confirmation of the immobilization of the ZnO nanoparticles on the KAO-ZnO, KAODMSO-ZnO, KAO-ZnO300 ◦C, KAODMSO-ZnO300 ◦C different surfaces (basal and lateral) of the kaolinite particles, as well as and MK800-ZnO samples respectively. These micrographs confirm the those having undergone either physical or chemical activation. This can heterogeneous texture of the different matrices on which the nanometric be explained with particular regard to the reduction in specific surface ZnO was immobilized. We can therefore observe the typically two- areas (Table 4). (Hai et al., 2015) reached the same conclusion with the dimensional morphology of kaolinite (Song et al., 2023; Tang et al., immobilization of TiO2 nanoparticles on a natural and activated Chinese 2017) (Fig. 7a). EDX analyzes also provide confirmation since they kaolinite. However, it was noted that this reduction in the specific sur-qualitatively highlight the presence of Zn on the surface of the kaolinite face area after immobilization of the ZnO nanoparticles is increasingly particles (Fig. 6). strong in composites made from kaolinite, having previously undergone physical or chemical activation. 3.5. Texture of the samples 3.6. Adsorption of dyes by composite materials The BET surface areas and pore volumes of all materials are listed in Table 4. Natural kaolinitic clay, the basis for the synthesis of all com- 3.6.1. Influence of contact time posites, has a specific surface area of 20 m2/g. After physical activation Table 5 gives the results of the study of the influence of the contact by heat treatment (600, 700 and 800 ◦C) or chemical activation by time for the adsorption of 20 mL of each of the dye solutions (MR19, treatment with DMSO, we see that the specific surface area increases DB53 and DG1) with a concentration 100 mg/L at pH 7.4 with 0.1 g of from 20 to 30 m2/g, despite the dehydroxylation obtained and the each of the adsorbent matrices (MK800-ZnO(1:1), MK800-ZnO(1:1/2), breakage of the strong hydrogen bonds present between adjacent layers. KAODMSO-ZnO(1)(1:1), KAODMSO-ZnO(2)(1:1), KAO-ZnO(1:1), This corresponds to a slight increase in the total porous volume from KAODMSO-ZnO(1)(1:1/2), KAO-ZnO(1:1/2) and KAO). The quantity of 0.09 to 0.1 cm3/g. However, the acid treatment of KAO and MK800 dye adsorbed as a function of time is represented by Fig. 8a, b and c. 13P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255Fig. 12. Langmuir model isotherm with (a) MR19, (b) DB53 and (c) DG1 for MK800-ZnO(1:1), MK800-ZnO(1:1/2), KAODMSO-ZnO(1)(1:1), KAODMSO-ZnO(2)(1:1) and KAO-ZnO(1:1). Table 6 Langmuir and Freundlich isotherm parameters of MK800-ZnO(1:1), MK800-ZnO(1:1/2), KAODMSO-ZnO(1)(1:1), KAODMSO-ZnO(2)(1:1) and KAO-ZnO(1:1).  Adsorbent Dyes Langmuir model Freundlich model qm (mg/g) KL (L/mg) R R2 K n R2 L F MK800-ZnO(1:1)  30.2 1.1696 0.0168 0.9998 12.56 4.71 0.5748 MK800-ZnO(1:1/2)  28.2 1.3460 0.0146 0.9998 11.66 4.64 0.5977 KAODMSO-ZnO(1)(1:1)  28.9 1.6321 0.0121 0.9996 12.21 4.76 0.5785 KAODMSO-ZnO(2)(1:1) MR19 24.0 1.5639 0.0126 0.9993 8.77 4.22 0.7298 KAO-ZnO(1:1)  23.0 0.9156 0.0214 0.9995 8.07 4.12 0.7706 Adsorbent MK800-ZnO(1:1)  37.7 1.4480 0.0136 0.9993 18.02 5.26 0.7214 MK800-ZnO(1:1/2)  35.6 1.9514 0.0101 0.9999 16.74 5.18 0.7138 KAODMSO-ZnO(1)(1:1) DB53 36.4 1.7188 0.0115 0.9999 16.64 4.94 0.7414 KAODMSO-ZnO(2)(1:1)  30.5 0.4391 0.0436 0.9989 20.19 4.46 0.9094 KAO-ZnO(1:1)  20.4 0.6945 0.0280 0.9996 9.53 6.21 0.8561 Adsorbent MK800-ZnO(1:1)  33.0 0.3890 0.0489 0.9997 8.81 3.12 0.8570 MK800-ZnO(1:1/2)  30.9 0.2351 0.0784 0.9995 7.09 2.96 0.9062 KAODMSO-ZnO(1)(1:1) DG1 31.6 0.3238 0.0582 0.9996 8.01 3.08 0.8778 KAODMSO-ZnO(2)(1:1)  21.7 0.1990 0.0913 0.9989 5.24 3.30 0.8284 KAO-ZnO(1:1)  19.5 0.2092 0.0873 0.9993 5.19 3.62 0.8322  Whatever the dye and the adsorbent matrix considered, the adsorp- 40%, 10%); (DG1: 95%, 94%, 89%, 80%, 70%, 53%, 30%, 10%) cor-tion dynamics between 0 and 60 min clearly show that the kinetics are responding respectively to MK800-ZnO(1:1), MK800-ZnO(1:1/2), divided into three phases: first, a linear and rapid increase in the KAODMSO-ZnO(1)(1:1), KAODMSO-ZnO(2)(1:1), KAO-ZnO(1:1), quantity of adsorbed dye is observed during the first 5 min. To this end, KAODMSO-ZnO(1)(1:1/2), KAO-ZnO (1:1/2) and KAO samples. This is we therefore record retention rates (RR (%)) of (MR19: 97%, 93%, 90%, followed by a transition regime which also extends over 5 min, and 85%, 75%, 45%, 34%, 14%); (DB53: 98%, 96%, 95%, 90%, 80%, 64%, finally a plateau reflecting an equilibrium reached after 10 min. 14P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255From the above and with regard to the RRmax (%) recorded where Ce (mg/L) is the equilibrium dye concentration, qe (mg/g) is the (Table 5), it is clear that the efficiency of adsorption strongly depends on equilibrium adsorption capacity, KL (L/mg) is the Langmuir equilibrium the zeta potential of the material. For this purpose, the higher the zeta constant and qm (mg/g) is the maximum adsorption capacity. The values potential is towards positive values, the better the adsorption capacities of KL and qm can be determined from the linear plot Ce/qe versus Ce. The of the nanocomposites. equilibrium parameter RL, which describes the feasibility of adsorption, However, this surface phenomenon, by which anionic azo dye mol- is defined as follows: ecules attach to the solid surface of nanocomposites, is potentially 1governed by physisorption forces. This can possibly be of three different RL = (5)  origins: electrostatic interactions between the sulfonate groups (SO− ) of 1 + KLC03the anionic dye molecules (MR19, DB53, DG1) and the positive surface where KL (L/mg) is the Langmuir equilibrium constant and C0 (mg/L) is charges of the nanocomposites (see Fig. 9); the polar forces resulting the initial dye concentration. Adsorption is more favorable as (RL → 0) from the presence of an electric field in the micropores of the nano- and more unfavorable as (RL → 1). composites; and finally from the hydrogen bonds that can be established The Langmuir isotherms are represented in Fig. 12. between the hydroxyl (-OH) and amine (-NH2) groups of the dye mol- Likewise, the Freundlich equation can be translated as follows: ecules (MR19, DB53, DG1), and the silanol (-SiOH) and aluminol (-AlOH) groups of certain nanocomposites. Consequently, kaolinite’s 1ln qe = ln KF + ln C e (6)  capability to eliminate anionic azo dye molecules can be considerably nimproved by modifying it by physical or chemical activation followed by immobilization of ZnO nanoparticles on the surface. where qe (mg/g) is the equilibrium adsorption capacity, KF is the Freundlich adsorption equilibrium constant, n is a Freundlich intensity 3.6.2. Adsorption kinetics factor and Ce (mg/L) is the concentration of the dye at equilibrium. To further evaluate the adsorption kinetics of dyes MR19, DB53 and The Freundlich isotherms are represented in Fig. S7. DG1, the experimental data were analyzed with the plot of the model s Table 6 lists the values of all Langmuir and Freundlich adsorption ’pseudo-first order (Figure S6a, b and c) and pseudo-second order isotherm parameters corresponding to the adsorption of MR19, DB53 (Fig. 10a, b and c) kinetics using equations (1) and (2). and DG1 on the nanocomposites MK800-ZnO(1:1), MK800-ZnO(1:1/2), The values of the rate constants (k1 and k2), the calculated equilib-KAODMSO-ZnO(1)(1:1), KAODMSO-ZnO(2)(1:1) and KAO-ZnO(1:1). The linear correlation coefficients (R2) of the Langmuir adsorption iso-rium adsorption capacities (qe, cal) and the linear correlation coefficients (R2) are listed in Table 5. In most cases, the pseudo-first-order kinetic therms are all greater than 0.99. While those of the Freundlich isotherm vary between 0.5748 and 0.9094, indicating that the adsorption data model showed a lower value of (R2). Furthermore, the values of qe, cal obtained from this model do not agree with those of experimental q , best fit the Langmuir adsorption isotherm. Moreover, the values of RL → e 0 whatever the composite, which suggests a very favorable adsorption of which suggests that the kinetics of adsorption of dyes on the different materials does not follow this model. On the other hand, the values of dye molecules on these composite materials. 2 To synthesize, from the different types of adsorbents, MK800-ZnO (R ) of the pseudo-second order kinetic model are all greater than 0.99, and the values of qe, cal are close to the experimental values q . The plot t⁄ (1:1) sample is the more efficient for the 3 dyes. This can be explained e by the structure of these dyes. To this end, some of these molecules have qt versus t is a straight line in all cases (Fig. 10a, b and c). The linear plot suggests a good fit of the experimental data with the pseudo-second- a higher auxochrome group density than others. This is the case with the DG1 dye which has 5 auxochromic groups, the DB53 dye has 8 aux-order kinetic model, indicating that this model is more appropriate to describe the adsorption of MR19, DB53 and DG1 on the different ochromic groups and finally the MR19 dye which has 3 auxochromic groups. In conclusion, the higher the auxochrome group density, the matrices. more intense the interaction with the surface of the adsorbent. 3.6.3. Adsorption isotherms The adsorption isotherms of MR19, DB53 and DG1 of 5 nano- 3.7. Limitations of this study composites: MK800-ZnO(1:1), MK800-ZnO(1:1/2), KAODMSO-ZnO(1) (1:1), KAODMSO-ZnO (2)(1:1) and KAO-ZnO(1:1) are shown in The study of the development of new composite materials, based on Fig. 11a, b and c. kaolinitic clay modified by ZnO, for the elimination of anionic azo Regardless of the material, initially the adsorption capacities of the textile dyes by adsorption in aqueous solution, may be limited by several three dyes increase linearly between 0 and 150 mg/L, then reach a methodological, material and analytical factors. plateau at 200 mg/L which reflects the saturation of the available active sites and beyond which they do not evolve anymore. Several adsorption 3.7.1. In the methodological framework isotherm models have been developed to study the adsorption phe- The Langmuir and Freundlich adsorption isotherm models used to nomenon. Among these, the Langmuir and Freundlich isothermal correlate with the characteristic properties of composite materials and models are the most used because they have proven useful in translating anionic azo dye molecules are based on simplifying assumptions that a number of adsorption processes. However, the underlying assumptions may not accurately reflect the complexities of the real system. of the two models are quite different. The Langmuir isothermal model In terms of experimental conditions, we note that the parameters in assumes that adsorption takes place on specific homogeneous active the adsorption process such as pH, temperature and stirring speed sites, of the same energy within the adsorbent. They can only complex a considerably influencing the interaction between the anionic molecules single molecule of the solute (monolayer adsorption) and there are no of azo dye and the different composite materials are not always interactions between the adsorbed molecules (Páez et al., 2012). The controlled or standardized to perfection. It should be noted that the Freundlich model is based on adsorption on a heterogeneous surface adsorption experiments were carried out at room temperature; but this (Páez et al., 2012). In the present study, we used both models to analyze can vary slightly from one experiment to another, from one day to the experimental data. The linearized Langmuir equation can be trans- another with the potential consequence of an erroneous result and lated as follows: conclusion on the thermodynamic equilibrium which could thus poorly C 1 C reflect the feasibility and spontaneous nature of the adsorption process. e e= + (4)  qe KLqm qm 3.7.2. At the material level The characteristics of adsorbents in terms of surface properties, 15P. Ngue Song et al.                                                                                                                                                                               R  e  s u  l t s  i n   S  u r  f a  c e  s  a  n d   I n  t e  r f a  c e s 16 (2024) 100255morphology, porosity and adsorption capacity can vary slightly. It is kinetics of dyes could be well described using the pseudo-second order sometimes difficult to accurately reproduce the same preparation when kinetic model. The adsorption of MR19, DB53 and DG1 on the nano-moving from one sample to another. It may also happen that the com- composites could be described using the Langmuir model, suggesting posite material degrades during the experiment; this can the case for that monolayer adsorption took place in all cases. example with the release of zinc in solution during experiments, when it is used several times in a row, which can thus affect the adsorption Ethical approval capacities. The authors declare that they have no known competing financial 3.7.3. On the analytical level interests or personal relationships that could have appeared to influence The precision and sensitivity of the spectrophotometer, for the work reported in this paper. measuring optical densities in the UV-VIS range, after the adsorbent is separated from the solution by centrifugation, may be a limitation in the Consent to participate context of this study to determine low concentrations of pollutants (MR19, DB53 and DG1). In this case, measuring the exact quantity of All authors agreed to participate in this work. adsorbed dye may prove difficult. Consent to publish 3.8. Statistical analysis and significance of the results All authors agreed to this version for publication. It should be emphasized that the scientific assertions, theories, hy-potheses and even the methodology used in this study are credible CRediT authorship contribution statement because the results leading to their formulation are reproducible. At this level it is necessary to remember that the methodology leading to the Pierre Ngue Song: Writing – review & editing, Writing – original different treatments (thermal, DMSO, acid and thermal + acid) of raw draft, Methodology, Investigation, Formal analysis, Conceptualization. kaolinite with the aim of causing activation of the latter is clearly set out Julien G. Mahy: Writing – review & editing, Writing – original draft, in the literature. The same is true for the immobilization of ZnO on a clay Supervision, Methodology, Formal analysis. Antoine Farcy: Writing – matrix by the sol-gel and co-precipitation processes, with a view to review & editing, Investigation, Formal analysis. Cédric Calberg: obtaining the different composite materials. Writing – review & editing, Investigation, Formal analysis. Nathalie In addition, the adsorption experiments (influence of contact time, Fagel: Writing – review & editing, Supervision, Project administration, influence of initial concentration), which made it possible to carry out Methodology, Formal analysis. Stéphanie D. Lambert: Writing – review the kinetic and isothermal studies of adsorption as part of this work, & editing, Supervision, Project administration, Methodology, Funding were carried out in triplicate for each of the samples. It is the same with acquisition, Conceptualization. the measurement of the zeta potential of each of the different samples of raw kaolinite and composite materials. Declaration of competing interest From the statistical analysis, it appears that the results obtained during the study of the pseudo-second order kinetic model are signifi- The authors declare that they have no known competing financial cant because the linear correlation coefficient values (R2) recorded are interests or personal relationships that could have appeared to influence all greater than 0.99. In addition, although the values of adsorption the work reported in this paper. capacities at equilibrium (qe), obtained during the experiments are close to those calculated at equilibrium (qm) from this model, it is clear that Data availability they remain lower than the latter. Data will be made available on request. 4. Conclusion Acknowledgments In this work, natural kaolinitic clay extracted from Cameroon is modified with ZnO, DMSO, and with thermal and acidic treatment to The authors acknowledge Joel Otten and Nicolas Delmelle for their enhance its adsorption properties for water depollution. technical support. Julien G. Mahy and Stéphanie D. Lambert thank the F. ZnO nanoparticles, synthesized by the sol-gel process, are immobi- R.S.-FNRS for their Postdoctoral Researcher position and Research Di-lized on the clay material producing a composite ZnO-clay material. rector position, respectively. After doping with ZnO, a sudden inversion of the nature of the surface charge of certain composite materials (KAODMSO-ZnO, MK600-ZnO, Appendix A. Supplementary data MK700-ZnO, MK800-ZnO) obtained was observed, through zeta poten-tial values ranging from − 31 mV before doping with ZnO to +36 mV Supplementary data to this article can be found online at https://doi. after doping. 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