Development and characterization of a clay-HDTMABr composite for the removal of Cr(VI) from aqueous solutions with special emphasis on the electrochemical interface

3.1.1 X-ray fluorescence analysis

The X-ray fluorescence data in Table 2 shows that the crude clay is mostly made from silicon oxide (72.2 wt%), confirming the high concentration of quartz suggested by the XRD diffraction plots. Additionally, this clay has a moderate level of alumina (15.8 wt%), indicating that there are not many clay minerals in it. The molar ratio exceeds the traditional value of bentonite, which is 2.7 wt%, indicating that this clay contains a significant amount of free quartz. Since other oxides make up 12 wt% of the mixture, the clay cannot be considered pure. The existence of smectite traces explains the MgO content of 0.1 wt%. A mass loss of around 2.93 wt% was caused by the loss of ignition (LOI). This can be explained by removing organic matter, dehydration of clay, and hydroxylation of clay minerals (Jiang, et al., 2019).

3.1.2 X-ray diffraction

The diffraction of raw clay and clay sintered to 550 °C are shown in Fig. 1. The diffraction pattern (Fig. 1a) indicates that the crude clay is mainly composed of quartz (Q), evident with a high peak intensity positioned at 26.7° and low peaks’ intensities at 20.71, 27.4, 45.4, and 50.2°, and kaolinite (K), which exhibits its characteristic lines around 12.2° and 25.4°, illite (I), which appears around 8.8°, and smectite, which was detected at 17.8°. However, (Fig. 1b) showing the sintered clay to 550 °C revealing the disappearance of the kaolinite and a decrease in the intensity of the main quartz peak at 26.7° (Zhang, 2021). Kaolinitic clays have been widely reported in many studies, they show similar diffraction spectrums, in a pervious study we have identified a triclinic kaolinite by diffraction peaks attributed to reticular planes (0 0 1), (0 2 0), (0 0 2) and (0 0 3) with respective d-spacings of 7.17 Å, 4.47 Å, 3.57 Å, and 2.38 Å, (Lahnafi et al., 2021).

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Fig. 1

X-ray diffractogram of raw clay (a) and clay sintered to 550 °C (b). I: illite, K: kaolinite, Q: quartz, S: smectite.

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3.1.3 Nitrogen (N₂) adsorption/ desorption

The N₂ adsorption/desorption technique based on the Brunauer–Emmett–Teller theory (BET) allows the determination of the specific surface area, pores’ volume, and pores’ diameter (Selim et al., 2017; Yenial et al., 2020). The clay materials were degassed overnight at T = 100 °C, then nitrogen adsorption isotherms were recorded at 77 K. Fig. 2 shows the adsorption–desorption isotherm curves of the raw clay and the HDTMABr/clay materials.

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Fig. 2

N₂ adsorption/desorption isotherms of (a) raw clay and (b) HDTMABr/clay.

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The porous clay isotherms are type IV, often known as H3 hysteresis-type characteristics for porous solids. The above results show a decrease in the BET specific surface area when the clay is modified by HDTMA-Br faced with an increase in the average pore diameter. It was due to the obstruction of mesopores by HDTMA⁺ cations, thus, impeding the diffusion of N₂ throughout its channels. The parameters characterizing the texture of the raw clay and HDTMABr/clay obtained from the adsorption/desorption of N₂ are summarized in Table 3.

The pore size, pore volumes and specific surface area are parameters that could be used to explain the mechanism of heavy metals removal using aluminosilicates in general and clays particularly. For instance, Selim et al. (Selim et al., 2017), demonstrated that a zeolite A material performed well in the removal of Cd²⁺ through a precipitation and ion exchange mechanisms due to its specific surface area of 374 m²/g, a total pore volume of 0.205 cm³/g and pore size of 1.08 nm.

3.1.4 Thermogravimetric (TGA) analysis

Fig. 3a shows the thermogravimetric analysis (TGA) of raw clay, and Fig. 3b gives the TGA of the HDTMABr/clay modified. The analysis is used to identify the temperature regimes at which large mass losses and possible phase transformations that are taking place while heating the materials. The first mass loss of 1.0 wt% and 3.0 wt% for clay and clay-HDTMABr, respectively, occurring between 25° and 300 °C, corresponding to the evaporation of interstitial water inserted between the plates or into the cavities of the crystal structure of the clay material as well as adsorbed waters on the surface or as moisture. The second important mass loss of approximately 2.3 wt% for the natural clay occurring between 300 and 500 °C are associated with the exit of structural water caused by the release of hydroxyl belonging to the crystal lattices of clay minerals such as illite and kaolinite. While clay-HDTMABr shows a second mass loss of 1.0 wt% between 250 and 350 °C, which could be attributed to the HDTMA⁺ degradation, Nunes et al. reported degradation of the same reagent at temperatures between 473 and 573 K (200–300 °C) (Nunes et al., 2020). Between 488 and 700 °C, the natural clay shows mass loss of 0.4 wt% which could be attributed to hydroxyl loss due to the transformation of kaolinite to metakaolinite and α-quartz to β-quartz for clay and clay modified (El-Kordy, 2022). The TGA of clay-HDTMABr shows a late mass loss of 2.0 wt%, between 365 and 590 °C, which could be attributed to continuous degradation of the HDTMA+, surfactant which is strongly bounded to the clay mineral surface, and this may be due to the electrostatic binding between the ammonium head and corresponding negative sites of interaction on the clay. Similar results were demonstrated by Aghaii et al. (Aghaii et al., 2013), where they demonstrated that a HDTMABr modified zeolite presented two main mass loss stages the first one below 200 °C attributed to unbound and physisorb water on the zeolite surface and a second stage at higher temperatures attributed to strong bounding of the surfactant on the zeolite surface.

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Fig. 3

Thermogravimetric analysis (TGA) of (a) raw clay and (b) HDTMABr/clay.

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3.1.5 FTIR analysis

The study of clays by infrared spectroscopy aims to determine different chemical functional groups present on the surface of these solids. This is a complementary method that often focuses on the molecular analysis of samples. Fig. 5a shows the infrared spectra of the crude clay, and Fig. 5b that of the HDTMABr-modified clay. In Fig. 5a, several bands are observed. The Si-OH, Al-OH, and Mg-OH, OH groups in the clay fractions, as well as the adsorbed water, are responsible for the two broad bands that occur at 3706 cm⁻¹ and 3433 cm⁻¹, respectively. A weak absorption band is detected at 1634 cm⁻¹ and is attributed to the bending of the structural water molecule of the clay mineral. The bands appearing at 540 cm⁻¹ and 1031 cm⁻¹ are attributed to the stretching vibrations of Si-O and Si-O-Si, respectively. Si-O bending vibrations and Si-OH stretching vibrations, respectively, are responsible for the vibration bands that emerge at 472 cm⁻¹ and 782 cm⁻¹. The band at 690 cm⁻¹ is due to the stretching vibration of the Si-O-Al bond of the kaolin (Lynch et al., 2018).

Fig. 4b, represents the FTIR of the clay-HDTMABr. The FTIR spectrum contains all bands recorded in the raw clay, including those appearing at 3706 cm⁻¹ and 3433 cm⁻¹, which were attributed to the stretching vibration of the OH group in the clay fraction (Si—OH, Al—OH, and Mg—OH) and adsorbed water, respectively. The bands detected at 472 cm⁻¹, and 1031 cm⁻¹ are attributed to the stretching vibrations of Si—O and Si—O—Si, respectively. Additionally, it should be highlighted that treating the clay with HDTMA-Br resulted in two new absorption bands at 2850 cm⁻¹ and 2920 cm⁻¹, which are attributed to the symmetric and asymmetric stretching vibrations of the CH₂ and CH₃ groups, respectively. These bands also have another vibration mode detected at 1472 cm⁻¹, corresponding to a bending mode. This could be used as evidence of the exchange between Na⁺ and HDTMA⁺, and, therefore, the intercalation of the clay by HDTMA-Br. Displacement of vibrations could also be assessed during the modification of the clay with HDTMA-Br. Slimani et al. (Slimani et al., 2012) attributed these displacement to exchange reactions occurring in the interlayer space of sodium modified bentonite, it was suggested that the interlayer water coordinating the Na⁺ cation is been lost during the replacement of Na⁺ by the HDTMA⁺ cation.

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Fig. 4

FTIR spectra of raw clay (a) and Clay/HDTMABr (b).

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Fig. 5

Micrographs and corresponding elemental analysis of raw clay (a, b) and HDTMABr/clay (c, d) using SEM coupled with EDAX.

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3.1.6 Scanning electron microscopy coupled to energy disperse spectroscopy (SEM-EDX)

The morphology of the clay was examined by scanning electron microscopy (SEM) before and after HDTMABr intercalation. The SEM micrograph in Fig. 5a shows clay powder particles with no defined shape. Furthermore, no agglomeration could be seen in the clay powder. The surface is homogenous, with the occurrence of small platelets indicating the crystallinity of the clay mineral. The peak intensity of silicon, aluminum, and calcium oxides (SiO₂ and Al₂O₃), which appear to be quite high in the clays according to XRD, are relatively low in the EDAX analysis (Fig. 5b), however still show that they are the main components of the clay. These observations are fully confirmed and supported by the elemental composition of the clay using XRF. Furthermore, it is important to note from the EDAX spectra that the elemental chemical composition of the clay is dominated by the elements Si, Al, Ca, and Fe. Dehmani et al. found that Si, Al, Fe and K were the predominant elements contained in a clay sampled from the Errachidia region (Dehmani, 2020).

In Fig. 5c, the two solids can be clearly distinguished from one another, the clay's porous structure could be clearly seen with small particles inside the pores. This is due to the intercalation of the clay by the HDTMA-Br surfactant and its adsorption on the outer surface of the clay. The intercalation of the clay by the surfactant is also demonstrated by EDAX analysis presented in Fig. 5d, which shows the appearance of a new carbon peak, originating from the HDTMABr surfactant, with high intensity. However, the percentage of Al and Si, decreases because of the elemental redistribution in the new HDTMA-Br/ clay material.

3.1.7 Point of zero charge

The point of zero charge (pH_pzc) is an important parameter that can be used to explain the adsorption, as it gives information about the surface charge of the adsorbent. Fig. 6, shows the evolution of the pH as a function of the volume of NaOH added to the clay suspension and the blank solution. The point of intersection of the two curves corresponds to pH_pzc. The intersection point was found at pH = 9.5, which indicates that below pH_pzc = 9.5, the surface of the clay is positively charged, while at higher pH above the pH_pzc = 9.5 the surface of the clay will be negatively charged. This would help in the explanation of the mechanism of adsorption of Cr(VI) at the surface of the adsorbent. For example, we demonstrated in a previous study that the pH_pzc = 9.8 for a zeolite membrane, which was found to control the rejection of heavy metals at the surface of the zeolite membrane. Filtrations were carried out at pH ≤ pH_pzc, the surface of the zeolite membrane was positively charged which played a high role in the rejection of heavy metals by repulsion effect, since these are positively charged also (Lahnafi et al., 2021).

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Fig. 6

Surface charge curves of the clay suspension and blank at an ionic strength of 0.001 M NaCl.

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3.2.1 Influence of pH on the adsorption of Cr(VI) at the surface of clay and HDTMABr/Clay

To evaluate the influence of pH on the adsorption of Cr(VI) on the raw and HDTMABr-modified clay, we have conducted a series of experiments using Cr(VI) solutions of 10^-4 M concentration which were placed in contact with the raw clay and HDTMABr modified clay at different pH values. Fig. 7 represents the results obtained. At pH = 1.0, the predominant form of chromium is ${\mathit{HCrO}}_4^{-}$ constituting almost 80% of the chromium speciation, $H_2{\mathit{CrO}}_4$ is present with 12% and ${{\mathit{Cr}}_{2}O}_7^{2-}$ with 8% (Trifi, 2013), it is expected that the surface of the clay and HDTMABr-modified clay will be highly charged positively at this pH. This will help ${\mathit{HCrO}}_4^{-}$ and ${{\mathit{Cr}}_{2}O}_7^{2-}$ to be adsorbed perfectly compared to the chromic acid ( $H_2{\mathit{CrO}}_4$ ) and explain the highest capacity of both materials at lower pHs. On the further increase of the pH to values above pH = 4, the number of positive charges of the clay and HDTMABr modified start declining. However, still charged positively since the pH was below the pHpzc = 9.5. At these pHs, the percentage of speciation predominance changes, ${\mathit{HCrO}}_4^{-}$ speciation predominance increases to 92%, and the ${{\mathit{Cr}}_{2}O}_7^{2-}$ remain constant, while the chromic acid disappear completely. The reduction of positive surface charge is much higher than ${\mathit{HCrO}}_4^{-}$ speciation increase, this has a direct effect on the decrease in the adsorption capacity of both materials, the raw clay and HDTMABr-modified clay. The highest capacity of HDTMABr-modified clay however, could be explained by the additional positive sites provided by the ammonium group.

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Fig. 7

pH effect on Cr(VI) removal capacity of the raw clay (a) and HDTMABr modified clay (b) at different pH values. Experimental conditions: C₀ = 10^-4 M, m = 50 mg, V = 20 mL, T = 25 °C, pH = Variable, t = 120 min.

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3.2.2 Influence of the contact time on the adsorption of Cr(VI) at the surface of clay and HDTMABr/Clay

To study the effect of contact time on the adsorption of Cr(VI) ions on the raw and HDTMABr/clay, we have used the batch experiment technique. In 9 test tubes, 50 mg of clay or HDTMABr/clay was introduced in each test tube, then 20 mL of Cr(VI) with 10^-4 M initial concentration and at pH = 2, since the highest capacities of removal are obtained at this pH. Test tubes were rota-mixed and kept for different timing ranging from 60 min to 580 min. Cr(VI) ion elimination by raw clay and clay-HDTMABr as a function of time are depicted in Fig. 8a and b, respectively. Fig. 8a shows that the adsorption of Cr(VI) ions increases linearly in the beginning, which is explained by the high affinity of the active sites to retain Cr(VI). Still, after 360 min, the sites are saturated and have reached equilibrium. The removal rate is about 9.2 mg/g. For Fig. 8b, we also observe a gradual increase until the equilibrium time of 380 min, which is consistent with active sites in the (clay-HDTMABr). The removal capacity of chromium (VI) by this organoclay is found to be 250 mg/g which is 25 folds higher than previously reported montmorillonite HDTMABr modified clay (Akar et al., 2009).

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Fig. 8

effect of contact time on the Cr(VI) removal capacity using raw clay (a) and using HDTMABr/clay (b). Experimental conditions: C₀ = 10^-4 M, V = 20 mL, T = 25 °C, pH = 2, m = 50 mg, t = Variable.

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3.2.3 Influence of the temperature on the adsorption of Cr(VI) on the surface of clay

Fig. 9a presents the adsorption of chromium (VI) by clay as a function of temperature. At room temperature of 25 °C, the adsorption capacity is quite low (9.30 mg/g). Increasing the temperature leads to a linear increase in the capacity with a stiff slope of 0.55 and a regression coefficient R² of 0.994 to a final value of 37.0 mg/g at 75 °C. The behavior of the material at high temperatures above 25 °C could be explained by the increase of the speed of movement of Cr(VI) speciated ions towards the surface of the adsorbent. The process could be classified as an endothermic possess. Slimani et al. found that the standard Gibbs free energy (ΔG°) is decreasing while increasing the temperature, which indicated that adsorption is much efficient at higher temperature (Slimani et al., 2014).

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Fig. 9

Influence of temperature on the capacity of Cr(VI) removal with raw clay (a), modeling of adsorption kinetics (b) pseudo-first-order and (c) pseudo-second-order. Experimental conditions: C₀ = 10^-4 M, V = 20 mL, T = Variable, pH = 2, m = 50 mg, t = 120 min.

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3.2.4 Adsorption kinetics

To study the adsorption kinetics of raw clay, pseudo first- and second-order kinetic models were used, while the Langmuir equation was used to determine the rate constants K₁, the equilibrium adsorption capacity (Q_e), and the correlation coefficients (R₁) of the lines obtained by plotting ln (Q_e - Q_t) as a function of times (Fig. 9a). In the same circumstance, we have also tested the pseudo-second-order kinetic model by plotting t/Q_t as a function of time (Fig. 9b). When using the pseudo-first-order kinetics model, it is found that the amount adsorbed at equilibrium determined experimentally (Q_exp = 9.2 mg/g), differs from that calculated theoretically (Q_th = 3 mg/g). The correlation coefficient between the two data is also found to be very low (R² = 0.55). Therefore, the pseudo-first-order kinetic model was considered non-suitable to model the kinetics of the reaction of adsorption of Cr(VI) on the raw clay material. However, the pseudo-second-order model (Fig. 9c) shows that an equilibrium adsorption capacity, Q_exp of 9.2 mg/g is found to be closer to a theoretically calculated, Q_calc of 12.8 mg/g, with a correlation coefficient R² = 0.99. Therefore, it is concluded that the pseudo-second-order model is more suitable and is the one that best describes the Cr(VI) adsorption process on the studied clay. Table 4 compares the experimental and theoretical results of Cr(VI) adsorption on the raw clay. In this study, we presented only the experimental results of Cr(VI) adsorption processes on clay. Similar results were reported for the removal Cr(VI) on a bentonite modified clay, which shows that the process was very fast and following pseudo second order reaction with similar capacity of 251.7 mg/g (Slimani et al., 2014).

3.2.5 Thermodynamic study

Thermodynamic parameters compared to kinetic give very precise information about the physical process of Cr(VI) adsorption on the clay. Estimation of the standard Gibbs free energy (ΔG°), standard enthalpy (ΔH°), and standard entropy (ΔS°) from Van't Hoff equation is not always valid because the temperature is not constant. However, general guidance about the process could be gained, such as whether the process is binding (enthalpy) or diffusion (entropy) controlled. The values of ΔH° and ΔS° are obtained from the slope and intercept of the linear plot of ln K_d versus 1/T, as shown by Eqs. (6) and (8) (Kali, 2022). K_d, is the thermodynamic stability constant that measures the strength of adsorption of Cr(VI) on the raw clay and is determined from the pseudo-second-order kinetic model described previously. Fig. 10 shows the linear plotting of the Van't Hoff equation, which gave a straight line with ΔH°/R as the slope and ΔS°/R as the intercept. However, ΔG° is determined from ΔG° = ΔH° -TΔS°.

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Fig. 10

Variation of Ln (K) as a function of 1/T. Experimental conditions: C₀ = 10^-4 M, V = 20 mL, T = Variable, pH = 2, m = 50 mg, t = 120 min.

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The adsorption process is spontaneous for the investigated temperatures, as evidenced by the negative value of the standard Gibbs free energies (Table 5). It may be claimed that the Cr(VI) adsorption process is endothermic, spontaneous and functions as physisorption since the standard Gibbs free energy is negative and smaller than 40 KJ/mol. Also, the process is entropically controlled since the term TΔS° is the predominant term at high temperatures. The positive value of the entropy shows that adsorption is accompanied by an increase in the disorder, which shows that Cr(VI) ions near the solid/liquid interface are less structured than those in the bulk of the solution. We have demonstrated previously that is not right to use Van't Hoff equation to estimate the thermodynamic parameters because this equation work on the assumption that the temperature is constant, however still the equation can give information about the spontaneity of the process, the sign of internal energy as well as whether the process is enthalpically or entropically controlled (Elgamouz, 2020).

3.2.6 Electrochemical testing

CV was used to evaluate the electrochemical properties of the unmodified and modified CPEs. An equimolar (5.0 mM) solution of K₄[Fe(CN)₆]/K₃[Fe(CN)₆] was used due to the efficiency and reversibility of Fe²⁺/Fe³⁺ redox species, and 0.1 M KCl was used as the supporting electrolyte. The analysis was carried out at a potential range of −0.25–0.75 V and a scan rate of 100 mV/s. The reduction and oxidation peaks of the Fe²⁺/Fe³⁺ redox species were observed at ∼ 0.1 V and ∼ 0.3 V, respectively (Fig. 11a-b). Unmodified CPE showed an anodic peak current of 47.7 μA, which was increased upon 10 wt% clay modification to 55.4, reached a maximum of 56.2 μA at 20 wt% clay modification, and then decreased significantly to ∼ 51.0 μA at higher clay content. Clay-HDTMABr showed a similar trend with a maximum peak current of 99.3 μA at 40 wt% modification. A voltage overpotential was observed at > 40 wt% and > 20 wt% of clay and clay-HDTMABr modification, respectively. The increasing in peak current with CPE modification using both clays and Clay-HDTMABr demonstrated the high electrochemical activity of both additives, and the higher current achieved by clay-HDTMABr indicated its higher electrocatalytic effect. The different concentrations of both additives at which the maximum peak current was achieved could be attributed to the surface change upon HDTMABr incorporation. The decreasing current and overpotential observed at high additive content could be explained by the increasing of electrode resistance due to the insulative nature of the additives, which stifled charge transfer at high concentrations.

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Fig. 11

CV and Nyquist plots of (a, c) CPE-clay with different clay content and (b, d) CPE-clay-HDTMABr with different clay-HDTMABr contents in 5.0 mM [Fe(CN)₆]^3-/4- prepared in 0.1 M KCl at a scan rate of 100 mV/s for CV scans and EIS at the frequency range of 10⁶-0.05 Hz.

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The electroactive surface area of CPEs modified with different loads (CPE-wt%clay) of clay or Clay-HDTMABr (CPE-wt%Clay-HDTMABr) was calculated by applying the Randles-Sevcik Eq (9), where C is the concentration of the analyte (5.0 mM), n is the number of electrons (1e^-) that contribute to the redox reaction on the CPE-Clay, and CPE/Clay-HDTMABr modified electrodes, A is the electroactive surface area of the electrodes (cm²), D is the diffusion coefficient (cm².s⁻¹), and γ is the scan rate (Vs⁻¹). With the aid of the CVs of CPE-%clay and CPE-%Clay-HDTMABr presented in Figs. S2-S12 (Supporting document). The electroactive surface areas were calculated and represented in Fig. 12a. The surface area of the CPE-wt%Clay electrodes and CPE-wt%Clay-HDTMABr were found to increase linearly with the increase of the modifier percentage. The maximum surface areas were presented by 20 wt% and 30 wt% for clay and Clay-HDTMABr, respectively. On the further increase of the percentages of load, the surface area for both materials declines dramatically. This decrease can be explained by the brittleness of the prepared paste due to the fact that the electrode releases imperfections in the solution of K₄[Fe(CN)₆]/K₃[Fe(CN)₆].

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Fig. 12

Change of (a) electrochemical surface area, (b) R_s and R_ct, and (c) anodic peak current of CPE-modified electrodes with clay and clay-HDTMABr additives. (d) LSV of 500 µM Cr(VI) in 5.0 M HNO₃ (pH = 0.5) using unmodified and modified CPEs at 100 mV/s and the accumulation condition used −1.2 V for 60 s.

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EIS was done to investigate the electrode resistance and study the influence of clay and clay-HDTMABr on the charge transfer efficiency (Fig. 11 c-d). The Nyquist plots were fitted using Randle’s circuit (R_s [C_dl-R_ct-W]), where R_s is the solution resistance corresponding to the intersection with the real-impedance axis in the high-frequency region, C_dl is the double-layer capacitance, R_ct is the charge-transfer resistance corresponding to the semi-circle in the high-to-medium frequency region, and W is Warburg diffusion in the low-frequency region corresponding to the ionic diffusion process. The unmodified CPE showed Rs and R_ct of 151 Ω and 1598 Ω, respectively (Fig. 12b). Upon CPE modification with clay, R_s showed only minor change, while R_ct decreased significantly with the increase in clay content reaching a minimum of 887.7 Ω at 40 wt% clay and then increased thereafter. Clay-HDTMABr showed a similar trend (Fig. 12b), however, with an enhanced sensitivity, where a drastic decrease in R_ct took place at 10 wt% modification reaching a low value of 349.1 Ω, and decreased further to 269.9 Ω at 20 wt%, and then started to increase thereafter. The decrease in R_ct with clay and clay-HDTMABr modification indicated enhanced electrode charge transfer kinetics, which could be attributed to the electrocatalytic effect of the oxides abundant in the clay. The significantly reduced R_ct in the case of clay-HDTMABr showed that HDTMABr further facilitated charge transfer. The increase in R_ct at high clay and modified clay concentrations could be due to their insulative nature that started to dominate the electrode as their ratio to graphite became too high. This corroborated the lower peak currents and overpotential observed in the abovementioned CV study. Based on the data above, 20 wt% modification was chosen as the optimal additive content, considering the high current achieved, low over potential, and low R_ct.

LSV was used for Cr(VI) detection by electrochemical reduction on the surfaces of modified CPEs with various additives percent, 20 wt% additive was found to yield the highest peaks for both materials clay and clay-HDTMABr (Fig. 12c). CPE-0wt%clay, CPE-20wt% clay, and CPE-20wt%clay-HDTMABr, were therefore, tested to explain the mechanism of accumulation of Cr(VI) at the interface between the clay or the clay-HDTMABr and the solution. 500 µM Cr(VI) prepared in 5.0 M HNO₃ (pH = 0.5) was used at a scan rate of 100 mV/s (Fig. 12d). A high Cr(IV) reduction peak was observed at 0.57 V in the case of CPE-20wt%clay-HDTMABr with a large peak current of 456 µA, which was 60% higher than that obtained using CPE-20wt%clay. Unmodified CPE only showed a minor peak with a very low peak current of 100 µA. The high peak current shown by the modified CPEs showed the significant electrocatalytic activity of clay and clay-HDTMABr, and the 60% peak current increase in the case of CPE-20wt%clay-HDTMABr, as compared to CPE-20wt%clay, indicated enhanced electrochemical activity upon HDTMABr incorporation within the clay. The high sensitivity of the modified electrodes towards Cr(IV) reduction could be explained by an anion exchange process involving the dominant Cr(IV) anion at pH = 0.5, (Cr₂O₇)^2-, at HDTMA⁺ site as explained in equations Eq. (10) (El Ouafy et al., 2015), the reaction of reduction of Chromium VI to Cr(III) indicated in Eqs (12) according to the study which was made by Kachoosangi et al. and Butter et al. (Kachoosangi and Compton, 2013; Butter et al., 2021). The high accumulation of Cr(VI) at the surface of the CPE-20wt%clay-HDTMABr, manifested by high peak current correlate will with adsorption data which indicates high adsorption of Cr(VI) by the HDTMABr modified clay. The mechanism of retention of Cr(VI) by HDTMABr modified clay could be dominated by charge repulsion, adsorption, and/or exchange capacity of the material.