Surfactant-Modified Bolivian Natural Zeolite for the Adsorption of Cr (VI) from Water

Abstract

The present study reports the surfactant modification of Bolivian natural zeolite with hexadecyltrimethylammonium bromide (HTDMA-Br) for the adsorption of hexavalent chromium Cr (VI) anions from water. The surfactant-modified natural zeolite was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption/desorption isotherms, and Fourier-transform infrared spectroscopy (FTIR) to analyze the effect of its modification with HTDMA-Br and to verify its charge on the zeolite surface. We report a maximum adsorption capacity of 17 mg/g of Cr (VI) anions, surpassing the findings of some of the previous investigations on surfactant-modified natural zeolites of different geological origins. The analysis of the equilibrium data described the Cr (VI) anions adsorption by Langmuir isotherm and the pseudo second-order kinetic model. In addition, thermodynamics revealed an exothermic adsorption. Furthermore, anion exchange, electrostatic attraction, and chemical reduction were indicated to be dominating sorption mechanisms by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) characterization techniques.

1. Introduction

Chromium has been reported as one of the most harmful chemicals to health as it can cause adverse health effects in living beings [1]. Chromium is a transition metal that can be found as a Cr (VI) and Cr (III), and it exists in the aquatic environment in the form of anions (H₂CrO_4, HCrO₄⁻, CrO₄²⁻, and Cr₂O₇²⁻) [2,3]. The contamination of water resources by chromium derives mainly from the discharge of industrial activities such as leather tanning [4], mining [5,6,7], wood preservation [8], and painting and dyeing [9]. Nonetheless, the most significant concern is the tanning industry, which disposes of chromium in its hexavalent form, which is considered the most toxic and hazardous due to its being carcinogenic and mutagenic [10]. The worldwide regulations governing the release of chromium into aquatic ecosystems provide specific limitations on the concentration of chromium allowed, often ranging from 0.05 to 2 mg/L [11]. However, there is still a need to improve the strict monitoring for compliance with the established regulations, especially in countries of Latin America [12]. Therefore, efficient, easily accessible, and regenerative alternatives should be sought to encourage the implementation of chromium treatment techniques in remote communities and small tanneries.

Different chromium removal methods have been studied, such as adsorption [13,14,15], biological remediation [16,17], electrochemical method [18,19], and membrane filtration [20,21,22,23]. Despite other methods being available, the adsorption method has generated the most interest due to its easy operation, affordability, efficiency, and regeneration [24,25]. In addition, the adsorption process offers a wide variety of adsorbent selection, resulting in a wider and more accessible range of options due to the possibility of using cost-effective and readily available materials, such as natural zeolites. This feature of the adsorption method enhances its practicality and applicability in various scientific and industrial fields [26,27]. Natural zeolites are porous materials with a hydrated aluminosilicate structure comprising a three-dimensional framework of AlO₄ and SiO units [28]. Moreover, natural zeolites have exhibited remarkable versatility upon modification, such as structural modification, to remove diverse contaminants including heavy metals, dyes, and others [29]. Additionally, the expansion of the adsorption range has been observed through the surface modification of natural zeolites using acids, bases, salts, metallics, and surfactants [29]. The modifications made to natural zeolites have been shown to enhance their adsorptive capacity towards anions, including chromium, which typically exhibits low or no affinity for raw natural zeolites [30,31]. The modification of natural zeolites is based on mechanisms that include hydrophobic interactions, ion exchange, electrostatic interaction, and van der Waals forces [32,33].

Efficient adsorption of chromium has been reported using cationic surfactants such as dilauryldimethylammonium bromide (DDAB) [34], cetylpyridinium chloride (CPC) [35,36], cetyltrimethylammonium bromide (CTAB) [37,38], hexadecylpyridinium bromide (HDPB) [39], hexadecyltrimethylammonium (HDTMA) [40], hexadecyltrimethylammonium bromide HTDMA-Br [32], and octadecyltrimethyl-ammonium bromide (ODTMA-Br) [41]. Likewise, adsorption ranges between 10–20 mg/g of Cr (III) and Cr (VI) were reported with surfactant-modified natural zeolites [30,42,43]. In the case of Bolivian natural zeolite [44], no studies have yet been reported on its modification with cationic surfactants for its application in removing metals such as chromium. This approach seeks to improve its efficiency, given that Bolivian zeolite is an excellent alternative for water treatment in Bolivian communities.

Therefore, this work aims to examine the efficacy of Bolivian natural zeolite in removing Cr (VI) after undergoing modification with HTDMA-Br, resulting in alterations to its surface negative charge and hydrophilicity. The inquiry commences by examining the zeolite structure through X-ray diffraction (XRD), scanning electron microscopy (SEM), and nitrogen adsorption–desorption isotherms. Subsequently, batch tests were performed to assess the adsorption efficiency, explicitly focusing on the influence of pH and initial concentration. Additionally, the investigation examines the kinetics, isotherms, and thermodynamics of the adsorption process. In addition, a comparison is made between previous studies on chromium adsorption on natural zeolites from other parts of the world and the present work. Furthermore, this investigation seeks to make a meaningful contribution towards the advancement of efficient and sustainable approaches for environmental remediation.

2. Materials and Methods

2.1. Preparation of Surfactant Modified Bolivian Zeolite

Bolivian natural zeolite (BZ), which has been previously reported [44], was subjected to modification using HTDMA-Br (≥98%, Sigma Aldrich, Darmstadt, Germany), as described in previous studies [42,45]. The modification involved adding BZ to a solution of 0.066 M HTDMA-Br, followed by stirring at room temperature (RT) for 24 h. The resulting mixture was subsequently filtered, thoroughly washed with distilled water, and dried at 80 °C for 5 h. The obtained sample was appropriately identified as a surfactant-modified Bolivian zeolite and labeled as SMBZ.

2.2. Characterization of the Adsorbents

The SMBZ was characterized by scanning electron microscope (SEM, JSM-IT300LV, JEOL GmbH, Freising, Germany), X-ray powder diffraction (XRD, ADP 2000 Pro, Novara, Italy) with Cukα radiation (λ = 1.5418 Å) in the 2θ range of 5–50 with a step of 0.02, Fourier transform infrared spectroscopy (FTIR, Vertex 70v vacuum-based, Billerica, MA, USA) in a range of 4000–400 cm⁻¹, and X-ray photoelectron spectroscopy (XPS, Kratos Analytical Ltd., Manchester, UK); the surface area, pore diameter, pore volume, and external surface area were calculated through the adsorption–desorption isotherm of nitrogen at −196 °C and the BET (Brunauer–Emmett–Teller) and BJH (Barret–Joyner–Halenda) methods of surface area analysis (Gemini VII 2390, Micromeritics, Norcross, GA, USA).

2.3. Adsorption Test of Chromium

A series of batch tests involving chromium adsorption were performed. Different concentrations of Cr (VI) were initially prepared from a potassium dichromate stock solution. (K₂Cr₂O₇) (Sigma Aldrich). Subsequently, 500 mg of zeolite was added to 50 mL of the Cr (VI) solution and stirred for 24 h at RT. The liquid and solid solutions were then separated using a 0.45 µm syringe filter. Finally, the liquid solution was analyzed by ICP-SFMS (USA) in a certified analytical laboratory (ALS Scandinavia, Luleå, Sweden), and the solid—in this case, chromium-loaded SMBZ—was characterized by the techniques mentioned in the previous section. The experiments were replicated three times, and the resulting values were averaged.

The effect of pH (4–10) and temperature (25–45 °C) on adsorption was investigated. The pH of the solution was adjusted using 0.5 M solutions of HCl and NaOH.

The equilibrium adsorption capacity of chromium on BZ and SMBZ (${\mathrm{q}}_e$), and adsorption percentage ($\mathrm{R}$) were calculated using Equations (1) and (2), respectively.

$${\mathrm{q}}_e=\frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)\mathrm{V}}{\mathrm{W}}$$

(1)

$$\mathrm{R}=\frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)}{{\mathrm{C}}_0}\times 100$$

(2)

The terms ${\mathrm{C}}_0$ and ${\mathrm{C}}_{\mathrm{e}}$ are the initial and equilibrium concentrations of Cr (VI) in the solution (mg/L), $\mathrm{V}$ is the volume of solution (L), and $\mathrm{W}$ is the weight of adsorbent (g).

The isotherm study was conducted with varying concentrations of Cr (VI) (30, 60, 90, 120, and 180 mg/L). These isotherms were analyzed using the Langmuir and Freundlich adsorption isotherm models [46,47], which are expressed in the following equations. By applying these models, the aim was to obtain a deeper insight into the adsorption behavior of Cr (VI) and evaluate the potential of the adsorbent material under different concentrations.

$$\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{q}}_{\mathrm{m}}{\mathrm{k}}_{\mathrm{L}}}+\left(\frac{1}{\mathrm{n}}\right){\mathrm{C}}_{\mathrm{e}}$$

(3)

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{k}}_{\mathrm{F}}{{\mathrm{C}}_{\mathrm{e}}}^{\frac{1}{\mathrm{n}}}$$

(4)

$${\mathrm{R}}_{\mathrm{L}}=\frac{1}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_0}$$

(5)

The equilibrium concentration (mg/L) is denoted as ${\mathrm{C}}_{\mathrm{e}}$, ${\mathrm{q}}_{\mathrm{e}}$ is the equilibrium adsorption (mg/g), ${\mathrm{q}}_{\mathrm{m}}$ is the maximum adsorption capacity (mg/g), ${\mathrm{k}}_{\mathrm{L}}$ is the Langmuir isotherm constant (L/mg), ${\mathrm{k}}_{\mathrm{F}}$ is the Freundlich isotherm constant (mg/g), and the parameter of 1/n represents the heterogeneity of the adsorbent sites and indicates the affinity between adsorbate and adsorbent.

In the case of Langmuir adsorption, the parameter ${\mathrm{R}}_{\mathrm{L}}$ (Equation (5)) can determine if the adsorption is favorable or unfavorable. If the value is between 0 and 1 the adsorption is favorable. On the contrary, if the value is higher than 1 the adsorption is unfavorable.

The interpretation of the adsorption kinetics was achieved through the linear equations of the pseudo-first order (Equation (6)), pseudo-second order (Equation (7)), order and intraparticle diffusion (Equation (8)), and kinetics models [48].

$$\mathrm{l}\mathrm{o}\mathrm{g} \mathrm{l}\mathrm{o}\mathrm{g}\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)=\mathrm{l}\mathrm{o}\mathrm{g} \mathrm{l}\mathrm{o}\mathrm{g}{\mathrm{q}}_{\mathrm{e}}-\frac{{\mathrm{k}}_1}{2.303}\mathrm{t}$$

(6)

$$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{k}}_2{{\mathrm{q}}_{\mathrm{e}}}^2}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}$$

(7)

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{i}\mathrm{d}}{\mathrm{t}}^{0.5}+\mathrm{C}$$

(8)

The parameters of ${\mathrm{q}}_{\mathrm{e}}$ and ${\mathrm{q}}_{\mathrm{t}}$ are the adsorption amount of Cr (VI) at equilibrium and adsorption time $\mathrm{t}$ (mg/g), ${\mathrm{k}}_1$ is the adsorption rate constant of pseudo first-order kinetics (L/min), ${\mathrm{k}}_2$ is the adsorption rate constant of pseudo-second order (g/mg m), ${\mathrm{k}}_{\mathrm{i}\mathrm{d}}$ is the diffusion model rate constant of intraparticle diffusion (mg/g min^0.5), and $\mathrm{C}$ is the intercept.

Additionally, the thermodynamic study was carried out at 298, 313, and 323 K temperatures. The parameters of Gibbs free energy ($∆{\mathrm{G}}^{°}$) (KJ/mol), enthalpy change ($∆{\mathrm{H}}^{°}$) (KJ/mol), and entropy change ($∆{\mathrm{S}}^{°}$) ((J/mol K) were calculated by the following equations.

$${\mathrm{K}}_{\mathrm{D}}=\frac{{\mathrm{q}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{e}}}$$

(9)

$$∆{\mathrm{G}}^{°}=-\mathrm{R}\mathrm{T}\mathrm{l}\mathrm{n}{\mathrm{K}}_{\mathrm{D}}$$

(10)

$$\mathrm{l}\mathrm{n}{\mathrm{K}}_{\mathrm{D}}=\frac{∆{\mathrm{S}}^{°}}{\mathrm{R}}-\frac{∆{\mathrm{H}}^{°}}{\mathrm{R}\mathrm{T}}$$

(11)

The constant at equilibrium is represented as ${\mathrm{K}}_{\mathrm{D}}$, ${\mathrm{C}}_{\mathrm{e}}$ is the concentration of chromium in solution at equilibrium (mg/L), ${\mathrm{q}}_{\mathrm{e}}$ is the adsorption amount of Cr (VI) at equilibrium (mg/g), $\mathrm{R}$ is the gas constant (8.314 J/mol K), and $\mathrm{T}$ is the temperature (K).

3. Results and Discussion

3.1. Structure and Properties of SMBZ

According to the results obtained from SEM images (Figure 1a), a stacked flake structure is observed on BZ. However, after modification, the stacked flakes are not clearly distinguishable, which would indicate the adhesion of the cationic surfactant on the BZ surface. The loading of the surfactant on the BZ surface was verified through the FTIR spectrum (Figure 1b) in which the appearance of three bands was detected at 2922 cm⁻¹, 2852 cm⁻¹, and 1452 cm⁻¹ corresponding to the asymmetric and symmetric stretching vibration of the hexadecyl chain of HTDMA-Br, and the stretching of N-H, respectively [49]. Moreover, the characteristic peaks of zeolites at 3625 cm⁻¹, 3427 cm⁻¹, 1639 cm⁻¹, 1207 cm⁻¹, 1051 cm⁻¹, 794 cm⁻¹, 730 cm⁻¹, 671 cm⁻¹, 609 cm⁻¹, and 460 cm⁻¹ did not show change, which is in agreement with the results of previous studies [50]. Also, X-ray diffractograms (Figure 1c) ratified the preservation of the natural zeolite structure since no alteration to the crystal structure of BZ was observed after its modification, indicating that the interaction mechanism between HTDMA-Br and the natural zeolite was by ion exchange, where quaternary ammonium cations replaced the natural zeolite cations. Similar results have been previously reported [32].

In contrast, the textural properties of BZ were modified due to surfactant loading on the surface and cavities of BZ [50], and showed a decrease in surface area (25.93 to 15.04 m²/g), pore diameter (13.88 to 3.93 Å), and pore volume (0.0051 to 0.0029 cm³/g), for BZ and SMBZ respectively.

3.2. Adsorption Behavior of Chromium on SMBZ

BZ has shown no affinity for chromium uptake. Therefore, the results and discussion were focused on chromium adsorption in SMBZ.

3.2.1. Effect of pH and Initial Concentration

The effect of the initial concentration and pH were evaluated in the adsorption process of Cr (VI) on SMBZ (Figure 2). According to Figure 2a, a proportional increase between the adsorption capacity of SMBZ and the initial chromium concentration was observed. This phenomenon is due to the increase in the mass propulsive force which enhance the interaction between SMBZ, and the chromium solution as previously reported [51,52,53,54]. In the case of the pH parameter, a decrease in the adsorption capacity of Cr (VI) was observed at higher pH values (Figure 2b), which indicates higher affinity for chromium in acidic media with the presence of the predominant HCrO₄⁻ coexisting with Cr₂O₇²⁻ species as can be observed in the Pourbaix diagram of Cr (Figure 2c) [55,56]. In addition, as the pH increases, two changes occur both in the chromium speciation that appears as CrO₄²⁻ (Figure 2c) and in the increase of the negative charge on the surface of SMBZ that causes a repulsion between the new chromium species, which results in the decrease of chromium adsorption [37,57].

3.2.2. Adsorption Modeling of Chromium on SMBZ

Isotherm Modeling

Langmuir’s and Freundlich’s adsorption isotherm models were applied to analyze chromium adsorption data on SMBZ. According to Figure 3a,b and

Table 1. Adsorption isotherms of SMBZ for the adsorption of chromium.

Sample	Langmuir			Freundlich
	${\mathbf{q}}_{\mathbf{m}}$ (mg/g)	${\mathbf{k}}_{\mathbf{L}}$ (L/mg)	R2	$\mathbf{n}$ (L/mg)	${\mathbf{k}}_{\mathbf{F}}$ (mg/g)	R2
SMBZ	17	0.85422	0.997	2.718	4.198	0.791

, the Langmuir model showed a higher correlation factor R² (0.997), indicating a monolayer adsorption of chromium occurring at identical and equivalent defined localized sites [52,55], and suggesting chemical adsorption as the adsorption mechanism [59,60]. The value of R_L calculated using Equation (5) was 0.006, indicating that the adsorption is favorable. In addition,

Table 2. Adsorption capacity of different surfactant modified natural zeolites for Cr (VI).

Adsorbate	Adsorbent	${\mathbf{q}}_{\mathbf{m}}$ (mg/g)	Reference
Cr (VI)	Mexican natural zeolite modified by HTDMA-Br	5.07	[42]
	Korean and Japanese natural zeolite modified by HTDMA-Br	3.55–8.83	[39]
	Mexican natural zeolite modified by HTDMA-Br	0.9–1.05	[61]
	Chinese natural zeolite modified by CTMAB and CPB	0.3–2	[62]
	Mexican natural zeolite modified by HTDMA	9.83	[40]
	Bolivian natural zeolite	No-affinity towards Cr (VI)	This work
	Bolivian natural zeolite modified by HTDMA-Br	17	This work

Note(s): CTMAB: cetyltrimethylammonium, CPB: cetylpyridinium bromide.

compares previous studies on chromium adsorption on natural zeolites from other parts of the world to SMBZ, where SMBZ showed higher adsorption capacity towards chromium, which may be due to a higher number of anionic sites of SMBZ and chromium anions in the solution.

Kinetic Modeling

As can be seen in Figure 3c, chromium was adsorbed speedily in SMBZ within the first 30 min of contact, reaching equilibrium at 60 min, which shows two adsorption stages (Figure 3f). The mentioned change during the adsorption process could be due to the availability of adsorption sites at the beginning of the process and its subsequent occupation with chromium [63]. Likewise, to better understand the mechanism of chromium adsorption on SMBZ and to obtain kinetic data, the pseudo-first order (Equation (5)), and pseudo-second order (Equation (6)) kinetic models, and intra-particle diffusion (Equation (7)) were applied. The experimental data (

Table 3. Adsorption kinetics of SMBZ for the adsorption of chromium.

Pseudo-First Order			Pseudo-Second Order			Intraparticle Diffusion			${{\mathbf{q}}_{\mathbf{e}}}_{\mathbf{e}\mathbf{x}\mathbf{p}}$ (mg/g)
${\mathbf{k}}_1$ (/min)	${{\mathbf{q}}_{\mathbf{e}}}_{\mathbf{c}\mathbf{a}\mathbf{l}}$ (mg/g)	R2	${\mathbf{k}}_2$ (g/mg min)	${{\mathbf{q}}_{\mathbf{e}}}_{\mathbf{c}\mathbf{a}\mathbf{l}}$ (mg/g)	R2	${\mathbf{k}}_{\mathbf{i}\mathbf{d}}$ (mg/g min0.5)	$\mathbf{C}$	R2
0.0506	1.58	0.768	0.076	16.33	1	0.118	14.9	0.799	16.17

) established that the pseudo-second order kinetic model gives the best fit with a correlation factor (R²) of 1 for all the ranges of concentrations. Also, the adsorption capacities obtained experimentally (2.98, 5.93, 8.81, 11.90, 16.17 mg/g) are similar to the adsorption capacities obtained by applying the second-order kinetic model (2.98, 5.92, 8.82, 11.87, 16.33 mg/g), which is not the case with the first-order kinetic model. According to previous studies, the similarity in the experimental and calculated adsorption capacity results, as well as the fit of the data with the pseudo-second order kinetic model, would propose chemisorption as the main adsorption mechanism [37,64].

3.3. Thermodynamics

In the thermodynamic study for Cr (VI) adsorption using SMBZ, the change of enthalpy ($∆{\mathrm{H}}^{°}$), entropy ($∆{\mathrm{S}}^{°}$), and Gibbs energy ($∆{\mathrm{G}}^{°}$) parameters were investigated where ΔH° and $∆{\mathrm{S}}^{°}$ were calculated from the slope and intercept of the Van’t Hoff plot of ln ${\mathrm{K}}_{\mathrm{D}}$ versus 1/T. The data in

Table 4. Thermodynamic parameters for adsorption of chromium on SMBZ.

∆H° (KJ/mol)	∆S° (J/K mol)	∆G° (KJ/mol)
		298 K	313 K	323 K
−123.57	415.49	306.60	−8003.83	−9659.19

revealed a negative value of ($∆{\mathrm{H}}^{°}$ and a positive value of $∆{\mathrm{S}}^{°}$, indicating an exothermic process and a favorable affinity of SMBZ towards chromium, respectively [65,66,67]. Also, according to previous research, it is indicated that if the value of $∆{\mathrm{H}}^{°}$ is between 20.9–418.4 KJ/mol, chemisorption is suggested as the mechanism. Therefore, as the calculated value of $∆{\mathrm{H}}^{°}$ is within the mentioned range (−123.57 KJ/mol) it could be indicated that the prevailing mechanism for chromium adsorption on SMBZ is chemisorption [68]. In addition, a change in Gibbs energy from a positive to a negative value could be identified, which indicates that the reaction is nonspontaneous at 298 K and spontaneous (favorable) at 313 and 323 K [69,70,71].

3.4. Mechanism Analysis of Cr (VI) Adsorption on SMBZ

The adsorption mechanism of Cr (VI) on SMBZ was evaluated using XPS and FTIR. The adsorption of chromium on SMBZ was initially verified by the appearance of two new bands in the XPS spectra between 573 and 593 eV, corresponding to Cr 2p _3/2 and Cr 2p _1/2, respectively (Figure 4a) [72]. In addition, a small increase in band intensity was identified between 1000 and 700 cm⁻¹ in the FTIR spectra (Figure 4c), confirming the presence of Cr (VI) as preliminarily reported [73]. No bands related to water deformation vibrations were detected, as the samples were oven dried at 70 °C to remove moisture prior to FTIR characterization.

According to the results obtained, three main mechanisms of chromium adsorption on SMBZ were identified (Figure 5). The first mechanism is the chemical reduction of Cr (VI) to Cr (IV) and Cr (III), as shown in Figure 4b. This process involves electron transfer by the functional groups, where Cr (IV) acts as an intermediate state before its final reduction to Cr (III) [74,75].

The second mechanism identified is electrostatic attraction, which occurs due to the positive charge of the surfactant-modified zeolite (SMBZ), which attracts chromium anions. This mechanism was evidenced by the small shifts observed in the band positions of the SMBZ functional groups, as shown in Figure 4c. These shifts indicate an interaction between the chromium anions and the positive charges on the SMBZ surface [32,76].

The third mechanism identified is anion exchange, in which the bromide anions present in the SMBZ structure are replaced by chromium anions. This mechanism was corroborated by the observation of a reduction in the amount of bromide after chromium adsorption, confirming the anion exchange process, as shown in Figure 4a [40,74].

4. Conclusions

In this study, BZ was modify using HTDMA-Br, and subsequently characterized through the utilization of XRD, SEM, and FTIR techniques. SMBZ was applied as adsorbent in the adsorption of Cr (VI) from water. The Langmuir isotherm model demonstrated the most optimal agreement with the equilibrium data, exhibiting a maximum adsorption capacity of 17 mg/g for Cr (VI). The second-order kinetic model was considered the most feasible for describing the kinetic behavior. Furthermore, XPS and FTIR analysis indicated that the mechanisms involved in this process include anion exchange, electrostatic attraction, and reduction. The process of Cr (VI) adsorption on SMBZ was found to be exothermic. These results demonstrate the satisfactory efficiency in the application of SMBZ in the adsorption of Cr (VI). However, adsorption tests in real waters are necessary for its implementation at pilot scale.