Sorptive Removal of Color Dye Safranin O by Fibrous Clay Minerals and Zeolites

The increased use of color dyes in industry imposes a great threat to the environment. As such, developing cost-eﬀective techniques for dye removal from wastewater attracted great attention. Earth materials, particularly those with large speciﬁc surface area (SSA) and high cation exchange capacity (CEC), were evaluated for their potential use for wastewater treatment. In this study, palygorskite, sepiolite, and clinoptilolite were evaluated for their removal of cationic dyes using safranin O (SO + ) as a model compound. The CEC values of the materials played a key role in SO + removal while other physicochemical conditions, such as temperature, equilibrium solution pH, and ionic strength, had less inﬂuence on SO + removal. Sorbed SO + cations were limited to the external surfaces of the minerals, as their channel sizes are less than the size of SO + cation. Molecular dynamic simulations showed dense monolayer SO + uptake on palygorskite due to its relatively large CEC value. In contrast, loosely packed monomer SO + uptake was adopted on sepiolite for its large SSA and low CEC. Dense multilayers or admicelles of SO + formed on zeolite surfaces. As such, for the best SO removal, palygorskite is better than sepiolite, though both are ﬁbrous clay minerals.


Introduction
Pigments and dyes are widely used in different types of industries including textile and leather dyeing, paper, printing, pharmaceutical, and cosmetics [1]. As such, largescale production and widespread applications resulted in many synthetic organic dyes to permeate into different compartments of water and soil bodies [2]. To prevent direct discharges of waters containing different types, amounts, and toxicities of dyes that threaten the environment, numerous studies were conducted recently to treat the dyecontaining water [3]. However, as there are more than 10,000 types of dyes used and each may have different physicochemical properties [4], and different approaches were undertaken for dye removal including physical [3], chemical [4], and biological [5], effective methods for removal of a particular dye might be limited. In addition, most of the studies were focused on developing new materials, mostly nanomaterials [3,6], due to their large specific surface area (SSA) and high reactivity. However, high material costs may limit their immediate practical values.
Earth materials have long been used to sorb color dyes for painting purposes. A typical example is Maya blue, a mixture of indigo dyes with palygorskite, used to color sculpture, artwork, and textiles around the Aztec area in Mexico over 1200 years. Due to the skyrocketing use of color dyes and inexpensive material cost, earth material was studied extensively for the removal of color dyes, most of them were of cationic and zwitterionic in nature, from solution because of large SSA and high cation exchange capacity (CEC) of many phyllo-and tectosilicates, such as different types of clay minerals and zeolites.
Safranin O (SO), also called Basic Red 2, Cotton Red, Gossypimine, Safranin T, and Safranin Y or A, is a watersoluble reddish-brown powder [7], commonly used in tracer and biological laboratory purposes [8]. As such, methods tested for its removal from water include physicochemical, biological, and chemical methods [9].
Studies on SO removal by clay minerals were limited. Montmorillonite (MMT), due to its high CEC and SSA values, could sorb SO to a capacity of 380 mg/g or 1.1 mmol/ g [10]. In a different study, SO removal capacity by bentonite collected from Khulais, Saudi Arabia, was 294 mg/g or 1.1 mmol/g [11], but neither the CEC value of the clay nor the mechanism of SO removal was discussed. An SO removal capacity of 24 mg/g or 68 mmol/kg was reported for a Saudi Arabia natural clay [8]; however, the major components of the clay were quartz, chamosite, albite, dickite, and microcline as determined by X-ray diffraction analyses, and the mechanism of SO removal was not mentioned.
SO sorption capacity on kaolinite was up to 16 mg/g or 45 mmol/kg as reported in one study [12]. On a different kaolinite, SO sorption capacity as high as 160 mmol/kg was also reported, and the relatively hydrophobic silica face of kaolinite contributed more to the SO sorption and aggregation [13]. SO sorption on a raw ferruginous kaolinite was up to 59.3 mg/g or 168 mmol/kg and was attributed to physical behavior, heterogeneous nature, and multilayer formation [14].
Palygorskite (PAL) and sepiolite (SEP) are fibrous phyllosilicates. Both have channels running parallel to the fiber length with channel sizes of 0.37 by 0.64 nm for SEP and 0.37 by 1.06 nm for SEP that are accessible by inorganic cations [15]. us, not only the surfaces but also the channel area is responsible for cation exchanges and specific sorption of cations. In one study, an SO sorption capacity of 200 mg/g was found on a natural Iraqi PAL, but the CEC value of the PAL was not reported [16]. No studies of SO sorption on natural sepiolite were reported, but an SO sorption capacity of 18.48 mg/g was reported on iron oxide/sepiolite magnetite composite (MSep) [17].
Zeolite is a group of minerals in tectosilicates with abundant resources, available globally, and has already found many applications because of its high CEC and SSA [18]. A clinoptilolite zeolite (CZ) was studied for the sorption and release of photodynamically active dyes hypericin (Hyp), methylene blue (MB), chlorin e6, Alphthalocyanine, and fluorescein under multiphoton microscopy, and the results suggested even distribution of the dyes on the surface of CZ with the potential release of Hyp from CZ surfaces in the presence of biomolecules, indicating that CZ could serve as an effective material for drug delivery and controlled release in biological systems [19]. Sorption of SO on CZ showed a capacity of 0.08 mg/g, and the sorption increased with initial solution pH [20], but the mechanism was not discussed. In a different study, even for the CZ modified by iron oxide nanoparticles, the MB sorption capacity was still at 52 mg/g, corresponding to 170 mmol/kg, and again, the mechanism of MB removal was not discussed [21]. Sorption of SO on a CZ from Yemen resulted in a capacity of 43 mg/g or 120 mmol/kg, and the SO uptake on ZEO was attributed to cation exchange and chemisorption [22]. On the opposite, after acid modification, the uptake of a color dye Red 24 on a CZ was only 0.9 mmol/kg [23]. A magnetic zeolite NaA achieved an SO removal capacity of 149 mg/g or 425 mmol/kg after sodium dodecyl sulfate (SDS) modification [18], while the purpose of SDS coating was not mentioned, nor noncoating zeolite as controls was compared.
is study aims to use the selected phyllo-and tectosilicates with open channels to assess their SO removal from water under different physicochemical conditions and to correlate changes in material properties of the sorbents after SO uptake as characterized by different instruments. In addition, the interactions between SO and the minerals were simulated by molecular dynamic simulations. e configuration of sorbed SO + cations on the mineral surfaces was derived from the sorption study and instrumental analyses in conjunction with the simulation. en, the SO removal mechanism by the minerals was proposed.

Materials.
e SO used is in a chloride form (CAS number of 477-73-6, M � 350.84 g/mol). Literature values are 50 g/L for solubility [24] and 11 for pKa [13]. Molecular sizewise, it is 1.16 nm long by 0.96 nm wide by 0.4 nm thick, as determined by a molecular editor Avogadro (https:// avogadro.cc/) ( Figure 1). e minerals used include fibrous phyllosilicates palygorskite (PAL) and sepiolite (SEP) and tectosilicate zeolite (ZEO), all with channel structures where sorption of inorganic cations is attainable. e PAL used is PFl-1, and the SEP used is SepSp-1. Both were reference clays obtained from the Source Clay Minerals Repository. e ZEO used is clinoptilolite obtained from the St. Cloud Mine in Winston, NM. e CEC values are 175 [25] and 15 [26] meq/kg, for PFl-1 and SpeSp-1, respectively, while their SSA values are 173 [27] and 250 m 2 /g [26]. For ZEO, the external CEC (ECEC) is 90 meq/kg and the SSA is 15.7 m 2 /g [28].

SO Uptake.
For SO removal by PAL and ZEO, 0.25 g of solid and 10 mL of SO solution were added to each 50 mL centrifuge tube. For SEP, 0.5 g of solid and 10 mL of solution were used due to its lower CEC value. e initial concentrations for the isotherm study varied from 0.3 to 2.5 mM for ZEO and SPE and up to 5 mM for PAL, and the mixtures were shaken on a reciprocal shaker at 150 rpm and room temperature (23 ± 1°C) for 24 h. For the kinetic study, mixing lasted from 0.1 to 24 h. To investigate the influence of solution pH on SO removal, the equilibrium pH was adjusted multiple times during the 24 h period to achieve final values between 3 and 11 with an interval close to 1. For the influence of solution ionic strength on SO removal, the SO solution was made with 0.001, 0.01, 0.1, and 1.0 M of NaCl instead of DI water. e influence of temperature on SO removal was tested at 23, 33, 43, and 53°C with a deviation of less than 1°C. In all these studies, the initial SO concentrations were fixed at 4.5, 2.0, and 1.0 mM for PAL, ZEO, and SEP, respectively, and shaken for 24 h. After mixing, samples were centrifuged at 3500 rpm for 10 min and the supernatants passed through 0.45 μm syringe filters before being analyzed for equilibrium SO concentrations using a UV-Vis method. e amount of SO sorbed was determined by the difference between the initial and equilibrium SO concentrations. Duplicates were tested for each experimental condition.

Instrumental Analyses.
e equilibrium SO concentrations were determined by a UV-Vis spectrophotometer. e wavelength used was 516 nm [29,30]. To assess the counterion Cl − concentration in solution, an ion chromatography (IC) with a PRP-100 anion exchange column was used. Calibration was made using SO concentrations of 0.1 to 5 mM with an r 2 of 0.999.
A Shimadzu 6100 X-ray diffractometer was used for X-ray diffraction (XRD) analyses of the samples after sorption experiments. A Ni-filtered CuKα radiation at 30 kV and 40 mA and a scanning speed of 2°min were set for the experimental conditions, and the samples were scanned from 5 to 40°(2θ). e FTIR spectra were acquired on a Shimadzu IRAffinity-1S FTIR spectrometer equipped with a quartz attenuated total reflection device from 400 to 4000 cm −1 with a resolution of 4 cm −1 .

Molecular Dynamic Simulation.
e molecular dynamic simulation was performed using the FORCITE module in Materials Studio 6.0 software to determine the surface configurations of sorbed SO + cations on different surfaces of the minerals. e number of SO + cations used for the simulation was based on the SSA of the minerals and the SO sorption capacity on each mineral. e unit cell parameters used were as follows: a � 12.78Å, b � 17.86Å, c � 5.24Å, β � 95.78°, and Z � 4 for PAL; a � 13.43Å, b � 26.88Å, c � 5.281Å, and Z � 4 for SEP; and a � 17.688Å, b � 17.902Å, c � 7.409Å, β � 116.5°, and Z � 4 for ZEO. Also, the supercells were made of 2bx4c for PAL and SEP due to their layered structure and 2bx4c, 2ax4c, and 2ax1.5b for ZEO due to its 3D structure. e constructed model was optimized geometrically.

Results and Discussion
3.1. Kinetics of SO Removal. SO removal by these minerals was relatively fast, and equilibrium could be reached within an hour for PAL and SEP (Figure 2(a)). In contrast, it may take up to 8 h to reach equilibrium for ZEO. is could be due to its larger particle size, and thus, longer diffusion time is needed to reach equilibrium. Similarly, uptake of SO on CMC-41 was thought as diffusion controlled [7]. e results were fitted to several kinetic models, and the pseudosecond-order kinetics fitted the data best. It has the following form: that can be rearranged into a linear form as follows: where k (kg/mmol-h) and kq 2 e (mmol/kg-h) are the rate constant and initial rate of SO removal by the minerals, while q t and q e (mmol/kg) are the amount of SO uptake at time t and at equilibrium. e initial rates and the rate constants are 1379, 213, and 23256 mmol/kg-h and 3.5, 0.04, and 0.7 kg/mmol-h for SO uptake on SEP, ZEO, and PAL, respectively. Meanwhile, calculated q e values were 20, 76, and 180 mmol/kg for SO uptake on SEP, ZEO, and PAL, respectively, with the coefficients of determination r 2 all greater than 0.999 (inset of Figure 2(a)). e very high rate, large rate constant, and larger q e for SO uptake on PAL correlated to its larger CEC and SSA values well. In comparison, the slow SO removal by ZEO confirmed the speculation that diffusion played an important role. In a previous study, SO sorption on MSep followed the pseudosecond-order kinetics with a k of 0.19 g/mg-min [17].

Isotherms of SO Uptake.
Uptake of SO by the tested minerals is illustrated in Figure 2(b). Results were fitted to both the Freundlich and Langmuir models, and the latter had a better fit. e Langmuir isotherm has the following form: that can be converted into a linear form as follows:  [28].
A previous study showed an SO sorption capacity of 200 mg/g or 570 mmol/kg on PAL [16]. Sorption of MB on a PAL showed a capacity of 200 mmol/kg [31]. However, the CEC and SSA values and the purity of the mineral in these studies were not provided. Moreover, the data in their isotherm study only showed a capacity slightly over 100 mg/ g or about 300 mmol/kg [16]. SO sorption on MSep also followed the Langmuir isotherm with a sorption capacity of 18.48 mg/g [17]. In a previous study, the SO uptake on CZ was 43 mg/g or 120 mmol/kg and was attributed to cation exchange and chemisorption [22]. e SO sorption on a synthetic ZEO was only 9.4 mg/g [32]. On a different type of  ZEO, the SO sorption only reached a capacity of 1.3 mg/g [33]. Even lower, sorption of SO on CZ was fitted to the Langmuir isotherm with a capacity of 0.055 mg/g, corresponding to only 0.155 mmol/kg [20], in comparison with 79 mmol/kg in this study (Figure 2(b)). As the SO used is SO + Cl − , the equilibrium counterion Cl − concentrations at the SO sorption capacity were about 30% lower than their initial concentrations as measured by ion chromatography, suggesting that some of the SO removal from solution was in a dimeric form balanced by counterion Cl − bridging the two cations. An increase in dimer concentration was associated with increased SO concentration [34].
For CZ, there are three channels of which two channels are running parallel to each other and to the c-axis with a channel consisting of a 10-member ring of a size of 0.44-0.72 nm and another channel consisting of an 8member ring of a size of 0.41-0.47 nm, while the third channel is parallel to a-axis and made of an 8-member ring of a size of 0.40-0.55 nm [35]. As the size of SO is greater than the channel size of the three channels, the uptake of SO is limited to the external surfaces of ZEO. Taking consideration of the limited SSA, the uptake of SO may form a multilayer sorption on ZEO.

Influence of Solution pH, Ionic Strength, and
Temperature on SO Uptake. Essentially, equilibrium solution pH had minimal influence of SO uptake on these minerals (Figure 3(a)). A similar pH effect on SO removal by PAL was reported earlier [16]. As the initial solution pH increased from 2 to 10, the amount of SO sorption on CZ increased from 3.5 to 7 mg/g [20]. Similarly, the SO uptake on ZEO increased from 30 to 60% as the initial solution pH increased from 2 to 10 [33]. A slight increase in SO removal from 82 to 95% was observed for a Yemen CZ [22]. With a pK a value of 11, SO is in a cationic form when equilibrium solution pH is <11. e negligible pH influences on SO + uptake indicated that organic cations have a higher affinity for negatively charged mineral surfaces in comparison with inorganic cations.
Similar to equilibrium solution pH, solution ionic strength also had a minimal influence of SO uptake (Figure 3(b)). A previous study showed that added NaCl, CaCl 2 , Na 2 SO 4 , and NH 4 Cl in the range of 0-10% (w/v) also had minimal influence on SO uptake on modified zeolite [18]. e results suggested again that the organic cation had higher affinity for the negatively charged mineral surfaces in comparison with inorganic cations.
Influence of equilibrium temperature on SO uptake revealed exothermic processes for SO removal using PAL, ZEO, and SEP (Figure 3(c)). e thermodynamic parameters of SO removal can be related to the SO distribution coefficient K d , the ratio of SO sorbed on solids to the equilibrium SO concentration in solution, by where the ΔH and ΔS are the changes in enthalpy and entropy after SO uptake, R is the gas constant, and T is the temperature in K. e ΔH and ΔS values are related to the free energy ΔG of SO uptake by ΔG � ΔH − TΔS. (6) e thermodynamic parameters of SO calculated from equations (5) and (6) showed small ΔS°values, moderate negative ΔH°, and ΔG°values in the ranges of −14 to −24 kJ/ mol (Table 1), indicating net attractive interactions via physical sorption or electrostatic interactions between the solute and sorbents. e ΔG°value was about −15 kJ/mol for SO uptake on MSep, suggesting physisorption [17].

XRD Analyses.
e SO used is crystalline as revealed by the XRD analysis (Figure 4 inset). None of the SO peaks showed up in the XRD patterns of the three minerals after uptake of different amounts of SO, confirming that the SO uptake from solution was via sorption instead of precipitation. Meanwhile, the d-spacings of the minerals did not change after SO uptake, indicating the location of sorbed SO + cations was limited to the external surfaces, although all three minerals have channel structures where the sites for the uptake of inorganic cations are available (Figure 4).

FTIR Analyses.
e full FTIR spectra of the three minerals together with the crystalline SO are displayed in Figure 5. For SO, the broad absorption bands at 3338 and 3183 cm −1 were assigned to the −OH and−NH 2 groups and to the N-H group [36]. However, in this study, they are located at 3313 and 3140 cm −1 (Figure 5). In a different study, bands at 1641 and 1610 cm −1 were assigned to the aromatic ring stretching vibration, 1493 cm −1 to the −CH 3 stretching vibration, and 1334 cm −1 to the aromatic C � N stretching vibration [37]. ese bands were located at 1639, 1609, 1485, and 1323 cm −1 in this study ( Figure 5). Meanwhile, the C � N stretching vibration band was located at 1320 cm −1 [38]. ese bands did not show up after SO uptake on PAL from the initial SO concentration of 0.9 mM, corresponding to an SO uptake of 36 mmol/kg, except the one at 1323 cm −1 . At an initial concentration of 2.0 mM, corresponding to an SO uptake of 80 mmol/kg, the bands at 1485 and 1528 cm −1 started to appear and shifted to 1489 and 1533 cm −1 . Meanwhile, the band at 1323 cm −1 shifted to 1337 cm −1 (Figure 6(a)). At the SO sorption capacity of 198 mmol/kg, the bands remained at the same location (no further band shift was observed), but their intensities increased accordingly. For SEP, the only band showed up was the one at 1323 cm −1 , even at the highest SO input concentration of 2.5 mM, corresponding to an SO uptake of 43 mmol/kg, and the band shifted to 1331 cm −1 after SO uptake on SEP (Figure 6(b)). For ZEO, the 1323 cm −1 band showed up at an initial concentration of 0.9 mM, corresponding to an SO uptake of 36 mmol/kg, and the bands at 1323, 1485, 1533, and 1609 cm −1 all appeared at the SO initial concentration of 1.5 mM, corresponding to an SO uptake of 60 mmol/kg, and Advances in Materials Science and Engineering   (Figure 6(c)). ese shifts, particularly the C � N stretching vibration band located at 1323 cm −1 , confirmed the participation of the C � N bond for the SO uptake on these mineral surfaces ( Figure 6). Also, taking the consideration of the formula mass of 350 g/mol for SO, the sensitivity of FTIR for detection of sorbed SO is at the level of about 40 mmol/kg or 1.4%.
3.6. Molecular Dynamic Simulation. Molecular dynamic simulation was used to decipher the surface configuration of sorbed SO + cations at the sorption capacity for the minerals. For PAL and SEP, a supercell made of 2bx4c was used for the simulation, and the number of SO + cations used per supercell was calculated from the SO sorption capacity and the SSA of the minerals. e sorbed SO + cations formed a condensed monolayer configuration on PAL (Figure 7). In contrast, at the SO sorption maximum, the SO + cations formed monomers on SEP surfaces due to its much lower CEC and large SSA values (Figure 8). In both cases, the phenazine rings are parallel to the surfaces. ZEO is a granular mineral and supercells were made of 2bx4c, 2ax4c, and 2ax1.5b, and the number of SO molecules used per supercell was calculated from the SO sorption capacity and the SSA of the minerals. Multilayer or admicelle formation of SO on ZEO was obtained because of the very low SSA value of ZEO (Figures 9-11).

Discussion.
e CEC values are 175 [25] and 15 [26] meq/kg for PAL and SEP, respectively, while the ECEC for ZEO is 90 meq/kg [28]. Meanwhile, the SO uptake capacity was 195, 45, and 79 mmol/kg for PAL, SEP, and ZEO, respectively (Figure 2(b)), strongly suggesting that cation exchange played a crucial role in SO removal. On the other hand, their SSA values are 173 [27] and 250 m 2 /g [26] and the SSA is 15.7 m 2 /g [28] for PAL, SEP, and ZEO, respectively. As the XRD results showed no changes in dspacings for all minerals (Figure 4), it suggested that the uptake of SO was on the external surfaces of the minerals. Using the CEC and SSA values, the calculated area per SO + occupied at the uptake capacity is 1.5, 9.3, and 0.33 nm 2 for PAL, SEP, and ZEO, respectively. Meanwhile, the dimension of SO molecule is 1.16 nm long by 0.96 nm wide by 0.4 nm thick (Figure 1). e molecular simulation showed flat-lying SO + cations on {100} surface of PAL and SEP (Figures 7 and 8). us, a close monolayer packing of SO + on PAL and loose monomer packing of SO + on SEP suggested that the influence of SSA on SO uptake may not be critical. For ZEO, the SO sorption density was 0.33 nm 2 per SO + cation, much smaller than the dimension of SO. As SO may form dimers in solution and its dimer concentration increased with increasing SO concentration [34], formation of SO multilayer or admicelles on ZEO is extremely likely due to its limited SSA and large ECEC and was confirmed by the molecular dynamic simulation (Figures 9-11).

Conclusion
In this study, the sorptive removal of SO using fibrous clay minerals palygorskite and sepiolite and tectosilicate clinoptilolite zeolite was assessed under different physicochemical conditions. e results revealed that the SO removal capacity by these minerals was primarily controlled by their cation exchange capacity, suggesting cation exchange between SO + and inorganic cations played a key role in SO removal.
Other physicochemical parameters such as equilibrium, solution pH, and ionic strength had minimal effect on SO removal. e sorbed SO + cations may form loosely packed monomers, densely packed monolayer, and multilayer or admicelle formations limited by the specific surface area of the minerals. e XRD results confirmed that the sorbed SO + cations were limited to the external surfaces of the minerals as the dimension of SO + is larger than the channel size of the minerals. e FTIR results suggested participation of N + in the benzene ring for the electrostatic interaction with negatively charged Si-O tetrahedral surface. e electrostatic interactions between SO + and negatively charged mineral surfaces and the SO + surface configurations on mineral surfaces were confirmed by molecular dynamic simulations. As a result, the clay minerals with high CEC and SSA values are good candidates for cationic dye removal from water.

Data Availability
e data used to support the findings of the current study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.