Natural Palygorskite as an Industrial Dye Remover in Single and Binary Systems

Layered or fibrous clay minerals are used as adsorbents because they have good performance to remove inorganic and organic species. In this work, a Brazilian fibrous clay mineral was applied to remove Remazol Yellow GR industrial dye from simple and binary systems; this was performed in the presence of lead in the latter case. The adsorption efficiency was investigated under various conditions of pH, time, temperature and dye concentration. The experimental data were fitted to the kinetic pseudo-firstand pseudo-second-order models, and the isotherms were fitted to the Langmuir and Freundlich models. The maximum adsorption capacity was 20.08 mg g-1 at pH 2, 240 minutes and 298 K, and these data were better fit by the kinetic pseudo-second-order model. The equilibrium study was fitted to the Langmuir model at 298 K and Freundlich for other temperatures and binary systems. The dye sorption in the presence of lead was decreased to 15 mg g-1. Therefore, palygorskite behaved as a promising solid for treating effluents.


Introduction
Environmental problems arise mainly from anthropic actions, during the exploration of natural resources and/or the processing of synthetic materials in the industry, and the consequence is unfavorable changes in ecosystems 1 .
Pollution control, particularly in the textile industries, is highly problematic due to the difficulty in purifying wastewater with a high content of dyes in the liquid effluents.The presence of dyes in water, although not always toxic, is harmful to its aesthetics; therefore, it is a type of visual pollution.Additionally, colorful effluents prevent light transmission for photosynthesis, causing ecological chaos, including in the food chain, which is by far the most critical of the effects produced 2,3 .The chemical substances responsible for the changed water color may also be harmful to human health, causing mutations, cancer, allergies and skin burns 4,5 .
Dyes have complex aromatic structures 6 containing chromophore groups, which produce color, and auxochromes, which interact with the textile fibers 7 .In general, the dyes used in industries include acid, basic, direct, sulfurous, reactive, disperse, and azo 8 , and the final category is more widely applied 9 .The complex structure of dyes is stable and not sensitive to biodegrade 6 .
The anionic dyes are distinguished by their negative charge 10 .Despite having several classes (azo, anthraquinone) and types (reactive, acid and azo) with distinct peculiarities in their structures, they share as a common characteristic the solubility favored by the presence of the ionic substituents 11 .
The adequate treatment of industrial effluent disposal is necessary to reuse the water; therefore, studies have been conducted aiming to obtain adequate materials and methods from the environmental and economic perspectives with optimal dye removal capacity.As a result, several techniques have been used to treat colored wastewater, e.g., flocculation, coagulation, filtration, oxidation (O 3 , UV, H 2 O 2 ), and processes using membranes, electrochemistry, photocatalytic oxidation and adsorption 12 .The advantages and disadvantages of each technique have been intensively studied, and adsorption is regarded as an efficient and economic process to remove dyes and pigments 13 .
Among these materials, clay minerals are very interesting solids because they are natural, abundant, easily extractable, usually nontoxic, and relatively inexpensive.They also have properties, e.g., mechanical-thermal-chemical stability, and their surface can be chemically modified.Their reactivity and pore structure usually determine the optimal adsorption capacity of these materials 28 [29][30][31] .Its structure has zeolitic water weakly bound to the micro-channels, structural water together with coordination water bonded to ions (Mg, Al) in the limits of each octahedral layer, and hydroxyl groups (e.g., Al-OH and Mg-OH) [32][33][34] .Due to the inversion in the tetrahedral silica sheet, the structure of palygorskite has parallel channels in the form of micro-tubes, providing a higher surface area and consequently an excellent adsorption capacity 34 .Therefore, these physical-chemical properties make palygorskite suitable to be used for the sorption of ions and dyes [35][36][37] as well as other organic contaminants [38][39][40] .
In this work, natural palygorskite was used to remove an industrial dye from an aqueous solution.First, the clay mineral sample was characterized by chemical analysis, X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and N 2 adsorption/desorption measurements.Next, adsorption experiments were performed to evaluate the effects of time, pH, dye initial concentration, and temperature.Kinetic and equilibrium models were fitted to the experimental adsorption data.The influence of lead on the dye removal was investigated.

Chemicals and materials
Sodium hydroxide (Synth), ethanol P.A. (Synth), potassium nitrate (Vetec), hydrochloric acid (Sigma), and lead nitrate, Pb(NO 3 ) 2 (Vetec) in analytical grade were used without further treatment.The Brazilian clay mineral sample originated from Guadalupe city, state of Piauí, where it was size-separated in a 200-mesh sieve.Remazol Yellow GR dye was supplied by Brazilian company Dystar Ltda.Deionized water was used in all preparations.

Characterizations
Semi-quantitative chemical analysis was performed in an X-ray fluorescence spectrometer model BOL-FRX.The results were expressed in oxides percentage (%), which was normalized to 100%.Initially, the samples were prepared by fusion in a borate mixture (Li 2 B 4 O 7 -LiBO 2 ) in a 1:10 w/w proportion.X-ray diffraction was performed on a Shimadzu XR-D600A diffractometer, in the 2θ range of 5 to 60º.The scanning rate was 5º min -1 , using the radiation source CuKα, at a wavelength of 1.54 Ǻ.The infrared spectrum was analyzed on an FTIR Bomem spectrophotometer, series MB, by the KBr pellet method containing 1% of the sample, in 96 scans, in the 4000-400 cm -1 range at a resolution of 4 cm -1 .The morphology of the clay particles was analyzed by Scanning Electron Microscopy, after the metallization of the samples with gold and palladium on the JEOL T-300 microscope.The specific surface area was measured by the Quantacrome NOVA 4200 analyzer through nitrogen adsorption/desorption measurements.The dye concentration was determined on the Varian CARY ® 300 UV/VIS Spectrophotometer.

Point of zero charge (pHPZC)
The point of zero charge was determined by the powder addition method 41,42 .A 50 mg aliquot of the palygorskite was suspended in 50.0 mL of 0.001 mol L −1 NaCl solutions.The pH of the system was adjusted using 0.10 mol L −1 HCl or NaOH aqueous solution.The suspensions were then shaken and allowed to equilibrate for 24 h, after which the pH of the supernatant was measured.The pH variation (ΔpH=pH f −pH i ) was plotted against the pH i , and the point of intersection of the resulting curve at which ΔpH=0 was the pH PZC value.

Adsorption Kinetics
In the kinetic study of Remazol Yellow GR dye removal, 30.0 mL of the 1000 mg L -1 solution of the dye was transferred to a series of Erlenmeyer flasks containing approximately 30.0 mg of the adsorbent, in a solution whose natural pH was 6.2.Next, the suspension was shaken at the temperature of 298 ± 2 K in time intervals of 15-300 min.After the pre-established intervals, the supernatant was separated by centrifugation at 3500 rpm for 10 min in a centrifuge (Centribio 80-2B).The dye concentration in the solution, for each time interval, was determined by UV-Vis Spectroscopy (Varian CARY ® 300) at 414 nm, which corresponds to the maximum dye adsorption.The adsorption capacity of the adsorbent (q) was calculated by the Equation 113 , ( ) where q (mg g -1 ) is the adsorbed quantity per gram of adsorbent; Ci and C f (mg L -1 ) are the concentrations before and after adsorption, respectively; V (L) is the solution volume; and m (g) is the adsorbent mass.
The sorption data were fitted to two kinetic models.The linear form of the pseudo-first-order rate equation in integrated form is described by Equation 2 43 , .
( ) where q e,exp (mg g -1 ) is the quantity adsorbed per gram of adsorbent in the equilibrium, q e , cal (mg g -1 ) is the adsorbed theoretical quantity per gram of adsorbents, q t (mg g -1 ) is the adsorbed quantity per gram of adsorbent in the time t (min), and k 1 (min -1 ) is the rate constant of the pseudo-first-order.By plotting log (q e,exp -q t ) versus t, it is possible to calculate the parameters of the pseudo-first-order equation, q e,cal and k 1 , using the angular and linear coefficients.
The pseudo-second-order model is described by Equations 3 44,45 : adsorption constant, which involves the adsorption energy; and q e (mg g -1 ) is the maximum adsorption capacity.
The Freundlich model is applied to a heterogeneous surface and the formation of multi-layers.Its linear form is defined by Equation 647 : ( ) e cal e cal 2

= +
Under the established conditions, the rate constant k 2 and the theoretical equilibrium sorption capacities q ecal were calculated from the slopes and intercepts of the linear plots of the pseudo-second-order kinetic model.

Influence of pH
The pH influence on the Remazol Yellow GR dye removal was verified by testing of the 1000 mg L -1 Remazol Yellow GR dye solution at values pH of 2, 3, 5, 7, 9 and 10, using 0.1 mol L -1 HCl and/or NaOH solutions.Next, 30.0 mL aliquots of each solution and 30.0 mg solid were transferred to Erlenmeyer flasks to shake in a thermostatic bath for 240 min at 298 ± 2 K.Then, the suspension was centrifuged at 3500 rpm for 10 min.The dye concentrations in the supernatant were determined using the UV-Vis Spectrophotometer.The quantity of the dye adsorbed was calculated by Equation 1.
To verify any alteration in the dye structure, the dye solution was analyzed at different pH by UV-Vis spectroscopy at previously described conditions.

Adsorption isotherms
The adsorption isotherms were obtained at 298, 308 and 318 K.The used dye solutions were in the range 100-500 mg L -1 , all adjusted to pH 2.0, which was previously determined to be the optimal pH.Thus, 30.0 mg of the adsorbent was suspended in 30.0 mL of each solution and the system was agitated in a thermostatic bath at the pre-established time.After the equilibrium time, the dye concentration of supernatant was quantified.The experimental adsorption data were fitted to the Langmuir and Freundlich models.
The Langmuir model assumes monolayer formation at homogeneous sites of the adsorbent.The maximum adsorption capacity (q m ) was calculated using the linear form of the Langmuir equation (Eq.4).R L is a parameter that indicates if the process was favorable or unfavorable, and it was obtained using Equation 546 : where q m (mg g -1 ) is the maximum adsorption capacity for the formation of the monolayer; C e (mg L -1 ) is the dye concentration in the equilibrium solution; K L is the Langmuir where q e represents the maximum adsorbed capacity in the equilibrium, C e is the equilibrium concentration, K F is the adsorption constant related to the bonding energy and n F is associated with the surface heterogeneity.

Influence of Pb 2+ ion on the dye adsorption
Adsorption was verified for a binary mixture (yellow dye and lead) at the same optimized conditions as the single experiments.Thus, 30.0 mg and 30.0 mL of the binary solution (dye or metal) at each substance concentration of 100 to 1000 mg L -1 were transferred to Erlenmeyer flasks, and the suspension was shaken in a thermostatic bath.The dye concentration was determined as described above.
The diffractograms of the natural palygorskite before and after the sorption of Remazol Yellow GR dye are shown in Figure 1i.The reflections were indexed with the JCPDS-ICCD crystallographic cards 31-0783 (palygorskite) and 85-0794 (quartz).The XRD of the pristine palygorskite shown typical indexed planes (110), (040), (121) and (161).The XRD patterns are similar to the ones published by Frini-Srasra and collaborators 48 .After the dye sorption, some alterations in the intensity of the (110), (040), ( 121) and (161) crystallographic planes were clearly observed.Evaluation of the crystallinity was accomplished based on the ratios of the intensity of the reflection of the palygorskite phase and the quartz one, (I quartz /I clay ) before and after dye interaction as shown in Table 1.A significant change in ratios was observed for the sample after dye sorption, suggesting an alteration in the original crystallinity of the inorganic matrix as the dye removal occurs.The infrared spectra (Figure 1ii) of the natural palygorskite showed typical absorption bands at 3617 and 3543 cm -1 , which were assigned to the stretching of the hydroxyl groups bound to the Mg 2+ and Al 3+ octahedral cations of the inner surface.The groups are characteristic of the channel ends.The broad band centered at 3405 cm -1 was attributed to the stretching of the O-H associated to zeolitic water of the clay mineral 49 .The band centered at 1654 cm -1 was assigned to the bending deformation of the zeolitic water molecules 49 .Typical bands at 1030 and 983 cm -1 were assigned to siloxane stretching, and the bands at 519 and 478 cm -1 were attributed to the vibrational deformation of the Si-O group on the tetrahedral sheet 50 .The band at 914 cm -1 was attributed to M-OH deformation 51 .These data were fundamental to confirm the palygorskite structure.
The SEM images of the palygorskite (Figure 2) showed the fibrous morphology of the particles 52 .The specific surface area of clay minerals and other materials is one of the most important properties in controlling surface phenomena.Normally microporous and fibrous clay minerals, e.g., sepiolite and palygorskite, and expanded clay minerals have high surface area 52,53 .In this case, the surface area of the natural palygorskite was 113 m² g -1 , which is consistent with palygorskite samples from Algeria (125 m² g -1 ) 52 , Spain(112 m² g -1 ) 53 and China (119 m² g -1 ) 54 .

Dye adsorption
pH PCZ measurements determine the pH influence on the adsorbent surface by protonation/deprotonation mechanisms 5,55,56 .Thus, acid or basic pH will interfere in the adsorption depending on the systems individual characteristics (adsorbent and adsorbate).Figure 3 shows that the point of zero charge was approximately 8. In other words, at this pH, the palygorskite surface has zero charge.Therefore, at pH above 8.0, the surface will be negatively charged with affinity to cationic species.For pH lower than 8, the opposite will occur.adsorption process was described by a pseudo-second-order kinetic, as indicated by the R 2 and the low difference between the calculated and experimental maximum capacities, q e,exp and q e,cal .Similar results are frequently cited in the literature on dye adsorption 5,14 .The Figure 5 presents the pH influence on dye-palygorskite interaction.At high pH, Remazol Yellow GR dye removal decreased due to deprotonation of palygorskite, which does not favor interaction with anionic species, such as dye in aqueous solution.At pH 2, the highest dye removal occurred because at lower pH, the palygorskite surface is protonated, which is favorable to its electrostatic interaction with the anionic reactive groups of Remazol Yellow GR dye 57 .Therefore, based on these results, a dye-clay reaction mechanism in an acid medium was proposed, as shown in Figure 6, whose structure of the dye at pH 2 was determined in software Marwin beans ® , with 100% probability of being dissociated species in the finish sulphonic groups, with no other change in the structure.From the results obtained at zero charge potential, it is shown that below pH 8 the clay surface is protonated and as prevailing in the clay groups are hydroxyl groups, these groups is will be protonated, as indicated in Figure 6.The reaction sequence shows the initial solubility of the reactive groups in an acid medium, forming nucleophilic species.As natural palygorskite in acid medium has electrophilic character, the dye-clay electrostatic interaction is predominant.
The UV-Vis spectra of the dye (Figure 7) showed the behavior of Remazol Yellow GR dye absorption after the pH adjustment (2, 3, 5, 7, 9 and 10).No alteration was observed in the absorptions.The maximum wavelength of Remazol Yellow GR dye after the pH alteration is the same, and no divergences were detected in the spectra.
The influence of temperature on adsorption is illustrated in Figure 8.The maximum adsorption capacities were 20.08, 15.00 and 15.10 mg g -1 at 298, 308 and 318 K, respectively.These results showed a small influence on the adsorption at higher temperature.
To verify the information collected by the isotherms, it is crucial to fit the experimental data to the physical chemical models because to get a better comprehension of the adsorbate-adsorbent interactions.Table 3 shows the results of fitting to the Langmuir and Freudlinch models for the dye removal on palygorskite in aqueous medium.The data indicated a better fit to the Langmuir model at 298 and to the Freundlich isotherm at 308 and 318 K, with linear increased adjustment to the model Freundlich with increasing temperature, favoring the formation of multilayer and different sites of interaction.These results are consistent with the results of a previous published study 56 .
The adsorption capacity of natural palygorskite for anionic dyes was compared with the adsorption capacities of other adsorbents, as shown in Table 4.These data showed that palygorskite has good potential for removing anionic dye from aqueous solutions.The kinetic study using the natural palygorskite for adsorption of the Remazol Yellow GR dye at 1000 mg L -1 was performed for time points from 15-300 min.As shown in Figure 4, the maximum removal was at an equilibrium time of 240 min.After 300 min, the removal capacity was practically constant.This equilibrium time is a fundamental value in wastewater treatment.The kinetic experimental adsorption data were fitted to the pseudo-first-and pseudo-second-order models.The parameters obtained are shown in Table 2.These data suggest that the  Pseudo-first-order Pseudo-second-order q e,cal K 1 R 2 q e,cal K 2 R 2 (mg g -1 ) (min -1 ) (mg g -1 ) (mg -1 min -1 )    A binary system was used to better understand the dye removal process by natural palygorskite.In this case, the influence of lead was investigated, because it is present in various pigment formulations.Hence, the experimental data presented in Figure 9 indicated that lead decreased dye removal to approximately 16 mg g -1 .In other words, the preference of adsorption sites for dye molecules was maintained.This result may be due to the preponderance of electrostatic interaction, as discussed previously.The data fits to the Langmuir and Freundlich models are shown in Table 5, showing that the experimental adsorption data were a good fit to the Freundlich model.The n F parameter was higher than unity, suggesting that the dye/solid interaction was favorable in the presence of lead.

Conclusions
In this work, the use of natural palygorskite for the removal of anionic dye showed an equilibrium time of (mg g -1 ) (L mg -1 ) (L g -1 ) Therefore, this study demonstrated that natural palygorskite can be used in Remazol Yellow GR dye removal under the investigated conditions.

Figure 1 :
Figure 1: i) X-ray diffractogram of natural palygorskite before (a) and after (b) dye adsorption ii) Infrared spectrum of natural palygorskite.

Figure 2 :
Figure 2: Scanning electron microscopy images of pure palygorskite with different amplifications.

Figure 3 :
Figure 3: Potential of zero charge for natural palygorskite.

Figure 4 :
Figure 4: Effect of reaction time on the Remazol Yellow GR dye adsorption of natural palygorskite at 298 K (Conditions: adsorbent mass of 30 mg, dye concentration of 1000 mg L -1 and pH 6).

Figure 5 :
Figure 5: Influence of pH on yellow dye adsorption by natural palygorskite (Conditions: adsorbent mass of 30 mg, dye concentration of 1000 mg L -1 and contact time of 240 min).

Table 2 :
Parameters obtained from the kinetic fit for adsorption of Remazol Yellow GR dye by natural palygorskite.q e,exp (mg g -1 )

Figure 6 :
Figure 6: Proposed mechanism for the interaction between Remazol Yellow GR dye and natural palygorskite.

Figure 8 :
Figure 8: Adsorption isotherms of the Remazol Yellow GR dye by natural palygorskite at different temperatures (Conditions: adsorbent mass of 30 mg, dye concentration 100-500 mg L -1 , pH 2 and contact time of 240 min).

Figure 9 :
Figure 9: Adsorption isotherms of the Remazol Yellow GR dye by natural palygorskite at different temperatures (Conditions: adsorbent mass of 30 mg, dye concentration 100-500 mg L -1 , pH 2 and contact time of 240 min) in the binay systems.
at 298 K, and the experimental data were fit to a pseudo-second-order kinetic model.Higher dye adsorption was achieved at pH 2 with a preponderance of electrostatic interactions at the dye/solid interface.The temperature influenced adsorption, with maximum removal of 20.08 mg g -1 at 298 K.The equilibrium data were a good fit to the Langmuir model at 298 K and to the Freundlich equation for the other temperatures investigated.The presence of lead in the solution affected the dye removal with a small change in dye retention, 14.00 mg g -1 .

Table 1 :
Estimation of the ratios of the intensity of some reflections of the palygorskite and the quartz phase (I quartz /I P ) before and after dye retention.

Table 3 :
Parameters obtained by the Langmuir and Freundlich isotherms for adsorption of Remazol Yellow GR dye by natural palygorskite.

Table 4 :
Comparison of different adsorbents for dye removal from aqueous solution.

Table 5 .
Parameters obtained by the Langmuir and Freundlich isotherms for the removal of Remazol Yellow GR dye in the presence of Pb 2+ ion by natural palygorskite at 298 K and pH 2.0.