Use of Brazilian Kaolin as a Potential Low-cost Adsorbent for the Removal of Malachite Green from Colored Effluents

This study investigated the potential of Brazilian kaolin as a low-cost adsorbent for the removal of Malachite Green (MG) from colored effluents. The morphology, chemical structure and surface properties of the adsorbent were investigated by characterization techniques such as X-ray diffraction, N2 adsorption-desorption isotherms, Fourier transform infrared spectroscopy, X-ray fluorescence spectrometry, scanning electron microscopy, thermogravimetric analysis and particle size distribution. A possible technological application of raw kaolin is the MG removal from aqueous media, which was investigated using batch adsorption experiments. The adsorption kinetics was studied using the pseudo-first order, pseudo-second order and Elovich models. The adsorption isotherms were studied using the Langmuir, Freundlich and Sips models. The Elovich model was the more adequate to represent the adsorption kinetic, while the equilibrium was well represented by the Langmuir model. The maximum adsorption capacity, at pH of 6.3 and temperature of 25oC, was 128 mg g-1, and this satisfactory result may be associated with some adsorbent properties. Therefore, the results revealed that raw kaolin can be utilized as a promising low-cost adsorbent to remove MG from colored effluents.


Introduction
Dyes are used as coloring agents in many industries, and if improperly discarded into the environment, it can cause adverse effects to the human life and aquatic ecosystem 1,2 .Some methods used for dye removal from industrial wastewater are flotation 3 , filtration 4 , adsorption [5][6][7][8] and photocatalysis 9,10 .Among the various available methods, the adsorption process is one of the most effective techniques, which has been successfully employed for dye removal from wastewater [11][12][13][14][15][16][17] .Adsorption process offers significant advantages such as lowcost, efficiency and operational ease 18 .Low-cost materials may be used as promising dye adsorbents in order to make the adsorption process less expensive.In this sense, several low-cost materials have been used as adsorbents to treat dye-containing wastewater [19][20][21][22][23] .Among them, clay minerals have been used as a potential adsorbents for this purpose 23,24 .
Kaolin is a clay predominantly composed by kaolinite (Al 2 Si 2 O 5 (OH) 4 ) 25 , which has been widely used in a variety of technological applications [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] .Due to the low-cost and large availability, the use of clays like kaolin becomes the adsorption process an attractive and promising technology.Although there are several works in literature reporting the removal of different classes of dyes onto kaolin [41][42][43][44] , no studies were found reporting the application of a natural kaolin sample from the Rio Grande do Sul State (Brazil) for the removal of Malachite Green dye.
In this context, the aim of the present work was to investigate the potential application of a Brazilian kaolin sample as an adsorbent for the removal of Malachite Green dye from aqueous solution.Kinetic and equilibrium models were studied in order to elucidate the adsorption process.

Materials
The natural kaolin was obtained from a mining company located in Rio Grande do Sul State, Brazil.The sample was used as received.Malachite Green (MG) (Sigma-Aldrich, CAS number: 569-64-2, C 23 H 25 ClN 2 , 364.91 g mol -1 ) was used as target dye pollutant.The chemical structure of MG dye is shown in Figure 1.

Characterization techniques
X-ray diffraction pattern (XRD) was obtained using a Rigaku Miniflex 300 diffractometer, where, the X-ray source was Cu-Kα radiation, powered at 30 kV and 10 mA.Scans were performed over 2θ angles ranging from 5 to 70º.N 2 adsorption-desorption isotherms were obtained at 77 K using an ASAP 2020 apparatus.The particle size distribution

Adsorption experiments
In order to determine the adsorption capacity of the kaolin adsorbent, various concentrations (100 -175 mg L -1 ) of the MG dye were prepared and, 100 mL of each were added to 0.1 g of adsorbent.All the adsorption experiments were performed at natural pH of dye solution (pH = 6.3).The mixtures were agitated on a shaker for 240 min at 100 rpm and 25 o C. After, the solutions were centrifuged (Centribio, 80-2B, Brazil) and immediately analyzed using a UV-vis spectrophotometer (Bel Photonics, SP1105) at 618 nm to verify the residual MG concentration.All experiments were carried out in triplicates and blanks were performed.The equilibrium adsorption capacity (q e ) and adsorption capacity at any time (q t ) were determined by Equations ( 1) and (2), respectively:

Kinetic and equilibrium modeling
Some kinetic and equilibrium models were employed to interpret the MG adsorption process on the kaolin adsorbent.Regarding to the kinetic models, pseudo first-order 45 , pseudo second-order 46 and Elovich models 47 were used.These models are given by the Equations ( 3), ( 4) and ( 5), respectively: where, C 0 is the initial dye concentration in liquid phase (mg L -1 ), C e is the equilibrium dye concentration in liquid phase (mg L -1 ), C t is the dye concentration in liquid phase at any time (mg L -1 ), m is the amount of adsorbent (g) and V is the volume of solution (L).
where, k 1 and k 2 are the rate constants of pseudo firstorder (min -1 ) and pseudo second-order (g mg -1 min -1 ) models, respectively; q 1 and q 2 are the theoretical values for the adsorption capacity (mg g -1 ), a is the initial velocity due to dq/dt with q t = 0 (mg g -1 min -1 ), b is the desorption constant of the Elovich model (g -1 mg -1 ) and t is the time (min).
The equilibrium curves were interpreted by the Freundlich 48 , Langmuir 49 and Sips 50 isotherm models.These models are given by the Equations ( 6), ( 7) and ( 8), respectively, where, K F is the Freundlich constant (mg g -1 )(mg L -1 ) -1/nF , 1/n F is the heterogeneity factor, q m is the maximum adsorption capacity (mg g -1 ), K L is the Langmuir constant (L mg -1 ), q S is the maximum adsorption capacity from Sips model (mg g -1 ), K S is the Sips constant (L mg -1 ) and ms is the Sips exponent.

Parameters estimation
The kinetic and equilibrium parameters were determined by the fit of the models with the experimental data using nonlinear regression.The parameters were estimated by the minimizing the least squares function using the Quasi-Newton estimation method.The calculations were carried out using the Statistic 9.1 software (Statsoft, USA) 51 .The fit quality was verified by the obtainment of determination coefficient (R 2 ), average relative error (ARE) and Akaike information criterion (AIC) 52,53 , which are presented in the Equations ( 9), (10) and (11), respectively: where, q i,model corresponds to each value of q predicted by the fitted model, q i,exp corresponds to each value of q experimentally measured, q i,exp is the average of q experimentally measured, n is the number of experimental points, p is the number of parameters of the fitted model and, the sum of squared errors (SSE) is given by the Equation ( 12): FTIR spectrum of raw kaolin is shown in Figure 3.The main characteristic bands of the kaolinite are highlighted in Figure 3.The bands at 3694 and 3619 cm -1 (OH stretching vibrations), 1114 and 694 cm -1 (Si-O stretching), 1031 cm -1 (Si-O-Si), 1008 and 540 cm -1 (Si-O-Al) and, 912 cm -1 (Al-OH) are typical of kaolinite mineral 56,57 .Bands located at 789, 753 and 468 cm -1 can be attributed to the presence of quartz [57][58][59][60] .Vibration at 432 cm These findings are consistent with other previously reported works 54,55 .Therefore, the sample showed a predominant phase as kaolinite, and also, quartz as minor impurity.The chemical composition (wt.%) obtained from XRF analysis of natural kaolin was: 43.50 (SiO 2 ), 40.03 (Al 2 O 3 ), 0.56 (Fe 2 O 3 ), 0.33 (K 2 O), 0.27 (MgO), 0.17 (CaO), 0.08 (TiO 2 ), 0.02 (MnO), 0.02 (SO 3 ), 15.01 (Mass loss on ignition, obtained from TGA analysis).Typically, a pure kaolinite mineral is made up of 46.5 wt.% of SiO 2 and 39.5 wt.% of Al 2 O 3 , and exhibits 14 wt.% of mass loss on ignition 56 .Therefore, the composition of the kaolin used in this work was close to a kaolinite mineral with high purity degree.The minor differences can are attributed to the presence of accessory minerals.Figure 4 shows the DTA/TGA curves for the natural kaolin.From these curves, it can be seen the first endothermic peak around 50 ºC, which corresponds to the adsorbed water loss (about 1.3 wt.%).The second peak centered at 500 ºC corresponds to the mass loss (about 13.7 wt.%) due to kaolinite dehydroxylation toward the formation of a noncrystalline phase (metakaolinite) 62,63 .The particle size distribution curve of kaolin is shown in Figure 5.It was found that the kaolin particles are in the range of 0.72-120 μm, resulting in an average particle size of 23.45 μm (0.2345 mm).This particle size can explain the q ,exp i mesoporosity of material due to a variety of pore voids among the particles caused by its agglomeration 64 .Therefore, this mesoporous structure is interesting for adsorption purposes.The morphology of kaolin clay observed in SEM image (Figure 6), indicates that the particles presented irregular shape, rough surface, and different particle sizes (smaller than 50 μm).The N 2 adsorption-desorption isotherms and the pore size distribution of kaolin adsorbent are exhibited in Figure 7.This figure indicates the N 2 adsorbed/desorbed from the sample in relation to the relative pressure (P/P 0 ) and is used to identify the pore constitution of the material.As shown in Figure 7, the N 2 adsorption-desorption isotherms of adsorbent can be classified as type IV, with type-H3 hysteresis behavior, which are indicative of predominantly mesoporous material.In addition, the material exhibited a narrow pore distribution with a maximum peak centered at 4.0 nm, indicating mesoporous characteristics, according to IUPAC.The BET specific surface area, total pore volume and average pore size of the adsorbent were 16.75 m 2 g -1 , 0.0628 cm³ g -1 and 13.5 nm, respectively.

Adsorption kinetic results
The experimental kinetic curves of MG adsorption on kaolin were constructed at different initial dye concentrations (100, 125, 150 and 175 mg L -1 ) and the respective data were fitted using the pseudo-first order, pseudo-second order and Elovich models.These results are presented in Figure 8 and Table 1.It can be noticed in Figure 8 that the curves were overlapped until the first 20 min.Until 60 min, more than 70% of saturation was attained independent of the initial dye concentration.The equilibrium was reached around 240 min.Based on the statistical parameters (R 2 > 0.96), (R 2 adj > 0.96), (ARE < 8.0%), (SSE < 630) and (AIC < 56) (Table 1), it can be clearly observed that the Elovich model was the most suitable to represent the MG adsorption onto kaolin.At higher initial dye concentrations, the parameter "b" from the Elovich model was lower.Since that the "b" units is g mg -1 , this fact shows that the MG adsorption capacity was favored at initial concentrations of 150 and 175 mg L -1 .The Elovich model was also suitable to represent other previous studies reporting the adsorption of food dyes onto chitosan 65 .

Adsorption isotherms
The adsorption isotherm of MG dye onto natural kaolin was obtained at 25 ºC and pH of 6.3 and, it is shown in Figure 9.The isotherm data were fitted using the Freundlich, Langmuir and Sips models, where the results are shown in Table 2.It can be observed in Figure 9 that the isotherm shows a strong initial inclination, followed by a plateau.The initial inclination shows a strong affinity between MG and kaolin and the plateau represents the maximum adsorption capacity.This behavior corresponds to a L2-type isotherm according to the Giles classification 66 .
The statistical parameters presented in Table 2 revealed that the Freundlich and Langmuir models were able to represent the experimental equilibrium data.Despite of Sips model to generate satisfactory statistical parameters, this model overestimated the maximum adsorption capacity and so, it cannot be used to predict the experimental data.The 1/n F value was lower than 1 and consequently, n F > 1, showing that the MG adsorption onto kaolin was a favorable process.The high K L value confirms the high affinity between the MG dye and kaolin.The q m parameter obtained from the Langmuir model was relatively high and, it was used to determine the quality of the kaolin adsorbent.
In order to verify the efficiency of raw kaolin for the MG dye adsorption, a comparison among the maximum adsorption capacities (q m, mg g -1 ) of several low-cost adsorbents reported in literature was performed, as shown in Table 3.The values in Table 3 were obtained under several different experimental conditions and, therefore, the maximum capacity (q m ) only from the each work was listed herein.Based on the results presented in Table 3, it can be affirmed that the natural raw kaolin used in this work can be utilized as a promising low-cost adsorbent to remove the MG organic dye from liquid effluents.

Conclusions
The removal of Malachite Green dye using kaolin clay as adsorbent was systematically investigated under different experimental conditions.The raw kaolin from the Rio Grande do Sul State/Brazil presented interesting characteristics for adsorption purposes.The results indicated that the Elovich model was the more adequate to represent the adsorption kinetic data.The maximum adsorption capacity toward Malachite Green was estimated at around 128 mg g -1 , according to the Langmuir model.In summary, the findings from this study demonstrated that the use of raw kaolin as

a
Department of Chemical Engineering, Federal University of Santa Maria, 97105-900, Santa Maria, Brazil Use of Brazilian Kaolin as a Potential Low-cost Adsorbent for the Removal of Malachite Green from Colored Effluents was measured using a laser particle size analyzer (Malvern Mastersizer 2000).Derivative thermogravimetric analysis (DTA) and thermogravimetric analysis (TGA) were carried out in a Netzsch STA 409 analyzer, at a heating rate of 10 o C min -1 and air flow rate of 35 mL min -1 .The sample morphology was examined by a scanning electron microscope (SEM, JEOL JSM-6610LV) at 15 kV.Chemical composition of the sample was determined by X-ray fluorescence spectrometry (XRF) (Bruker S8 Tiger equipment).Fourier transform infrared (FTIR) spectrum was recorded on a Shimadzu IR-Prestige-21 spectrophotometer in the range of 4000-375 cm -1 .

Figure 1 .
Figure 1.Chemical structure of MG dye.

Figure 4 .
Figure 4. DTA and TGA curves of natural kaolin.

Figure 5 .
Figure 5. Particle size distribution curve of kaolin.

Table 1 .
Kinetic parameters for the adsorption of MG onto natural kaolin.

Table 2 .
Equilibrium parameters for the adsorption of MG onto natural kaolin.

Table 3 .
Comparison of Brazilian natural kaolin with other low-cost adsorbents for the adsorption of MG dye.