3.1 Characterization of palygorskite samples (Pal and PalSIL)
The X-ray diffractograms (Fig. 1A) of Pal and PalSIL samples displayed the reflections related to Pal and quartz in higher intensity. Small amounts of kaolinite, diaspore and magnetite were also observed. In the diffractogram of the PalSIL sample, the reflection typical of Pal at 2θ = 9.9° was slightly shifted to a lower 2θ value (8.48°), indicating the incorporation of the functionalizing agent in the structural channels and in the external part of the clay mineral fibers. However, the difference was not important, as this clay mineral is non-swellable. The other diffraction peaks did not change with treatment, indicating that the modification of Pal did not alter its structure (Moreira et al. 2017).
The thermogravimetric (TG) curves of the Pal and PalSIL samples are shown in Fig. 1B. Three main loss stages were observed for both samples: The first thermal event below 130 ºC was attributed to the elimination of the adsorbed water; the second event in between 130–290°C was associated with the loss of zeolitic water, found in the Pal channels; and the third event, from 290 to 750 ºC, was related to the dehydroxylation of structural water and OH groups (Wang et al. 2020; Meneguin et al. 2021; Suárez et al. 2022). The total Pal mass loss (between 120–750 ºC) was around 7.3%. PalSIL sample presented mass loss of 13.36% between 120–750 ºC, involving the dehydroxylation of the structural water and also the thermal decomposition of the APTMS (Xue et al. 2010). The degree of incorporation of APTMS in the PalSIL was determined by comparing the difference in the mass loss percentage from 120–750 ºC, observed for PalSIL and neat Pal sample, indicating 6.0% of incorporation.
The FTIR spectra of Pal and PalSIL in the range of 4000 to 500 cm− 1 are shown in Fig. 1C. The absorption bands in the range of 3692–3400 cm− 1 (band a) was attributed to the vibrational stretching of O-H groups linked to Si, Mg and zeolite water present in the clay channel. The band at 1651 cm− 1 (band b) was attributed to angular deformation of water molecules. The band at 1031 cm− 1 was related to the Si-O-Si vibrational stretching and at 988 cm− 1 was attributed to Al-OH-Al deformation (band c). The spectrum of the functionalized PalSIL showed a very small band at 2933 cm− 1 (band d) assigned to the C–H stretching vibration of CH2 groups in the APTMS structure, indicating the small incorporation of amino-silane at the Pal surface, as also indicated by TG analysis (Bertuoli et al. 2014; Xue et al. 2010).
Figure 1D shows the zeta potential curve of Pal and PalSIL, using KCl as indifferent electrolyte. The Pal zeta potential profile exhibited negative surface charges over the entire pH range. According to the literature, this charge can occur from isomorphic substitutions of the active sites of Pal Si4+ by trivalent ions and Al3+ by divalent cations (Middea et al. 2013; Silva et al. 2014). The zeta potential profile of the PalSIL functionalized sample indicated a positive charge in the pH range between 2 and 10, attributed to the grafting of NH2 groups on its surface that can be protonated generating NH3+ cations and, at pH above 10, it presents a negative charge due to a higher density of Si-O- groups than protonated amino groups. Furthermore, the residual hydroxyl groups present on the surface of the clay mineral end up generating negative charges induced by the OH− ions in the aqueous solution (Agmon et al. 2016).
Figure 2 presents the SEM images of Pal and PalSIL. In both cases, it was possible to observe the typical fibrous morphology of the Pal and the formation of clusters of ribbons and needles, with flat or straight shapes, randomly oriented. The surface functionalization resulted in the formation of larger aggregates with a preferential orientation, probably due to hydrogen bond between amine groups and hydroxyl and also due to the silylation between the fibers.
Properties such as specific surface area, surface area of micropores, volume of micropores and average pore size were obtained through N2 physisorption, using the BET method. The surface area of the Pal sample was 149 m² g− 1, with a pore volume of 0.36 cm3 g− 1 and an average pore size of 9.70 nm. This pore size value characterizes Pal as a mesoporous material, according to IUPAC standards (Sing 1985), which includes pores with internal widths between 2 and 50 nm. The particle size found for this sample was approximately 40 nm.
The PalSIL sample had a surface area of 35 m² g− 1, with a pore volume of 0.17 cm3 g− 1 and an average pore size of 18.36 nm, that is, the functionalization decreased the surface area due to the incorporation of aminosilane at the outer surface of the Pal. The particle size also showed differences, with a value of 157 nm being reported. These results are in agreement with the aggregates shown in the SEM micrograph. In general, smaller particles have a relatively large surface area compared to larger ones. The decrease in surface area in the PalSIL was due to silylation between the fibers.
Although functionalization decreased the specific surface area, the presence of amine groups can generate specific and selective adsorption sites. Furthermore, the average pore size increased in the functionalized sample, facilitating the accessibility of the adsorbate molecules to the internal adsorption surface.
3.2 Adsorption studies
The results of the removal rate and maximum adsorption capacity of Pal and PalSIL for Cr(III) are summarized in Table 1. According to the results, PalSIL showed the highest percentage of removal of Cr(III) ions from the aqueous solution in the highest part of the combinations of the tests. In other words, PalSIL presented itself as an adsorbent with better performance in different experimental conditions, which expands the application of this material. From the adsorptive results, the removal percentage, used as a measure of the adsorbent's performance from a given solution, and the adsorption capacity (q), which refers to the amount of adsorbate that an adsorbent can retain on its surface, were analyzed. The maximum experimental adsorption capacity found for PalSIL was around 21 mg g− 1 and a maximum removal percentage of 99.8%, calculated by Eq. 1 and Eq. 2. In comparison, the Pal sample reached maximum adsorption capacity of 4.5 mg g− 1 and a maximum removal percentage of 95%, achieved only at low concentrations of Cr(III) and high amounts of adsorbent. These are crucial parameters in evaluating the performance of adsorption processes, but they provide different perspectives. Therefore, in some experimental tests, some discrepancies were observed in relation to these parameters. For example, tests 7 and 8 for PalSIL, which show the highest values of mg of Cr(III) adsorbed per g of adsorbent, 19 mg/g and 21 mg/g, respectively, also show low values of removal percentages.
Table 1
Results of removal percentage and maximum adsorption capacity of Cr(III) (mg/g) for the adsorbents tested in the experimental tests of the factorial design
Tests | Time (min) | pH | Adsorbent (g) | [Cr(III)] (mg L− 1) | Removal (%) | | qe (mg g− 1) |
---|
Pal | PalSIL | | Pal | PalSIL |
---|
1 | 30 | 1 | 0.050 | 25.00 | 19.23 | 20.77 | | 2.00 | 2.16 |
2 | 360 | 1 | 0.050 | 25.00 | 15.77 | 94.24 | | 1.64 | 9.80 |
3 | 30 | 5 | 0.050 | 25.00 | 26.54 | 83.08 | | 2.76 | 8.64 |
4 | 360 | 5 | 0.050 | 25.00 | 28.46 | 98.85 | | 2.96 | 10.28 |
5 | 30 | 1 | 0.050 | 125.00 | 7.94 | 6.35 | | 4.00 | 3.20 |
6 | 360 | 1 | 0.050 | 125.00 | 8.73 | 2.38 | | 4.40 | 1.20 |
7 | 30 | 5 | 0.050 | 125.00 | 7.94 | 37.70 | | 4.40 | 19.00 |
8 | 360 | 5 | 0.050 | 125.00 | 7.14 | 41.82 | | 3.60 | 21.08 |
9 | 30 | 1 | 0.200 | 25.00 | 64.23 | 97.65 | | 1.67 | 2.53 |
10 | 360 | 1 | 0.200 | 25.00 | 68.077 | 98.85 | | 1.77 | 2.57 |
11 | 30 | 5 | 0.200 | 25.00 | 93.46 | 98.85 | | 2.43 | 2.57 |
12 | 360 | 5 | 0.200 | 25.00 | 95.00 | 98.85 | | 2.47 | 2.57 |
13 | 30 | 1 | 0.200 | 125.00 | 27.46 | 99.26 | | 3.46 | 12.5 |
14 | 360 | 1 | 0.200 | 125.00 | 18.25 | 99.75 | | 2.30 | 12.5 |
15 | 30 | 5 | 0.200 | 125.00 | 36.19 | 99.64 | | 4.56 | 12.5 |
16 | 360 | 5 | 0.200 | 125.00 | 36.11 | 99.76 | | 4.55 | 12.5 |
*17 | 195 | 3 | 0.125 | 75.00 | 29.15 | 99.52 | | 3.01 | 10.3 |
*18 | 195 | 3 | 0.125 | 75.00 | 31.47 | 99.52 | | 3.25 | 10.3 |
*19 | 195 | 3 | 0.125 | 75.00 | 36.28 | 99.39 | | 3.74 | 10.3 |
*central point | | | | | |
This difference can be attributed to the use of low concentrations of the adsorbent, reducing the available adsorption sites and, consequently, the percentage values of removed Cr(III) were lower. However, with this amount of adsorbent it was possible to retain a satisfactory amount of chromium at its saturation point. High removal percentages are observed with greater amounts of adsorbent used. However, in these cases, the maximum adsorption capacity per gram of adsorbent ends up decreasing, probably due to the overlapping of adsorption sites caused by overcrowding of the adsorbent (Kong et al. 2019).
The removal of Cr(III) was mainly due to direct complexation with the organosilane groups present in the modified sample. As previously predicted, the presence of APTMS amino groups on the PalSIL surface improved the adsorption of Cr(III) ions through the chelating effect of the lone pair of electrons of a nitrogen atom, overcoming the electrostatic repulsion between Cr(III) species and the APTMS. It is possible to form a coordination bond through the sharing of electron pairs between the nitrogen atom of the amine, which acts as a ligand, and the central chromium atom (Lei et al. 2020)
Figure 3 compares the FTIR spectra of PalSIL adsorbent before and after the adsorption of Cr. A fluctuation in the intensity of the band of the peaks obtained was observed, indicating the change in frequency in the functional groups due to the adsorption of chromium. The division of the band at 2933 cm− 1 by two new bands at 2982 and 2906 cm− 1 with the incorporation of the APTMS was the most important difference between the spectra, suggesting its involvement on the adsorption of trivalent chromium ions. Furthermore, the band around 3540 cm− 1, referring to the hydroxyl groups and at 1645 cm− 1 associated with the vibration of the N-H bond were observed, indicating the involvement of hydroxyl an amino functional groups in the fixation of chromium ions (Moreira et al. 2017; Ba et al. 2018; Hashem et al. 2020).
3.3 The factorial analysis
The estimated effects of the statistical analysis of the factorial design can be seen in Table 2, with a confidence level of 95%, that is, only a 5% chance of error, the significance value (p-value) is considered as 0.05 (Antony et al., 2014). The factorial analysis of these results was carried out with the PalSIL adsorbent, due to the better performances presented in the adsorption.
Table 2
Estimated effects, coefficients and p-value of variables and significant interactions for CR(III) removal
Variable | Factor | Effect | Coefficient | p-value |
---|
Time (min) | A | 11.40 | 5.7 | 0.122 |
pH | B | 17.41 | 8.7 | 0.031 |
Maximum concentration of Cr(III) (mg L− 1) | C | 50.93 | 25.46 | 0.000 |
Amount of adsorbent (g) | D | -25.56 | -12.78 | 0.006 |
Interaction [Cr(III)]/ Amount of adsorbent | CD | 26.61 | 13.31 | 0.005 |
Interaction pH/ Amount of adsorbent | BC | -17.02 | -8.51 | 0.034 |
For PalSIL, the factorial analysis indicates that the maximum concentration of Cr(III), the pH and the amount of adsorbent were the main variables that significantly influenced the percentage values of chromium removal. As observed in Table 2, the maximum concentration of Cr(III) (C) represented the factor that most influenced the system response, in a directly proportional way, due to the presence of the positive sign. That is, the higher initial concentration provides a greater driving force that helps to overcome all resistances to mass transfer between the aqueous and solid phases (Priyadarshini et al. 2018).
The variable pH (B) also directly affected the system response, but with lesser significance. Through the results of zeta potential, we can verify that the surface of the adsorbent is positive in the range of pH used in the experiments. This could not favor the adsorption due to the electrostatic repulsion. However, at low pH closer to 5, the adsorption was more favored. The amine functional groups at the surface of the adsorbent tend to be protonated, thus, they can attract trivalent chromium cations in solution, as electron donor sites, demonstrating adsorption via chemical bonding. The variable referring to the amount of adsorbent (D) indirectly affected the system response, that is, the influence on the response was better with minimum values of the variable. In this sense, we saw that the adsorptive efficiency is good even with smaller amounts of adsorbent. However, time (A) was not a variable that significantly influenced the percentage of removal in this factorial study, so this variable was better analyzed through a subsequent kinetic study (Priyadarshini et al. 2018).
In addition, the Pareto chart presented in Fig. 4 shows in a more visual way that the greatest effect was the concentration of Cr(III) (C) followed by the interaction between the concentration of Cr(III) with the amount of adsorbent used (CD). Subsequently, in decreasing order, the amount of adsorbent (D) and the pH (B) appeared as significant factors, as well as the interaction between the amount of adsorbent and the pH of the solution (BC), corroborating the results presented above.
The significance of interactions demonstrates how the relationship between a factor and the system's response is dependent on a second or third factor of the term. The most significant interaction was between the Cr(III) concentration and the amount of adsorbent, demonstrated in Fig. 5A. The graph showed that the combination of the lower levels of both variables reached removal percentages of ≈ 74%, that is, with low concentrations of Cr(III) the adsorption was less effective due to the low disposition of the adsorbate. However, the combination of the lower amount of adsorbent with the highest availability of Cr(III) at high concentration resulted in removal percentages of 99%, that is, the lowest amount of adsorbent displayed a surface with enough adsorption sites to almost completely remove the higher concentration of Cr(III) used, before reaching the saturation limit.
The interaction of the Cr(III) concentration with the pH of the solution demonstrated that, with higher levels of both variables, removal percentages of 99% were achieved, confirming that the variables were directly proportional to the system response (Fig. 5B). Since the pH values used were 1, 3 and 5, it is suggested that the adsorption is efficient in more acidic pH, since the chromium ions tend to precipitate in alkaline media. At high concentrations of Cr(III), it was observed that the adsorption was efficient in the chosen pH range, demonstrating a greater influence of this variable than the pH on the adsorption efficiency. These analyses corroborate the results of the effects presented in Table 1.
Residual plots are used to examine the goodness of fit in the ANOVA. Examining them helps determine whether ordinary least squares assumptions are being met, producing unbiased coefficient estimates with minimal variance. To verify the assumption that the residuals are normally distributed, the normal probability graph of the residuals was used (Fig. 6A), verifying the asymmetry of the line values. Using the plot of residuals versus adjusted values (Fig. 6B), it was verified that the residuals were randomly distributed with constant variance, confirming the normality and adequacy of the proposed model for the statistical calculations.
3.4 Kinetic study
The effect of the contact time between the adsorbent and the metal ion on the adsorption ability was illustrated in Fig. 7. This study was carried out by varying the contact time and maintaining the other factors studied fixed. A plateau corresponding to the adsorption capacity (qe) around 20 mg g− 1 was achieved after 60 min, suggesting the saturation of the active sites of the adsorbent. After this time interval, the adsorptive process balance occurred.
The kinetic study is very important to determine the speed of the adsorption reaction: the faster the more practical and effective is the application of the adsorbent. Thus, this maximum qe value reached in 60 min demonstrated that PalSIL had a readily available surface for adsorption, that is, a large number of sites available for good adsorption of ions.
The validity of the models was tested by linearizing the curve applying pseudo-first order and pseudo-second order kinetic models, as shown in Fig. 8 (Nascimento, et al. 2023). The analysis of the results demonstrated a better adjustment with the pseudo-second order model, presenting a higher correlation coefficient (R² = 0.99), compared to the other studied model (Table 3). Thus, this model proposed that the adsorption rate was dependent on the amount of ions on the surface of the adsorbent and the amount of ions adsorbed at equilibrium, corroborating the observation of the most significant variable in the factorial analysis. This indicates that the fast mechanism of the adsorptive process was chemical in nature, where there was electron transfer and formation of chemical bonds between the adsorbate and the adsorbent surface, forming a single adsorbed molecular layer (monolayer).
Table 3
Parameters referring to pseudo-first order and pseudo-second order kinetics related to Cr(III) adsorption by PalSIL
Pseudo-first order | | Pseudo-second order |
---|
k1 (g mg− 1 min− 1) | qe (mg g− 1) | R2 | | k2 (g mg− 1 min− 1) | qe (mg g− 1) | R2 |
0.0084 | 1.008 | 0.52 | | 0.0001 | 21.32 | 0.9955 |
Statistical analysis suggested that the time variable was not significant in the Cr(III) removal process by PalSIL. Thus, aligning this result with the kinetic study, it can be concluded that 60 min was enough to reach the highest amount of adsorbed ions, that is, since most of the values of the levels of the factorial study exceeded the proposed equilibrium time, the tendency is for time to demonstrate that it does not influence the response variable, since the saturation of the adsorptive sites was indicated in this period.
Table 4 compares the adsorption ability of the system studied in the present work for Cr(III) with others already reported in the literature. The values found in the present study was quite similar to other employing biochar (Chen et al., 2015), kaolinite (Petrović et al. 2023), diatomite (Gürü et al. 2008) and polyethyleneimine cryogel (Bagdat et al. 2023). Thus, by using Pal, a low cost and available clay mineral, functionalized with aminosilane as adsorbent, it was possible to obtain high levels of removal of Cr(III) ions from the solution (above 99%) and a maximum adsorption capacity of 23 mg g− 1 in the batch system at 27 ºC. The optimal conditions recorded in this work can be applied economically on an industrial scale and are ecologically correct. Thus, the results achieved in the present work can be considered compatible and even higher than some results relate to the natural adsorbents found in the literature. In addition, the economical adsorbent, widely available, with high adsorption capacity, fast adsorption kinetics and low operational cost of the process make this study promising for technological application. This justifies the better applicability and cost-effectiveness of the functionalization of the clay mineral sample in relation to other less accessible and high-cost treatments already reported, adding value to a national product. The significant influence of the studied variables (pH, initial concentration of the Cr(III) solution and amount of adsorbent) on the removal efficiency was also in agreement with the literature. Noting that the highest concentration levels of the Cr(III) solution and the amount of adsorbent used directly affect the element removal percentage (Casqueira & Lima 2016).