Sorption of Cu 2 + Ions by Bentonite Modiﬁed with Al Keggin Cations and Humic Acid in Solutions with pH 4.5

: The sorption of Cu 2 + onto bentonite modiﬁed with Al Keggin cations and humic acid from CuCl 2 solutions at pH 4.5 was studied. Modiﬁcation of Na-bentonite with Al Keggin cations was found to result in an increase in the basal spacing of montmorillonite from 1.29 nm for N-form to 1.85 and 1.78 nm for HAl 13 and Al 13 forms respectively, in a reduction of CEC (cation exchange capacity) and in the formation of additional sites with a variable charge with pH PZC 4.2. Al 13 -bentonite is not a ﬀ ected by heat. Under the conditions of the experiments at pH of 4.5 Na-bentonite adsorbs more Cu 2 + from CuCl 2 solutions then Al 13 forms of bentonites. The main mechanism of copper sorption on Na-bentonite is the cation exchange Cu 2 + –Na + . The reduction of CEC of Na-bentonite after modiﬁcation with Al Keggin cations leads to a decrease in the Cu 2 + sorption. pH-dependent sorption sites on Al 13 -bentonites have a pH PZC of 4.2 and, therefore, under conditions of the experiment have positive charge which prevents Cu 2 + sorption. Na-bentonite adsorbs more humic acid solution (HA) then Al 13 -bentonite and the proportion of adsorbed HA remains constant over the entire concentration range. Treatment of the Al 13 -bentonite with HA leads to the formation of the additional sorption sites. The amount of sorbed Cu 2 + and the percentage of their extraction from solutions by HAAl 13 -bentonite is similar to those values for Na-bentonite.


Introduction
Natural bentonite clays are widely used as sorbents for heavy metals and radionuclides due to their high sorption capacity and relatively low price [1].
The sorption capacity of bentonite clays towards these pollutants depends on their composition and varies in clays from different deposits [2][3][4][5][6][7][8][9][10][11][12]. It was demonstrated that depending on the pH, ionic strength, composition of the solution and the conditions of the experiments, bentonites with different mineral composition and physical properties, adsorb from 13.2 to 32.7 mg Cu 2+ per gram clay [13][14][15][16]. The main mechanisms of sorption of Cu(II) ions on bentonite are ion exchange and proton substitution of aluminol and silanol groups on the edge surfaces of clay minerals. Ion exchange is independent of pH. Sorption on aluminol and silanol groups depends on pH.
The obtained solution was kept at room temperature for 24 h and then centrifuged. Smectite saturated with polyhydroxy-aluminum (HAl 13 -smectite), was washed with distilled water to remove Cl − ions and then dried at room temperature. The removal of hydroxyl groups from the Al Keggin cations was performed by calcination of the HAl 13 -bentonite at 400 • C for 4 h. The calcined HAl 13 -bentonite form hereinafter will be denoted as Al 13 -bentonite. Saturation with polyhydroxy-aluminum decreased the pH of the clay suspension compared to the Ca and Na forms to the values 5.38 and 5.45 for the HAl 13and Al 13 -bentonites respectively.
Experiments on the sorption of Cu 2+ on the Na-, HAl 13 -, and Al 13 -bentonites. The first experiment studied the dependence of the clay sorption capacity towards Cu 2+ on the reaction time with 0.5 mM CuCl 2 solution at pH 4.5, which was maintained by adding 0.1 N HCl solution. Experiments were carried out at a solid:liquid ratio of 1:1000. Suspensions were shaken at 250 rpm for 5, 10, 20, 30, 40 and 60 min. After that, the pH was measured and the suspension was centrifuged for 5 min at 5000 rpm. Then the supernatant was passed through the 0.45 µm filter and the Cu(II) concentration was determined.
The second experiment studied the dependence of the Na-and Al-bentonites sorption capacity towards Cu 2+ in CuCl 2 solutions. Experiments were carried out at a solid:liquid ratio of 1:1000 where the concentrations of Cu(II) were 0.025, 0.05, 0.125, 0.250 and 0.5 mM, all at pH 4.5. The ionic strength of the resultant copper chloride solutions ranged from 7.5.10-5 to 1.5.10-3 M. The suspensions were shaken for 30 min at 250 rpm. After measuring the pH, the suspension was centrifuged for 5 min at 5000 rpm. Then the supernatant was passed through a 0.45 µm filter and the Cu(II) concentration was measured.
Based on the data obtained it was established that the maximum sorption of Cu 2+ is achieved within 30 min. Thus, this time interval was chosen as standard for the main sorption experiments.

Experiments on the Sorption of Humic Acid by Na-and Al 13 -bentonite
A humic acid solution (HA) was prepared from a dry sample, produced by the company "Biolar". A sample with a mass of 0.735 g was poured into 500 mL of distilled water, then 0.75 mL of 1 M NaOH was added dropwise and the whole volume was adjusted to 1000 mL with distilled water. The obtained solution was filtered through a White Ribbon filter and the Corg concentration was determined. The dependencies of the HA sorption by the different bentonite forms on different factors were studied. The experiments on the effect of the sorbent content were carried out at clay:solution ratios of 1:1000, 1:750 and 1:500 at constant pH of 4.5. The experiments were carried out at an ionic strength of 0.1 M, which was maintained using NaCl solution; the interaction time was 30 min. The ionic strength is actually high but such values of ionic strength is commonly applied in similar kinds of experiments [25][26][27].
The dependence of the HA sorption on the time of interaction was studied using solution with concentration Corg 24.35 mM at pH 4.5 with a clay:solution ratio of 1:750 and ionic strength of 0.1 M. The suspensions were shaken at 200 rpm for 0.25, 0.5, 1, 2, 4, 6 and 8 h. Then the supernatants were passed through a White Ribbon filter. The HA concentrations were determined by the photometric method.
The dependence of the HA sorption on pH was studied using 24.35 mM and 6.09 M solutions at pH values of 3.0, 3.5, 4.5 and 7.0 at a constant ionic strength of 0.1 M and a solid:liquid ratio-1:750 as was chosen according to the preliminary experiments. It was found that the highest sorption by the Al 13 -smectite is achieved at pH 3 so it was chosen for the following experiment.
To determine the sorption dependence on the initial concentration of humic acid the following concentrations were used: 6.09, 12.18, 18.26, 24.35 mM. The experiments were carried out at pH 3 with a clay:solution ratio of 1:750 and ionic strength of 0.1 M. In both experiments, the suspensions were shaken for 6 h at 200 rpm. Then the supernatants were filtered through a White Ribbon filter and the concentrations of Corg were determined.
The concentration of the Corg solution, at which the adsorption of HA on the Al 13 -bentonite was the highest turned out to be 12.18 mM and was used for the preparation of the HA-Al 13 -bentonite for the following experiment.
2.3. Experiment on the Sorption of the Cu 2+ on the HA-Al 13 -Bentonites Al 13 -bentonite was saturated with HA (HA-Al 13 ) in a solution with a Corg concentration of 12.18 mM, I = 0.1 M, pH 3 and at a clay:solution ratio of 1:750 for 6 h. The experiment was conducted under the same conditions as the experiments on Cu 2+ sorption by the Na-, HAl 13 -and Al 13 -bentonite forms.
Sorption isotherms of Cu 2+ and HA ions were described by the Freundlich equation: where Q is the amount of sorbed Cu 2+ , in mmol/g; C eq is the equilibrium concentration of Cu(II), in mmol/L; and k and n are empirical coefficients.
All the experiments were carried out in duplicate.
The potentiometric method was used to determine the pH of the suspensions and solutions, and for the determination of the pH of the point of zero charge (pH PZC ). The potentiometric analysis was carried out with the Mettler Toledo Sevengo Pro (Mettler Toledo Inc., New York, NY, USA) and autotitrator Mettler Toledo DL 58 (Mettler-Toledo AG, Schwerzenbach, Switzerland).
The concentration of Cu(II) was measured by atomic absorption on the spectrophotometer ContrAA 300 (Analytik Jena spectrometer, Jena, Germany).
The content of Corg in the initial HA solution was determined by the Tyurin method with the photometric ending at 590 nm wavelength [28]. The Corg concentration in the equilibrium solutions was determined by spectrophotometry at 350 nm wavelength using a UNICO 1201 spectrophotometer (United Products & Instruments Inc., Dayton, NJ, USA), since the absorption maxima at this wavelength correlated well with the DOC concentration.

Mineral Composition and Properties of Various Bentonite Forms
On the XRD pattern of Ca-bentonite, a series of peaks with d/n values of 12.9 (001), 6.1 (002), 4.04 (003), and 3.13 (004) Å was observed ( Figure 1). After saturation with ethylene glycol (EG), the interplanar distance increases to 16.9 Å, and after calcination, it decreases to 9.9Å. It can be concluded that the <1 µm fraction consists almost entirely of smectite. The smaller d(001) values of the Ca-montmorillonite in the air-dry state than what has been described in the literature [29,30] can be explained by a slight modification of the montmorillonite's crystal lattice after the treatment with the 10% HCl solution [5,6,31]. The saturation of the smectites with Na + ions leads to a decrease of the d(001) value of montmorillonite to 12.4 Å while the intercalation with polyhydroxy-aluminum cations increases it up to 18.5 Å. Calcination of HAl13-bentonite at 400 °C causes compaction of the lattice along the c-axis which is observed by the d(001) value of 17.8 Å. The decrease in the interplanar distance after the calcination is caused by the reactions of dehydroxylation and the formation of aluminum oxide, which has a smaller size, compared to the Al Keggin cations. Considering the facts that the thickness of the montmorillonite layer is of about 9.6 Å [32], and the diameter of Cu(H2O)6 2+ is about 5.4 Å [18], the obtained HAl13 and Al13-smectites with interlayer distances of 8.9 Å and 8.2 Å respectively, will be available for intercalation with Cu 2+ ions.
After saturation with ethylene glycol, the interplanar distances of smectites in Ca-and Na-bentonite increase by 3.9 Å and 4.7 Å respectively. Montmorillonite in HAl13-bentonite swells to a lesser extent and its interplanar distance is increased by only 0.6 Å. The interplanar distance of montmorillonite in Al13-bentonite after saturation with ethylene glycol does not change. After calcination at 350 °C and 550 °C, the interplanar distances of montmorillonite in the Ca-and Na-bentonites reduces to 9.6-9.8 Å, whereas Al13-montmorillonite, even after calcination at 550 °C, retains a broad peak with d(001) = 17.1 Å (Figure 1).
From the data obtained, it can be concluded that montmorillonite in the Al13-bentonite sample has a stable crystal structure and its interplanar spacings were not changed either after saturation with ethylene glycol or after calcination at of 550 °C.

Kinetics of Cu 2+ Adsorption
In the investigated time interval, Na-bentonite adsorbs more than 2 times as much Cu 2+ ions compared with the Al forms of bentonite and after 5 min of interaction adsorbs 50-60% of the Cu 2+ from the 0.5 mM CuCl2 solution with pH 4.5 (Figure 2a,b). Further increasing the interaction time has no significant effect on the sorption of copper. In the range from 10 to 60 min, about 70% or 0.3 The saturation of the smectites with Na + ions leads to a decrease of the d(001) value of montmorillonite to 12.4 Å while the intercalation with polyhydroxy-aluminum cations increases it up to 18.5 Å. Calcination of HAl 13 -bentonite at 400 • C causes compaction of the lattice along the c-axis which is observed by the d(001) value of 17.8 Å. The decrease in the interplanar distance after the calcination is caused by the reactions of dehydroxylation and the formation of aluminum oxide, which has a smaller size, compared to the Al Keggin cations. Considering the facts that the thickness of the montmorillonite layer is of about 9.6 Å [32], and the diameter of Cu(H 2 O) 6 2+ is about 5.4 Å [18], the obtained HAl 13 and Al 13 -smectites with interlayer distances of 8.9 Å and 8.2 Å respectively, will be available for intercalation with Cu 2+ ions. After saturation with ethylene glycol, the interplanar distances of smectites in Ca-and Na-bentonite increase by 3.9 Å and 4.7 Å respectively. Montmorillonite in HAl 13 -bentonite swells to a lesser extent and its interplanar distance is increased by only 0.6 Å. The interplanar distance of montmorillonite in Al 13 -bentonite after saturation with ethylene glycol does not change. After calcination at 350 • C and 550 • C, the interplanar distances of montmorillonite in the Ca-and Na-bentonites reduces to 9.6-9.8 Å, whereas Al 13 -montmorillonite, even after calcination at 550 • C, retains a broad peak with d(001) = 17.1 Å (Figure 1).
From the data obtained, it can be concluded that montmorillonite in the Al 13 -bentonite sample has a stable crystal structure and its interplanar spacings were not changed either after saturation with ethylene glycol or after calcination at of 550 • C.

Kinetics of Cu 2+ Adsorption
In the investigated time interval, Na-bentonite adsorbs more than 2 times as much Cu 2+ ions compared with the Al forms of bentonite and after 5 min of interaction adsorbs 50-60% of the Cu 2+ from the 0.5 mM CuCl 2 solution with pH 4.5 (Figure 2a,b). Further increasing the interaction time has no significant effect on the sorption of copper. In the range from 10 to 60 min, about 70% or 0.3 mmol/g (19.1 mg/g) Cu 2+ is sorbed. The obtained values of the maximum amount of sorbed Cu 2+ ions agrees well with those described in the literature (13-33 mg/g) for unmodified bentonites [13][14][15][16]21].
Minerals 2020, 10, x FOR PEER REVIEW 6 of 16 mmol/g (19.1 mg/g) Cu 2+ is sorbed. The obtained values of the maximum amount of sorbed Cu 2+ ions agrees well with those described in the literature (13-33 mg/g) for unmodified bentonites [13][14][15][16]21]. The sorption of copper by the Al forms of bentonite in the entire time interval does not exceed 0.07 mmol/g. The non-calcined bentonite sample extracts 2-18%, while the calcined bentonite sample extracts 1-10%, of the Cu 2+ from a copper chloride solution.
As a result of the interaction of Na-and HAl13-bentonite with the 0.5 mM solution of CuCl2, the equilibrium pH of the suspension increased, compared to the initial pH of copper chloride solution by 0.1 and 0.3 average, respectively. This increase turned out to be independent of the interaction time ( Figure 3). The increase of pH can be associated with the neutralizing effect of clay, the pH values of the suspension of which, as already mentioned above, are 6.39 and 5.45 for the Na and HAl13 forms respectively. In the first 30 min of the interaction with Al13-bentonite, the pH decreases in comparison with the initial value of the copper chloride solution. After 5 min of interaction, the pH decreases by 0.1 and after 30 min it returns almost to the initial value of the copper chloride solution.

Sorption of Cu 2+ Depending on the Concentration of the Initial Solution
In the range of initial concentrations of copper chloride solution, Na-bentonite adsorbs rather more Cu 2+ ions compared to the Al forms (Figure 4a,b). With an increase in the concentration of The sorption of copper by the Al forms of bentonite in the entire time interval does not exceed 0.07 mmol/g. The non-calcined bentonite sample extracts 2-18%, while the calcined bentonite sample extracts 1-10%, of the Cu 2+ from a copper chloride solution.
As a result of the interaction of Na-and HAl 13 -bentonite with the 0.5 mM solution of CuCl 2 , the equilibrium pH of the suspension increased, compared to the initial pH of copper chloride solution by 0.1 and 0.3 average, respectively. This increase turned out to be independent of the interaction time ( Figure 3). The increase of pH can be associated with the neutralizing effect of clay, the pH values of the suspension of which, as already mentioned above, are 6.39 and 5.45 for the Na and HAl 13 forms respectively.
Minerals 2020, 10, x FOR PEER REVIEW 6 of 16 mmol/g (19.1 mg/g) Cu 2+ is sorbed. The obtained values of the maximum amount of sorbed Cu 2+ ions agrees well with those described in the literature (13-33 mg/g) for unmodified bentonites [13][14][15][16]21]. The sorption of copper by the Al forms of bentonite in the entire time interval does not exceed 0.07 mmol/g. The non-calcined bentonite sample extracts 2-18%, while the calcined bentonite sample extracts 1-10%, of the Cu 2+ from a copper chloride solution.
As a result of the interaction of Na-and HAl13-bentonite with the 0.5 mM solution of CuCl2, the equilibrium pH of the suspension increased, compared to the initial pH of copper chloride solution by 0.1 and 0.3 average, respectively. This increase turned out to be independent of the interaction time ( Figure 3). The increase of pH can be associated with the neutralizing effect of clay, the pH values of the suspension of which, as already mentioned above, are 6.39 and 5.45 for the Na and HAl13 forms respectively. In the first 30 min of the interaction with Al13-bentonite, the pH decreases in comparison with the initial value of the copper chloride solution. After 5 min of interaction, the pH decreases by 0.1 and after 30 min it returns almost to the initial value of the copper chloride solution.

Sorption of Cu 2+ Depending on the Concentration of the Initial Solution
In the range of initial concentrations of copper chloride solution, Na-bentonite adsorbs rather more Cu 2+ ions compared to the Al forms (Figure 4a,b). With an increase in the concentration of In the first 30 min of the interaction with Al 13 -bentonite, the pH decreases in comparison with the initial value of the copper chloride solution. After 5 min of interaction, the pH decreases by 0.1 and after 30 min it returns almost to the initial value of the copper chloride solution.

Sorption of Cu 2+ Depending on the Concentration of the Initial Solution
In the range of initial concentrations of copper chloride solution, Na-bentonite adsorbs rather more Cu 2+ ions compared to the Al forms (Figure 4a,b). With an increase in the concentration of copper chloride, the amount of sorbed copper on Na-bentonite increases significantly. This dependence was less significant for the Al-bentonites (Figure 4a).
Minerals 2020, 10, x FOR PEER REVIEW 7 of 16 copper chloride, the amount of sorbed copper on Na-bentonite increases significantly. This dependence was less significant for the Al-bentonites (Figure 4a). In the whole range of the concentrations tested, Na-bentonite adsorbs about 80% of the Cu(II) ions of the initial content ( Figure 4b). The proportion of sorbed copper on Al-bentonites decreases with an increase in the initial solution concentration from 40-50% to 25% and from 30% to 10% for Al13 and HAl13 respectively.
In the range of concentrations of the initial copper chloride solutions from 0.025 to 0.125 mmol/L, the equilibrium pH values in the experiments with Na-bentonite do not vary and exceed the pH value of the initial solution consistently by 1 unit. However, beyond this, the equilibrium pH decreases sharply and at 0.5 mmol/L, the pH value is higher than the initial only by 0.3 units ( Figure  5). The opposite dependence is observed for Al-bentonites ( Figure 5). In this case, an increase in pH compared to the initial value does not exceed 0.3 units. In general, HAl13-bentonite equilibrium pH appeared consistently higher by 0.2 units compared with Al13-bentonite. In the whole range of the concentrations tested, Na-bentonite adsorbs about 80% of the Cu(II) ions of the initial content (Figure 4b). The proportion of sorbed copper on Al-bentonites decreases with an increase in the initial solution concentration from 40-50% to 25% and from 30% to 10% for Al 13 and HAl 13 respectively.
In the range of concentrations of the initial copper chloride solutions from 0.025 to 0.125 mmol/L, the equilibrium pH values in the experiments with Na-bentonite do not vary and exceed the pH value of the initial solution consistently by 1 unit. However, beyond this, the equilibrium pH decreases sharply and at 0.5 mmol/L, the pH value is higher than the initial only by 0.3 units ( Figure 5).
Minerals 2020, 10, x FOR PEER REVIEW 7 of 16 copper chloride, the amount of sorbed copper on Na-bentonite increases significantly. This dependence was less significant for the Al-bentonites (Figure 4a). In the whole range of the concentrations tested, Na-bentonite adsorbs about 80% of the Cu(II) ions of the initial content (Figure 4b). The proportion of sorbed copper on Al-bentonites decreases with an increase in the initial solution concentration from 40-50% to 25% and from 30% to 10% for Al13 and HAl13 respectively.
In the range of concentrations of the initial copper chloride solutions from 0.025 to 0.125 mmol/L, the equilibrium pH values in the experiments with Na-bentonite do not vary and exceed the pH value of the initial solution consistently by 1 unit. However, beyond this, the equilibrium pH decreases sharply and at 0.5 mmol/L, the pH value is higher than the initial only by 0.3 units ( Figure  5). The opposite dependence is observed for Al-bentonites ( Figure 5). In this case, an increase in pH compared to the initial value does not exceed 0.3 units. In general, HAl13-bentonite equilibrium pH appeared consistently higher by 0.2 units compared with Al13-bentonite. The opposite dependence is observed for Al-bentonites ( Figure 5). In this case, an increase in pH compared to the initial value does not exceed 0.3 units. In general, HAl 13 -bentonite equilibrium pH appeared consistently higher by 0.2 units compared with Al 13 -bentonite.

Sorption of Humic Acid by Na-and Al 13 -bentonites
It was found that the maximum amount of humic acid from the solution with concentration C HA = 23.4 mmol/L, I = 0.1 mmol/L and pH 4.5 is adsorbed in 6 h of interaction with the calcined Al 13 -bentonites. This maximum corresponds to about 38% of the initial amount of humic acid in solution. A longer reaction time (8 h) leads to the desorption of HA ( Figure 6).

Sorption of Humic Acid by Na-and Al13-bentonites.
It was found that the maximum amount of humic acid from the solution with concentration CHA = 23.4 mmol/L, I = 0.1 mmol/L and pH 4.5 is adsorbed in 6 h of interaction with the calcined Al13-bentonites. This maximum corresponds to about 38% of the initial amount of humic acid in solution. A longer reaction time (8 h) leads to the desorption of HA ( Figure 6). The sorption of HA on Na-bentonite from a solution with low concentration is virtually independent of the pH of the solution, while from higher concentration HA solutions the sorption is largely reduced with increasing pH (Figure 7a,b). The sorption of humic acid on Al13-bentonite increases slightly at pH < 3.5 and practically does not depend on pH in the range of pH 3.5-8 (Figure 7a,b).
Na-bentonite adsorbs more HA compared to Al13-bentonite and the proportion of the sorbed HA does not change in the whole range of concentrations (Figure 8). The sorption of HA on Na-bentonite from a solution with low concentration is virtually independent of the pH of the solution, while from higher concentration HA solutions the sorption is largely reduced with increasing pH (Figure 7a,b).

Sorption of Humic Acid by Na-and Al13-bentonites.
It was found that the maximum amount of humic acid from the solution with concentration CHA = 23.4 mmol/L, I = 0.1 mmol/L and pH 4.5 is adsorbed in 6 h of interaction with the calcined Al13-bentonites. This maximum corresponds to about 38% of the initial amount of humic acid in solution. A longer reaction time (8 h) leads to the desorption of HA ( Figure 6). The sorption of HA on Na-bentonite from a solution with low concentration is virtually independent of the pH of the solution, while from higher concentration HA solutions the sorption is largely reduced with increasing pH (Figure 7a,b). The sorption of humic acid on Al13-bentonite increases slightly at pH < 3.5 and practically does not depend on pH in the range of pH 3.5-8 (Figure 7a,b).
Na-bentonite adsorbs more HA compared to Al13-bentonite and the proportion of the sorbed HA does not change in the whole range of concentrations (Figure 8). The sorption of humic acid on Al 13 -bentonite increases slightly at pH < 3.5 and practically does not depend on pH in the range of pH 3.5-8 (Figure 7a,b).
Na-bentonite adsorbs more HA compared to Al 13 -bentonite and the proportion of the sorbed HA does not change in the whole range of concentrations ( Figure 8).

Sorption of Cu 2+ Ions by HAAl13-bentonite.
The amount of sorbed Cu 2+ ions increases with their concentration in the initial and equilibrium solutions and the proportion of adsorbed ions decreases expectedly with increasing concentration of the initial copper chloride solution (Figure 9). The maximum amount of sorbed Cu 2+ ions on the HAAl13-bentonite is 0.25 mmol/g and this value is close to the maximum quantity of sorbed Cu 2+ ions by Na-bentonite which equates to 0.33 mmol/g in the conditions of these experiments (Figures 4a and 9a). The proportion of the adsorbed Cu(II) ions on the HAAl13-bentonite at initial concentrations of copper chloride <0.05 mmol/L varies from 60 to 90%, which is comparable with those for the Na-bentonite and significantly higher than those of the HAl13 and Al13 forms (Figures 4b and 9b)

Sorption of Cu 2+ Ions by HAAl 13 -bentonite
The amount of sorbed Cu 2+ ions increases with their concentration in the initial and equilibrium solutions and the proportion of adsorbed ions decreases expectedly with increasing concentration of the initial copper chloride solution (Figure 9).

Sorption of Cu 2+ Ions by HAAl13-bentonite.
The amount of sorbed Cu 2+ ions increases with their concentration in the initial and equilibrium solutions and the proportion of adsorbed ions decreases expectedly with increasing concentration of the initial copper chloride solution (Figure 9). The maximum amount of sorbed Cu 2+ ions on the HAAl13-bentonite is 0.25 mmol/g and this value is close to the maximum quantity of sorbed Cu 2+ ions by Na-bentonite which equates to 0.33 mmol/g in the conditions of these experiments (Figures 4a and 9a). The proportion of the adsorbed Cu(II) ions on the HAAl13-bentonite at initial concentrations of copper chloride <0.05 mmol/L varies from 60 to 90%, which is comparable with those for the Na-bentonite and significantly higher than those of the HAl13 and Al13 forms (Figures 4b and 9b)  The maximum amount of sorbed Cu 2+ ions on the HAAl 13 -bentonite is 0.25 mmol/g and this value is close to the maximum quantity of sorbed Cu 2+ ions by Na-bentonite which equates to 0.33 mmol/g in the conditions of these experiments (Figures 4a and 9a). The proportion of the adsorbed Cu(II) ions on the HAAl 13 -bentonite at initial concentrations of copper chloride <0.05 mmol/L varies from 60 to 90%, which is comparable with those for the Na-bentonite and significantly higher than those of the HAl 13 and Al 13 forms (Figures 4b and 9b)

Regularities of Sorption of Cu 2+ by Different Bentonite Forms
Calculations performed in the Visual MINTEQ program showed that the proportion of Cu 2+ ions in the solutions used for the sorption experiments is 0.99.
Bentonite saturated with Na + ions under experimental conditions, i.e., at pH 4.5 in the range of ionic strengths from 7.5 × 10 −5 to 1.5 × 10 −3 mol/L sorbs 2-6 times more Cu 2+ than HAl 13 -and Al 13 -bentonite (Figure 4). The values of the coefficient k in the Freundlich equation, which satisfactorily approximates to obtained adsorption isotherms, are significantly higher for Na-bentonite than for HAl 13 and Al 13 -bentonites (Table 1). There are two main mechanisms of sorption of cations by montmorillonite-ion exchange and adsorption on the surface [33][34][35][36][37]. Saturation of bentonite with Al Keggin cations resulted in a significant decrease of CEC from 104 cmol(+)/kg for Na-bentonite, to 29 and 39 cmol(+)/kg for HAl 13 -and Al 13 -bentonite respectively ( Table 1). Given the fact that, in the experiments at the initial concentration of CuCl 2 ≤ 0.125 mmol/L, the pH of the equilibrium solution remains constant ( Figure 5), it can be assumed that the main mechanism of sorption of Cu 2+ ions on Na-bentonite in the given range of concentration is cation exchange.
When using more concentrated solutions of copper chloride, an insignificant decrease of pH by 0.5-0.7 units is observed, which is probably caused by the substitution of H + in aluminol groups by Cu 2+ ion and the formation of surface complexes on the side edges of montmorillonite. It was shown that the pH of the point of zero net proton charge (pH PZC ) of the montmorillonite edge surfaces is about 3.4 units [38]. According to other authors, the pH PZC of montmorillonite varies between 4.7-7.8 [39][40][41][42]. According to calculations conducted in [34] the pKa value of silanol (≡ Si − O− ⇔≡ SiOH) and aluminol (≡ Al(OH)2 ⇔≡ OHOH2) groups of montmorillonites are 7.0 and 8.3, respectively.
Taking the above into account, we can conclude that the pH-dependent edge-sites on montmorillonites are sufficiently protonated in the conditions of the experiment and do not make significant contributions to the adsorption of Cu 2+ ions. The value of the parameter n in the Freundlich equation for Na-bentonite is 1 which might be evidence of the homogeneity of sorption sites.
Therefore, it can be concluded that in the investigated range of concentrations and at a pH of 4.5 of the initial copper chloride solutions, the cation exchange reaction Cu 2+ ↔ Na + is the main mechanism of Cu 2+ sorption on Na-bentonite The saturation of bentonite with Al Keggin cations leads to a decrease of CEC (Table 1) and to an increase in the number and proportion of pH-dependent sorption sites due to the "pillar" structures in montmorillonite interlayers. The titration curves of Na-bentonite obtained using a solution of sodium chloride of different concentrations do not intersect, which indicates the predominance of a constant charge (Figure 10a). The predominance of the pH-dependent charge in Al 13 -bentonite ensures the intersection of the titration curves at the point corresponding to pH PZC = 4.2 (Figure 10b). The obtained pH PZC values for Al 13 -bentonite are consistent with published data [26]. Given the fact that in the experiments with Al13-bentonite the equilibrium pH values are equal to or higher than the pHPZC by less than 0.2-0.4 units, it can be concluded that a significant proportion of the reaction surface of Al13-bentonite is neutrally charged or has a positive charge, which complicates the sorption of Cu 2+ ions on these surfaces from a solution of copper chloride with a pH 4.5. Thus, the saturation of bentonite with Al Keggin cations leads to a significant decrease in CEC, and the additionally formed pH-dependent adsorption sites, for the mentioned reasons, provide much lower sorption of Cu 2+ ions in comparison with ion-exchange reactions. The pHPZC of HAl13-bentonite was not determined in this work, however, it can be assumed that its value would not differ significantly from the pHPZC of Al13-bentonite, which would explain the deterioration of the sorption characteristics of HAl13-bentonite in comparison with Na-bentonite towards copper ions. Similar results were obtained in experiments on the sorption of Cu(II) ions on Al13-bentonite at pH 4.9 [16,43].
A reduction in the equilibrium pH values due to the sorption of Cu 2+ ions on HAl13-and Al13-bentonite in the initial copper chloride solution concentration range from 0.025 to 0.25 mol/L ( Figure 5) can be explained by deprotonation of aluminol groups and the formation of inner-sphere surface complexes. The presence of the dependence of the pH value on the concentration of the initial copper chloride solution indicates that the sorption of Cu 2+ ions on Al-bentonite is mainly carried out by the formation of surface complexes. For both forms of Al-bentonite, the Freundlich parameter n turned out to be < 1, which can be explained by the heterogeneity of the sorption centers, which is more pronounced in the case of HAl13 bentonite (Table 1).

Regularities of HA Sorption on Na-and Al13-bentonites
Humic acids can be adsorbed on montmorillonite by ligand exchange, electrostatic interactions, hydrogen bonding, van der Waals and hydrophobic interactions. Sorption of humic acids on the montmorillonite surface depends on several factors, including the pH, the structure and concentration of the HA, the ionic strength of the solution, and the interaction [44,45]. Both external basal and edge surfaces of montmorillonite are available for the sorption of humic acids.
Siloxane surfaces are mainly hydrophobic (in parts where there is no charge effect and accordingly no influence of the charge-compensating cations) and the edge surfaces have hydrophilic properties due to silanol and aluminol groups. Therefore, the sorption of HA on the basal surfaces can be carried out by hydrophobic interactions. However, sorption by this mechanism does not depend on pH. At montmorillonite's basal surfaces, a constant negative charge is concentrated and therefore compensated for by Na + cations. Since the Na + cations are weakly bonded to the montmorillonite surface in the form of outer-sphere complexes, it can be assumed that at low pH there is a possibility for competition between protonated functional groups of HA and Na + ions for the sorption sites on the external basal surfaces, which leads to a significant increase of sorption with decreasing pH. An increase in pH is accompanied by deprotonation of the functional groups of Given the fact that in the experiments with Al 13 -bentonite the equilibrium pH values are equal to or higher than the pH PZC by less than 0.2-0.4 units, it can be concluded that a significant proportion of the reaction surface of Al 13 -bentonite is neutrally charged or has a positive charge, which complicates the sorption of Cu 2+ ions on these surfaces from a solution of copper chloride with a pH 4.5. Thus, the saturation of bentonite with Al Keggin cations leads to a significant decrease in CEC, and the additionally formed pH-dependent adsorption sites, for the mentioned reasons, provide much lower sorption of Cu 2+ ions in comparison with ion-exchange reactions. The pH PZC of HAl 13 -bentonite was not determined in this work, however, it can be assumed that its value would not differ significantly from the pH PZC of Al 13 -bentonite, which would explain the deterioration of the sorption characteristics of HAl 13 -bentonite in comparison with Na-bentonite towards copper ions. Similar results were obtained in experiments on the sorption of Cu(II) ions on Al 13 -bentonite at pH 4.9 [16,43].
A reduction in the equilibrium pH values due to the sorption of Cu 2+ ions on HAl 13 -and Al 13 -bentonite in the initial copper chloride solution concentration range from 0.025 to 0.25 mol/L ( Figure 5) can be explained by deprotonation of aluminol groups and the formation of inner-sphere surface complexes. The presence of the dependence of the pH value on the concentration of the initial copper chloride solution indicates that the sorption of Cu 2+ ions on Al-bentonite is mainly carried out by the formation of surface complexes. For both forms of Al-bentonite, the Freundlich parameter n turned out to be < 1, which can be explained by the heterogeneity of the sorption centers, which is more pronounced in the case of HAl 13 bentonite (Table 1).

Regularities of HA Sorption on Na-and Al 13 -bentonites
Humic acids can be adsorbed on montmorillonite by ligand exchange, electrostatic interactions, hydrogen bonding, van der Waals and hydrophobic interactions. Sorption of humic acids on the montmorillonite surface depends on several factors, including the pH, the structure and concentration of the HA, the ionic strength of the solution, and the interaction [44,45]. Both external basal and edge surfaces of montmorillonite are available for the sorption of humic acids.
Siloxane surfaces are mainly hydrophobic (in parts where there is no charge effect and accordingly no influence of the charge-compensating cations) and the edge surfaces have hydrophilic properties due to silanol and aluminol groups. Therefore, the sorption of HA on the basal surfaces can be carried out by hydrophobic interactions. However, sorption by this mechanism does not depend on pH. At montmorillonite's basal surfaces, a constant negative charge is concentrated and therefore compensated for by Na + cations. Since the Na + cations are weakly bonded to the montmorillonite surface in the form of outer-sphere complexes, it can be assumed that at low pH there is a possibility for competition between protonated functional groups of HA and Na + ions for the sorption sites on the external basal surfaces, which leads to a significant increase of sorption with decreasing pH. An increase in pH is accompanied by deprotonation of the functional groups of HA and, as a consequence, to the repulsion of HA from the negatively-charged montmorillonite basal surfaces.
At pH < 4.2 silanol and aluminol groups on the edges of montmorillonite are mainly in a protonated state and do not take part in the sorption of HA. Increasing the pH results in the deprotonation of these functional groups and a stronger repulsion of HA from the edge surfaces of montmorillonite.
It is shown that the adsorption of organic compounds involving van der Waals interactions and hydrogen bonding is virtually pH-independent [45].
Based on the above, it can be assumed that the main mechanism of HA sorption from a 24.35 mmol/L solution on Na-bentonite (Figure 7b) is electrostatic interaction with external basal surfaces. Sorption of HA on Na-bentonite from solutions with concentrations C HA from 6.09 to 24.35 mmol/L at pH 3 and I = 0.1 mol/L is satisfactorily approximated by the Freundlich equation and a value of n = 1 may indicate the uniformity of sorption sites (Table 1).
Al 13 -bentonite adsorbs much less humic acid compared to Na-bentonite (Figure 8a,b). It can be explained by the fact that after the saturation of Na-bentonite with AL Keggin cations a significant proportion of negative charge is compensated by a highly charged cation [A1O 4 A1 12 (OH) 24 (H 2 O) 12 ] 7+ , which is strongly bonded on the surface and in the interlayer of montmorillonite and probably only a small amount might be substituted with protonated functional groups of HA with decreasing pH. Consequently, the possibility of HA sorption due to electrostatic interaction on the basal surfaces of Al 13 bentonite decreases. A slight increase in HA sorption by Al 13 -bentonite at pH < 3.5 can be explained by the incomplete substitution of exchangeable cations with Na + during the preparation of Na-bentonite. In the range of pH from 3.5 to 7, HA adsorption on the Al 13 -bentonite occurs to a lesser extent than on the Na-bentonite and does not depend on pH. Artificially created additional pH-dependent sites on the surface of Al 13 -bentonite in highly acidic conditions are becoming protonated and have a positive charge, while at pH > 4.2 they become deprotonated, and the proportion of their negative charge increases. Therefore, the electrostatic interactions between the functional groups of the HA and the montmorillonite surface under these conditions are most likely to be reduced. The lack of pH dependence of the sorption at pH > 3.5 indicates that the main mechanisms of HA sorption may be hydrophobic, van der Waals interactions and hydrogen bonding that are essentially independent of pH.
The value of the parameter n = 1 in the Freundlich equation describing the sorption of HA on Al 13 -bentonite in the studied concentration range at pH 3 and I = 0.1 mol/L may indicate the homogeneity of the sorption centers ( Table 1).
The lack of a dependence, or the presence of a weak dependence, of the amount of sorbed HA of pH at C HA = 6.09 mmol/L (Figure 7a) can be explained by small concentrations of C HA and errors in a determination by the spectroscopic method.
In addition to the considered mechanisms of HA sorption on different forms of bentonite, coagulation at low pH values cannot be excluded. However, this process has not been studied in this present work.

Sorption of Cu 2+ Ions by HAAl 13 -bentonite
The sorption of Cu 2+ ions by HAAl 13 -bentonite in the range of studied concentrations is described by the Freundlich equation, although with a lower coefficient of determination in comparison with Na and Al forms of bentonite (Table 1). An increase in sorption and in the degree of extraction of Cu 2+ ions from solutions by HAAl 13 -bentonite in comparison with HAl 13 -and Al 13 -bentonite can be explained by the formation of additional sorption sites on the surface of humic acid-modified Al 13 -bentonite. It was shown that the sorption of humic acids on Na-bentonite, Ca-bentonite and goethite leads to a significant increase in the amount of adsorbed metal cations by increasing the amount of the sorption sites on the surface of minerals available for complexation [21,46,47]. The value n = 0.5 in the Freundlich equation probably indicates the inhomogeneity of sorption sites on the obtained organomineral complex.
As already mentioned above, the sorption experiments of Cu 2+ ions by HAAl 13 -bentonite was carried out at constant ionic strength of 0.1 mol/L, and in experiments, with Na-and Al 13 -bentonites ionic strength ranged between 10 −5 up to 10 −3 mol/L. It is known that an increase of ionic strength leads to a decrease of sorption of both metal ions and humic acid [39,40,[48][49][50]. However, a change in the ionic strength in the range from 10 −5 to 10 −3 mol/L affects the sorption of Cu 2+ ions slightly. With an increase in ionic strength to 0.1 mol/L, the amount of sorbed metal ions decreases significantly [51]. Despite the fact that in the sorption experiments of Cu 2+ by HAAl 13 -bentonite, the ionic strength was in 2-4 times higher than that in the experiments with Na-bentonite, the amount of sorbed Cu 2+ ions and the degree of extraction from solutions are similar to those for Na-bentonite

Conclusions
1. The modification of Na-bentonite with Al Keggin cations leads to an increase in the interplanar spacing of montmorillonite from 1.29 nm for the Na-form to 1.85 and 1.78 nm for HAl 13 -and Al 13forms respectively; a reduction of CEC; and the formation of additional surfaces with a variable charge having pH PZC 4.2. Al 13 -bentonite is not affected by heat.
2. Under the conditions of the experiments carried out, Na-bentonite adsorbs more Cu 2+ ions from CuCl 2 solutions with a pH of 4.5, compared to Al 13 forms of bentonites. The main mechanism of copper sorption on Na-bentonite is the cation exchange Cu 2+ ↔ Na + . The reduction of CEC of Na-bentonite after modification with Al Keggin cations leads to a decrease in the sorption capacity of Cu 2+ . pH-dependent sorption sites formed on Al 13--bentonites have a pH PZC of 4.2 and, therefore, under the conditions of the experiment carried out, have a significant positive charge and prevent intensive sorption of Cu 2+ ions. Sorption of Cu 2+ ions by all studied bentonite forms can be satisfactorily approximated by the Freundlich equation.
3. Na-bentonite adsorbs more HA, compared with Al 13 -bentonite and the proportion of adsorbed HA does not change over the entire concentration range. A decrease in pH leads to an increase in the sorption of HA by Na-bentonite and weakly affects the HA sorption by Al 13 -bentonite.
The main sorption mechanism of HA by Na-bentonite is electrostatic interaction with the external basal surfaces, while for Al 13 -bentonite it is a combination of hydrophobic, van der Waals interactions and hydrogen bonding that are virtually independent of pH. At low pH values, the coagulation of HA is possible and may affect the obtained results.
4. Treatment of the Al 13 -bentonite surface with HA leads to the formation of the additional sorption sites. The amount of sorbed Cu 2+ ions and the degree of their extraction from solutions by HAAl 13 -bentonite is close to those values for Na-bentonite.