Sorption of copper(II) ions from aqueous solution by activated carbon BAU-A and coal sorbent MIU-S. The relationship between the structure of sorbents and their sorption properties

Bivalent copper ions are one of the most dangerous environmental pollutants. They are capable of causing acute poisoning in humans and animals, and have a wide range of toxic effects with diverse clinical manifestations (Desai & Kaler 2008). In this regard, there are serious restrictions on the maximum content of copper(II) ions in the water discharged into the environment. For example, according to Russian's regulatory documents the concentration of copper(II) ions in wastewater discharged into fishery basins must not be exceed 1 μg/L. However, it is impossible to achieve such low concentrations of copper(II) ions in solutions to be purified by the traditional method using milk of lime. Therefore, the development of methods for deep purification of wastewater to such a low residual concentration of copper(II) ions in them is a very important task, and the use of sorbents can be one of the ways to solve it, especially at the stage of final post-treatment of wastewater. In this regard, the search for new effective sorbents for copper ions continues.

The efficiency of using sorbents for cleaning contaminated solutions is largely determined by their specific surface area, as well as their ability to sorb undesirable impurities. It is obvious that the larger these values, the more efficiently this sorbent will purify the contaminated solution.

An increase in the specific surface of sorbents can be achieved both by reducing the size of their particles and by increasing the porosity of the latter. However, a decrease in the particle size of sorbents complicates the process of separating them from the solution after its purification. Carbon sorbents obtained from natural raw materials do not have this drawback, due to the fact that their particles, as a rule, have a porous structure. Therefore, despite the large particle size, these sorbents have a high specific surface area. Another advantage of carbon sorbents is the possibility of their use in solutions of almost any chemical composition, both acidic and alkaline. All this predetermines their advantage in industrial technologies for the purification of contaminated solutions over other types of sorbents.

Activated carbons and different carbon sorbents are applied for removal of pollutants from natural water and wastewater for a long time. They are most often used for removal of oil and dangerous organic compounds (Newcombe et al. 1993; Babel & Kurniawan 2003; Stafiej & Pyrzynska 2007; Sulaymon et al. 2009; Shen et al. 2017; Zhao et al. 2020; Gomes et al. 2022). Besides, they are applied for removal of heavy metals (Goyal et al. 2001; Babel & Kurniawan 2003; Machida et al. 2005; Rao et al. 2007; Imamoglu & Tekir 2008; Barcat et al. 2009; Harrelkas et al. 2009; Kuo 2009; Pyrzyńska & Bystrzejewski 2010; Gorbacheva et al. 2017; Duan et al. 2020; Serna-Carrizales et al. 2021; Usanmaz et al. 2021; Gomes et al. 2022) and rare earth ions from polluted water (Qiu et al. 2019).

Sorption of copper(II) ions by various carbon sorbents was studied in a number of works (Newcombe et al. 1993; Goyal et al. 2001; Machida et al. 2005; Rao et al. 2007; Stafiej & Pyrzynska 2007; Imamoglu & Tekir 2008; Kuo 2009; Sulaymon et al. 2009; Pyrzyńska & Bystrzejewski 2010). In this case, most often these were sorbents synthesized in laboratory experiments. In this work, we studied the sorption properties of two Russian industrial carbon sorbents, BAU-A and MIU-S, with respect to copper(II) ions. Both sorbents consist of almost 100% carbon, but are obtained in different ways from natural raw materials.

Sorbent BAU-A is a crushed activated carbon obtained under the influence of water vapor at temperatures of 800–950 ^oC. This sorbent has a great specific surface due to its high porosity. It is used in industry to remove oil, phenols, and other organic impurities from polluted solutions (Gorbacheva et al. 2017).

The MIU-S coal sorbent differs from activated carbons and other carbon sorbents. It is produced from natural long-flame coals (grade D coal). The MIU-S structure is a solid inorganic polymer consisting of several carbon layers that are interconnected by Van der Waals's forces (Tarnopol'skaya 2006; Tarnopol'skaya & Solov'yova 2011; Tarnopol'skaya & Hagger 2012). These layers consist of aromatic structures and aliphatic chains of hydrocarbons with atoms of hydrogen, oxygen, nitrogen and sulfur. Currently MIU-S is used in industry to remove iron ions, ammonium, heavy metals, bacterial pollutions, hydrogen sulphide, phenols, dyes, and other pollutants from wastewater (Tarnopol'skaya 2006; Tarnopol'skaya & Solov'yova 2011; Tarnopol'skaya & Hagger 2012).

Despite the widespread use of these sorbents in Russian industry, their sorption properties in relation to heavy metals, and in particular copper(II) ions, in a wide range of pH are still studied insufficiently. This makes it difficult to purposefully use them for the purification of polluted water. Mechanism of sorption of copper(II) ions by MIU-S and BAU-A sorbents and parameters of this process depending of pH are unknown. This work is an attempt to fill this gap. In addition, the relationship between the structure of sorbents and their sorption properties is of interest.

Commercial samples of sorbents were used in the study: technically pure (the content of the main component is above 95%) activated carbon BAU-A (Russia, GOST 6217-74) and coal sorbent MIU-S. Note, the commercial coal sorbent MIU-S has several modifications (MIU-S1, MIU-S2, MIU-S3, and MIU-S4) with different size of particles. Modification MIU-S4 with size of particles of 0.06–0.6 mm was chosen for our laboratory studies. This choice was due to the fact that with this size of sorbent particles, they could easily be transferred to a suspended state by stirring the solution with a magnetic stirrer.

All experiments were carried out at room temperature (25 ± 2 °C) with a model solution simulating natural sulfate mine water and some types of industrial pickling washings wastewater. The model solution was prepared by mixing specified volumes of concentrated solutions of sodium and copper(II) sulfates (Na₂SO₄ and CuSO₄) in distilled water. The concentration of sodium sulfate in the prepared model solution was 400 mg/L.

To prepare the solutions, Russian-made reagents were used. NaOH had the qualification ‘chemically pure’, Na₂SO₄ – ‘pure’, CuSO₄ – ‘analytical grade’. The solutions were stirred during the experiments using an MR Hei-Tec mixer (Heidolph Instruments GmbH & Co. KG). The stirring speed was 650 rpm to maintain in suspension the copper hydroxide precipitate formed in the solution in preliminary experiments and the BAU-A and MIU-S sorbents in sorption runs. The pH of solutions was controlled and maintained at a constant level using an ANION 4100 pH meter (Russia). The pH meter electrodes were continuously immersed in the model solution throughout the experiments.

Prior to the sorption experiments, preliminary runs on a precipitation of copper(II) hydroxide from the model solution by caustic soda were performed. The purpose of these experiments was to determine the residual concentration of copper(II) ions in the model solution, depending on the pH of the latter. This was necessary in order to eliminate the precipitation of copper(II) from the solution in the form of hydroxide in further experiments on sorption.

In these experiments, various amounts of NaOH solution (5 g/L) were added to the model solution (pH = 5.43–6.29) with a concentration of copper(II) ions in it of 13.05 mg/L (this concentration of copper(II) ions is close to that found in natural sulfate mine water and some types of industrial pickling washings wastewater) with continuous stirring on a magnetic stirrer. Then the solution, with its continuous stirring with a magnetic stirrer, was kept for at least 2 hours until the pH became constant. After the end of stirring, the pH of the solution was measured, and the formed precipitate of copper hydroxide was separated from the solution by filtration through a paper filter. The residual content of copper(II) ions was determined in the filtrate.

In addition to experiments on a precipitation of copper hydroxide, several runs were conducted to study the interaction of sorbents with the model solution. In these experiments, the samples of sorbents (BAU-A or MIU-S) were kept in the solution of sodium sulfate (400 mg/L) for some time with continuous stirring of the suspension by a magnetic stirrer. During the runs the pH of the solution was periodically measured, and at the end of the experiments chemical analysis of the solution was carried out.

Sorption experiments were performed at different pH values of the model solution. In these runs, a different amount of sorbent (BAU-A or MIU-S) was added to the model solution with its continuous stirring on a magnetic stirrer. Then, using 0.1 N solutions of H₂SO₄ and NaOH the pH of the model solution was adjusted to the desired value. After 15–120 minutes of exposure, the sorbent was separated from the solution by filtering through a paper filter. The filtration process took 20–30 min. Thus, the total contact time of the solution with the sorbent was 35–150 min.

The initial concentration of copper(II) ions in the model solution in sorption experiments was different and depended on pH of the model solution. It was selected on the basis of the results of preliminary experiments on the precipitation of cooper(II) hydroxide with caustic soda. Thus, for pH 4, 5, 6, and 6.7 it was 12.72, 12.72, 6.36, and 3.816 mg/L, respectively.

The concentration of copper(II) ions in the solution was determined by a photocolorimetric method with sodium diethyldithiocarbamate on a photocolorimeter KFK-2 (Russia) (Novikov et al. 1990). In total, three parallel determinations of the concentration of copper(II) ions in the solution were made. The results obtained were averaged. In order to exclude possible distortion of the analysis results during filtration due to the sorption of a part of copper(II) ions on a paper filter, the first portions of the filtered solutions with a volume of about 100 mL were discarded and the remaining volumes of the filtrate were taken for analysis.

The pH of the point of zero charge (pH_ZPC) at which the absolute charge on the surface of sorbent is zero was determined by the method of potentiometric titration. For this purpose, one sample with a volume of 50 mL of 400 mg/L Na₂SO₄ solution was titrated with 0.0105 N H₂SO₄ solution, and another such sample was titrated with 0.01017 N NaOH solution. Then, a similar titration was carried out for a suspension of 0.5 g of a sorbent (BAU or MIU-S) in a 400 mg/L Na₂SO₄ solution. Note that before this, the sorbent samples were thoroughly washed in distillation water to a constant pH of the solution in order to remove soluble impurities from them (CaCO₃, MgCO₃, etc.). The point of intersection of the titration curves of the 400 mg/L Na₂SO₄ solution and the suspension of 0.5 g of sorbent (BAU-A or MIU-S) in the 400 mg/L Na₂SO₄ solution gives the pH_ZPC of a given sorbent.

To study the properties of the surface of the sorbents, the Raman and IR spectra were recorded. Raman spectra were recorded in the range of 50–4000 cm⁻¹ at room temperature on a RENISHAW-1000 spectrometer (λ = 532 nm, P = 25 mW). The IR spectra were recorded on a Vertex 80 IR Fourier spectrometer (Bruker) in the range 4000–400 cm⁻¹ from powder samples pelleted with CsI.

The specific surface area of sorbents was determined by the BET method of nitrogen adsorption on a Gemini VII 2390 V1.03 (USA).

The morphology of sorbents was determined by the scanning electron microscopy (SEM) method on a JSM JEOL 6390LA device.

X-ray phase analysis of sorbents was performed on a STADI-P X-ray powder diffractometer (STOE, Germany).

The results obtained on the precipitation of copper(II) ions from the model solution, depending on its pH, are shown in Figure 1. It is seen that, as expected, with an increase in the pH of the solution, the concentration of copper(II) ions in it decreases. Comparison with the theoretical calculations of Markov et al. (2007) shows that at pH = 7–9 they are very close to our experimental results. However, at pH < 7 a significant deviation is observed (Figure 1). In particular, the residual concentration of copper(II) ions found by us is below the theoretical curve (Markov et al. 2007). This can be explained by the formation of small amounts of insoluble Cu₂(OH)CO₃ (malachite) and Cu₃(OH)₂(CO₃)₂ (azurite) in our experiments.

From the data presented in Figure 1, it follows that in order to avoid the deposition of copper(II) ions in the form of hydroxide (or malachite, or azurite), their concentration, for example at pH = 6.7, should be 5 mg/L or lower. However, in order to guarantee the prevention of the latter process, in further sorption experiments we set the copper concentration in the model solution at pH = 6.7 equal to 3.816 mg/L, and for pH 4, 5, and 6 it was 12.72, 12.72, and 6.36 mg/L, respectively.

SEM images of the sorbents are shown in Figure 2. As can be seen, BAU-A and MIU-S have different morphology and structures. BAU-A has a pronounced porous composition (Figure 2(a) and 2(b)). This is confirmed by measuring the specific surface of BAU-A. It showed that the specific surface area of BAU-A is 984.94 m²/g. The pore surface is 674.6 m²/g. Thus, the fraction of pore surface area is about 69% of the total surface area of BAU-A. The MIU-S in contradiction to BAU-A has a layered structure (Figure 2(c) and 2(d)). Measurement of the specific surface of MIU-S showed that it is 22.27 m²/g. It was found that the specific surface of pores is only 0.25 m²/g. Thus, the contribution of pores to the specific surface of the MIU-S sorbent is very insignificant.

Raman spectra of BAU-A and MIU-S sorbents in Figure 3a show significant differences.

It can be seen from Figure 3(a) that the Raman spectrum of BAU-A contains two lines characteristic of amorphous carbon: the D-line at ∼1332 cm⁻¹ and the G-line at 1593 cm⁻¹. The position and intensity of the G-line at 1593 cm⁻¹ make it possible to determine the degree of carbon graphitization, since it corresponds to vibrations of sp2-hybridized atoms located in well-ordered graphite planes. The line at 1332 cm⁻¹ is due to the bonds of C–C with sp3-type hybridization and is responsible for structural disordering and serves as a characteristic of the degree of defects of the carbon phase, and its significant broadening and high intensity indicate the amorphous state of carbon in the sorbent. Lines of low intensity around 2600–3000 cm⁻¹ are overtones of lines D and G.

For MIU-S along with D-line at 1372 cm⁻¹ and G-line at 1602 cm⁻¹ there are second order peaks at 2271 cm⁻¹ and 2920 cm⁻¹. They are characteristic of amorphous carbon. The line at 3491 cm⁻¹ corresponds to stretching vibrations of OH-groups, and the other line at about 1100 cm⁻¹ corresponds to the bending vibrations of the hydroxyl.

The ratio between the integral intensities of the G and D bands (i.e. G/D) is known to be an indicator of crystallinity. Calculation in accordance with Figure 3(a) gives G/D = 0.70 for BAU-A and G/D = 1.51 for MIU-S. Thus, the MIU-S coal sorbent has a higher crystallinity compared to BAU-A activated carbon.

The IR spectrum of the MIU-S coal sorbent is shown in Figure 3(b). For BAU-A, the IR spectrum could not be recorded, since this sorbent is certified by Raman spectroscopy as amorphous carbon.

Figure 3(b) shows that in the IR spectrum of MIU-S, as well as in the Raman spectrum, there are characteristic bands absorption of water: stretching vibrations at 3424 cm⁻¹ and bending vibrations at 1613 cm⁻¹. Two narrow intense bands at 3694 and 3621 cm⁻¹ correspond to the stretching vibrations of hydroxyl groups, and lines near 1035 cm⁻¹ are the bending vibrations of OH bonds. Asymmetrical and symmetrical stretching vibrations of the C-H bonds are characterized by intense narrow bands at 2960, 2927 and 2856 cm⁻¹, and bending vibrations by a weak band at 1452 cm⁻¹. In the field of frequencies of bending vibrations of the C-H group, a narrow band of low intensity at 1279 cm⁻¹ is observed. The band at 1359 cm⁻¹ belongs to asymmetrical stretching vibrations of nitrate ion (impurity in the matrix of CsI). The presence of absorption bands of C-H and OH bonds indicates the presence of alcohols in MIU-S.

Figure 4 shows the X-ray powder diffraction patterns of BAU-A and MIU-S sorbents. As can be seen, BAU-A is an amorphous material (Figure 4(a)). In MIU-S, lines of small amounts of impurities in the form of quartz (SiO₂) and kaolinite (Al₄(OH)₈Si₄OH₁₀) were recorded (Figure 4(b)).

The change in the pH of the model solution when the sorbents BAU-A and MIU-S are introduced into it is shown in Figure 5.

As seen in Figure 5(a), after the introduction of BAU-A into the sodium sulfate solution, within the first 20 minutes after stirring, a rather significant increase in the pH of the solution to pH ≈ 9.5 was observed (Figure 5(a), run 1). The analysis of the solution after experiment showed the presence in the solution of bicarbonate (34.2 mg/L), calcium (4.5 mg/L), and magnesium (3.6 mg/L) ions. Probably, it was due to the presence of small amounts of calcium and magnesium carbonates in BAU-A. Apparently, these impurities are present in BAU-A in very small amounts, since X-ray phase analysis did not detect them (please see Figure 4(a)).

Then, after about 220 minutes, the pH of the model solution decreased to about 7.7. The reason for this change in the alkalinity of the solution is not entirely clear. It can only be assumed that the decrease in the pH of the solution may be associated with the sorption of OH⁻ anions on the surface of the sorbent.

After second exposure of a sample of the sorbent from run 1 in a new solution of sodium sulfate, a significant rise in the pH of the solution, such as it was in run 1, was not observed. The hydrogen index increased slightly (up to pH ≈ 7) and stabilized at this value after 20 minutes from the beginning of stirring of the suspension (Figure 5(a), run 2). In this case, the content of bicarbonate (24.4 mg/L) and magnesium (2.4 mg/L) ions reduced in the solution, and calcium was found only in trace amounts. A similar pattern was observed in run 3, when a sample of the sorbent from run 2 was again added into the fresh sodium sulfate solution (Figure 5(a), run 3). As is seen in Figure 5(a), the experimental points of runs 2 and 3 formed almost one curve. Chemical analysis of the solution after run 3 showed a further decrease in the concentration of bicarbonate (21.9 mg/L) and magnesium (1.4 mg/L) ions. All this confirms the assumption about the presence of small amounts of calcium and magnesium carbonate compounds in BAU-A, the transition of which in the liquid phase causes an increase in the pH of sodium sulfate solution.

The interaction of the sorbent MIU-S with sodium sulfate solution proceeds in a similar way (Figure 5(b)). However, in this case there is only a slight increase in the pH of the model solution, also, apparently, due to the presence of small amounts of calcium and magnesium carbonates in MIU-S. This, in particular, is indicated by the results of the analysis of the solution after experiment, which showed the presence of bicarbonate (30.5 mg/L), calcium (4 mg/L), and magnesium (2.4 mg/L) ions in the model solution. These impurities are also not detected by X-ray phase analysis. The kaolinite present in MIA-C (see Figure 4(b)) can also shift the pH of the solution to the alkaline region.

From Figure 5(b) it is seen that the pH of the solution is stabilized after about 80 minutes from the beginning of stirring MIU-S in it.

The curves of potentiometric titration of suspensions of sorbents by 0.0105 N H₂SO₄ solution and 0.01017 N NaOH solution are shown in Figure 6. Taking into account the results obtained in previous experiments (Figure 5), before determining the zero charge point (pH_ZPC), both sorbents were thoroughly (several times) washed in distilled water to a constant pH of the solution in order to remove soluble impurities (CaCO₃, MgCO₃, etc.).

In Figure 6 it is seen that the point of zero charge of the surface of the activated carbon BAU-A corresponds to pH = 7.3 (pH_ZPC = 7.3, Figure 6(a)), and of the MIU-S coal sorbent – pH = 6.4 (pH_ZPC = 6.4, Figure 6(b)).

In Figure 6 it also follows that BAU-A and MIU-S adsorb hydroxide ions (OH⁻) on their surface well. This is indicated by the significant difference between titration curves 1 and 2 at V(NaOH) > 0 in Figure 6. So, when alkali is added to a pure solution of sodium sulfate, the pH of the latter sharply increases, while when the BAU-A and MIU-S suspensions are alkalized, the increase in pH is not so strong.

During acidification, a similar picture is observed: the introduction of sulfuric acid into a pure solution of sodium sulfate leads to a sharp drop in its pH, and upon acidification of BAU-A and MIU-S suspensions, the pH shift occurs to a lesser extent. This indicates that hydrogen ions (H⁺) are also well adsorbed on the surface of the sorbents. However, taking into account the fact that when BAU-A is introduced into a pure solution of sodium sulfate, a shift in the pH of the latter to an alkaline region from pH ≈ 5.5 to pH ≈ 7 (see Figure 6(a)), it can be concluded that H⁺ ions are adsorbed on the surface of the sorbent in large quantities compared to OH⁻ ions. In MIU-S, this shift is much less (from pH ≈ 5.8 to pH ≈ 6.1). Therefore, we can conclude that this sorbent adsorbs H⁺ ions weaker than BAU-A.

The effect of contact time on removal Cu(II) ions is presented in Figure 7, which indicates that the sorption equilibrium in the solution is established after 30 minutes from the start of its mixing.

Taking into account the dependencies in Figure 7, in further sorption experiments, their duration was chosen to be 30 min in order to guarantee the completion of the process of removing Cu(II) ions from the solution under equilibrium conditions.

Figure 8 shows the effect of BAU-A and MIU-S concentrations on the residual concentration of copper(II) ions in the model solution at pH = 5. Similar dependences were obtained for other pH values.

As can be seen, with an increase in the concentration of BAU-A and MIU-S in the model solution, the content of copper(II) ions in it decreases (Figure 8(a)). Moreover, BAU-A removes copper ions better than MIU-S. However, for a correct comparison of the sorption capacity of these two carbon materials, one should take into account their specific surface area, namely, the total surface area of the sorbents placed in the model solution (Figure 8(b)).

From Figure 8(b) it can be seen that although both sorbents are almost entirely composed of carbon, their sorption properties with respect to copper(II) ions are significantly different. For example, with a total surface area of sorbents in a solution of 1000 m²/L, the residual concentration of copper(II) ions in the model solution for MIU-S sorbent is about 3 mg/L, and for BAU-A sorbent it is 11 mg/L (Figure 8(b)). Similar dependences were obtained for other pH values of the model solution.

Sorption isotherms of copper(II) ions at different pH values of the model solution onto BAU-A activated carbon and MIU-S coal sorbent are shown in Figure 9.

Isotherms of adsorption of copper(II) ions by BAU-A activated carbon and MIU-S coal sorbent at different pH values of the model solution in the coordinates of linear forms of Equations (3) and (4) are shown in Figure 10.

As one can see, all experimental points are well approximated by straight lines. This indicates that the sorption of copper(II) ions by activated carbon BAU-A and the carbon sorbent MIU-S in the model solution can be described with satisfactory accuracy by the classical isotherms of Langmuir and Freundlich. The parameters of Equations (3) and (4) calculated from the experimental data are given in Tables 1 and 2.

From Tables 1 and 2 it can be seen that Equations (3) and (4) describe, with approximately the same accuracy, the sorption of copper(II) ions by sorbents BAU-A and MIU-S. This is a fairly typical situation (see, for example, Imamoglu & Tekir 2008). However, the parameters of the Langmuir equation, in contrast to the empirical formula of Freundlich, have a clear physical meaning. This allows them to be compared with similar values for other sorbents.

For both sorbents, BAU-A and MIU-S, an increase in the sorption capacity of a monolayer with an increase in the pH of the model solution is observed. However, the adsorption equilibrium constants (K_L) behave differently. If for BAU-A K_L decreases with increasing pH of the model solution, then for MIU-C, on the contrary, it increases. At the same time, in terms of mg/g, the sorption capacity of the BAU-A sorbent is almost twice that of MIU-S. However, if we take into account the specific surface area of the sorbents, the opposite ratio is obtained: the sorption capacity of MIU-S in the dimension of mg/m² is almost two orders of magnitude higher than q_∞ for BAU-A. So, at pH = 4, the sorption capacity of a monolayer for BAU-A is 0.00092 mg/m², and for MIU-S it is 0.01191 mg/m² (see Tables 1 and 2). Such significant differences in the sorption capacity in terms of mg/m² indicate the different ability of the BAU-A and MIU-S materials to adsorb copper(II) ions, and the observed excess of the sorption capacity of BAU-A in terms of mg/g over the same value for MIU-S is due to its higher specific surface area.

It is known that if E lies in the range of 8–16 kJ/mol, then the adsorption process proceeds by ion exchange. At E < 8 kJ/mol, physical adsorption takes place (Donat et al. 2005; Yang et al. 2009).

The results of recalculating the experimental data in the coordinates of Equation (8) are shown in Figure 11, and its parameters are given in Tables 1 and 2.

From Figure 11 one can see that the Dubinin-Radushkevich equation describes the experimental data somewhat worse than the Langmuir and Frendlich isotherms. This is also indicated by the lower correlation coefficients of this equation (Tables 1 and 2). The capacities of the adsorption monolayer of BAU-A and MIU-S sorbents calculated according to Equation (7) at different pH values of the model solution, with respect to copper(II) ions, turned out to be somewhat lower than the values found from the Langmuir isotherm. However, the tendency for their values to increase with increasing pH of the model solution persists. The calculated values of the free energy of adsorption (E) do not exceed 8 kJ/mol (Tables 1 and 2). This indicates the physical nature of adsorption and excludes the ion-exchange interaction of copper(II) ions with sorbents.

Obviously, the differences found in the sorption parameters of BAU-A and MIU-S are largely due to differences in the properties of the sorbent surfaces. So, from the data obtained above, it can be seen that BAU-A and MIU-S have different morphology (see Figure 2) and structure (Figures 3 and 4). As a result of this, the pH of the point of zero charge (pH_ZPC) of BAU-A and MIU-S also has different values. In addition, it can be seen that the G/D ratio of the MIU-S sorbent (the ratio of the integrated intensities of the G and D bands in the Raman spectra, Figure 3(a)) is higher than that of BAU-A activated carbon, and the adsorption capacity of MIU-S in the dimension of mg/m² is also higher than the adsorption capacity of BAU-A. For the pH_ZPC we have the opposite picture: pH_ZPC of MIU-S is less of pH_ZPC of BAU-A. In this regard, it is of interest to compare the adsorption and other properties of BAU-A activated carbon and MIU-S coal sorbent with similar properties of other carbon sorbents.

Thus, activated carbon obtained in the work by Imamoglu & Takiri (2008) at pH = 5.7 has a sorption capacity for copper(II) ions of 6.645 mg/L (or 0.0061 mg/m²). In the mg/g dimension this slightly exceeds the similar value for BAU-A, but in the mg/m² dimension it is much lower than the sorption capacity of BAU-A and MIU-C.

In Pyrzyńska & Bystrzejewski (2010), the sorption of copper(II) ions was studied at different pH values of a solution of three carbon sorbents: activated carbon (AC), carbon nanotubes (CNTs), and carbon-encapsulated nanoparticles (CEMNPs). The work lacks data on the sorption capacity of the studied sorbents, but calculations according to Figure 3 and Table S1 of Pyrzyńska & Bystrzejewski (2010) give the next values of adsorption at pH = 5: q(AC) = 0.038 mg/g = 0.00009 mg/m², q(CNTs) = 0.104 mg/g = 0.0014 mg/m², and q(CEMNPs) = 1.098 mg/g = 0.0229 mg/m². For pH = 6 we have: q(AC) = 0.422 mg/g = 0.001 mg/m², q(CNTs) = 0.514 mg/g = 0.0069 mg/m², and q(CEMNPs) = 1.506 mg/g = 0.0314 mg/m². Note that we did not use the data of Pyrzyńska & Bystrzejewski (2010) for adsorption at pH 7 and 8, as they are questionable. In particular, the initial concentration of copper(II) ions in solution in Pyrzyńska & Bystrzejewski (2010) was 10 mg/L. However, as can be seen above from Figure 1, at pH 7 and 8 and at an initial concentration of copper(II) ions in a solution of 10 mg/L, the precipitation of copper(II) ions in the form of Cu(OH)₂ will be observed. Therefore, this process can distort the results (Pyrzyńska & Bystrzejewski 2010) when determining adsorption at pH 7 and 8.

To compare the sorption properties of sorbents AC, CNTs and CEMNPs with BAU-A and MIU-S, the adsorption values for BAU-A and MIU-S were calculated at the same pH, at the same initial concentration of copper(II) ions and for the same total surface area of the sorbents in solution, as for AC, CNTs and CEMNPs. The results are shown in Table 3. It also presents the values for pH_ZPC and the ratio G/D for these sorbents.

It can be seen from Table 3 that at pH = 5 the adsorption values for CNTs (Pyrzyńska & Bystrzejewski 2010) and BAU-A activated carbon are almost the same, but at pH = 6 some difference is observed. At the same time, the adsorption values for the MIU-S coal sorbent at pH 5 and 6 are almost an order of magnitude higher than the adsorption values for CNTs and BAU-A.

For sorbent CEMNPs (Pyrzyńska & Bystrzejewski 2010) we observe an opposite picture: values of adsorption of CEMNPs and MIU-S practically coincide but the adsorption of BAU-A is almost one order of magnitude less.

The adsorption of activated carbon AC (Pyrzyńska & Bystrzejewski 2010) at pH = 5 has a very low value but at pH = 6 it increases 10 times. However, it still is less then the values of adsorption of BAU-A and MIU-S.

The removal of copper(II) ions by as-grown multi-walled carbon nanotubes (As-CNTs) was studied by Kuo (2009). The Langmuir isotherm coefficients for pH = 6 and various solution temperatures are presented in Table 2 of Kuo (2009). For 300 K one can find q_∞ = 9.4 mg/g = 0.1202 mg/m². This is close to the capacity of the adsorption monolayer of MIU-S at pH = 6 (please see above Table 2). However, the found value for adsorption constant K_L = 0.022 L/mg is 20 times less than that of MIU-S. In this case, the recalculation of the data from Figure 3 of Kuo (2009) gives close values for adsorption of As-CNT and MIU-S at pH 5 and 6 (please see Table 3). There are only some differences at pH = 4.

The adsorption of copper(II) ions by granular activated carbon (GAC) was studied in Machida et al. (2005). According to the data given in Table 1 of Machida et al. (2005) the parameters of the Langmuir equation for GAC at pH = 6 have the following values: K_L = 12 L/mmol = 0.189 L/mg and q_∞ = 0.056 mmol/g = 0.0036 mg/m². As can be seen, the last value is close to the capacity of the adsorption monolayer of BAU-A at pH = 6 (please see Table 1). Calculating the data from Figure 1 of Machida et al. (2005) gives a close value of adsorption only for pH = 6 (please see Table 3).

According to Sulaymon et al. (2009), the parameters of the Langmuir equation for the studied granular activated carbon at pH = 6–6.9 have the following values: q_∞ = 5.845 mg/g = 0.0097 mg/m² and K_L = 0.710 L/mg. From the data in Table 1 for this range of pH for BAU-A it follows q_∞ = 0.00258–0.01113 mg/m² and K_L = 0.771–0.232 L/mg.

As you can see, in row (R3) BAU-A and MIU-S changed places, but AC remained in its place.

Thus, in the series for pH_ZPC (R3, R6, R10) there is no clear relationship with the sorption value (q). However, in the series for the G/D ratio (R2, R5, R9), in most cases, it is found: a smaller G/D value corresponds to a smaller sorption value q. Moreover, a linear relationship is found between these two quantities (Figure 12). So, from Figure 12 it is clearly seen that the coefficient of determination (R²) of these dependencies has a sufficiently high value confirming their significance.

The revealed relationship between the G/D ratio and the adsorption value requires, of course, further verification. Unfortunately, we still cannot find an explanation for this dependence. However, in some cases, this regularity can be used to select effective sorbents for removing copper(II) ions from contaminated solutions.

The sorption of copper(II) ions in a wide range of pH by two Russian industrial carbon sorbents BAU-A and MIU-S was studied. Sorbents varied in structure, surface area, pH of the zero charge point (pH_ZPC), and degree of graphitization.

The removal of copper(II) ions from the solution by both sorbents at pH 4, 5, 6, and 6.7 is described with satisfactory accuracy by the classical adsorption isotherms of Freundlich and Langmuir.

With an increase in pH from 4 to 6.7, the sorption capacity of sorbents increases, from 0.910 to 7.163 mg/g for BAU-A, and from 0.265 to 3.307 mg/g for MIU-S.

The calculated values of the free energy of adsorption for both sorbents do not exceed 8 kJ/mol. This indicates the physical nature of adsorption and excludes the ion-exchange interaction of copper(II) ions with sorbents.

This study shows that there is a relationship between the G/D ratio and the sorption properties of the sorbents. This regularity can be used to select effective sorbents for removing copper (II) ions from contaminated solutions.

This work was carried out in accordance with the scientific and research plans and state assignment of the Institute of Solid State Chemistry of the Ural Branch of the Russian Academy of Sciences.

All relevant data are included in the paper or its Supplementary Information.