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Article

Study of Sorption Activity of Carbon Nanomaterials for Capture of Chlorine-Containing Gases

by
Yulia Ioni
1,* and
Victoria Ibragimova
1,2
1
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii Prospect, 31, 119991 Moscow, Russia
2
Department of Materials Science, Lomonosov Moscow State University, Building 73, Leninskie Gory 1, 119991 Moscow, Russia
*
Author to whom correspondence should be addressed.
Clean Technol. 2025, 7(2), 39; https://doi.org/10.3390/cleantechnol7020039
Submission received: 19 November 2024 / Revised: 22 February 2025 / Accepted: 31 March 2025 / Published: 8 May 2025
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Abstract

:
Chlorine gas and hydrogen chloride are highly reactive chemicals that pose a significant hazard to living organisms upon direct contact. Also, chlorine-containing gases are often by-products of industrial chemical synthesis and can be released into the air as a result of accidents. This can lead to great pollution of the environment. To remove toxic gases, various filter systems can be used. Filters based on carbon nanomaterials can be suitable for capturing gaseous chlorine-containing substances, preventing their spread into the air. In this work, the sorption activity of various carbon-based nanomaterials (graphene oxide, modified graphene oxide, reduced graphene oxide, multi-walled carbon nanotubes, carbon black) in relation to gaseous chlorine and hydrogen chloride was investigated for the first time. It has been shown that employed carbon nanomaterials have an excellent ability to remove chlorine and hydrogen chloride from the air, exceeding the performance of activated carbon. Modified graphene oxide with an increased surface area showed the highest sorption capacity of 73.1 mL HCl and 200.0 mL Cl2 per gram of the sorbent, that is almost two and five times, respectively, higher than that of activated carbon. The results show that carbon nanomaterials could potentially be used for industrial filters and membrane fabrication.

1. Introduction

Chlorine and chlorine-containing compounds are highly reactive and are able to cause significant dangers to the respiratory, nervous and circulatory systems of humans [1,2,3,4,5]. Chlorine can be found in the atmosphere in the form of chlorine gas (Cl2) or hydrogen chloride (HCl), as well as hydrochloric acid, chlorides and organochlorines [6,7]. Chlorine is approximately 2.5 times heavier than air (density 3.214 kg/m3), and as a result, it usually accumulates in lowlands, tunnels, basements and wells. It is soluble in water and possesses high chemical reactivity, making it one of the most toxic and dangerous substances [8,9]. The use of chlorine as a poison gas has led to the severe pollution of rivers and riverine areas, and the death of animals and people. Today, some factories and plants release chlorine into the air, causing great harm to the planet. Chlorine gas can be easily detected due to its specific odor and green color. The first symptom of exposure to chlorine gas is irritation of the mucous membranes, while higher concentrations may cause pulmonary edema [10,11]. HCl is a colorless gas with a pungent, suffocating odor. It is also heavier than air (density 1.49 kg/m3) and can dissolve in water to form hydrochloric acid. Hydrogen chloride, like chlorine, is a toxic and dangerous substance. The accumulation of hydrogen chloride in the atmosphere leads to acid rain. HCl exposure leads to irritation of the mucous membranes, coughing and lung damage, and at high concentrations it can be fatal. Contact with concentrated hydrochloric acid can cause burns to the skin [12,13].
A number of industries involve the use of chlorine and its derivatives [14,15,16,17,18]. Chlorine and hydrogen chloride can also be by-products of synthesis, e.g., in the production of caustic soda or chlorinated organic solvents [19]. Their toxicity depends on the dose and duration of exposure [20]. The maximum permissible concentration of Cl2 and HCl in the air of the working area is 1 mg/m3 and 5 mg/m3, respectively [10]. Due to the large scale of industrial application and extremely high reactivity, accidental industrial or domestic release of chlorine or hydrogen chloride into the atmosphere can occur [21]. Air purification from chlorine-containing substances or filtration systems are critically needed. The release of chlorine gas is also possible as a result of an accident during its transportation. Consequently, the contamination of air and water reservoirs can take place [1,22,23,24]. This can lead to great pollution of the environment and the death of living beings.
One of the strategies for removing harmful and dangerous substances from the air is the use of sorbents or filters. For a long time, carbon-containing materials have attracted a great deal of attention due to their sorption properties. These materials are widely used as sorbents for the removal of harmful pollutants from air, water and soil [25,26,27]. The large surface area, chemical and thermal stability and scalable production can give them an advantage when used in adsorption processes [28]. A number of studies have already shown the efficiency of using carbon nanomaterials to remove harmful and hazardous gases that are either used in the chemical industry or produced as its waste [29]. They have been used to capture gases such as CO2, NH3, CH4, SO2. Carbon-containing materials can be components of membranes or filters for physical gas adsorption. In addition, such materials are easy to modify to improve adsorption processes. For example, SWCNTs are able to capture carbon dioxide from a mixture with air well, but modifying their surface with amino groups improves this process greatly [30,31]. Activated carbon is one of the most widely used sorbents for gas capturing [32,33]. However, recent studies have shown that aminated graphene oxide, under equal conditions, exhibits higher sorption capacity for CO2 [34]. Therefore, the efficiency of gas adsorption by carbon materials strongly depends not only on the surface area, but also on their structure. Consequently, the exploration and application use of new carbon-containing nanomaterials to remove pollutants from the atmosphere can contribute to environmental improvements and a reduction of air pollution [35,36].
In this work we examined the sorption activity of a wide range of carbon-containing materials (graphene oxide (GO), thermal expanded graphene oxide (TEGO), reduced graphene oxide (RGO), multi-walled carbon nanotubes (MWCNTs) and carbon black (CB)) toward gaseous chlorine and hydrogen chloride. For the first time, it has been shown that most graphene-like materials have an excellent efficiency to remove chlorine and hydrogen chloride from the air, surpassing the performance of activated carbon (AC).

2. Materials and Methods

2.1. Chemicals

Potassium permanganate (KMnO4, CAS No. 7722-64-7), concentrated sulfuric acid (H2SO4, CAS No. 7664-93-9), hydrogen peroxide (30% H2O2, CAS No. 7722-84-1), concentrated hydrochloric acid (37% HCl, CAS No. 7647-01-0), sodium hydroxide (NaOH, CAS) and phenolphthalein (C20H14O4, CAS No. 77-09-8) were of reagent grade, purchased from “Khimmed”, Moscow, Russia. All reagents were used without additional purification.
Commercial multi-walled carbon nanotubes (CAS No. 1333-86-4), activated carbon (CAS No. 7440-44-0) and carbon black (CAS No. 1333-86-4), used as sorbents, were purchased from “Khimmed”, Moscow, Russia. To synthesize graphite oxide using the modified Hummers method [37], synthetic graphite (99%, 20 µm size, Sigma Aldrich, Burlington, MA, USA) was used. Thermally expanded graphite oxide and reduced graphite oxide were obtained in the laboratory from GO using previously described methods [38,39].
Briefly, 2 g of natural graphite powder was dispersed in 80 mL of concentrated H2SO4. Then, 7.2 g of KMnO4 was slowly added and the mixture was stirred for 12 h. After that, 120 mL of cold water was added dropwise and system was stirred at a temperature ≤ 30 °C. Then, 200 mL of distillated water and 5 mL of hydrogen peroxide H2O2 were poured into the reaction mixture. The obtained GO was washed with a large amount of water by centrifugation, and the final product was dried in air at a temperature of 40 °C.
The modification of GO was carried out on the flame spraying equipment (Powder Gun 5 PM-2 (Metal Coat, Jodhpur, India), working on propane–oxygen flame). The GO powder (150–200 µm fraction) was spread into a disposable hopper, then fed into the gas gun nozzle and shot out with a burning gas mixture. The total heating temperature of the propane–oxygen gas mixture was 750–800 °C. The parameters of the flame spraying were as follows: oxygen consumption—90 r.u., propane consumption—60 r.u., air pressure—1.8 atm., propane pressure ~2 atm. and oxygen pressure ~3.5 atm. The TEGO was collected and placed in a clean container.
For RGO synthesis, 100 mg GO powder was dispersed in 5.72 mL of 2-propanol into a quartz test tube. The test tube was placed into a steel autoclave, further heated to 275 °C and kept for a day in a furnace. Then, after cooling, the autoclave was opened and the supernatant was removed. The obtained RGO was washed with 2-propanol and dried in the furnace at a temperature of 80 °C.

2.2. Characterization

Identification of the phase composition by powder X-ray diffraction analysis (XRD) was performed using a Bruker D8 Advance (Bruker, Karlsruhe, Germany) diffractometer operating in reflection mode with CuKα radiation (40 kV, 40 mA, λ = 1.54056 Å) and a scanning step of 4° per minute. The specific surface area of the samples was determined by low-temperature nitrogen adsorption using an ATX 06 analyzer (KATAKON, Novosibirsk, Russia). Based on the obtained data, the specific surface area was calculated using the BET model at five points in the range of partial pressures of 0.05–0.25. The pore size distribution was calculated according to the Barret–Joyner–Halenda (BJH) model using adsorption branches of isotherms in the partial pressure range of 0.4–0.97. The surface morphology was studied using a Carl Zeiss Supra 40 scanning electron microscope (SEM) (Carl Zeiss AG, Oberkochen, Germany). The samples were placed on a holder inside a vacuum chamber (~10−6 mbar). The accelerating voltage during imaging in secondary scattered electrons was 10 kV, the aperture was 30 μm. Transmission electron microscopy (TEM) for all samples was performed using a JEOL JEM-1011 microscope (JEOL, Tokyo, Japan) at an accelerating voltage of 80 kV. The samples were applied to copper grids coated with carbon film by US sputtering. X-ray fluorescence analysis (XRF) was performed on an Olympus Vanta M analyzer (Olympus, Tokyo, Japan) with a 3-beam mode (scanning time—175 s). Samples of 10 mg weight were uniformly distributed on the table support within the closed chamber equipped with irradiator. When the sample was irradiated, excitation and characteristic fluorescent radiation of the atoms occurred, where each atom emitted a photon with an energy of a strictly defined value. After excitation, the spectrum was recorded on a detector and then the chemical composition of the sample was determined by the peaks of the obtained spectrum. Carbon, oxygen and hydrogen were determined as light elements in total. To determine chlorine, the spectra were compared with the spectrum obtained during irradiation of a standard sample (spectra library). FTIR spectra were recorded on a Bruker Alpha spectrometer (Bruker, Karlsruhe, Germany) with a Platinum ATR attachment in the range of 400–4000 cm−1 with scanning step 4 cm−1. The content of C, H and N (in wt%) in the samples was determined on an EA1108 Carlo Ebra Instruments analyzer (Carlo Erba Instruments, Milan, Italy). Combustion of samples was ensured by adding Co2O3 to the melting pot. A sample with the weight up to 1 mg was burned in the reaction tube of the analyzer at 1000 °C with a pulsed supply of oxygen. The combustion products were analyzed using a thermal conductivity detector with computer processing of the obtained chromatographic data.

2.3. Sorption Experiments

2.3.1. Chlorine Sorption

A total of 50 mg of dry sorbent was placed in a 15 mL vial. Then, the vial was filled with dried gaseous Cl2 generated by dropwise addition of HCl (36%) to solid KMnO4 with continuous stirring (Equation (1)):
2 KMnO4 (s) + 16 HCl → 5 Cl2 (g) + 2MnCl2 + 2KCl + 8H2O.
The synthesized gas was passed through a trap containing calcined CaCl2 to remove water impurities in the chlorine. After filling the vial with green gas, it was tightly sealed with a stopper with a Teflon liner, wrapped with Teflon film and retained for 24 h. After opening, the vial with the sorbent was kept in an oven at a temperature of 30 °C until the residual chlorine was completely removed. The presence of chlorine in the sample was confirmed by the Beilstein test, specifically the appearance of a green flame in the burner.
Gas sorption experiments were conducted under standardized conditions for all samples (760 mm Hg, 20 °C, relative air humidity 50%). To ensure the reproducibility, all experiments were repeated at least three times.

2.3.2. Hydrogen Chloride Sorption

A total of 50 mg of dry sorbent was placed in a 15 mL vial. Then, the vial was filled with dried gaseous HCl generated by dropwise addition of H2SO4 (96%) to solid NaCl with continuous stirring (Equation (2)):
NaCl (s) + H2SO4 → HCl (g) + NaHSO4.
The synthesized gas was passed through a trap containing calcined CaCl2 to remove water impurities in the hydrogen chloride. After filling the vial with the green gas, it was tightly sealed with a stopper with a Teflon liner, wrapped with Teflon film and retained for 24 h. After opening, the vial with the sorbent was kept in an oven at a temperature of 30 °C until the residual chlorine was completely removed. The presence of chlorine in the sample was confirmed by the Beilstein test.
Gas sorption experiments were conducted under standardized conditions for all materials (760 mm Hg, 20 °C, relative air humidity 50%). To ensure the reproducibility, all experiments were repeated at least three times.

2.3.3. Quantitative Measurement of Absorbed Gases

The volume of absorbed gases was determined by acid–base titration of the solution obtained by dissolving chlorine (or hydrogen chloride) in water. A total of 100 mg of sorbent was taken after exposure to the gas, placed in 100 mL of water and treated with high-power ultrasound (US) in cavitation regime (specific power 1.5 W/cm3) for 30 min to prepare the aqueous dispersion. The solution with pH ≤ 7 was separated from the solid phase by filtration and centrifugation (15,000 rpm, 10 min) and diluted with water. During US treatment, the carbon-containing sorbent was destroyed, and absorbed chlorine or hydrogen chloride was released into water (Equation (3)):
Cl2 + H2O → HClO + HCl.
The complete release of the gas from the sorbent was confirmed by the Beilstein test and X-ray fluorescence spectroscopy performed after drying the sorbent sample. The solution obtained after chlorine sorption was additionally exposed to UV radiation for a day to decompose hypochlorous acid (Equation (4)):
2   HClO   → h v 2   HCl + O 2
During experiments with HCl adsorption, no additional UV aging of the resulting solution was performed. The resulting supernatant solution was titrated with a freshly prepared 0.01 M NaOH solution, standardized to HCl using phenolphthalein as an indicator.

3. Results and Discussion

To study the sorption of chlorine and hydrogen chloride, it was necessary to select carbon-containing materials with properties that facilitate the effective capture of gas molecules. These materials are characterized by a large surface area, layered or highly dispersed structure, a large number of pores or active surface sites such as oxygen-containing groups. Thus, GO and RGO, CB and MWCNTs were used for this study. AC and graphite were used as reference points. Additionally, in this work, TEGO with an increased surface area, which provided good results in previous experiments on the adsorption of the methylene blue dye from water solution, was used [38]. The structures and surface morphology of the sorbents used are shown in Figure 1.
GO is a structure with sp2 and sp3 carbon atoms bound into a graphene lattice, covered with a large amount of oxygen-containing groups such as hydroxyl, carboxyl, carbonyl and epoxy groups (Figure 1a). It has a layered structure; the lateral size of GO flakes can reach several μm [40]. In aqueous dispersion, GO exists as plane-parallel monolayers. When dried, these layers agglomerate into a multilayer structure, but they can be easily redispersed into a monolayer state using US treatment. The surface area of dry powdered GO is ~1.4 m2/g, according to our experimental and literature data (Table 1) [38]. Due to its unique structure, GO can be an excellent adsorbent. The high oxygen content makes the surface of GO extremely reactive and it can easily capture organic dyes, heavy metal cations and radionuclides, removing them from aqueous solutions [37,41]. In previous studies it has been shown that when GO interacts with gaseous chlorine under normal conditions, no pure chemical interaction occurs; however, GO can adsorb Cl2 on its surface and in the interlayer space [42].
Thermal treatment or chemical reduction of GO leads to the deoxygenation of its surface, with the removal of functional oxygen-containing groups. This process transforms GO into a layered material with sp2-hybridized carbon atoms bound into a hexagonal graphene lattice (Figure 1b). The surface of graphene monolayers can be covered with residual oxygen-containing groups. According to [43], this material is specifically named “reduced graphene oxide” to distinguish it from mono-, bi- and multilayer graphene produced by physical methods [44,45]. However, the surface of RGO contains a large number of defects retained from the original GO, and additionally formed during the reduction process. In general, the structure of RGO is similar to the structure of GO and also it can be redispersed to a monolayer state in low-polarity solvents. It is easily confirmed using TEM.
CNTs are a promising material for manufacturing various devices, such as temperature and humidity sensors, gas sensors, filters, membranes, etc. [46,47,48]. CNTs consist of graphene sheets rolled into a cylinder (Figure 1c). Depending on the number of layers in the structure, nanotubes can be single-walled or multi-walled. The MWCNTs used in this work consist of two or more cylinders formed by graphene sheets, with the distance between layers being 0.34 nm. Their outer diameter is ~20 nm and the length of the nanotubes reaches several μm. According to the TEM and SEM, MWCNTs also contain a small amount of metallic nanoparticles. In industry, MWCNTs are produced using ferrocene as a catalyst. After synthesis, the resulting nanotubes contain a certain amount (up to 1–8% by weight) of Fe nanoparticles, which are sealed inside the nanotubes and do not affect their adsorption properties.
CB, or soot, is a less commonly used material that is currently used as an alternative to activated carbon in adsorption processes [49]. It is a highly dispersed, amorphous, hydrophobic synthetic material primarily composed of carbon [50]. CB particles are globules made up of residual graphite structures. Depending on the production method, hydroxyl, carboxyl, carbonyl, ether and epoxy groups can be formed on the surface of carbon black. Adsorbed residues of undecomposed hydrocarbons can also be present (Figure 1d). Its surface area ranges from 5 to 500 m2/g depending on synthesis conditions [51]. The commercial CB used in this work has a surface area of ~150 m2/g. Using SEM and TEM we observed that the CB used consists of 14 nm particles, which tend to form aggregates.
Despite being a promising material for adsorption, GO has a relatively low surface area (Table 1). To increase the surface area and, in the long term, improve its sorption properties, an additional modification of GO was carried out. In this method, powdered graphene oxide particles are quickly passed through a propane–oxygen torch flame in less than one tenth of a second. This rapid heat treatment removes surface and interlayer water molecules and leads to a significant increase in the interlayer distance of GO while maintaining its overall structure (Figure 1e). The bulk density of TEGO is 0.116 g/cm3, which is half that of the original GO (0.333 g/cm3). In addition, the surface area of TEGO increases by 27 times compared to the original GO reaching 38.2 m2/g (Table 1), mainly due to the formation of pores ranging from 3 to 8 nm. It has been shown that TEGO conserves the basic chemical structure of original GO with partial deoxygenation, while some TEGO layers have quite extended graphenic domains with the size of ≥30 nm. TEGO, along with the original graphene oxide, reveals good sorption properties with respect to the molecules of the organic dye methylene blue. Gas molecules have a size significantly smaller than dye molecules; therefore, modified graphene oxide with an increased interplanar distance can also serve as sorbent for chlorine-containing gaseous substances.
In the present study, two reference points were also tested: the original graphite, which has zero sorption capacity for Cl2 and HCl, and AC, which are commonly used as a sorbent for the removal of harmful and hazardous substances [52,53].
AC is a porous amorphous material, consisting of randomly oriented amorphous carbon that forms cylindrical pores. (Figure 1f). The structure of AC is similar to that of graphite and contains a large number of pores; the size and number of them are determined on the starting materials used for its production. Carbon-containing materials such as wood, coal, bamboo, bitumen coal, coconut shell, petroleum resin and others are used as raw materials for AC production [54,55]. The starting materials are first carbonized and then subjected to physical or chemical activation which significantly increases the surface area of the resulting substances. In the present study, AC with a surface area of up to 500 m2/g was used. The large surface area determines excellent AC adsorption properties. It is used for groundwater purification, greenhouse gas reduction, hydrogen storage, etc. [55,56,57].
Graphite is a three-dimensional structure consisting of a large number of graphene layers (Figure 1g). Its structure and properties can vary greatly depending on the method of production, origin, size, etc. [53]. In this study, synthetic fine graphite with a flake size up to 20 μm was used. Finely dispersed graphite has a higher surface area compared to natural or pyrolytic graphite and, therefore, can exhibit some sorption activity towards gas molecules, though its capacity is minimal. Graphite and activated carbon are three-dimensional bulk structures and are not transparent to an electron beam. Therefore, their TEM images are not provided in the work.
Figure 2 shows the general scheme of the operations performed. The vial containing the sorbent was filled with gas (chlorine or hydrogen chloride) and kept in the dark to establish equilibrium (Figure 2a). The gases were first passed through a drying trap to remove impurity water (omitted from the scheme for simplicity). The vials were chosen with an excess volume to ensure that the green color of chlorine did not disappear after some time (i.e., there was always an excess of gas). In this way, we could record the maximum gas capture value achieved by the sorbent.
After opening the vial, the excess chlorine/hydrogen chloride was completely removed. Then the sorbent samples were dispersed in water using US treatment (Figure 2b). Almost complete destruction of the used carbon-containing materials occurred due to strong cavitation, causing the release of adsorbed Cl2 and HCl molecules into the solution. The complete transfer of the adsorbed gas into the solution was confirmed using the Beilstein test. For all the materials studied, including activated carbon and graphite, US treatment resulted in the complete solution of gases in the water. No green coloration of the burner flame was observed during the combustion of the sorbent after opening the vial.
The Beilshtein method is a sufficiently accurate and sensitive method of qualitative analysis, allowing the detection of chlorine with minimal content [58,59]. For comparison, all samples were also analyzed using a modern method of X-ray fluorescence analysis. The obtained spectra used are presented in the Supplementary File (Figure S1). The summary results are presented in Table 2. From the data obtained, it is obvious that both methods are adequately sensitive and differ only when synthetic graphite is used as a sorbent for gaseous substances.
To determine the nature of the interaction between the sorbents and gaseous chlorine, we used the FTIR spectroscopy. The obtained spectra are shown in Figure 3. For GO, the experiments were carried out on powders separated into several fractions (75–100 μm, 100–150 μm, 150–200 μm) to determine the effect of the GO flakes size on gas sorption (Figure 3a). It was shown that the FTIR spectra of GO samples of different sizes after the interaction with chlorine are almost identical to the spectrum of the original graphene oxide. Vibrations corresponding to the C-Cl bond were absent, since the spectrum does not contain a characteristic band in the region of 600–800 cm−1. At the same time, changes in the “fingerprint” region were recorded. There was a decrease in the bands of ~1720 cm−1, corresponding to the stretching vibrations of C=O bonds, and the band of ~1200 cm−1, corresponding to the stretching vibrations of C-O, compared to the spectrum of the original GO. When interacting with the surface of graphene oxide, chlorine molecules can (1) be physically adsorbed on the graphene plane through edge or intraplane defects, forming a weak coordination interaction with epoxy or carboxy groups, thereby closing them; (2) enter the interlayer space of GO, interacting with adsorbed water molecules.
For RGO, the FTIR spectra before and after the interaction with chlorine were almost identical. In the obtained FTIR spectra of RGO, a slight change in the intensities of the vibrations of the bands of residual oxygen-containing C-O groups was recorded (Figure 3b). This is similar to GO and can be explained as above. For MWCNTs and CB, no noticeable changes in the FTIR spectra before and after the interaction with chlorine were detected (Figure 3c,d).
Acid–base titration was used for quantitative determination of adsorbed chlorine and hydrogen chloride. When gas molecules are released into the solution, they interact with water and form either hydrochloric acid or a mixture of hydrochloric and hypochlorous acids. The latter is unstable and decomposes under UV light in the closed system, forming hydrochloric acid. The titration results per g of sorbent are shown in Figure 4.
From the obtained data, it is obvious that AC is a good sorbent for gaseous Cl2 and HCl (Figure 4a). However, it was also found that carbon-containing nanomaterials such as GO, CNTs and CB show good results in removing chlorine-containing gases from the air. The volume of chlorine adsorbed by AC was 41.4 mL/g of sorbent. CB and MWCNTs are able to adsorb 25.2 and 37.5 mL of Cl2/g of sorbent, respectively, under the same conditions. As expected, graphite showed the lowest sorption capacity for chlorine, the volume of adsorbed gas was only 1.9 mL/g of sorbent. RGO also shows a low sorption capacity for gas (5 mL of Cl2 per g of sorbent).
GO is a layered two-dimensional material. Due to its unique structure, it is able to effectively adsorb gaseous chlorine (49.1–54.3 mL of gas/g of sorbent). In this work, the dependence of the sorption capacity on the lateral size (fraction) of GO flakes was also investigated. It was found that the amount of adsorbed Cl2 weakly depends on the lateral size of the GO flake. The maximum achieved value was 54.3 mL of Cl2/g of sorbent with flake sizes from 75 to 100 μm. GO with flake sizes of 100–150 μm and 150–200 μm adsorbed 49.1 and 48.7 mL of gas/g of sorbent, respectively. Thus, finer grinding of GO leads to a slight increase in the surface area and active sites, which also increases the sorption capacity.
TEGO demonstrated the record sorption capacity for chlorine gas. A large number of nanosized pores with a diameter of 3–8 nm (Figure 5a) increased interlayer distance, and preservation of oxygen-containing functional groups resulted in the production of a unique sorbent (Figure 5b). TEGO demonstrated improved sorption capacity for chlorine equal to 200 mL of Cl2/g of sorbent, which was 4 times higher than that of AC.
Similar results were achieved when studying the ability of the selected materials to adsorb gaseous HCl (Figure 4b). Graphite and RGO were unable to remove hydrogen chloride from the air. AC adsorbed 47.9 mL of HCl/g of sorbent, which correlates with the data obtained for gaseous chlorine. The same situation was observed for GO with a flake size of 75–100 μm; its sorption capacity reached 53.1 mL of gas/g of sorbent. MWCNTs exhibited a lower sorption capacity of hydrogen chloride molecules compared to chlorine, only 25.5 mL of gas/g of sorbent. TEGO also demonstrated a lower sorption capacity of 75.2 mL of gas/g of sorbent. This phenomenon can be explained as follows. The HCl molecule is a dipole, unlike the non-polar Cl2 molecule. For AC, which shows physical sorption in the volume (absorption), the polarity of the molecule does not matter, only its radius. As for the TEGO, when hydrogen chloride is adsorbed, partial protonation of the surface occurs, so the structure of TEGO undergoes changes. Negatively charged GO sheets begin to interact with positively charged areas where hydrogen chloride molecules have attached. Thus, the interlayer distance is reduced and the efficiency of the sorbent decreases. The Figure 5a,b shows SEM images of AC and TEGO structures before and after sorption experiments, which confirms the proposed mechanism. The structure of TEGO after interaction with hydrogen chloride is quite different from the original; the GO layers are highly disordered, although they retain a large volume. AC in the original state is a finely dispersed powder. After interaction with HCl, we observe its swollen structure after the physical sorption process. The presented XRD data also correlate with the SEM images (Figure 6c,d). The XRD pattern of the initial GO contains the diffraction peak at 2θ = 11.3° which corresponds to interlayer distance of ~7.8 Å. The XRD pattern for TEGO was somehow different, namely as a combination of minimum two peaks. The main peak at 2θ = 10.6° (~8.3 Å) is retained, but another broad signal appears at 2θ ~13.5°. The new broad peak corresponds to interlayer distances from 5.8 to 7.4 Å. After the sorption of hydrogen chloride, a strong broadening of the diffraction pattern is observed in the range of 2θ = 7.5–14.5°, which corresponds to disordering of layers with different interlayer distances (Figure 6c). AC is an amorphous substance; therefore, the diffraction patterns of the sorbents before and after adsorption are identical (Figure 6d). There is a common opinion that high surface area and porosity of the sorbent are of primary importance. However, based on the data obtained, we found that the flat structure of GO-based materials with a large number of defects and surface functional groups is the most suitable for capturing chlorine-containing gases. Figure 7 demonstrates the relationship between the adsorbed volume of chlorine and hydrogen chloride and surface area of the sorbent used. Interestingly, the AC sample with the highest surface area does not exhibit the maximum sorption capacity for both gases. However, the TEGO sample, whose surface area is average, captures as well as chlorine and hydrogen chloride much more efficiently.
Table 3 provides some data on the sorption of a number of gases, the emissions of which can cause significant damage to the environment and humans. As it can be seen from the data provided, AC is indeed one of the most effective sorbents due to its pore structure. However, carbon-based nanomaterials, in many cases, can be used to the same extent effectively for the sorption of industrially important harmful and dangerous gases. Therefore, these materials and composites can be used to produce filter systems for capturing harmful gases and their mixtures.

4. Conclusions

A unique comparative study of the sorption capacity of carbon-containing materials in relation to chlorine-containing gases was conducted in this work. GO, RGO, CB, modified GO (TEGO) and MWCNTs were used for the work as a sorbent for Cl2 and HCl gases. Additionally, AC and synthetic graphite were used as reference points. It was found that TEGO with the increased surface area showed the highest sorption capacity of 73.1 mL HCl and 200.0 mL Cl2 per gram of sorbent, almost two and five times more than the sorption capacity of activated carbon. The present study showed, for the first time, that carbon-containing nanomaterials can be used to effectively remove chlorine-containing gases from air under static conditions.
The results obtained are of interest from both fundamental and practical points of view. The synthesis of GO by the modified Hummers method has now become scalable for industrial production. Modification of GO to TEGO using gas flame treatment can also be performed at a scalable level due to the availability of the required equipment. Therefore, carbon nanomaterials, especially those based on GO, could be used in the future to create filters and membranes with better properties, making industries working with hazardous chemicals and waste safer for workers and for the air environment.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cleantechnol7020039/s1, Figure S1: XRF spectra of carbon nanomaterials after chlorine (a) and hydrogen chloride (b) sorption experiments. The broadened bands in the region of 2.9–3.8 are related to the radiation of the Ar–Rh anode.

Author Contributions

Y.I.: conceptualization, data curation, resources, writing—review and editing. V.I.: investigation, visualization, writing—original draft. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author(s).

Acknowledgments

The authors express their gratitude to Alexandra Ivannikova and Ivan Sapkov for their assistance in the present research. This work was supported by the Ministry of Science and Higher Education of the Russian Federation as part of the State Assignment of the Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences. The XRD analysis was performed at the Shared Facility Center of the Kurnakov Institute of General and Inorganic Chemistry (IGIC RAS).

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 1. Structure, SEM and TEM of carbon nanomaterials used: (a) GO, (b) RGO, (c) MWCNTs, (d) CB, (e) TEGO. Structure and SEM of AC (f) and graphite (g).
Figure 1. Structure, SEM and TEM of carbon nanomaterials used: (a) GO, (b) RGO, (c) MWCNTs, (d) CB, (e) TEGO. Structure and SEM of AC (f) and graphite (g).
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Figure 2. Scheme of sorption experiments (a) and quantitative determination of absorbed chlorine (b).
Figure 2. Scheme of sorption experiments (a) and quantitative determination of absorbed chlorine (b).
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Figure 3. FTIR spectra before and after chlorine sorption of (a) GO with different lateral sizes of flakes; (b) RGO; (c) CB; (d) MWCNT.
Figure 3. FTIR spectra before and after chlorine sorption of (a) GO with different lateral sizes of flakes; (b) RGO; (c) CB; (d) MWCNT.
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Figure 4. Quantitative determination of absorbed chlorine (a) and hydrochloride (b).
Figure 4. Quantitative determination of absorbed chlorine (a) and hydrochloride (b).
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Figure 5. Pore volume distribution (a) and structures (b) of GO and TEGO.
Figure 5. Pore volume distribution (a) and structures (b) of GO and TEGO.
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Figure 6. SEM of TEGO (a) and AC (b) before and after hydrochloride sorption, and XRD pattern of TEGO (c) and AC (d) before and after hydrochloride sorption.
Figure 6. SEM of TEGO (a) and AC (b) before and after hydrochloride sorption, and XRD pattern of TEGO (c) and AC (d) before and after hydrochloride sorption.
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Figure 7. Relationship between the adsorbed gas volume and surface area of the sorbent.
Figure 7. Relationship between the adsorbed gas volume and surface area of the sorbent.
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Table 1. BET surface area of the carbon-containing materials used.
Table 1. BET surface area of the carbon-containing materials used.
SampleSBET (m2/g) *
Graphite1.1
AC500.1
GO1.4
RGO161.5
MWCNT92.9
CB150.0
TEGO38.2
* SBET: BET surface area.
Table 2. Results of qualitative determination of chlorine by XRF and Beilstein test methods (+ positive, − negative).
Table 2. Results of qualitative determination of chlorine by XRF and Beilstein test methods (+ positive, − negative).
SorbentBeilstein TestXRF
After SorptionAfter US TreatmentAfter SorptionAfter US Treatment
Cl2HClCl2HClCl2HClCl2HCl
GO++−−++−−
RGO+− − −+−−−
MWCNTs++ − −++−−
CB++ − −++−−
TEGO++ − −++−−
AC++ − −++−−
Graphite+− − −+−−−
Table 3. Sorption capacity of carbon nanomaterials for industrially important gases.
Table 3. Sorption capacity of carbon nanomaterials for industrially important gases.
SorbentGasConditionsCapture Capacity (mmol·g−1)Reference
ACCO2105 °C, 100 kPa2.04[32]
MWCNTs25 °C, 100 kPa1.88[30]
Aminated GO100 °C, 100 kPa2.9[34]
RGO0 °C, 100 kPa2.89[60]
MWCNT modified60 °C, 100 kPa5[31]
ACCH425 °C, 3.5 MPa9.7[33]
AC0 °C, 2 MPa6[61]
RGO1.25
CB0.6
MWCNTsNH335 °C, 744 kPa5.29[62]
GO25 °C, 100 kPa2.46[63]
SWCNTsSO225 °C, 375 kPa22.8[64]
GO25 °C, 25 kPa4
ACCl220 °C, 100 kPa1.88This study
GO2.42
TEGO8.93
ACHCl20 °C, 100 kPa2.14This study
GO2.37
TEGO3.36
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Ioni, Y.; Ibragimova, V. Study of Sorption Activity of Carbon Nanomaterials for Capture of Chlorine-Containing Gases. Clean Technol. 2025, 7, 39. https://doi.org/10.3390/cleantechnol7020039

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Ioni Y, Ibragimova V. Study of Sorption Activity of Carbon Nanomaterials for Capture of Chlorine-Containing Gases. Clean Technologies. 2025; 7(2):39. https://doi.org/10.3390/cleantechnol7020039

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Ioni, Yulia, and Victoria Ibragimova. 2025. "Study of Sorption Activity of Carbon Nanomaterials for Capture of Chlorine-Containing Gases" Clean Technologies 7, no. 2: 39. https://doi.org/10.3390/cleantechnol7020039

APA Style

Ioni, Y., & Ibragimova, V. (2025). Study of Sorption Activity of Carbon Nanomaterials for Capture of Chlorine-Containing Gases. Clean Technologies, 7(2), 39. https://doi.org/10.3390/cleantechnol7020039

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