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Article

Sustainable Chromium Remediation: Sorption of Chromium from Leaching Solutions of Refined

by
Gita Sultanbayeva
1,*,
Raushan Kaiynbayeva
1,
Raissa Chernyakova
1,*,
Hamdi Temel
2,*,
Umirzak Jussipbekov
1 and
Khaidar Tassibekov
1
1
A.B. Bekturov Institute of Chemical Sciences, JSC Kazakh-British Technical University, 06, Sh. Ualikhanov Str., 050010 Almaty, Kazakhstan
2
Department of Pharmacology, Faculty of Medicine, Yozgat Bozok University, 66900 Yozgat, Turkey
*
Authors to whom correspondence should be addressed.
Sustainability 2025, 17(6), 2726; https://doi.org/10.3390/su17062726
Submission received: 4 February 2025 / Revised: 21 February 2025 / Accepted: 11 March 2025 / Published: 19 March 2025
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Abstract

:
Chromium pollution has emerged as a critical environmental concern, prompting extensive research into the chemical and mineralogical properties of refined ferrochrome (RFC) slag, the leaching of chromium using sulfuric acid, and the adsorption of chromium cations onto natural zeolite. The aim of the study is to analyze the chemical and mineralogical properties of purified ferrochrome slag (RFC) from the Aktobe Ferroalloy Plant and its leaching with sulfuric acid, as well as to study the effectiveness of Shankanai zeolite in the adsorption of chromium cations from a sulfuric acid solution to improve waste management in the ferrochrome industry. Semi-quantitative X-ray analysis reveals that the dominant phase in RFC slag is olivine (50.7% Ca2SiO4). The optimal chromium transition rate (16.67%) occurs in dilute H2SO4 (23%) after 145 min of leaching, while the highest transition efficiency (18.0–18.5%) is achieved at 90 °C with a leaching duration of 145–180 min. Chromium in the RFC slag cake is predominantly in the divalent state, existing as pentahydrate chromium (II) sulfate (CrSO4•5H2O). The chromium sorption process was studied in a sulfuric acid solution obtained after leaching of ferrochrome slags. The process of chromium sorption by Shankanai zeolite from sulfuric acid has been studied for the first time, and the influence of the main technological parameters of the process on the degree of its purification has been established. It was determined that the highest degree of purification of a chromium-containing sulfuric acid solution is achieved with a ratio of zeolite:chromium-containing sulfuric acid solution equal to 1:10, heated to 35 °C for 15 min, and it reaches (63.6–69.0%). The natural zeolite of the Shankanai deposit is an effective, and inexpensive sorbent for cleaning aggressive media, particularly media contaminated with chromium-containing sulfuric acid. X-ray diffraction analysis further confirmed that both chromium and sulfur ions participate in the sorption process, as evidenced by microstructural changes in the zeolite, including pore filling and smoothing observed in microphotographs. These findings underscore the potential of natural zeolite as an efficient and cost-effective adsorbent for the remediation of chromium-contaminated solutions following sulfuric acid leaching. Its ability to adsorb chromium ions highlights its significant applicability in environmental cleanup efforts. This study contributes to sustainability by offering an environmentally friendly and cost-effective method for chromium removal, reducing industrial waste impact and promoting circular economy principles by utilizing natural zeolite, a readily available and recyclable adsorbent.

1. Introduction

Chromium is a highly toxic element with severe environmental and health implications. Chromium compounds are carcinogenic and mutagenic, contaminating water and soil ecosystems, thus endangering biodiversity and public health [1,2]. Industrial activities, including ferrochrome production, contribute significantly to chromium pollution, making it crucial to develop cost-effective and efficient remediation strategies [2]. Sulfuric acid leaching combined with sorption techniques using natural materials such as zeolite presents a promising approach to mitigating chromium pollution [3,4].
Technogenic waste from ferrochrome production contains chromium, which is one of the most toxic components of industrial waste. Chromium and its compounds are highly toxic and carcinogenic, poison water and soil, and negatively affect the functioning of all living organisms. Given the toxicity of chromium, wastewater containing chromium(VI) and chromium (III) compounds must be treated before being discharged into surface sources [1,5].
The problem of processing chromium-containing slag and extracting metal components from it, in particular chromium, with its subsequent use as secondary raw materials is an urgent problem [6,7].
The method described by John Clark Stouter (1980) involves extracting chromium from chromite ores using acid leaching. This process combines chromite ore with manganese dioxide-containing raw materials, introducing concentrated sulfuric acid in varying ratios (1:1:1 to 1:2:10), followed by grinding [8]. Similarly, Zhao et al. (2020) explore chromium recovery from low Fe(II)-chromite, highlighting sulfuric acid as a leaching agent in environmentally friendly conditions. Both studies emphasize optimizing reagent ratios and process parameters to enhance chromium extraction efficiency while minimizing environmental impact [9].
Sulfuric acid leaching has emerged as a promising and environmentally favorable method for the production of chromium salts; however, its applicability to low Fe(II)-chromite ores has yet to be fully validated. Experimental studies have identified the optimal leaching conditions as a temperature of 176 °C, a dichromic acid-to-chromite mass ratio of 0.12, and a sulfuric acid concentration of 81%. Additionally, the dissolution kinetics of chromite during the leaching process, as well as the catalytic influence of dichromic acid, were systematically examined to elucidate their roles in enhancing chromium extraction efficiency [9,10,11]. The findings indicate that the efficiency of chromite decomposition is significantly influenced by its Fe (II) content, with higher Fe(II) concentrations facilitating improved dissolution rates. Moreover, dichromic acid was observed to function both as an oxidizing agent and as a catalytic enhancer during the leaching process. Based on these results, a novel method for processing low-Fe(II) chromite has been proposed, offering an optimized approach for chromium recovery [9,10].
In thesis work [12], a high-grade chromite concentrate obtained from Pınarbaşı, Kayseri region of Turkey was reacted with sulfuric acid solution to determine the optimum conditions of leaching of chromite ores. In this study, only trivalent chromium remained in the leach solution and did not change to the hexavalent state. The product obtained after leaching was chromium (III) sulfate. Chemical precipitation using lime or magnesia is another attractive method for the removal of chromium from industrial wastewater [10,12,13,14].
It should be noted that a significant part of the work is aimed at processing (neutralizing) sludge, including the toxicity of sludge containing hexavalent chromium, from current production. Most of the proposed solutions are related to high-temperature processes [15] using sufficiently aggressive reagents such as mineral acids [16].
The available developments in the processing of slag ferrochrome are mainly related to the use of elevated temperatures (40–90 °C) and different concentrations of acid solutions [17]. More promising is the extraction of chromium in the sulfuric acid process of ferrochrome slag processing with further production from the residues of safe building materials.
The process of producing magnesium oxide from the chromium-containing raw material of one of the deposits of Kazakhstan [18] is investigated. In this study, raw materials were processed as follows: The particle size of the original ore was taken up to 20 mm. After mechanical grinding, the particle size changed to 0.5 mm. Then, the ore was subjected to wet magnetic separation. The next step was sulfuric acid leaching (GOST 2184-2013) at different concentrations of the non-magnetic fraction with and without heating.
The analysis of the modern scientific and patent literature on acid processing and the utilization of chromium-containing slags from the production of refined and high-carbon ferrochrome showed that the various methods of processing ferrochrome production slags, despite their availability, are characterized by a multi-stage nature and do not allow for complete processing of ferrochrome production slags in Kazakhstan [19]. Currently, there is an excess of sulfuric acid production in Kazakhstan, so it is advisable to use sulfuric acid as a reagent for leaching chromium from slag from the production of ferrochromium. It can be expected that in sulfuric acid under certain conditions, such as heating, a sufficiently high degree of extraction of chromium (III) will be achieved in slags from the production of high-carbon chromium and in refined ferrochrome slags [20,21].
Today, the most universal and effective methods for purifying chromium-containing solutions, wastewater, and other liquid waste are sorption methods using natural sorption materials such as non-activated and activated carbons, zeolites, bentonites, glauconites, etc. Sorption materials can also be derived from liquid waste. For instance, reference [22] describes how iron ferrocyanide sorbent, which is effective in purifying rare metals, was produced from cotrel milk.
A promising area is the development of sorption processes for purification of chromium-containing solutions using natural aluminosilicates with a layered and crystalline lattice.
Refs. [23,24] investigated the influence of a number of parameters on the process of removing Cr(VI) from aqueous solutions by the sorption method, with montmorillonite and clay being used as sorbents, which are good adsorbents for removing chromium (VI) from aqueous media, including from wastewater. It was noted that bentonite turned out to be more effective in removing Cr (VI) than activated carbon under the same experimental conditions [25].
In a number of scientific works on the processes of sorption treatment of chromium-containing solutions, including wastewater, natural zeolite is used. Due to a system of channels and cavities that permeate zeolite crystals, they have a well-developed internal surface, available for adsorbed molecules [26].
In Reference [27], the process of sorption of chromium (III) by clinoptilolite on model aqueous solutions containing Cr3+, Pb2+, Cu2+, and Fe3+ cations were studied. It has been shown that during the sorption of these cations by clinoptilolite, their competitive absorption occurs.
Natural zeolites (clinoptilolites) are very promising as sorbents in the purification of aqueous media, including wastewater and mineral acids, from cations of rare earth toxic elements [25,28,29,30]. It should be noted that zeolite (clinoptilolite) will be effective in chromium (III) sorption processes in mild and medium acidic environments [28,29,30].
In the scientific literature, there are works on the study of the sorption capacity for Cr (III) and Cr (VI) ions by zeolite from the Taizhuzgen deposit (Eastern Kazakhstan) [31]. In article, the process of purification of chromium-containing wastewater with Taizhuzgen zeolites was studied under laboratory conditions. The prospect of using natural zeolites in sorption processes of purification is also in the possibility of recycling used zeolites as soil meliorants, building materials, fillers, etc. [31,32]. This application of natural zeolites is based on the content in the species of clinoptilolite and their physical–chemical properties. Since the zeolite from the Shankanai deposit has a higher Si/Al ratio and sorption capacity, is more represented by clinoptilolite than the Taizhuzgen zeolite, and is acid-resistant, it can be expected that the Shankanai zeolite will have a high sorption capacity with respect to chromium cations not only in aqueous environments, but also in aggressive (acidic) environments.
This study aims to analyze the chemical and mineralogical properties of refined ferrochrome slag (RFC) from the Aktobe ferroalloy plant, focusing on its leaching with sulfuric acid. Additionally, this research investigates the efficiency of Shankani zeolite for adsorbing chromium cations from the chromium-containing sulfuric acid solution, aiming to assess its potential for purifying chromium-contaminated solutions and improving waste management in the ferrochrome industry.
The sustainable management of chromium-containing waste is crucial to minimizing environmental hazards and promoting industrial circularity. Conventional treatment methods often generate secondary pollutants or require energy-intensive processes. This study introduces a more sustainable alternative using natural zeolite, a cost-effective and abundant sorbent which aligns with the principles of green chemistry and waste valorization.

2. Experimental Part

Materials and Methods

Refined ferrochrome slag (RFC), a by-product of the Aktobe ferrochrome production (Aktobe Ferroalloy Plant, Aktobe, Kazakhstan) process, contains more than 5% Cr(VI), classifying it as hazardous toxic waste [9,33]. Mineralogical analysis of RFC slag was performed using an automated diffractometer (DRON-3) with CuKa radiation at 35 kV and 20 mA. X-ray diffraction patterns were analyzed semi-quantitatively using the ICDD PDF-2 database and pure mineral standards.
The phase analysis was semi-quantitative, based on powder sample diffraction patterns, employing the equal portions and artificial mixtures method to determine the quantitative ratios of crystalline phases.
The interpretation of the X-ray diffraction patterns of the slag was performed using data from the ICDD PDF-2 database (Release 2022) and diffraction patterns of pure, impurity-free minerals. Microphotographs of zeolite samples, both before and after sorption, were captured using a scanning electron microscope (SEM) (JEOL brand “JXA-8230” (Akishima, Japan), with an acceleration voltage of 18 kV and a magnification of 10,000×.
In order to establish the complex dependence and quantitative evaluation of the experiment, the method of mathematical planning of an orthogonal rotatable 3-factor experiment of the second order was applied [34]. Using computer processing of experimental data, regression analysis of the results was performed. The input parameters selected were independent factors influencing the extraction process: temperature T, °C (z1), mixing time τ per. (z2), and a sulfuric acid concentration of CH2SO4 (z3). The output parameter was the chromium content after leaching (mg/L) Y (response). Sulfuric acid concentration varied from 5 to 95% by diluting concentrated acid. The duration of the process was from 10 to 180 min at S:L = 1:30, and temperature ranged from 22 to 90 °C. Stirring rate was 200–250 rpm.
A study on the extraction of total chromium (III and VI) from sulfuric acid leaching solutions was carried out depending on time (5–90 min), temperature (22–60 °C), and sorbent rate (T:L = (5–30:100)).
The chromium sorption process was carried out in a sulfuric acid solution obtained after leaching slags from ferrochrome production [35]. The natural zeolite of the Shankanai deposit was used as a sorbent, which has high sorption properties, is stable in acidic environments [27], and is a promising sorbent.
In this work, we used granulated (5 mm) zeolite from the Shankanai deposit with the following composition, wt.%: K2O—1.38; Na2O—0.95; Fe2O3—0.16; Al2O3—10.81; CaO—2.32; MgO—0.93; SiO2—65.28, p.p.p.—18.15, related to high-silica zeolites with Ca–K, Na-form of clinoptilolite (Si/Al = 5.4 and CaO/(Na2O + K2O) = 1) [30,31]. Zeolite has the following sorption characteristics: volumetric weight—1.093 g/cm3; total pore volume—0.068 g/cm3; total porosity—0.0231 g/cm3; sorption capacity for iodine—3.67 mEq/g; SEC of cation exchanger—0.997 mEq/g; SEC—anion exchanger 0.4985 mEq/g.
Zeolite was introduced into a reactor containing a sulfuric acid solution (23%), which was preheated to a specified temperature, at a defined zeolite:acid ratio (S:L). The mixture was stirred for a duration determined by the experimental conditions.
To assess the impact of time on chromium extraction, the process was conducted at a constant temperature of 22 °C, with an initial total chromium concentration (C(Crtotal)) of 98 mg/L and a zeolite leaching solution ratio of 1:10, over a range of process times from 5 to 120 min. The effect of temperature on chromium sorption by zeolite was studied in a sulfuric acid leaching solution containing CCr total. equal to 98 mg/L at 22 °C, a ratio of S:L = 1:10, and an optimal process time of 15 min. The effect of the consumption of natural zeolite on the degree of purification of a sulfuric acid chromium-containing solution was studied at 35 °C for 10 min in a sulfuric acid solution with a content of 98 mg/l of total chromium. The consumption of zeolite varied from 5 to 30 g per 100 g of the solution to be cleaned.
The total chromium concentration in the resulting filtrate was measured using an atomic absorption spectrophotometer (AA-7000, Shimadzu Corporation, Almaty, Kazakhstan).

3. Results and Discussion

The chemical composition of the slag samples from the ferrochrome production process is provided in Table 1. As shown, the maximum content of CaO ranges from 49.48% to 52.50%, the MgO content is between 8.12% and 12.29%, SiO2 varies from 22.59% to 27.11%, and the maximum Al2O3 content is 4.42%.
Semi-quantitative X-ray diffraction analysis of RFC slag powder enabled the determination of the quantitative ratios of crystalline phases, as shown in Figure 1. Chromium in RFC slag exists in two primary phases: magnesium chromium oxide (Mg7.92Cr1.08)Cr16O32 and calcium chromium oxide (CaCr2O4), as detailed in Table 2.
Calcium chromite should be considered as a double compound of chromium oxide (Cr2O3) with CaO, that is, as a compound CaO•Cr2O3 (Nurgali et al. 2019) [36]. Moreover, in calcium chromite, chromium is in the trivalent state. Calcium chromite CaCr2O4 is practically insoluble in water and mineral acids. The chromium-containing phases Mg-Cr-Oxide (6.3%) and Ca-Cr-Oxide (4.6%) account for a total of 10.9%. Considering that calcium chromite CaCr2O4 is insoluble in H2O and acids, apparently, the water-soluble form of chromium is in the magnesium chromium oxide (Mg7.92Cr1.08)Cr16O32 (6.3%) phase.
The predominant crystalline phase in RFC slag is OlivineCa, constituting 50.7% (Ca2SiO4). The second most abundant phase is MgO at 20.3%, followed by Ca-Mg-Fe Silicate at 11.8%. Quantitative analysis indicates that the OlivineCa phase contains 17.7% silicon dioxide (SiO2). These findings underscore the significant presence of silicate-based compounds in the slag, reflecting its mineralogical composition. According to Table 3, the content of the SiO2 quartz phase in RFC slag is low (6.3%). In addition, the Ca-Mg-Fe Silicate phase (11.8%) is present in the RFC slag. However, judging by the coefficient in its formula Ca2Fe1.2Mg0.4Si0.4O5, the SiO2 content does not exceed 0.53%. Based on the data in Table 3, the total amount of RFC slag contains SiO2—24.53%.
In the process of sulfuric acid leaching of ferrochrome production slag, a contaminated sulfuric acid solution is formed as a waste, containing, along with chromium, other associated components. Analysis of the oxide content in the slag cake (a solid residue from pulp filtration obtained during ore leaching) showed that associated compounds transfered from the slag into H2SO4 (23%) (Table 3). Thus, in the cake from RFC slag, the content of all oxides is less than in its original slag, namely Cr2O3, CaO, MgO, Al2O3, SiO2, and FeO respectively decrease by 1.9, 1.6, 5.9, 2.1, 1.4, and 6.8 times compared to their content in the RFC slag. In addition, sulfur-containing compounds are formed in significant quantities in the slag cake of the RFC (46.67%).
To neutralize sulfuric acid from chromium (total), as well as its reuse and preservation of activity with its further reuse in the process of leaching a new batch of slag, the sulfuric acid leaching solution was purified from contaminants and, above all, from chromium.
There are no data in the scientific literature on the behavior of ferrochrome production slags in highly concentrated sulfuric acid solutions; therefore, the influence of a number of process parameters such as temperature, the duration of the process, and the concentration of sulfuric acid from chromium to solution were studied. The influence of the ratio of ferrochrome layer to acid mass (S:L) was also investigated.
The results of chromium conversion from refined ferrochrome slag to sulfuric acid depending on its concentration, temperature, and process time are shown in Table 4 and Figure 2.
All calculations are performed on the developed mathematical model of the leaching process, which automatically presents the regression equation adequately describing this process:
Y = 4.679 + 0.482X1 + 0.681X2 + 0.264 X3 − 0.484X22 + 0.406 X1X2 + 1.280 X1X3 − 0.88 X2X3
Analysis of the regression equation shows that the time factor of the process is related to the sulfuric acid concentration and the temperature of the process.
The results of the transition of chromium from refined ferrochrome slag into a sulfuric acid solution are presented in Table 4 and Figure 2.
The study of the influence of sulfuric acid concentration and temperature showed that as the duration of the process increases for all studied temperature ranges, the chromium transition to a sulfuric acid solution increases (Figure 2). Based on the results, in dilute H2SO4 (23%), the highest chromium transition rate (16.67%) occurs in 145 min (Figure 2a). Moreover, with a process duration of up to 180 min in the entire temperature range (22–90 °C), the chromium content has the same value (15.56%). As the temperature of the process increases over time, the chromium content in the acid increases (Figure 2b,c).
With the increase of the sulfuric acid concentration to 95%, the chromium content in the solution decreases at 22 °C. In the case of longer contact of refined ferrochrome with sulfuric acid (95–180 min), chromium content is drastically reduced. The reduction of chromium content in sulfuric acid under these conditions is really possible due to passivation of the material. It is known that the degree of chromium passivation is very high and is 1.94 in the Chemistry Handbook [37] (pp. 191, 364, 349). As stated in the manual, chromium does not dissolve in concentrated sulfuric acid by oxide film hardening, i.e., chromium becomes passive (p. 197). That is, the concentrated solution of H2SO4 passes the least amount of chromium. The sulfuric acid concentration under these conditions has no significant effect on chromium leaching. The leaching curves of RFC at temperatures up to 56 °C are different. As the leaching process lasts longer, chromium transition increases at low acid concentrations (5–23.2%), and increases in concentration and duration simultaneously reduce the transition rate.
In high-temperature processes with increasing sulfuric acid concentration, the chromium yield to the solution increases, and the maximum content of chromium in 95% sulfuric acid is 9.6 mg/L, which corresponds to 26.5% in the solution (Figure 2c).
From the experimental experiments above, it follows that the greatest chromium transition from ferrochrome slag is observed at low concentrations of sulfuric acid. As can be seen from the obtained data, the optimal conditions for the decomposition of refined ferrochrome slag are the ratio S:L = 1:20, C(H2SO4) = 23%, temperature—75 °C, and leaching time—145 min, at which the maximum transition of chromium (16.67%) into the sulfuric acid solution occurs.

Study of Chromium Cation Sorption by Natural Zeolite

The objectives of the work were to study the effect of time, temperature, chromium concentration, and sorbent rate on the content of Crtot. in the leaching solution formed after slag treatment with 23% sulfuric acid. Studies on the sorption of chromium from leached sulfuric acid solution using natural zeolite from the Shanghai deposit, depending on the duration of the process, showed that the degree of sorption decreases slightly between 5 and 30 min (Figure 3a). During this period, the degree of purification (R) decreases by only 0.48%. Further increasing the process time initiates the process of chromium desorption, i.e., its transition from zeolite back into the solution. From 30 to 120 min, the degree of sorption significantly decreases (by 11.19%).
The study of the effect of temperature on chromium sorption by zeolite in a sulfuric acid leaching solution containing Crtot., a ratio of 98 mg/L, S:L = 1:10, and the optimal process time of 15 min is shown in Figure 3. The process temperature ranged from 22 to 60 °C. As shown in Figure 3b, the sorption capacity of zeolite relative to chromium increases slightly, reaching a maximum at 50 °C. Further temperature increases have no significant impact on increasing zeolite sorption capacity. It should be noted that in the range from 35 to 50 °C, the degree of sorption increases by only 3%. Therefore, the process of sorption at elevated temperatures is not advisable.
The effect (influence) of natural zeolite consumption on the degree of purification of a chromium-containing sulfate solution was studied at 35 °C for 10 min in a sulfate solution containing 98 mg/L of total chromium. The zeolite consumption varied from 5 to 30 g per 100 g of solution to be purified.
Increasing the zeolite rate to 15 g per 100 g of chromium-containing sulfate solution being purified significantly increased the degree of purification (Figure 3c). With an increase in zeolite consumption from 5 g to 15 g per 100 g of solution, the degree of purification increased from 49.49 to 54.05%, i.e., by 4.56%. A more significant load of zeolite (20–30 g per 100 g of solution) increased the degree of sorption (R) by only (0.45–0.5)% due to an increase in the mass of the sorbent. It is preferable to carry out the process at a zeolite consumption of 10–15 g per 100 g of solution being purified.
The effect of chromium concentration (total) on the sorption capacity of natural zeolite was studied at constant values of the S:L ratio of 1:10 and 35 °C for 10 min (Table 5). The concentration of Crtotal in the sulfuric acid solution was varied from 20 to 98 mg/L. As shown in Table 4, in the range from 20 to 50 mg/L, the degree of chromium sorption by zeolite increased from 40.0% to 69.0%. Increasing the chromium concentration in the sulfuric acid solution to 70 mg/L reduced the degree of sorption by 14.7%. Further increases in the concentration of chromium in the solution being purified had virtually no effect on the degree of its sorption by zeolite, which is due to saturation of the adsorption site of the zeolite. The lowest degree of sorption was observed in solutions with low chromium concentrations. The results obtained were consistent with the data from [36].
EDX spectra of zeolite samples before and after the sorption of chromium cations from chromium-containing sulphate solution are presented in Figure 4 and Figure 5. Energy-dispersive X-ray diffraction analysis of zeolite after 10 min of contact with chromium-containing sulfuric acid solution showed the presence of new maxima related to chromium, which can be seen in the spectrum (Figure 4). The intensity of lines characteristic of iron and aluminum increased compared to the data in Figure 5 due to sorption from sulfuric acid solution. The presence of a new line characteristic of sulfur is due to the sorption of sulfur ions from a sulfuric acid solution.
Micrographs of adsorption products provide additional evidence regarding adsorption phenomena and the structure of the sorbent surface. A comparative analysis of the surface of natural zeolite before and after sorption of chromium cations was carried out using the electron microscopic method. As can be seen from Figure 6a,b, the micrograph of natural zeolite shows the presence of dark and light spots of small size, which correspond to cavities and voids in the zeolite (Figure 6a). In the micrograph of the zeolite isolated from chromium-containing sulfuric acid after the sorption of chromium cations (Figure 6b), the number of voids and channels, as well as their size, decreases slightly compared to the original zeolite. Chromium sorption by natural zeolite occurs mainly through ion exchange and adsorption on the surface. For example, in the case of chromium (III), its cations can replace cations (for example, calcium or magnesium) present in zeolite at exchange centers. In the case of chromium (VI), in dissolved forms (for example, in the form of chromates), its anions can bind to cations in zeolite structures.
Thus, the study of natural zeolite and zeolite isolated after the sorption of chromium cations showed that, judging by the results of X-ray diffraction analysis and micrographs, not only chromium cations but also sulfur are involved in the sorption processes. As a result, the porosity of the sorbent changes, which is reflected in the micrograph of the zeolite isolated from the acid after sorption of chromium.
Based on the sorption properties of Shangkanai zeolite, it can be expected that water containing only chromium cations will be the most effective sorbent in relation to them. The purified rinsing water is returned to the process to wash the next batch of cake. In this case, it is possible to reuse the zeolite until it is more completely saturated with chromium.

4. Conclusions

Semi-quantitative X-ray phase analysis of the RFC slag indicated that chromium-containing phases constitute 10.9% of the total slag. Chromium in the slag exists as magnesium chromium oxide (Mg7.92Cr1.08)Cr16O32 and calcium chromium oxide (CaCr2O4). The predominant phase is OlivineCa (50.7% Ca2SiO4). It was determined that in the slag cake, chromium is present in the divalent state as chromium (II) sulfate pentahydrate (CrSO4·5H2O).
Chromium leaching studies showed that the highest chromium transition (16.67%) from RFC slag occurred in dilute H2SO4 (23%) after 145 min, with the transition stabilizing at 15.56% after 180 min across all temperatures (22–90 °C). The highest chromium transition to the acid solution was observed at 90 °C with a leaching time of 145–180 min (18.0–18.5%).
The process of chromium sorption by Shangkanai zeolite from a sulfuric acid solution contaminated with bivalent and trivalent metal cations has been studied for the first time, and the influence of the main technological parameters of the process on the degree of its purification has been established. It was determined that the highest degree of purification of a chromium-containing sulfuric acid solution is achieved with a ratio of zeolite:chromium-containing sulfuric acid solution equal to 1:10, heated to 35 °C for 15 min, and reaches (63.6–69.0%). Chromium sorption by natural zeolite occurs mainly through ion exchange and adsorption on the surface. The natural zeolite of the Shankanai deposit is an effective inexpensive sorbent for cleaning aggressive media, in particular sulfuric acid contaminated with chromium.
This study highlights the potential of natural zeolite as a sustainable and efficient sorbent for chromium remediation. The findings offer valuable insights into the optimization of leaching and sorption processes, contributing to the development of eco-friendly strategies for industrial waste management.
In conclusion, natural zeolite from the Shankanai deposit proves to be an effective, accessible, and cost-efficient sorbent for the purification of chromium-containing solutions following sulfuric acid leaching.
The use of natural zeolite as a sorbent not only enhances chromium removal efficiency but also supports the principles of sustainable resource management by utilizing naturally occurring materials with minimal environmental impact. Furthermore, the potential for zeolite regeneration and reuse further strengthens its role in sustainable wastewater treatment systems.

Author Contributions

G.S.: experimental, interpretation and writing stages; R.K.: experimental studies; R.C.: interpretation and writing stages; H.T.: experimental, interpretation and writing stages; U.J.: experimental, interpretation and writing stages; K.T.: interpretation and writing stages. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the JSC A.B. Bekturov Institute of Chemical Sciences under the framework of the Research Program (BR27101179 Fundamental principles for obtaining innovative environmentally friendly multifunctional chemical products and materials).

Institutional Review Board Statement

This study did not involve any experiments on humans or animals, and therefore, ethics approval was not required. The research complies with all ethical standards for environmental and chemical studies.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The article was prepared with financial support of the Research Program (BR27101179 Fundamental principles for obtaining innovative environmentally friendly multifunctional chemical products and materials).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. X-ray diffraction pattern of RFC slag.
Figure 1. X-ray diffraction pattern of RFC slag.
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Figure 2. Effect of temperature (a), time (b) and H2SO4 concentration (c) on the leaching of chromium into an acidic solution.
Figure 2. Effect of temperature (a), time (b) and H2SO4 concentration (c) on the leaching of chromium into an acidic solution.
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Figure 3. Influence of time (a), temperature (b), and ratio (c) of sorbent on sorption of Cr by zeolite.
Figure 3. Influence of time (a), temperature (b), and ratio (c) of sorbent on sorption of Cr by zeolite.
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Figure 4. EDX spectra of zeolite before Cr sorption from chromium-containing sulfuric acid solution.
Figure 4. EDX spectra of zeolite before Cr sorption from chromium-containing sulfuric acid solution.
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Figure 5. EDX spectra of zeolite after sorption of Cr from a chromium-containing sulfuric acid solution.
Figure 5. EDX spectra of zeolite after sorption of Cr from a chromium-containing sulfuric acid solution.
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Figure 6. Micrograph of natural zeolite (a) and zeolite isolated after sorption of Cr (b).
Figure 6. Micrograph of natural zeolite (a) and zeolite isolated after sorption of Cr (b).
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Table 1. Oxide content in RFC slag.
Table 1. Oxide content in RFC slag.
SpectrumOxides Content, %
Cr2O3CaOMgOAl2O3SiO2FeOTotal
Spectrum 17.0449.4812.294.1425.961.09 100
Spectrum 26.5051.659.384.3927.051.05 100
Spectrum 36.5852.508.124.4227.111.27 100
Average6.7151.219.934.3226.711.13 100
Standard Deviation0.301.562.140.150.650.12
Max.7.0452.5012.294.4227.111.27
Min.6.5049.488.124.1425.961.05
Note: Treatment parameters: oxygen by stoichiometry (normalized).
Table 2. Semi-quantitative analysis of crystalline phases of RFC slag.
Table 2. Semi-quantitative analysis of crystalline phases of RFC slag.
Mineral (Phase)FormulaC, %
Olivine-(Ca)Ca2SiO450.7
Magnesium OxideMgO20.3
QuartsSiO26.3
Mg-Cr-Oxide(Mg7.92Cr1.08)Cr16O326.3
Ca-Cr-OxideCaCr2O44.6
Ca-Mg-Fe SilicateCa2Fe1.2Mg0.4Si0.4O511.8
Table 3. Average chemical composition of RFC slag and cake by element oxides.
Table 3. Average chemical composition of RFC slag and cake by element oxides.
Oxides Content, %
Cr2O3totalCaOMgOAl2O3SiO2FeOSO3
RFC slag
6.9344.5712.125.1124.631.56 -
Cake from RFC slag
3.6127.752.072.4017.370.23 46.57
Table 4. Output parameters of the experiment on leaching from RFC slag into a sulfuric acid solution. Initial content of total chromium—35.8 mg/L.
Table 4. Output parameters of the experiment on leaching from RFC slag into a sulfuric acid solution. Initial content of total chromium—35.8 mg/L.
№X1
T °C		X2
t, Min		X3 CH2SO4, %Y1,
Residual Content Cr (mg/L)		Degree of Cr Leaching, Rel., %
135.844.423.24.1511.53
276.244.423.27.2620.17
335.8145.623.24.9613.78
476.2145.623.24.5612.67
535.844.476.83.158.75
676.244.476.83.8510.69
735.8145.676.83.158.75
876.2145.676.83.7810.5
92295503.7410.5
109095506.6718.53
115610503.208.88
1256180506.0416.78
13569556.9619.33
145695953.068.5
155695505.7215.89
165695503.9611.0
175695506.1317.03
185695504.8013.33
195695503.8010.55
205695503.8110.58
Table 5. Effect of Cr concentration on the sorption capacity of natural zeolite.
Table 5. Effect of Cr concentration on the sorption capacity of natural zeolite.
CCr, mg/LCr Content After Sorption, mg/LExtracted Cr Content, mg/LR, %
2012.08.040.0
3010.019.163.6
5015.534.569.0
7032.038.054.3
9844.453.654.7
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Sultanbayeva, G.; Kaiynbayeva, R.; Chernyakova, R.; Temel, H.; Jussipbekov, U.; Tassibekov, K. Sustainable Chromium Remediation: Sorption of Chromium from Leaching Solutions of Refined. Sustainability 2025, 17, 2726. https://doi.org/10.3390/su17062726

AMA Style

Sultanbayeva G, Kaiynbayeva R, Chernyakova R, Temel H, Jussipbekov U, Tassibekov K. Sustainable Chromium Remediation: Sorption of Chromium from Leaching Solutions of Refined. Sustainability. 2025; 17(6):2726. https://doi.org/10.3390/su17062726

Chicago/Turabian Style

Sultanbayeva, Gita, Raushan Kaiynbayeva, Raissa Chernyakova, Hamdi Temel, Umirzak Jussipbekov, and Khaidar Tassibekov. 2025. "Sustainable Chromium Remediation: Sorption of Chromium from Leaching Solutions of Refined" Sustainability 17, no. 6: 2726. https://doi.org/10.3390/su17062726

APA Style

Sultanbayeva, G., Kaiynbayeva, R., Chernyakova, R., Temel, H., Jussipbekov, U., & Tassibekov, K. (2025). Sustainable Chromium Remediation: Sorption of Chromium from Leaching Solutions of Refined. Sustainability, 17(6), 2726. https://doi.org/10.3390/su17062726

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