In nature, chromium is most often found in the trivalent form as the mineral chromite (FeCr₂O₄) and in the hexavalent form as crocoite PbCrO₄ (Das et al., 2021). The presence of Cr⁶⁺ and Cr³⁺ in groundwater may be a consequence of natural and anthropogenic processes occurring in different areas. The natural cause of the occurrence of chromium, biological leaching, and others in the environment can be weathering. Such natural processes do not lead to global pollution of the environment, but they should not be omitted during monitoring and assessing the state of water bodies located near deposits of this metal (Ukhurebor et al., 2021).

The leather industry is one of the most polluting industrial sectors (Gautam et al., 2017; Monira and Mostafa, 2023). Almost every leather industrial plant uses a large amount of chemicals in the processing of raw materials into leather. At each technological stage of a tannery, wastewater with different physical and chemical characteristics is generated (Laxmi and Kaushik, 2020; Xu et al., 2024). Thus, pickling and chrome tanning effluents contain sulphuric acid, chromium, chlorides, sodium bicarbonate and sulphates. Such waters often have a temperature of 25°C. Chrome tanning effluents usually have the lowest pH value, i.e. 3.0 ± 0.5 (Chowdhury et al., 2015). The low pH of such effluents may be due to the addition of sulphuric acid during the pickling step to produce a pickled hide (Gibert et al., 2002).

Chromium is a common pollutant introduced into the soil and natural waters due to the discharge of a variety of industrial wastewaters (Shi et al., 2020). Chromium most often exists in the environment as Cr³⁺ and Cr⁶⁺, and the latter is harmful for people and animals (Han et al., 2025). In living organisms, the reduction of Cr⁶⁺ takes place at the presence of Fe²⁺, which is part of haemoglobin. In this case, adverse biochemical reactions occur inside living cells (James and Bartlett, 1983; Naghipour et al., 2020).

The danger of such pollutant lies in the fact that its removal from groundwater and soil is a very difficult process. The traditional technology of Cr⁶⁺ removal from water is related to its reduction using a FeSO₄ solution and precipitation (Equations 1, 2), resulting in the creation of an insoluble form of Cr³⁺. Such Cr⁶⁺ reduction should be carried out at a minimum concentration of dissolved oxygen, which can oxidize Fe²⁺ and thereby reduce the efficiency of the process.

The technology of Cr⁶⁺ removal is based on the following chemical reactions (Simon et al., 2002; Zheng et al., 2020).

1.Acidic conditions (pH = 3.0 ± 0.5)

Contaminated groundwater can be treated using various methods such as drilling water wells, pumping the polluted water to ground facilities to perform different chemical reagents of treatment (Gheju, 2018; 2011; Noubactep, 2013; Simon et al., 2002). The system of pumping groundwater to the surface, treating it, and reinjecting it into the ground poses several environmental risks. Firstly, it can disrupt the balance of aquifers, causing a decline in groundwater levels and negatively affecting local ecosystems (Seo et al., 2018). Secondly, there is a risk of re-contaminating the aquifer if the treatment process is not sufficiently effective (Meuser and Meuser, 2013; Simon et al., 2002). Additionally, changes in the physicochemical properties of water during treatment may lead to sedimentation, reducing soil permeability (Reddi et al., 2000; Simon et al., 2002). Finally, high energy consumption and potential disruption of natural processes make this system not always a sustainable solution for the environment (Keely, 2019).

In this context, several studies have been conducted to find effective removal chromate by their immobilization (Eary and Rai, 1991; Liu et al., 2025; Naghipour et al., 2020) presented information about using pure minerals, e. g., natural iron and aluminum (oxy)hydroxides (Ajouyed et al., 2010; Madhusudan et al., 2023) and activated carbons (Mohan and Pittman Jr, 2006) were applied. In addition, natural or secondary polymineral mixtures such as soil, claystone, coal, and peat were also used. Also, suitable adsorbents of chromates were organo-silicates. Clay minerals and zeolites modified with quaternary alkylammonium cations were frequently studied because of their potential application as environmental remediation materials (Brigatti et al., 2000b; Jiang et al., 2018; Naghipour et al., 2020; Yin and Ellis, 2009).

Taking into account the disadvantage’s technology of pumping water, scientific research is focused on the possibility of treatment groundwater directly in the ground using the same treatment principles in situ (Hori et al., 2015; Langlois and James, 2015; Li et al., 2005; Simon et al., 2002). The most promising, affordable and environmentally-economically profitable is the application of iron-based technologies (Cundy et al., 2008; Gheju, 2018; 2011; Konadu-Amoah et al., 2022). Different materials of synthetic and natural origin can be used as Cr⁶⁺ reducing agents and such synthetic materials include metallic iron Fe⁰ (Morrison et al., 2002; Zheng et al., 2020; Zuo et al., 2025). A disadvantage of using Fe⁰ is that very often, groundwater is weakly acidic and contains carbonates in the form of hydrocarbonates (HCO₃ ⁻) (Eriksson and Khunakasem, 1966; Lai and Lo, 2008). Soil contaminated with chromium-containing wastewater also has a low pH value. In such conditions, which are additionally anaerobic, Fe⁰ is quantitatively oxidized to Fe²⁺ and the environment becomes weakly alkaline (Barker et al., 2018; Constantinou et al., 2023). As presented in paper (Eba et al., 2020; Michiels et al., 2015) FeCO₃ was formed on the surface of the Fe⁰ grain, thereby blocking its ability to further participate in the reduction of Сr⁶⁺.

Results of using Fe-rich aluminosilicates for the reduction of Cr⁶⁺ by goethite are presented in (Luo et al., 2020; Tomaszewski et al., 2017), by chlorite in (Brigatti et al., 2000a; Brookshaw et al., 2014; Carnicelli et al., 1997; Kohut and Warren, 2018) and by glauconite (Bajda and Kłapyta, 2013; Naghipour et al., 2020). A number of authors also observed the reduction of Cr⁶⁺ by Fe(II)-rich minerals (Eary and Rai, 1988; Gould, 1982; Ilton and Veblen, 1994; White and Peterson, 1996). In particular, in paper (White and Peterson, 1996) proposed that structural Fe²⁺ acts as a stronger reducing agent than aqueous Fe²⁺ under low pH conditions.

Glauconite concentrate is also an aluminosilicate that contains Fe²⁺ in its crystal structure (Baldermann et al., 2017; McRae, 1972). This mineral is able to adsorb heavy metals and participate in redox reactions (Chayka et al., 2021; Franus et al., 2019). The peculiarity of the presence of glauconite in natural conditions is that it often occurs in sands (Naghipour et al., 2020). Such sands are very common in Ukraine. They can lie at depths from 0 to 50 m, and the content of glauconite can range from 8% to 50% (Natkaniec-Nowak et al., 2017; Trach et al., 2021a). Glauconite can be extracted from sands using wet or dry magnetic separation (Franzosi et al., 2014; Skiba et al., 2014; Tripathy et al., 2017). The process of enrichment of natural minerals is always an economically expensive process. Its feasibility is always determined by the special values of the natural mineral.

Data of the Eh-pH diagram for chromium-water systems under standard state conditions show that at pH exceeding 5, Cr⁶⁺can be transformed into the insoluble form Cr₂О₃ (Barnhart, 1997). Thus, after the reduction of Cr⁶⁺, it is necessary to increase the water pH to reduce the concentration of the formed Cr³⁺. The use of alkaline reagents, such as NaOH and Ca(OH)₂, which are used for this purpose in natural (groundwater, surface water) treatment technologies, is not suitable for technologies applied directly in the environment. This is because when they come into contact with water, the pH of the water rises very quickly. For the environment, a change in pH from 3.5 to nine can lead to various negative phenomena (Antonelli et al., 2017).

Taking into account numerous studies on the properties of natural limestone and its occurrence, the complex solution of the problem of Cr⁶⁺ recovery and reduction of the migration of the Cr³⁺ formed in the environment is of great interest. During the normal operation of the water treatment station, additional use of natural materials is not required. The need to use natural materials may arise in the event of a possible emergency at the treatment water station of an industrial plant. As a result, very large areas of contaminated ground may appear.

The aim of this work was to study the possibility of using inexpensive natural materials of local origin to mitigate the toxic effects of acidic chromium-containing wastewater, which may enter the environment as a result of emergency situations. The relevance of this issue is especially high in the context of Ukraine, where the environmental situation, due to the ongoing war, remains strained in almost all regions, and financial resources for the implementation of complex water treatment technologies are extremely limited.

Quartz-glauconite sand, volcanic tuff, and glauconite concentrates were used in experimental studies on the possibility of Cr⁶⁺ recovery in acidic waters. Natural limestone was used to increase water pH and decrease the concentration of the formed Cr³⁺ (as oxide or hydroxide) in the environment. These natural materials have been earlier partially studied and the results have been published.

The studied quartz-glauconite sand contains 15% of glauconite, has grain sizes of 0.1–0.6 mm, and a filtration coefficient of 0.011–0.2 cm/s (9.84–17.28 m/d) (Trach et al., 2021a). Deposits of such sands are not thick (up to 3.5–4.0 m). They very often comprise the overburden, are not used in any way, and stored in special dumps. Their properties were partly studied by (Natkaniec-Nowak et al., 2017; Trach et al., 2021a). As shown in experimental studies performed in static conditions, when such sand is in contact with water (at different initial water pH and mineralization), an increase in the water pH is observed.

Volcanic tuff from the Khmelnitsky region was collected for the experimental study. The volcanic tuff lies at the depth of 0–27 m and has no regional industrial value. Its mineralogical composition is as follows: chlorite 30%, pyroxene 10%, kaolinite 18%, hematite 12%, and quartz 20% (Trach et al., 2021b). It is important to emphasize that among these minerals occurs chlorite and pyroxene, which contain Fe²⁺, a ion capable of reducing Cr⁶⁺.

The glauconite concentrate taken for comparative studies is a commercial product. The product comprises 70% glauconite, 25% sand and 5% limestone. It was purchased at a working quarry in the Khmelnitsky region, Ukraine [https://ua.all.biz/uk/glaukonit-g1619865].

The studied limestone (Ternopil region, Ukraine) is a secondary product of an active limestone quarry and is accumulated every year. According to its petrographic composition, the limestone is very homogeneous. Its properties were partially investigated and the results are presented in the article (Trach et al., 2021c).

In particular sections, the limestone consists of 50%–80% fine crystalline calcite (grain size less than 0.001 mm) and up to 30% of calcite fossils. In some cases, the calcite is more recrystallized and has dimensions between 0.01 and 0.05 mm. The texture is massive, porous and spotted. Recrystallized (dense) limestones are characterized by a CaСО₃+MgCO₃ content from 85.4% to 99.81% and insoluble residue from 0.89% to 9.5%. The content of magnesium carbonate varies from 0.79% to 3.43%, with an average of 1.91%.

The general appearance of the investigated natural materials and their geographic location are presented in Figure 1.

The results of the experimental studies showed that at the same initial conditions (С = 0.1 mg/dm³, pH 3.2, TDS = 550 mg/dm³ and the same doses of the applied materials), the effectiveness of Cr⁶⁺reduction to Cr³⁺ by the studied natural materials was different. The lowest efficiency reduction of Cr⁶⁺, i.e. 10%, was at the contact of QGS at 0.5 g/50 mL. GC showed the highest efficiency of 86%. The results of the experimental studies are presented in Figure 3.

Moreover, after 24 h of contact of the studied solution with natural materials, the pH, Eh and TDS were determined. This was done in order to evaluate the change of water parameters after Cr⁶⁺ reduction. Figure 3 shows that the pH increase of the solution depended on the doses and type of the applied natural materials. Regarding the increased pH, this process is very important due to the need for the formation of inactive chromium (Cr³⁺). With increased doses of VT, QGS, GK and at a constant volume of the solution, the pH had always increased. The highest ΔpH = 2 was after the contact with VT, the lowest ΔpH = 0.6 after the contact with QGS.

Determination of the Eh of the study solutions has shown the following. After reduction of Cr⁶⁺, the value of Eh increased after the contact with QGS, and decreased after the contact with GC and VT (Figure 3). Comparing ΔEh, at increasing dose of VT and GC, the ΔEh was greater for VT. For example, at a dose of 2.5 g/50 mL of VT, the ΔEh was −245, and for GC the ΔEh was −143.

An important water parameter that was monitored after Cr⁶⁺ reduction by the tested materials was TDS. The obtained results are graphically presented in Figure 3. The analysis of this chart shows that after 24 h of contact, TDS decreased in all cases, thus showing that the reduction of Cr⁶⁺ to Cr³⁺ was taking place. Hexavalent chromium was in the form of the anion CrО₄ ^2-, and then turned into the cationic form Cr³⁺. Therefore, there is a decrease of the mass of the inorganic form of chromium. A higher TDS decrease was observed at the highest dose of VT (2.5 g/50 mL). This was obviously associated with the increase of the solution pH to 5.2, at which the formed Cr³⁺ partially passed into the inactive form Barnhart (1997).

Simultaneously with the determination of the Cr⁶⁺ concentration in each solution, the Cr³⁺concentration was also determined. Its highest concentration was in the solution after contact with GC, and the lowest after contact with QGS. As shown in Figure 3, the highest efficiency of Cr⁶⁺ reduction was for GC at 2.5 g/50 mL, compared to VT in the same conditions. However, the concentration of the formed Cr³⁺ was significantly lower in the solution that was in contact with 2.5 g/50 mL of VT. This is obviously related to the increase of the solution pH after contact with VT.

After natural limestone sampling, according to the above method, its grain composition and grain size module were determined. Equations 3, 4 was used for the calculation. The results are presented in Table 1.

The next stage in the study of the filtration properties of the investigated limestone was the determination of the filtration coefficient according to the above method. Thus, the filtration coefficient of natural limestone 0–20 mm was K_f = 1.54 m/day; the bulk density was 1,400.5 g/cm³, and the porosity was 48.13%.

The analysis of the grain-size composition of the limestone studied showed that fractions 0–0.16 mm contribute to less than 1.73%. When such limestone comes into contact with acidic water, it will be neutralized and its smallest fractions will dissolve. Thus, over time, the filtration ratio will increase. Therefore, the authors have determined the limestone filtration coefficients for fractions 0.16–20 mm and 1.25–20 mm. For 0.16–20 mm the Kf was 3.9 m/day, the bulk density was 1,394.1 g/cm³, and porosity was 52.31%.

It was experimentally established that in the studied limestone about 70% of the fractions are 1.25–20 mm in size. The filtration coefficient of this fraction was Kf = 12.92 m/day, the bulk density was 1,381.6 g/cm³, and porosity was 58.39%. According to the classification of engineering soils following DSTU B V.2.1-2-96, such limestone belongs to highly permeable media.

During determination of the filtration coefficients for fractions 0–20 mm and 0.16–20 mm by KF-OOM, karst-suffusion process are manifested (Figure 4). Under the pressure of water, particles of calcium carbonate are removed from the filtration column. As a result of the mechanical removal of small particles by the water flow, the silt fraction of limestone I accumulated at the bottom of the outer casing of the device. As a result of suffusion of small limestone particles, a significant shrinkage of 3–5 mm is visible in the inner cup at the height of 100 mm, which corresponds to 3%–5%. Karst holes with a diameter of about 1–3 mm are visible on the surface of the filtered, finely dispersed limestone.

The rate of Fe²⁺release is linked with the mineral structure, the strength of the covalent or ionic bond, environmental degradation, particle size, moisture, and the soil pH (Fu et al., 2018). Chlorite, as an iron-rich aluminosilicate occurring in the volcanic tuff studied, is formed mainly as a result of metamorphic changes in biotite, phlogopite, hornblende, and augite. The rate of Fe²⁺ release is higher in the case of phlogopite and biotite in neutral conditions due to the fusion between soil water and minerals (Feigenbaum et al., 1981). The release of Fe²⁺ from clay minerals, such as glauconite, takes place at a slower rate under acidic conditions (Baldermann et al., 2017).

Glauconite crystals are characterized by a high content of K⁺ and Fe²⁺ (mica layers) and a low content of Fe³⁺ (smectite layers) (McRae, 1972; Yamashita et al., 2019). The reduction of hexavalent chromium, in contact with this mineral, is due to the presence of Fe²⁺ in the octahedral layer. Reports suggest that K⁺ may be bioavailable from glauconite in soil (Rudmin et al., 2020; 2019). This is due to the slightly acidic environment. Since Fe²⁺ is in the same octahedral leaf, Fe²⁺ is available at the same time.

GC and VT had a similar effect on Cr⁶⁺ reduction. With approximately the same efficiency of hexavalent chromium reduction by glauconite concentrate and volcanic tuff, the increase in the pH value of the solution was different.

This difference in the pH change of the solutions can be explained by the difference in the mineralogical composition of glauconite concentrate and volcanic tuff. As present in (Trach et al., 2021b), volcanic tuff contained up to 10% CaCO₃ (calcite). In an acidic solution, this compound is dissolved and thus increases the pH value (Trach et al., 2021c). Regulation (increase) of pH values of the solution, after Cr⁶⁺ recovery, is one of the main objective goals of the studied process, since the resulting pH of the solution is one of the most important factors affecting the mobility and migration of Cr³⁺ in the environment.

The results of experimental studies have shown that the efficiency of Cr⁶⁺ reduction was the lowest in contact with quartz-glauconite sand. This is due to the low content (15%) of glauconite. To increase the ability to reduce hexavalent chromium by quartz-glauconite sand, it would be advisable to add volcanic tuff to this medium. VT cannot be used on its own, as it contains a significant amount of clay minerals (Trach et al., 2021b). Their presence hampers water filtration by volcanic tuff.

To increase the efficiency of Cr⁶⁺ reduction, the studied glauconite concentrate can also be added to the quartz-glauconite sand. At the same time, the cost of glauconite concentrate should be taken into account. The studied volcanic tuff occurs on the ground surface and has no regional industrial value. Given the proximity of volcanic tuff deposits and quartz-glauconite sands, this can result in a high ecological and economic effect of their application. For Cr⁶⁺ reduction, it is advisable to install permeable reactive barriers (PRB) when using Fe-rich aluminosilicates and quartz-glauconite sand.

It is important to note that today there are technologies of Fe³⁺ reduction to Fe²⁺ in PRBs. In such technologies, the redox altering reagent is often sodium dithionite (Na₂S₂O₄) (Drits and Manceau, 2000). The dithionite ion is a strong reductant, particularly in strongly alkaline solutions. Reduction reactions with the dithionite anion proceed in two steps: dissociation of the dithionite ion to form a two sulphoxyl radical anion (SO₂ ⁻); the reaction of these radicals with the oxidized species Fe³⁺ yields a reduced species Fe²⁺ and oxidized sulphite (SO₃ ^2–) or bisulphite (HSO₃ ⁻). Thus, the use of such materials can be long-term and have an ecological and economic effect.

The peculiarity of the mechanism of Cr⁶⁺ reduction, which is in anionic form, is that it passes into Cr³⁺, which is in cationic form. As commonly known, the extraction of this cation from water is possible by sorption with the help of natural materials, in particular aluminosilicates (Aziz et al., 2008; Laxmi and Kaushik, 2020; Trach et al., 2021b) and by conversion into a stable form of Cr₂О₃/Cr(OH)₃ due to increase in the pH value of the aqueous medium (Barnhart, 1997).

The sorption process can only take place up to a certain point, i.e., until the sorbent’s sorption capacity is exhausted. After that, sorption cannot occur and the desorption process begins. Moreover, the desorption of heavy metals from the sorbent’s surface is possible when the physical and chemical indicators of water quality change (Belousov et al., 2019). Based on this, it can be concluded that the formation of the immobile form Cr³⁺ is relatively more reliable than its sorption by natural materials. Thus, in this work, experimental studies on the use of natural limestone (CaCO₃) were conducted to decrease the migration of Cr³⁺ in the environment. The use of natural limestone to neutralize or increase the water pH to 7.8–8.2 is a well-known and very common practice in the world (Angeler et al., 2017; Bernes, 1991; Trach et al., 2021c; Yi et al., 2017). This process occurs due to the dissociation of CaCO₃ in water and its decomposition into ions (Ca²⁺ and CO₃ ^2-). After dissociation, the formed Ca²⁺ ions are hydrolysed, and CaOH⁺ and Ca(OH)2 are formed. CO₃ ^2- anions undergo proton reactions, and form HCO₃ ⁻ and H₂CO₃.

Considering the above, when natural limestone is added to water containing Cr³⁺, the following chemical processes occur: CaCO 3   →   Ca 2 + + CO 3 2 ‐  pH > 7 (6) Ca 3 + + H 2 O   pH > 7 →  Cr OH 3 / ( Cr 2 O 3 ) ↓ (7)

The possibility to decrease Cr³⁺ concentration by naturally crushed limestone in static conditions, as well as the filtration properties and removal of this cation in dynamic conditions, were studied in this work. The results of such experimental studies have shown its high efficiency. Natural limestone can be used in different ways to achieve the set purpose. The method of its application depends on the topography of the contaminated area. The formed Cr³⁺ together with the surface runoff can migrate to the lowest point of the terrain and accumulate there. Very often, lakes form in the lowest points of the terrain (Born et al., 1979). The source of their nutrition is surface runoff and atmospheric precipitation. To solve the problem, natural crushed limestone can be used by direct introduction into the polluted lake. The scheme of the possible application of quartz-glauconite sand, volcanic tuff and naturally crushed limestone, taking into account the topographic features of the contaminated area, is presented in Figure 6A.

The second option of using limestone is possible by organizing semi-permeable reaction barriers. The scheme of the application of the studied natural materials located close to the contaminated site with a slight difference in relief is presented in Figure 6B.

In practice, it is commonly known that natural limestone can be used as a reaction material of PRB for the extraction of Fe, Mn and neutralization of acidic waters (Wang et al., 2016). The article reported that an increase in water pH and a decrease in the concentration of these pollutants was observed during the three observation years. The use of PRB with limestone is also presented in (Amos and Younger, 2003). It was used to neutralize acidic mine waters and treat coal mine leachates. The filter material was composed of 50% limestone grains at 10 mm and 50% gravel. Such filter material showed that the maximum alkalinity, acidity reduction and heavy metal removal were achieved within 24 h. Due to its filtration properties, natural limestone is used to reduce the concentration of SO₄ ^2- in acidic mine waters. When such waters come into contact with limestone, partial dissolution of CaСО₃ occurs and, as a result, an increase in the water pH and the formation of poorly soluble CaSO₄ (O et al., 2002).

In the event of a possible emergency situation in industrial enterprises, it is important to study local deposits of natural materials. In general, it is necessary to have information about possible existing mining waste (overburden, secondary products). Such natural materials can be environmentally friendly and economically beneficial. Thus, to ensure the recovery of Cr⁶⁺ and reduce the migration of the formed Cr³⁺, it is advisable to use iron-containing aluminosilicates mixed with sand and natural limestone.

The results of experimental studies are of great environmental importance in solving the problem of possible emergencies in industrial enterprises that produce wastewater with a high content of hexavalent chromium. This is mainly due to the fact that all the studied natural materials are located close to each other, being waste or by-products of the mining industry.

The studied quartz-glauconite sand, volcanic tuff and glauconite concentrate showed that at pH = 3.2 for 24 h they are able to reduce the Cr⁶⁺ concentration. Glauconite concentrate and volcanic tuff showed same efficiency of Cr⁶⁺ reduction, while quartz-glauconite sand showed the lowest efficiency. After Cr⁶⁺ reduction in all solutions studied, the presence of Cr³⁺ was established. Its lowest concentration was in a solution, in which Cr⁶⁺ was reduced by volcanic tuff. This is due to the fact that the composition of volcanic tuff includes minerals such as kaolinite, calcite, which are able to increase the pH of the solution.

To reduce the migration of the formed Cr³⁺ in the environment, the properties of natural limestone (0–20 mm), which is a secondary product of limestone quarries, were studied. Experimental studies have shown the ability of limestone to reduce the Cr³⁺ concentration in static and dynamic conditions. Thus, due to the chemical characteristics and ability to filter water (for 0–20 mm K_f 1.54 m/day, for 0.16–20 mm K_f 3.9 m/day, for 1.25–20 mm K_f 12.92 m/day), the studied accumulated limestone is potentially suitable for the construction of PRBs for the treatment of surface and groundwater or for direct application in surface water bodies.