In order to discharge the water bodies and soils contaminated with hexavalent chromium, it is essential to predict the physico-chemical conditions that occur in the aqueous environment and soil. I hereby declare that this research thesis is the result of my own work and to the best of my knowledge that it does not contain any material previously published or written by another person; all sources of information, including figures, tables and other material are expressly acknowledged in the thesis.' All chromium reserves are located in the western part of Kazakhstan, the Aktobe region (Koshim et al., 2015).
The Khromtau chromite deposits are located in the Kempirsay massif on the southern edge of the Ural mountain system, and the Donskoy chromite deposits occur in the main ore field. Bioaccumulation is defined as an increase in the concentration of a chemical element in a biological organism over time compared to its concentration in the environment. According to many research findings (Kudarov et al., 1989), the territory of the Khromtau region established a stable biogeochemical province of chromium, which led to an increase in the mobility of chromium in the biosphere: the soil-plant-animal-human cycle.
The spread of chromium in soil and groundwater in the vicinity of the Aktobe region is of increasing concern. The general aim of the research thesis is to investigate the reduction of toxic Cr(VI) to non-toxic Cr(III) in the Aktobe region. The study focuses on the screening of various approaches to toxic chromium reduction that can be applied in the environmental environment of the city of Khromtau.
The reduction of hexavalent chromium to trivalent chromium in the groundwater of the Aktobe region is important because of the threat that the distribution of chromium poses to the environment and the population of that region.
2 LITERATURE REVIEW
- Basics of Chromium
- Generalities on chromium geochemistry in the environment
- Geographical and geological setting of Aktobe region chromite mines .1 Ural mountains and Kempirsay massif
- Health effects of chromium and environmental contamination
- Reduction strategies of Cr(VI)
- Screening factors for chromium reduction methods in Khromtau
Because of the unstable environment in the oceanic crust, the chromite deposits are irregular in shape and elongated (Stowe, 1994). The host rocks of chromite orebodies are dunite, serpentine or peridotite (Mosier et al., 2012). The serpentine rocks are produced as a result of a hydrous alteration of ultramafic rocks, including peridotite or pyroxenite, at low temperatures (Chrysochoou et al., 2016).
In addition to chromium, the stratiform chromite deposits are also rich in platinum group elements, including platinum, osmium, rhodium, iridium, and ruthenium (Schulte et al., 2010). In contrast, the hexavalent state of chromium is a highly toxic and highly soluble metal and is considered a Group I human carcinogen (Stefansson et al., 2015; Das et al., 2021). In the presence of oxygen, Cr(VI) is commonly found as chromate (CrO42-) or dichromate (Cr2O72-) oxyanions.
In addition, hexavalent chromium is more abundant in oxygenated waters, while trivalent species increase in anoxic environmental conditions (Cranston et al., 1978). Trivalent Cr species predominate under acidic conditions, while hexavalent Cr species are released when the pH is basic (Oze et al., 2004). Specifically, the Kempirsay ultramafic massif in the southern end is widely known for the aforementioned deposits of podiform chromite.
A common source of Cr in these soil types is chromite and Cr-magnetite (Oze et al., 2004). The heavy metal particles can stick to the soil surface and thus contaminate surrounding lakes and rivers and leach into groundwater and soil (Das et al., 2021). Furthermore, trivalent and hexavalent chromium have been shown to accumulate in human and animal tissues (Das et al., 2021).
However, not all bacteria that are Cr(VI)-resistant can convert Cr(VI) to Cr(III), as their development is considerably slower at high chromate concentrations (Tumolo et al., 2020). It is used as a reducing agent and can transform (degrade) or bind a wide range of pollutants in both groundwater and soil (Shi et al., 2011). Cr(VI) removal is best done with anion exchangers, whereas Cr(III) is removed with cation exchangers (Coetzee et al., 2020).
Since chromate is the most desirable of the common anions in water, anion exchange using synthetic resins is an ideal approach for chromium removal (Tumolo et al., 2020). The part of the Ilek River that flows near the city of Khromtau has a pH between 10–12.9, which is considered very alkaline (Zhakashov et al., 2011).
3 PROJECT PLAN
4 METHODOLOGY
Geochemical modeling basic principles
Screening criteria of chromium remediation methods
5 DATA ANALYSIS AND RESULTS
Development of a pe-pH diagram
These methods are evaluated by Cr(VI) concentrations in soil and water bodies, weather conditions and environmental pH. Based on the reactions, ΔG and logK values were automatically calculated for each reaction and helped determine the pe and pH values (Figure 7). The blue lines on the pe-pH diagram below indicate the limits of water stability (Figure 7).
The dominant Cr(VI) species include HCrO4- and CrO42-; sometimes the solutions contain Cr2O72 dichromate species, but only in strongly acidic solutions.
Application of Cr(VI) remediation strategies in the Aktobe region
Furthermore, most soils and water bodies in mining sites have their native microbiota, which may exhibit tolerance to toxic chromium. This ability of bacteria to survive is of widespread importance in industrial environments such as in Khromtau, because anthropogenic pollutants increased the pH of soil and water to 12.9. At a pH higher than 7, the chromium reduction rate decreases due to the osmosis and hydrolyzing effect.
Nevertheless, this restriction is not essential as the chromium concentrations are not extremely high, but it is still toxic for domestic use and drinking. In general, the conditions for growing bacterial strains in Khromtau are satisfactory due to the favorable temperature values, which vary between 30-40°C. Chemical reduction is one of the most efficient methods of Cr(VI) reduction in wastewater, and it has been observed that with Fe(II) sulfate, the removal is 100% perfect.
Furthermore, while the Cr(VI) compounds are reduced, Fe(II) ions are oxidized to Fe(III) compounds, which are excellent coagulants. Reduction is made under acidic conditions, while the pH of water bodies in Khromtau is more alkaline. Sodium hydrosulfite can be used directly in alkaline conditions, but is usually not cost effective.
In chromium-contaminated soils, remediation by chemical reduction is not feasible due to the enormous labor costs and ineffectiveness. Observing temperature is not important as it has no influence on the chromium reduction rate and chemical reduction efficiency. Ion exchange is a physical chromium remediation method that involves the construction of barriers using engineering approaches.
According to the EPA's recommendation for chromium removal, ion exchange is one of the best methods available. It is a reliable technology for removing low chromium concentrations that is effective to use in Khromtau. The pH of the bodies of water is not of great importance, as the synthetic resins used as filters can be chosen taking into account the pH of the water.
6 CONCLUSION
However, this limitation will not cause any difficulty if the synthetic resins are available in large quantities and can replace each other in an effective manner. Based on the literature review, the main conclusion that can be drawn is that chromium removal from water is affected by the species of chromium present and their concentrations and pH. A literature review and screening criteria revealed that the most preferred method for chromium reduction in Khromtau is bioremediation.
Compared to other disposal methods, bacterial reduction is more sustainable and produces no waste. It was taken into account due to the massive contamination of soil and water bodies in the Khromtau region with several heavy metals other than hexavalent chromium. The main disadvantage of bacterial reduction is the long time lag between bacterial growth and reduction.
Consequently, the bacterial reduction is preferred as the purification of highly contaminated water bodies and soils will be expensive. Chemical reduction with zero-valent iron and Fe(III) is considered the most effective method, but the effects of this treatment can improve the deterioration of the pollution level in Khromtau. The chemical reduction is also preferred for the treatment of waste water in facilities in a short time, as the rate of reduction is fast compared to the other two methods.
Among other proposed technologies, bioremediation and ion exchange, this method is the oldest and does not foresee the ecological aspects of chromium removal. This research was limited in scope due to restrictions on visiting mining sites due to the pandemic. Further research could investigate aquifer conditions in the Khromtau, Kazakhstan groundwater system and allow tracing the pathways of different chromium transport mechanisms if water and soil sampling at the mine site becomes possible.
The chemical analysis could research chromium geochemistry in the Ilek, Emba, Kosestek, Kobda, and Or’ rivers;. The groundwater geochemical analysis in Khromtau would provide quantitative data regarding the amount of hexavalent chromium in water. Moreover, the detailed characterization of the weathering profile of the mine site and groundwater would provide important information on chromium exposure routes.
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Petrogenesis of the giant ophiolitic chromite deposits of Kempirsai, Kazakhstan: a study of solid and fluid inclusions in chromite.Journal of Petrology. National report on the state of the environment and the use of natural resources.
