Oxidation-reduction reactions in geochemistry

Cite this paper: Vietnam J. Chem., 2021, 59(2), 133-145 Review DOI: 10.1002/vjch.202000196 133 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Oxidation-reduction reactions in geochemistry Salah Uddin Biswas, Kripasindhu Karmakar, Bidyut Saha * Homogeneous Catalysis Laboratory, Department of Chemistry, The University of Burdwan, Burdwan- 713104, West Bengal, India Submitted December 7, 2020; Accepted January 12, 2021 Abstract There are more than hundred elements in the periodic table and many of them are associated with various geochemical processes. Most of the elements can show more than one oxidation states, therefore, reactions involving oxidation and reduction are of very much importance in geochemistry. Since every change (chemical or physical) is associated with the change of energy, hence for every single process, there should be a reliable way for quantitative measurement of energy change. In case of any redox process, the energy change can be quantitatively expressed in terms of reduction potential of that process. For better understanding of a geochemical process, previously known reduction potentials can be used. The main importance of reduction potentials in geochemistry is for understanding the frequent concentration and enrichment of various elements in deposits formed under extremely reducing or oxidizing conditions. Keywords. Elements, Geochemistry, Oxidation, Reduction, Redox process, Potential. 1. INTRODUCTION Geochemistry utilizes the principles of chemistry to describe various mechanisms that regulates the processes of the major geological systems viz. earth‟s mantle, earth‟s crust, ocean and its atmosphere. This branch is also important for the understanding of various terrestrial and planetary processes, namely the origin of granite and basalt, sedimentation, the origin of mineral deposits etc. [1] The dynamics of earth are controlled by numerous mechanisms and most of them work in opposing direction. For the case oxidation and reduction reaction in geochemistry, oxidation and reduction are two opposite processes by which various changes occur in earth‟s environment and of course these two processes are collectively known as redox process is applicable to the most of the elements since they may exists in several oxidation states in the earth‟s crust.[2] Nonbiodegradable metals are accumulative in nature and their deposition can lead to metal contamination at the surface environment, they are harmful to human body, specially cadmium, lead etc. [3] Again all the oxidation states of a element or a metal may not be harmful, e.g, trivalent chromium is a trace mineral which is essential to human nutrition but the hexavalent chromium is toxic. So, to remove or to control the enriched deposition of various metals from earth‟s surface, especially from soil the application of oxidation and reduction in geochemistry is of great interest. In the ocean, trace metals form different complexes with major anions such as hydroxide ion, carbonate ion, chloride are present and their chemical speciation changes depending on the environment whether it is oxidized or reduced. [4] When the complexation is stronger, the reactivity of the metal ion is lower, the consequence that is chelation tends to stabilize the metals in aqueous solution instead of in solids. [5] A hybrid distributions in the ocean is observed for iron and copper. A limiting nutrient iron is in vast areas of the oceans, and is of high abundance along with manganese near hydrothermal vents. Iron forms precipitate of sulfides and oxidized iron oxyhydroxide compounds. Million fold concentrations of iron can be found at hydrothermal vents concentrations found in than that of the open ocean. [6] Concept of oxidation and reduction and Nernst’s equation The word „oxidation‟ originally implied reaction with oxygen to form an oxide or as the removal of hydrogen (or any other electropositive element). Later, the term (oxidation) was expanded to encompass oxygen-like substances such as chlorine etc. Later on, these definitions have been given on the basis of the addition of electron(s) to an element Vietnam Journal of Chemistry Bidyut Saha et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 134 or the removal of electron(s) from an element, the so called electronic theory of oxidation and reduction which states that oxidation is a process that results in the loss of electron(s) from an atom or ion whereas reduction is a process in which one or more electrons are gained by an atom or ion. Hence, an oxidizing agent (oxidant) is the one which accepts electron(s) and thereby it is reduced to a lower oxidation state; a reducing agent (reductant) is one that loses electron(s) and then it is oxidized to a higher oxidation state. For general purpose the relationship between the oxidized form and reduced form can be expressed by the equation given below Ox + ne = Red The electrode potential i.e. here the reduction potential (it is the potential arising depending upon the tendency of the oxidized form of any substance to accept electron(s) from the electrode and thereby to be converted to the reduced form) is given by the Nernst equation as ( ) ( ) The above equation, at 298 K transforms into: ( ) ( ) where n is the number of electron(s) transferred, F ꞊ 96500 C, R ꞊8.314 volt coulombs, T is temperature in Kelvin scale, E is electrode potential in volt, is standard potential in volt, terms are respective activities of the components. A brief concept about geochemistry The term „geochemistry‟ was introduced by the Swiss-German chemist Christian Friedrich Schöbein in the year 1838 as “a comparative geochemistry ought to be launched, before geochemistry can become geology, and before the mystery of the genesis of our planets and their inorganic matter may be revealed.”[7] Geochemistry has become a separate discipline after the establishment of the major laboratories, the beginning being started with the United States Geological Survey (USGS) in the year 1884, with the systematic surveys of the chemistry of rocks and minerals. Frank Wigglesworth Clarke, the principal chemist of USGS summarized his work in The Data of Geochemistry which reveals that the abundance of elements, in general, decreases as their atomic weights increase. [2,8] Different branches of geochemistry There are many subfields of geochemistry of which seven subfields are immensely dominating. [9] (a) Aqueous geochemistry: It describes the role of various elements in watersheds viz. copper (Cu), sulfur (S), mercury (Hg) and how elemental fluxes are exchanged through atmospheric-terrestrial- aquatic interactions. [10] (b) Biogeochemistry: It focuses on the effect of life on the chemistry of the Earth. [11] (c) Cosmo geochemistry: It is the subfield of geochemistry that involves analysis of elemental and isotopic distribution in the cosmos. [7] (d) Isotope geochemistry: It includes determination of the absolute and relative elemental and isotopic concentrations in the Earth and on Earth's surface. [12] (e) Organic geochemistry: It is the field study where the role of processes and compounds that are derived from living organisms are of focus. [13] (f) Photo geochemistry: Here light-induced chemical reactions that occur or may occur among natural components of the Earth's surface are of interest. [14] (g) Regional geochemistry: This subfield of geochemistry includes applications to environmental, hydrological and mineral exploration studies. [15] Broad idea of oxidation-reduction reactions in geochemistry There are hundreds of elements in the periodic table of elements, many of which are capable of and do occur in various oxidation states in earth‟s crust depending upon the surrounding environment of them. Few of these elements can be named among which the most common elements can be named as iron (Fe) which occurs as native metal in zero oxidation state as well as in an oxidized form as ferric ion (Fe 3+ ) and in a reduced form as ferrous ion (Fe 2+ ) in various compounds and vanadium (V), chromium (Cr), manganese (Mn), cobalt (Co), copper (Cu), arsenic (As) etc. The existence and stability of a particular oxidation state of a particular element is directly related to the energy change during the removal or addition of electron(s) to convert the element to a higher or lower oxidation state and the quantitative measurement of the energy change is given by reduction potential value. [2] According to the latest convention by IUPAC, the electrode potential is given a positive sign if there occurs taking up of electron(s) from the electrode when connected to the standard hydrogen electrode and a negative sign if there occurs liberation of electron(s). Now, what is reduction potential? We can Vietnam Journal of Chemistry Oxidation-reduction reactions in geochemistry. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 135 understand it in an easier way. Suppose there is a solution of equal concentrations of oxidized and reduced form of the same element. Now if we dip an electrode which is not attacked by the solution, a potential will originate in that electrode depending on the tendency of the oxidized form to accept electron(s) from the electrode and it will go to the lower oxidation state i.e., it is reduced form of the element. Now this potential is called reduction potential of that particular element by which the oxidized form is converted to the reduced form and it is denoted by . Determination of electrode potential Like temperature, there is no procedure by which one can measure the absolute value of the electrode potential which is the reduction potential value. Therefore, it is measured with respect to other suitable reference electrode system i.e., reduction potential value is a relative quantity. The reference electrode being the hydrogen electrode and the standard potential of the hydrogen electrode is taken as zero i.e., 0 volt and the corresponding reaction is given below: 2H + + 2e = H2 If we make a series by placing the different metals in order of their increasing reduction potential value, the series we will get is the electrochemical series where the most powerful oxidizing agents are those having their highest positive reduction potential value. For further clarification we can say that if there are two different reduction potential systems, one having higher reduction potential value than the other, then if the reaction condition permits, the oxidized form of the higher reduction potential valued system will be capable of oxidizing the reduced form of the lower reduction potential valued system or we can say that the reduced form of the lower reduction potential valued system will be able to reduce the oxidized form of the higher reduction potential valued system. The reduction potential value depends on the ratio of the activities of the oxidized and reduced form and the for dilute solutions activity can be replaced by the respective concentration terms, the variation of reduction potential with the concentration is very important for those reactions that involve H + and OH  ions. According to the pH scale, the range of pH is 0-14. Therefore, in aqueous solution the concentration of hydrogen ion may vary from 1 to 10 -14 . This kind of changes in concentration produce change in reduction potential value where hydrogen and/or hydroxyl ion(s) are involved in the reaction and in order to reach accuracy this must be taken into account in applying values to actual reactions.[2] The effect of pH on the reduction potential values of some reactions is shown graphically as follows in figure 1. [2] Figure 1: Variation of reduction potential values with pH change [2] and the respective reactions are as follows: A. 2H + + 2e = H2 Aˊ. H2 + 2OH = 2H2O + 2e B. 2H2O = O2 + 4H + + 4e Bˊ. 4OH = O2 + 2H2O + 4e Vietnam Journal of Chemistry Bidyut Saha et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 136 C. Fe 2+ + 2e = Fe C. Fe(OH)2 + 2e = Fe + 2OH D. Pb 2+ + 2e = Pb D. PbO + H2O +2e = Pb + 2OH E. Fe 3+ + e = Fe 2+ E. Fe(OH)3 + e = Fe(OH)2 + OH F. NO2 + 10H + + 8e = NH4 + + 3H2O F. NO3 + 6H2O + 8e = NH3 + 9OH H. PbO2 + 4H + + 2e = Pb 2+ + 2H2O H. PbO2 + H2O + 2e = PbO + 2OH K. Mn 3+ + e = Mn 2+ K. Mn(OH)3 + e = Mn(OH)2 + OH L. NiO2 + 4H+ 2e = Ni 2+ L. NiO2 + 2H2O + 2e = Ni(OH)2 + 2OH M. Co 3+ +e = Co 2+ M. Co(OH)3 + e = Co(OH)2 + OH Redox reactions in natural environment From literature we can collect a large number of reduction potential data for various redox processes from which we can theoretically expect the oxidation of the reduced form of the lower reduction potential valued system by the oxidized form of the higher reduction potential valued system. But, the expectations of us are not always true i.e., there are many reactions that never occurs naturally, from which we may conclude that there is a certain range of reduction potential value within which natural processes occur. This limitation may arise as chemical reactions at or near the earth's surface generally occurs in aqueous solution. Theoretically, it limits the reactions which can takes place to those with reduction potentials lying between those for the reactions [2] O2 + 4H + + 4e = 2H2O, volt 4H + + 4e = 2H2, volt Now the oxidized form of any redox couple which has higher reduction potential value than 0 volt will decompose water and hence the reaction cannot occur in aqueous medium. Similarly, for any redox couple whose reduction potential value is more negative than 1.23 volt, redox reaction involving the redox couple cannot occur in aqueous solution, at least theoretically. In the same way, we can explain the existence and nonexistence of elements in their zero oxidation state. The lower the reduction potential value, the easier is the removal of electron(s) from the metal(s) and thereby the lesser is the possibility of existence of metal(s) in their zero oxidation state and if any metal has reduction potential value higher than standard hydrogen electrode, it is not possible to generate its constituent ions in aqueous medium but in nonaqueous medium they can be generated. Redox geochemistry of some elements: Vanadium The concentration of vanadium(V) is not same everywhere in the world, namely in soils and sediments and it is very much toxic and also hazardous to the entire living system in the world. The toxicity controlling parameter is the soil redox potential. Vanadium being a transition element, can show more than just one oxidation states of which +5 oxidation state is the most toxic form in the environment instead of the most common +4 oxidation state in normal condition. Though vanadium(IV) is stable in acidic medium but above pH 5 it is transformed into vanadium(V), whereas at lower pH this transformation is slower. [16] The aqueous chemistry of vanadium(IV) is dominated by vanadyl ion (VO 2+ ) which is formed by dissolving VO 2+ in acids which is the reason why the +4 oxidation state of vanadium is stable in acidic condition. The vanadyl ion is not strongly oxidizing or reducing agent which is revealed from the following data: VO 2+ + 2H + + e = V 3+ + H2O, volt [V(OH)4] + + 2H + + e = VO 2+ + 3H2O, volt. The geochemistry of vanadium is largely dependent on its oxidation state in different compounds of it. Since vanadium(V) disturbs different enzyme activities and also able to act as of Na + K + -ATPase inhibitor of plasma membranes, it is more toxic to human being than vanadium(IV) and this is why it may be categorized in the same class of mercury, lead arsenic; the reduction of vanadium(V) with hydrogen sulfide produces less soluble vanadium(IV) and the potential at which different vanadium(IV) species precipitates have been reported such as vanadyl phosphate sincosite [CaV2(PO)4(OH)4.3H2O], vanadium hydroxides. [17- 19] The geochemistry of vanadium is affected by iron oxide, aluminum oxide, manganese oxide. Because of almost equal size of vanadium(III) and high spin iron(III), they show similarities in some aspects and co-exist in soil and ores and vanadium is capable of replacing iron in many cases from octahedral sites. Since vanadium is strongly bound by iron hydroxides, the chemistry of vanadium in different redox conditions can be regulated by the redox behavior of iron. Vietnam Journal of Chemistry Oxidation-reduction reactions in geochemistry. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 137 Chromium Chromium(Cr) is another transition element with atomic number 24 which is essential for both plants and animals for their metabolism, can be extremely harmful if accumulated at high level. The stable oxidation state of chromium is Chromium(III) which forms the most stable trivalent cation in aqueous solution. The geochemistry of chromium is important since steel works, leather tanning, chemical manufacturing etc. produce high chromium waste. Variation of chromium content (in µmol/g) with the nature and type of rocks and sediments is shown in table 1. [20] The data reveals that granite, carbonates, sandy sediment and sandstone contain the lowest chromium content whereas river suspended matter, soil, deep-sea clay and shale contain higher chromium content. The table 2 data show that the concentration (nmol/l) of chromium in natural water is even higher and the dissolved chromium is associated with soluble chromate species. [20] Table 1: Chromium content in naturally occurring solids [20] Name of solid Typical amount (in µ mol/g) Range of amount (in µ mol/g) Lithosphere 2.4 1.5-3.8 Granite 0.4 0.02-0.5 Sandstone 0.7 0.2-1.9 Shale 1.7 1.7-7.7 Carbonate 0.2 0.02-0.3 Coastal suspended matter - 0.01-0.21 Deep-sea clay 1.8 1.1-2.1 Marine sediment - 0.2-0.7 River sediment - 0-2 River suspended matter 3.6 - Sandy sediment 0.5 0.3-0.7 Clayey sediment 1.2 0.7-1.6 Clay 2.3 0.6-11.3 Soil 1.9 0.02-58 Although chromium has electronic configuration [Ar]3d 5 4s 1 and it can have span of oxidation state from zero to six, in natural aqueous medium only two of them, namely chromium(III) and chromium(VI) are important according to their existence. However, the stable chromium(III) species in water solution at lower reduction potential value are Cr(OH) 2+ , Cr(OH)4  , and also Cr(OH)3 in the following way: 2Cr 3+ (aq) + 3CO3 2- + 3H2O = 2Cr(OH)3 + 3CO2 Vietnam Journal of Chemistry Bidyut Saha et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 138 But, the superiority of the chromium(III) is encountered at pH below 3.6 which is shown in the figure-2. [20] Table 2: Concentration of chromium in various water systems [20] Type of the water Typical concentration (nmol/l) Range of concentration (nmol/l) Seawater 3 0.1-16 Interstitial water - 1.0-6.6 River 10 0-2200 Lake - < 2-33 Ground water