Oxidation reduction reactions in geochemistry

Cite this paper: Vietnam J Chem., 2021, 59(2), 133-145 Review DOI: 10.1002/vjch.202000196 Oxidation-reduction reactions in geochemistry Salah Uddin Biswas, Kripasindhu Karmakar, Bidyut Saha* Homogeneous Catalysis Laboratory, Department of Chemistry, The University of Burdwan, Burdwan713104, 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 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 133 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Vietnam Journal of Chemistry 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 Bidyut Saha et al 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-terrestrialaquatic 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 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 (Fe3+) and in a reduced form as ferrous ion (Fe2+) 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 © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 134 Vietnam Journal of Chemistry 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., 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 Oxidation-reduction reactions in geochemistry 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 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 © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 135 Vietnam Journal of Chemistry C Fe2+ + 2e = Fe D Pb2+ + 2e = Pb E Fe3+ + e = Fe2+ F NO2 + 10H+ + 8e = NH4+ + 3H2O H PbO2 + 4H+ + 2e = Pb2+ + 2H2O K Mn3+ + e = Mn2+ L NiO2 + 4H+ 2e = Ni2+ M Co3++e = Co2+ Bidyut Saha et al C Fe(OH)2 + 2e = Fe + 2OH D PbO + H2O +2e = Pb + 2OH E Fe(OH)3 + e = Fe(OH)2 + OH F NO3 + 6H2O + 8e = NH3 + 9OH H PbO2 + H2O + 2e = PbO + 2OH K Mn(OH)3 + e = Mn(OH)2 + OH L NiO2 + 2H2O + 2e = Ni(OH)2 + 2OH 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 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 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 (VO2+) which is formed by dissolving VO2+ 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: VO2+ + 2H+ + e = V3+ + H2O, volt [V(OH)4]+ + 2H+ + e = VO2+ + 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.[1719] 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 © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 136 Vietnam Journal of Chemistry Oxidation-reduction reactions in geochemistry Table 1: Chromium content in naturally occurring solids[20] 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 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] Although chromium has electronic configuration [Ar]3d54s1 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 Name of solid Lithosphere Granite Sandstone Shale Carbonate Coastal suspended matter Deep-sea clay Marine sediment River sediment River suspended matter Sandy sediment Clayey sediment Clay Soil Typical amount (in µ mol/g) 2.4 0.4 0.7 1.7 0.2 - Range of amount (in µ mol/g) 1.5-3.8 0.02-0.5 0.2-1.9 1.7-7.7 0.02-0.3 0.01-0.21 1.8 3.6 1.1-2.1 0.2-0.7 0-2 - 0.5 1.2 2.3 1.9 0.3-0.7 0.7-1.6 0.6-11.3 0.02-58 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: 2Cr3+(aq) + 3CO32- + 3H2O = 2Cr(OH)3 + 3CO2 © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 137 Vietnam Journal of Chemistry Bidyut Saha et al But, the superiority of the chromium(III) is encountered at pH below 3.6 which is shown in the figure-2.[20] E  E  0.059 14log  H   i.e, Table 2: Concentration of chromium in various water systems[20] Type of the water Seawater Interstitial water River Lake Ground water Polluted water Tap water Typical concentration (nmol/l) - Range of concentration (nmol/l) 0.1-16 1.0-6.6 10 < 20 0-2200 < 2-33 10-4000 - 960-27000 0-700 Figure 3: pe-pH relationships for dissolve aqueous chromium species in presence of Cr(OH)3(s)[20] Figure 2: Speciation of chromium(III) as a function of pH[20] Whereas CrO42 ion predominates at pH greater than and that HCrO4 and Cr2O72 exist in equilibrium between pH and where chromium exists as chromium(VI) which is shown in figure 3.[20] The pH dependence stability of Cr(III) and Cr(VI) can be explained in the following way: Cr2O72 + 14H+ = 2Cr3+ + 7H2O – 6e, V Now, applying the Nernst‟s equation, we get: 14 Cr2O72   H   0.059 E  E  log [Cr 3 ]2 If the concentration of the H+ ion is taken as unity, the above equation can be rewritten as Cr2O72  0.059 0.059  E  E  14log  H   log 6 [Cr 3 ]2 For equimolar concentration of dichromate ion and chromium(III), the above equation becomes: Now, as the pH increases, the effective potential of Cr2O72|Cr3+ system decreases from 1.33 volt and becomes 0.83 volt at pH 3.6, therefore its oxidizing power reduces and it cannot be reduced to chromium(III) Previously it is known that chromium(III) is stable only at a pH lower than 3.6 However, at higher pH range chromium(III) can be oxidized to chromium(VI) by MnO2 (manganese dioxide) where the oxidizing agent is reduced to Mn2+ Theoretical stoichiometry of this reaction has been suggested as below [20] Cr3+ + 1.5 MnO2(s) + H2O = HCrO + 1.5 Mn2+ + H+ The above reaction is supposed to occur in three steps (a) adsorption of chromium(III) on the surface sites of MnO2, (b) oxidation of chromium(III) to chromium(VI) by Mn(IV) from MnO2 and (c) at the end the chromium(VI) are desorped from the surface and Mn(IV) is converted to Mn(II) The geochemistry of this particular redox reaction can be understood very easily if we look at the standard reduction potential values of the two couple The value for the Cr(VI)|Cr(III) system is 1.33 volt and that of Mn(IV)|Mn(II) system is 1.23 volt So, theoretically the reaction should not proceed forward, but at a pH around 3.6 the reduction potential of Cr(VI)|Cr(III) system falls much below than that of Mn(IV)|Mn(II) system, so the reaction can happen © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 138 Vietnam Journal of Chemistry Oxidation-reduction reactions in geochemistry Similar to the oxidation of chromium(III) to chromium(VI), the exactly opposite process can happen i.e., reduction of chromium(VI) to chromium(III), but this time the reducing agent being the dissolved Fe(II) produced from the weathering of Fe(II) containing minerals such as biotite, hematite etc and industrial wastes, the reaction is given below[20] [3FeO] + 6H+ + Cr(VI)(aq) = Cr(III)(aq) + 3Fe(III)(aq) + 3H2O The entire process is very fast, probable reason may be the formation of ferric hydroxide (Fe(OH)3) in alkaline medium which has very low solubility product If we consider the values of the two systems, we can easily predict that the reaction is thermodynamically favorable, but if we incorporate the solubility product data of the product along with the pH value, we can justify the very faster rate of the reaction to the completion From literature, the values of the Fe3+|Fe2+ and Cr(VI)|Cr(III) systems are respectively 0.77 volt and 1.33 volt But, in aqueous medium both Fe3+ and Fe2+ forms insoluble hydroxides Fe(OH)3 and Fe(OH)2 that have the solubility products as given below K sp ( Fe3 )  1038  [ Fe3 ][OH  ]3 and K sp ( Fe2 )  1015  [ Fe2 ][OH  ]2 And now for the process, Fe3+ + e = Fe2+ assuming the unit concentration of H+ ion, from Nernst‟s equation, we can write  Fe3  E  E   0.059log  Fe2  ability of Fe3+|Fe2+sytem increases and the reaction becomes very much feasible and proceed to completion rapidly In recent times, scientists are very much attentive for the removal of hazardous and harmful metal ions from various systems, especially from aqueous system The most stable oxidation states of chromium are chromium(III) and chromium(VI), while chromium(VI) is toxic and labile than chromium(III) and there is various reason behind this difference like concentration, oxidation state etc.[21] So, the geochemistry and hence the removal of toxic chromium, specially chromium(VI) is dependent on its oxidation state and by applying this its removal from environment will be easier Since these mentioned two different oxidation states of chromium is different from the viewpoint of pH, pH has a pivotal role for the separation and removal of chromium from our mother nature Like many other elements, chromium(II) is adsorbed by iron and manganese oxides, clay minerals and also by sand where as chromate ion where chromium exists as chromium(VI) is adsorbed by colloids and aluminium oxides in addition to the materials that adsorb chromium(III) But, when the solution is dilute, with decrease in pH the adsorption of chromium(VI) increases and does not depend on the types of adsorbent.[20] This may be due to the fact that at lower pH, the standard reduction potential is lower for Cr(VI)|Cr(III) system and therefore tendency of getting oxidized of chromium(III) to chromium(VI) is higher therefore increasing the total amount of chromium(VI) The adsorption behavior of sawdust suspension for Cr(VI) is shown in the following figure[22] In dilution solution, incorporating the solubility product values in the above equation, we get K E  E   0.059 log  sp  Fe3    K sp Fe2 OH     On simplification, the above equation becomes E  E  0.059[log K sp Fe3  log K sp Fe2 ] 0.059 pOH     And now putting the corresponding values in the above equation, we get Now from the final equation, it is very much clear that with increasing the pH, the reduction potential falls to very lower value and the reducing Figure 4: Adsorption behavior of sawdust suspension for Cr(VI)[22] Again managanese oxides minerals are ones among various constituents in soils and sediments that are very active and are of higher sorption and oxidizing capability.[23-26] The oxidation of chromium(III) by the action of manganese oxides influences and controls various characteristics like © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 139 Vietnam Journal of Chemistry transport, transformation, bio toxicity etc of chromium in the natural environment.[27-29] Among various factors that influences the oxidation of chromium(III), pH factor has complex effects for the interaction of manganese oxides with chromium(III).[30] It is well known that in manganese oxides, manganese exists in higher oxidation state and it is very willing to accept electron and acts as very strong oxidizing agent and can oxidize chromium(III) at lower pH range because at this condition chromium(III) is not precipitated so remains available to be oxidized The following figure represents the aqueous geochemistry of chromium Bidyut Saha et al catalyst for shaping chemical gradients in the oxygen-deficient zones in the coastal ocean and marginal seas Manganese is rapidly recycled by bacterial process and thus various transformations of manganese can serve as electron-transfer system for many other chemical cycles, very common of which is the consumption of hydrogen sulfide in the Black Sea by the oxidized manganese.[34] Now, the reduced manganese during this process when defuses upward consumes welling oxygen and thereby again oxidizing the manganese that creates a catalytic loop.[33] In the following figure the catalytic cycle is shown[33] Figure 6: Catalytic cycle involving Mn(II), O2, H2S[33] Figure 5: Aqueous geochemistry of chromium[20] Under oxidizing condition chromium(VI) is soluble as chromate (CrO42 ) while Mn(IV) is scavenged as MnO2 and if the condition is reducing, chromium(III) forms chromium hydroxide and Mn2+ is soluble.[20] Thus this is a useful procedure to separate toxic chromium(VI) for the removal of it by taking the advantage of redox process in geochemistry Manganese Manganese, 3d transition metal, has electronic configuration of [Ar]3d54s2 and is the second most abundant element in the earth‟s crust Though manganese can exists in different oxidation states ranging from -1 to +7, in natural environment the two most commonly observed states are +2 and +4; but, Mn2+ containing species of are the dominating ones in soil solution.[31] Not only in soil but also in some marine reduction oxidation cycles Mn(II) and Mn(IV) play an vital role and during bacterial oxidation of Mn(II) intermediate forms is Mn(III) as suggested.[32] Therefore, there is a big possibility of leaking of intermediate Mn(III) to the natural environment.[33] Manganese being capable of existing in various oxidation states, can act as Also sedimentary geochemistry of manganese is dependent on the existence and stability of its various oxidation states The higher oxidation states +3 and +4 are stable as insoluble hydroxides while the lower oxidation state +2 is soluble in oxygen deficient condition.[35] Again, in the sea water manganese has mainly occurrence as Mn(II), Mn(III) and Mn(IV) Though Mn2+ and [MnCl]+ are the most dominant forms of manganese in toxic water, still Mn2+ is thermodynamically unstable in the presence of oxygen and at neutral pH it is oxidized to Mn3+ and Mn4+ species forming manganese oxides.[35] Here it is important to mention that if the environment is strongly acidic in nature, the potential of Mn3+|Mn2+ is independent of pH but because of formation of insoluble hydroxides in alkaline condition, the potential becomes dependent on pH and thus in acidic media Mn3+ can liberate oxygen which is not possible in alkaline medium which is easily understood from the reduction potential of the two reactions below at unit concentration of hydroxyl ion volt Mn  OH 3  e  Mn  OH 2  OH  , O2  2H 2O  2e  4OH  , volt Another important aspect of redox reaction in geochemistry involving manganese with iron is in the bank filtration setting The four key geochemical processes which play an important role are (a) © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 140 Vietnam Journal of Chemistry reduction near the bank, (b) oxidation near production well, (c) dissolution of carbonate minerals in aquifer and (d) sorption-desorption to the aquifer material which is shown in the figure given below [36,37] Figure 7: Redox reaction in geochemistry involving manganese with iron (a) geochemical change along a natural; (b, c) bank filtration flow path; d) The relative concentration of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in bank filtration are shown in after Bourg and Bergin (1993)[36] Previously we have known that manganese has three common oxidation states +2, +3, +4 and iron also exists as Fe2+ and Fe3+ Among them iron(II) and manganese(II) are soluble at low to neutral pH condition whereas they precipitate as ferrous hydroxide and manganous hydroxide at a higher value of pH Now if there is enough carbonate concentration, these two metal ions are capable of precipitating as Fe(CO3) and Mn(CO3) Manganese(III) and manganese(IV) are very much insoluble and exists as manganese oxides and manganese oxyhydroxides and the same is also true for iron(III) But, the iron compounds are susceptible to dissolution under reducing conditions in lesser amount than Mn(III) and Mn(IV) compounds.[36] Both reductive dissolution of their oxides and oxidative precipitation of iron(II), manganese(II) are microbally mediated.[38-41] Reductive dissolution of Mn and Fe oxides are caused by influx of organic carbon from the overlying body of water into shallow sediments as it increases sufficient microbial activity by consuming the supply of oxygen and often nitrate [42] On the other hand Oxidation-reduction reactions in geochemistry reduction of oxygen, nitrate, and Mn oxides along the flow path is promoted by solid organic carbon in the aquifer sediments.[43] If the reductive dissolution of solid-phase oxides of the sediments occurs, iron and manganese are released in the pore water High level of dissolved iron and manganese in bank filtrate can problematic though both iron and manganese are major sinks for trace metals.[36] For the oxidation near the production well three major routes for entry of oxygen to the shallow groundwater are: diffusive flux from the overlying unsaturated zone, vertical infiltration of toxic rainwater, and gas entrapment due to water table oscillations Figure 7b reflects that the reaeration of bank filtrate in the vicinity of the cone of depression as the production well is turned on and off type The toxic zone near the production well has remarkable implications In the presence of oxygen dissolved Mn and Fe may precipitate as oxides from the reducing zone Due to the rapid oxidation kinetics of iron(II), the extent of iron oxidation is expected to be alike with the oxygen profile (figure 7b) rather than the manganese profile (figure 7c) Manganese oxidation can be assumed to be microbially mediated and only takes place in the presence of oxygen.[36] Iron The geochemistry of iron is very important Iron is the another 3d transition metal with the electronic configuration [Ar]3d64s2 and the fourth most abundant element in the earth crust In the period of early evolution and differentiation of the Earth, most of the iron along with nickel and sulfur was incorporated into the core and that significant amount of it remained in the crust.[44] Owing to its abundance, iron plays an dominant role in Earth surface system and its geochemistry thus becomes very much significant Being a transition metal and as it has partially filled 3d orbital, iron can have more than one oxidation states ranging from -2 to +6, but the commonest of all and the most stable two oxidation states of iron are +2 and +3 But the elemental iron is the rarest form at the Earth‟s surface since it is readily oxidized to iron(III) oxides Iron, due to its variable oxidation states, can produce an enormous number of geologically, environmentally and economically important iron minerals some of which are listed below: The oxidation of iron(II) to iron(III) and the reverse process of it is of great importance for various geological processes, mainly for elementcycling processes The reduction process i.e., transformation of iron(III) into iron(II), in absence © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 141 Vietnam Journal of Chemistry Bidyut Saha et al of free sulfide at lower temperature is mediated by dissimilatory iron-reducing bacteria and the reduced iron i.e., iron(II) is expelled into the natural environment and the bacteria gain energy in this process The electrons, required for the reduction of iron(III) come from hydrogen or from organic molecules.[44] Not only for bacteria, the process of oxidation and reduction of iron is also important for other elemental cycles and often it is observed that the reduction of iron(III) is associated with the oxidation of sulfur by means of oxidation of sulfide to sulfate The two half-reactions are as follows Table 3: Common Iron minerals present at, or near Earth‟s surface[44] Mineral class Native form Oxides/oxyhydroxides Name Native iron Ferrihydrite Goethite Lepidocrocite Hematite Maghemite Magnetite Green rusts Carbonates Siderite Ankerite Pyrite Marcasite Pyrrhotite Mackinawite Greigite Vivianite Strengite Berthierine Chamosite Glauconite Greenalite Odinite Sulfides Phosphates Silicates Fe3+ + e = Fe2+, H2S + 2O2 = SO42− + 2H+ + 8e The figure given below shows us how redox cycling of iron is linked to different elemental cycles.[44] Figure 8: Interactions of the iron redox cycle with other elemental cycles Another important side of iron in geochemistry Formula Fe Fe3+ 4-5(OH,O)12 FeO(OH) Fe3+O(OH) Fe2O3 Fe2.67O4 Fe3O4 Fe(2+,3+) hydroxysalts General formula: [Fe2+(1-x) Fe3+x(OH)2]x+· [(x/n)An−.(m/n)H2O]x−, where x is the ratio Fe3+/Fetot FeCO3 Ca(Fe,Mg,Mn)(CO3)2 FeS2 FeS2 Fe1-xS FeS Fe3S4 Fe3(PO4)2·8H2O FePO4·2H2O (Fe2+,Fe3+,Al)3(Si,Al)2O5(OH)4 (Fe2+,Mg,Al,Fe3+)6(Si,Al)4O10(OH,O)8 KMg(FeAl)(SiO3)6·3H2O (Fe2+,Fe3+)2-3Si2O5(OH)4 (Fe3+,Mg,Al,Fe2+)2.5(Si,Al)2O5(OH)4 involves the archean phosphorus liberation An experiment was carried out to establish the involvement of iron redox chemistry in the reduction of phosphate to phosphite by Herschy et al.[45] The experimental observation showed the presence of phosphate, pyrophosphate, and phosphate though previous result indicates production of pyrophosphate only.[46] Different experiment with various form of iron confirms the role of ferrous iron in the reduction process Again, a number of experiments of pyrite oxidation unraveled the mechanisms of the formation of acid mine waters drainage The initial step of the oxidation was supposed to be the release of ferrous ions and elemental sulfur [47] FeS2 = Fe2+ + S2 + 2e This was true for other dipositive metal ions as iron And in the subsequent step the sulfur is oxidized to sulfate as below: © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 142 Vietnam Journal of Chemistry S2 + 8H2O = 2SO42− + 16H+ + 12e The produced iron(II) will be in its reduced form till the pH is about At higher pH iron hydroxides (ferric hydroxide and ferrous hydroxide) are formed and an equilibrium exists as: Fe(OH)3 + e = Fe(OH)2 + OH− Both the hydroxides mentioned above are very less soluble with the solubility product of 10-38 and 10-15, respectively for iron(III) and iron(II) hydroxides The standard reduction potential value for Fe3+|Fe2+ system is 0.77 volt, with these data from the Nernst‟s equation we will have And finally we will have that is equivalent to According to the above equation, as the pH increases, the reduction potential value goes to the more negative side and becomes strongly reducing and in that condition ferrous ion will be converted to ferric ion Though oxygen is the overall oxidant but several investigators have identified ferric ion as the primary oxidant that directly react with pyrite surface.[48,49] Accordingly, the reaction will be dependent on the concentration of ferric ion and hence the reaction rate depends upon the reaction below Fe3+ + e = Fe2+ And the step is the rate-determining step in the formation of acid mine drainage.[50] Cobalt Cobalt (Co) is a bio-essential trace element categorized as first row transition metal with electronic configuration [Ar]3d74s2 It has been proposed based on the greater utilization of Co by anaerobic microbes relative to plants and animals that Co was more abundant in poorly ventilated Precambrian oceans.[51] However, cobalt concentration in modern seawater may vary from 3120 pM depending upon interaction with other metals present In coastal and deep water, Co behaves as a scavenged-type metal, oxidized and adsorbed to Mn(III,IV) oxides as they precipitate Cobalt does not form carbonates and the precipitation of Co(OH)3 is possible only at very high reduction potential, pH conditions The geochemistry of cobalt in aqueous medium is mainly controlled by +2 oxidation state of it i.e., Co(II) at ambient conditions This can easily be understood if we look at the standard reduction potential values of Co3+|Co2+ (1.82 volt) and that of Co2+|Co (-0.27 volt) Oxidation-reduction reactions in geochemistry that is the first couple has a very high standard reduction potential value for which it can easily oxidize most of the redox couple and in the second case the reduction potential value being very less and negative, it can reduce many redox couple whence in both cases the result is the formation of Co2+ [52] Arsenic Presence of arsenic (As) in natural waters, its geochemical mobility and toxicity to human beings, make it one of the most difficult challenges of present water research Arsenic accumulation generates owing to reductive dissolution of arsenic enrich sediments or arsenic leaching and transport by alkaline oxidizing groundwater Arsenic has two common stable oxidation states +3 and +5 The geochemistry of As is controlled by reactions that forms stable oxyanions of As, AsO33−or AsO43− and that transitions among various As(V) species is pH dependent, H2AsO− and HAsO42− being the two stable As(V) species Similarly, H3AsO3 should be the dominant As(III) species in an environment of As(III).[53] But, the geochemistry of arsenic is pH dependent which can be explained on the basis of the Nernst‟s equation: H3AsO4 + 2H+ + 2e = H3AsO3 + H2O, volt From the reduction potential value of the above system, it is clear that the value is in between volt, the reduction potential of hydrogen electrode and 1.23 volt which is the reduction potential for the dissociation of water So, the reaction is possible in aqueous medium The fact is not so straight, there is a pH dependence [ ][ ] [ ] Now if we consider the concentrations of both arsenate and arsenite as unity, we will have the above equation as The potential of the system decreases on increasing the pH and when the pH is enough high, the potential will become negative and thus it will decompose water with the evolution of hydrogen Cerium The one of the lanthanoid elements that closely resembles manganese, a 3d transition element is cerium(Ce), in its marine geochemistry In water system it undergoes oxidation from Ce(III) to Ce(IV) and Ce(IV) is rarely soluble in seawater and the probable reason of its insolubility is its association with other particles Due the similarity of © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 143 Vietnam Journal of Chemistry cerium and manganese in their geochemistry, it is suggested that Ce(III) and Mn(II) are closely related A microbial mediated path for oxidation of Ce(III) and Mn(II) in seawater collected from the Sargasso Sea, Massachusetts and Chesapeake Bay was suggested as the reaction was inhibited by azide Variation of specific oxidation rates of Ce(III) and Mn(II) is significant for two reasons First, for a given sample small variation (factors to 3) in oxidation rates for each element may be due to similarities in Ce and Mn geochemistry Second, though absolute rate constants varied ~ l04 in these environments but ratio of rates is similar This clearly indicate a common mechanism of oxidation, probably involving the same organism and (or) enzyme system.[54] CONCLUSION Bidyut Saha et al 10 11 12 Now it is clear from the above discussion that oxidation and reduction reactions have a bigger role in understanding the geochemistry of various processes, specially the geochemistry of metals This writing shows how oxidation and reduction reactions can be made useful in determining the concentration and enrichment of various elements in deposits which are formed under extreme redox conditions using the previously known electrode potential data and the procedure is very effective since most of the elements occur in more than one oxidation states And the redox process has a great role in geochemistry to remove or to control the enriched deposition of various metals from earth‟s surface as well Thus this area of research is becoming a broader and effectively useful area in chemistry which we name as oxidation and reduction reactions geochemistry 13 14 15 16 17 18 REFERENCES A Francis Geochemistry: An introduction Translated from the French (5th ed.), Cambridge: Cambridge Univ Press, 2007 B Mason Oxidation and reduction in geochemistry, J Geol., 1949, 57, 62-72 C S C Wong, X Li, I Thornton, Urban environmental geochemistry of trace metals, Environ Pollut., 2006, 142, 1-16 T J Nameroff, L S Balistrieri, J W Murray, Suboxic trace metal geochemistry in the 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Sci Lett., 2014, 390, 253-263 R N Collins, A S Kinsela The aqueous phase speciation and chemistry of cobalt in terrestrial environments, Chemosphere, 2010, 79, 763-771 A Aiuppa, W D‟Alessandro, C Federico, Ba Palumbo, M Valenza The aquatic geochemistry of arsenic in volcanic groundwaters from southern Italy, J Appl Geochem., 2003, 18, 1283-1296 I Grenthe, W Stumm, M Laaksuharju, A.C Nilsson, P Wikberg Redox potentials and redox reactions in deep groundwater systems, Chem Geol., 1992, 98, 131-15 Corresponding author: Bidyut Saha Homogeneous Catalysis Laboratory, Department of Chemistry The University of Burdwan, Burdwan713104, West Bengal, India E-mail Id: b_saha31@rediffmail.com (B Saha) Tel.: +91- 9476341691 / +91- 342-2533913 (O), Fax: +91-342-2530452 (O) © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 145 ... role in understanding the geochemistry of various processes, specially the geochemistry of metals This writing shows how oxidation and reduction reactions can be made useful in determining the... 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 Oxidation- reduction reactions in. .. controlling effect of pH on oxidation of Cr(III) by manganese oxide minerals, J Colloid Interf Sci, 2006, 298, 258-266 31 H S Hundal, K Singh, D Singh Geochemistry of manganese in neutral to alkaline