The Role of Carbonate in Catalytic Oxidations.

CO2, HCO3-, and CO32- are present in all aqueous media at pH > 4 if no major effort is made to remove them. Usually the presence of CO2/HCO3-/CO32- is either forgotten or considered only as a buffer or proton transfer catalyst. Results obtained in the last decades point out that carbonates are key participants in a variety of oxidation processes. This was first attributed to the formation of carbonate anion radicals via the reaction OH• + CO32- → CO3•- + OH-. However, recent studies point out that the involvement of carbonates in oxidation processes is more fundamental. Thus, the presence of HCO3-/CO32- changes the mechanisms of Fenton and Fenton-like reactions to yield CO3•- directly even at very low HCO3-/CO32- concentrations. CO3•- is a considerably weaker oxidizing agent than the hydroxyl radical and therefore a considerably more selective oxidizing agent. This requires reconsideration of the sources of oxidative stress in biological systems and might explain the selective damage induced during oxidative stress. The lower oxidation potential of CO3•- probably also explains why not all pollutants are eliminated in many advanced oxidation technologies and requires rethinking of the optimal choice of the technologies applied. The role of percarbonate in Fenton-like processes and in advanced oxidation processes is discussed and has to be re-evaluated. Carbonate as a ligand stabilizes transition metal complexes in uncommon high oxidation states. These high-valent complexes are intermediates in electrochemical water oxidation processes that are of importance in the development of new water splitting technologies. HCO3- and CO32- are also very good hole scavengers in photochemical processes of semiconductors and may thus become key participants in the development of new processes for solar energy conversion. In this Account, an attempt to correlate these observations with the properties of carbonates is made. Clearly, further studies are essential to fully uncover the potential of HCO3-/CO32- in desired oxidation processes.

Key References

.[1]This study was the first one to point out that carbonate plays a key role in advanced oxidation processes.

.[2]This study points out that OH

.[3]This study is the most detailed study of the role of carbonate in electrocatalytic water oxidation.

.[4]This study describes the use of pulse radiolysis in the study of the mechanisms of oxidations by CO.

Introduction

All aerated solutions contain a mixture of CO2/HCO3–/CO32–, and their speciation depends on the CO2 solubility under the partial pressure of CO2 in the air and on the following equilibria:[5]Therefore, one commonly considers the role of CO2/HCO3–/CO32– present in solution as a buffer and/or as a proton transfer agent. However, results in recent years have pointed out that the role of CO2/HCO3–/CO32– in redox processes is often of major importance. This is due to three different reasons:

(a) CO2 reacts with peroxides via[6]This reaction is clearly slow in neutral solutions and requires relatively high concentrations of H2O2 in order to contribute to redox processes. However, the equilibrium in reaction might shift to the right in the presence of metal cations because of reaction :This reaction was shown to be of importance in catalytic oxidations for M = Mn.[7,8] Furthermore, reaction was shown to be exothermic and fast, at least for M = CoII:[1]

(b) The redox potential of the CO3•–/CO32– couple is 1.57 V vs NHE,[9,10] and that for the (CO3•– + H+)/HCO3– couple is clearly somewhat higher because of reaction . These potentials are considerably lower than those of the OH•/OH– and (OH• + H+)/H2O couples. Thus, at all pH values CO3•– has the potential to oxidize water, i.e., it is expected to be involved in oxygen evolution reactions (OERs). Therefore, it is thermodynamically easier to oxidize bicarbonate/carbonate than to oxidize water. Not surprisingly, adsorbed carbonates on semiconductors facilitate photocatalytic water oxidation.[11−14] Therefore, it is also not surprising that HCO3– and CO32– catalyze the Fenton reaction, forming CO3•– and not OH•.[2,15] HCO3– and CO32– also act as electrocatalysts for the OER process.[3,4,16−19]In the absence of other substrates, CO3•– decomposes via[6,20,21]The CO3•– anion radical is considerably less reactive than the OH• radical, and its reactions are more selective.[21] The CO3•– anion radical reacts in most systems via the inner-sphere mechanism.[9,22]

The carbonate anion radical is also formed via the following reactions:[23,24]In biological systems it is formed via the following reactions:[25]The CO3•– thus formed is believed to be one of the sources of oxidative stress induced by superoxide.[26−28] CO3•– is also formed in several enzymatic processes, e.g., in superoxide dismutase.[29] The above-mentioned processes are shown graphically in Scheme .

Scheme 1

Mechanisms for the Formation of CO3•– and HCO4– in the Absence of Transition Metal Complexes

(c) As a strong hard base, carbonate is a very good ligand for high-valent transition metal cations and therefore stabilizes transition metal complexes with higher valence, e.g., MnIII,[30] FeIV,III,[19,31] CoV/IV/III,[3] NiIV/III,[32] CuIV/III,[4] RuIV,[32] etc. In these complexes, the carbonate is a noninnocent ligand and is therefore involved in electrocatalytic OER processes and photoelectrocatalytic processes.

In the next sections these processes are discussed separately.

Electrocatalytic Water Oxidation by Metal Carbonate Complexes

Water oxidation[3] is of major importance in understanding the fundamental mechanism of photosynthesis[33] and in addressing modern-day energy challenges by means of water electrolysis[34,35] and solar energy conversion via photochemical water splitting.[36−39] As water oxidation involves the loss of 4e–/4H+ and the formation of the O–O bond, it has a high activation energy. Thus, the use of a catalyst is indispensable. All catalysts reported contain a transition metal, M. It is commonly accepted that water oxidation proceeds via reactions 14–17 in Scheme .[4]

Scheme 2

Outline of Water Oxidation Mechanisms; M Is the Metal, and n Represents the Highest Oxidation State

The formation of a percarbonate complex in the presence of carbonate has been reported (reaction ):[40]It is evident that in reactions 15–17 in Scheme , O2– and OH– behave as noninnocent ligands, where the reactions proceed via radical pathways to form the O–O bond. A similar radical pathway can be expected when other noninnocent ligands are used.

Carbonate can act as a catalyst/cocatalyst in electrocatalytic water oxidation on the basis of the arguments raised in the Introduction. On the basis of the redox properties of carbonate, the reactions shown in Scheme , where M is formed electrochemically, have to be taken into account to describe the participation of bicarbonate/carbonate in electrocatalytic water oxidation processes. In all of these reactions, the formed peroxide is easily further oxidized to form molecular oxygen at lower potentials. The involvement of bicarbonate/carbonate in homogeneous and heterogeneous electrocatalytic water oxidation, including mechanisms involving specific metal ions (e.g., Cu, Co, Ni), are discussed in the following sections.

Scheme 3

Various Plausible Processes Involved during Electrocatalytic Water Oxidation by Metal Carbonates in Aqueous Bicarbonate/Carbonate Solutions

Homogeneous Electrocatalysis

The first study reporting homogeneous electrocatalytic water oxidation in the presence of CO2/HCO3–/CO32– was by Chen and Meyer.[17] They showed that CuII(aq) with CO2/HCO3–/CO32– in the medium acts as an efficient water oxidation catalyst on a variety of working electrodes. The catalytic current increases with [CuII] with a cathodic shift of the onset potential. The catalytic current depends linearly on [CuII] in neutral media, while under alkaline conditions it depends on [CuII]2. Therefore, at pH 10.8 a bimolecular mechanism involving the active intermediate was proposed, while at pH 6.7 a single copper site was suggested to be the active species. The redox potential of CuIII/II(H2O) is >2.3 V vs NHE,[41] which is obviously shifted cathodically by the strong carbonate ligand. However, the authors did not clarify the involvement of either CuIII or CuIV as the active species in the catalytic cycle. Later, a density functional theory (DFT) study at pH 8.3 suggested the possibility of a CuIV complex as the active intermediate.[4,40]

Pulse radiolysis is a useful tool to study the chemical properties of complexes in unstable oxidation states.[42,43] This technique was used to study the properties of [CuIII(CO3)]3–2 formed by the oxidation of [CuII(CO3)]2–2 by CO3•–. The results suggested the following oxidation mechanism:[4]DFT calculations of the NBO charges suggested that significant charge transfer from the CO32– to the central metal ion in CuIII(CO3)3–2 takes place. The CuIII(CO3)3–2 thus formed decomposes in a process that obeys second-order kinetics irrespective of the pH of the medium:[4]The discrepancy with the electrochemical results in neutral solutions is probably due to the fact that in neutral media the electrocatalytic process proceeds via reactions 30–34 in Scheme . This hypothesis cannot be tested by the pulse radiolysis technique.[23]

The analogous systems containing other divalent first-row transition metal cations in the presence of HCO3–/CO32– pointed out that CoII in the presence of >1 × 10–3 M HCO3–/CO32– is an excellent catalyst for electrocatalytic water oxidation.[3] During chronoamperometry, a precipitate is formed that serves as a heterogeneous catalyst for the same (vide infra). Electrochemically, three homogeneous processes are observed:It should be noted that in order to calculate the redox potential, the simplest structures, [CoIII/IV/V(CO3)3]3–/2–/– with octahedral geometries (Figure ), were considered.

Figure 1

Structure of [CoV(CO3)3]− obtained at the B3LYP/6-311+G(2d,p) level. Reprinted with permission from ref (3). Copyright 2020 Wiley-VCH.

A relatively small wave is observed at Ep,a ≈ 0.71 V vs Ag/AgCl that is due to the CoIII/II redox couple, i.e., the carbonate ligands shift the redox potential of the CoIII/II couple cathodically by ca. 0.9 V.

A second wave is observed at Ep,a ≈ 1.10 V vs Ag/AgCl that has a considerably larger current (by a factor of >20) than the first peak. This process is attributed to the CoIV/III(CO3)32–/3– redox couple. The observed potential is in good agreement with the results of DFT calculations. The large current is attributed to catalytic oxidation of bicarbonate/carbonate by the CoIV complex to form HCO4– via reactions analogous to reactions 26–28 and 31 in Scheme .

A process with very large currents, with a current density of 10.50 mA·cm–2 at [CoII] = 0.50 mM at a peak plateau that starts at >1.2 V vs Ag/AgCl, is due to the formation of CoV(CO3)3–, and the observed redox potential is in accord with that calculated using DFT for the CoV/IV(CO3)3–/2– couple. The CoV/IV(CO3)3–/2– complex oxidizes both water and bicarbonate/carbonate. The results suggest that the oxidation proceeds via reaction and reactions 25–28 and 30–33 in Scheme . If one assumes, as these equations indicate, that the rate-determining step in this catalytic process involves a two-electron oxidation process, then kcat = 350 s–1. During long-time electrolysis at these potentials, a precipitate forms that is a heterogeneous OER catalyst (vide infra).

In the examples given above, carbonate was the only ligand lowering the redox potential of the central transition metal cation. However, carbonate can also be a second ligand, where it is the ligand getting oxidized. The following are such systems: NiII(1,4,8,11-tetraazacyclotetradecane)2+, NiIIL22+,[16] CuII(N,N′-bis(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate), CuIIL3,[44] and AlIII(TMPyP) (TMPyP = 5,10,15,20-tetrakis(1-methylpyridin-1-ium-4-yl porphyrinate)).[45] In the latter, the central cation is clearly not oxidized, and they act as homogeneous water oxidation catalysts in the presence of bicarbonate/carbonate.

In the system containing the NiL22+ complex, the role of carbonate is due to the lowering of the redox potentials of the NiIII/II and NiIV/III couples and to the reactions shown in Scheme . Recently the process of water oxidation by NiIIL22+ was reinvestigated, and it was shown that the formation of nickel oxide on the electrode surface is mainly responsible for the catalysis. However, the large catalytic wave in the cyclic voltammogram and the isomerization of the complex were not addressed. Under long-term chronoamperometry, the oxidation of the organic ligand and the formation of nickel oxide and/or nickel carbonate as nanocomposites cannot be ruled out.

Scheme 4

Electrocatalytic Processes Occurring during Electrocatalytic Water Oxidation Using the NiIIL22+ Complex in HCO3–/CO32–

CuL3 also acts as an active catalyst for water oxidation in the presence of carbonate at pH 9.0–11.0. The proposed mechanism is a “proton shuttle” mechanism, as shown in Scheme .[44] However, the possible oxidation of carbonate to HCO4– and C2O62– was not considered[44] and cannot be ruled out. Carbonate was also shown to act as a cocatalyst in electrocatalytic water oxidation by an aluminum porphyrin (Al(TMPyP)).[45] This catalytic system forms H2O2 as the major product. The suggested mechanism involves the formation of an AlIII–percarbonate complex as the key intermediate.[45]

Scheme 5

Structure of the Complex CuII(N,N′-bis(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate) (CuIIL3) and its Electrocatalytic Role in Water Oxidation in the Presence of Bicarbonate

Reprinted from ref (44). Copyright 2017 American Chemical Society.

Heterogeneous Catalysis

Bicarbonate and carbonate are involved also in heterogeneous electrocatalytic water oxidation processes via precipitates on the anode. Reactions analogous to reactions –34 are expected on the electrodes. Here the role of the carbonate is also dual: it lowers the redox potential of the central cation, and it has a lower oxidation potential than OH–/H2O. Processes analogous to reactions 22, 23, 29, and 33 in Scheme require carbonate in the homogeneous medium only to replace the carbonate loss from the precipitate. Processes analogous to reactions 21, 24–27, 31, 32, and 34 in Scheme can also proceed without any carbonate in the precipitate on the electrode:An example of the latter type of catalysis is the report that NiII(aq) adsorbed on a SiO2 sol–gel matrix and mixed with graphite acts as an OER electrocatalyst in solutions containing HCO3–/CO32– with a current proportional to [HCO3–/CO32–].[46,47] Clearly one cannot rule out that ligand exchange in the sol–gel matrix transforms the NiII(aq) into carbonate complexes. In this study it was proposed that the catalytic process involves the formation of CO3•– radical anions.[46]

In the study of homogeneous electrocatalysis of the OER in solutions containing CoII and HCO3–/CO32–, it was noted that during chronoamperometry a green precipitate of Na3[Co(CO3)3] is formed.[3] This precipitate on the anode surface serves as an excellent heterogeneous catalyst in the presence of bicarbonate/carbonate. This catalytic process was shown to be in agreement with reactions analogous to reactions 22 and 26–34 in Scheme . DFT calculations suggest that the active species in these process is the CoV(CO3)3– complex.[3]

In an analogous study,[18] a precipitate was formed on an anode by electrolysis of a CO2-saturated solution containing FeII(aq) and HCO3–. The thin film thus formed contained FeIII, O2–, OH–, and CO32–. This film was shown to be a good OER electrocatalyst for which the current increased with [HCO3–/CO32–].[18] The detailed mechanism causing the electrocatalytic process was not discussed. Clearly reactions analogous to reactions 22 and 26–34 are probably responsible for the electrocatalytic properties of this precipitate. The observation that the film is stable during electrolysis in carbonate solutions that do not contain FeII ions proves that the mechanism does not involve reduction of the Fe ions in the precipitate to FeII and dissolution to the aqueous phase.

An analogous study involving nickel carbonate solutions[48] resulted in the formation of an analogous NiIII(HCO3–)3–/NiIII(CO32–)3–2 precipitate on the anode that is a good electrocatalyst for water oxidation in solutions containing NiII and HCO3–/CO32–. However, the precipitate dissolves during electrolysis when no NiII ions are present in solution. This proves that the catalytic step involves the reduction of a NiIV complex into a NiII complex that is soluble, i.e. via reactions analogous to reactions 21, 22, 25, and 26–28 in Scheme . In another study,[49] an amorphic nickel carbonate nanowire array on a nickel foam (NiCO3/NF) anode was prepared. This modified electrode is a very good electrocatalyst for water oxidation in solutions containing only HCO3–/CO32–.[49] The source of the discrepancy between these two studies might be that the nickel foam supplies the needed nickel to preserve the precipitate. The use of a mixture of salts of iron and nickel in carbonate solutions was also reported to form an oxide precipitate that acts as an efficient catalyst in carbonate media to oxidize water.[19]

To sum up this section, the results suggest that precipitates of transition metal oxides and/or carbonates, where the central cation can be oxidized to a high oxidation state, on anodes serve as good electrocatalysts for water oxidation in solutions containing HCO3–/CO32–. The detailed mechanisms of these catalytic processes depend on the properties of the central cation. The advantage of these catalytic processes is that all of the components are stable inorganic species that are not consumed during the catalytic process.

Photocatalysis

The difference in the oxidation potentials of the CO3•–/CO32– and OH•/OH–(aq) or (OH• + H3O+)/2H2O couples suggests that the holes formed photochemically in semiconductors will oxidize HCO3–/CO32– faster and in higher yields than the oxidation of water. This was verified computationally in a DFT study of the photocatalysis by GaN (Figure ).[13] The condition for this is that adsorption of HCO3–/CO32– on the surface of the semiconductor, i.e., a relatively high point of zero charge (the pH at which the net charge of the adsorbent’s surface is zero or positive), is favorable. The products of these oxidations are either CO3•– or C2O62–. Indeed, several studies have pointed out that the presence of HCO3–/CO32– in the system catalyzes photochemical water oxidation.[14,50,51] Furthermore, it has been shown that photochemical oxidation of SO2 in aqueous media is enhanced by the presence of carbonate.[11] It was proposed that the formation of CO3•– is involved.

Figure 2

Structure of carbonated GaN. Ga, yellow; C, brown; H, white; O, red; N, blue. Reprinted with permission from ref (13). Copyright 2016 The PCCP Owner Societies.

In the previous section, the role of metal carbonate precipitates as efficient heterogeneous electrocatalysts for water oxidation was reviewed. It seemed to be of interest to check whether these precipitates are photoactive, and indeed, preliminary results[32] point out that precipitates of cobalt and nickel carbonate act as photoelectrocatalysts for water and methanol oxidations.

Percarbonate as an Oxidation Agent

In principle, percarbonates are involved in two types of processes:

An inorganic percarbonate salt, e.g., sodium percarbonate (Na2CO4·1.5H2O2), is used as the oxidizing agent. These percarbonates are used mainly in advanced oxidation processes (AOPs) (vide infra). Their mechanisms of oxidation are suggested to proceed via formation of H2O2 upon dissolution followed by Fenton-like and/or photolytic processes and/or reaction with O3, often involving CO3•– anion radicals.[52−55]

The percarbonate is formed in situ via the reaction of H2O2 with a transition metal carbonate complex, analogous to reaction followed by 5, or via reactions analogous to reaction or reaction below.

The first peroxocarbonate complex of a transition metal was reported by Hashimoto et al.[56] It was formed via the reaction of a bis(μ-hydroxo)diiron(III) complex with H2O2 and CO2. Oxidative degradation of an organic dye, Orange II, by MnIII(TPPS) (TPPS = 5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphyrin) in carbonate buffer was proposed to involve the formation of a percarbonate–metal complex that undergoes decomposition to form a MnIV=O species and degrades Orange II via reactions –56:[8]Catalytic oxidation of azo dyes by [MnII(bpy)2Cl2] and [Mn2III/IV(μ-O)2(bpy)4](ClO4)3 also involves a similar mechanism.[57] As reactions –55 probably do not involve an oxygen atom transfer, it is reasonable to propose that intermediates in which the percarbonate is ligated to the central MnIII are formed (Scheme ).

Scheme 6

Proposed Mechanism for the Degradation of Substrate (S) by the Complex Mn(bpy)2L2 (L = Water or Carbonate) Formed from Mn(bpy)2Cl2 or [Mn2III/IV(μ-O)2(bpy)4](ClO4)3

Reprinted with permission from ref (57). Copyright 2010 Royal Society of Chemistry.

Oxidation of calmagite (H3CAL) dye in aqueous solution using H2O2 at pH 7.5–9.0 is accelerated considerably in the presence of HCO3–.[58] The proposed mechanism involves the following steps:It should be pointed out that [MnIII(CAL)(HCO4)]− might also decompose to form a MnIV complex and CO3•– anion radical. The degradation of various dyes using Mn/H2O2/HCO3– has been reported.[7] It was shown that under visible light percarbonate catalyzes the degradation of rhodamine B in the presence of FeOCl.[59] Oxidations of primary alcohols to carbonyl compounds using catalytic amounts of percarbonate and dichromate were also reported.[60]

Role of Bicarbonate in Fenton and Fenton-like Reactions

The Fenton reaction, Fe(H2O)62+ + H2O2, is of major importance. Thus, a search in SciFinder for “Fenton” limited to articles in English for 2019 yielded 2371 results. The major source of this importance is in its role in inducing oxidative stress[61−66] and its role in AOPs.[67−70] The Fenton reaction and Fenton-like reactions, in which another low-valent metal ion replaces Fe and/or another ligand L replaces H2O and/or another peroxide replaces H2O2, were shown to proceed via a variety of mechanisms:[71]where RH is a substrate. The reactions always proceed via the inner-sphere mechanism.

Still, nearly all of the recent articles cite the Fenton reaction as proceeding via the formation of OH•. Furthermore, recent results point out that in systems where it is difficult to oxidize the central cation, the central cation is not oxidized in the process, and the reaction proceeds via the following mechanism:[72,73]Reaction indicates that H2O2 ligated to a central cation can oxidize another ligand that has no bond with it. DFT calculations verified this for other ligands, including carbonate.[74]

For the carbonate-containing systems, three cases were studied in detail:

(I) The reaction of H2O2 with Co(H2O)62+ (10–25.0 mM) in the presence of 0–0.6 mM HCO3– was studied. Under these conditions, [CoII(H2O)62+] > [CoII(H2O)5(HCO3)+]. The following reactions were observed:[1]These results point out the following: (a) No OH• radicals are formed. Though a percarbonate ligand is formed as an intermediate in the process, the active oxidizing agent formed is the CO3•– anion radical. (b) The central CoII cation is not oxidized during the process. The process is analogous to reactions and 66 with k = 2. Thus, in AOPs with CoII(H2O)62+ and H2O2, the active species is CO3•– and not OH• as usually assumed.[75] The mechanism is outlined in Scheme .

Scheme 7

Proposed Mechanisms of the Fenton Reaction for M(H2O)62+ (M = Fe, Co) in H2O

(II) The Fenton reaction at pH 7.4 in solutions containing 2.0 × 10–5 M FeII(H2O)62+, 0–3.0 mM HCO3–, and 0–0.8 mM H2O2 was studied. Under these conditions, [FeII(H2O)3(HCO3–)]+ constitutes less than 3% of the FeII(H2O)62+. However, the observed rate constants increase dramatically with [HCO3–]. The following reactions were observed:This reaction sequence is the major one for [HCO3–] < 1.0 mM. For [HCO3–] > 1.0 mM, reactions and 75 lead to the formation of (CO32–)FeII(OOH–)(H2O)2–, i.e., the same intermediate formed in reaction :The mechanism is shown in Scheme . In the Fe Fenton reaction, metal ion oxidation occurs, whereas in the Co Fenton-like reaction, the metal oxidation state does not change. One more difference is that for Co a cyclic percarbonate complex is formed, but this does not occur in the case of Fe. The (CO32–)FeII(OOH–)(H2O)2– thus formed decomposes via the following reactions:Thus, also in this system, the reactive oxygen species (ROS) formed under physiological conditions (i.e., in the presence of ∼1.0 mM HCO3–) is not the OH• radical as commonly assumed.

(III) The results concerning the Fenton reaction in the presence of HCO3– suggest that under physiological conditions, the Fenton reaction yields CO3•– and not OH• radicals as commonly assumed, and this is of major importance. However, since in biological media FeII ions are not present as FeII(H2O)62+ but appear in the mobile pool mainly as FeII(citrate), the Fenton reaction was studied[15] in solutions containing 2.0 × 10–5 M FeII(H2O)62+, 0–2.0 mM sodium citrate, 0–8.0 mM NaHCO3, and 0–0.39 mM H2O2. It was shown that the kinetics of the process and the composition of the final products in the presence of dimethyl sulfoxide depend dramatically on the concentration of HCO3–. The following mechanism was proposed:[15]The mechanism of decomposition of FeIV(citrate)(HCO3–)aq was not clarified. It can decompose via the formation of CO3•– or via oxidation of the citrate ligand. Alternatively, the mechanism might involve reactions and 80,where (citrate)FeII(H2O2)(HCO3–)2– is formed in a reaction analogous to reaction . It was suggested that this mechanism fits the results better.

The results of these three studies of the mechanisms of Fenton and Fenton-like reactions point out that the role of bicarbonate present in biological systems and in wastewater cannot be overlooked. This conclusion was recently verified.[28]

Other Biological Sources of CO3•–

As stated in the Introduction, CO3•– is formed biologically via the reaction of peroxonitrite with CO2 (reactions –13).[76] The formation of NO2• and CO3•– is believed to be the main mechanism via which O2•– causes oxidative stress.[77,78] CO3•– is also formed enzymatically by the enzymes superoxide dismutase (Cu,Zn-SOD, SOD-1)[79] and xanthine oxidase.[80]

The CO3•– anion radicals formed in biological systems can oxidize nucleobases[26−28,77,81,82] and proteins[83,84] and peroxidize low-density lipoprotein.[85]

Advanced Oxidation Processes/Technologies

The treatment of polluted water and soil is of major importance. Many organic pollutants are treated by advanced oxidation processes/technologies.[86] There is still no optimal treatment for pollutants, and a variety of technologies have been studied. These include chemical oxidation by peroxides, Fenton and Fenton-like processes (including photo-Fenton and electrochemical Fenton), ozone, photochemical processes (including solar light using TiO2 as a photocatalyst), electrochemical processes, microwave processes, ultrasonic processes, ionizing radiation, hydrodynamic cavitation, and various combinations of these techniques.[86] Carbonate is involved in these processes via the addition of percarbonate as an oxidizing agent,[52,55,87−91] the involvement of the HCO3–/CO32– that is always present in water,[75,92−96] and sometimes by the addition of HCO3–/CO32– to the system.[97]

The role of the added percarbonate is clear. However, the role of the bicarbonate/carbonate present in the solution is more complicated. For many systems it has been proposed that bicarbonate/carbonate reacts with the OH• radical initially formed, thus decreasing the reactivity of the oxidizing species formed but increasing its lifetime and its selectivity in the choice of substrates.[52,94,95] On the other hand, for some systems involving Fenton and photochemical reactions, at least the carbonate is involved in the ROS formation and therefore increases the pollutant decomposition yield.[75] It should be pointed out that the Fenton reaction in neutral solutions in the absence of bicarbonate/carbonate forms FeIV(aq) and not OH• radicals.[98] This is commonly not addressed in publications on the Fenton reaction discussed in AOPs. Thus, in these systems the presence of bicarbonate/carbonate exchanges FeIV(aq) with CO3•– as discussed above.

Conclusions and Perspectives

CO2, HCO3–, and CO32– are present in all aquatic media at pH > 4 if no effort to remove them is made. Usually one considers their role only as buffers and/or proton transfer agents. The recent results discussed in this Account point out the important role of HCO3–/CO32– in a variety of catalytic oxidation processes in aqueous media. This role is due to the following properties of carbonates:

Carbonate is a strong hard ligand that stabilizes transition metal complexes in high oxidation states. These complexes are key intermediates in electrochemical water oxidation processes.

The redox potential of the CO3•–/CO32– and (CO3•– + H+)/HCO3– couples is considerably lower than that of the OH•/OH– and (OH• + H+)/H2O couples. Therefore, HCO3– and CO32– act as cocatalysts in water oxidation and are involved in Fenton-like processes.

Percarbonate, as a bidentate ligand, is easily formed in the presence of transition metal cations with fast ligand exchange properties, HCO3–/CO32–, and peroxides.

The results obtained thus far point out that the presence of HCO3–/CO32– dramatically changes the mechanisms of the Fenton and Fenton-like reactions. These findings require reassessment of the major sources of oxidative stress in biological systems and of the role of carbonate anion radicals in oxidative stress. Furthermore, the results point out the activity of HCO3–/CO32– as catalysts/cocatalysts for water oxidation electrochemically and photochemically. This is of importance in developing technologies for efficient water splitting processes.

Extremely little has been done to date on the application of HCO3–/CO32– in catalytic oxidations of specific substrates performed electrochemically, photochemically, and photoelectrochemically. Also, the role of HCO3–/CO32– in Fenton-like processes using other peroxides (e.g., S2O82–, HSO5–, and ROOH, where R is an aliphatic residue) has not been studied. These studies might be of importance in the development of new advanced oxidation technologies.