Photogeochemistry

The domain of photogeochemistry

The context of a photogeochemical reaction is implicitly the surface of Earth, since that is where sunlight is available (although other sources of light such as chemiluminescence would not be strictly excluded from photogeochemical study). Reactions may occur among components of land such as rocks, soil and detritus; components of surface water such as sediment and dissolved organic matter; and components of the atmospheric boundary layer directly influenced by contact with land or water, such as mineral aerosols and gases. Visible and medium- to long-wave ultraviolet radiation is the main source of energy for photogeochemical reactions; wavelengths of light shorter than about 290 nm are completely absorbed by the present atmosphere, and are therefore practically irrelevant, except in consideration of atmospheres different from that of Earth today.

Photogeochemical reactions are limited to chemical reactions not facilitated by living organisms. The reactions comprising photosynthesis in plants and other organisms, for example, are not considered photogeochemistry, since the physiochemical context for these reactions is installed by the organism, and must be maintained in order for these reactions to continue (i.e. the reactions cease if the organism dies). In contrast, if a certain compound is produced by an organism, and the organism dies but the compound remains, this compound may still participate independently in a photogeochemical reaction even though its origin is biological (e.g. biogenic mineral precipitates or organic compounds released from plants into water).

The study of photogeochemistry is primarily concerned with naturally occurring materials, but may extend to include other materials, inasmuch as they are representative of, or bear some relation to, those found on Earth. For example, many inorganic compounds have been synthesized in the laboratory to study photocatalytic reactions. Although these studies are usually not undertaken in the context of environmental or Earth sciences, the study of such reactions is relevant to photogeochemistry if there is a geochemical implication (i.e. similar reactants or reaction mechanisms occur naturally). Similarly, photogeochemistry may also include photochemical reactions of naturally occurring materials that are not touched by sunlight, if there is the possibility that these materials may become exposed (e.g. deep soil layers uncovered by mining).

Except for several isolated instances, studies that fit the definition of photogeochemistry have not been explicitly specified as such, but have been traditionally categorized as photochemistry, especially at the time when photochemistry was an emerging field or new facets of photochemistry were being explored. Photogeochemical research, however, may be set apart in light of its specific context and implications, thereby bringing more exposure to this "poorly explored area of experimental geochemistry". Past studies that fit the definition of photogeochemistry may be designated retroactively as such.

Early photogeochemistry

The first efforts that can be considered photogeochemical research can be traced to the "formaldehyde hypothesis" of Adolf von Baeyer in 1870, in which formaldehyde was proposed to be the initial product of plant photosynthesis, formed from carbon dioxide and water through the action of light on a green leaf. This suggestion inspired numerous attempts to obtain formaldehyde in vitro, which can retroactively be considered photogeochemical studies. Detection of organic compounds such as formaldehyde and sugars was reported by many workers, usually by exposure of a solution of carbon dioxide to light, typically a mercury lamp or sunlight itself. At the same time, many other workers reported negative results. One of the pioneer experiments was that of Bach in 1893, who observed the formation of lower uranium oxides upon irradiation of a solution of uranium acetate and carbon dioxide, implying the formation of formaldehyde. Some experiments included reducing agents such as hydrogen gas, and others detected formaldeyhde or other products in the absence of any additives, although the possibility was admitted that reducing power may have been produced from the decomposition of water during the experiment. In addition to the main focus on synthesis of formaldehyde and simple sugars, other light-assisted reactions were occasionally reported, such as the decomposition of formaldehyde and subsequent release of methane, or the formation of formamide from carbon monoxide and ammonia.

In 1912 Benjamin Moore summarized the main facet of photogeochemistry, that of inorganic photocatalysis: "the inorganic colloid must possess the property of transforming sunlight, or some other form of radiant energy, into chemical energy." Many experiments, still focused on how plants assimilate carbon, did indeed explore the effect of a "transformer" (catalyst); some effective "transformers" were similar to naturally occurring minerals, including iron(III) oxide or colloidal iron hydroxide; cobalt carbonate, copper carbonate, nickel carbonate; and iron(II) carbonate. Working with an iron oxide catalyst, Baly concluded in 1930 that "the analogy between the laboratory process and that in the living plant seems therefore to be complete," referring to his observation that in both cases, a photochemical reaction takes place on a surface, the activation energy is supplied in part by the surface and in part by light, efficiency decreases when the light intensity is too great, the optimal temperature of the reaction is similar to that of living plants, and efficiency increases from the blue to the red end of the light spectrum.

At this time, however, the intricate details of plant photosynthesis were still obscure, and the nature of photocatalysis in general was still actively being discovered; Mackinney in 1932 stated that "the status of this problem [photochemical CO₂ reduction] is extraordinarily involved." As in many emerging fields, experiments were largely empirical, but the enthusiasm surrounding this early work did lead to significant advances in photochemistry. The simple but challenging principle of transforming solar energy into chemical energy capable of performing a desired reaction remains the basis of application-based photocatalysis, most notably artificial photosynthesis (production of solar fuels).

After several decades of experiments centered around the reduction of carbon dioxide, interest began to spread to other light-induced reactions involving naturally occurring materials. These experiments usually focused on reactions analogous to known biological processes, such as soil nitrification, for which the photochemical counterpart "photonitrification" was first reported in 1930.

Classifying photogeochemical reactions

Photogeochemical reactions may be classified based on thermodynamics and/or the nature of the materials involved. In addition, when ambiguity exists regarding an analogous reaction involving light and living organisms (phototrophy), the term "photochemical" may be used to distinguish a particular abiotic reaction from the corresponding photobiological reaction. For example, "photooxidation of iron(II)" can refer to either a biological process driven by light (phototrophic or photobiological iron oxidation) or a strictly chemical, abiotic process (photochemical iron oxidation). Similarly, an abiotic process that converts water to O₂ under the action of light may be designated "photochemical oxidation of water" rather than simply "photooxidation of water", in order to distinguish it from photobiological oxidation of water potentially occurring in the same environment (by algae, for example).

Thermodynamics

Photogeochemical reactions are described by the same principles used to describe photochemical reactions in general, and may be classified similarly:

1. Photosynthesis: in the most general sense, photosynthesis refers to any light-activated reaction for which the change in free energy (ΔG^o) is positive for the reaction itself (without considering the presence of a catalyst or light). The products have higher energy than the reactants, and therefore the reaction is thermodynamically unfavorable, except through the action of light in conjunction with a catalyst. Examples of photosynthetic reactions include the splitting of water to form H₂ and O₂, the reaction of CO₂ and water to form O₂ and reduced carbon compounds such as methanol and methane, and the reaction of N₂ with water to yield NH₃ and O₂.
2. Photocatalysis: this refers to reactions that are accelerated by the presence of a catalyst (the light itself is not the catalyst as may be erroneously implied). The overall reaction has a negative change in free energy, and is therefore thermodynamically favored. Examples of photocatalytic reactions include the reaction of organic compounds with O₂ to form CO₂ and water, and the reaction of organic compounds with water to give H₂ and CO₂.
3. Uncatalyzed photoreactions: photoinduced or photoactivated reactions proceed via the action of light alone. For example, photodegradation of organic compounds often proceeds without a catalyst if the reactants themselves absorb light.

Nature of reactants

Any reaction in the domain of photogeochemistry, either observed in the environment or studied in the laboratory, may be broadly classified according to the nature of the materials involved.

1. Reactions among naturally occurring compounds. Photogeochemistry, both observational and exploratory, is concerned with reactions among materials known to occur naturally, as this reflects what happens or may happen on Earth.
2. Reactions in which one or more of the reactants are not known to occur naturally. Studies of reactions among materials related to naturally occurring materials may contribute to understanding of natural processes. These complementary studies are relevant to photogeochemistry in that they illustrate reactions that may have a natural counterpart. For example, it has been shown that soils, when irradiated, can generate reactive oxygen species and that clay minerals present in soils can accelerate the degradation of synthetic chemicals; it may therefore be postulated that naturally occurring compounds are similarly affected by sunlight acting on soil. The conversion of N₂ to NH₃ has been observed upon irradiation in the presence of the iron titanate Fe₂Ti₂O₇. While such a compound is not known to occur naturally, it is related to ilmenite (FeTiO₃) and pseudobrookite (Fe₂TiO₅), and can form upon heating of ilmenite; this may imply a similar reaction with N₂ for the naturally occurring minerals.

Direct catalysts

Direct photogeochemical catalysts act by absorbing light and subsequently transferring energy to reactants.

Semiconducting minerals

The majority of observed photogeochemical reactions involve a mineral catalyst. Many naturally occurring minerals are semiconductors that absorb some portion of solar radiation. These semiconducting minerals are frequently transition metal oxides and sulfides and include abundant, well-known minerals such as hematite (Fe₂O₃), magnetite (Fe₃O₄), goethite and lepidocrocite (FeOOH), and pyrolusite (MnO₂). Radiation of energy equal to or greater than the band gap of a semiconductor is sufficient to excite an electron from the valence band to a higher energy level in the conduction band, leaving behind an electron hole (h⁺); the resulting electron-hole pair is called an exciton. The excited electron and hole can reduce and oxidize, respectively, species having suitable redox potentials relative to the potentials of the valence and conduction bands. Semiconducting minerals with appropriate band gaps and appropriate band energy levels can catalyze a vast array of reactions, most commonly at mineral-water or mineral-gas interfaces.

Organic compounds

Organic compounds such as "bio-organic substances" and humic substances are also able to absorb light and act as catalysts or sensitizers, accelerating photoreactions that normally occur slowly or facilitating reactions that might not normally occur at all.

Indirect catalysts

Some materials, such as certain silicate minerals, absorb little or no solar radiation, but may still participate in light-driven reactions by mechanisms other than direct transfer of energy to reactants.

Production of reactive species

Indirect photocatalysis may occur via the production of a reactive species which then participates in another reaction. For example, photodegradation of certain compounds has been observed in the presence of kaolinite and montmorillonite, and this may proceed via the formation of reactive oxygen species at the surface of these clay minerals. Indeed, reactive oxygen species have been observed when soil surfaces are exposed to sunlight. The ability of irradiated soil to generate singlet oxygen was found to be independent of the organic matter content, and both the mineral and organic components of soil appear to contribute to this process. Indirect photolysis in soil has been observed to occur at depths of up to 2 mm due to migration of reactive species; in contrast, direct photolysis (in which the degraded compound itself absorbs light) was restricted to a "photic depth" of 0.2 to 0.4 mm. Like certain minerals, organic matter in solution, as well as particulate organic matter, may act as an indirect catalyst via formation of singlet oxygen which then reacts with other compounds.

Surface sensitization

Indirect catalysts may also act through surface sensitization of reactants, by which species sorbed to a surface become more susceptible to photodegradation.

True catalysis

Strictly speaking, the term "catalysis" should not be used unless it can be shown that the number of product molecules produced per number of active sites is greater than one; this is difficult to do in practice, although it is often assumed to be true if there is no loss in the photoactivity of the catalyst for an extended period of time. Reactions that are not strictly catalytic may be designated "assisted photoreactions". Furthermore, phenomena that involve complex mixtures of compounds (e.g. soil) may be hard to classify unless complete reactions (not just individual reactants or products) can be identified.

Experimental approaches

The great majority of photogeochemical research is performed in the laboratory, as it is easier to demonstrate and observe a particular reaction under controlled conditions. This includes confirming the identity of materials, designing reaction vessels, controlling light sources, and adjusting the reaction atmosphere. However, observation of natural phenomena often provides initial inspiration for further study. For example, during the 1970s it was generally agreed that nitrous oxide (N₂O) has a short residence time in the troposphere, although the actual explanation for its removal was unknown. Since N₂O does not absorb light at wavelengths greater than 280 nm, direct photolysis had been discarded as a possible explanation. It was then observed that light would decompose chloromethanes when they were absorbed on silica sand, and this occurred at wavelengths far above the absorption spectra for these compounds. The same phenomenon was observed for N₂O, leading to the conclusion that particulate matter in the atmosphere is responsible for the destruction of N₂O via surface-sensitized photolysis. Indeed, the idea of such a sink for atmospheric N₂O was supported by several reports of low concentrations of N₂O in the air above deserts, where there is a high amount of suspended particulate matter. As another example, the observation that the amount of nitrous acid in the atmosphere greatly increases during the day lead to insight into the surface photochemistry of humic acids and soils and an explanation for the original observation.

Photogeochemical reactions

The following table lists some reported reactions that are relevant to photogeochemical study, including reactions that involve only naturally occurring compounds as well as complementary reactions that involve synthetic but related compounds. The selection of reactions and references given is merely illustrative and may not exhaustively reflect current knowledge, especially in the case of popular reactions such as nitrogen photofixation for which there is a large body of literature. Furthermore, although these reactions have natural counterparts, the probability of encountering optimal reaction conditions may be low in some cases; for example, most experimental work concerning CO₂ photoreduction is intentionally performed in the absence of O₂, since O₂ almost always suppresses the reduction of CO₂. In natural systems, however, it is uncommon to find an analogous context where CO₂ and a catalyst are reached by light but there is no O₂ present.