Photogeochemistry
Photogeochemistry merges photochemistry and geochemistry into the study of light-induced chemical reactions that occur or may occur among natural components of Earth's surface. The first comprehensive review on the subject was published in 2017 by the chemist and soil scientist Timothy A Doane, but the term photogeochemistry appeared a few years earlier as a keyword in studies that described the role of light-induced mineral transformations in shaping the biogeochemistry of Earth; this indeed describes the core of photogeochemical study, although other facets may be admitted into the definition.
The domain of photogeochemistry
The context of a photogeochemical reaction is implicitly the surface of Earth, since that is where sunlight is available. 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. 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.
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. 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.
File:Ancienne carrière d'ocre.JPG|thumb|188x188px|Iron oxides and oxyhydroxides, such as these cliffs of ochre, are common catalysts in photogeochemical reactions.
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" ; some effective "transformers" were similar to naturally occurring minerals, including iron oxide or colloidal iron hydroxide; cobalt carbonate, copper carbonate, nickel carbonate; and iron 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 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.
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, the term "photochemical" may be used to distinguish a particular abiotic reaction from the corresponding photobiological reaction. For example, "photooxidation of iron" can refer to either a biological process driven by light or a strictly chemical, abiotic process. 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.Thermodynamics
Photogeochemical reactions are described by the same principles used to describe photochemical reactions in general, and may be classified similarly:- Photosynthesis: in the most general sense, photosynthesis refers to any light-activated reaction for which the change in free energy is positive for the reaction itself. 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.
- Photocatalysis: this refers to reactions that are accelerated by the presence of a catalyst. 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.
- 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
- 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.
- 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 and pseudobrookite, and can form upon heating of ilmenite; this may imply a similar reaction with N2 for the naturally occurring minerals.