A survey of photogeochemistry

Abstract

The participation of sunlight in the natural chemistry of the earth is presented as a unique field of study, from historical observations to prospects for future inquiry. A compilation of known reactions shows the extent of light-driven interactions between naturally occurring components of land, air, and water, and provides the backdrop for an outline of the mechanisms of these phenomena. Catalyzed reactions, uncatalyzed reactions, direct processes, and indirect processes all operate in natural photochemical transformations, many of which are analogous to well-known biological reactions. By overlaying photochemistry and surface geochemistry, complementary approaches can be adopted to identify natural photochemical reactions and discern their significance in the environment.

Keywords: Atmosphere; Minerals; Natural photoreactions; Photocatalysis; Photochemistry; Soil; Surface geochemistry; Water.

Fig. 1

Photogeochemistry is the study of sunlight-induced chemical reactions among substances that are found naturally on Earth’s surface and intermingle across its domains. Examples of photochemical reactions are shown that occur in the basic domains of land, air, and water. Reaction details and references can be found in Table 1

Fig. 2

Photogeochemical reactions, if enough information is known, can be classified using general principles of photochemistry. Examples are given for each of four categories in a simple scheme of classification based on the mechanism of reaction. Light-absorbing materials are shaded and catalysts are shown in italics. Intermediate processes in indirect reactions are indicated as separate reactions below the main reaction arrow. For additional explanation of these mechanisms, see the text and the references for specific reactions listed in Table 1

Fig. 3

Simplified representations and some examples of processes that occur in photochemical reactions of natural substances: a promotion of electrons (e ⁻) and generation of electron vacancies (holes, h ⁺) upon irradiation of a semiconductor, which may then reduce and oxidize other substances; b excitement of organic compounds by sunlight which then directly react with other substances or are themselves altered, with examples of photochemical acidification, dissolution, and crosslinking; c photocatalysis via surface adsorption, which makes a species, here N₂O, susceptible to the effect of light; d indirect generation, via a photosensitizer, of electrons and holes in a semiconductor: the difference between the highest occupied molecular orbital (HOMO) of the sensitizer and its lowest unoccupied molecular orbital (LUMO) is smaller than the band gap of the semiconductor, and therefore less energy is required to excite the sensitizer; e cooperative generation of transient reactive species by compounds that do not individually absorb sunlight; f generation of transient reactive species by light-absorbing compounds. Arrows with shadows indicate reactions induced by light (hν), asterisks (*) indicate excited species (electrons promoted to higher energy levels), single brackets (]) indicate mineral surfaces, and dotted lines (^…) indicate surface adsorption. The references cited in the text offer additional, detailed explanations of these processes

Fig. 4

The study of photogeochemistry reflects the overlap between surface geochemistry and photochemistry. The curved arrows represent three different but complementary approaches which can lead to the discovery of natural photoreactions: observing natural phenomena, extending known natural photoreactions, and contextualizing photoreactions that are not known to occur naturally