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.

Doane Geochem Trans (2017) 18:1 DOI 10.1186/s12932-017-0039-y

A survey ofphotogeochemistry

Timothy A. Doane*

Background

Photogeochemistry has been dened as the photochemistry of Earth-abundant minerals in shaping biogeochemistry [1], and this can be extended to the entire interface between photochemistry and geochemistry to include any chemical reaction induced by sunlight among naturally occurring substances. The term has been used previously on only several other isolated occasions [2, 3], but if existing research is surveyed for studies that t this definition, an appreciable body of knowledge emerges.

The context of a photogeochemical reaction is implicitly the surface of the earth, since that is where sunlight is available (ignoring other sources of light such as bioluminescence). Reactions may occur among constituents of land such as minerals, plant residue, and the organic and inorganic components of soil; constituents of surface water such as sediment and dissolved organic matter; and constituents of the atmospheric boundary layer directly inuenced by contact with land or water, such as organic aerosols, mineral aerosols, and gases. Figure 1 shows some examples of photochemical reactions among these substances. Sunlight penetrates up to approximately 0.3mm in soils and particulate minerals, depending on the wavelength of light and the nature of the particles

[4], and many meters in clear water, depending on the concentration of light-absorbing molecules [5, 6]. Light of wavelengths less than about 290 nm is completely absorbed by the present atmosphere and therefore does not reach Earths surface [7, 8].

Photogeochemistry describes photochemical reactions on Earth that are not facilitated by living organisms. The reactions that comprise photosynthesis in plants and other organisms, for example, are not included, since the physiochemical context for these reactions is installed by the organism, and must be maintained in order for the reactions to continue (the photoreactions cease if the organism dies). However, if a certain substance is produced by an organism, and the organism dies but the substance remains (e.g., plant residue or biogenic mineral precipitates), photoreactions involving this substance still contribute to photogeochemistry.

History

The most famous example of a photochemical reaction involving natural compounds is the production of indigoid dyes from the secretions of marine mollusks, known since antiquity [9]; the role of sunlight was emphasized in a study by William Cole in 1685 [10]. The development of modern photochemistry in general was fostered by similar adventitious observations of the eect of sunlight on natural compounds. For example, Hyde Wollaston in 1811 [11] observed that guaiac, a tree resin, rapidly

*Correspondence: [email protected] of Land, Air and Water Resources, University of California, Davis, Davis, CA 95616-5270, USA

The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

Web End =http://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/

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Doane Geochem Trans (2017) 18:1

Page 2 of 24

turned green in the air when exposed to sunlight (due to photooxidation). Natural photodegradation was also known, as described by Berzelius in 1829 [12]: Light fades and destroys the majority of plant colorants. Every day we see that of the sun weakening the dyes of our fabrics. This phenomenon was also mentioned by John William Draper in 1845 [13]. Georges Witz in 1883 described the degradation of cellulose by sunlight, remarking on the inuence of air and moisture, and further noted that degradation was greatly accelerated by ferric oxide [14]. By the end of the 19th century, photodegradation of organic matter in natural waters was recognized as a universal phenomenon [15]. In addition to degradation, other light-induced transformations were also recorded. Louis Pasteur described how a dark-colored material is produced in cinchona bark under the inuence of sunlight, an observation that he conrmed in the laboratory with specic compounds [16], and Hermann Trommsdor [17] and Karl Fritzsche [18] were also among those who observed changes in natural organic substances when they were illuminated. Many inorganic substances were also known to change (e.g., in color or crystal structure) upon exposure to light [13]. For example, since 1881 it has been known that zinc sulde, normally white, becomes dark when exposed to sunlight [19]; John Cawley remarked that I have prepared pigments so sensitive as to be turned almost black when exposed to bright sunlight for one or two minutes [20]. Investigation of the light-induced reactions of this compound [21], which occurs as a natural mineral, provided some additional empirical contributions to photochemistry and the photochemical metallurgy of zinc, and its photocatalytic

properties are still studied at present [22, 23]. Many natural inorganic compounds used throughout the ages as pigments in painting also slowly degrade by exposure to sunlight; artists like Van Gogh were aware of this [24]. Some of these compounds, such as mercury(II) sulde, undergo a number of light-mediated reactions [25] which are environmentally relevant.

Around the time of these and other observations, experiments increased in an eort to reproduce natural processes. The hypothesis of von Baeyer in 1870 [26], in which formaldehyde was proposed to be the initial product of plant photosynthesis followed by polymerization into sugars, inspired numerous attempts to obtain formaldehyde from carbon dioxide and water. For example, the formation of lower uranium oxides was observed upon irradiation of a solution of uranium acetate and carbon dioxide, implying the formation of a reducing agent assumed to be formaldehyde [27]. Some experiments included reducing agents such as hydrogen gas [28], and others reportedly detected formaldehyde and other products in the absence of additives [29, 30], suggesting that reducing power was produced from the decomposition of water during exposure to light. In addition to this main focus on the synthesis of formaldehyde and simple sugars, other light-driven reactions were occasionally noted, such as the decomposition of formaldehyde and subsequent release of methane [28]. Many experiments explored the eect of a catalyst in converting light energy into chemical energy; some eective transformers (as they were sometimes called) were similar to naturally occurring minerals, including iron(III) oxide or colloidal iron(III) hydroxide [3032], zinc oxide [33], and cobalt,

Doane Geochem Trans (2017) 18:1

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copper, nickel, and iron carbonates [30, 33]. By this time, interest had spread to other light-induced reactions involving naturally occurring materials. These studies sometimes reported photoreactions analogous to biological processes, such as oxidation of simple carbon compounds [34] or nitrication in soil [35].

Overview ofphotogeochemical reactions

Table1 presents a selection of documented photochemical reactions (with light >290 nm) among naturally occurring substances, ranging from general reactions such as mineralization of organic matter to specic reactions such as methylation and demethylation of mercury. This compilation is by no means exhaustive, either in reactions or references, but illustrates the general scope and diversity of abiotic photochemical reactions that may occur at the surface of the earth.

Classication ofphotogeochemical reactions

The same principles that form the foundation of photo-chemistry can also be used to describe and explain photo-geochemical reactions. If specic reactions are known, they may be distinguished as either photosynthetic reactions, photocatalytic reactions, or uncatalyzed reactions. In the most general sense, photosynthesis refers to any photo-chemical reaction for which the change in energy (G) is positive. The energy of the products is greater than that of the reactants, and therefore the reaction is thermodynamically unfavorable, except through the action of light in conjunction with a catalyst [36] or a chromophoric system, for example, that mimics what occurs in plants [37]. Examples of photosynthetic reactions include the production of H2

and O2 from water and the reaction of CO2 and water to form O2 and reduced carbon compounds such as methane and methanol. Photocatalysis refers to photochemical reactions, accelerated by the presence of a catalyst, that have a negative change in energy and are therefore thermodynamically favored [36], such as the reaction of organic compounds with O2 to form CO2 and water. Finally, uncatalyzed photoreactions proceed through the action of light alone. For example, many organic compounds absorb light and suer decomposition as a result. Figure 2 depicts a simple scheme for classifying photoreactions based on the requirement for a catalyst and whether a reaction proceeds by a direct or indirect mechanism, as further described below. Figure3 shows some of the processes that operate in these reactions, also discussed below.

Catalysis

A catalyst is a substance that increases the rate of a chemical reaction due to a change in mechanism, but does not experience any net change itself during the course of the

reaction [37, 38]. A photocatalyst does this by absorbing light, but as described below, other substances that do not absorb light may nevertheless catalyze light-induced reactions. 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 on a substance (the turnover number) is greater than one [39]; this is difficult to do in practice, although it is often assumed to be true if there is no loss in the activity of the substance for an extended period of time [36]. Reactions which are not denitively catalytic may be designated as assisted photoreactions [36, 38] or photosensitized reactions. Photosensitized reactions involve transfer of energy from a light-absorbing species (photosensitizer) to another, nonabsorbing species, and therefore facilitate reaction of this nonabsorbing species [40]. If the photosensitizer remains intact it is eectively a photocatalyst. Furthermore, a substance may initially act as a photocatalyst in a reaction even if it eventually suers light-induced decomposition. Descriptors such as those given here are most applicable when all of the participants in a specic reaction can be identied, not just individual reactants or products. In contrast, it is hard to classify observations in complex matrices such as soil if the complete reactions responsible for the observations are not rst discerned.

Direct reactions

Photochemical reactions can be further categorized as either direct or indirect. Direct reactions involve the substance that initially absorbs light [4143] which reacts with other substances or is itself changed. Many photo-chemical reactions on Earth may be directly mediated by naturally occurring semiconductors that absorb ultra-violet and visible radiation. These are mostly transition metal oxides and suldes and include abundant, widely distributed minerals such as hematite (Fe2O3), magnetite (Fe3O4), goethite and lepidocrocite (FeOOH), anatase and rutile (TiO2), pyrolusite (MnO2), pyrite (FeS2) chalcopy-rite (CuFeS2), and sphalerite (ZnS) [44, 45]. Other types of minerals are also known to absorb light and directly participate in photoreactions, including silicates such as Ag6Si2O7 [46] and phosphates such as Cu2(OH)PO4 [47].

Light of energy equal to or greater than the band gap of a semiconductor is sufficient to promote electrons from the valence band to a higher energy level in the conduction band, leaving behind electron vacancies or holes (Fig.3a). The excited electron and hole in the semiconductor can then, respectively, reduce and oxidize other compounds having appropriate redox potentials relative to the potentials of the valence and conduction bands [48]. The band gaps and absolute energy levels of many minerals are suitable, in theory, for a diverse array of

Doane Geochem Trans (2017) 18:1

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[124, 125, 166(CO 2implied), 167]

ReactionDescriptorFacilitatorsReferences

[70, 109, 185187]

[190, 192194]

Plant material (litter and living foliage) COPhotochemical decomposition (mineralization)[125, 168171]

Plant material (foliage) CH 4(Reductive) photochemical mineralization[171, 173176]

(Reductive) photochemical decomposition[171, 177]

Photochemical dissolution[115, 182, 183]

photochemical decomposition + acidication[135, 186]

[172174]

[136, 178, 179]

[134, 135, 196]

[181]

[188191]

Sand[180]

Plant material dissolved organic matterPhotochemical decomposition + dissolution[115]

Photochemical decomposition (depolymerization)[184]

Aqueous and solid iron(III) species

No facilitator

(mineralization/methanication)

Photochemical decompositionNo facilitator

photochemical oxidation + acidication[185]

Photochemical decomposition[189, 197]

TiO 2

Photochemical aliphatization[63, 193]

[195]

Photochemical priming (encouraging subsequent

(mineralization/methanication)

Photochemical priming (encouraging subsequent

Table 1 Photochemical reactions ofnaturally occurring substances

Plant material CO 2(Oxidative) photochemical decomposition

plant material (litter) CH 4(Reductive) photochemical decomposition

Solid organic matter CO 2(Oxidative) photochemical decomposition

Soil organic matter CH 4(Reductive) photochemical decomposition

Dissolved organic matter CO(Oxidative) photochemical decomposition

Dissolved organic matter CO 2(Oxidative) photochemical decomposition

(mineralization)

biotic decomposition)

(mineralization/methanication)

(mineralization)

Plant material ethane, ethene, propene, butane,

Dissolved organic matter CH 4(Reductive) photochemical decomposition

(mineralization)

(mineralization)

biotic decomposition)

Plant material biologically more labile

Sorbed or particulate organic matter dissolved

Dissolved and colloidal organic matter amino

(Nonspecic) decomposition of dissolved organic

Dissolved organic matter biologically more labile

Humic substances humic substances with

increased carboxylic acid content

Dissolved organic matter organic matter with

Humic substances small carboxylic acids;

increased hydrophobicity of remaining organic

Humic substances simple carbonyl compounds

Carbon compounds

other hydrocarbons

increased aliphatic content

(e.g., formaldehyde, acetone, pyruvate)

compounds

organic matter

acids

matter

compounds

matter

Doane Geochem Trans (2017) 18:1

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ReactionDescriptorFacilitatorsReferences

[14, 50, 96, 97, 199]

[98, 202, 203]

[210, 211]

[214218]

[138140, 219]

Photochemical condensation[193]

Carbohydrates and lipids oxidized productsPhotochemical oxidationWith and without ZnO[198]

(Nonspecic) decomposition of cellulosePhotochemical decompositionNo facilitator

Photochemical depolymerization + oxidation[96, 200]

(Nonspecic) decomposition of chitosanPhotochemical decomposition[201]

(Nonspecic) decomposition of woolPhotochemical decomposition[99]

(Nonspecic) decomposition of ligninPhotochemical decompositionNo facilitator

Lignin CH 4, ethane(Reductive) photochemical decomposition[204]

Lignin quinones(Oxidative) photochemical decomposition[99, 204, 205]

Lignin aromatic and aliphatic aldehydes(Oxidative) photochemical decomposition[206]

Fe(III) compounds, ZnO, ZnS, TiO 2

Photochemical crosslinking[207]

Photochemical isomerization, condensationObserved in seawater[208]

(Oxidative) photochemical crosslinking[209]

Photochemical oxidationTiO 2[211, 212]

Long-chain alkanes ketones, alcohols, acidsPhotochemical oxidationNaphthol, xanthone, anthraquinone[101]

Dienes + NO x carboxylic acidsPhotochemical oxidation[213]

Photochemical nitrationNo facilitator

Photochemical decompositionNo facilitator

Polycyclic aromatic hydrocarbons quinonesPhotochemical oxidationAl 2O 3[78]

(Oxidative) photochemical decomposition[63, 220, 221]

Soot oxygen-containing speciesPhotochemical oxidation[222]

Crude oil CO 2Photochemical oxidation (mineralization)Sand containing magnetite and ilmenite[223]

Amino acids CO 2Photochemical oxidation (mineralization)Cu(II) (aq)[224, 225]

Organic dyes

No facilitator

TiO 2, Fe 2O 3

Algae (live or dead)

TiO 2

TiO 2

TiO 2

Photochemical oxidation, cleavage,

dimerization

, or NO 3

Dissolved organic matter condensed aromatic

structures (soluble and particulate)

Cellulose less polymerized cellulose with

increased carbonyl and carboxyl content

via intermolecular tyrosine dimerization

Unconjugated unsaturated lipids conjugated

unsaturated lipids + insoluble material

Polyunsaturated lipids humic substances

Fatty acids

CO 2, alkenes, aldehydes, ketones, fatty acid dimers

Hydrocarbons e.g., ethane, ethene, propane, butane,

(Nonspecic) decomposition of polycyclic aromatic

Table 1 continued

Proteins larger, aggregated proteins e.g.,

Aromatic compounds +NO x, NO 2

nitrated aromatic compounds

Condensed aromatic compounds (dissolved black

carbon) nonspecic products, CO 2

(proposed reaction)

paraffin CO 2

hydrocarbons

Doane Geochem Trans (2017) 18:1

Page 6 of 24

ReactionDescriptorFacilitatorsReferences

Photochemical cyclizationHgS, ZnS, CdS[227, 228]

Photochemical oxidationFe 2O 3, TiO 2[211, 229, 230]

Quinones quinone dimersPhotochemical coupling/dimerization[235, 236]

[219, 231, 232]

[217, 233, 234]

[121, 122, 244]

[122, 211, 244]

[245247]

[122, 250]

[71, 211, 253, 254]

Phenols phenol dimersPhotochemical coupling/dimerizationFe(III) (aq)[102]

Ketones CH 4, ethanephotochemical reduction[174, 240]

Cinnamic acid cinnamic acid dimerPhotochemical coupling/dimerization[243]

[226]

absence of O 2); TiO 2, Fe 2O 3, SrTiO 3plus an electron

(Oxidative) photochemical decomposition, miner-

Phenolic ketones and aldehydes brown carbonPhotochemical oxidation, oligomerization[155]

Photochemical oxidationHumic and fulvic acids, avins

Photochemical oxidationNo facilitator

Photochemical coupling[237]

Ketones carboxylic acidsPhotochemical cleavage + acidication[238240]

Photochemical condensation[241]

Vicinal diols ketones, aldehydes, carboxylic acidsPhotochemical cleavage + oxidationFe(III) porphyrins[242]

Acetic acid CH 4 + CO 2Photochemical disproportionation/dismutationTiO 2; -Fe 2O 3; Fe 2O 3on montmorillonite (in the

Various-Fe 2O 3; TiO 2, Fe 2O 3, SrTiO 3, WO 3plus an electron

Acetate, terpenes + O 2 organic (hydro)peroxidesPhotochemical peroxidationNo facilitator

Unsaturated lipids + O 2 lipid hydroperoxidesPhotochemical peroxidationChlorophyll[248, 249]

Photochemical decarboxylationFe 2O 3alone or on montmorillonite

Lactic acid pyruvic acid + H 2Photochemical oxidation + dehydrogenationZnS[251]

Lactic acid acetaldehyde + CO 2(Oxidative) photochemical decarboxylationAqueous Cu(II) and Fe(III)[251, 252]

Glucose CO 2Photochemical oxidationTiO 2[211]

Algae (live or dead)

ZnO, organic sensitizers

Oxalic acid CO 2Photochemical oxidationTiO 2, sand, ash,

acceptor

acceptor

Algae (live or dead)

-Fe 2O 3, -Fe 2O 3,

-FeOOH, -FeOOH,

-FeOOH, -FeOOH

NO 3

alization

Amino acids and peptides smaller carboxylic

acids, amines, and amides, NH 3, CO 2

Phenol hydroquinone, catechol further oxida-

Decomposition of aqueous phenol, naphthol, meth-

Lysine pipecolinic acid

ylphenols, methoxyphenols, anilines

Quinones + benzocyclic olens addition prod-

Acetic acid CO 2, CH 4, ethane; methanol, ethanol,

Table 1 continued

Phenols quinones, naphthols, aminonaph-

Aromatic ketones condensed aromatic ring

ornithine proline

tion products, CO 2

thols naphthoquinones

propionic acid, other products

Propionic acid ethane + CO 2

Butyric acid propane + CO 2

Salicylic acid phenol + CO 2

ucts

systems

Doane Geochem Trans (2017) 18:1

Page 7 of 24

[30, 31, 33, 262268]

[132, 184, 193, 194, 283286]

ReactionDescriptorFacilitatorsReferences

[269, 270]

[271, 272]

[104, 287]

Photochemical oxidationFerritin[255]

Pyruvic acid pyruvic acid oligomersPhotochemical oligomerization[256]

Salicylic acid humic-like substancesPhotochemical condensationAccelerated in the presence of algae[250]

Photochemical decomposition[257]

CH 3 ClPhotochemical decomposition + chlorination[257]

Photochemical coupling + dehydrogenationZnS in the absence of air[258]

Photochemical oxidation[259]

Phototoxicity[260, 261]

(aq), ZnO, TiO 2, ZnS, CdS, ZrO 2, WO 3, CaFe 2O 4,

Elemental Cu on silicate rocks such as granite and

SiC, ZnS, BiVO 4, montmorillonite-modied TiO 2[273277]

[115]

BiVO 4, hydrous Cu 2O, transition metal ions and

CO 2 CO, HCOOH, HCHO, CH 3OH, CH 4Photochemical reduction (one-carbon products)Fe(III) oxides, FeCO 3, NiCO 3, CoCO 3, CuCO 3, Mn(II)

CO 2 + H 2 CH 4Photochemical reduction-Fe 2O 3and Zn-Fe oxide in the presence of water,

CO 2 + H 2 CO, HCOOH, CH 3OHPhotochemical reduction-Fe 2O 3and Zn-Fe oxide in the presence of water[269]

CO 2 HCOOHPhotochemical reductionPorphyrins, phthalocyanines

Photochemical oxidationTiO 2[211, 278]

CH 4 ethane + H 2Photochemical coupling + dehydrogenationSiO 2-Al 2O 3-TiO 2[279]

Plant foliage NO x[280]

Plant foliage N 2O[281]

Dissolved organic N biologically more labile NPhotochemical priming[282]

oxides in zeolites

shale

No facilitator

Organic matter,

NiO

Fe 2O 3, soil

Photochemical reduction (products with more than

Photochemical decomposition (dissolution +

Photochemical decomposition (mineralization/

one carbon)

mineralization)

ammonication)

(Oxidative) photochemical decomposition

(mineralization)

Tartaric, citric, oxalic, malonic acids oxidized

Syringic acid and other methoxybenzoic

Isoprene methylthreitol and methylerythritol

Table 1 continued

Syringic acid and related compounds + Cl

(Specic) plant compounds compounds toxic to

CO 2 tartaric, glyoxylic, oxalic acids

Particulate organic N dissolved organic N and

+

acids methanol

Methanol ethylene glycol + H 2

Ethanol butane-2,3-diol + H 2

CO 2 ethane, ethene, propane, propene

Amino acids and other organic N (including

biologically recalcitrant organic N) NH 4

products

(aerosols)

other organisms

CO 2 ethanol

CH 4 HCOOH

CH 4 CO, CO 2

Nitrogen compounds

Humic substances NO 2

NH 4+

Doane Geochem Trans (2017) 18:1

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[288290]

[299305]

[103, 306309]

[157, 310, 311]

[94, 95, 151, 312]

[2, 229, 315321]

[58, 321, 322]

TiO 2, ZnO, Fe 2O 3, soil[292, 293]

[320, 324, 325]

Al 2O 3[294]

ReactionDescriptorFacilitatorsReferences

Photochemical oxidation (nitrication)TiO 2, ZnO, Al 2O 3, SiO 2, MnO 2, soil

NH 3 N 2O, N 2Photochemical oxidationTiO 2[290, 291]

Photochemical oxidationTiO 2 [295, 296]

NO 2 HONO, NO, N 2OPhotochemical reductionTiO 2[296]

Photochemical oxidationTiO 2 , ZnO, Fe 2 O 3 , WO 3 [297]

NO 3 NH 3 Photochemical reductionTiO 2 plus electron acceptor[298]

N 2O N 2 + O 2Photochemical dissociationZnO, Cu(I) zeolites[313, 314]

N 2 + H 2O N 2H 4 + O 2Photochemical reduction + oxidationTiO 2in the absence of O 2[322]

+ H 2 Photochemical oxidation + reductionZnO-Fe 2 O 3 under N 2 [326]

-Fe 2O 3, ZnO, CuCrO 2, Na zeolite, sand

Iron(III) oxide, soil, organic matter; TiO 2plus humic

ZnO, Al 2O 3, Fe 2O 3, Ni 2O 3, CoO, CuO, Fe(III) in TiO 2,

Aqueous suspensions of TiO 2, ZnO, CdS, SrTiO 3, Ti(III)

Hydrous iron(III) oxide in the absence of O 2

TiO 2in air[323]

Suspension of ZnO in the absence of O 2

Al 2O 3, TiO 2, SiO 2,

Observed in snow

Humic and fulvic acids

Aerated suspension of hydrous iron(III) oxide

Observed in seawater

Fe 2O 3-Fe 3O 4, MnO 2,

(+ O 2 )Photochemical reduction (+oxidation)No facilitator

N 2O N 2Photochemical reductionZnO, Fe 2O 3, sand

N 2 + H 2O NH 3 + O 2Photochemical reduction + oxidationTiO 2in the absence of O 2, -Fe 2O 3,

Observed in ice

acids

Sand, soil

zeolites

Fe(III)-doped TiO 2

Photochemical decomposition + decomplexation[327, 328]

NH 4NO 3 N 2OPhotochemical oxidation + reduction

NO 3 or HNO 3 N 2 O, NO, HONO, NO 2 Photochemical reduction (denitrication/

Photochemical oxidation + reduction

NO 2 HONOPhotochemical reductionHumic acids, soot, soil

TiO 2, soil

N 2 NH 3Photochemical reduction/(reductive) photochemical

N 2 N 2H 4Photochemical reductionSand[2]

Photochemical oxidation (oxidative) photochemical

N 2 + O 2 NOPhotochemical oxidation (oxidative) photochemical

(denitrication)

(denitrication)

renoxication)

xation

xation

xation

Organic complexes of Fe, Al, Co, Ni (aq) ionic Fe,

N 2

Table 1 continued

urea, protein [NH 4NO 2] N 2

NH 3 NO 2

NH 3 NO 3

NH 4+ + NO 2

NO x NO 3

NO 2 NO 3

NO 3 NO 2

N 2 NO 3

N 2 + H 2O NO 2

Metal compounds

Al, Co, Ni (aq)

N 2 NO 2

Doane Geochem Trans (2017) 18:1

Page 9 of 24

[70, 71, 92, 122, 331338]

ReactionDescriptorFacilitatorsReferences

[339, 340]

[337, 343347]

[352, 354357]

Photochemical decomposition +precipitation[328]

Photochemical occulation[193, 329]

FeOH+ (aq) FeOOHPhotochemical oxidation[330]

Fe(III)-carboxylate complexes (aq) Fe(II) (aq)Photochemical reduction + decomplexation[66, 70, 341, 342]

Mn(II) (aq) MnO x(x = 1 to 2)Photochemical oxidationOrganic matter, TiO 2[348, 349]

Cu(II) (aq) Cu(I)Photochemical reductionAmino acids[224, 225]

Cr(VI) (aq) Cr(III) (aq)Photochemical reductionFerritin, phenol[350, 351]

ZnS Zn(0) + S(0)Photochemical oxidation + reduction[21]

Coprecipitated or dissolved organic matter, HSO 3 ,

[25]

Cl [25]

(Reductive) photochemical dissolution of

FeOOH + photochemical oxidation of organic

matter (if present)

Fe(II) (aq)/Fe(OH) 2 + H 2O Fe(III) + H 2Photochemical oxidation + reductionNo facilitator

Mn(IV) oxide Mn(II) (aq)(Reductive) photochemical dissolutionDissolved organic matter

ZnS + H 2O H 2S H 2Photochemical reduction + dissolution[21, 251]

CdS Cd(II) + S(0)Photochemical oxidation[211]

HgS Hg(II) (aq) + H 2SPhotochemical dissolution[228, 352]

Hg(0) (aq) Hg(II) (aq)Photochemical oxidation[352, 353]

+ Photochemical methylation[358]

montmorillonite

HgS [Hg 2Cl 2and other intermediates] HgCl 2Photochemical oxidation, reduction/

Hg(II) (aq) Hg(0) (aq)photochemical reductionFe(III) species, TiO 2, organic matter

HgCH 3+ Hg(II)Photochemical demethylation[359, 360]

HgCH 3Cl Hg(II) + Hg(0) + CHCl 3 + HCHOPhotochemical demethylation + reduction[361]

Plant material H 2(Reductive) photochemical decomposition[362, 363]

Dissolved organic P inorganic phosphatePhotochemical decomposition (mineralization)[364]

HS /S2 H 2 Photochemical reductionCdS, -Fe 2 O 3 [367, 368]

2 Photochemical oxidationTiO 2 , Fe 2 O 3 , ZnO, CdS[369372]

Observed in freshwater, seawater, and snow

2 oxidized productsPhotochemical oxidationFerritin[255]

Chromophores such as chlorophyll

No facilitator

Accelerated in ice

Accelerated in ice

Photochemical desorption[161, 365, 366]

Photochemical oxidation[373]

HgS Hg(0) + S(0)Photochemical oxidation + reductionCl

photochemical dissolution

Organic complexes of Fe, Cu, Cr, Pb, V (aq) col-

Organic matter (aq) + iron (aq)

Phosphate adsorbed to Fe(III) oxides or Fe(III)-organic

Table 1 continued

loidal Fe, Cu, Cr, Pb, V

organic matter + iron (s)

Fe(III) (hydr)oxides (s)

Fe(II) (aq)

matter complexes free phosphate

Alkyl suldes + NO x aldehydes, sulfonic acids,

Hg(II) (aq) HgCH 3

Other elements

Thiols and SO 3

2

SO 2 SO 4

SO 2, SO 4

Doane Geochem Trans (2017) 18:1

Page 10 of 24

[34, 107, 246, 298, 374376]

[21, 22, 262, 315, 377382]

[60, 137, 321, 322, 391393]

ReactionDescriptorFacilitatorsReferences

[123, 377]

[383390]

[158, 394, 395]

O 2 H 2O 2Photochemical reductionZnO, TiO 2, sand in the presence of organic electron

H 2O H 2Photochemical reductionNumerous catalysts, usually in the absence of O 2, e.g.,

H 2O O 2Photochemical oxidation-Fe 2O 3 + Fe(III) (aq), BiVO 4 + electron acceptor,

H 2O H 2 + O 2Photochemical water splitting (oxidation + reduc-

As 4S 4 As 4S 4(polymorph)Photochemical structural (crystal) modication[396]

Volatile organic compounds + NO x O 3Photochemical oxidation[398]

2 (dichloride radical anion)Photochemical oxidationChlorophyll, Hg(II)[65, 352]

Cl + O 3 Cl 2 Photochemical oxidation[399]

Br 2 Photochemical oxidation[400]

A suggested descriptor is given for each reaction as well as substances reported to facilitate the reaction (if any) and some relevant notes. These facilitating substances also occur naturally, or (in just a few instances)

are reasonably similar to something that might occur naturally. About 15% of the studies cited here can be considered eld studies, which means that a reaction was observed with both natural sunlight and natural

substances as well as under representative environmental conditions, as opposed to the use of articial light and/or laboratory-prepared equivalents of natural compounds

Note on terminologyThe term photochemical can be used to maintain a clear distinction between abiotic photoreactions and analogous reactions involving light and living organisms (phototrophy). For example,

iron(II) photooxidation can refer to either a biological process driven by light (photobiological/phototrophic iron(II) oxidation) or a strictly chemical, abiotic process (photochemical iron(II) oxidation). Similarly, an abiotic

process that converts water to O 2under the action of light may be described as photochemical oxidation of water rather than simply photooxidation of water (even though the latter is shorter and often understood to

mean a photochemical reaction); this distinguishes it from light-induced biological oxidation of water that might occur simultaneously in the same environment

Aqueous Fe(III)-carboxylic acid complexes

TiO 2, ZnS, -Fe 2O 3, hydrated Cu 2O, tungstosilicate

Tryptophan and tyrosine

Porphyrins and phthalocyanines

on TiO 2, Ti(III)-zeolite, graphite oxide

Mn 2O 3, -MnO 2, Mn 3O 4, Co 3O 4 + sensitizer, AgCl,

layered double hydroxide minerals

TiO 2, Fe 2O 3-Fe 3O 4, Fe 2O 3-FeS 2, Cu 2O, ZrO 2, Ag zeolite,

Algae (live or dead)

Dissolved Fe and humic substances

diverse two-mineral systems

As(III) (aq) As(V) (aq)Photochemical oxidationNo facilitator

Ferrihydrite, kaolinite

O 2 H 2OPhotochemical reduction-Fe 2O 3

donors

(a catalytic cycle)

Fe(OH)2+ (aq)

Photochemical oxidation/dissolutionWater[396, 397]

tion)

Table 1 continued

As 2S 3 [As + S] + O 2 As 2O 3

As 4S 4 As 2O 3

Cl Cl

NO 3 + Br

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photoreactions at interfaces with water, gases, and other solids. Naturally occurring semiconductors are almost exclusively inorganic compounds, with notable exceptions (notable because they occur widely) being melanin [49] and possibly cellulose [50, 51] and peptides [5254].

Natural semiconducting minerals, like most minerals, are rarely pure; additional metals are almost always present [44], and these substitutional impurities can cause changes in energy levels and conductivity [44, 55]. Such alterations are manifested in photocatalytic activity. For example, the band gap of TiO2 decreases due to Fe impurities [56, 57], which extends its response to a wider range of solar radiation compared to pure TiO2; the efficiencies of photochemical oxidation and reduction reactions of TiO2 are also greater if Fe impurities are present [57, 58]. Similarly, the presence of Ti or V in magnetite enhances its photocatalytic activity relative to pure magnetite [59]. In addition to atoms of foreign elements, another common defect in minerals is deviation from stoichiometry due to vacancies (missing atoms), and this can also aect photochemical properties. For example, sulfur deciencies in ZnS crystals impart increased photocatalytic activity under visible light to a material that normally absorbs little or no visible light [23]. In addition

to chemical alterations, the photocatalytic activity of materials like these is also inuenced by physical properties such as crystal structure and specic surface area [23, 56, 60].

Like inorganic minerals, many natural organic compounds also absorb sunlight and can react directly with other compounds or undergo reactions themselves (Fig.3b); these include dissolved organic matter [6163], bioorganic substances [64], chlorophyll [65], atmospheric humic-like substances [42], and soot or black carbon [42, 66]. Moreover, two species may combine to form a new species with even greater propensity to undergo direct photoreactions, as is often the case with intra-molecular or intermolecular charge-transfer complexes among components of organic matter [67] or between transition metals and organic matter [68]. Sometimes this even leads to catalytic or autocatalytic cycles [6971].

Finally, materials that do not absorb sunlight, such as silica, may nonetheless enable direct photoreactions. These materials are usually catalysts and act primarily via surface adsorption, which can alter the bond lengths and energies of a substance when it is bound to the catalyst [72, 73] and consequently alter the amount or wavelengths of sunlight absorbed by this substance [74, 75].

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The bound substance then becomes susceptible to photolysis and other photoreactions (Fig. 3c). Depending on the nature of a substance, however, adsorption onto materials such as clay and ash can sometimes impede rather than facilitate photoreactions [7678].

Indirect reactions

Indirect photochemical reactions are initiated by substances that absorb radiation and subsequently facilitate other reactions that do not involve the original light-absorbing substance [42]. For example, excited electrons and holes can be indirectly generated in semiconductors by light of lower energy than the band gap: the semiconductor itself does not absorb this light, but another substance (possibly even another semiconductor) that does absorb this light may be excited, and if this substance is in contact with the semiconductor and has appropriate energy levels, electrons can then be transferred between the excited substance and the semiconductor [48, 68, 79 81] (Fig.3d). The semiconductor, now carrying additional electrons or holes, can participate in redox reactions that would not otherwise occur. For example, TiO2 has a large band gap and is not normally excited by visible light; however, organic matter and natural chlorophyll derivatives are excited upon absorption of visible light, and in proximity to TiO2 can transfer electrons to TiO2 [82, 83].

This process is called charge injection, and is an example of photosensitizationreactions of TiO2 with additional substances are facilitated by the initial presence of organic matter or chlorophyll derivatives.

A substance may also participate indirectly in photo-chemical reactions by generating reactive species upon irradiation; these reactive species then engage in other reactions that do not involve the original light-absorbing substance [42]. For example, some aluminosilicates (e.g., zeolites) and non-transition-metal oxides (e.g., SiO2,

Al2O3, MgO) can react with the oxygen in air upon irradiation to produce reactive oxygen species (ROS) such as singlet oxygen and superoxide [84, 85]. Photodegradation of an organic compound was observed in the presence of kaolinite and montmorillonite, for example, and was attributed to the formation of ROS on the surface of these minerals in the presence of molecular oxygen and water [86]. Since the organic compound in question does not absorb sunlight and the ROS are produced in a separate reaction, this is an indirect photoreaction, facilitated by the clay minerals which presumably act as catalysts by generating ROS from O2 upon exposure to light (Fig.3e).

Along with minerals [87], other substances can indirectly facilitate photoreactions by generating reactive species in sunlight: dissolved and particulate organic matter [8895], dissolved organic matter and silicate minerals in synergy [63], cellulose [50, 96, 97], lignin [98, 99], leaves

of phototoxic plants [100], chlorophyll [101], nitrite and nitrate [102104], avins [41, 105], tryptophan and tyrosine [99, 106, 107], and aqueous iron(III) species [108 110]. In contrast to the typically strong oxidizing action of ROS, a strongly reducing species can also be generated which is usually represented as e (aq), a hydrated electron, although its true nature and features are not completely understood. Hydrated electrons are evident upon irradiation of dissolved organic matter, for example [94, 95]. As might be expected, reactive species are formed on exposed soil surfaces [111, 112]; both the mineral and organic components of soil contribute to this process [113]. Indirect photolysis of organic compounds in soil has been observed to occur at depths of up to 2mm due to migration of reactive species; in contrast, direct photolysis (in which the degraded compound itself absorbs light) is restricted to a photic depth of about ten times less [114, 115]. Both light penetration and transport processes such as diusion inuence the extent to which compounds are degraded by light in soil and similar media [116]. Indirect processes may operate during photodegradation of plant material as well [117]. In certain instances, however, the same substances listed above may also inhibit the formation of reactive species and therefore retard indirect photoreactions, as observed for chlorophyll [118], carotenoids [119], and organic matter in soil and water [76, 120].

Experimental approaches

Studies in photogeochemistry may take several dierent paths, depending on the source of inspiration for identifying and investigating natural photochemical reactions (Fig. 4). Oftentimes photogeochemistry distinctly parallels biogeochemistry. As mentioned above, early research sometimes intentionally used biological phenomena as a starting point to search for analogous photochemical reactions. Other studies simply explored the eect of light on dierent materials, and as a result also discovered photochemical reactions analogous to biological processes. Photochemical counterparts have since been conrmed for many well-known biochemical reactions. These include photochemical disproportionation of acetic acid [121, 122] which is analogous to acetoclastic methanogenesis, and light-induced depletion of O2 via a catalytic cycle involving iron and organic matter [123], analogous to consumption of O2 by microorganisms. Estimates of the environmental signicance of photochemical reactions relative to biological reactions have been oered on occasion, as for photochemical production of gases from plant litter [124, 125], and the photoxation of N2 in deserts, estimated as 20 kg N ha1 year1, which is about one third of that xed by lightning and about 10% of that xed biologically on Earth [126]. In contrast to these processes, in which

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biological reactions predominate (at least on a global level), the rate of degradation of dissolved lignin in rivers by photochemical mechanisms was found to be several times larger than by biological mechanisms [127]. Witz, based on his (nonbiological) studies with cellulose and other plant bers [14], concluded that light is indeed an integral participant in natural decomposition: In nature, once the life of plants is extinguished, cellulosic matter and other structured matter must no doubt pass progressively under the inuence of light, air, and humidity and are eventually transformed into gaseous compounds and colored humic materials.

Extension ofknown photoreactions

The most obvious experimental precedent in photogeo-chemistry is a natural photoreaction that has already been ascertained. Known reactions may be further investigated as to their context, mechanisms, and environmental signicance. For example, the greenhouse gases CO2, CH4, and N2O are the subject of a large amount of ongoing interdisciplinary research. Natural production and consumption of these gases at the earths surface are ascribed largely to biological activity [128131], which remains the focus of most research, in spite of studies that have demonstrated photochemical production and consumption (see Table1). Similarly, mineralization of organic carbon, nitrogen, and phosphorus in soil and water, the biological drivers of which are extensively studied, may also proceed photochemically. It is interesting to note that biologically recalcitrant portions of organic matter can be quite

susceptible to photodegradation [132, 133]; the consequent release of labile organic and inorganic compounds can stimulate biological activity [134136].

Sometimes a particular reaction, when placed in a certain environmental context, may even aect existing paradigms. For example, it is generally (and logically) assumed that in water classied as anoxic there can be no reactions involving molecular oxygen, including aerobic metabolism. However, some naturally occurring minerals are known to facilitate the photochemical oxidation of water to molecular oxygen; such photochemical sources of oxidizing power in low-oxygen environments [137] may be active alongside or in place of other sources of oxygen such as air or photosynthetic organisms. Similarly, organic acids known to be produced during the photodecomposition of organic matter may form a connection between light exposure and soil acidity, a simple but unestablished possibility next to the usual factors that determine soil pH.

While investigation of known natural photoreactions can be extended by pursuing additional work with the same substances, knowledge of natural photoreactions may also support inquiry into photoreactions of distinct but related substances. For example, the susceptibility to photodegradation of polycyclic aromatic hydrocarbons and related condensed aromatic compounds has been reported [e.g., 78, 138140]. These studies focus on relatively simple molecules which are either regarded as naturally occurring pollutants or are components of dissolved organic matter. At the same time, the incomplete

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combustion of natural organic materials leaves solid residues (charcoal, biochar, or pyrogenic black carbon) that contain analogous extended aromatic structure [141143]. It may therefore be suggested that this ubiquitous material, commonly deemed environmentally persistent [63, 140, 143, 144] and therefore paradoxical (since it does not accumulate in the environment) [145, 146], is also degraded upon exposure to sunlight.

The study of photogeochemistry, while purely chemical in nature, may even venture into the domain of biology and identify more of the ways in which compounds derived from living organisms can inuence abiotic photochemistry [e.g., 81], as well as more of the unique relationships between photochemical reactions and biological metabolism known as photobiocatalysis [147149].

Observation ofnatural phenomena

Specic photoreactions are often planned and conveniently observed in the laboratory, using articial light sources or sunlight itself, where it is easy to conrm the identity of the substances involved, design reaction vessels, characterize the light, and adjust the reaction environment. However, observations of natural phenomena can oer opportunities to consider unknown photochemical reactions possibly associated with these phenomena. For example, by the 1970s it was generally agreed that nitrous oxide (N2O) has a short residence time in the troposphere, although the explanation for its removal was incomplete. Since N2O does not absorb light of wavelengths greater than 290nm, direct photolysis had been discarded as a possible explanation. It was then observed that light would decompose chloromethanes when they were adsorbed on silica sand [150], and this occurred at lower energies (longer wavelengths) than the absorption spectra for the free compounds. The same phenomenon was observed for N2O on natural sand, leading to the conclusion that particulate matter in the atmosphere is responsible for the destruction of N2O via surface-sensitized photolysis [151]. Indeed, the idea of such a sink for atmospheric N2O was supported by reports of low concentrations of N2O in the air above deserts, where there is a large amount of suspended particulate matter [152]. In general, simple atmospheric gases (e.g., CO2, CO, CH4,

N2O, N2, H2O, H2, O2) do not absorb ultraviolet and visible sunlight at the earths surface, and the cooperation of particulate matter is necessary for photoreactions involving these gases; such reactions are therefore heterogeneous. Other gases, however, such as some of the volatile compounds emitted from living plants [153, 154], burning plants [155] and soils [156], do absorb sunlight and can undergo homogeneous as well as heterogeneous reactions.

As another example, the observation that the amount of nitrous acid in the atmosphere greatly increases during the day led to insight into the surface photochemistry of humic acids and soils and an explanation for the original observation [157]. Fluctuations such as this are often a clue to the existence of photochemical reactions, which operate only during the day. Diurnal photogeochemical cycles often have a signicant inuence on the amounts of redox-sensitive elements in aqueous environments [70, 158160]. Furthermore, multiple elemental cycles can be linked via photoreactions that directly aect both elements, as occurs during the concurrent oxidation of organic matter and reduction of iron [92]. The eect of light on one element can also indirectly aect other elements: a daily cycle of photoreduction, reoxidation, and precipitation of iron(III) species aects dissolved As, Cu, and P, which adsorb to iron(III) oxides as they reappear at night and may be subsequently released the next day upon photoreduction of the same iron oxides [158, 159, 161].

Contextualization ofnonnatural photoreactions

Although photogeochemistry describes reactions among substances known to occur naturally, studies of similar substances may nonetheless point towards greater understanding of natural processes. A general example demonstrates this: it has been shown that samples of clay minerals found in soils can accelerate the photodegradation of synthetic chemicals via production of reactive oxygen species [e.g., 86]; it may therefore be assumed that many naturally occurring compounds are similarly aected. The conversion of N2 to NH3 and NO3 has been observed upon irradiation with visible light in the presence of Fe2Ti2O7 [162, 163]. While such a compound is not known to occur naturally, it is related to known minerals like ilmenite (FeTiO3), ulvospinel (Fe2TiO4), pseudorutile (Fe2Ti3O9), and various titanium-substituted iron oxides, and can form when ilmenite is heated [162, 164]; these naturally occurring minerals might therefore also react with N2 under certain conditions.

Outlook

Principles of photochemistry can be readily merged with geochemistry in investigation as well as education. Given the broad response of natural substances to light, recognizing photochemical reactions in the environment is part of understanding its fabric of interconnected processes, particularly on land, where this has not been explored as much as in water or the atmosphere. As remarked by Formenti and Teichner [40] concerning heterogeneous photochemistry, there are so many dierent possibilities, an outlook reiterated by Cooper and Herr [165] for aqueous photochemistry which is

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easily extended to photogeochemistry: there are a seemingly endless number of combinations and permutations to study. This does not enjoin an unattainable research agenda, but rather affirms ample opportunity for geoscientists to incline their curiosity toward what happens on Earth when the sun appears.

Acknowledgements

It is a pleasure to acknowledge discussions with colleagues and guidance from the editor and three reviewers which helped me considerably improve this paper.

Competing interests

I have no competing interests.

Received: 30 August 2016 Accepted: 28 January 2017

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