. 2017 Feb 10;18:1. doi: 10.1186/s12932-017-0039-y

Table 1.

Photochemical reactions of naturally occurring substances

Reaction	Descriptor	Facilitators	References
Carbon compounds
Plant material → CO₂	(Oxidative) photochemical decomposition (mineralization)		[124, 125, 166 (CO₂ implied), 167]
Plant material (litter and living foliage) → CO	Photochemical decomposition (mineralization)		[125, 168–171]
plant material (litter) → CH₄	(Reductive) photochemical decomposition (mineralization/methanification)		[172–174]
Plant material (foliage) → CH₄	(Reductive) photochemical mineralization		[171, 173–176]
Plant material → ethane, ethene, propene, butane, other hydrocarbons	(Reductive) photochemical decomposition		[171, 177]
Plant material → dissolved organic matter	Photochemical decomposition + dissolution		[115]
Plant material → biologically more labile compounds	Photochemical priming (encouraging subsequent biotic decomposition)		[136, 178, 179]
Solid organic matter → CO₂	(Oxidative) photochemical decomposition (mineralization)	Sand	[180]
Soil organic matter → CH₄	(Reductive) photochemical decomposition (mineralization/methanification)		[181]
Sorbed or particulate organic matter → dissolved organic matter	Photochemical dissolution		[115, 182, 183]
Dissolved and colloidal organic matter → amino acids	Photochemical decomposition (depolymerization)		[184]
(Nonspecific) decomposition of dissolved organic matter	Photochemical decomposition	No facilitator Aqueous and solid iron(III) species	[70, 109, 185–187]
Dissolved organic matter → CO	(Oxidative) photochemical decomposition (mineralization)		[188 – 191]
Dissolved organic matter → CO₂	(Oxidative) photochemical decomposition (mineralization)	No facilitator TiO₂	[190, 192–194]
Dissolved organic matter → CH₄	(Reductive) photochemical decomposition (mineralization/methanification)		[195]
Dissolved organic matter → biologically more labile compounds	Photochemical priming (encouraging subsequent biotic decomposition)		[134, 135, 196]
Humic substances → humic substances with increased carboxylic acid content	photochemical oxidation + acidification		[185]
Dissolved organic matter → organic matter with increased aliphatic content	Photochemical aliphatization		[63, 193]
Humic substances → small carboxylic acids; increased hydrophobicity of remaining organic matter	photochemical decomposition + acidification		[135, 186]
Humic substances → simple carbonyl compounds (e.g., formaldehyde, acetone, pyruvate)	Photochemical decomposition		[189, 197]
Dissolved organic matter → condensed aromatic structures (soluble and particulate)	Photochemical condensation		[193]
Carbohydrates and lipids → oxidized products	Photochemical oxidation	With and without ZnO	[198]
(Nonspecific) decomposition of cellulose	Photochemical decomposition	No facilitator Organic dyes Fe(III) compounds, ZnO, ZnS, TiO₂	[14, 50, 96, 97, 199]
Cellulose → less polymerized cellulose with increased carbonyl and carboxyl content	Photochemical depolymerization + oxidation		[96, 200]
(Nonspecific) decomposition of chitosan	Photochemical decomposition		[201]
(Nonspecific) decomposition of wool	Photochemical decomposition		[99]
(Nonspecific) decomposition of lignin	Photochemical decomposition	No facilitator TiO₂	[98, 202, 203]
Lignin → CH₄, ethane	(Reductive) photochemical decomposition		[204]
Lignin → quinones	(Oxidative) photochemical decomposition		[99, 204, 205]
Lignin → aromatic and aliphatic aldehydes	(Oxidative) photochemical decomposition		[206]
Proteins → larger, aggregated proteins e.g., via intermolecular tyrosine dimerization	Photochemical crosslinking		[207]
Unconjugated unsaturated lipids → conjugated unsaturated lipids + insoluble material	Photochemical isomerization, condensation	Observed in seawater	[208]
Polyunsaturated lipids → humic substances (proposed reaction)	(Oxidative) photochemical crosslinking		[209]
Fatty acids → CO₂, alkenes, aldehydes, ketones, fatty acid dimers	Photochemical oxidation, cleavage, dimerization	No facilitator TiO₂	[210, 211]
Hydrocarbons e.g., ethane, ethene, propane, butane, paraffin → CO₂	Photochemical oxidation	TiO₂	[211, 212]
Long-chain alkanes → ketones, alcohols, acids	Photochemical oxidation	Naphthol, xanthone, anthraquinone	[101]
Dienes + NO_x → carboxylic acids	Photochemical oxidation		[213]
Aromatic compounds + NO_x, NO₂ ⁻, or NO₃ ⁻ → nitrated aromatic compounds	Photochemical nitration	No facilitator TiO₂, Fe₂O₃	[214–218]
(Nonspecific) decomposition of polycyclic aromatic hydrocarbons	Photochemical decomposition	No facilitator Algae (live or dead) TiO₂	[138–140, 219]
Polycyclic aromatic hydrocarbons → quinones	Photochemical oxidation	Al₂O₃	[78]
Condensed aromatic compounds (dissolved black carbon) → nonspecific products, CO₂	(Oxidative) photochemical decomposition		[63, 220, 221]
Soot → oxygen-containing species	Photochemical oxidation		[222]
Crude oil → CO₂	Photochemical oxidation (mineralization)	Sand containing magnetite and ilmenite	[223]
Amino acids → CO₂	Photochemical oxidation (mineralization)	Cu(II) (aq)	[224, 225]
Amino acids and peptides → smaller carboxylic acids, amines, and amides, NH₃, CO₂	(Oxidative) photochemical decomposition, mineralization		[226]
Lysine → pipecolinic acid ornithine → proline	Photochemical cyclization	HgS, ZnS, CdS	[227, 228]
Phenolic ketones and aldehydes → brown carbon	Photochemical oxidation, oligomerization		[155]
Phenol → hydroquinone, catechol → further oxidation products, CO₂	Photochemical oxidation	Fe₂O₃, TiO₂	[211, 229, 230]
Decomposition of aqueous phenol, naphthol, methylphenols, methoxyphenols, anilines	Photochemical oxidation	Humic and fulvic acids, flavins Algae (live or dead)	[219, 231, 232]
Phenols → phenol dimers	Photochemical coupling/dimerization	Fe(III) (aq)	[102]
Phenols → quinones, naphthols, aminonaphthols → naphthoquinones	Photochemical oxidation	No facilitator NO₃ ⁻	[217, 233, 234]
Quinones → quinone dimers	Photochemical coupling/dimerization		[235, 236]
Quinones + benzocyclic olefins → addition products	Photochemical coupling		[237]
Ketones → carboxylic acids	Photochemical cleavage + acidification		[238 – 240]
Ketones → CH₄, ethane	photochemical reduction		[174, 240]
Aromatic ketones → condensed aromatic ring systems	Photochemical condensation		[241]
Vicinal diols → ketones, aldehydes, carboxylic acids	Photochemical cleavage + oxidation	Fe(III) porphyrins	[242]
Cinnamic acid → cinnamic acid dimer	Photochemical coupling/dimerization		[243]
Acetic acid → CH₄ + CO₂	Photochemical disproportionation/dismutation	TiO₂; α-Fe₂O₃; Fe₂O₃ on montmorillonite (in the absence of O₂); TiO₂, Fe₂O₃, SrTiO₃ plus an electron acceptor	[121, 122, 244]
Acetic acid → CO₂, CH₄, ethane; methanol, ethanol, propionic acid, other products	Various	α-Fe₂O₃; TiO₂, Fe₂O₃, SrTiO₃, WO₃ plus an electron acceptor	[122, 211, 244]
Acetate, terpenes + O₂ → organic (hydro)peroxides	Photochemical peroxidation	No facilitator ZnO, organic sensitizers	[245–247]
Unsaturated lipids + O₂ → lipid hydroperoxides	Photochemical peroxidation	Chlorophyll	[248, 249]
Propionic acid → ethane + CO₂ Butyric acid → propane + CO₂ Salicylic acid → phenol + CO₂	Photochemical decarboxylation	Fe₂O₃ alone or on montmorillonite Algae (live or dead)	[122, 250]
Lactic acid → pyruvic acid + H₂	Photochemical oxidation + dehydrogenation	ZnS	[251]
Lactic acid → acetaldehyde + CO₂	(Oxidative) photochemical decarboxylation	Aqueous Cu(II) and Fe(III)	[251, 252]
Glucose → CO₂	Photochemical oxidation	TiO₂	[211]
Oxalic acid → CO₂	Photochemical oxidation	TiO₂, sand, ash, α-Fe₂O₃, γ-Fe₂O₃, α-FeOOH, β-FeOOH, γ-FeOOH, δ-FeOOH	[71, 211, 253, 254]
Tartaric, citric, oxalic, malonic acids → oxidized products	Photochemical oxidation	Ferritin	[255]
Pyruvic acid → pyruvic acid oligomers	Photochemical oligomerization		[256]
Salicylic acid → humic-like substances	Photochemical condensation	Accelerated in the presence of algae	[250]
Syringic acid and other methoxybenzoic acids → methanol	Photochemical decomposition		[257]
Syringic acid and related compounds + Cl⁻ → CH₃Cl	Photochemical decomposition + chlorination		[257]
Methanol → ethylene glycol + H₂ Ethanol → butane-2,3-diol + H₂	Photochemical coupling + dehydrogenation	ZnS in the absence of air	[258]
Isoprene → methylthreitol and methylerythritol (aerosols)	Photochemical oxidation		[259]
(Specific) plant compounds → compounds toxic to other organisms	Phototoxicity		[260, 261]
CO₂ → CO, HCOOH, HCHO, CH₃OH, CH₄	Photochemical reduction (one-carbon products)	Fe(III) oxides, FeCO₃, NiCO₃, CoCO₃, CuCO₃, Mn(II) (aq), ZnO, TiO₂, ZnS, CdS, ZrO₂, WO₃, CaFe₂O₄, BiVO₄, hydrous Cu₂O, transition metal ions and oxides in zeolites	[30, 31, 33, 262–268]
CO₂ + H₂ → CH₄	Photochemical reduction	α-Fe₂O₃ and Zn-Fe oxide in the presence of water, NiO	[269, 270]
CO₂ + H₂ → CO, HCOOH, CH₃OH	Photochemical reduction	α-Fe₂O₃ and Zn-Fe oxide in the presence of water	[269]
CO₂ → HCOOH	Photochemical reduction	Porphyrins, phthalocyanines Elemental Cu on silicate rocks such as granite and shale	[271, 272]
CO₂ → ethanol CO₂ → ethane, ethene, propane, propene CO₂ → tartaric, glyoxylic, oxalic acids	Photochemical reduction (products with more than one carbon)	SiC, ZnS, BiVO₄, montmorillonite-modified TiO₂	[273–277]
CH₄ → HCOOH CH₄ → CO, CO₂	Photochemical oxidation	TiO₂	[211, 278]
CH₄ → ethane + H₂	Photochemical coupling + dehydrogenation	SiO₂-Al₂O₃-TiO₂	[279]
Nitrogen compounds
Plant foliage → NO_x			[280]
Plant foliage → N₂O			[281]
Particulate organic N → dissolved organic N and NH₄ ⁺	Photochemical decomposition (dissolution + mineralization)		[115]
Dissolved organic N → biologically more labile N	Photochemical priming		[282]
Amino acids and other organic N (including biologically recalcitrant organic N) → NH₄ ⁺	Photochemical decomposition (mineralization/ammonification)	No facilitator Organic matter, Fe₂O₃, soil	[132, 184, 193, 194, 283–286]
Humic substances → NO₂ ⁻	(Oxidative) photochemical decomposition (mineralization)		[104, 287]
NH₃ → NO₂ ⁻ NH₃ → NO₃ ⁻	Photochemical oxidation (nitrification)	TiO₂, ZnO, Al₂O₃, SiO₂, MnO₂, soil Observed in seawater	[288 – 290]
NH₃ → N₂O, N₂	Photochemical oxidation	TiO₂	[290, 291]
NH₄ ⁺ + NO₂ ⁻ → N₂ urea, protein → [NH₄NO₂] → N₂	Photochemical oxidation + reduction (denitrification)	TiO₂, ZnO, Fe₂O₃, soil	[292, 293]
NH₄NO₃ → N₂O	Photochemical oxidation + reduction (denitrification)	Al₂O₃	[294]
NO_x → NO₃ ⁻	Photochemical oxidation	TiO₂	[295, 296]
NO₂ → HONO, NO, N₂O	Photochemical reduction	TiO₂	[296]
NO₂ ⁻ → NO₃ ⁻	Photochemical oxidation	TiO₂, ZnO, Fe₂O₃, WO₃	[297]
NO₃ ⁻ → NH₃	Photochemical reduction	TiO₂ plus electron acceptor	[298]
NO₃ ⁻ or HNO₃ → N₂O, NO, HONO, NO₂	Photochemical reduction (denitrification/renoxification)	Al₂O₃, TiO₂, SiO₂, α-Fe₂O₃, ZnO, CuCrO₂, Na zeolite, sand Observed in snow	[299–305]
NO₃ ⁻ → NO₂ ⁻ (+ O₂)	Photochemical reduction (+oxidation)	No facilitator Iron(III) oxide, soil, organic matter; TiO₂ plus humic acids	[103, 306–309]
NO₂ → HONO	Photochemical reduction	Humic acids, soot, soil Observed in ice	[157, 310, 311]
N₂O → N₂	Photochemical reduction	ZnO, Fe₂O₃, sand Humic and fulvic acids	[94, 95, 151, 312]
N₂O → N₂ + O₂	Photochemical dissociation	ZnO, Cu(I) zeolites	[313, 314]
N₂ → NH₃	Photochemical reduction/(reductive) photochemical fixation	ZnO, Al₂O₃, Fe₂O₃, Ni₂O₃, CoO, CuO, Fe(III) in TiO₂, Fe₂O₃-Fe₃O₄, MnO₂, Sand, soil Aqueous suspensions of TiO₂, ZnO, CdS, SrTiO₃, Ti(III) zeolites Hydrous iron(III) oxide in the absence of O₂	[2, 229, 315–321]
N₂ + H₂O → NH₃ + O₂	Photochemical reduction + oxidation	TiO₂ in the absence of O₂, α-Fe₂O₃, Fe(III)-doped TiO₂	[58, 321, 322]
N₂ → N₂H₄	Photochemical reduction	Sand	[2]
N₂ + H₂O → N₂H₄ + O₂	Photochemical reduction + oxidation	TiO₂ in the absence of O₂	[322]
N₂ + O₂ → NO	Photochemical oxidation (oxidative) photochemical fixation	TiO₂ in air	[323]
N₂ → NO₂ ⁻ N₂ → NO₃ ⁻	Photochemical oxidation (oxidative) photochemical fixation	Suspension of ZnO in the absence of O₂ Aerated suspension of hydrous iron(III) oxide TiO₂, soil	[320, 324, 325]
N₂ + H₂O → NO₂ ⁻ + H₂	Photochemical oxidation + reduction	ZnO-Fe₂O₃ under N₂	[326]
Metal compounds
Organic complexes of Fe, Al, Co, Ni (aq) → ionic Fe, Al, Co, Ni (aq)	Photochemical decomposition + decomplexation		[327, 328]
Organic complexes of Fe, Cu, Cr, Pb, V (aq) → colloidal Fe, Cu, Cr, Pb, V	Photochemical decomposition + precipitation		[328]
Organic matter (aq) + iron (aq) → organic matter + iron (s)	Photochemical flocculation		[193, 329]
FeOH⁺ (aq) → FeOOH	Photochemical oxidation		[330]
Fe(III) (hydr)oxides (s) → Fe(II) (aq)	(Reductive) photochemical dissolution of FeOOH + photochemical oxidation of organic matter (if present)	No facilitator Coprecipitated or dissolved organic matter, HSO₃ ⁻, montmorillonite Accelerated in ice	[70, 71, 92, 122, 331–338]
Fe(II) (aq)/Fe(OH)₂ + H₂O → Fe(III) + H₂	Photochemical oxidation + reduction	No facilitator Chromophores such as chlorophyll	[339, 340]
Fe(III)-carboxylate complexes (aq) → Fe(II) (aq)	Photochemical reduction + decomplexation		[66, 70, 341, 342]
Mn(IV) oxide → Mn(II) (aq)	(Reductive) photochemical dissolution	Dissolved organic matter Accelerated in ice	[337, 343–347]
Mn(II) (aq) → MnO_x (x = 1 to 2)	Photochemical oxidation	Organic matter, TiO₂	[348, 349]
Cu(II) (aq) → Cu(I)	Photochemical reduction	Amino acids	[224, 225]
Cr(VI) (aq) → Cr(III) (aq)	Photochemical reduction	Ferritin, phenol	[350, 351]
ZnS + H₂O → H₂S → H₂	Photochemical reduction + dissolution		[21, 251]
ZnS → Zn(0) + S(0)	Photochemical oxidation + reduction		[21]
CdS → Cd(II) + S(0)	Photochemical oxidation		[211]
HgS → Hg(II) (aq) + H₂S	Photochemical dissolution		[228, 352]
HgS → Hg(0) + S(0)	Photochemical oxidation + reduction	Cl⁻	[25]
HgS → [Hg₂Cl₂ and other intermediates] → HgCl₂	Photochemical oxidation, reduction/photochemical dissolution	Cl⁻	[25]
Hg(0) (aq) → Hg(II) (aq)	Photochemical oxidation		[352, 353]
Hg(II) (aq) → Hg(0) (aq)	photochemical reduction	Fe(III) species, TiO₂, organic matter Observed in freshwater, seawater, and snow	[352, 354–357]
Hg(II) (aq) → HgCH₃ ⁺	Photochemical methylation		[358]
HgCH₃ ⁺ → Hg(II)	Photochemical demethylation		[359, 360]
HgCH₃Cl → Hg(II) + Hg(0) + CHCl₃ + HCHO	Photochemical demethylation + reduction		[361]
Other elements
Plant material → H₂	(Reductive) photochemical decomposition		[362, 363]
Dissolved organic P → inorganic phosphate	Photochemical decomposition (mineralization)		[364]
Phosphate adsorbed to Fe(III) oxides or Fe(III)-organic matter complexes → free phosphate	Photochemical desorption		[161, 365, 366]
HS⁻/S²⁻ → H₂	Photochemical reduction	CdS, α-Fe₂O₃	[367, 368]
SO₂ → SO₄ ²⁻	Photochemical oxidation	TiO₂, Fe₂O₃, ZnO, CdS	[369–372]
Thiols and SO₃ ²⁻ → oxidized products	Photochemical oxidation	Ferritin	[255]
Alkyl sulfides + NO_x → aldehydes, sulfonic acids, SO₂, SO₄ ²⁻	Photochemical oxidation		[373]
O₂ → H₂O₂	Photochemical reduction	ZnO, TiO₂, sand in the presence of organic electron donors Aqueous Fe(III)-carboxylic acid complexes Tryptophan and tyrosine Porphyrins and phthalocyanines Algae (live or dead)	[34, 107, 246, 298, 374 – 376]
O₂ → H₂O	Photochemical reduction	α-Fe₂O₃ Dissolved Fe and humic substances (a catalytic cycle)	[123, 377]
H₂O → H₂	Photochemical reduction	Numerous catalysts, usually in the absence of O₂, e.g., TiO₂, ZnS, α-Fe₂O₃, hydrated Cu₂O, tungstosilicate on TiO₂, Ti(III)-zeolite, graphite oxide	[21, 22, 262, 315, 377–382]
H₂O → O₂	Photochemical oxidation	α-Fe₂O₃ + Fe(III) (aq), BiVO₄ + electron acceptor, Mn₂O₃, λ-MnO₂, Mn₃O₄, Co₃O₄ + sensitizer, AgCl, layered double hydroxide minerals Fe(OH)²⁺ (aq)	[383–390]
H₂O → H₂ + O₂	Photochemical water splitting (oxidation + reduction)	TiO₂, Fe₂O₃-Fe₃O₄, Fe₂O₃-FeS₂, Cu₂O, ZrO₂, Ag zeolite, diverse two-mineral systems	[60, 137, 321, 322, 391–393]
As(III) (aq) → As(V) (aq)	Photochemical oxidation	No facilitator Ferrihydrite, kaolinite	[158, 394, 395]
As₄S₄ → As₄S₄ (polymorph)	Photochemical structural (crystal) modification		[396]
As₂S₃ → [As + S] + O₂ → As₂O₃ As₄S₄ → As₂O₃	Photochemical oxidation/dissolution	Water	[396, 397]
Volatile organic compounds + NO_x → O₃	Photochemical oxidation		[398]
Cl⁻ → Cl⁻ ₂ (dichloride radical anion)	Photochemical oxidation	Chlorophyll, Hg(II)	[65, 352]
Cl⁻ + O₃ → Cl₂	Photochemical oxidation		[399]
NO₃ ⁻ + Br⁻ → Br₂	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 field 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 artificial light and/or laboratory-prepared equivalents of natural compounds

Note on terminology The 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 O₂ under 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