Introduction
There are environmental chemical processes which require light to react. These processes are called photochemical reactions. Photochemical reaction is commonly initiated due to the absorption of solar energy or light. When molecules absorb energy in the form of light, the atoms are converted in a transient excited state (Larson & Weber, 1994). This transition state or excited has completely different physical and chemical properties with the initial state of the molecule. The excited states also have more tendencies to react since they are considered stronger acids and stronger reductants that the initial state of the molecule. The excited state of the molecule is also considered a new chemical species which could react to any substance.
Photochemical reactions are common in the environment and some of its examples are considered to be very important to most of the living things on earth. One of the major examples of a photochemical reaction is the photosynthesis. Photosynthesis is process of converting solar energy or light energy into a stored chemical energy. It could also be referred to as the use of light energy to convert water and carbon dioxide into oxygen and glucose (Larson & Weber, 1994). The ability of the eye to see things also starts with a photochemical reaction. In addition, the production or Vitamin D on the body is commonly initiated by solar or light energy.
Understanding of photochemical reactions is both important in the applications of inorganic and organic chemistry. In most cases, photochemical reactions are described as its counterpart which is the thermal reactions. However, these two types of reaction are completely different (Carey & Sundberg, 2000). The objective of this term paper is to discuss some of the important aspects of environmental photochemical reactions. Aside from understanding of the basic mechanism of photochemical reactions, this paper also opts to analyse and evaluate some of the major examples of photochemical reactions in the environment and in general applications.
Brief History of Photochemistry and Photochemical Reactions
Humans started to use or apply photochemical reactions during the Bronze Age or during the 1500s. During these periods of time, the applications of photochemical reactions have been observed in various groups of people around the world. For example, the Canaanite people applied the use of photochemical reaction in colouring their cloaks. They use the photochemical reaction by preparing a fast dye from a local mollusk (Larson & Weber, 1994). In the modern times, this fast dye is called 6,6’-dibromoindigotin. These applications of photochemical reaction are also mentioned in some of the famous literary works such as the Pentateuch and the epics of Homer.
Luminescence is a common cause of the photo-excitation of molecules when exposed to solar energy or light. Although people from the past could not understand it mechanism, it was used to several applications. For example, several scientists or experts discovered that some of the substances or chemicals emit low intensity of radiation when exposed to light. In 1565, a Spanish physician named Nicolas Monardes discovered that a water-based extract of a Mexican wood glow a blue colour when it is exposed to sunlight (Larson & Weber, 1994). In 1853, quinine solution was discovered by George Stokes to re-emit low intensity of blue radiation when exposed to lightning. This phenomenon is also called fluorescence. By the late 19th century, fluorescence and phosphorescence are distinguished from each other.
The modern understanding and application of photochemical reactions started during the late 19th century (Holick et al., 1980). Scientists and experts began to formulate the common understanding of the process and mechanism of fluorescence and phosphorescence. The understanding of quantum mechanics helped in developing the fundamental understanding for the basis of photochemical reactions. Scientists found out the absorption of solar energy or light by the molecules involved promotion of the electrons to a low-energy orbital making the molecule to be excited. This process could result to photochemical reactions (Larson & Weber, 1994). The further understanding of the concept of photochemical reactions is developed during the 20th century due to the discoveries and concepts provided by Albert Einstein.
The Sunlight
In order to understand the aspects and mechanism of photochemical reactions, one should also understand the nature of sunlight. Environmental photochemical reactions are commonly carried out through the use of solar energy or energy from the sun. Although some of the applications of photochemical reactions are carried out through the use of artificial light such as the use of mercury lamps, most of the photochemical reactions in the environment are carried out due to sunlight. The sun is considered to be an intense, hot and a big ball of gases which is heated due to the nuclear reactions. These nuclear reactions are the ones which produced the radiant energy which could be experienced by the living things on earth.
The continuous input of radiant energy could be experienced by the upper boundary of the earth’s atmosphere. Almost 50 % of this radiant energy is sent back to space through reflection (Carey & Sundberg, 2000). The remaining radiant energy are scattered around the earth’s surface through direct radiation or through scattering with the use of smoke, clouds and atmospheric dusts. The radiant energy coming from the sun is mainly composed of a mixture of invisible and visible radiation. In general, this radiant energy is characterized by different wavelengths (Larson & Weber, 1994). The visible light is composed of the radiant energy with a wavelength ranging between 400-720 nm. This visible light is commonly the main reasons for the photochemical reactions. Some of the photochemical reactions are carried out with the use of ultraviolet and infrared waves.
The modern day understanding of the mechanism and nature of photochemical reactions are based on the concept of the wave-particle duality of light. It was a concept developed by Planck and Einstein during the start of the 20th century. It is a concept proposing the light could achieve the properties of both a wave and a particle. In this case, the light could behave as a wave and a particle which is commonly referred to as “photon”. The introduction of the concept of the wave-particle duality of light also provided the means of quantifying the energy, transmittance and absorbance of radiant energy. It also brought methods and strategies to fully understand and study several important environmental photochemical reactions.
Interfacial Photochemical Reactions
Air and Water Interface
Organic surface layers are commonly found in most of the natural water bodies. These micro-layers or interfaces are enriched with several classes of compounds which could be subjected to some of the photochemical reactions. The organic surface layers could be considered an environment filled with mixed water-solvent solution or organic solvent solution rather than an environment filled with pure aqueous solution. In general, photochemical reactions could be more favoured at the micro-layers than the compounds at the water column due to large concentrations of the organic materials at this region (Larson & Weber, 1994). The compounds at the organic surface layers are in the state of equilibrium since the input of new materials in this region is compensated by the photodecomposition reactions and microbial degradation.
In most of the studies, photochemical reactions occur on the organic surface layers. For example, the unsaturated fatty acids, which are common compounds at the organic surface layers of most of the water bodies, are considered to be unstable to sunlight. Transformations of lipids such as the linoleic acid and the linolenic acid could occur through the help of sunlight. The chemical formula of the linoleic acid and the linolenic acid is illustrated in Figure 1.
Figure 1.The chemical formula of the linoleic acid (4) and the linolenic acid (5).
Source: Larson, R., & Weber, E. (1994). Reaction mechanisms in environmental organic chemistry.CRC Press.
Photochemical reactions could also be observed at the air-water interfaces during an oilspill. Photochemical aging and weathering of petroleum could occur through the exposure to low intensity of ultraviolet waves (Gilbert & Baggott, 1991). The photochemical aging of fuel and petroleum could result to the formation of water-soluble products such as peroxides and phenols. Photo-oxidation of petroleum could also occur through the help of long exposure to ultraviolet waves. The termination steps of these reactions could produce alcohols, carbonyl compounds, carboxylic acids, phenols and other water-soluble products. In general, photochemical reactions which occur at the water-air interfaces are due to the large amounts of organic compounds at these layers (Holick et al., 1980). Most of the organic compounds are not soluble in water which is why photochemical reactions commonly occur at the air and water interfaces.
Solid-Water and Solid-Air Interfaces
Photochemical reactions commonly occur at the interface of soil and mineral boundaries due to the presences of organic and inorganic matter (Carey & Sundberg, 2000). One of the main photochemical reactions which occur at soil and clay surfaces includes photo-transformation and photolysis. Several scientists revealed that photo-transformations at soil and clay interfaces could occur due to the presence of organic and inorganic materials. In general, light could not easily penetrate to soil and sediments below 0.2 mm (Larson & Weber, 1994). However, surface photochemical reactions could occur at significant rates. Most of the toxic substances such as insecticides could react to sunlight through photolysis. In some cases, this photolysis could result to more toxic substances especially when the soil is high in organic carbon. Some of the toxic substances present at soil and clay interfaces which could proceed into photolysis include dichlorodiphenyldichloroethylene (DDE), parathion, paraoxon and fenpopathrin. The chemical formulas of these compounds are illustrated at Figure 2.
Figure 2.Chemical formula of dichlorodiphenyldichloroethylene (DDE) (6), parathion (7), paraoxon (8) and fenpopathrin (9).
Source: Larson, R., & Weber, E. (1994). Reaction mechanisms in environmental organic chemistry.CRC Press.
Photochemical reactions could also occur at the surfaces of organisms such as plants and bacteria. The compounds present at insecticides could remain at the cuticular layer or ourter layer of plants if insecticides are sprayed onto them. The surfaces of plants are considered to be hydrophobic since it is mainly composed of fatty acids, fatty alcohols, long-chain alkenes, esters and other types of polymers. The compounds present at the insecticides could proceed to photochemical reactions when they are suspended at the surfaces of plants. Some of these chemicals could be bound to plant tissues when exposed to sunlight through photochemical reactions. For example, scientists reported that the dichlorodiphenyltrichloroethane (DDT) could form products which could result to bounding of these substances to the plant tissues.
Photochemical Reactions of Several Compounds
Natural Organic Matter
Natural organic matters such as humic materials commonly undergo bleaching or changes in colour when exposed to radiant energy or solar energy. This is a common understanding among experts such as scientists. However, the fundamental understanding for these reactions is not yet well developed until several studies have been made. The details of these photochemical reactions are developed with the help of modern studies regarding light-induced processes of natural organic matter (Larson & Weber, 1994). Some of these studies revealed that several reactive species could form photolysis of natural organic matter. This is one of the reasons why most of the photochemical reactions are common at the interface of solids and liquids due to the large quantities of natural organic matter.
Aromatic Hydrocarbons
Photochemical reactions are not common in aromatic hydrocarbons or benzene derivatives since it does not absorb much visible light (Carey & Sundberg, 2000). In this case, excitation of the benzene molecules does not commonly occur. However, some of the aromatic hydrocarbons or benzene derivatives form complexes with oxygen when exposed to sunlight. The absorption spectra of the aromatic hydrocarbons which formed complexes with oxygen could increases and the yield for photochemical reaction is also high. Some of the benzene derivatives form long-chain conjugated di-aldehydes through photochemical reactions (Gilbert & Baggott, 1991). Other products include alcohols and aldehydes from oxidation of the alkyl-substituted benzenes. The reaction mechanisms of these photochemical reactions are still being studied.
Although most of the benzene derivatives do not absorb light strongly in the solar ultraviolet region, the polycyclic aromatic hydrocarbons have relatively high absorbance. For example, naphthalene could absorb light strongly in water than in organic solvents. There are also several studies which suggest that photochemical reactions of benzene derivatives of aromatic hydrocarbons are different when they are suspended in non-polar environments than it aqueous solution. For example, the photochemical reaction of benzo[a]pyrene results to the formation of quinones. The chemical formula of benzo[a]pyrene is illustrated in Figure 3.
Figure 3.Chemical formula of benzo[a]pyrene (18).
Source: Larson, R., & Weber, E. (1994). Reaction mechanisms in environmental organic chemistry.CRC Press.
Halogenated Hydrocarbons
Aliphatic mono-halogenated compounds have low amount of sunlight absorption. However, the tail of the C-X (carbon-halogen) absorption band shift into the solar region when there are two or more chlorine are added to the same carbon atoms (Larson & Weber, 1994). This is the reason why some halogenated hydrocarbons such as chloroform and carbon tetrachloride are unstable to decomposition through photochemical reaction.
Carbonyl Compounds
Mechanism of photochemical reactions of carbonyl groups such as the aldehydes and the ketones are well understood through the help of empirical studies. For example, the ketone such as the acetone could react photochemically to form alkyl-acyl radical pair through the cleavage mechanism called the Norrish type I. The Norrish type 1 cleavage mechanism is illustrated in Figure 4.
Figure 4. Norrish type 1 cleavage mechanism.
Source: Larson, R., & Weber, E. (1994). Reaction mechanisms in environmental organic chemistry.CRC Press.
Examples of Naturally Occurring Photochemical Reactions
Photosynthesis
Photosynthesis is considered to be the most common and popular example of environmental photochemical reaction. Photosynthesis is process of converting solar energy or light energy into a stored chemical energy. It could also be referred to as the use of light energy to convert water and carbon dioxide into oxygen and glucose (Carey & Sundberg, 2000). The chemical energy which is produced from the process of photosynthesis is stored in the form of a carbohydrate molecule which is the main product of the synthesis carbon dioxide and water. In the process of photosynthesis, oxygen is released as a waste product (Gilbert & Baggott, 1991).
There are several types of species and organisms which could perform photosynthesis. These organisms are commonly referred to as photoautotrophs (Larson & Weber, 1994). Some of these photoautotrophs include plants, several species of algae and cyanobacteria. In general, photosynthesis is considered to be the most important photochemical reaction since it supplies most of the organic compounds, energy which is necessary for most of the living things on earth. In addition, it also maintains the concentration of the atmospheric oxygen level on earth.
In general, the overall process of photosynthesis could be represented in a simple method of stoichiometry. The plants or the organism that executes photosynthesis take in carbon dioxide and water in order to produce a stored energy in the form of a sugar such as glucose and oxygen. The overall chemical formula or equation of a simple photosynthesis could be as follows:
6CO2+ 6H2O → 6O2+ C6H12O6
or
carbon dioxide+water →oxygen+glucose
The equation could be balanced depending on the amount of carbon dioxide and water reacted during the process of photosynthesis. Plants could absorb water at any time and carbon dioxide is always available in the atmosphere. However, the process of photosynthesis requires sunlight or radiant energy in order to proceed.
Absorption of light is necessary in order for the photosynthesis to proceed. In plants, there have a pigment called the chlorophyll in order to absorb the radiant energy from the visible spectrum. The pigment called the chlorophyll commonly reflects the green light which is the main reason why the leaves of the plants are green. The absorbed light is directly funnelled to the reaction centres and energy transfer takes place through the movement of electrons from one molecule to another. The process of photosynthesis commonly takes place at the chloroplasts which thylakoids. These parts of the plant contain important chemicals for the transfer of electrons during the process of photosynthesis such as the adenosine di-phosphate (ADP) and the nicotinamide adenine dinucleotide (NADP).
Human Formation of Vitamin D
Vitamin D is an important substance in the body which could be acquired through photochemical reactions within the body or by inducing Vitamin D supplement. It also refers to the group of fat-soluble secosteroids which helps in improving the absorption of the important minerals such as iron, magnesium, calcium, zinc and phosphate into the body. The most important types of Vitamin D to the body are the cholecalciferol or the Vitamin D3 and the ergocalciferol or the Vitamin D2 (Larson & Weber, 1994). The major natural source of Vitamin D3 in the body is through synthesis in the skin using photochemical reaction. In general, the synthesis of the Vitamin D3 in the skin is controlled and dependent on the exposure of the skin to the solar energy or light.
The synthesis of the Vitamin D in the skin is also accompanied by a negative feedback loop which could help the body from preventing toxic results. However, doctors and other health practitioners also do not advised individuals to expose their skin to sunlight due to the larger risk of acquiring cancer (Robertson, 2006). The Vitamin D3 is produced through photochemical reaction of the 7-dehydrocholesterol from the skin which is common to most of the skin of animals. The 7-dehydrocholesterol is produced by the human body in large quantities through the help of hormones and enzymes (Carey & Sundberg, 2000). The 7-dehydrocholesterol could react with the ultraviolet light at wavelengths ranging from 270 to 300 nm (Gilbert & Baggott, 1991). This range of radiant energy is considered to be dangerous for humans since it could increase the risk of acquiring cancer (Klessinger & Michl, 1994). According to most of the studies, the synthesis of Vitamin D3 in the skin could acquire its peak at the wavelength of 295 to 297 nm. These wavelengths are common in the sunlight. Direct exposure of the skin to the sunlight is required since the ultraviolet light is completely blocked by most of the glass (Klessinger & Michl, 1994).
The 7-dehydrocholesterol could react to ultraviolet light to generate the pre-Vitamin D3 at several layers of the skin. This pre-Vitamin D3 will be isomerized in order to produce the Vitamin D3. This Vitamin D3 will bind to its carrier protein and transported to the liver where hydroxylation of the Vitamin occurs. Small amounts of Vitamin D3 could be produced when the individual exposed his or her arms, face and legs to the sun about 10 to 60 minutes per week. This range is commonly advisable in order to avoid skin problems such as sunburn and skin cancer. Individuals who have dark skin requires more time of exposure to sunlight in order to produce sufficient amount of Vitamin D since it absorb less sunlight (Robertson, 2006).
Photo degradation of Substances
Another common environmental photochemical reaction which naturally occurs is the photo-degradation of several materials through the use of sunlight. In general, photo-degradation is a combination of the mechanism or action of air and sunlight to various materials (Klessinger & Michl, 1994). The most common type of photo-degradation which occurs in the environment is oxidation and hydrolysis with the use of sunlight. In some cases, photo-degradation is avoided since it could destroy some of the man-made structures or objects. However, it could also be helpful to humans since it could be used for remineralisation of the biomass and photo-degradation of toxic substances such as insecticides and pesticides.
There are certain products or materials in which photo-degradation could be useful. In pesticides, photo-degradation of toxic substances is required in agriculture in order to prevent damages from the nearby environment especially in the water bodies. Substances in pesticides do not readily proceed to photo-degradation even with the help of sunlight. Several additives must be mixed to the pesticides in order to enhance the photo-degradation of toxic substances such as photo-catalysts, photosensitizers, and hydrogen peroxide (Larson & Weber, 1994). Pharmaceuticals are also interested in the photo-degradation of toxic chemicals since it has deleterious effects on the aquatic life. They also prevent photo-degradation of some of the substances in the product.
Some of the food and beverages are susceptible from photo-degradation which could lead to the damage of the substances in the food or in the beverage. In most cases, food and beverage manufacturers protect their product from photo-degradation. Some of the nutrients from these foods and beverages could be damaged due to the process of photo-degradation. One major example of food and beverage which are greatly affected by photo-degradation is beer. The exposure of the beer from ultraviolet light could lead to photo-degradation of the hop bitter compounds to 3-methyl-2-buten-1-thiol (Carey & Sundberg, 2000). This photo-degradation process commonly changes the taste of the beer. In this case, beer bottles are often made of amber glass which commonly absorbs ultraviolet radiation to avoid photo-degradation of compounds.
The mechanism of the photo-degradation of several compounds is similar to most of the photochemical reactions which are naturally occurring in the environment. In the photo-degradation, the reaction is commonly initiated by the absorption of a photon which is within the wavelength range of 200 to 700 nm. The absorbed photon will increase the energy of the molecule by slightly changing the configuration of the molecule. It will result to the changing of the configuration of the molecule from the ground state into an excited state (Klessinger & Michl, 1994). The excited state of the molecules is not stable in the presence of H2O and O2. In this case, these molecules commonly decompose and are the result of the direct and indirect photolysis.
Bioluminescence
Another common type of environmental photochemical reaction is the bioluminescence. It is referred to as a type of photochemical reaction which produces and emits light. Several living organisms have the ability to perform bioluminescence. This type of photochemical reaction is common to several marine animals as well as fungi and microorganisms. Terrestrial vertebrates such as the fireflies also have the ability to perform bioluminescence. There are also light producing activities through symbiotic relationship of animals and Vibrio bacteria. Bioluminescence is a unique type of photochemical reaction since the photons or the particle responsible for emitting of light is at the product side of the reaction sequence.
The chemical reaction mechanism of bioluminescence is not similar to most of the photochemical reactions. It involves production and emitting of light using the pigment luciferin and enzyme luciferase. In some species, these proteins are assisted by several compounds such as aequorin. The oxidation of the luciferin is catalysed by enzymes. It is also accompanied with the use of molecules which carry energy such as the adenosine triphosphate (ATP) and co-factors such as magnesium and calcium ions. Bioluminescence could also be a tool for studying evolution since it rises over forty times in the history of evolution.
The chemical reaction mechanism of bioluminescence is unique and it is not similar with the other common environmental photochemical reactions (Larson & Weber, 1994). It this process, the light energy is released through a chemical reaction. In bioluminescence, the luciferin reacts with oxygen to create light energy. It is accompanied by a co-factor such as magnesium and calcium ions as well as enzymes such as luciferase. It is also accompanied by stored energy in the molecule such as the ATP. The general chemical reaction of bioluminescence is as follows:
Luciferin+O2+ATP →oxy-Luciferin+CO2+AMP+PP+Light
The adenosince monophosphate, the carbon dioxide and the phosphate groups are considered waste products of the reaction.
Environmental Applications of Photochemical Reactions
Photochemical Reactions for Wastewater Treatment
One of the main applications of photochemical reactions is the photo-degradation of organic materials in the wastewater (Carey & Sundberg, 2000). Exposure to ultraviolet radiation is one of the chemical methods of wastewater treatment which is widely used in the industry. The organic materials suspended in the water as well as some of the substances at the water and air interface are photo-degraded or decomposed using photochemical reactions. The use of photochemical reaction methods in the degrading of undesirable compounds in the wastewater is also being developed in the modern times. The use of catalysts and co-factors in the photochemical reactions of the undesirable substances in the wastewater has been studied by many experts in the modern times. Some of the new techniques in the photo-degradation of undesirable substances in wastewater includes the use of soluble sensitizers, use of metal ions and oxides as co-factor and advanced technological methods for oxidation of toxic substances.
The use of oxygen, solar energy and dissolved substances which are capable in absorbing ultraviolet radiation for the treatment of wastewater was first formulated in 1977 by Acher and Rosenthal. They added the methylene blue to the wastewater in order to visualize or to quantify the absorbance of sunlight. They exposed the wastewater in the ultraviolet radiation of the sun and they analyze the losses in the amounts of the detergent as well as the chemical oxygen demand (COD) of the wastewater. The decrease in the chemical oxygen demand of the wastewater could be associated with the photo-degradation of the natural organic matter in the wastewater. In their study, almost 50% decrease in the COD was reported. In addition, there is about 90% loss in the amount or concentration of the detergents in the wastewater.
In the modern times, various approaches were studied in the use of photo-degradation of organic matter or the use of photochemical reactions in the treatment of wastewater. The use of organic substances for forming complexes has been studied for further improvement of the wastewater treatment of sewage. One of the popular substances which are used to form complexes with organic substances is the riboflavin. It is used to form complex substances with the organic substances with electron-rich donor molecules such as phenols, polycyclic aromatic hydrocarbons and anilines. These complexes could increase the absorbance of sunlight of the substances or it could enhance the photo-degradation or photochemical reactions of these substances. The chemical formula of riboflavin is illustrated in Figure 4.
Figure 4. Chemical formula of riboflavin (32)
Source: Larson, R., & Weber, E. (1994). Reaction mechanisms in environmental organic chemistry.CRC Press.
Another approach in improving the photochemical reactions or photo-degradation of undesirable substances in the wastewater is to improve or enhance the quantity or intensity of sunlight (Carey & Sundberg, 2000). In 1990, the Sandia National Laboratories and the Solar Energy Research Institute devised a method in order to focus the sunlight on the reactor which contains the wastewater. They used parabolic mirrors in order to enhance or to increase the intensity of sunlight which could increase the absorbance of sunlight of the undesirable organic and inorganic materials (Gilbert & Baggott, 1991). In order to enhance the treatment method through photo-degradation, a photo-catalyst is mixed with the wastewater (Larson & Weber, 1994). The most common photo-catalyst which is used in the wastewater treatment is the TiO2. In some of the wastewater reactors, the tracking mirrors are attached and hydrogen peroxide is added in order to enhance the photochemical reactions. According to their laboratory scale studies, the photo-degradation of several organic compounds could be observed. Some of the compounds which are removed from the wastewater include tetrachloroethylene and salicylic acid.
Photochemical Reactions for Degradation of Chemical Wastes
Photo-degradation of toxic chemicals is also one of the main applications of environmental photochemical reactions (Carey & Sundberg, 2000). Most of the compounds and substances in the insecticides and pesticides require photochemical reactions in order to decompose them. These substances and toxic and could damage the nearby environment especially the aquatic life. The compounds present at the insecticides could proceed to photochemical reactions when they are suspended at the surfaces of plants. Some of these chemicals could be bound to plant tissues when exposed to sunlight through photochemical reactions. For example, scientists reported that the dichlorodiphenyltrichloroethane (DDT) could form products which could result to bounding of these substances to the plant tissues.
Conclusion
Photochemical reaction is commonly initiated due to the absorption of solar energy or light. When molecules absorb energy in the form of light, the atoms are converted in a transient excited state. This transition state or excited has completely different physical and chemical properties with the initial state of the molecule. Understanding of photochemical reactions is both important in the applications of inorganic and organic chemistry. In most cases, photochemical reactions are described as its counterpart which is the thermal reactions. However, these two types of reaction are completely different. Environmental photochemical reactions are commonly carried out through the use of solar energy or energy from the sun. Although some of the applications of photochemical reactions are carried out through the use of artificial light such as the use of mercury lamps, most of the photochemical reactions in the environment are carried out due to sunlight. The sun is considered to be an intense, hot and a big ball of gases which is heated due to the nuclear reactions. The modern day understanding of the mechanism and nature of photochemical reactions are based on the concept of the wave-particle duality of light. It was a concept developed by Planck and Einstein during the start of the 20th century. Some of the common environmental photochemical reactions could be observed at the interfaces of solid, air and water. Some of the most common photochemical reactions in the ecosystem include photosynthesis, formation of Vitamin D, photo-degradation of chemicals and bioluminescence.
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