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Environmental Impacts of Ink
Approximately 942 million inkjet cartridges were sold across the world in 2012. This amounts to nearly 19 billion kilograms of plastic from the cartridge itself and 7.5 million liters of ink. Aside from the plastic pollution of the cartridges, the ink itself is a complex mixture that can contain hundreds of different chemicals used as solvents, pigments, aerosols, stabilizers, and media fasteners, many of which may have environmental effects that have been rarely, if ever, studied.

There are almost no present studies addressing the effects of printer ink mixtures directly on the environment, however, there are some studies presently available that have investigated some of those ink chemicals or their more broad chemical families, either in isolation or in other systems outside of printer ink mixtures.

Two chemical families, Glycol Ethers and Phthalocyanines, have been provided here as they are both used in printer ink mixtures. Both of these chemical families are lacking studies analyzing the environmental effects of them in the context or application of printer ink mixtures.

Glycol Ethers
Glycol ethers are widely used as solvents and are of interest because of their contribution to the formation of photochemical air pollution especially in urban and regional areas. Glycol ethers primarily enter the environment from manufacturing effluents and emissions and during its use in commercial products. In addition, the improper disposal of ink cartridges can result in the ink mixture leaking out into the environment and eventually can reach the water supply or the atmosphere due to its high vapor pressure. Glycol ethers fall under a family of volatile organic compounds (VOCs) which can undergo photolysis and chemical reactions in the atmosphere. One extension of glycol ethers that are of interest, are propylene glycol methyl ether acetate (PGMEA). PGMEA's are prone to general ozonolysis, direct photolysis processes, and free radical reactions shown below. One of the main focuses on glycol ethers is their ability to react with hydroxyl radicals and undergo hydrogen atom transfer. Specifically, in the troposphere, ethers and glycol ethers can be photo oxidized through a mechanism in which OH abstracts an H atom from VOCs to produce a carbon-centered radical. This carbon radical can react with atmospheric oxygen and form a peroxy radical, which can react with nitrous oxide to form an unstable peroxynitrite adducts and either stabilize or dissociate to produce organic nitrate.
 * 1) CH3CH(OCOCH3)CH2OCH3 + O3 → Oxidation products
 * 2) CH3CH(OCOCH3)CH2OCH3 + hv → Oxidation products
 * 3) CH3CH(OCOCH3)CH2OCH3 + OH → Oxidation products

VOC   + OH(+ O2)   → RiO2·(1)

RiO2·   + NO            → RiOONO*                    (2)

RiOONO*                → RiO· + NO2(3a)

RiOONO*                → RiOONO2                    (3b)

NO2    + hv              → NO             + O(3P)     (4)

O(3P)  + O2             → O3                                (5)

In reaction 1, a VOC reacts with a hydroxy radical that produces a carbon-centered radical. Following, atmospheric oxygen reacts with the carbon-centered radical to produce a peroxy radical. In reaction 2, the peroxy radical reacts with nitrous oxide in the atmosphere and yields a unstable peroxynitrite adduct. The fate of this adduct is either stabilizing and forming an organic nitrate, or dissociating and forming an peroxy radical and NO2. Starting from reaction 3a, it is possible that the products can be followed by reactions 4 and 5 and contribute to the formation of ozone in the environment. The formation of the organic nitrate serves as a termination step consuming the peroxy radical and NO2 while also competing with the reaction 3b. The NO2 product in 3a is of interest due to its potential association with pollutants while affecting air quality by its contribution to photochemical smog. In particular, the NO2 product from 3a can react with light and yield NO and a singlet oxygen, as seen in reaction 4. This singlet oxygen can react with diatomic oxygen and contribute to ozone production in the troposphere, as seen in reaction 5. Whereas NO can react with diatomic oxygen and produce more NO2, a photochemical smog contributor.

Summary of Glycol Ethers
Glycol ethers play a role in chemistry of today’s world due to its potential of being a notable pollutant. One of the main impacts from glycol ethers (e.g. PGMEA) is its ability to react with hydroxy radicals generated by photolysis of water and O3 and contribute to the accumulation of photochemical smog. Not only does this affect the quality of air by generating reactive oxygen species (ROS) and NO2, but it also affects biological life. ​

Phthalocyanines
Phthalocyanines are a family of large aromatic, multi-ringed compounds with different metal centers, used as pigments in printer ink mixtures and are of interest primarily in their generation of reactive oxygen species (ROS) in the presence of light. Phthalocyanines have also been used as dyes and pigments in other applications, as well as oncology in photodynamic cancer therapy, however, the focus of this article is on phthalocyanines in inkjet mixtures.

Phthalocyanines have also been noted to catalyze many different redox reactions of different substrates, but this entry focuses primarily on the ROS’ produced. Due to the different types of phthalocyanines that can be included in the mixture, i.e. with different functional groups or metal centers, these confer different solubilities that can enable the phthalocyanine to become mobile and leach out to the environment.

Reaction Chemistry
These ROS’ are generated through the excitation of the phthalocyanine by light (of a specific wavelength dependent upon the functional groups and metal center of that phthalocyanine) through one of two mechanisms that generate either radical products or singlet oxygen, via type I or type II mechanisms, respectively, For both mechanisms, the phthalocyanine absorbs photons of light, and is excited from its ground state (S0) to an excited singlet state (S1). From the singlet state, it quickly drops down to a lower energy excited triplet state (T1) through fluorescence or thermal decay. This T1 state is more stable and temporally longer lasting than the S1 state, and through either type I or type II mechanisms, produces ROS’.

The T1 state can follow the type I mechanism, generating reactive oxygen species by one of two processes, detailed below. The first process (process A) starts first with either a T1 triplet excited state (3Psen*) or a singlet excited state (Psen*) of phthalocyanine (only the singlet excited state is shown in these descriptions) initiating a radical through transfer of an electron from some substrate (Subs) to the singlet excited state of phthalocyanine, generating a radical of the substrate (Subs•+oxi) and a radical of the phthalocyanine (Psen•-). This phthalocyanine radical then propagates by reacting with molecular oxygen (O2) to produce the ground state of phthalocyanine (Psen) and a radical of oxygen (O2•-). The oxygen radical can further react with other substrates (such as iron, protons, or hydrogen peroxide) to generate the triplet excited state of oxygen (3O2), in addition to the particularly cytotoxic hydroxyl (•OH), superoxide, and hydrogen peroxide radicals.

The second process (process B) starts by initiating a radical through transfer of a proton to the excited state of phthalocyanine from some substrate, generating a protonated radical of phthalocyanine (Psen-H•) and radical of the substrate (R•). The radical of the substrate (R•) can further react with other substrates such as other ROS to generate even more ROS such as the aforementioned hydroxyl, superoxide, and hydrogen peroxide radicals.

These reaction processes are:

Type I mechanism, process A:

Psen*   + Subs       → Psen•−        + Subs•+oxi

Psen•−  + O2          → Psen           + O2•-

Psen*   + O2           → Psen•+        + O2•-

O2•- + Fe3+        → 3O2             + Fe2+

2O2•- + 2H+        → 3O2             + H2O2

Fe2+     + H2O2       → Fe3+            + •OH

Type I mechanism, process B:

Psen*  + R-H          → Psen-H•      + R•

R•       + O2             → RO2•

RO2•   + R-H          → RO2H          + R•

The T1 state can also follow the type II mechanism and transfer energy in the reaction of a triplet excited state of oxygen with a triplet excited state of phthalocyanine to generate a singlet state of oxygen (1O2) and a singlet state of phthalocyanine (1Psen). The singlet state of oxygen, which is considered the primary agent of cell damage, reacts with some substrate to cause oxidative damage.

Type II mechanism:

3Psen  + 3O2           → 1Psen  + 1O2

1O2+ Subs        → Oxidative damage

The effects of reactive oxygen species (ROS) are detailed elsewhere, but can be summarized into stimulating apoptosis and oxidative damage that unleashes a cascade of radical reactions. The purpose of this is to demonstrate the initial generation of those ROS through photosensitizers such as phthalocyanines.

Both reactions generate ROS, but much of the oncologic literature suggests that the type II mechanism, generating the singlet oxygen, is the primary agent of cell death, and further, that the effects of photodynamic therapy are generally attributed to the effects of this singlet oxygen. However, it must be noted that most of the literature addressing phthalocyanines focus on the effects within the context of oncology, and most studies of phthalocyanine toxicity are conducted within the same context.

Environmental Effects of Phthalocyanines
There are only a small handful of studies presently available that address the environmental impacts of phthalocyanines, but their results are mixed and attribute phthalocyanine toxicity to either the chemistry of the compound as a whole (especially the functional groups on the rings), or the generation of ROS. In Synechococcus nidulans, a representative cyanobacteria, phthalocyanine-generated ROS’ did not correlate to inhibition or cell death. They find that instead, the properties of the phthalocyanine itself, such as the functional groups and their positioning, led to different charge distributions and atom densities and thus, cell death. This particular mechanism was suggested to work by direct interaction of the phthalocyanine with the membranes of the cyanobacteria, such that toxicity is induced through strong binding of positively charged groups on the phthalocyanines to the negatively charged groups of the biological membrane. In a system of the green alga  Pseudokirchneriella subcapitata  and the cyanobacteria Synechoccus nidulans, the same team attributed the toxicity of phthalocyanines more to cell membrane interactions than generation of ROS. Contrary to this, phthalocyanines were found to contribute toxicity via generation of reactive oxygen species in antifungal systems of Magnaporthe grisea. As pointed out in a 2011 review, at that time of writing, there were no studies addressing the effects of phthalocyanines on larger species such as fish, macrophytes, or benthic invertebrates.

Biodegradability of Phthalocyanines
Literature analyzing the biodegradability of phthalocyanines is very limited and is complicated by the wide variation in phthalocyanines, with variation in different metal centers, and functional groups, that all confer different biodegradabilities. One team has studied the biodegradability of a particular turquoise-blue copper phthalocyanine, however, its decomposition was very limited and occurred only in a system with the lignin-degrading fungus Phanerochaete chrysosporium.

Summary of Phthalocyanines
Phthalocyanines are common ingredients in printer ink mixtures and are used in many other applications despite clear understandings of their effects on the environment. The chemistry has been thoroughly studied and is well understood, but the toxicity and mechanisms of toxicity arising from that chemistry in the environment is less clear and currently focuses on generation of reactive oxygen species, additional phthalocyanine-catalyzed redox reactions and its effects of its chemical structure on cells. There is a small handful of literature presently available, some of which are listed, that address phthalocyanines and their use in photodynamic therapy, as algicides, fungicides, bacteriocides, and control of cyanobacterial blooms. However, there presently lacks a concise synthesis of the effects over all systems that would shed light on the predominant mechanisms of phthalocyanine toxicity in the environment.