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Environmental Effects

Mascara is often used on a daily basis around the world. In 2016 alone, U.S. consumers spent 335.6 million USD on the top 10 leading mascara brands. With an estimated cost of 8 USD per unit and roughly 13 mL of product per unit, 550,000 L of mascara is bought per year in the U.S. Mascara has a short shelf life; according to manufacturers after two to four months of use, mascara should be discarded. When it is thrown away, most containers can still contain leftover product. These containers can pile up in landfills or get dumped in the ocean, but it is their pigments that can have important effects on the environment.

Cosmetic Pigments

Mascara is composed of a base mixture of pigments, waxes, and oils with varying supporting components. Mascara pigments commonly include iron oxides and titanium oxides which provide mascara with its desired color. Titanium dioxide (TiO₂) accounts for over 65% of inorganic pigments sales volume. TiO₂ gives the pigment a white color while different iron oxides provide a variety of colors such as red, yellow, brown, and black. The color of a pigment is produced by its opaque quality; the light cannot penetrate the pigment due to particles of the pigment scattering and reflecting visible light at different wavelengths. The particle size of opaque pigments ranges from 0.2-0.3 µm.

Iron Oxides

Methanogenesis Replacement

Under conditions favoring iron reduction, the process of iron oxide reduction can replace at least 80% of methane production occurring by methanogenesis. This phenomenon occurs in a nitrogen-containing (N₂) environment with low sulfate concentrations. Methanogenesis, an Archaean driven process, is typically the predominate form of carbon mineralization in sediments at the bottom of the ocean. Methanogenesis completes the decomposition of organic matter to methane (CH₄). The specific electron donor for iron oxide reduction in this situation is still under debate, but the two potential candidates include either Titanium (III) or compounds present in yeast. The predicted reactions with Titanium (III) serving as the electron donor and phenazine-1-carboxylate (PCA) serving as an electron shuttle is as follows: 

Ti(III)-cit + CO₂ + 8H⁺ → CH₄ + 2H₂O + Ti(IV) + cit               ΔE=-240 + 300 mV

Ti(III)-cit + PCA (oxidized) → PCA (reduced) + Ti(IV) + cit               ΔE=-116 + 300 mV

PCA (reduced) + Fe(OH)₃ → Fe²⁺ + PCA (oxidized)                         ΔE=-50 + 116 mV

Note: cit = citrate.

Titanium (III) is oxidized to Titanium (IV) while PCA is reduced. The reduced form of PCA can then reduce the iron hydroxide (Fe(OH)₃). 

Hydroxyl Radical Formation

On the other hand when airborne, iron oxides have been shown to harm the lung tissues of living organisms by the formation of hydroxyl radicals, leading to the creation of alkyl radicals. The following reactions occur when Fe₂O₃ and FeO, hereafter represented as Fe³⁺ and Fe²⁺ respectively, iron oxide particulates accumulate in the lungs.

O₂ + e⁻ → O₂•⁻

The formation of the superoxide anion (O₂•⁻) is catalyzed by a transmembrane enzyme called NADPH oxidase. The enzyme facilitates the transport of an electron across the plasma membrane from cytosolic NADPH to extracellular oxygen (O₂) to produce O₂•⁻. NADPH and FAD are bound to cytoplasmic binding sites on the enzyme. Two electrons from NADPH are transported to FAD which reduces it to FADH₂. Then, one electron moves to one of two heme groups in the enzyme within the plane of the membrane. The second electron pushes the first electron to the second heme group so that it can associate with the first heme group. For the transfer to occur, the second heme must be bound to extracellular oxygen which is the acceptor of the electron. This enzyme can also be located within the membranes of intracellular organelles allowing the formation of O₂•⁻ to occur within organelles.

2O₂•⁻ + 2H⁺ → H₂O₂ + O₂

The formation of hydrogen peroxide (H₂O₂) can occur spontaneously when the environment has a lower pH especially at pH 7.4. The enzyme superoxide dismutase can also catalyze this reaction. Once H₂O₂ has been synthesized, it can diffuse through membranes to travel within and outside the cell due to its nonpolar nature.

Fe²⁺ + H₂O₂→ Fe³⁺ + HO• + OH⁻

Fe³⁺ + H₂O₂ → Fe²⁺ + O₂•⁻ + 2H⁺

H₂O₂ + O₂•⁻ → HO• + OH⁻ + O₂

Fe²⁺ is oxidized to Fe³⁺ when it donates an electron to H₂O₂, thus, reducing H₂O₂ and forming a hydroxyl radical (HO•) in the process. H₂O₂ can then reduce Fe³⁺ to Fe²⁺ by donating an electron to it to create O₂•⁻. O₂•⁻ can then be used to make more H₂O₂ by the process previously shown perpetuating the cycle, or it can react with H₂O₂ to form more hydroxyl radicals. Hydroxyl radicals have been shown to increase cellular oxidative stress and attack cell membranes as well as the cell genomes.

HO• + RH → R• + H₂O

The HO• radical produced from the above reactions with iron can abstract a hydrogen atom (H) from molecules containing an R-H bond where the R is a group attached to the rest of the molecule, in this case H, at a carbon (C).

Titanium Dioxide

Nanoparticle Introduction

Titanium Dioxide (TiO₂) is mostly introduced into the environment as nanoparticles via wastewater treatment plants. Mascara pigments including titanium dioxide enter the wastewater when the product is washed off into sinks after cosmetic use. Once in the plants, pigments separate into sewage sludge which can then be released into the soil when injected into the soil or distributed on its surface. 99% of these nanoparticles wind up on land rather than in aquatic environments due to their retention in sewage sludge. Once in the environment, the titanium dioxide nanoparticles have incredibly low to negligible dissolution and have been shown to be very stable once particle aggregates are formed in soil and water surroundings. In the process of dissolution, water soluble ions typically dissociate from the nanoparticle into solution when thermodynamically unstable. TiO₂ dissolution increases when there are higher levels of dissolved organic matter and clay in the soil. However, aggregation is promoted by pH at the isoelectric point of TiO₂ (pH = 5.8) which renders it neutral and solution ion concentrations above 4.5 mM.

Titanium dioxide has been found to be toxic to plants and small organisms such as worms, nematodes, and insects. The toxicity of TiO₂ nanoparticles on nematodes increases with smaller nanoparticle diameter specifically 7 nm nanoparticles relative to 45 nm nanoparticles, but growth and reproduction are still affected regardless of the TiO₂ nanoparticle size. The release of titanium dioxide into the soil can have a detrimental effect on the ecosystem in place due to its hindrance of proliferation and survival of soil invertebrates; it causes apoptosis as well as stunts growth, survival, and reproduction in these organisms. These invertebrates are responsible for the decomposition of organic matter and the progression of nutrient cycling in the surrounding ecosystem. Without the presence of these organisms, the soil composition would suffer.

Hydroxyl Radical Formation

Although TiO₂ pigment does not absorb visible light, it does strongly absorb ultraviolet (UV) radiation (hv), leading to the formation of hydroxyl radicals. This occurs when photo-induced valence bond holes (h⁺_vb) are trapped at the surface of TiO₂ leading to the formation of trapped holes (h⁺_tr) that cannot oxidize water.

TiO₂ + hv → e⁻ + h⁺_vb

h⁺_vb →h⁺_tr

O₂ + e⁻ → O₂•⁻

O₂•⁻+ O₂•⁻+ 2H⁺→ H₂O₂ + O₂

O₂•⁻+h⁺_vb→ O₂

O₂•⁻+h⁺_tr→ O₂

OH⁻ +h⁺_vb→HO•

e⁻ +h⁺_tr→ recombination

Note: Wavelength (λ) = 387 nm

This reaction has been found to mineralize and decompose undesirable compounds in the environment, specifically the air and in wastewater.

Chloride Process

The production of TiO₂ via the chloride process also produces chlorine gas as a byproduct. This process separates the titanium from the rest of the ore by chlorination, then purifying and reacting it with oxygen at a high temperature to reform the titanium dioxide compound. First, titanium tetrachloride (TiCl₄) is formed through carbothermal chlorination at temperatures from about 900-1000°C.

2TiO₂(s) + Cl₂(g) + 3C(s) → 2TiCl₄(g) + 2CO(g) + CO₂(g)

From there the TiCl₄(g) undergoes further purification before oxidation; the chlorination process shown above creates metal chlorides from metal impurities in the TiO₂(s) ore which can then be precipitated out by cooling the mixture. The oxidation reaction to reform TiO₂ is conducted at 648.89°C with TiCl₄ still in the gas phase.

TiCl₄(g) + O₂(g) + heat → TiO₂(s) + 2Cl₂(g)