Environmental impact of silver nanoparticles

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In 2015, 251 million tubes of toothpaste were sold in the United States. [1] A single tube holds roughly 170 grams of toothpaste, so approximately 43 kilotonnes of toothpaste get washed into the water systems annually. [2] Toothpaste contains silver nanoparticles, also known as nanosilver or AgNPs, among other compounds. [2]

Contents

Each tube of toothpaste contains approximately 91 mg of silver nanoparticles, with approximately 3.9 tonnes of silver nanoparticles entering the environment annually. [3] Silver nanoparticles are not entirely cleared from the water during the wastewater treatment process, possibly leading to detrimental environmental effects. [2]

Silver nanoparticles in toothpaste

Silver nanoparticles are used for catalyzing chemical reactions, Raman imaging, and antimicrobial sterilization. [4] Along with its antimicrobial properties, its low mammalian cell toxicity makes these particles a common addition to consumer products. [4] Washing textiles embedded with silver nanoparticles results in the oxidation and transformation of metallic Ag into AgCl. [5]

Silver nanoparticles have different physicochemical characteristics from the free silver ion, Ag+ and possess increased optical, electromagnetic, and catalytic properties. [4] Particles with one dimension of 100 nm or less can generate reactive oxygen species. Smaller particles less than 10 nm may pass through cellular membranes and accumulate within the cell. [4] Silver nanoparticles were also found to attach to cellular membranes, eventually dissipating the proton motive force, leading to cell death. [4]

Silver nanoparticles that are larger than the openings of membrane channel proteins can easily clog channels, leading to the disruption of membrane permeability and transport. [4] However, the antimicrobial effectiveness of silver nanoparticles has been shown to decrease when dissolved in liquid media. [4]

The free silver ion are potentially toxic to bacteria and planktonic species in the water. [4] The positively charged silver ion can also attach to the negatively charged cell walls of bacteria, leading to deactivation of cellular enzymes, disruption of membrane permeability, and eventually, cell lysis and death. [4] However, its toxicity to microorganisms is not overtly observed since the free silver ion is found in low concentrations in wastewater treatment systems and the natural environment due to its complexation with ligands such as chloride, sulfide, and thiosulfate. [4]

Some occurrences of interactions of AgNPs in wastewater treatment systems are depicted. Silver Nanoparticle Interactions in Wastewater Treatment Systems.png
Some occurrences of interactions of AgNPs in wastewater treatment systems are depicted.

Wastewater treatment

A majority of silver nanoparticles in consumer products go down the drain and are eventually released into sewer systems and reach wastewater treatment plants. [5] Primary screening and grit removal in wastewater treatment does not completely filter out silver nanoparticles, and coagulation treatment may lead to further condensation into wastewater sludge. [2] The secondary wastewater treatment process involves suspended growth systems which allow bacteria to decompose organic matter within the water. [2] Any silver nanoparticles still suspended in the water may collect on these microbes, potentially killing them due to their antimicrobial effects. [2] After passing through both treatment processes, the silver nanoparticles are eventually deposited into the environment. [2]

A majority of the submerged portions of wastewater treatment plants are anoxic and rich in sulfur. [6] During the wastewater treatment process, silver nanoparticles either remain the same, are converted into free silver ions, complex with ligands, or agglomerate. [7] Silver nanoparticles can also attach to wastewater biosolids found in both the sludge and the effluent. [7] Silver ions in wastewater are removed efficiently because of their strong complexation with chloride or sulfide. [8]

A majority of the silver found in wastewater treatment plant effluent is associated with reduced sulfur as organic thiol groups and inorganic sulfides. [8] Silver nanoparticles also tend to accumulate in activated sludge, and the dominant form of the silver found in sewage sludge is Ag2S. [8] Therefore, most of the silver found in wastewater treatment plants is in the form of silver nanoparticles or silver precipitates such as Ag2S and AgCl. [7]

The amount of silver precipitate formed depends on silver ion release, which increases with increasing dissolved oxygen concentration and decreasing pH. [9] Silver ions account for approximately 1% of total silver after silver nanoparticles are suspended in aerated water. [9] In anoxic wastewater treatment environments, silver ion release is therefore often negligible, and most of the silver nanoparticles in wastewater remain in the original silver nanoparticle form. [9] The presence of natural organic matter can also decrease oxidative dissolution rates and therefore the release rate of free silver ions. [9] The slow oxidation of silver nanoparticles may enable new pathways for its transfer into the environment. [9]

Transformation in the environment

The silver nanoparticles that pass through wastewater treatment plants undergo transformations in the environment through changes in aggregation state, oxidation state, precipitation of secondary phases, or sorption of organic species. [10] These transformations can result in the formation of colloidal solutions. Each of these new species potentially have toxic effects which have yet to be fully examined. [10]

Most silver nanoparticles in products have an organic shell structure around a core of Ag0. [10] This shell is often created with carboxylic acids functional groups, usually using citrate, leading to stabilization through adsorption or covalent attachment of organic compounds. [10] In seawater, glutathione reacts with citrate [10] to form a thioester via esterification. [11]

Esterification reaction of citrate and glutathione Citrate and glutathione reaction.png
Esterification reaction of citrate and glutathione

Thioesters exhibit electrosteric repulsive forces due to amine functional groups and their size, which prevents aggregation. These electrostatic repulsive forces are weakened by counterions in solution, such as Ca2+ found in seawater. Ca2+ ions are naturally found in seawater due to the weathering of calcareous rocks, and allow for dissolution of the oxide-coated particle at low electrolyte concentrations. [6]

This leads to the aggregation of silver nanoparticles onto thioesters in seawater. [6] When aggregation occurs, the silver nanoparticles lose microbial toxicity, but have greater exposure in the environment for larger organisms. [6] These effects have not been completely identified, but may be hazardous to an organism’s health via biological magnification. [6]

Chemical reactions in seawater

Solubility Products (Ksp) of Silver-Containing Solids [12]
Ag2O4.00 x 10−11
Ag2CO38.46 x 10−12
AgCl1.77 x 10−10
Ag2S5.92 x 10−51
Ag2SO41.20 x 10−5

Silver nanoparticles are thermodynamically unstable in oxic environments. [5] In seawater, silver oxide is not thermodynamically favored when chloride and sulfur are present. On the surface where O2 is present in much greater quantities than chloride or sulfur, silver reacts to form a silver oxide surface layer. [13] This oxidation has been shown to occur in nanoparticles as well, despite their shell. [13]

Dissolution of Ag2O in Water:

Ag2O + H2O → 2Ag + 2OH [11] [13]

The nano-size of the particles aids in oxidation since their smaller surface area increases their redox potential. [14] The silver oxide layer easily dissolves in water because of its low Ksp value of 4×10−11. [14]

Possible Oxidation Reactions of Silver:

Ag + O2 → Ag+ + O2

4Ag + O2 → 4Ag+ + 2O2 [15]

In aerobic, acidic seawater, oxidation of Ag can occur through the following reaction:

Oxidation of Silver in Seawater:

2Ag(s) + ½ O2(aq) + 2H+(aq) ⇌ 2Ag+(aq) + H2O(l) [15]

The formation of these Ag+ ions are a concern for environmental health, as these ions freely interact with other organic compounds, such as humic acids, and disrupt the normal balance of an ecosystem. [15] These Ag+ ions will also react with Cl to form complexes such as AgCl2, AgCl32−, and AgCl43−, which are bioavailable forms of silver that are potentially more toxic to bacteria and fish than silver nanoparticles. [15] The etched structure of silver nanoparticles provides the chloride with the preferred atomic steps for nucleation to occur. [16]

Reaction of Silver with Chloride:

Ag+ + Cl → AgCl

AgCl(s) + Cl(aq) → AgCl2(aq) [16]

Ag has also been shown to readily react with sulfur in water. [17] Free Ag+ ions will react with H2S in the water to form the precipitate Ag2S. [17]

Silver and Sulfur Reaction in Seawater:

2Ag(aq) + H2S(aq) → Ag2S(s) + H2(aq) [18]

H2S is not the only source of sulfur that Ag will readily bind to. Organosulfur compounds, which are produced by aquatic organisms, form extremely stable sulfide complexes with silver. [18] Silver outcompetes other metals for the available sulfide, leading to an overall decrease in bioavailable sulfur in the community. [18] Thus, the formation of Ag2S limits the amount of bioavailable sulfur and contributes to a reduction in toxicity of silver nanoparticles to nitrifying bacteria. [13]

Effect on bacteria

Silver nanoparticles are experimentally shown to inhibit autotrophic nitrifying bacterial growth (86±3%) more than Ag+ ions (42±7%) or AgCl colloids (46±4%). [4] Silver nanoparticle-inhibited heterotrophic growth (55±8%) in Escherichia coli is best observed at lower concentrations, between 1.0 uM and 4.2 uM. [4] This is less than Ag+ ions (~100%), but greater than AgCl colloids (66±6%). [4] The actual cause of these results is undetermined as growth conditions and cell properties differ between nitrifying bacteria and heterotrophic E. coli . [4] Studies conducted in natural lake environments show less response from bacterioplankton than in laboratory environments when exposed to similar concentrations of silver nanoparticles. This may be due to the binding of free Ag+ ions to dissolved organic matter in lake environments, rendering the Ag+ unavailable. [19]

Within toothpaste, Ag+ ions have been shown to have a stronger effect on gram-negative bacteria than on gram-positive bacteria. [3] In comparison to other nanoparticles, such as gold, silver tends to have a broader antimicrobial effect, which is another reason why it is incorporated into so many products. [3] Ag+ is less effective on gram-positive bacteria due to the thick layer of peptidoglycan around them that gram-negative species lack. [3] Approximately half of the peptidoglycan wall is composed of teichoic acids linked by phosphodiester bonds, which results in an overall negative charge in the peptidoglycan layer. [20] This negative charge may trap the positive Ag+ and prevent them from entering the cell and disrupting the flow of electrons. [20]

Toxicology in aquatic environments

The most environmentally relevant species of these nanoparticles are silver chloride within marine ecosystems and organic thiols within terrestrial ecosystems. Once Ag0 enters the environment, it is oxidized to Ag+. [21] Of the potential species formed in seawater, such as Ag2S and Ag2CO3, AgCl is the most thermodynamically favored due to its stability, solubility, and the abundance of Cl in seawater. [21] Research has shown that partially oxidized nanoparticles may be more toxic than those that are freshly prepared. [4]

It was also found that Ag dissolutes more in solution when the pH is low and bleaching has occurred. [21] This effect, coupled with ocean acidification and increasing coral reef bleaching events, leads to a compounding effect of Ag accumulation in the global marine ecosystem. [21] These free formed Ag+ ions can accumulate and block the regulation of Na+ and Cl ion exchange within the gills of fish, leading to blood acidosis which is fatal if left unchecked. Additionally, fish can accumulate Ag through their diet. Phytoplankton, which form the base level of aquatic food chains, can absorb and collect silver from their surroundings. [22]

As fish eat phytoplankton, the silver accumulates within their circulatory system, which has been shown to negatively impact embryonic fish, causing spinal cord deformities and cardiac arrhythmia. [22] The other class of organisms heavily affected by silver nanoparticles is bivalves. [22] Filter feeding bivalves accumulate nanoparticles to concentrations 10,000 times greater than was added to seawater, and Ag+ ions are proven to be extremely toxic to them. [22]

The base of complex food webs consists of microbes, and these organisms are most heavily impacted by nanoparticles. [22] These effects cascade into the problems that have now reached an observable scale. [23] As global temperatures rise and oceanic pH drops, some species, such as oysters, will be even more susceptible to the negative impacts of nanoparticles as they are stressed. [23]

See also

Related Research Articles

<span class="mw-page-title-main">Silver</span> Chemical element, symbol Ag and atomic number 47

Silver is a chemical element; it has symbol Ag ) and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. The metal is found in the Earth's crust in the pure, free elemental form, as an alloy with gold and other metals, and in minerals such as argentite and chlorargyrite. Most silver is produced as a byproduct of copper, gold, lead, and zinc refining.

The term chloride refers either to a chloride ion, which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond. Many inorganic chlorides are salts. Many organic compounds are chlorides. The pronunciation of the word "chloride" is.

<span class="mw-page-title-main">Brine</span> Concentrated solution of salt in water

Brine is water with a high-concentration solution of salt. In diverse contexts, brine may refer to the salt solutions ranging from about 3.5% up to about 26%. Brine forms naturally due to evaporation of ground saline water but it is also generated in the mining of sodium chloride. Brine is used for food processing and cooking, for de-icing of roads and other structures, and in a number of technological processes. It is also a by-product of many industrial processes, such as desalination, so it requires wastewater treatment for proper disposal or further utilization.

<span class="mw-page-title-main">Hydrogen sulfide</span> Poisonous, corrosive and flammable gas

Hydrogen sulfide is a chemical compound with the formula H2S. It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. Swedish chemist Carl Wilhelm Scheele is credited with having discovered the chemical composition of purified hydrogen sulfide in 1777.

<span class="mw-page-title-main">Sodium hypochlorite</span> Chemical compound (known in solution as bleach)

Sodium hypochlorite, commonly known in a dilute solution as (chlorine) bleach, is an alkaline inorganic chemical compound with the formula NaOCl, consisting of a sodium cation and a hypochlorite anion. It may also be viewed as the sodium salt of hypochlorous acid. The anhydrous compound is unstable and may decompose explosively. It can be crystallized as a pentahydrate NaOCl·5H
2
O
, a pale greenish-yellow solid which is not explosive and is stable if kept refrigerated.

<span class="mw-page-title-main">Silver nitrate</span> Chemical compound

Silver nitrate is an inorganic compound with chemical formula AgNO
3
. It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called lunar caustic because silver was called luna by ancient alchemists who associated silver with the moon. In solid silver nitrate, the silver ions are three-coordinated in a trigonal planar arrangement.

Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. Sulfide also refers to large families of inorganic and organic compounds, e.g. lead sulfide and dimethyl sulfide. Hydrogen sulfide (H2S) and bisulfide (SH) are the conjugate acids of sulfide.

<span class="mw-page-title-main">Triclosan</span> Antimicrobial agent

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<span class="mw-page-title-main">Acid mine drainage</span> Outflow of acidic water from metal or coal mines

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<span class="mw-page-title-main">Silver chloride</span> Chemical compound with the formula AgCl

Silver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water and its sensitivity to light. Upon illumination or heating, silver chloride converts to silver, which is signaled by grey to black or purplish coloration in some samples. AgCl occurs naturally as a mineral chlorargyrite.

<span class="mw-page-title-main">Industrial wastewater treatment</span> Processes used for treating wastewater that is produced by industries as an undesirable by-product

Industrial wastewater treatment describes the processes used for treating wastewater that is produced by industries as an undesirable by-product. After treatment, the treated industrial wastewater may be reused or released to a sanitary sewer or to a surface water in the environment. Some industrial facilities generate wastewater that can be treated in sewage treatment plants. Most industrial processes, such as petroleum refineries, chemical and petrochemical plants have their own specialized facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the regulations regarding disposal of wastewaters into sewers or into rivers, lakes or oceans. This applies to industries that generate wastewater with high concentrations of organic matter, toxic pollutants or nutrients such as ammonia. Some industries install a pre-treatment system to remove some pollutants, and then discharge the partially treated wastewater to the municipal sewer system.

A silver chloride electrode is a type of reference electrode, commonly used in electrochemical measurements. For environmental reasons it has widely replaced the saturated calomel electrode. For example, it is usually the internal reference electrode in pH meters and it is often used as reference in reduction potential measurements. As an example of the latter, the silver chloride electrode is the most commonly used reference electrode for testing cathodic protection corrosion control systems in sea water environments.

<span class="mw-page-title-main">Silver chromate</span> Chemical compound

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<span class="mw-page-title-main">Triclocarban</span> Antimicrobial agent

Triclocarban is an antibacterial chemical once common in, but now phased out of, personal care products like soaps and lotions. It was originally developed for the medical field. Although the mode of action is unknown, TCC can be effective in fighting infections by targeting the growth of bacteria such as Staphylococcus aureus. Additional research seeks to understand its potential for causing antibacterial resistance and its effects on organismal and environmental health.

Green nanotechnology refers to the use of nanotechnology to enhance the environmental sustainability of processes producing negative externalities. It also refers to the use of the products of nanotechnology to enhance sustainability. It includes making green nano-products and using nano-products in support of sustainability.

<span class="mw-page-title-main">Silver nanoparticle</span> Ultrafine particles of silver between 1 nm and 100 nm in size

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<span class="mw-page-title-main">Gold cycle</span>

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