Silver Nanotechnologies and the Environment: Old Problems or New Challenges Samuel N. Luoma An overview presented by Todd Kuiken, Ph.D. Woodrow Wilson International Center for Scholars Project on Emerging Nanotechnologies

Silver Basics Extremely rare element in the Earth’s crust Background concentrations are extremely low Addition of only a small mass of silver to a water body will result in proportionally large deviations from natural conditions

Silver’s regulated history Silver is designated as a priority pollutant by EPA Added to the priority pollutant list in 1977 Based upon its persistence in the environment and high toxicity to some life forms Released to natural waters from photographic facilities, smelters, mines or urban wastes

Commercial Use of Nanosilver One of the most rapidly growing classes of nanoproducts Silver is used in more manufacturer identified consumer products than any other nanomaterial Silver is used in more manufacturer identified consumer products than any other nanomaterial Hundreds of nanosilver products are currently on the market, and their number is growing rapidly

Why Silver? Effectiveness in killing a wide range of bacteria Including some of the strains that have proven resistant to modern antibiotics Including some of the strains that have proven resistant to modern antibiotics Can be readily incorporated into plastics, fabrics and onto surfaces Delivers toxic silver ions in large doses directly to sites where they most effectively attack microbes

Chemistry and Toxicity

Silver Chemistry Speciation has the greatest influence on how much silver is available to affect living organisms When an abundance of chloride atoms are available silver precipitates out of the water column as silver chloride Making it unavailable for uptake by organisms Making it unavailable for uptake by organisms The strong reactions of silver with free sulfides, dissolved organic materials and chloride can reduce silver availability to near zero in freshwaters

Silver Chemistry As silver precipitates out of solution it can accumulate in sediments Geochemical reactions bind more silver ions to particulate matter than silver in solution Between 10,000 and 100,000 ions of silver bind with PM for every ion that remains in solution Between 10,000 and 100,000 ions of silver bind with PM for every ion that remains in solution Risk assessments should consider the long-term implications of accumulation, storage, remobilization, form and bioavailability from sediments

Environmental Toxicity Ionic silver is one of the most toxic metals known to aquatic organisms Persists and accumulates to elevated concentrations in water, sediments, soils and organisms where human wastes are discharged Silver contamination in water and mud corresponds strongly with ecological damage

Factors that affect toxicity Ability to be taken inside cells Tendency to bind to biological sites that perform important functions Degree to which the metal is excreted Degree to which the metal is sequestered in nontoxic forms inside cells

Mitigated Risks or Trojan Horse Silver ions tend to form strong complexes that lower their bioavailability and toxicity Complexes with sulfides strongly reduce bioavailability under some circumstances Complexes with sulfides strongly reduce bioavailability under some circumstances It’s not clear how this will affect the toxicity of nanosilver It’s not clear how this will affect the toxicity of nanosilver If organic/sulfide coatings or complexation in natural waters reduce bioavailability of nanosilver particles, risks to natural waters will be reduced

Mitigated Risks or Trojan Horse It’s possible nanoparticles shield silver ions from complexation reactions which then can deliver free silver ions to membranes of organisms or into cells Accentuation of environmental risks is therefore greater compared to a similar mass of silver itself This Trojan Horse mechanism is an important area of future research

A better approach… Interdisciplinary study is essential Nanoparticles can aggregate or change form during experiments affecting exposure and effects Nanoparticles can aggregate or change form during experiments affecting exposure and effects Nanoparticles need to be physically characterized and; Nanoparticles need to be physically characterized and; any effects of residual chemicals added to promote stability be understood any effects of residual chemicals added to promote stability be understood

...a better approach Studies with whole living organisms remain rare in the study of nanoparticles In vitro tests with isolated cells is a powerful tool to address mechanisms and likelihood of toxicity In vitro tests with isolated cells is a powerful tool to address mechanisms and likelihood of toxicity It cannot address dose response It cannot address dose response Realistic in vivo tests are necessary to determine what concentrations in nature will be toxic Methodologies exist that fully examine a stage in the life cycle or exposure from diet

Environmental Risks Pathways into and effects on the environment

Ecological Hazards A chemical or particle’s ecological hazard is determined by its persistence, its tendency to bioaccumulate and its toxicity Silver is persistent in the environment and is one of the most toxic of the trace metals to many species Has a tendency to bioaccumulate to high concentrations in bacteria, humans and other organisms It is biomagnified to higher concentrations in predators than in their prey

Bioavailability Strongly influenced by the form of silver Microscopic plants at the bottom of the food web have bioaccumulation rates between 10, ,000 times the concentration of the water Uptake rates of silver are exceeded only by mercury among metals High concentrations of silver will occur at the base of food webs wherever silver contamination occurs in estuaries, coastal waters or the ocean

Uses and Form Make a Difference Different uses release silver in different forms and varying quantities Complex geochemical reactions determine how those releases translate into concentrations in food, water, sediments etc. Complex geochemical reactions determine how those releases translate into concentrations in food, water, sediments etc. The concentration in the environment determines the impact Concentrations in the environment are low and obtaining reliable data on environmental trends is difficult Concentrations in the environment are low and obtaining reliable data on environmental trends is difficult

A picture speaks a thousand words Traditional photography established a precedent for how a silver-based technology could constitute an environmental risk Small amounts used by millions of people Release of silver to waste streams was the primary cause of silver contamination in water bodies

Silver Concentrations in Water Most trace metals in water are reported in the parts per billion (ppb) or micrograms per liter (µg/L) Silver is reported in parts per trillion (ppt) Silver is reported in parts per trillion (ppt)

Water Quality Standards U.S. for streams and coastal waters are set between 1,920-3,200 ng/L The European Union does not list silver among its 33 designated priority hazardous pollutants Levels are much higher than were found in even the most contaminated open waters

Water Quality Standards Bielmyer et al. (2006) suggested that water quality standards are well above the concentration at which toxicity occurs in zooplankton Suggested that EPA’s standard is a 10 to 100 fold underestimation of the silver toxicity threshold for many natural waters, particularly estuaries, coastal zones and the oceans

Mass Discharges to the Environment It is not clear whether silver lost from products will be nanosilver or silver itself Estimates are based on the assumption that the baseline risk is from silver metal Additional risks will occur if nanosilver is more toxic than silver metal

Mass Discharges to the Environment: Factors to Consider Nature of the potential sources Number of sources and potential for growth Potential for dispersal to the environment Concentration of silver associated with each source

Removal from wastewater Its argued that most nanosilver will be removed from wastewaters and deposited in sludges by waste treatment Silver concentrations in discharges correlate with silver in the incoming wastewater

Removal from wastewater Sewage treatment helps, but it is not a cure all for environmental risk if incoming loads are large enough The degree to which nanoparticles containing silver might be captured by wastewater treatment is unknown

Pathways to the Environment Nature and form of nanosilver could influence its’ fate and implication to the environment Silver nanoparticles may: Stay in suspension as individual particles Stay in suspension as individual particles Aggregate Aggregate Dissolve; or Dissolve; or React with natural materials like dissolved matter or natural particulates React with natural materials like dissolved matter or natural particulates

Potential Environmental Risks If single nanoparticles in suspension prove to be a form of high toxicity then their persistence will affect its ranking as an environmental hazard Once silver nanoparticles enter aquatic environments they are subject to reactions in that environment indefinitely Longer the particle or traits that aid dispersal resist such reactions, the greater the buildup of such forms in natural waters

Factors that should be considered when evaluating environmental risks Sources of nanosilver must be understood in order to manage risks Concentrations in the environment determine risk The pathways of nanosilver in the environment also influence risk Receptor: Bioavailability of nanosilver is a crucial consideration in determining impacts Impact: Toxicity is determined by the internally accumulated, bioavailable nanosilver in each organism Impact: Effects on ecological structure and function are determined by how many and what kinds of organisms are most affected by nanosilver at the bioavailable concentrations that are present in the environment.

The way forward

Nanosilver raises new questions A research strategy is necessary to address them Questions will need long-term exploratory research before answers are found Opportunities exist to address other questions in a timelier manner If research is strategically targeted If research is strategically targeted

What is the strategy? Neither a bottom-up, principal investigator- led research nor a top-down wish list of research needs is likely to result in adequately targeted studies What knowledge is needed? What knowledge is needed? How we are to generate it? How we are to generate it? Identify basic research needs and immediate opportunities Identify basic research needs and immediate opportunities

Priority research goals An agenda that addresses these four areas would quickly position better understanding and regulation of the impact of nanosilver Source Source Pathways Pathways Receptor Receptor Impact Impact Significant investment will be necessary to address just the immediate opportunities available to better manage this one set of nanoproducts

Where research and policy connect Integrate nanosilver risk research needs into a unified, multi-agency, stakeholder- vetted nanotech dialogue Assign responsibilities, resources and timelines for implementing the research strategy, and clearly identify mechanisms that will lead to better and more effective translation of the new knowledge into decision making

Where research and policy connect Integrate research among international research programs to leverage resources and ensure timely and relevant progress Develop and share appropriate terminologies to underpin research and oversight Define clear rules for defining a product’s ingredients that take into account its unique physical and chemical attributes

Where research and policy connect Assess what information is needed to oversee safe use of nanosilver, over and above that for managing the impact of ionic silver Assess the relevance and shortcomings of current silver-relevant regulations

Final thoughts Existing knowledge provides a powerful baseline from which to identify research priorities and begin making scientifically defensible policy decisions The sophisticated advances in engineering nanosilver products have created new challenges to accompany the new products All institutions must rise to the challenge if we are to see the benefits these new technologies promise

Thank You Todd Kuiken, Ph.D. Woodrow Wilson International Center for Scholars Project on Emerging Nanotechnologies