Silver Nanotechnologies and the Environment: Old Problems or New Challenges Samuel N' Luoma

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Silver Nanotechnologies and the Environment Old
Problems or New ChallengesSamuel N. Luoma

• An overview presented by
• Todd Kuiken, Ph.D.
• Woodrow Wilson International Center for Scholars
• Project on Emerging Nanotechnologies

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Silver Basics

• Extremely rare element in the Earths 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

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Silvers 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

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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
• Hundreds of nanosilver products are currently on
  the market, and their number is growing rapidly

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Why Silver?

• Effectiveness in killing a wide range of bacteria
• 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

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Chemistry and Toxicity
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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
• The strong reactions of silver with free
  sulfides, dissolved organic materials and
  chloride can reduce silver availability to near
  zero in freshwaters

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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
• Risk assessments should consider the long-term
  implications of accumulation, storage,
  remobilization, form and bioavailability from
  sediments

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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

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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

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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
• Its 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

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Mitigated Risks or Trojan Horse

• Its 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

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A better approach

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

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...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
• 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

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Environmental Risks

• Pathways into and effects on the environment

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Ecological Hazards

• A chemical or particles 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

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Bioavailability

• Strongly influenced by the form of silver
• Microscopic plants at the bottom of the food web
  have bioaccumulation rates between 10,000-70,000
  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

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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.
• The concentration in the environment determines
  the impact
• Concentrations in the environment are low and
  obtaining reliable data on environmental trends
  is difficult

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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

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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)

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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

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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 EPAs standard is a 10 to 100 fold
  underestimation of the silver toxicity threshold
  for many natural waters, particularly estuaries,
  coastal zones and the oceans

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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

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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

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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

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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

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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
• Aggregate
• Dissolve or
• React with natural materials like dissolved
  matter or natural particulates

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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

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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.

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The way forward
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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

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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?
• How we are to generate it?
• Identify basic research needs and immediate
  opportunities

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Priority research goals

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

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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

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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 products
  ingredients that take into account its unique
  physical and chemical attributes

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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

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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

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Thank You
Todd Kuiken, Ph.D. Woodrow Wilson International
Center for Scholars Project on Emerging
Nanotechnologies todd.kuiken_at_wilsoncenter.org 202-
691-4398