Soil Washing

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Soil Washing

• There is a need for increased use of new
  separation technologies (such as soil washing)
  that reduce the quantity of waste requiring
  solidification/stabilization, or allow the
  recycling of valuable metals.

--EPA 1993
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Need for the Technology

• EPA estimates that over 20 million cubic yards of
  soil at current NPL sites (national priority
  list) are contaminated with metals
• DOE (Department of Energy) estimates 10s of
  millions of cubic yards
• Leaking Underground Storage Tanks (UST)
  contribute 56 million cubic yards
• DOD (Department of Defense) ?
• Volume reduction technologies!!

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DOE
Containing the Cold War Mess Restructuring the
Environmental Management of the U.S. Nuclear
Weapons Complex Marc Fioravanti Arjun Makhijani,
Ph.D. October 1997

• The nuclear arms race and its aftermath have
  created the largest and most complex problem of
  environmental remediation and waste management in
  U.S. history
• The problem is so complicated and costly that
  there is a tendency in the nuclear establishment
  to simply bury the problem, literally and
  figuratively, creating what have been called
  national sacrifice zones
• The current Department of Energy (DOE) best
  estimate for partial environmental restoration
  and waste management and disposal is 227 billion
  over a 75-year period
• 90 to 400 per cubic yard operating cost

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DOE Waste Management

• LLW Low-Level WasteMLLW Mixed Low-Level
  WasteTRU TransuranicHLW High-Level
  WasteSNF Spent Nuclear FuelGCD Greater
  Confinement DisposalNTS Nevada Test SiteLANL
  Los Alamos National LaboratoryORNL Oak Ridge
  National Laboratory

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Soil Washing

• A technology for volume reduction of contaminated
  soil
• Potentially removes mixed wastes from
  contaminated soil so the soil can be returned to
  the original site
• References
• Griffiths, R. A. 1995. Soil-washing technology
  and practice Journal of Hazardous Materials
  40(2) 175-189.
• Semer, R. and K. R. Reddy. 1996. Evaluation of
  soil washing process to remove mixed contaminants
  from a sandy loam Journal of Hazardous Materials
  45(1) 45-57.

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Examples of Contaminated Waste Sites with Mixed
Wastes

• DOE/DOD sites with radioactive metals plus
  organic scintillation cocktails.
• Cornell chemical dump near airport organic
  solvents, metal salts, ... all dumped in close
  proximity to each other.
• Coal gas plants
• pyrolysis was used to get coal gas (for street
  lamps)
• also produced coal tars containing polycyclic
  aromatic hydrocarbons (PAHs) and coal ash that
  has high metal concentration.
• Printers ink contained both organic dyes and lead
• Dumps

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Possible Site Management Strategies

• Seal off site (clay barriers, concrete cover,
  fences to keep people and animals out...)
• Remove contaminated soil - huge volumes!
• Remove contaminants using an in-situ separation
  process
• pump and treat
• in situ bioremediation
• electro-chemical remediation
• Remove contaminants from excavated soil
• soil washing, bioremediation, thermal desorption

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Pump and Treat Limitations

• Pollutants absorb (within), adsorb (on surface),
  sorb (dont know precise mechanism), or attach to
  the soil. High retardation factors. Immobile.
• Soil properties
• negatively charged at neutral pH
• SiO2 has a PZC of pH 2-3 (at this pH enough
  hydrogen ions have reacted with the surface to
  make it neutral). At lower pH the surface would
  be positively charged.
• Therefore cations (metals) easily bind to the
  negatively charged soil.

Point of zero charge
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Soil Washing

• Separation of fine soil particles from larger
  soil particles
• contaminants adhere to particle surfaces
• small particles have more surface area/unit mass
• removal of fines from a contaminated soil also
  removes most of the contaminants
• Separation of contaminants from the fines
• solubilize contaminants in the wash water
• surfactants, acids, bases, chelating agents,
  alcohols,...

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Soil Washing - Process Description

• Excavate contaminated soil
• Remediate the contaminated soil
• Remove large debris or particles larger than 2
  in.
• Separate all contaminants from the soil
• remove sand after initial water wash
• silt/clay fraction requires further treatment
• Treat or dispose of residues
• Return soil

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EPA Mobile Soil-Washing System
Feed Soil
Drum Washer
Contaminant water
Wash Water
Coarse Fraction
Trommel
Hydrocyclones
Fine Fraction
Stirred Tank
Stirred Tank
Stirred Tank
Stirred Tank
Clean Product
Chemical Additives
Clean Water
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Wash Water Additives

• Generally undesirable
• complicate recycling or disposal of wash water
• additional unit processes needed to remove
  additives
• Additives are contaminant specific
• acids and chelating agents ________ _______
• bases improve extraction of _______ _____
• surfactants and organic-solvents improve
  extraction of ____ _________ organics

solubilize metals
organic acids
low solubility
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Soil Washing Pros and Cons
Pros
Cons

• closed system controlled conditions
• significant volume reduction of contaminated soil
• applicable to varied waste groups
• hazardous waste remains on site
• lower cost than removing contaminated soil

• no reduction in contaminant toxicity if only
  physical separation is used
• potentially hazardous chemicals used that may be
  difficult to remove from soil
• effectiveness limited by
• complex waste mixtures
• high humic content in soils
• undesired solvent-soil reactions (ex. soil ANC)
• high fine-grained clay content

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Synthetic Contaminated Soil (Zinc and Methylene
Blue)

• Zinc properties
• cation (positive charge)
• Methylene blue properties
• cation
• organic
• hydrophobic
• can be oxidized

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Remediation Strategies

• Zinc
• Neutralize the negative soil charge with acid
• Solubilize the metals with chelating agents
• Methylene blue
• Neutralize the negative soil charge with acid
• Oxidize methylene blue with acid or an oxidant
  such as chlorine, ozone, or hydrogen peroxide
• Solubilize with a surfactant (soap) or with a
  solvent

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Expectations

• extractant Zn Methylene Blue problems Environmenta
  l impact
• water
• acid
• organic solvent
• surfactants
• chelating agent
• oxidant

lots of ANC in soil CO2
Would need to neutralize acid
solubilize
solubilize/oxidize
remove solvent
solubilize
?
Use biodegradable surfactants
Solubilize soil particles
?
solubilize
complex and solubilize
complex may sorb to soil
very reactive/ short life
oxidize
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Analytical Methods

• Methylene Blue
• UV-Visible Spectrophotometer
• Zinc
• Atomic Absorption Spectrophotometer
• Each method requires that the compound be in the
  ______ phase!
• Extraction from the soil is required for the
  analysis!
• How can we know how much contaminant is on the
  soil initially? ________________________

liquid
Contaminate it with known amount!
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Soil WashingPotential Experiment Objectives

• Effect of extractant concentration
• Optimize extractant dose (consider stoichiometry)
• Effect of soil type
• Are organic contaminants more difficult to remove
  if soil contains more organics?
• Effect of multiple extractants
• Acid followed by surfactant or chelating agent
• Effect of a series of extractions (mimics real
  system)
• Can additional contaminant be removed by a series
  of extractions?

Distilled water Sodium Dodecyl Sulfate Triton
X-100 Acetone HCl NaOH EDTA