1. 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

2. 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 3.3 to 200 million cubic yards
Leaking Underground Storage Tanks (UST) contribute 56 million cubic yards
DOD (Department of Defense) ?
Volume reduction technologies!!

3. 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 “Soil-washing technology and practice” Journal of Hazardous Materials 40(2):
Semer, R. and K. R. Reddy “Evaluation of soil washing process to remove mixed contaminants from a sandy loam” Journal of Hazardous Materials 45(1):

4. 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

5. 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

6. Pump and Treat Limitations
Pollutants absorb (within), adsorb (on surface), sorb (don’t 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

7. 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,...

8. 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

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

10. 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

11. Soil Washing: Pros and 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

12. Synthetic Contaminated Soil (Zinc and Methylene Blue)
Zinc properties
cation (positive charge)
Methylene blue properties
cation
organic
hydrophobic
can be oxidized

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

14. Expectations
extractant Zn Methylene Blue problems Environmental impact
water
acid
organic solvent
surfactants
chelating agent
oxidant
lots of ANC in soil
Would need to neutralize acid
solubilize
solubilize/oxidize ?
solubilize
remove solvent
Solubilize soil particles
Use biodegradable surfactants ?
solubilize
complex and solubilize
complex may sorb to soil
very reactive/ short life
oxidize

15. Contaminate it with known amount!
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!

16. Soil Washing: Potential 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?