Soil WashingSoil 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.”

“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

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!!

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.

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

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

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 chargePoint of zero charge

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,...

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

EPA Mobile Soil-Washing System

Feed SoilFeed SoilWash WaterWash Water

Coarse FractionCoarse Fraction

Chemical AdditivesChemical AdditivesClean Water

Clean Produc

t

HydrocyclonesHydrocyclones

Drum Washer

Trommel

Fine FractionFine Fraction

StirredTank

StirredTank

StirredTank

StirredTank

StirredTank

StirredTank

StirredTank

StirredTank

Contaminant + waterContaminant + water

solubilize metalssolubilize metals

organic acidsorganic acids

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 ____ _________ organicslow solubilitylow solubility

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

Pros Cons

Synthetic Contaminated Soil (Zinc and Methylene Blue)

Zinc properties cation (positive charge)

Methylene blue properties cation organic hydrophobic can be oxidized

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

solubilizesolubilize solubilize/oxidizesolubilize/oxidize lots of ANC in soil

lots of ANC in soil

Would need to neutralize acidWould need to neutralize acid

??

solubilizesolubilize??

solubilizesolubilize

complex and solubilize

complex and solubilize

complex may sorb to soil

complex may sorb to soil

oxidizeoxidizevery reactive/

short lifevery reactive/

short life

remove solventremove solvent

Use biodegradable surfactants

Use biodegradable surfactants

Solubilize soil particles

Solubilize soil particles

ExpectationsExpectations

extractant Zn Methylene Blue problems Environmental impact

water

acid

organic solvent

surfactants

chelating agent

oxidant

extractant Zn Methylene Blue problems Environmental impact

water

acid

organic solvent

surfactants

chelating agent

oxidant

liquidliquid

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? ________________________

Contaminate it with known amount!Contaminate it with known amount!

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?