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

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 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 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

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

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

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 Oxidize methylene blue with acid or an oxidant such as chlorine, ozone, or hydrogen peroxide Solubilize with a surfactant (soap) or with a solvent

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

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!

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?