Groundwater Pollution Solidification and Stabilization

These slides are adapted from: 1.34 Waste Containment and Remediation Technology , As taught in: Spring 2004, by Dr. Peter Shanahan , MIT OpenCourseWare, Creative Commons License, http://ocw.mit.edu/OcwWeb/Civil-and-Environmental-Engineering/1-34Spring2004/CourseHome/ with added material from the EPA’s Citizen’s Guide Series

Solidification/stabilization (S/S) Solidification: covering waste in cement or other rock-like material Stabilization: mixing of stabilizer with waste to alter the chemistry of the waste and make it less toxic, less soluble, and/or less mobile.

Solidification/stabilization refers to a group of cleanup methods that prevent or slow the release of harmful chemicals from polluted soil or sludge. These methods usually do not destroy the chemicals - they just keep them from moving into the surrounding environment.

Solidification refers to a process that binds the polluted soil or sludge and cements it into a solid block. Stabilization refers to changing the chemicals so they become less harmful or less mobile. These two methods are often used together to prevent exposure to harmful chemicals.

Solidification/stabilization are used both in-situ and ex-situ. Their use ex-situ is most common.

S/S is the second most common source-control technology (U.S.)

Wastes treated by S/S

Select agents by bench-scale testing S/S agents Organic agents: Urea formaldehyde, polyethylene, bitumen, asphalt Inorganic agents: Cement Lime Pozzolans Proprietary mixtures and additives ($$$) Select agents by bench-scale testing

Reaction generates heat Examples: Pozzolans Pozzolan = alumino-silicate minerals that form cements when combined with lime and water Reaction generates heat Examples: Volcanic pumice (pozzolana) Kiln dust Fly ash

Inorganic agents are used more than organic agents Inorganic agents are used on for heavy metals, soils, sludges, radioactive waste Oil and grease, surfactants, chelating agents might prevent inorganic agents from working well. Inorganic agents are not likely to be effective with volatile organic compounds.

Ex-situ Solidification/stabilization Process

Ex-Situ Stabilization Screening soil before mixing in pug mill

In-situ methods include shallow soil mixing – to about 10 meters deep. Cost: ~ $50-80/m3

In-situ methods include deep soil mixing Cost: ~ $190-300/m3 Vacuum hoods may be needed to control vapor and dust. The volume increase is usually about 15%.

In situ S/S

Solidification/stabilization may take weeks or months to complete, depending on several factors that vary from site to site: • types and amounts of chemicals present • size and depth of the polluted area • types of soil and geologic conditions • whether the mixing occurs in place or in mixing tanks

Solidification/stabilization provides a relatively quick and low cost way to protect from the threat of harmful chemicals, especially metals. For example, soil polluted with metals can be mixed with lime (석회). The lime reacts with metals to form metal hydroxides. The metal hydroxides do not move as easily.

In-situ vitrification Formation of glass to encase waste Rarely used – most use at radioactive waste sites Cost at one Superfund site: $350/m3 (cost varies with cost of electricity)

Vitrification is a process that permanently traps harmful chemicals in a solid block of glasslike material. This keeps them from leaving the site. Vitrification can be done either in place or above ground.

Vitrification uses electric power to create the heat needed to melt soil. Four rods, called electrodes, are drilled in the polluted area. An electric current is passed between the electrodes, melting the soil between them. Melting starts near the ground surface and moves down.

As the soil melts, the electrodes sink further into the ground causing deeper soil to melt. When the power is turned off, the melted soil cools and vitrifies, which means it turns into a solid block of glass-like material.

When vitrified, the original volume of soil shrinks When vitrified, the original volume of soil shrinks. This causes the ground surface in the area to sink slightly. To level it, the sunken area is filled with clean soil.

Vitrification

The heat used to melt the soil can also destroy some of the harmful chemicals and cause others to evaporate. The evaporated chemicals rise through the melted soil to the ground surface. A hood, which covers the heated area, collects the gasses. These gasses are sent to a treatment system where they are cleaned up.

Any harmful chemicals that remain underground become trapped in the vitrified block, which is left in place. This prevents rainfall, groundwater flow, and wind from transporting the chemicals offsite.

In general, in situ vitrification is faster than most methods. The time it takes for in situ vitrification to clean up a site depends on several factors: • size and depth of the polluted area • types and amounts of chemicals present • how wet the soil is (wet soil must be dried, which takes more time) In general, in situ vitrification is faster than most methods. Cleanup can take from weeks to months, rather than years.

Vitrification can clean up several types of chemicals and soils Vitrification can clean up several types of chemicals and soils. By cleaning up soil in place, it avoids the expense of digging up soil or taking it to a landfill for disposal. Vitrification often is faster than other methods.

In-situ vitrification process Install surface electrodes Pass high electrical current through starter path of graphite and glass frit Starter path and then soils start to melt at 1600 to 2000°C Electrodes advanced through soil as molten mass enlarges Can melt about 1000 tons of soil per melt Melted soil hardens into monolithic, chemically inert vitreous slag.

Chemical containment Metal containment by chemical containment with organosulfur compound Marketed as MRC – Metals Remediation Compound Chemical first binds to metals Organic portion is then biodegraded leaving metal sulfide precipitate

Chemical containment