Hazardous or toxic waste destruction or containment – Containment – Solidification – vitrification – or cementation
Reexamination Certificate
1999-07-16
2001-09-18
Langel, Wayne (Department: 1754)
Hazardous or toxic waste destruction or containment
Containment
Solidification, vitrification, or cementation
C405S129250, C405S129650, C405S129950, C588S252000, C588S259000
Reexamination Certificate
active
06290637
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to disposal and storage of contaminated waste materials. More particularly the invention relates to using reactive barriers as containment systems for contaminated waste materials. Most particularly the invention relates to phosphate mineral based reactive barrier containment systems that use the targeted waste materials to form and seal the barrier by chemically and physically reacting with the reactive barrier material.
BACKGROUND OF THE INVENTION
Approximately 400 million cubic yards of sediment are dredged from harbors and waterways in the U.S. each year and up to 12 million cubic yards of very contaminated sediments are handled with special remediation strategies. Heavy metals are one of the most frequently reported contaminants and are problematic with respect to dredge material management. They impact sediment restoration activities throughout the U.S. The lack of available disposal space, the presence of multiple contaminants, and the lack of cost efficient technologies to treat the materials imposes a bottleneck on dredging operations, thus impacting navigation in harbors and waterways. The continued pressing needs for navigable waterways means that innovative dredge material management strategies are needed so that dredging can occur in an environmentally beneficial and cost effective manner.
Disposal sites are used for either the temporary or permanent disposal of contaminated sediments and dredged materials. Until recently, ex situ or terrestrial-based confined disposal systems (landfills) had been widely used for contaminated sediment and dredged materials disposal. However, difficulties in siting landfills as well as the premium placed on the disposal of municipal, industrial, and hazardous wastes in landfills, means that consideration is again being given to coastal, near-shore, off-shore or subsurface disposal in confined disposal sites.
FIG. 1
(from NRC, 1997) depicts some of the types of disposal sites that are currently used for waste containment and disposal. These facilities are designed to meet storage requirements for the contaminated materials and to ensure the control of contaminant release. Each is described below:
Terrestrial landfills
A terrestrial landfill involves the placement of contaminated sediments within an engineered disposal site featuring an hydraulically impervious liner system (either clay or high density polyethylene liners). The landfill, when closed, is then capped with a hydraulically impervious cap system. Traditional liners and cap systems are know to work well. However, over time they may be subjected to geotechnical stresses or environmental deterioration that cause the liners or caps to fail, or be compromised in some other way.
Confined Disposal Sites
Confined disposal sites involve the placement of contaminated sediments within diked near shore, island, or land-based disposal systems. The deposit final grade is above high water (tidal or storm). Materials are typically transported to this site for disposal. Typically, confinement is achieved by the use of retention dikes or structures that enclose the disposal area so that the contaminants are isolated. In some cases man-made islands are made with these systems. Retention dikes have performed well in the estuarine environment. However, they may be subjected to geotechnical stresses or erosion that cause the dikes to lose integrity, allowing release of contaminated materials.
Confined Aquatic Disposal
Confined aquatic disposal involves the placement of contaminated sediments or dredged materials at an open water location within engineered disposal site within an underwater dike or berm system, or in a natural depression, where the system is lined with containment systems. Materials are typically transported to this site for disposal. The system, when closed, is then capped with a with a top containment system. Although the performance of subsurface disposal systems has been generally adequate, the system may be subjected to geotechnical stresses, biological perturbations, or environmental deterioration that causes the containment system to fail.
In-Situ Capping
In-situ capping involves the covering of contaminated sediments (or historically dredged materials) that are left in place in underwater environments. They are not typically moved to another disposal location. Here, the capping concept is used just to cover the deposit. The deposit is left in place. Traditional subsurface caps have usually worked well. However, they may be subjected to geotechnical stresses, or biological perturbations that cause the cap to fail, or be compromised in some other way.
Chemical Reaction
Chemical stabilization of inorganic waste materials offers the potential to reduce the leachability of heavy metal contaminants present in the waste. The principal objective during stabilization is to precipitate new solid mineral phases with both reduced solubilities and increased geochemically stability with respect to solution phase ligands, pH, or redox (E
h
). One stabilization agent of recent interest for heavy metals is orthophosphate: PO
4
3−
. Phosphate combines with over 30 elements to form about 300 naturally-occurring minerals. Metal phosphates are ubiquitous secondary minerals in the oxidized zones of lead ore deposits and as assemblages around ore bodies. They also occur in soils, sediments, and phosphatic or phosphorite beds. As such, they are very stable geochemically with respect to pH, E
h
, and mineral authigenesis. Isomorphic substitutions are common in nature for these phosphate minerals for both divalent cations (e.g. Pb
2+
for Ca
2+
) and oxyanions (e.g. AsO
4
3−
for PO
4
3−
). They are also very insoluble minerals. Notable among the phosphate minerals are the apatite family of minerals; e.g., Ca
5
(PO
4
)
3
F (fluoroapatite), Ca
5
(PO
4
)
3
OH (hydroxyapatite), Ca
10
(PO
4
,CO
3
)
6
(OH)
2
(carbonate apatite), etc.
Past research efforts have shown that phosphate minerals, including apatites, are likely controlling solids for Ca
2+
, Cd
2+
, Cu
2+
, Pb
2+
and Zn
2+
in soil systems. As controlling solids, these mineral phases are both stable geochemically and, by virtue of their insolubility, able to control the aqueous concentration of their heavy metal constituents at very low levels.
The use of orthophosphate to immobilize metals has been advocated for industrial wastewaters, and metal-bearing industrial wastes such as municipal solid waste combustion residues. Both soluble orthophosphate and phosphate-containing minerals have been promoted as sources of orthophosphate for the stabilization process.
Possible stabilization mechanisms can involve a continuum from surface sorption processes to existing particulate surfaces in a waste material, through the formation of surface precipitates, to the formation of discrete heterogeneous or homogeneous precipitates.
Mechanisms
To date, heavy metals have been successfully chemically stabilized in terrestrial environments in soils, mining wastes, and industrial wastes using orthophosphate (PO
4
3−
) as a chemical stabilization agent. This process is used commercially in many venues in both the U.S. and abroad at low cost. Phosphate can react with a many heavy metals (e.g. Cd, Cu, Ni, Pb, Zn) and metalloids (e.g. AsO
4
3−
) to precipitate out and form Ca-based apatite family minerals (e.g. Pb
5
(PO
4
)
3
Cl or Ca
5
(AsO
4
)
3
Cl). Further, many marine phosphorites, which a phosphate-based and largely contain apatite-family minerals also contain heavy metals such as Pb and Cd which have substituted for the Ca in the crystal lattice.
The apatite family of minerals is well documented. In nature, the apatite mineral structure conforms to the 6/m class of mineral with hexagonal crystal structure and the generic formula Me
5
(XO
4
)
3
Z where Me is Ca, Sr, Ba, Cd, and Pb (typically), X=P, As, V, Mn, and Cr; and Z=OH, F, Cl, and Br. The propensity to form very insoluble apatite family minerals with end members that contain
Devine, Millimet & Branch P.A.
Kohler, Esq Kristin
Langel Wayne
Remus, Esq Paul C.
University of New Hampshire
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