Rao, Pagilla, and Wagh
cement S/S is offset by poor waste loading efficiency and
potentially poor structural performance of waste forms
in disposal.
mercury phosphate is low in the groundwater pH range,
but not sufficiently low to meet the stringent universal
treatment standard (UTS) limit of 0.025 mg/L. The leach-
ing level of Hg can be further reduced by converting it
into HgS, which has an extremely low solubility prod-
uct.10
Brookhaven National Laboratory has developed two
thermoplastic processes for treating hazardous and mixed
waste.3 The modified sulfur cement waste forms in these
two processes can encapsulate up to 43-wt % incinerator
fly ash, while the maximum quantity of this waste in
hydraulic cement is 16 wt %. Although the comparison is
impressive, 57-wt % binder is required for waste S/S using
sulfur cement thermoplastic methods. This will not re-
duce the final volume of waste, as any increase in weight
has an associated volume increase.
The chemically bonded phosphate ceramic (CBPC)
technology, developed at Argonne National Laboratory
(ANL), has been very successful in the S/S of hazardous
and low-level mixed wastes that are part of the mixed
waste stream inventory at several DOE sites.4 This tech-
nology is based on synthetically produced, newberyite-
rich magnesium phosphate ceramic and is intended for
stabilization of the DOE’s low-level mixed waste streams.5
Since newberyite6 has high strength and low solubility
(10–6), the magnesium phosphate matrix can be a very
stable ceramic in an aqueous environment, hence mak-
ing it a good candidate for stabilizing waste streams. CBPC
is formed by an exothermic acid-base reaction between a
supersaturated solution of calcined magnesium oxide
(MgO) at a pH of 10.4 and a solution of phosphoric acid
(H3PO4) at a pH of 0. Free-energy calculations show an
estimate of heat evolution.
There has been no specific attempt in the past at vol-
ume reduction of waste using CBPC binders. So far, little
research has been done in finding a viable technology
that would ultimately decrease the final waste volume
even after the addition of binders. Researchers at Lawrence
Livermore National Laboratory conducted a comparative
cost analysis study in which low-level mixed waste was
compacted using various industrial compactors.11 Al-
though their study provided some insight into available
compaction systems, it did not include the stabilization
aspect of waste disposal.
The objective of this research was to develop a high
waste loading compaction technique for stabilizing the
most common waste streams such as ash and soil using a
CBPC binder, without compromising effective contami-
nant stabilization and high compression strength of waste
forms. Maximizing binder loading and reducing the final
volume of waste translates to shorter treatment time, an
increase in available landfill space, and ultimately big sav-
ings in transportation and disposal costs.
EXPERIMENTAL METHODS
In order to develop a viable S/S technology, bench-scale
samples were fabricated to determine a standard binder
composition and optimum waste loading for compacting
ash and soil waste. Brick-size samples were prepared us-
ing both a uniaxial press at ANL and a harmonic press at
Denver University. Bricks produced using both presses
were subjected to various tests in order to determine if
compaction method dictates the production of a good
waste form. Tests were conducted to evaluate their physi-
cal properties, compression strength, leach resistance (by
toxicity characteristic leaching procedure [TCLP]), and
long-term leaching resistance (using American Nuclear
Society’s ANS 16.1 test). The metals in the leachate were
determined by atomic adsorption spectroscopy (AAS).
Specific methods and materials are described in the fol-
lowing sections.
H3PO4 + MgO + 2H2O → MgHPO4·3H2O
∆G = –97.94 kJ/mol
(1)
In order to form newberyite, it is necessary to react a sto-
ichiometric proportion of 40.3 g MgO with 98 g H3PO4
and 36 g of water.
The most important fixation mechanism for metals
in S/S systems is chemical precipitation as low-solubility
species. During formation of chemically bonded phos-
phate ceramics, the contaminants are first converted into
their low solubility phosphates and then physically en-
capsulated in the magnesium phosphatematrix, yielding
excellent waste forms for hazardous and radioactive
waste.5 Removal of lead from wastewater has been most
efficient when it is precipitated as phosphate.7 Solubility
of metal sulfides and phosphates are much lower than
hydroxides.8 In fact, phosphates prove to be the least
soluble of the heavy metal species. An exception to this is
Hg, where HgS has a higher pKsp value (52) compared with
mercury phosphate (pKsp = 45). Typically, phosphates have
the ability to form insoluble complexes of hazardous
metals over a relatively wide pH range.9 The solubility of
Surrogate Waste Preparation
The surrogate waste was prepared in the laboratory based
on steps similar to those involved in the formation of
actual waste. The DOE surrogate ash waste composition
given in Table 1 closely represents one of the DOE ash
waste streams.1 It consists of fly ash, coal bottom ash, ver-
miculite, and activated carbon as its major constituents
and salts of certain metals as hazardous contaminants.
1624 Journal of the Air & Waste Management Association
Volume 50 September 2000