Page 546 - Materials Chemistry, Second Edition

CAT3525_C16.qxd 1/27/2005 12:42 PM Page 517
Hazardous Waste Treatment 517
stabilization serves to physically sorb, encapsulate, or alter the physical or chemical form of the
contaminants, producing a less leachable material.
Stabilization techniques also improve the handling characteristics of the waste for transport on-
site or to an off-site TSDF. Stabilization techniques designed to limit the solubility or mobility of
hazardous constituents are required for RCRA hazardous wastes containing heavy metals.
The ideal stabilization agent is inert, readily available, and nondegradable. In selecting a stabi-
lization agent, considerations should include the quantity required to eliminate free liquid, compat-
ibility with the waste, and binding properties.
Pozzolanic materials such as fly ash form a solid monolithic mass when mixed with hydrated
lime. Stabilization of waste using lime and pozzolanic materials requires that the waste be mixed
with water to create an optimal consistency. Numerous treatment processes incorporate Portland
cement as a binding agent along with pozzolanic materials to improve the strength and chemical
resistance of the solidified waste. Soluble silicates may be added to a pozzolan–cement mixture to
further enhance performance and to reduce interference from metals. Emulsifiers may be added to
better incorporate organic liquids. Solidification and fixation processes are generally adjusted for
the waste on a case-by-case basis.
To reduce the final volume of stabilized waste for disposal, wastes should be dewatered before
stabilization reactions. Pretreatment, such as a chemical treatment to scavenge toxic materials, may
contribute to more cost-effective treatment of the waste.

EXAMPLE 16.4

An aqueous sludge containing high amounts of free liquids is stabilized with a cement–slag mix-
ture at a secure landfill. The wastes are mixed with the cement–slag using a front-end loader. The
blended material is then transported to the secure landfill and spread in uniform lifts with a bull-
dozer. Waste properties are as follows: 45 mg/kg Cd, 1055 mg/kg Pb, and 575 mg/kg Zn. In order
to optimize stabilization, the required ratio of fly ash to waste is estimated at 1.5:1. Calculate the
reduction in contaminant concentrations due to dilution.

SOLUTION

C W C (W W ) (adapted from LaGrega et al., 1994) (16.17)
FA
o
w
f
w
Where C is the original containment concentration, W the weight of waste, C the final concen-
o
f
w
tration, and W FA the weight of fly ash.
C /C (W W )/W (1.0 1.5)/1 2.5/1 2.5
f
w
FA
o
w
C /C W /(W W ) 1.0/(1.0 1.5) 1/2.5 0.40
w
FA
f
o
w
There is a reduction of 60% of contaminant concentrations.
A major drawback to stabilization processes is the significant increase in the volume of mate-
rial to be disposed. Waste mixtures may require several pretreatment steps that might make the
process cost-prohibitive (Wentz, 1995).
REFERENCES
Blackman, W.J. Jr., Basic Hazardous Waste Management, 3rd ed., Lewis Publishers, Boca Raton, FL, 2001.
Camp, Dresser and McKee, Inc., Technical Assessment of Treatment Alternatives for Wastes Containing
Corrosives, Prepared for U.S. EPA under contract No. 68–01–6403, Boston, MA, September 1984.
Dawson, G.W., and Mercer, B.W., Hazardous Waste Management, Wiley, New York, NY, 1986.