17    EFFICIENCIES OF SOIL WASHING SOLUTIONS FOR THE
REMEDIATION OF LEAD AND ORGANICALLY
CONTAMINATED SOILS
Steven R. Cline, Graduate Assistant
Brian E. Reed, Assistant Professor
Department of Civil Engineering
West Virginia University
Morgantown, West Virginia 26506-6101
Mark R. Matsumoto, Associate Professor
Department of Civil Engineering
State University of New York at Buffalo
Buffalo, New York 14260
INTRODUCTION
Heavy metals are often found in soils at high concentrations as a result of man's neglect for the
environment. Unlike many organic pollutants that are subject to biodegradation, heavy metals will
not degrade to harmless end products. Soil washing has recently become a popular ex-situ technique
for remediating sites contaminated with both organic and inorganic pollutants. In soil washing,
excavated soil is vigorously mixed with a solution(s) (typically water) that separates the contaminants
from the large particle size fractions. The recyclable washwater is then treated to remove the contaminants. Unlike most conventional soil washing processes, all particle size fractions were washed and
recovered during the course of this research.
Several retention mechanisms can be operative within a soil system. Cation exchange (weak outer
sphere complexation) and specific adsorption (strong inner sphere complexation) are two mechanisms
controlling metal adsorption. Heavy metals can also be retained by solid state diffusion and precipitation
reactions. Heavy metal retention has been found to generally increase with increases soil pH, CEC,
organic content, clay content, and the metal oxide content of the soil. While metal oxides can be both
crystalline and amorphous in nature, amorphous metal oxides such as allophanes usually sequester heavy
metals to a larger extent than do crystalline metal oxides such as geothite. The strength of metal retention
generally increases as the initial concentration of the contaminant decreases. The average binding
strength of a soil decreases once the high energy binding sites are selectively filled first.
This research focused on lead contamination, since lead is one of the most common inorganic
contaminants at Superfund sites across the nation,1 and most soil types have a particular affinity for
lead. It is plausible to expect washing solutions that are capable of removing lead could also remove
other divalent heavy metals. Lead is commonly found at sites where past industrial activities include
battery breaking and recycling, oil refining, paint manufacturing, metal molding and plating, and
smelting.1 Lead pollution from the automobile industry is also extensive. Eight soils were characterized and contaminated with various lead concentrations. Some samples were also contaminated with
TCE (200 mg/kg) or naphthoic acid (1,000 mg/kg) to study the effects of organic contamination upon
lead removal from contaminated soils. The efficiencies of six washing solutions in removing lead from
the contaminated soils were investigated via lab-scale batch washing experiments. Kinetic studies were
also performed to determine the time required for the samples to reach quasi-equilibrium conditions.
RESEARCH OBJECTIVES
Four research objectives were defined at the start of the experimental study:
1. Determine the lead retention capacity of each study soil.
2. Identify parameters controlling the retention and release of lead from soils.
3. Identify the washing solution(s) most effective in removing lead from soils.
4. Identify the soil type(s) most amenable to treatment by soil washing.
48th Purdue Industrial Waste Conference Proceedings, 1993 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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