Stabilization/Solidification (S/S) Pb and W Contaminated Soils using Type I/II Portland Cement, Silica Fume Cement and Cement Kiln Dust

Stabilization/solidification (S/S) processes were utilized to immobilize lead (Pb) and tungsten
(W) in contaminated soils, the inclusion of W motivated by the use of the new W-based
ammunition. Artificially contaminated soils were prepared by mixing either kaolinite or
montmorillonite with 10% Pb and 1% W (all percentages by dry weight). Type I/II Portland
cement (PC), silica fume cement (SFC) and cement kiln dust (CKD) were used as S/S agents.
The S/S agents were added at 5, 10 and 15 % for a curing time of 1-, 7- and 28-days. The
toxicity characteristic leaching procedure (TCLP) and synthetic precipitation leaching
procedure (SPLP) were used to evaluate the effectiveness of the treatments. X-ray powder
diffraction (XRPD) was used to investigate the crystalline mineral phases responsible for Pb
and W immobilization. The TCLP results showed that regardless of clay type and stabilizing
agents, the Pb concentrations decreased with increasing pozzolan content. PC and SFC
exhibited similar performance depending on the particular sample (not consistent with soil
type, dosage, and curing time). The most effective stabilizing agent on Pb leachability was PC
despite SFC being silica-enriched which should have contributed to its greater immobilization
of Pb. TCLP-W was immobilized below 1 mg l-1 in every case. The most effective stabilizing
agent on the SPLP-Pb leachability was CKD, consistently demonstrating among the lowest
concentrations for each soil type due to pH control. PC and SFC exhibited similar
performance depending on the particular sample (not consistent with soil type, dosage, and
curing time). The W concentrations in SPLP leachate were very low in most samples
indicating that W could be immobilized upon S/S processes even though W solubility is very
high at elevated pH conditions (662.9 mg l-1 at pH~11), and would remain immobilized under
SPLP exposure conditions. The XRPD results revealed that the formation of lead silicate
(Pb4SiO6), stolzite (PbWO4) and lead tungsten oxide (Pb0.29WO3) were strongly associated
with the immobilization of Pb in the S/S matrix.