Stabilization/solidification (S/S) of Pb and W contaminated soils using type I/II portland cement, silica fume cement and cement kiln dust

Abstract

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. © 2009 Global NEST.