Toxicity Evaluation of Cement-Soda Residue Stabilized and Solidified Zn-Contaminated Soil under Cyclic Freezing–Thawing Condition

Abstract

The efficiency of stabilization/solidification (S/S) treated heavy metal contaminated soils may be deteriorated by the changeable external environment. Thus, the toxicity characteristics leaching procedure (TCLP) and pH-dependent leaching test were carried out to evaluate the toxicity of zinc-contaminated soil treated by cement blended with soda residue with consideration of freezing–thawing (F-T) cycles. Results showed that F-T cycles had little impact on the toxicity of Zn-contaminated soils treated with cement-soda residue, and the leached Zn2+ concentration didn’t exceed the regulatory limit defined by the standards. Dissolution/precipitation of zinc hydroxides was suggested to be the controlling mechanism for Zn2+ leachability. Besides, Zn2+ immobilized by chemical incorporation and encapsulation by calcium silicate hydrates (CSHs) and calcium aluminate hydrates (CAHs) still remained stable in an acidic condition. Results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) verified that F-T cycles impose little impact on the chemical properties of the specimens, but only alter the micro- and macrostructure characteristics.