Lead leachability in stabilized/solidified soil samples evaluated with different leaching tests Chuanyong Jing ∗ , Xiaoguang Meng, George P. Korfiatis Center for Environmental Systems, Stevens Institute of Technology, Hoboken, NJ 07030, USA Received 5 August 2003; received in revised form 17 May 2004; accepted 9 July 2004 Available online 27 September 2004 Abstract Leaching tests and model calculations were performed to investigate the immobilization mechanisms of Pb and compare different leaching protocols. Stabilization/solidification (S/S) treatments reduced Pb concentrations in the toxicity characteristic leaching procedure (TCLP) leachate from 5.9 mg/L for untreated soil to less than 0.7 mg/L. The results of eight different leaching protocols show that: (1) the main factor controlling the Pb concentration in the leachate is the final pH; (2) the final pH is a function of the leachant acidity; and (3) for a given final pH, the type of leachant has a relatively minor effect on leachability. The diffuse layer adsorption model, aqueous and precipitation reactions were employed in the MINTEQA2 program to describe the Pb leaching behavior. Both leaching tests and model simulations indicate that the Pb leaching behavior can be divided into three stages based on the leachate pH: a high alkalinity leaching stage at pH > 12, where Pb formed soluble hydroxide anion complexes and leached out; a neutral to alkaline immobilization stage in the pH range of 6–12, which was characterized by low Pb leachability caused by adsorption and precipitation; and an acid leaching stage with pH < 6, where the acid neutralizing capacity (ANC) of the S/S materials was totally consumed and therefore free Pb-ion leached out. © 2004 Elsevier B.V. All rights reserved. Keywords: Lead; Stabilization/solidification; Leaching; Acid neutralizing capacity; Adsorption model 1. Introduction Stabilization/solidification (S/S) technology has been widely applied to immobilize heavy metals in contaminated soils, sediments, and sludge with additives such as cement, lime, and fly ash [1]. Quantifying the environmental impact of stabilized/solidified materials in real environmental sce- narios is crucial for selecting proper disposal and reuse alter- natives and for certification of immobilization technologies. The performance of S/S-treated wastes is generally measured in terms of leaching tests. Although numerous leaching tests are available to evaluate the S/S treatment, no single test can describe the complex leaching behavior of the treated mate- rials. The toxicity characteristic leaching procedure is the most commonly used regulatory protocol. However, it may under- ∗ Corresponding author. Tel.: +1 201 216 8994; fax: +1 201 216 8303. E-mail address: cjing@stevens-tech.edu (C. Jing). estimate the leachability of some redox sensitive elements, such as As, since redox reactions may happen during the extraction [2]. The multiple extraction procedure (MEP), the availability test (NEN 7341), and sequential chemical extrac- tion are also applied to a wide range of materials for differ- ent purposes, which creates confusion in how the results are to be interpreted and which extraction test should be used in determining the leachability. An approach to harmonize different leaching tests was proposed by van der Sloot [3]. However, the interpretation of different protocols is depen- dent on an accurate understanding of leaching mechanisms. This requires validation of the consistency of results between different types of test methods and different types of wastes. Pb is the most common contaminant in the environment and has chronic toxicity. Pb contamination has been found in 604 out of 1221 superfund sites on the National Priorities List (NPL). There have been intensive studies about Pb im- mobilization mechanisms in S/S-treated wastes. In a review of heavy metal immobilization in ettringite and the calcium 0304-3894/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2004.07.017