New phosphate-based binder for stabilization of soils contaminated with heavy metals: Leaching, strength and microstructure characterization Yan-Jun Du a, * , Ming-Li Wei a , Krishna R. Reddy b , Fei Jin c , Hao-Liang Wu a , Zhi-Bin Liu a a Institute of Geotechnical Engineering, Southeast University, Nanjing 210096, China b Department of Civil & Materials Engineering, University of Illinois at Chicago, IL 60607, USA c Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, UK article info Article history: Received 10 May 2014 Received in revised form 23 July 2014 Accepted 26 July 2014 Available online 28 August 2014 Keywords: Solidification/stabilization Binder Heavy metals Leachability Strength abstract Cement stabilization is used extensively to remediate soils contaminated with heavy metals. However, previous studies suggest that the elevated zinc (Zn) and lead (Pb) concentrations in the contaminated soils would substantially retard the cement hydration, leading to the deterioration of the performance of cement stabilized soils. This study presents a new binder, KMP, composed of oxalic acid-activated phosphate rock, monopotassium phosphate and reactive magnesia. The effectiveness of stabilization using this binder is investigated on soils spiked with Zn and Pb, individually and together. Several series of tests are conducted including toxicity characteristic leaching (TCLP), ecotoxicity in terms of lumi- nescent bacteria test and unconfined compressive strength. The leachability of a field Zn- and Pb- contaminated soil stabilized with KMP is also evaluated by TCLP leaching test. The results show that the leached Zn concentrations are lower than the China MEP regulatory limit except when Zn and Pb coexist and for the curing time of 7 days. On the other hand, the leached Pb concentrations for stabilized soils with Pb alone or mixed Zn and Pb contamination are much lower than the China MEP or USEPA regu- latory limit, irrespective of the curing time. The luminescent bacteria test results show that the toxicity of the stabilized soils has been reduced considerably and is classified as slightly toxic class. The unconfined compressive strength of the soils decrease with the increase in the Zn concentration. The stabilized soils with mixed Zn and Pb contaminants exhibit notably higher leached Zn concentration, while there is lower unconfined compressive strength relative to the soils when contaminated with Zn alone. The X-ray diffraction and scanning electron microscope analyses reveal the presence of bobierrite (Mg 3 (PO 4 ) 2 $8H 2 O) and K-struvite (MgKPO 4 $6H 2 O) as the main products formed in the KMP stabilized uncontaminated soils; the formation of hopeite (Zn 3 (PO 4 ) 2 $4H 2 O), scholzite (CaZn 2 (PO 4 ) 2 $2H 2 O), zinc hydroxide (Zn(OH) 2 ), and fluoropyromorphite (Pb 5 (PO 4 ) 3 F) in the soils are the main mechanisms for immobilization of Zn and Pb with the KMP binder. The change in the relative quantities of the formed phosphate-based products, with respect to the Zn concentration and presence of mixed Zn and Pb contaminants, can well explain the measured impact of the Zn concentration levels and presence of both Zn and Pb contaminants on the unconfined compressive strength of the KMP stabilized soils. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Abandoned industrial sites exist in urban locations in China, United States and other locations worldwide and are host to a number of problems due to improper waste disposal practices and accidental spills in the past. The soil at many of these sites is contaminated with high levels of heavy metals such as zinc (Zn) and lead (Pb) (Du et al., 2014a; Sharma and Reddy, 2004; The World Bank, 2011; Xue et al., 2013). Heavy metals are not only hazardous to the environment and public health, but they also lead to the * Corresponding author. Institute of Geotechnical Engineering, Southeast Uni- versity, Si Pai Lou#2, Nanjing 210096, China. Tel.: þ86 25 83793729; fax: þ86 25 83795086. E-mail addresses: duyanjun@seu.edu.cn (Y.-J. Du), weimingli830716@sina.com (M.-L. Wei), kreddy@uic.edu (K.R. Reddy), leonking1987@gmail.com (F. Jin), wuhaoliang90@163.com (H.-L. Wu), seulzb@seu.edu.cn (Z.-B. Liu). Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman http://dx.doi.org/10.1016/j.jenvman.2014.07.035 0301-4797/© 2014 Elsevier Ltd. All rights reserved. Journal of Environmental Management 146 (2014) 179e188