Stabilization and solidification of a heavy metal contaminated site soil using a hydroxyapatite based binder

Synthetic hydroxyapatite (HA) is an efficient and environment-friendly material for the remediation of heavy metal contaminated soils. However, the application of conventional HA powder in stabilizing contaminated soils is limited, due to high cost of final products, difficulties in synthesizing purified HA crystals. A new binder named SPC, which composes of single superphosphate (SSP) and calcium oxide (CaO), is presented as an alternative in this study. HA can form in the soil matrix by an acid-base reaction between SSP and CaO, resulting in a dense structure and improved mechanical properties of treated soils. Therefore, the SPC is capable of effectively immobilizing heavy metals and elevating strength of contaminated soils, meanwhile, maintaining relatively low cost. This paper presents a systematic investigation of the performance, reaction products, and microstructural properties of a lead (Pb), zinc (Zn), and cadmium (Cd) contaminated industrial site soil stabilized with SPC binder. The effects of SPC content and curing time on the pH, leachability and strength properties of the stabilized soils are evaluated. Furthermore, modified European Communities Bureau of Reference (BCR) sequential extraction procedure (SEP), mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscope (SEM) analyses are performed to interpret the mechanisms controlling the changes in these macro-properties. The results show that the soil pH and unconfined compressive strength (UCS) increase with increasing SPC content and curing time. After 28 days of curing, the UCS values of stabilized soils are approximately 2.2–5.7 times those of the untreated soil. The leachability of Pb, Zn and Cd is significantly reduced after stabilization, and the SPC content and curing time have considerable influences on the leached concentrations of heavy metals. The SEP results confirm that SPC significantly reduces the acid soluble fractions of Pb, Zn and Cd while increases their residual fractions. The MIP test results show that pore volume reduces notably and pore profile of the soil changes remarkably after SPC stabilization. The mineralogical (XRD) and microstructural (SEM) analyses reveal that the formation of heavy metal-bearing hydroxyapatites and phosphate-based precipitates are the primary mechanisms of immobilization of Pb, Zn and Cd in the SPC stabilized soil