Journal of Environmental Management 146 (2014) 179e188
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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 b c
Institute of Geotechnical Engineering, Southeast University, Nanjing 210096, China Department of Civil & Materials Engineering, University of Illinois at Chicago, IL 60607, USA Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 May 2014 Received in revised form 23 July 2014 Accepted 26 July 2014 Available online 28 August 2014
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 luminescent bacteria test and unconfined compressive strength. The leachability of a field Zn- and Pbcontaminated 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 regulatory 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 (Mg3(PO4)2$8H2O) and K-struvite (MgKPO4$6H2O) as the main products formed in the KMP stabilized uncontaminated soils; the formation of hopeite (Zn3(PO4)2$4H2O), scholzite (CaZn2(PO4)2$2H2O), zinc hydroxide (Zn(OH)2), and fluoropyromorphite (Pb5(PO4)3F) 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.
Keywords: Solidification/stabilization Binder Heavy metals Leachability Strength
1. Introduction
* Corresponding author. Institute of Geotechnical Engineering, Southeast University, Si Pai Lou#2, Nanjing 210096, China. Tel.: þ86 25 83793729; fax: þ86 25 83795086. E-mail addresses:
[email protected] (Y.-J. Du),
[email protected] (M.-L. Wei),
[email protected] (K.R. Reddy),
[email protected] (F. Jin),
[email protected] (H.-L. Wu),
[email protected] (Z.-B. Liu). http://dx.doi.org/10.1016/j.jenvman.2014.07.035 0301-4797/© 2014 Elsevier Ltd. All rights reserved.
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
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degradation of mechanical properties of soils, which results in unfavorable conditions for the redevelopment of those contaminated sites (Du et al., 2014a). Therefore, it is necessary to identify and implement effective, economical remediation technologies to treat soil that is contaminated by heavy metals, and to ensure that the mechanical behavior of the soils is not adversely affected so the treated soils can be reused or infrastructures (such as roads and buildings) can be constructed over the treated soil. Solidification/stabilization (S/S) is a widely-used remediation technology that involves mixing binders and contaminated soils to reduce the mobility of heavy metals and enhance the strength of the soils by physical and chemical means (Du et al., 2014a; Terzano et al., 2005). The commonly used binders in S/S practice are highalkali cementitious materials, such as Portland cement (PC), quicklime and pulverized fly ash. Previous studies have shown that when exposed to the long-term external conditions such as acid rain, sulfate attack, freeze-thaw cycling, and carbonation, heavy metals in the high-alkaline binder stabilized soils would leach easily (Du et al., 2014b). In addition, the presence of certain heavy metal in the soils, such as Zn, has a significant negative effect on the hydration of the cement-based binders, since Zn could react with calcium (Ca) and hydroxide (OH) ions to form calcium zinc hydroxide (Ca(Zn(OH)3)2$2H2O) (Du et al., 2014a). Here, Ca(Zn(OH)3)2$2H2O would wrap around the grains of the cement-based binders to form a barrier that separates the binder from water hydration. In turn, this would lower the soil pH, and then hinder the hydration of the cement-based binders. Previous studies have demonstrated that the strength and modulus of cement stabilized soils decreased rapidly as Zn concentrations increased (Du et al., 2014a). Moreover, the presence of both Zn and Pb contaminants, commonly encountered in industrial contaminated soils, has a more retardant effect on the hydration reaction of cement-based binders than the presence of Zn or Pb alone (Li et al., 2001). Therefore, it is necessary to develop alternative binders to stabilize soils that have relatively high concentrations of Zn and both Zn and Pb contaminants. This study develops KMP as a new binder. KMP consists of oxalic acid-activated phosphate rock, monopotassium phosphate (KH2PO4) and reactive magnesia (MgO). The leachability, ecotoxicity and strength characteristics of KMP stabilized soils with Zn or Pb contaminant individually and Zn and Pb contaminants together are investigated using several series of toxic characteristics leaching procedures (TCLP), luminescent bacteria test (LBT) and unconfined compression tests (UCTs). The mechanisms of Zn and Pb immobilization by the new KMP binder are interpreted based on the results of X-ray diffraction (XRD) and scanning electron microscope (SEM) analyses. 2. Background 2.1. Immobilization mechanisms of Pb and Zn with phosphate rock Phosphate rock is an effective binder for stabilizing soils that are rich in Pb, Zn, Cu, and Cd (Zhang et al., 2010; Fang et al., 2012; Tropek et al., 2013; Heneberg et al., 2014). The main chemical composition of phosphate rock is apatite (Ca5(PO4)3X, X ¼ Cl, OH, F) with small amounts of quartz and calcite (Mignardi et al., 2012). Pb immobilization by phosphate rock is mainly attributed to two mechanisms. The primary mechanism is dissolution/precipitation, in which phosphate ions (PO3 4 ) that are dissolved from the phosphate rock upon water hydration react with Pb to form highly insoluble pyromorphite (Pb5(PO4)3X, X ¼ Cl, OH, F) (Basta and McGowen, 2004; Cao et al., 2004). The second mechanism is adsorption/substitution, in which Pb is adsorbed on the surface of the phosphate rock and then Pb substitutes calcium (Ca) in the
lattice structure of apatite during re-crystallization and coprecipitation of CaxPb5-x(PO4)3X (X ¼ Cl, OH, F) (Mavropoulos et al., 2002; Park et al., 2011a, 2011b). The Zn immobilization by phosphate rock is attributed to the fact that (1) Zn substitutes Ca in the lattice structure of apatite and becomes fixed during re-crystallization and co-precipitation (Chen et al., 1997); and, (2) PO3 4 dissolved from apatite could react with Zn to form amorphous to poorly crystalline-metal Zn phosphates such as hopeite (Zn3(PO4)2$4H2O) and scholzite (CaZn2(PO4)2$2H2O) precipitated on the surface of apatite grains (Cao et al., 2004; Debela et al., 2013; Mignardi et al., 2012). In the cement-based solidified Pb-contaminated soils, Pb exists mainly as Pb hydrate phases and Pb hydroxide (Pb(OH)2) precipitated on the surface of calcium hydroxide (Ca(OH)2) or calcium silicate hydrate (CSH) (Du et al., 2014a). Compared to Pb(OH)2 (Ksp ¼ 1.6 104), pyromorphite has much lower solubility (Ksp ¼ 1060e1085) and a greater capacity to resist acid or alkaline attack (Gougar et al., 1996; Mignardi et al., 2012; Navarro et al., 2011). In addition, the solubility of Zn3(PO4)2$4H2O (Ksp ¼ 1.2 1017) or CaZn2(PO4)2$2H2O (Ksp ¼ 1034.1) is much lower relative to zinc hydroxide (Zn(OH)2) (Ksp ¼ 1.6 105), the main products involved in the immobilization of Zn with Portland cement (Desogus et al., 2013). It is suitable to directly apply ground and sieved phosphate as a binder to immobilize Pb and Zn in acidic soils. This is because under acidic condition, phosphate rock can easily dissolve and convert the phosphorus into soluble phosphates that, in turn, can readily react with Pb and Zn to form insoluble hopeite (Zn3(PO4)2$4H2O) and scholzite (CaZn2(PO4)2$2H2O) (Chen et al., 1997). However, when the soil is neutral or alkaline, the Pb immobilization process would take a relatively long time since the release of PO3 4 from phosphate rock occurs at a slow rate (Chen et al., 2010). Meanwhile, the dissolution of apatite would occur at a relatively slow rate, which then slows down the kinetic reaction between Zn and PO3 4 to form Zn3(PO4)2$4H2O or CaZn2(PO4)2$2H2O (Chen et al., 1997). To accelerate the dissolution of phosphate rock and subsequently accelerate the Pb or Zn immobilization, it is necessary to acidify the phosphate rock using a suitable acid such as oxalic acid prior to the soil stabilization (Jiang et al., 2012). Nevertheless, using phosphate rock or acidified phosphate rock alone has little capacity to enhance the strength of the soils. 2.2. Immobilization mechanisms of Pb and Zn with KMP It is hypothesized that immobilization of Pb or Zn in the soils with KMP may be due to three mechanisms: (1) Pb can react easily with PO3 4 released from the acidified phosphate rock and KH2PO4 to form highly insoluble pyromorphite, as explained earlier; (2) Pb or Zn precipitates as metal hydroxides that are adsorbed on the surface of soil matrix under the alkaline environment created by the hydration of reactive MgO (Buj et al., 2009, 2010); and, (3) Zn substitutes Ca in the lattice structure of remained un-acidified apatite and then is fixed in the apatite, as shown in the earlier section. It has been reported that magnesium phosphate cement (MPC), comprised of dead-burned MgO and KH2PO4, possesses significantly high strength due to the acidebase reaction of MgO and KH2PO4 and the formation of amorphous or crystallized k-struvite (MgKPO4$6H2O) and crystallized bobierrite (Mg3(PO4)2$8H2O) (Chau et al., 2011; Wang et al., 2013). Similarly, for the KMP binder, PO3 4 released from the acidified phosphate rock is able to react with magnesium (Mg2þ) ions and potassium (Kþ) ions dissolved from hydrated reactive MgO and KH2PO4, respectively, to form MgKPO4$6H2O and Mg3(PO4)2$8H2O. Therefore, when relatively high amounts of either Zn or Pb alone or co-existing Zn and Pb (i.e., Zn- and Pb- contaminants) are found in the soils, it is reasonable to
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expect that the KMP stabilized soils would exhibit relatively high strength.
Table 1 Properties of soil used in this study. Valuea
Property
2.3. Comparison of KMP and MPC binders MPC has been used to immobilize plutonium (Pu), Neptunium (Np), Actinium (Am), Cesium (Cs), Strontium (Sr), technetium (Tc), and Selenium (Se) contained in low level nuclear wastes and cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), Pb, and Zn contained in electronic wastes (Buj et al., 2009, 2010; Singh et al., 2006; Vinokurov et al., 2009). The immobilization of metals with MPC is mainly attributed to the formation of metal hydroxides (for heavy metals) and the encapsulation of metal-precipitates in the matrix (for radionuclides) (Buj et al., 2009, 2010; Singh et al., 2006). Nevertheless, studies on stabilizing heavy metal contaminated soils with MPC are limited. The KMP binder proposed here may be more advantageous than the MPC for stabilizing either Zn or Pb alone or Zn- and Pb- contaminated soils for the following reasons: (1) KMP stabilized soils may have higher early strength when the binder content, curing time or condition and other parameters remain unchanged. This is because the hydration rate of reactive MgO contained in the KMP is faster relative to the dead-burned MgO contained in the MPC, and therefore greater amounts of Mg2þ and OH would present in the KMP stabilized soils within a relatively short period (Wagh, 2004). Consequently, the acidebase reaction between the reactive MgO and PO3 4 released from the acidified phosphate rock is stronger in the KMP stabilized soils, which is favorable for the development of early strength of the stabilized soils (Ding et al., 2012); (2) KMP is more cost-effective as the cost of oxalic acidified phosphate rock is approximately 5e10 times lower than that of KH2PO4 (either chemical reagent or industrial-grade); while the cost of reactive MgO is approximately the same as dead-burned MgO. This means that the cost of KMP is only about 25e50% that of MPC; and, (3) the KMP is more environmentallyfriendly than MPC. The mass ratio of dead-burned MgO to KH2PO4 generally varies from 1:1 to 1.5:1 for the MPC binder (Buj et al., 2009, 2010; Iyengar and Al-Tabbaa, 2007), whereas the mass ratio of reactive MgO, KH2PO4 and acidified phosphate rock is 2:1:1 for the KMP binder. Hence, the quantity of KH2PO4 used for making the KMP binder is estimated to be 40e50% lower than that used for making the MPC binder. This is significant in terms of the environmental impact as the production of KH2PO4 (either chemical analytical reagent or industrial-grade) is often accompanied with production of dilute acid pollutant (approximately 0.3 t/t, Wagh, 2004). Although this needs to be examined in a future study, the authors believe that the KMP has higher performance in immobilizing Zn or Pb alone or Zn- and Pb- contaminants due to the three advanced immobilization mechanisms discussed in the Section 2.2, relative to the formation of metal hydroxide alone, the main immobilization mechanism of heavy metals by MPC. 3. Materials and methods 3.1. Raw materials The uncontaminated soil used in this study was collected from Nanjing City, China. The field-contaminated soil with approximately 2.4% Zn and 1.3% Pb concentrations was collected form Baiyin City, China. The basic physico-chemical properties of the soils are summarized in Table 1. The uncontaminated soil used in this study is classified as low plasticity clay based on Unified Soil Classification System (ASTM D2487). The Atterberg limits were measured as per ASTM D4318. The pH of the soil was measured by employing a pH meter HORIBA D-54 as per ASTM D4972 (ASTM,
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Natural water content, wn (%) Plastic limit, wp (%) Liquid limit, wL (%) Specific gravity, Gs pH
Clean soil
Field-contaminated soil
21.4 20.6 45.3 2.72 7.43
28.1 24.9 36.3 2.73 7.96
a Number of replicate is 3, Confidence interval (CI) is 95%, and standard deviation (SD) is
