Cement-stabilized contaminated soil: Understanding Pb retention with XANES and Raman spectroscopy
Graphical abstract
Introduction
Since the beginning of the industrialization era and the growth of urban societies, the development of humankind has started to pose a serious threat to the environment, because of the unregulated exploitation of its resources. Recently, soil contamination by heavy metals and metalloids has been recognized as one of the major environmental urgencies, which is estimated to affect more than 50% of the European Union's land surface (Jarsjö et al., 2019). In contrast to conventional and highly impacting remediation techniques, like soil washing and landfill disposal, cement-based solidification/stabilization (S/S) processes have gained attention as economically affordable and environmentally friendly technologies able to reduce heavy metal mobility and bioavailability (Tajudin et al., 2016), thanks to binder addition in the contaminated soil. Despite a widespread use of such techniques, the knowledge about the involved mechanisms of metal retention is still limited. This is especially because the high variability of soils and metal contaminants dramatically increases the complexity of the system, leading to difficulties in the development of standardized approaches.
When dealing with complex and hybrid systems, such as cement-solidified heavy metal contaminated soils, routinely used analytical techniques are not always enough to fully characterize such systems. The conventional approach to the remediation of a historically contaminated site usually starts with a chemical-mineralogical analysis of the contaminated matrix: a chemical analysis is performed to assess the severity of the contamination, which provides heavy metals concentration, while a mineralogical analysis permits to detect the crystalline phases present in the soil. However, with such routinely applied analyses, such as X-ray fluorescence (XRF) and X-ray diffraction (XRD), the gained information is restricted by some limitations. As an example, XRF does not provide the chemical speciation of a given element, which is of major importance in environmental research since it can provide crucial information about the behavior of contaminants, in terms of ecotoxicity, mobility and bioavailability. XRD provides a signal only from the crystalline fraction of the sample and for routine laboratory-diffractometer measurements on complex systems as soils the limit of detection is estimated to be around 1% by volume (Cullity and Stock, 2001), this leaving the amorphous/poorly crystalline phases unknown. Furthermore, in severely contaminated soils the heavy metals are often associated with poorly crystalline minerals and organic matter.
When the soil is submitted to S/S process, understanding the behavior of heavy metals and metalloids in the newly formed system is even more challenging. This is because the newly formed phases, which possibly incorporate contaminants, are usually poorly crystalline and in low concentrations, difficult to detect by conventional techniques. The variety of chemical forms that heavy metals can assume in the soil, prior and after the stabilization process, pushed the use of more advanced analytical techniques for better investigating the chemical fixation of contaminants by the soil and binder matrix. To achieve this, several sequential chemical extraction methods have been developed to investigate heavy metals partitioning within a multitude of different matrices, like soils, sediments and wastes (Gleyzes et al., 2002; Janoš et al., 2010; Pueyo et al., 2003; Tessier et al., 1979; Van Herreweghe et al., 2003), as well as solidified/stabilized materials (Li et al., 2001). According to these procedures, the solid material can be partitioned into specific fractions and the contaminants can be extracted selectively by using appropriate reagents (Okoro and Fatoki, 2012). However, there are evidences indicating an alteration of the samples during the different extraction steps, with consequent development of artifacts determining misleading results (Calmano et al., 2001; Scheckel et al., 2005). Furthermore, some portions of soil usually present in a contamination scenario (e.g. sulfates, sulfides, phosphates) are not considered properly in the extraction procedures (Orlow et al., 2005).
In this context, research in the field of S/S has started to take advantage of X-Ray Absorption Fine Structure (XAFS) spectroscopy (Gutsalenko et al., 2018; Roy and Stegemann, 2017; Hashimoto et al., 2011; Karamalidis et al., 2008) using synchrotron radiation, which is a chemically selective technique, able to selectively probe the local coordination chemistry, electronic state and local atomic structure around the absorbing atoms in complex compounds. These information, widely complementary to the one provided by techniques such as XRF and XRD above described, contribute to achieve a deep and reliable knowledge about the sample, in particular in this study, concerning the fate of heavy metal contaminants, such as Pb. The prerequisite needed for applying this technique is an a priori knowledge of the studied system, which is usually obtained with the X-ray techniques previously described. Another spectroscopic technique – Raman spectroscopy (RS) – provides information about the vibrational modes of a specific molecule, yielding a fingerprint of a sample even if it is composed of multiple phases. While RS has received attention in the past three decades for its application in cement chemistry, its application in the field of soil contamination regards a limited number of works (Frost et al., 2003; Hassan, 2019; Jones et al., 2009; Lanfranco et al., 2003; Timchenko et al., 2015).
Stabilization of Pb ion has been extensively studied by means of leaching and compressive strength tests (Badreddine et al., 2004; Cartledge et al., 1990; Du et al., 2014; El-Eswed et al., 2017; Gollmann et al., 2010; Guo et al., 2017; Li et al., 2015, Li et al., 2019; Li and Poon, 2017; Moon et al., 2013; Wang et al., 2018). These studies provided important evidences of different Pb S/S performances, depending on pH, binder used, curing time, metal loading, which are crucial for environmental management. However, in place of real contaminated soils, artificial soils spiked with Pb salts were used: even if this strategy surely facilitated the experimental procedures and the interpretation of results, it could lead to a certain risk of overlooking some actual processes occurring in real systems. Recently, XAFS was applied for studying Pb speciation in soils (Barrett et al., 2010; Guo et al., 2006; Luo et al., 2016; Nevidomskaya et al., 2016; Wilson, 2018), but only a limited number of works used XAFS for investigating Pb local environment in stabilized soils (Chrysochoou et al., 2007; Hashimoto et al., 2011; Sanderson et al., 2015).
This study investigates the mechanisms involved in the S/S of a Pb sulfate-contaminated soil, by taking advantage of not routinely used techniques, as XAFS and Raman spectroscopy. The studied contaminated soil was treated by using the High Performance Solidification/Stabilization process (HPSS®), which is a S/S process already applied for the reclamation of several contaminated sites (Calgaro et al., 2019; Scanferla et al., 2009, Scanferla et al., 2012). The study of complex real systems, i.e. soil prior and after the S/S process with different binders, was coupled with the investigation of simplified artificial systems. The latter were realized aiming at modeling the interactions of Pb with single binder components both in presence and in absence of sulfates, thus simulating the reactions occurring in the Pb contaminated soil amended with the binders. XAFS was used to assess Pb coordination environment in soil before and after the S/S treatment, while RS was applied for the characterization of the artificial systems where conventional techniques like XRD resulted insufficient for detecting and understanding the phases involved.
Section snippets
Samples description and experimental set up
The contaminated soil was sampled from a dismissed agrarian consortium located in Bagnolo Mella (BS, Italy), which was devoted to fertilizer production and included a sulfuric acid production plant by means of pyrite (FeS2) roasting process (Contessi et al., 2020). The soil was excavated from the top to 1.5 m depth, collecting 1 m3 of material. As in the already established HPSS® industrial procedure (Scanferla et al., 2009), the contaminated soil was air-dried up to 10% weight/weight (w/w)
Physical, chemical and mineralogical characterization
The pellets produced by applying the HPSS® process to the air-dried, sieved and homogenized contaminated soil showed the particle size distributions reported in Fig. S1 and Table S3. As reported, most of the pellets (51.3, 55.0 and 48.3% in weight for OPC-, CAC- and MK-pellets, respectively) were in the range 6.3–8.0 mm. Pb concentration in the studied systems is shown in Table 3. The mineralogical composition of soil, OPC, CAC, MK, OPC-pellets, CAC-pellets and MK-pellets is reported in our
Conclusions
The mechanisms responsible for Pb retention in different solidified/stabilized systems were investigated by means of XANES and Raman spectroscopic analyses. We studied both real systems (i.e. a Pb contaminated soil prior and after the solidification with different binders) and artificial ones, realized to model Pb behavior in simplified systems. From our results we can draw the following conclusions:
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Pb local environment in the Pb-sulfate contaminated soil undergoes a transformation when
CRediT authorship contribution statement
Silvia Contessi: Conceptualization, Investigation, Writing - original draft, Visualization. Maria Chiara Dalconi: Conceptualization, Investigation, Writing - review & editing, Supervision. Simone Pollastri: Investigation, Data curation, Formal analysis. Loris Calgaro: Conceptualization, Investigation, Writing - review & editing. Carlo Meneghini: Formal analysis, Writing - review & editing. Giorgio Ferrari: Conceptualization, Project administration. Antonio Marcomini: Conceptualization, Project
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the University of Padova with funds of the Project of National Interest PRIN2017 “Mineral reactivity, a key to understand large-scale processes: from rock forming environments to solid waste recovering/lithification”.
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