Research articleConsecutive thermal and wet conditioning treatments of sedimentary stabilized cementitious materials from HPSS® technology: Effects on leaching and microstructure
Graphical abstract
Introduction
One of the critical aspects of industrial development over the last century is the long-lasting contamination at sites where large-scale industrial installations were operating. Often, contamination is not limited to industrial sites, but it also affects the surrounding surface and underground water bodies, thus causing concern for both human health and the environment. This entails the need for relevant efforts for environmental risk assessment and management, which should take into account the implications of different remediation approaches and, in particular, the effects and costs of possible dredging and disposal of large volumes of contaminated soils or sediments.
The planning of the most suitable management strategy must consider the combination of: i) the nature and distribution of the contamination, ii) the characteristics of the polluted matrix, iii) the availability and applicability of suitable remediation technologies and iv) the necessity of evaluation and monitoring technologies to verify the performance and to ensure the sustainability of the remediation processes (Mulligan et al., 2010).
In this context, many technical solutions have been developed to reduce the toxicity, mobility or amount of contaminants, depending on their nature and concentration, as well as on the polluted matrix. These interventions usually include processes such as extraction (Gomes et al., 2013), chemical and/or biological degradation (Khalid et al., 2017), washing (Yao et al., 2012), thermal treatment (Guemiza et al., 2017) or immobilization of the contaminants in a given matrix (Liu et al., 2018).
In particular, the processes used to immobilize pollutants into a matrix by modifying its physical-chemical characteristics are known as solidification and stabilization (S/S) treatments, and are performed by mixing the matrix with different ligands (Bates and Hills, 2015). The term “solidification” indicates the physical retention of pollutants through their encapsulation in a solid matrix characterized by low levels of porosity and permeability, while the term “stabilization” refers to the chemical transformation of contaminants into less soluble, mobile or toxic forms. As a result, the associated hazard is reduced even without changing the physical nature of the material to which these processes are applied.
The S/S treatment can generally be applied either “in-situ”, where the binder is mixed or injected into the contaminated matrix using special augers, or “ex-situ”, where the matrix to be treated is excavated and brought to a specific treatment plant, mixed with the binder system and stored (Bates and Hills, 2015). The product thus obtained can be either reused in the original site or turned to other uses.
Among “ex-situ” processes, Mapei and In.T.Ec. (Ferrari and Pellay, 2007; Scanferla et al., 2009). have developed an innovative technology known as High Performance Solidification/Stabilization (HPSS®) for the treatment of soils, sediments, and wastes with a dominant inorganic matrix. This process is conducted in suitable mixing and granulation installations and consists in the treatment of a finely divided (≤2 mm) contaminated matrix with a hydraulic binder (typically Portland cement) and special additives, to obtain a hardened granular material. The latter is expected to show reduced leaching of inorganic contaminants and a proper mechanical resistance in order to be classified as building material (Ferrari and Pellay, 2007; Scanferla et al., 2009). These properties reflect the combined effect of the cement and the additives (superplasticizers and water-repellent) (Ferrari et al., 2010). The cement causes the immobilization of metals as poorly soluble compounds (hydroxides, carbonates) and their incorporation in the hydration products (tobermorite gels and aluminosilicate phases AFt and AFm) (Careghini et al., 2010), while the additives ensure the formation of a dense solid material characterized by a low residual porosity and low leaching of pollutants into the environment. However, it has been shown that, in the case of soils or sediments contaminated by both organic and inorganic pollutants, the HPSS® technology should be ameliorated to obtain higher quality materials (Ferrari et al., 2008).
Here we report a study on the improvement of the HPSS® technology applied to the treatment of freshwater sediments contaminated by mercury and heavy hydrocarbons (C12-40), which aims at obtaining safe materials that could be reused, for example, in the construction sector. The investigated sediments were dredged from the construction site of a new navigation basin along the Mincio river at Valdaro (Italy). Sediment pollution is a consequence of poor industrial waste management and accidental spills that affected this area since the 1950s, when several chemical and petrochemical factories and oil refineries were built along the Mincio river. The cementitious granular material obtained from the dredged sediments in the first stage of the HPSS® process was subjected to superheated steam distillation (Thermal Treatment - TT) to eliminate both volatile and semi-volatile substances, such as mercury and heavy hydrocarbons (C12-40). Temperature and thermal treatment time were optimized in a specific pilot plant, developed within this research work, and a Wet Conditioning step (WC) was added to the traditional HPSS® process to further improve the mechanical characteristics of the granular material, which are usually negatively affected by the degradation of the granules' microstructure caused by the high temperature used in the thermal treatment. An in-depth study of both contaminant leaching and microstructure of the cementitious granular material was carried out after each step of the implemented process, in order to highlight the overall improvement of the granules’ quality in terms of mechanical performance and contaminant retention.
Section snippets
Materials
Portland cement CEM I 42.5 R was purchased from Italcementi S.p.A. (Heidelberg Cement Group, Heidelberg, Germany), while additives Mapeplast ECO 1-B and Mapeplast ECO 1-A were purchased from Mapei S.p.A. (Milan, Italy). High purity HNO3, HF, HCl, and H3BO3 for trace metal analysis were purchased from PanChem (AppliChem GmbH, Darmstadt, Germany), while ultrapure water was obtained with the MilliQ system from Millipore (Merck KGaA, Darmstadt, Germany). The tap water used was characterized in
Results and discussion
Innovative technologies such as HPSS®, taking advantage of the principles of “circular economy”, are of great interest from both the environmental and economic point of view, since the reuse of contaminated materials is usually a more sustainable approach than landfill disposal.
In this context, we tried to improve this technology by varying thermal and time conditions of the TT and by adding a wet conditioning step, to further increase the performance of the pellets produced with the
Implications for environmental management
With the aim of limiting the unsustainable practice of landfill disposal for contaminated soil and sediments, the development, testing and application of innovative and efficient techniques for their treatment are needed. Sustainable practices should promote those technological approaches that allow to maintain soil or sediment quality or that are capable of transforming the contaminated matrices into reusable products, following the principles of the circular economy.
This research will
Conclusions
In this study, the HPSS® (High Performance Solidification/Stabilization) technology developed by Mapei and In.T.Ec. was successfully applied to remediate freshwater sediments from the Mincio river, contaminated by Hg and C12-40 petroleum hydrocarbons. The technology was improved by adding a wet conditioning process to a thermal treatment, in an attempt to produce granulated materials accomplishing all the Italian regulatory requirements for reuse. The thermal treatment yielded good results
Acknowledgments
The authors are grateful to In.T.Ec. S.r.l. and University Ca’ Foscari of Venice for funding the Ph.D. fellowship of L. C. and to Mapei S.p.A. for additional support. Dr. Elisa Giubilato and Dr. Cinzia Bettiol (University Ca’ Foscari of Venice) are acknowledged for useful discussions.
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