Stabilization and solidification of Pb in cement matrices
Introduction
The growing number of papers and conferences devoted to waste immobilization reflects a genuine need for economic solutions for restoring a safe, clear, green environment. This manner stabilization/solidification (S/S) processes are routinely used for the final treatment of hazardous wastes to reduce contaminant leaching prior to land disposal. Among various types of S/S binders, cement-based systems are the mostly widely used, due to relatively low cost, wide availability and versatility [1], [2], [3], [4].
S/S process can be used to convert heavy metals into less mobile form, applicable to several types of wastes which are not suitable to physical, chemical or biological processing [2], [5], [6], [7], [8]. Industrial activities in the product of materials and chemicals give rise to very large quantities of heavy metal-bearing wastes each year. With the increasing concern regarding environmental pollution and growing interest in sustainable development, the problem of heavy metal immobilization becomes even more significant [9]. Heavy metals are dangerous because they tend to bioaccumulate and can enter into water supply systems by industrial and consumer waste, or even from acidic rain breaking down soils and releasing heavy metals into streams, lakes, rivers, and groundwater. An improper disposal of heavy metals can cause serious environmental and ecological problems. Reduction of impact of those residues is desirable by disposing them into very stable and leaching free conditions. Metals like As, Cr, Pb, Zn, Cu, Hg are issues of a lot of studies and researches [10], [11], [12], [13], [14], [15], due to the hazardous effects that can cause to environmental and the human health.
The application of the materials resulting from S/S processes in civil engineering, for instance, depends on the degree of residues leaching, as well as on their structural resistance, minimizing therefore, potential environmental problems. Among metal wastes, those containing Pb represents a serious issue. Pb derivatives are employed in industries for fuel and asphalt. Furthermore, it is also present in mining and metallurgic industries, in processes involving corrosion of metal objects, manufacture of tin pigments, imprint and typographer and plastic incineration (utilized in polymer stabilization) [16]. In humans, Pb exposure can lead to a wide range of biological effects depending on the level and duration of exposure. Various effects occur over a broad range of doses, with the developing fetus and infant being more sensitive than the adult. High levels of exposure may result in toxic biochemical effects in humans which in turn cause problems in the synthesis of haemoglobin, effects on the kidneys, gastrointestinal tract, joints and reproductive system, and acute or chronic damage to the nervous system [17].
A great number of studies have been done about the S/S efficiency in cement matrices. For example, the incorporation of metals (Ni, Pb and Cd) results in a decrease of the Ca(OH)2 content of the cement paste and increases its vulnerability. The leaching rates of the metals increased in the following order: Pb ≪ Ni ≪ Cd [18]. Heavy metals (Pb, Cu, Zn, Cd and Mn) added as salts are very effectively immobilized in hydrated matrices (from 99.82 to 99.99%); Cr is an exception immobilized in lower degree (from 85.97 to 93.33%) [10]. The microstructure of cementation wastes containing Pb, Cd, As and Cr was also investigated. The metal leaching in the pH region of 6–8 decreased in the following order: Cr(VI) > Cd(II)Pb(II) > As(V). In another study in which Pb, Zn, Cu, Fe and Mn were immobilized by S/S process, mechanical strength was shown to increase [19].
In a previous study, we evaluated the desorption of lead immobilized into cement and concrete modified matrices. Lead desorbed content was monitored by graphite furnace atomic absorption spectroscopy (GF-AAS) [20], and was shown to be pH dependent. At present we investigated the changes in the cement matrices caused by the incorporation of Pb derivatives. The effect of cement cure time on Pb metal leaching and on the matrix mechanical strengths was evaluated. Data are discussed in terms of metal mobility along the cement block monitored by X-ray fluorescence (XRF) spectrometry. Complementary techniques, namely, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), thermal gravimetric analysis (TGA), small angle X-ray scattering (SAXS) and X-ray diffraction spectroscopy (XRD), were employed in the characterization of the modified matrices.
Section snippets
Materials and methods
All the tests were carried on using ordinary portland cement (OPC). The chemical composition and physical properties of the employed Portland cement are shown in Table 1. The tests were performed with three different matrices for the control of the effect of the presence of Pb. The first one (reference matrix) consisted solely of cement portland without the metal. In the second one (blank matrix), Pb (10 wt.%) was added. Such matrices were not exposed to pH solutions. Finally, the third one, Pb
Results and discussion
In the following discussion, the label S states for sample, SB for blank (with Pb), SR for reference (without Pb) and the numbers 5, 7 and 8 are used to describe the pH of the solutions. Thereafter, the letter S is used for short (7 days) and L for long (28 days) cure time. Then, for instance, S5-L means a sample with Pb that was cured in humid air for 28 days and then was exposed to a pH 5 solution. The samples SB and SR were not exposed to pH solutions, and they served to evaluate the effect
Conclusion
In the present study, it was observed that Pb stabilization and solidification in mortar is possible. The introduction of Pb into the matrices engenders a reduction in the resistance, not by hydration excess, but rather by an exchange reaction between Pb and the phases of the cement. Furthermore, the stability of the resulting mortars is sensitive to the external environment. Ca leaching, in basic medium, seems to be the main cause which contributes to resistance decreasing in the matrices.
Acknowledgments
This project was partially financed by CNPq. M.A. Gollmann thanks CNPq for the grant. The authors are thankful to LNLS (Project D11A SAXS1 #5296) for measurements in the SAXS beamline.
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