Abstract
Alkali-activated matrices are suitable materials for the immobilization of hazardous materials such as heavy metals. This paper is focused on the comparison of immobilization characteristics of various inorganic composite materials based on blast furnace slag and on the influence of various dosages of the heavy metal Pb on the mechanical properties and fixation ability of prepared matrices. Blast furnace slag (BFS), fly ash, and standard sand were used as raw materials, and sodium water glass was used as an alkaline activator. Pb(NO3)2 served as a source of heavy metal and was added in various dosages in solid state or as aqueous solution. The immobilization characteristics were determined by leaching tests, and the content of Pb in the eluate was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES). The microstructure of matrices and distribution of Pb within the matrix were determined by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). Increasing the dosage of the heavy metal had negative impacts on the mechanical properties of prepared matrices. The leaching tests confirmed the ability of alkali-activated materials to immobilize heavy metals. With increasing addition of Pb, its content in eluates increased.
1. Introduction
Heavy metals such as Pb belong to hazardous materials, which are harmful to the human beings. Lead is highly toxic element, which attacks mainly the nervous system. Lead exposure mostly occurs through the ingestion of contaminated water or food. Therefore, it is important to prevent its leakage into the environment by stabilization or solidification of waste materials before their deposition into the landfill [1]. A possible way on how to immobilize heavy metals is using alkali-activated materials (AAMs). AAMs present a broad range of materials (geopolymers, activated blast furnace slag, etc.), but all of them are activated by high pH during their preparation. The AAMs can be divided into two big groups: high-calcium alkali-activated materials (HCAAMs) and low-calcium alkali-activated materials (LCAAMs). HCAAMs are represented mainly by activated BFS, and LCAAMs are represented mainly by activated fly ash or metakaolin. The structure of HCAAMs consists of the C-A-S-H gel as a major hydration product. This gel has a similar structure as the C-S-H gel in hydrated ordinary Portland cement and is made up of tetrahedrally coordinated silicate chains, where aluminium is located in the bridging position instead of silicon. The structure and composition of the C-A-S-H gel depend on the nature and concentration of the used activator. AFm-type phases (NaOH-activated binders), hydrotalcite (BFS with high content of MgO), and zeolites (binders with high content of Al2O3) are usually formed as secondary hydration products [2–4].
Heavy metals can be immobilized by two types of fixation–physical and chemical. Mostly, both types of immobilization occur together. Physical fixation is linked with mechanical properties and porosity of the matrix. Chemical fixation means that there is a chemical bond between the heavy metal and the matrix. Heavy metals can be inhibited by transformation into a less soluble form as well. After alkali activation, the heavy metals usually occur in the form of silicate or hydroxide [5–7]. The immobilization of heavy metals within AAMs can be influenced by various factors, such as the composition of the matrix, the type of alkaline activator, the immobilized heavy metal, and the leaching medium [8, 9]. Previous researches proved that Pb reached high efficiency of immobilization in AAMs, but there is only little information about the influence of Pb dosage on immobilization characteristics of AAMs [10–13].
The aim of this work was to compare the ability of three different alkali-activated matrices based on BFS to immobilize Pb within their structure. The influence of different Pb dosages on the immobilization efficiency was also investigated.
2. Materials and Methods
2.1. Materials, Sample Preparation, and Leaching Tests
BFS and high-temperature fly ash were used as raw materials. The particle size distribution of both raw materials is listed in Table 1. Liquid sodium silicate with the SiO2/Na2O ratio of 1.85 served as an alkaline activator. As fine aggregates, three different fractions of siliceous Czech standard sand complying with ČSN EN 196-1 were used (the mass ratio of the fractions was 1 : 1 : 1). Pb was used in the form of Pb(NO3)2.
Three types of matrices were prepared. The first one was based only on BFS. In the second one, 50 wt.% of BFS was replaced by fly ash. And the third one consisted of BFS and standard sand. The sample nomenclature and the mixture designs are listed in Tables 2 and 3, respectively. Pb was added in the dosages of 1, 2.5, and 5 wt.% to binder mass and was added in two ways: as a solution and as a solid. In both cases, the water-to-binder ratios were the same.
The whole mixing procedure took four minutes. In the beginning, all materials without aggregates were put together and sand, if needed, was added after 30 s of mixing. The samples were cast in steel molds measuring 20 × 20 × 100 mm. All analyses were performed after 28 days of moist curing (98% relative humidity) at laboratory temperature (23 ± 2°C).
The leaching tests were based on ČSN EN 12457-4. The demineralized water served as leaching agent. The solid/liquid ratio was 1 : 10, and the mixture was agitated for 24 hours. After the leaching time ended, the mixture was filtered by a membrane filter with the pore size of 0.45 µm, and the concentration of Pb in solution was determined by ICP-OES.
2.2. Methods
The compressive strength was measured according to the ČSN EN 196-1 using Betonsystem Desttest 3310. To determine the porosity, prepared matrices were investigated by mercury intrusion porosimetry using Quantachrome instrument PoreMaster 33. The surface tension of mercury was 480 mN/m, and the contact angle was 140° based on the recommendation. The pore sizes were determined in the range from over 170 μm to 0.0064 μm. The ICP-OES data were obtained using Horiba Jobin Yvon Ultima II Spectrometer. The parameters of measurement were as follows: RF power, 1350 W; gas, argon; plasma gas, 13 L·min−1; auxiliary gas, 0.1 L·min−1; nebuliser gas, 0.85 L·min−1; plasma view, radial; nebuliser, Meinhard; and nebuliser pressure, 3 bar. The scanning electron microscopy (SEM) and the energy dispersive X-ray spectroscopy (EDS) analyses were performed using Zeiss EVO LS 10 equipped with an Oxford X-Max 80 mm2 detector in the backscattering mode. The working distance for all samples was 12 mm and the accelerate voltage was 30 kV. All samples were sputtered by carbon to obtain good surface conductivity.
3. Results and Discussion
3.1. Compressive Strength
The compressive strength of prepared matrices is shown in Figure 1 and Table 4. Following the results, the type of matrix had the highest influence on the compressive strength. The highest values had the matrix with aggregates, and the lowest values reached the matrix with fly ash. The results correspond with the findings in literature that, by replacing BFS with fly ash, the compressive strength decreases [2]. No clear behavior correlation was observed between the Pb dosage amount and mechanical properties. Increasing the dosage of Pb from 1 up to 2.5 wt.% led, in some cases, to the compressive strength increase and in some cases to the strength decrease. The addition of 5 wt.% of Pb caused the decrease of compressive strength in all types of matrices.
3.2. Porosity
The porosity of the prepared materials depended mainly on the type of the matrix. The total porosity of chosen samples is listed in Table 5. The matrix with fly ash showed the highest porosity. High-temperature fly ash consists of spherical particles, which can be filled in with similar smaller particles or can be hollow. Hence, partially reacted fly ash particles increased the porosity. The lowest porosity was observed in the matrix based on BFS. It is due to a very compact microstructure of this matrix, which was confirmed by SEM analysis (Figure 2). The dosage of Pb influenced the porosity of matrices too. The total porosity of matrices increased with increasing Pb content. As can be seen in Figure 3, around the areas, where Pb was cummulated, the matrix was not compact and the cracks and pores were formed. This caused the increase in porosity. The state of Pb addition—liquid/solid—did not affect the porosity.
3.3. Leaching Tests
To determine the efficiency of Pb immobilization, the leaching test based on ČSN EN 12457-4 was performed. This test serves to determine whether the material is suitable for landfilling and whether it does not represent any hazard for the environment. Figure 4 shows the concentration of Pb in eluates from the prepared matrices. The immobilization of Pb in all matrices was very high and reached up to 99%. The results correspond with previous research and correlate with the total porosity of the matrices [11]. The highest Pb release occurred from matrices with fly ash, which also had the highest porosity and the lowest mechanical properties. The dosage of Pb had the influence on immobilization too. With increasing Pb addition, the content of Pb released into the leaching medium increased as well. It should be taken into consideration that when the Pb addition rose five times, the release increased only maximally three times. The efficiency of Pb immobilization was better with higher dosages of Pb. The pH of eluates is listed in Table 6. The matrices with sand reached the lowest pH, because they contained the smallest amount of the binder. The presence of fly ash led to the decrease of pH. The highest dosage of Pb (5 wt.%) caused the decrease of pH as well.
3.4. SEM
The SEM analysis showed three different microstructures depending on the type of matrix. The samples S2, P2, M2, and SR2 were investigated. The matrix based on BFS had a compact structure with unreacted particles of BFS and hydration products between them (Figure 3, Area 2; Table 7). A similar structure was observed in the matrix containing fly ash, but moreover, unreacted particles of fly ash were identified. The addition of fly ash led to the increase in the content of Si and Al and the decrease in the content of Ca (Table 8). A quite different microstructure occurred in the matrix with aggregates. The unreacted aggregates filled the major area of the sample and in between them there was the matrix with the same composition as observed in the S2 sample (Figure 5, Table 9, and Area 6). The aggregates were composed of quartz (Table 9 and Area 7).
When Pb was added as a solid, it behaved in the same manner in all types of matrices. Pb was cumulated in pores and formed specific structures (Figures 3, 5, and 6). These structures consisted mainly of Pb and O. It can be assumed that Pb transformed into its insoluble salt Pb(OH)2 after the alkali activation. These findings correspond to previous research [14]. Minor amount of Pb was dispersed throughout the matrix. A quite different situation came up with the addition of Pb as a solution. Pb was dispersed equally throughout the matrix and did not create any specific structures (Figure 2, Table 10).
4. Conclusions
The immobilization of Pb in three different matrices was investigated. All matrices showed good ability to immobilize Pb. The increase in the Pb dosage led to the increase of Pb concentration in eluates, but the immobilization efficiency remained up to 99% in all cases. After the alkali activation, Pb formed its insoluble salt Pb(OH)2. Both the compressive strength and the porosity were influenced by the Pb dosage. The decrease of mechanical properties after the addition of higher Pb dosage (5 wt.%) was observed. The concentration of Pb in eluates correlated with the porosity: when the porosity was higher, more Pb was released in eluates. It can be assumed that Pb was fixed by physical fixation, which was linked with the mechanical properties of matrices. Pb was also well immobilized, thanks to the formation of its insoluble salt. There was a difference in Pb behavior when added either as a solid or as a solution. In the case of solution, Pb was dispersed throughout the matrix equally, but when added as a solid, it formed specific structures, which cumulated in pores.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Acknowledgments
This outcome has been achieved with the financial support by the project Materials Research Centre at FCH BUT-Sustainability and Development (REG LO1211), National Programme for Sustainability I (Ministry of Education, Youth and Sports), and “Development of shrinkage reducing agents designed for alkali activated systems” (GA17-03670S, Czech Science Foundation).