Solidification/stabilization mechanism of Pb(II), Cd(II), Mn(II) and Cr(III) in fly ash based geopolymers
Introduction
At present, with the development of industry, there is an increasing amount of heavy metal wastes generated from metalwork industry, chemical industry, electrical equipment manufacturing and other fields. It brings great damage to human beings and the environment. High concentrations of heavy metals in soil, water and air may cause bioaccumulation of heavy metals in animals and humans [1]. Heavy metals are classified as first pollutants in the world. Heavy metal pollution has become a very serious global environmental problem, and it is very serious [2].
Most of the conventional methods are inefficient in heavy metal recovery, only transferring the contaminants from one phase to another one [3]. In recent years, solidification has become the central research focus due to its method simplicity, effectiveness, and low cost, and the solidified body which can be easily moved is a complete monolithic solid [4]. Geopolymers possess solidified properties that can effectively remove heavy metals [5], [6]. Geopolymers solidification of heavy metals has many advantages [4]. Firstly, geopolymer has a three-dimensional network structure, and heavy metal pollutants can be “locked” in its structure. Secondly, the permeability coefficient of the geopolymer is very low, which can effectively prevent the rapid infiltration of heavy metal elements. Furthermore, in the process of geopolymer synthesis, Al (III) is a fourfold coordinated atoms, so [AlO4] has the ability to negatively charge and adsorb positive charge. In addition, the geopolymer is durable and resistant to weather, and simple processing.
Geopolymer is a hotspot in recent international research of new green cementitious materials. It is a kind of silicon oxygen tetrahedron and aluminum oxygen tetrahedron polymerized amorphous to semi crystalline aluminosilicate polymer [7]. It has a wide application prospect due to the excellent performance of organic polymer, ceramics and cement, and has the benefits of easily available raw materials, low prices, and simple preparation energy conservation and emission reduction [8]. Geopolymer can be used for coating adhesives, waste or toxic solid materials and new types of cement, and their raw materials can be some natural mineral or aluminosilicate waste [9]. Fly ash is the principal solid waste discharged from coal-fired power plants. In many countries, coal that produces a lot of fly ash is taken as the important foundation energy. The accumulation of large amount of the fly ash will cause serious environmental pollution every year [10]. Using fly ash to prepare geopolymer the solid waste can turn into treasure and at the same time it can reduce the pollution to environment [11]. The main reaction process of fly ash geopolymer: first of all, some aluminosilicate on the surface of fly ash has melted with the help of alkali-activator, and then migration, concentration and dispersed polymerization, and finally formed new amorphous gel phase of aluminosilicate [12]. The main source of fly ash geopolymer strength is the three-dimensional hydrated sodium aluminosilicate gel “N-A-S-H” produced during the process of geopolymer reaction [13].
Previous studies demonstrated that geopolymers that composed from metakaolin or wastes, such as fly ash, slag, and tailing could be solidified some heavy metals. Mallow et al. studies showed that heavy metal ions were involved in the reaction process of geopolymer [14]. In the study of Zhang et al. [15], they used the blast furnace slag and metakaolin as raw material to prepare geopolymer, and studied the solidify performance of geopolymer to Cu (II), and Pb (II), found that when slag 50%, geopolymer solid has the highest compressive strength, the solidification rate of Cu (II), and Pb (II) were above 98.5%. The Bankowski et al. [16] used the kaolinite geopolymers to solidify the harmful elements in the fly ash and found that when added fly ash 40%, the metakaolin geopolymers were able to solidify most of the harmful elements in the fly ash, the most significant solidifying effect among them were As, Cr and Cu, and the solidifying effect of heavy metal ions was reduced when the addition amount was too large. Between alkali solution and aluminosilicate powder weight ratio is usually between 0.2 and 0.5, but many studies reported that this weight ratio is above 0.35. Researchers tend to publish their research results in the form of patents and periodical articles, and still less realization with industrialization, which restricts the process of industrialization. At present, from bench test to commercial-scale application, solidification technology of geopolymer still has many problems to be investigated and solved. Many people do not understand characteristics of the geopolymer, and the geopolymer can replace cement in some areas are not accepted, which geopolymer applications have greatly hindered the promotion.
This study added heavy metal ions Pb (II), Cd (II), Mn (II), and Cr (III) respectively in fly ash geopolymers. The products and microstructures of the geopolymers material (solidified bodies) were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FIIR) and scanning electron microscopy (SEM). Finally, some pilot-tests were conducted on the basis of this study. The goals of this article were to research and evaluate the properties of solidified bodies and explain the mechanism of the solidification of heavy metal ions in fly ash geopolymers. This study provided the technical basis for industrialization of this kind of material. At the same time, this work can be used as evidence for the treating arsenic containing Pb (II), Cd (II), Mn (II) or Cr (III) waste materials. Preparation of geopolymers from heavy metals waste and fly ash can achieved the purpose of solidification of heavy metals, and the solidified bodies had certain value of use.
Section snippets
Materials
Fly ash was obtained from a power plant in Ningxia (China). NaOH (AR), Cd (NO3)2·4H2O (AR) and Cr (NO3)3·9H2O (AR) were obtained from Tianjin Kermel Chemical Reagent Co. Ltd. MnSO4·H2O (AR) was obtained from Beijing chemical plant. Sodium silicate solution was obtained from Panyu chemical plant, (8 wt% Na2O, 25 wt% SiO2 and 67 wt% H2O, initial modulus (M = n (SiO2)/n(Na2O)) is 3.23). The composite activator was composed of NaOH and sodium silicate solution. NaOH was used to regulate sodium
Compressive strength
Fig. 3 shows compressive strength of samples. In blank sample FA-G, the compressive strength were 35.98 MPa, 37.44 MPa and 41.29 MPa on 3 day, 7 day and 28 day. When dosage of heavy metal ion were 1 wt% and 1.5 wt%, respectively, Mn (II) and Cr (III) made no improvement to compressive strength of samples. But Pb (II) and Cd (II) were different from the others. The addition of 1 wt%Pb (II), 1.5 wt%Pb (II) and 1 wt%Cr (III) was improved to compressive strength of geopolymers, respectively. Sample
Conclusion
Based on this study, the following conclusions were drawn:
Aiming at the problem of fly ash and heavy metal pollution, this study is feasible to solidify heavy metal with fly ash geopolymers.
The leaching concentration of heavy metal ions in the solidified body is much lower than that of the relevant regulations, and the solidification rate of heavy metals is above 99.9% after Pb (II), Cd (II), Mn (II)and Cr (III) respectively. When the same amount is added, the solidification effect of the fly
Acknowledgments
Thanks to China EU International Cooperation Project (SQ2013ZOG300003) and Ningxia Hui Autonomous Region science and technology support project (2014ZYH50).
References (25)
- et al.
Biosorption of heavy metals by Saccharomyces cerevisiae: a review
J. Biotechnol. Adv.
(2006) - et al.
Detoxification and immobilization of chromite ore processing residue with metakaolin-based geopolymer
J. Environ. Chem. Eng.
(2014) - et al.
Geopolymers for immobilization of Cr(6+), Cd(2+), and Pb(2+)
J. Hazard. Mater.
(2008) - et al.
Microstructure and self-solidification/stabilization (S/S) of heavy metals of nano-modified CFA–MSWIFA composite geopolymers
J. Constr. Build. Mater.
(2014) - et al.
Mechanical properties and microstructure analysis of fly ash geopolymeric recycled concrete
J. Hazard. Mater.
(2012) - et al.
Analysis of compressive strength development of concrete containing high volume fly ash
J. Constr. Build. Mater.
(2015) - et al.
Study on surface morphology and physicochemical properties of raw and activated South African coal and coal fly ash
J. Phys. Chem. Earth
(2010) - et al.
Microstructure development of alkali-activated fly ash cement: a descriptive mode
J. Cem. Concr. Res.
(2005) - et al.
Environmental, physical and structural characterisation of geopolymer matrixes synthesised from coal(co-)combustion fly ashes
J. Hazard. Mater.
(2008) - et al.
The effect of ionic contaminants on the early-age properties of alkali-activated fly ash-based cements
J. Cem. Concr. Res.
(2002)
Reaction kinetics, microstructure and strength behavior of alkali activated silico-manganese (SiMn) slag – Fly ash blends
J. Constr. Build. Mater.
Efficiency and mechanism of stabilization/solidification of Pb(II), Cd(II), Cu(II), Th(IV) and U(VI) in metakaolin based geopolymers
J. Appl. Clay Sci.
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