Detoxification and solidification of heavy metal of chromium using fly ash-based geopolymer with chemical agents
Introduction
With the accelerated process of industrialization, a variety of solid wastes contained heavy metals are produced, especially a lot of chromium compounds are produced in the process of iron and steel smelting and electroplating process. They have various chemical valences and complex binding modes, they are not only hard to be decomposed by microorganism, but also easy to chemically generate in organisms and become more toxic compounds, which have been a threaten to the health and living environment of human.
Traditional cementitious materials were recognized as solidify materials in early stage, but the shortages of cement solidified bodies are high permeability and heavy metals leaching concentration, as well as poor durability [1]. J. Davidovits et al. believed that cement and other traditional inorganic cementitious materials were not suitable to deal with solid wastes contained heavy metals [2], [3]. Geopolymers are new generation of aluminosilicate inorganic cementitious materials. They are generally synthesized by activation of an aluminosilicate source (natural mineral, artificial silicon aluminum compound and solid wastes) with an alkaline hydroxide or silicate solution [4], [5]. Due to the “Cage” structure of the geopolymer gel like zeolite, they have great advantages in solidifying/stabilizing heavy metals.
Fly ash is an industrial wastes generated from the coal-fired power station, metallurgical industry, and chemical industry. Especially using coal to generate electricity that produce a large number of fly ash, accounting 15–40% weight of the raw coal. Recently, annual production of fly ash in the world is about 800 million ton, and dramatically increases due to the large demand of power in China and India since 2004 [6]. Fly ash is rich SiO2 and Al2O3 which is potential to be prepared as geopolymers.
Chemical agents can make toxic and harmful components to be low solubility, low mobility, and low toxic substance through chemical reaction. According to the types of heavy metals which contained in wastes, chemical agents can be divided into inorganic chemical agents and organic chemical agents. Inorganic chemical agents are including gypsum, bleaching powder, phosphate, sulfide, etc. [7], [8], [9], [10]. Organic chemical agents are including dithiocarbamates (DTCR or DTC), ethylenediaminetetraacetic acid (EDTA), etc. [11], [12], [13]. They have good effects in the treatment of heavy metals.
This work planned to use fly ash-based geopolymer to solidify/stabilize heavy metals, and chemical agents were used as supplementary. The effects of chromium on the compressive strength, reaction products, and pore structures of fly ash-based geopolymer were studied. The mechanism of solidification/stabilization of chromium by geopolymers was explored. The detoxification effectiveness of chemical agents for geopolymers that with heavy metals was researched. This work helps to open up a new approach for reusing solid wastes contained chromium, and give reference to broaden the application and security guarantee of the wastes-based geopolymers.
Section snippets
Materials
Fly ash used in this experiment is from a company in Shanghai, whose specific surface area is about 370 m2/kg. Chemical components are listed in Table 1 and XRD pattern is shown in Fig. 1. The main phases of this fly ash are Quarts, Mullite, Calcium Oxide, and Corundum.
The solid mass ratio of sodium silicate is 42.7% (13.2% Na2O and 29.6% SiO2, initial modulus is 2.32). NaOH is from the Chinese Medicine Group Chemical Reagent Co., Ltd., purity is 96.0%. NaOH was used to adjust sodium silicate to
Effects of chromium on compressive strength
Compressive strength of geopolymers that with chromium reagents is shown in Fig. 2. It can be seen that when the dosage of Cr(NO3)3 ≤ 1.0%, there is few effects on early strength, and the compressive strength of solidified geopolymer bodies can reach 20.0 MPa. But with the increasing of curing age, their growth trends become different. The sample F-Cr(NO3)3-0.5 shows better compressive strength at the age of 7 day and 28 day, but the compressive strength of F-Cr(NO3)3-1.0 decreased by 2.6% and 11.9%
Conclusions
- 1.
Compressive strength of geopolymers could be improved when dosage of small content of Cr(NO3)3. Reinhardbraunsite (Ca5(SiO4)2(OH)2) was generated in geopolymers that with Cr(NO3)3 or CrO3 because of ‘ions exchange’. Respectively dosing Cr2O3, Cr and CrO3 could make total pore volume of geopolymers smaller and make geopolymers more compact.
- 2.
Chemical bonding and physical encapsulation both existed in geopolymers. Chemical bonding played main role in geopolymers with Cr(NO3)3, and physical
Acknowledgement
The authors acknowledge the financial supports received from the National Natural Science Foundation of China (No. 51478328), the Natural Science Foundation of Shanghai of China (No. 17ZR1442000), and the Fundamental Research Funds for the Central Universities (No. 0500219225).
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