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Detoxification and solidification of heavy metal of chromium using fly ash-based geopolymer with chemical agents
Construction and Building Materials 151 (2017) 394–404
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Detoxification and solidification of heavy metal of chromium using fly ash-based geopolymer with chemical agents Xiaolu Guo a,b,⇑, Liyan Zhang b, Jiabao Huang b, Huisheng Shi a,b a b
Key Laboratory of Advanced Civil Engineering Materials (Tongji University), Ministry of Education, Shanghai 201804, China School of Materials Science and Engineering, Tongji University, Shanghai 201804, China
h i g h l i g h t s Different dosages and chemical valences of chromium obviously affect geopolymers. Chromium affects compressive strength, reaction products and pore structures of geopolymers. Chemical bonding and physical encapsulation coexisting in chromium containing geopolymers. DTCR has a good detoxification performance in chromium containing geopolymers.
a r t i c l e
i n f o
Article history: Received 10 October 2016 Received in revised form 9 May 2017 Accepted 23 May 2017
Keywords: Geopolymers Fly ash Chromium Solidification/stabilization Chemical detoxification agents
a b s t r a c t Geopolymers are new cementitious materials that with 3 dimensional networks, which can effectively solidify/stabilize heavy metals. Utilization of fly ash as precursor to prepare geopolymers, and influences of dosages and chemical valences of chromium reagents on geopolymers, as well as detoxification effectiveness of chemical agents for geopolymers were studied. The results showed that compressive strength of geopolymers could be improved when dosage of Cr(NO3)3 is small. Reinhardbraunsite (Ca5(SiO4)2(OH)2) was generated in geopolymers with Cr(NO3)3 or CrO3 due to ‘ions exchange’. Respectively dosing Cr2O3, Cr and CrO3 could make total pore volume of geopolymers smaller and make geopolymers more compact. Chemical bonding and physical encapsulation both existed in geopolymers. Chemical bonding played main role in geopolymers with Cr(NO3)3, and physical encapsulation played main role in geopolymers with Cr2O3, Cr and CrO3, respectively. Chemical agent DTCR could effectively improve compressive strength and capture Cr3+ of geopolymers, enhancing their abilities of anti-erosion. Ó 2017 Elsevier Ltd. All rights reserved.
1. 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, ⇑ Corresponding author at: School of Materials Science and Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China. E-mail address: [email protected] (X. Guo). http://dx.doi.org/10.1016/j.conbuildmat.2017.05.199 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
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.
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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. [7–10]. Organic chemical agents are including dithiocarbamates (DTCR or DTC), ethylenediaminetetraacetic acid (EDTA), etc. [11–13]. 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 wastesbased geopolymers.
2. Experimental procedure 2.1. 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 obtain composite chemical activator whose modulus (M = n(SiO2)/n(Na2O)) is 1.5. Four types of chromium reagents, Cr(NO3)3, Cr2O3, Cr and CrO3, are also from the Chinese Medicine Group Chemical Reagent Co., Ltd., The purities of all the chromium reagents are 99.0%. In this experiment, four kinds of detoxification agents, i.e., chemical agents of Na2S, NaH2PO4, (2, 4, 6-trithione-1, 3, 5-triazine trisodium salt, TMT), and dithiocarbamate (DTCR) were used to improve the solidifying/stabilizing effectiveness of the fly ash-based geopolymer with heavy metals. The purities of Na2S and NaH2PO4 are 98.0% and 99.0%, respectively. The solid mass ratios of TMT and DTCR are 15.0% and 40.0%, respectively.
Fig. 1. XRD pattern of fly ash.
In order to improve the solidifying/stabilizing effectiveness of these solidified geopolymer bodies with Cr(NO3)3, four chemical detoxification agents (Na2S, NaH2PO4, TMT and DTCR) were chosen and dosed in fly ash-based geopolymers. In the solidified geopolymer samples, these four chemical detoxification agents simply marked as S, P, T, D, respectively. These solidified bodies with chemical detoxification agents were compared to the blank sample F-Cr(NO3)3. The detoxification of chemical agents has been analyzed through compressive strength, leaching toxicity, as well as microstructures that before leaching and after leaching for 14 days. The mix proportions of these solidified geopolymer bodies with different chemical detoxification agents are listed in Table 4. 2.3. X-ray diffraction spectrometer (XRD) The minerals composition of the geopolymer samples curing for 28 days (Samples in Tables 2 and 3) were tested by XRD. A Siemens-Bruker D5000 powder diffractometer with Cu-Ka radiation in the theta/h configuration was used for measurements. The diffractometer was operated at 40 kV and 30 mA. Measurements were made from 5° to 75° 2h at a rate of 1°/min with a step size of 0.02° (2h). 2.4. Brunauer-Emmett-Teller (BET) The specific surface area of 28 days geopolymer samples in Tables 2 and 3 were tested by BET. Specific surface area test range of 3H-2000PS1/2 BET is above 0.01 m2/g, and aperture measurement range is tested from 0.35 nm to 200 nm.
2.2. Geopolymers synthesis 2.5. X-ray photoelectron spectroscopy (XPS) By referring to the preparation technology of cement paste, geopolymers were prepared according to mix proportion. They were slowly stirred for 120 s, then stopped for 15 s, at last quickly stirred for 120 s. The pastes were put into mould with the size of 20 mm 20 mm 20 mm, and were vibrated to remove bubbles, then were slicked. They were cured for 24 h in the temperature of 20 ± 1 °C and relative humidity of 95% ± 1%, then removed the mould. At last, they were cured for design ages which are sealed with plastic film. In this experiment, four different dosages (0, 0.5%, 1.0% and 1.5%) of Cr(NO3)3, were chosen, and the effects of dosage of Cr(NO3)3 on geopolymers were studied, including compressive strength, reaction products, and pore structures on geopolymers curing for 28 days. The mix proportions of geopolymers samples that with different dosages of Cr(NO3)3 are listed in Table 2. Four types of chromium reagents (Cr(NO3)3, Cr2O3, Cr and CrO3) were mixed in fly ash-based geopolymers. Compared with the blank gyopolymer sample F, the effects of different chemical valences of chromium reagents on fly ash-based gyopolymer samples curing for 28 days, including compressive strength, reaction products and pore structures were studied. The mechanism of solidification/stabilization of chromium reagents by geopolymers were analyzed. The mix proportions of geopolymers that with different chemical valences of chromium reagents are listed in Table 3.
Geopolymer samples in Table 3 curing for 28 days were tested by XPS. Through XPS, change of binding energy of O1s, Si2p and Al2p will be obtained. Vacuum degree of ESCALAB 250Xi XPS is 4.3 10 10 mbar, energy resolution is 0.44 eV, and sensitivity is 3.28 Mcps. 2.6. Environmental scanning electron microscope (ESEM) Geopolymer samples in Table 3 curing for 28 days, as well as the system of F-Cr (NO3)3-1.0 with chemical detoxification agents before leaching and after leaching for 14 days were tested by ESEM. Acceleration voltage of Quanta 200 ESEM is 500V–30 kV. Resolution ratios both in high vacuum mode and environmental scanning mode are <2 nm. Energy spectrum resolution is 132 eV. 2.7. Inductively coupled plasma-atomic emitted spectrometer (ICP-AES) Leaching tests of geopolymer samples in Table 4 curing for 28 days were conducted according to the ‘‘Solid Waste-Extraction Procedure for Leaching ToxicityHorizontal Vibration Method” (HJ 557-2010). ICP-AES was used to obtain concen-
Table 1 Chemical compositions of fly ash. Chemical compositions
Na2O
MgO
Al2O3
SiO2
P2O5
SO3
Fe2O3
K2O
CaO
TiO2
Loss
Content/%
0.45
0.85
22.40
40.70
0.71
2.17
5.34
0.69
9.46
1.16
16.09
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Table 2 Mix proportions of geopolymers with different dosages of Cr(NO3)3. Samples
Fly ash/g
Cr(NO3)3/g
Activator/g
Water/g
F F-Cr(NO3)3-0.5 F-Cr(NO3)3-1.0 F-Cr(NO3)3-1.5
100.0 100.0 100.0 100.0
– 0.5 1.0 1.5
53.9 53.9 53.9 53.9
1.9 1.9 1.9 1.9
Table 3 Mix proportions of geopolymers with different chemical valences of chromium reagents. Samples
Kinds of heavy metals
Fly ash/g
Chromium reagent/g
Activator/g
Water/g
F F-Cr(NO3)3-1.0 F-Cr2O3-1.0 F-Cr-1.0 F-CrO3-1.0
– Cr(NO3)3 Cr2O3 Cr CrO3
100.0 100.0 100.0 100.0 100.0
– 1.0 1.0 1.0 1.0
53.9 53.9 53.9 53.9 53.9
1.9 1.9 1.9 1.9 1.9
Table 4 Mix proportions of Cr(NO3)3 solidified bodies with different chemical detoxification agents. Samples
Fly ash/g
Activator/g
Cr(NO3)3/g
Kinds of chemical agents
Chemical agent (solid content)/g
Water/g
F-Cr(NO3)3-1.0 F-Cr(NO3)3-1.0-S F-Cr(NO3)3-1.0-P F-Cr(NO3)3-1.0-T F-Cr(NO3)3-1.0-D
100.0 100.0 100.0 100.0 100.0
53.9 53.9 53.9 53.9 53.9
1.0 1.0 1.0 1.0 1.0
– Na2S NaH2PO3 TMT DTCR
– 2.0 2.0 2.0 2.0
1.9 1.9 1.9 1.9 1.9
F-Cr(NO3)3-3.0 F-Cr(NO3)3-3.0-S F-Cr(NO3)3-3.0-P F-Cr(NO3)3-3.0-T F-Cr(NO3)3-3.0-D
100.0 100.0 100.0 100.0 100.0
53.9 53.9 53.9 53.9 53.9
3.0 3.0 3.0 3.0 3.0
– Na2S NaH2PO3 TMT DTCR
– 2.0 2.0 2.0 2.0
1.9 1.9 1.9 1.9 1.9
tration of total chromium content. The wavelength range of OPTIMA 2100DVⅡICPAES is 160–900 nm, resolution ratio is <0.003 nm in 200 nm, event frequency is 40.68 MHz.
3. Results and discussion 3.1. Effects of dosages and valences of chromium on geopolymers 3.1.1. 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% at the age of 7 day and 28 day, respectively. Particularly when the dosage of Cr(NO3)3 is 1.5%, the compressive strength has been influenced greatly, decreasing by 14.1%, 20.9% and 31.9% at the age of 3 day, 7 day and 28 day, respectively. The results show that only when the dosage of heavy metals in reasonable range, the compressive strength appears a good developing trend, which is corresponded to the results of Zhang et al. [14]. Different chemical valences of chromium affect compressive strength greatly. At the age of 3 day, 7 day and 28 day, the
Fig. 2. Compressive strength of geopolymers that with chromium reagents. (A) Different dosages of Cr(NO3)3. (B) Different chemical valences of chromium reagents.
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Fig. 3. XRD patterns of reaction products of geopolymers that with chromium reagents. (A) Different dosages of Cr(NO3)3. (B) Different chemical valences of chromium reagents.
Table 5 Pore structures of geopolymers with different dosages of Cr(NO3)3. Samples
Total pore volume/mlg
F F-Cr(NO3)3-0.5 F-Cr(NO3)3-1.0 F-Cr(NO3)3-1.5
0.0511 0.0484 0.0865 0.0660
1
Average hole diameter/nm
11.41 13.23 12.19 9.59
Pore distribution/% <2 nm
2 nm–20 nm
20 nm–50 nm
>50 nm
0 0 0 0
63.2 66.5 63.2 62.1
23.5 21.9 23.1 24.0
13.3 11.6 13.7 13.9
Table 6 Pore structures of geopolymers with different chemical valences of chromium reagents. Samples
F F-Cr(NO3)3-1.0 F-Cr2O3-1.0 F-Cr-1.0 F-CrO3-1.0
Total pore volume/mlg
0.0511 0.0865 0.0426 0.0315 0.0378
1
Average hole diameter/nm
11.41 12.19 18.45 15.34 17.19
compressive strength of geopolymer samples of F-Cr2O3-1.0, F-Cr1.0 and F-CrO3-1.0 are all larger than that of the blank sample F. When Cr2O3, Cr and CrO3 are dosed into geopolymers, some of them will be soluble in alkali, but most of them still keep the initial formations, so physical encapsulation is the main way of geopolymer to solidify/stabilize chromium. The compressive strength of FCr(NO3)3-1.0 apparently decreases at the age of 7 day and 28 day, and the main way of geopolymers to solidify/stabilize chromium is chemical bonding. Making comparison of these samples, it is found that when the dosage of chromium reagent is 1%, physical encapsulation can keep better compressive strength than chemical bonding. The reason may be that the oxide or element of chromium can work as physical filler, resulting in a more compact system.
3.1.2. Effects of chromium on reaction products XRD patterns of reaction products of geopolymers that with chromium reagents are shown in Fig. 3. In Fig. 3(A and B), the main phases of blank sample F are mullite and quarts. Dispersion peaks in the range of 20°–40° are non-crystalline phases alkali aluminum silicate (N-A-S-H) and calcium silicate hydrate (C-S-H).
Pore distribution/% <2 nm
2 nm–20 nm
20 nm–50 nm
>50 nm
0 0 0 0 0
63.2 63.2 65.8 69.7 63.5
23.5 23.1 15.4 14.5 16.3
13.3 13.7 18.8 15.8 20.2
In Fig. 3(A and B), except mullite and quarts, geopolymers that with Cr(NO3)3 or CrO3 also have reinhardbraunsite Ca5(SiO4)2(OH)2. Because chromium compounds show different solubility deliquescence. Thus Cr(NO3)3 and CrO3 can give out a lot of heavy metals Cr3+ and Cr6+. These heavy metals break the bonds of CaAl in N-A-S-H gel and the bonds of Ca-Si in C-S-H gel, then replace original Ca2+. The Ca2+ which has been replaced will react with [SiO4]4 and OH , and form stable reinhardbraunsite Ca5(SiO4)2(OH)2 [15]. There is no peak of reinhardbraunsite in samples of F-Cr-1.0 and F-Cr2O3-1.0, since Cr and Cr2O3 don not supply many Cr3+ or Cr6+, and they are enwrapped by geopolymers with the initial formation.
3.1.3. Effects of chromium on pore structures According to the International Union of Pure and Applied Chemistry (IUPAC), pores can be divided into macropores (>50 nm), mesopores (2–50 nm) and micropores (<2 nm) [16]. In geopolymers, macropores represent gaps between unreacted fly ash, mesopores represent typical pores between geopolymer gels, and micropores often appear in the network structure of gels [17].
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Fig. 4. Binding energies of O1s, Si2p and Al2p of geopolymers that with different chemical valence of chromium reagents/eV. (A) Binding energies of O1s. (B) Binding energies of Si2p. (C) Binding energies of Al2p.
Table 7 Binding energies and chemical shifts of O1s, Si2p and Al2p of geopolymers with different chemical valences of chromium reagents/eV. Samples
F F-Cr(NO3)3-1.0 F-Cr2O3-1.0 F-Cr-1.0 F-CrO3-1.0
O1s
Si2p
Al2p
Binding energies
Chemical shifts
Binding energies
Chemical shifts
Binding energies
Chemical shifts
531.27 531.72 531.31 531.43 531.34
– +0.45 +0.04 +0.16 +0.07
102.09 102.41 102.24 102.43 102.27
– +0.32 +0.15 +0.34 +0.18
73.94 74.31 74.23 74.31 74.16
– +0.37 +0.29 +0.37 +0.22
Table 8 Leaching tests of Cr(NO3)3 solidified bodies with chemical detoxification agents/mgL
1
.
Samples
F-Cr(NO3)3-1.0
F-Cr(NO3)3-1.0-S
F-Cr(NO3)3-1.0-P
F-Cr(NO3)3-1.0-T
Total chromium
0.07
0.07
0.08
0.06
0.04
Samples
F-Cr(NO3)3-3.0
F-Cr(NO3)3-3.0-S
F-Cr(NO3)3-3.0-P
F-Cr(NO3)3-3.0-T
F-Cr(NO3)3-3.0-D
Total chromium
1.93
1.27
1.70
0.59
1.05
Pore structures of geopolymers that with different dosages of Cr (NO3)3 are shown in Table 5. It is can be seen that only the total pore volume of sample F-Cr(NO3)3-0.5 is smaller than that of the blank sample F, and the number of pores in the range of
F-Cr(NO3)3-1.0-D
2–20 nm increases. It represents the lower dosage of Cr(NO3)3 can decrease pore volume of geopolymers and make them more compact, resulting in better development of compressive strength.
X. Guo et al. / Construction and Building Materials 151 (2017) 394–404
In Table 5, average pore diameter and total pore volume of sample F-Cr(NO3)3-1.5 are smaller than that of the sample F-Cr (NO3)3-1.0. Compressive strength of F-Cr(NO3)3-1.5 would not decrease concluded from the total pore volume results, but in fact, it decreases dramatically. Only through pore structures cannot explain this phenomenon. It may have relationship with chemical reaction between Cr3+ and gels (N-A-S-H and C-S-H), which is similar to the result of Jin et al. [18]. Due to 4 Al-O bonds around Al, and every O contributes one electron to Al3+, so the valence of alumina tetrahedron is 1. When Cr3+ participates in charge balance, a Cr3+ can balance 3 alumina tetrahedrons. Due to space volume of
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alumina tetrahedron is large, the length of Cr-[AlO4] bond is long, making binding energy weak. In result, network structures of geopolymers are not stable, and the compressive strength dramatically decreases. Pore structures of geopolymers that with different chemical valences of chromium reagents are shown in Table 6. Total pore volume and average pore diameter of sample F-Cr(NO3)3 are both larger than that of the blank sample F, thus its compressive strength decreases. Average pore diameter of samples F-Cr2O3-1.0, F-Cr-1.0, and F-CrO3-1.0 are larger than that of the blank sample F, but their total pore volume are smaller. After inter-
Fig. 5. Pore structures of geopolymers that with different chemical valences of chromium reagents. (A) F. (B) F-Cr(NO3)3-1.0. (C) F-Cr2O3-1.0. (D) F-Cr-1.0. (E) F-CrO3-1.0.
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action, total pore volume dominates main influence, these three samples show better compressive strength. 3.2. Solidification/stabilization of chromium by geopolymers 3.2.1. XPS analysis Elemental binding energies and chemical shifts are directly related with effective atomic charges. Elemental binding energy is the energy that electrons reach fermi level by overcoming
effects of nuclear and surrounding electrons. Chemical shift is regular displacement that resulted from the changes of the chemical compounds structures or element oxidation states. On one hand, the inner electrons have a certain binding energy, which are strongly affected by the atomic nucleus in Coulomb. On the other hand, they are affected by the shielding effect of the outer electrons. If valence of element or negativity of surrounding elements changes, binding energies of inner electrons will also change at the same time.
Fig. 6. Reaction products of geopolymers that with different chemical valences of chromium reagents. (A) F. (B) F-Cr(NO3)3-1.0. (C) F-Cr2O3-1.0. (D) F-Cr-1.0. (E) F-CrO3-1.0.
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Fig. 4 and Table 7 shows the binding energies and chemical shifts of O1s, Si2p and Al2p of geopolymers that with different chemical valences of chromium reagents. In Fig. 4(A) and Table 7, 531.27 eV represents binding energy of O of the blank sample F. when dose different chromium reagents to geopolymers, the binding energies of O of samples of F-Cr(NO3)3-1.0, F-Cr2O3-1.0, F-Cr-1.0, and F-CrO3-1.0 are risen by 0.45 eV, 0.04 eV, 0.16 eV and 0.07 eV, respectively. It represents that the influence of Cr3+ from Cr(NO3)3 on O1s is larger than that from Cr2O3, Cr and CrO3. Because Cr(NO3)3 is easy to dissolve and disperse in the system, so it is easy to participate in valence balance. In Fig. 4(B) and Table 7, 102.09 eV represents binding energy of Si of the blank sample F. The binding energies of Si of samples of FCr(NO3)3-1.0, F-Cr2O3-1.0, F-Cr-1.0, and F-CrO3-1.0 are risen by 0.32 eV, 0.15 eV, 0.34 eV and 0.18 eV, respectively. If polymerization degree of Si-O is high, it will be responsible for high binding energy [19]. A research showed that after dosing Cr3+, there is a very strong polymerization in network of silicate and/or aluminum silicate, and is similar to the phenomenon of immobilizing chromium by cement [20]. The results of this experiment show that different chemical valences of chromium can improve polymerization degree of Si-O. In Fig. 4(C) and Table 7, 73.94 eV represents binding energy of Al of blank sample F. The binding energies of Al of F-Cr(NO3)31.0, F-Cr2O3-1.0, F-Cr-1.0, and F-CrO3-1.0 are risen by 0.37 eV, 0.29 eV, 0.37 eV and 0.22 eV, respectively. Combining the change of O1s, Si2p and Al2p, it is can be seen that electron density of O, Si and Al all decrease, showing heavy metals make initial electron density close to Cr3+ or Cr6+. Because Cr3+ and Cr6+ have lost 3 or 6 electrons, they need to be balanced by electrons of other nucleus. Among them, Cr(NO3)3 have great effects on the binding energies of 3 elements, because it easily chemically bonds with geopolymer gels. In contrary, Cr2O3, Cr and CrO3 must dissolve in alkali at first, but there is little alkali for dissolution. So the numbers of Cr3+ and Cr6+, as well as the effects on O1s, Si2p and Al2p are both small. In summary, when chemical valences of heavy metals are different, the main reason to change electron density is the different dissolving ability of heavy metal compounds. Those heavy metal compounds which are easy to dissolve in alkali will have a large number of heavy metal ions to participate in valence balance.
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3.2.2. ESEM analysis Pore structures and reaction products of geopolymers with different chemical valences of chromium reagents testing by ESEM are shown in Figs. 5 and 6, respectively. In blank sample F (Fig. 5(A)), fly ash generates spongy gels with an alkali-activator, and enwraps those unreacted fly ash. There are many honeycomb micropores in the surface of geopolymer. Some fibrous C-S-H gels distribute in the surface or micropores of the sample. In Fig. 5(B), honeycomb micropores distribute more widely after dosing Cr(NO3)3, increasing internal specific surface energy of gels and promoting physical adsorption of Cr3+ by gropolymers. Besides, more and more unreacted fly ash reduces N-A-S-H gels and C-S-H gels. Cr(NO3)3 seriously affects dissolution rate of silicon aluminum phase, and slows down reaction process of geopolymers. It makes physical encapsulation and chemical bonding weak, and then affects the results of solidification/stabilization of Cr3+. It shows that heavy metal ions can slow down reaction process of geopolymers, which is corresponded to the results of our preliminary experiment [21]. In Fig. 5(C–E), it is obvious to find some big irregular substances. They may be Cr2O3, Cr and CrO3, which are enwrapped by N-A-S-H gels. Once Zhang et al. put element Pb into geopolymers, and cannot observe any element Pb in SEM figures. It represents those heavy metal compounds or elements that difficult to dissolve are mainly physically enwrapped by geopolymer gels [22]. In blank sample F (Fig. 6(A)), there are lots of fibrous C-S-H gels in the surface of the sample. However, after the addition of Cr (NO3)3, some small irregular substances was generate. It maybe shows that a portion of Cr(NO3)3 are enwrapped by N-A-S-H gels. The same phenomenon was observed in Fig. 6(C–E), but irregular substances were bigger in size, which is corresponded to the observation in Fig. 5(C–E). According to Fig. 2, compressive strengths of geopolymers have been improved after dosing Cr2O3, Cr and CrO3. The reasons may be that, on one hand, some heavy metals from Cr2O3, Cr and CrO3 participate in valence balance, on the other hand, Cr2O3, Cr and CrO3 physically fill into geopolymers. That decreases total pore volumes and makes structures more compact. In summary, chemical bonding and physical encapsulation affect heavy metal solidified bodies at the same time, but which effect plays the main role is up to the formation of heavy metals. Those heavy metal compounds which are easy to dissolve in alkali
Fig. 7. Compressive strength of Cr(NO3)3 solidified bodies that with chemical detoxification agents. (A) System of F-Cr(NO3)3-1.0. (B) System of F-Cr(NO3)3-3.0
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are mainly chemically bonded by geopolymers, and the others are mainly physically enwrapped by geopolymers. 3.3. Detoxification effectiveness of chemical agents for geopolymers 3.3.1. Detoxification effectiveness on compressive strength Fig. 7 shows compressive strength of Cr(NO3)3 solidified geopolymer bodies with four chemical detoxification agents, respectively. When dosage of Cr(NO3)3 is 1.0%, chemical
detoxification agents of Na2S, NaH2PO3 and TMT make no improvement to solidified bodies. But chemical detoxification agents of DTCR on the sample F-Cr(NO3)3-1.0-D is different from the others, and it is improved by 3.4%, 1.5% and 3.7% on 3 day, 7 day and 28 day, respectively. Sample F-Cr(NO3)3-1.0 possesses a good development of compressive strength, but F-Cr(NO3)3-3.0 shows worse than F-Cr (NO3)3-1.0. In Fig. 7(B), they are decreased by 45.9%, 44.4% and 32.3% on 3 day, 7 day and 28 day, respectively. It is easy to find that
Fig. 8. ESEM figures about Cr(NO3)3 solidified bodies with chemical detoxification agents before and after leaching. (A1) F-Cr(NO3)3-1.0 before leaching. (A2) F-Cr(NO3)3-1.0 after leaching. (B1) F-Cr(NO3)3-1.0-S before leaching. (B2) F-Cr(NO3)3-1.0-S after leaching. (C1) F-Cr(NO3)3-1.0-P before leaching. (C2) F-Cr(NO3)3-1.0-P after leaching. (D1) F-Cr (NO3)3-1.0-T before leaching. (D2) F-Cr(NO3)3-1.0-T after leaching. (E1) F-Cr(NO3)3-1.0-D before leaching. (E2) F-Cr(NO3)3-1.0-D after leaching.
X. Guo et al. / Construction and Building Materials 151 (2017) 394–404
403
Fig. 8 (continued)
compressive strength at early age are seriously affected when dose 3.0% Cr(NO3)3. As the same as sample F-Cr(NO3)3-1.0, Na2S, NaH2PO3 and TMT give no advantage to compressive strength at early age and later age, except DTCR. It is worth noting that compressive strength of F-Cr(NO3)3-3.0-D is 47.2 MPa, which is higher than F-Cr (NO3)3-1.0-D. Compared with F-Cr(NO3)3-3.0, the compressive strength of F-Cr(NO3)3-3.0-D even has been improved by 59.5%. It shows that DTCR can improve compressive strength of solidified geopolymer bodies, and the more dosage of heavy metals, the more obvious the effect is. The reason for DTCR improving compressive strength may be that macromolecules can be physically adsorbed by C-S-H gels, and intercalate into the interlayer of the C-S-H gels, which make structures of C-S-H gels different [23]. In geopolymers, both of TMT and DTCR can make change of C-S-H gels through intercalation reaction, and DTCR makes solidified bodies more compact than TMT which is implied from compressive strength. TMT may not have good compatibility with C-S-H gels, and results in a bad performance even intercalate into C-S-H gels.
When the dosage of Cr(NO3)3 is up to 3.0%, leaching toxicity of F-Cr(NO3)3-3.0 is 1.93 mgL 1, which is 27.6 times as large as F-Cr (NO3)3-1.0. But it is still in the range of threshold limit of total chromium content. After adding four chemical agents, there are different improvements between them. It is obvious to find TMT has the best performance, then DTCR. Both of them are organic chemical agent, which indicate that organic chemical agents are better than inorganic chemical agents in geopolymers. Once Li et al. used TMT to deal with heavy metals in municipal solid waste incineration fly ash (MSWIFA), and found it is better than the other inorganic chemical agents [24], which is corresponded to our experiment results. Besides, Cr3+ is very easy to oxidize to Cr6+, and that will increase its harm to environment. The sulfur atom in the thiol group of TMT has an empty d orbit. It is easy to lose electrons, and capture heavy metals by negative electric field which generated from polarization distortion. It can make Cr6+ reduce as Cr3+, then forms stable precipitate through complexation reaction. DTCR also has the same sulfur atoms, so the results of leaching test are well, too.
3.3.2. Leaching tests Table 8 is the results of leaching tests of Cr(NO3)3 solidified bodies that with chemical agents. Threshold limit of total chromium content is 15 mgL 1. Leaching toxicity of blank sample F-Cr (NO3)3-1.0 is 0.07 mgL 1, which is far below threshold limit of total chromium content. It indicates that geopolymers have a great advantage in solidifying/stabilizing Cr(NO3)3.
3.3.3. Micrographs of geopolymers before and after leaching As is shown in Figs. 7 and 8, when dosage of Cr(NO3)3 is 1.0%, compressive strength of F-Cr(NO3)3-1.0 is high on 28 day, results of extraction toxicities is also good, solidified bodies of the sample F-Cr(NO3)3-1.0 was chosen to be observed by ESEM before leaching and after leaching. Fig. 8 shows the ESEM figures about Cr(NO3)3 solidified bodies with four chemical detoxification agents before leaching and after
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leaching, respectively. As is shown in Fig. 8(A1 and A2), there are some difference between two samples F-Cr(NO3)3-1.0 that before and after leaching. It is easy to find that the sizes of micropores are becoming larger, the number of micropores is increasing, and the surroundings of micropores become loose. After adding different chemical agents, small differences also appear in different samples. Especially for Na2S, there are much more crackles surround pores after leaching which are shown in Fig. 8(B2). According to Griffith theory, these pores and crackles will cause cracks due to stress concentration [25]. After leaching, the number of pores and crackles increases, which indicates that liquid will make more erosion to solidified bodies and leach more heavy metals. After leaching for 14 days, although the number of pores and cracks increases, it’s not obvious for geopolymers that with TMT or DTCR in Fig. 8(D2 and E2). The reasons can be listed as follows. (1) Maybe there is an intercalation reaction when TMT or DTCR capture Cr3+, so the samples are hard to be eroded. (2) There is some unreacted fly ash in geopolymers’ system before leaching. But when water comes into interior through capillary pores, the unreacted fly ash will generate N-A-S-H gels with water and alkali, furtherly filling solidified bodies. 4. 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 encapsulation played main role in geopolymers with Cr2O3, Cr and CrO3, respectively. 3. Chemical detoxification agent DTCR could effectively improve compressive strength, and the more dosage of Cr(NO3)3, the more improvements it will make. DTCR also could effectively capture Cr3+ of geopolymers, enhancing their abilities of antierosion.
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). References [1] J.G. Jiang, X.U. Xin, Y. Zhang, Investigation of leaching characteristics of heavy metals during cement stabilization of fly ash from municipal solid waste incinerator, Environ. Sci. 27 (12) (2007) 2564–2569.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Detoxification and solidification of heavy metal of chromium using fly ash-based geopolymer with chemical agents Xiaolu Guo a,b,⇑, Liyan Zhang b, Jiabao Huang b, Huisheng Shi a,b a b
Key Laboratory of Advanced Civil Engineering Materials (Tongji University), Ministry of Education, Shanghai 201804, China School of Materials Science and Engineering, Tongji University, Shanghai 201804, China
h i g h l i g h t s Different dosages and chemical valences of chromium obviously affect geopolymers. Chromium affects compressive strength, reaction products and pore structures of geopolymers. Chemical bonding and physical encapsulation coexisting in chromium containing geopolymers. DTCR has a good detoxification performance in chromium containing geopolymers.
a r t i c l e
i n f o
Article history: Received 10 October 2016 Received in revised form 9 May 2017 Accepted 23 May 2017
Keywords: Geopolymers Fly ash Chromium Solidification/stabilization Chemical detoxification agents
a b s t r a c t Geopolymers are new cementitious materials that with 3 dimensional networks, which can effectively solidify/stabilize heavy metals. Utilization of fly ash as precursor to prepare geopolymers, and influences of dosages and chemical valences of chromium reagents on geopolymers, as well as detoxification effectiveness of chemical agents for geopolymers were studied. The results showed that compressive strength of geopolymers could be improved when dosage of Cr(NO3)3 is small. Reinhardbraunsite (Ca5(SiO4)2(OH)2) was generated in geopolymers with Cr(NO3)3 or CrO3 due to ‘ions exchange’. Respectively dosing Cr2O3, Cr and CrO3 could make total pore volume of geopolymers smaller and make geopolymers more compact. Chemical bonding and physical encapsulation both existed in geopolymers. Chemical bonding played main role in geopolymers with Cr(NO3)3, and physical encapsulation played main role in geopolymers with Cr2O3, Cr and CrO3, respectively. Chemical agent DTCR could effectively improve compressive strength and capture Cr3+ of geopolymers, enhancing their abilities of anti-erosion. Ó 2017 Elsevier Ltd. All rights reserved.
1. 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, ⇑ Corresponding author at: School of Materials Science and Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China. E-mail address: [email protected] (X. Guo). http://dx.doi.org/10.1016/j.conbuildmat.2017.05.199 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
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.
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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. [7–10]. Organic chemical agents are including dithiocarbamates (DTCR or DTC), ethylenediaminetetraacetic acid (EDTA), etc. [11–13]. 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 wastesbased geopolymers.
2. Experimental procedure 2.1. 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 obtain composite chemical activator whose modulus (M = n(SiO2)/n(Na2O)) is 1.5. Four types of chromium reagents, Cr(NO3)3, Cr2O3, Cr and CrO3, are also from the Chinese Medicine Group Chemical Reagent Co., Ltd., The purities of all the chromium reagents are 99.0%. In this experiment, four kinds of detoxification agents, i.e., chemical agents of Na2S, NaH2PO4, (2, 4, 6-trithione-1, 3, 5-triazine trisodium salt, TMT), and dithiocarbamate (DTCR) were used to improve the solidifying/stabilizing effectiveness of the fly ash-based geopolymer with heavy metals. The purities of Na2S and NaH2PO4 are 98.0% and 99.0%, respectively. The solid mass ratios of TMT and DTCR are 15.0% and 40.0%, respectively.
Fig. 1. XRD pattern of fly ash.
In order to improve the solidifying/stabilizing effectiveness of these solidified geopolymer bodies with Cr(NO3)3, four chemical detoxification agents (Na2S, NaH2PO4, TMT and DTCR) were chosen and dosed in fly ash-based geopolymers. In the solidified geopolymer samples, these four chemical detoxification agents simply marked as S, P, T, D, respectively. These solidified bodies with chemical detoxification agents were compared to the blank sample F-Cr(NO3)3. The detoxification of chemical agents has been analyzed through compressive strength, leaching toxicity, as well as microstructures that before leaching and after leaching for 14 days. The mix proportions of these solidified geopolymer bodies with different chemical detoxification agents are listed in Table 4. 2.3. X-ray diffraction spectrometer (XRD) The minerals composition of the geopolymer samples curing for 28 days (Samples in Tables 2 and 3) were tested by XRD. A Siemens-Bruker D5000 powder diffractometer with Cu-Ka radiation in the theta/h configuration was used for measurements. The diffractometer was operated at 40 kV and 30 mA. Measurements were made from 5° to 75° 2h at a rate of 1°/min with a step size of 0.02° (2h). 2.4. Brunauer-Emmett-Teller (BET) The specific surface area of 28 days geopolymer samples in Tables 2 and 3 were tested by BET. Specific surface area test range of 3H-2000PS1/2 BET is above 0.01 m2/g, and aperture measurement range is tested from 0.35 nm to 200 nm.
2.2. Geopolymers synthesis 2.5. X-ray photoelectron spectroscopy (XPS) By referring to the preparation technology of cement paste, geopolymers were prepared according to mix proportion. They were slowly stirred for 120 s, then stopped for 15 s, at last quickly stirred for 120 s. The pastes were put into mould with the size of 20 mm 20 mm 20 mm, and were vibrated to remove bubbles, then were slicked. They were cured for 24 h in the temperature of 20 ± 1 °C and relative humidity of 95% ± 1%, then removed the mould. At last, they were cured for design ages which are sealed with plastic film. In this experiment, four different dosages (0, 0.5%, 1.0% and 1.5%) of Cr(NO3)3, were chosen, and the effects of dosage of Cr(NO3)3 on geopolymers were studied, including compressive strength, reaction products, and pore structures on geopolymers curing for 28 days. The mix proportions of geopolymers samples that with different dosages of Cr(NO3)3 are listed in Table 2. Four types of chromium reagents (Cr(NO3)3, Cr2O3, Cr and CrO3) were mixed in fly ash-based geopolymers. Compared with the blank gyopolymer sample F, the effects of different chemical valences of chromium reagents on fly ash-based gyopolymer samples curing for 28 days, including compressive strength, reaction products and pore structures were studied. The mechanism of solidification/stabilization of chromium reagents by geopolymers were analyzed. The mix proportions of geopolymers that with different chemical valences of chromium reagents are listed in Table 3.
Geopolymer samples in Table 3 curing for 28 days were tested by XPS. Through XPS, change of binding energy of O1s, Si2p and Al2p will be obtained. Vacuum degree of ESCALAB 250Xi XPS is 4.3 10 10 mbar, energy resolution is 0.44 eV, and sensitivity is 3.28 Mcps. 2.6. Environmental scanning electron microscope (ESEM) Geopolymer samples in Table 3 curing for 28 days, as well as the system of F-Cr (NO3)3-1.0 with chemical detoxification agents before leaching and after leaching for 14 days were tested by ESEM. Acceleration voltage of Quanta 200 ESEM is 500V–30 kV. Resolution ratios both in high vacuum mode and environmental scanning mode are <2 nm. Energy spectrum resolution is 132 eV. 2.7. Inductively coupled plasma-atomic emitted spectrometer (ICP-AES) Leaching tests of geopolymer samples in Table 4 curing for 28 days were conducted according to the ‘‘Solid Waste-Extraction Procedure for Leaching ToxicityHorizontal Vibration Method” (HJ 557-2010). ICP-AES was used to obtain concen-
Table 1 Chemical compositions of fly ash. Chemical compositions
Na2O
MgO
Al2O3
SiO2
P2O5
SO3
Fe2O3
K2O
CaO
TiO2
Loss
Content/%
0.45
0.85
22.40
40.70
0.71
2.17
5.34
0.69
9.46
1.16
16.09
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Table 2 Mix proportions of geopolymers with different dosages of Cr(NO3)3. Samples
Fly ash/g
Cr(NO3)3/g
Activator/g
Water/g
F F-Cr(NO3)3-0.5 F-Cr(NO3)3-1.0 F-Cr(NO3)3-1.5
100.0 100.0 100.0 100.0
– 0.5 1.0 1.5
53.9 53.9 53.9 53.9
1.9 1.9 1.9 1.9
Table 3 Mix proportions of geopolymers with different chemical valences of chromium reagents. Samples
Kinds of heavy metals
Fly ash/g
Chromium reagent/g
Activator/g
Water/g
F F-Cr(NO3)3-1.0 F-Cr2O3-1.0 F-Cr-1.0 F-CrO3-1.0
– Cr(NO3)3 Cr2O3 Cr CrO3
100.0 100.0 100.0 100.0 100.0
– 1.0 1.0 1.0 1.0
53.9 53.9 53.9 53.9 53.9
1.9 1.9 1.9 1.9 1.9
Table 4 Mix proportions of Cr(NO3)3 solidified bodies with different chemical detoxification agents. Samples
Fly ash/g
Activator/g
Cr(NO3)3/g
Kinds of chemical agents
Chemical agent (solid content)/g
Water/g
F-Cr(NO3)3-1.0 F-Cr(NO3)3-1.0-S F-Cr(NO3)3-1.0-P F-Cr(NO3)3-1.0-T F-Cr(NO3)3-1.0-D
100.0 100.0 100.0 100.0 100.0
53.9 53.9 53.9 53.9 53.9
1.0 1.0 1.0 1.0 1.0
– Na2S NaH2PO3 TMT DTCR
– 2.0 2.0 2.0 2.0
1.9 1.9 1.9 1.9 1.9
F-Cr(NO3)3-3.0 F-Cr(NO3)3-3.0-S F-Cr(NO3)3-3.0-P F-Cr(NO3)3-3.0-T F-Cr(NO3)3-3.0-D
100.0 100.0 100.0 100.0 100.0
53.9 53.9 53.9 53.9 53.9
3.0 3.0 3.0 3.0 3.0
– Na2S NaH2PO3 TMT DTCR
– 2.0 2.0 2.0 2.0
1.9 1.9 1.9 1.9 1.9
tration of total chromium content. The wavelength range of OPTIMA 2100DVⅡICPAES is 160–900 nm, resolution ratio is <0.003 nm in 200 nm, event frequency is 40.68 MHz.
3. Results and discussion 3.1. Effects of dosages and valences of chromium on geopolymers 3.1.1. 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% at the age of 7 day and 28 day, respectively. Particularly when the dosage of Cr(NO3)3 is 1.5%, the compressive strength has been influenced greatly, decreasing by 14.1%, 20.9% and 31.9% at the age of 3 day, 7 day and 28 day, respectively. The results show that only when the dosage of heavy metals in reasonable range, the compressive strength appears a good developing trend, which is corresponded to the results of Zhang et al. [14]. Different chemical valences of chromium affect compressive strength greatly. At the age of 3 day, 7 day and 28 day, the
Fig. 2. Compressive strength of geopolymers that with chromium reagents. (A) Different dosages of Cr(NO3)3. (B) Different chemical valences of chromium reagents.
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Fig. 3. XRD patterns of reaction products of geopolymers that with chromium reagents. (A) Different dosages of Cr(NO3)3. (B) Different chemical valences of chromium reagents.
Table 5 Pore structures of geopolymers with different dosages of Cr(NO3)3. Samples
Total pore volume/mlg
F F-Cr(NO3)3-0.5 F-Cr(NO3)3-1.0 F-Cr(NO3)3-1.5
0.0511 0.0484 0.0865 0.0660
1
Average hole diameter/nm
11.41 13.23 12.19 9.59
Pore distribution/% <2 nm
2 nm–20 nm
20 nm–50 nm
>50 nm
0 0 0 0
63.2 66.5 63.2 62.1
23.5 21.9 23.1 24.0
13.3 11.6 13.7 13.9
Table 6 Pore structures of geopolymers with different chemical valences of chromium reagents. Samples
F F-Cr(NO3)3-1.0 F-Cr2O3-1.0 F-Cr-1.0 F-CrO3-1.0
Total pore volume/mlg
0.0511 0.0865 0.0426 0.0315 0.0378
1
Average hole diameter/nm
11.41 12.19 18.45 15.34 17.19
compressive strength of geopolymer samples of F-Cr2O3-1.0, F-Cr1.0 and F-CrO3-1.0 are all larger than that of the blank sample F. When Cr2O3, Cr and CrO3 are dosed into geopolymers, some of them will be soluble in alkali, but most of them still keep the initial formations, so physical encapsulation is the main way of geopolymer to solidify/stabilize chromium. The compressive strength of FCr(NO3)3-1.0 apparently decreases at the age of 7 day and 28 day, and the main way of geopolymers to solidify/stabilize chromium is chemical bonding. Making comparison of these samples, it is found that when the dosage of chromium reagent is 1%, physical encapsulation can keep better compressive strength than chemical bonding. The reason may be that the oxide or element of chromium can work as physical filler, resulting in a more compact system.
3.1.2. Effects of chromium on reaction products XRD patterns of reaction products of geopolymers that with chromium reagents are shown in Fig. 3. In Fig. 3(A and B), the main phases of blank sample F are mullite and quarts. Dispersion peaks in the range of 20°–40° are non-crystalline phases alkali aluminum silicate (N-A-S-H) and calcium silicate hydrate (C-S-H).
Pore distribution/% <2 nm
2 nm–20 nm
20 nm–50 nm
>50 nm
0 0 0 0 0
63.2 63.2 65.8 69.7 63.5
23.5 23.1 15.4 14.5 16.3
13.3 13.7 18.8 15.8 20.2
In Fig. 3(A and B), except mullite and quarts, geopolymers that with Cr(NO3)3 or CrO3 also have reinhardbraunsite Ca5(SiO4)2(OH)2. Because chromium compounds show different solubility deliquescence. Thus Cr(NO3)3 and CrO3 can give out a lot of heavy metals Cr3+ and Cr6+. These heavy metals break the bonds of CaAl in N-A-S-H gel and the bonds of Ca-Si in C-S-H gel, then replace original Ca2+. The Ca2+ which has been replaced will react with [SiO4]4 and OH , and form stable reinhardbraunsite Ca5(SiO4)2(OH)2 [15]. There is no peak of reinhardbraunsite in samples of F-Cr-1.0 and F-Cr2O3-1.0, since Cr and Cr2O3 don not supply many Cr3+ or Cr6+, and they are enwrapped by geopolymers with the initial formation.
3.1.3. Effects of chromium on pore structures According to the International Union of Pure and Applied Chemistry (IUPAC), pores can be divided into macropores (>50 nm), mesopores (2–50 nm) and micropores (<2 nm) [16]. In geopolymers, macropores represent gaps between unreacted fly ash, mesopores represent typical pores between geopolymer gels, and micropores often appear in the network structure of gels [17].
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Fig. 4. Binding energies of O1s, Si2p and Al2p of geopolymers that with different chemical valence of chromium reagents/eV. (A) Binding energies of O1s. (B) Binding energies of Si2p. (C) Binding energies of Al2p.
Table 7 Binding energies and chemical shifts of O1s, Si2p and Al2p of geopolymers with different chemical valences of chromium reagents/eV. Samples
F F-Cr(NO3)3-1.0 F-Cr2O3-1.0 F-Cr-1.0 F-CrO3-1.0
O1s
Si2p
Al2p
Binding energies
Chemical shifts
Binding energies
Chemical shifts
Binding energies
Chemical shifts
531.27 531.72 531.31 531.43 531.34
– +0.45 +0.04 +0.16 +0.07
102.09 102.41 102.24 102.43 102.27
– +0.32 +0.15 +0.34 +0.18
73.94 74.31 74.23 74.31 74.16
– +0.37 +0.29 +0.37 +0.22
Table 8 Leaching tests of Cr(NO3)3 solidified bodies with chemical detoxification agents/mgL
1
.
Samples
F-Cr(NO3)3-1.0
F-Cr(NO3)3-1.0-S
F-Cr(NO3)3-1.0-P
F-Cr(NO3)3-1.0-T
Total chromium
0.07
0.07
0.08
0.06
0.04
Samples
F-Cr(NO3)3-3.0
F-Cr(NO3)3-3.0-S
F-Cr(NO3)3-3.0-P
F-Cr(NO3)3-3.0-T
F-Cr(NO3)3-3.0-D
Total chromium
1.93
1.27
1.70
0.59
1.05
Pore structures of geopolymers that with different dosages of Cr (NO3)3 are shown in Table 5. It is can be seen that only the total pore volume of sample F-Cr(NO3)3-0.5 is smaller than that of the blank sample F, and the number of pores in the range of
F-Cr(NO3)3-1.0-D
2–20 nm increases. It represents the lower dosage of Cr(NO3)3 can decrease pore volume of geopolymers and make them more compact, resulting in better development of compressive strength.
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In Table 5, average pore diameter and total pore volume of sample F-Cr(NO3)3-1.5 are smaller than that of the sample F-Cr (NO3)3-1.0. Compressive strength of F-Cr(NO3)3-1.5 would not decrease concluded from the total pore volume results, but in fact, it decreases dramatically. Only through pore structures cannot explain this phenomenon. It may have relationship with chemical reaction between Cr3+ and gels (N-A-S-H and C-S-H), which is similar to the result of Jin et al. [18]. Due to 4 Al-O bonds around Al, and every O contributes one electron to Al3+, so the valence of alumina tetrahedron is 1. When Cr3+ participates in charge balance, a Cr3+ can balance 3 alumina tetrahedrons. Due to space volume of
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alumina tetrahedron is large, the length of Cr-[AlO4] bond is long, making binding energy weak. In result, network structures of geopolymers are not stable, and the compressive strength dramatically decreases. Pore structures of geopolymers that with different chemical valences of chromium reagents are shown in Table 6. Total pore volume and average pore diameter of sample F-Cr(NO3)3 are both larger than that of the blank sample F, thus its compressive strength decreases. Average pore diameter of samples F-Cr2O3-1.0, F-Cr-1.0, and F-CrO3-1.0 are larger than that of the blank sample F, but their total pore volume are smaller. After inter-
Fig. 5. Pore structures of geopolymers that with different chemical valences of chromium reagents. (A) F. (B) F-Cr(NO3)3-1.0. (C) F-Cr2O3-1.0. (D) F-Cr-1.0. (E) F-CrO3-1.0.
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action, total pore volume dominates main influence, these three samples show better compressive strength. 3.2. Solidification/stabilization of chromium by geopolymers 3.2.1. XPS analysis Elemental binding energies and chemical shifts are directly related with effective atomic charges. Elemental binding energy is the energy that electrons reach fermi level by overcoming
effects of nuclear and surrounding electrons. Chemical shift is regular displacement that resulted from the changes of the chemical compounds structures or element oxidation states. On one hand, the inner electrons have a certain binding energy, which are strongly affected by the atomic nucleus in Coulomb. On the other hand, they are affected by the shielding effect of the outer electrons. If valence of element or negativity of surrounding elements changes, binding energies of inner electrons will also change at the same time.
Fig. 6. Reaction products of geopolymers that with different chemical valences of chromium reagents. (A) F. (B) F-Cr(NO3)3-1.0. (C) F-Cr2O3-1.0. (D) F-Cr-1.0. (E) F-CrO3-1.0.
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Fig. 4 and Table 7 shows the binding energies and chemical shifts of O1s, Si2p and Al2p of geopolymers that with different chemical valences of chromium reagents. In Fig. 4(A) and Table 7, 531.27 eV represents binding energy of O of the blank sample F. when dose different chromium reagents to geopolymers, the binding energies of O of samples of F-Cr(NO3)3-1.0, F-Cr2O3-1.0, F-Cr-1.0, and F-CrO3-1.0 are risen by 0.45 eV, 0.04 eV, 0.16 eV and 0.07 eV, respectively. It represents that the influence of Cr3+ from Cr(NO3)3 on O1s is larger than that from Cr2O3, Cr and CrO3. Because Cr(NO3)3 is easy to dissolve and disperse in the system, so it is easy to participate in valence balance. In Fig. 4(B) and Table 7, 102.09 eV represents binding energy of Si of the blank sample F. The binding energies of Si of samples of FCr(NO3)3-1.0, F-Cr2O3-1.0, F-Cr-1.0, and F-CrO3-1.0 are risen by 0.32 eV, 0.15 eV, 0.34 eV and 0.18 eV, respectively. If polymerization degree of Si-O is high, it will be responsible for high binding energy [19]. A research showed that after dosing Cr3+, there is a very strong polymerization in network of silicate and/or aluminum silicate, and is similar to the phenomenon of immobilizing chromium by cement [20]. The results of this experiment show that different chemical valences of chromium can improve polymerization degree of Si-O. In Fig. 4(C) and Table 7, 73.94 eV represents binding energy of Al of blank sample F. The binding energies of Al of F-Cr(NO3)31.0, F-Cr2O3-1.0, F-Cr-1.0, and F-CrO3-1.0 are risen by 0.37 eV, 0.29 eV, 0.37 eV and 0.22 eV, respectively. Combining the change of O1s, Si2p and Al2p, it is can be seen that electron density of O, Si and Al all decrease, showing heavy metals make initial electron density close to Cr3+ or Cr6+. Because Cr3+ and Cr6+ have lost 3 or 6 electrons, they need to be balanced by electrons of other nucleus. Among them, Cr(NO3)3 have great effects on the binding energies of 3 elements, because it easily chemically bonds with geopolymer gels. In contrary, Cr2O3, Cr and CrO3 must dissolve in alkali at first, but there is little alkali for dissolution. So the numbers of Cr3+ and Cr6+, as well as the effects on O1s, Si2p and Al2p are both small. In summary, when chemical valences of heavy metals are different, the main reason to change electron density is the different dissolving ability of heavy metal compounds. Those heavy metal compounds which are easy to dissolve in alkali will have a large number of heavy metal ions to participate in valence balance.
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3.2.2. ESEM analysis Pore structures and reaction products of geopolymers with different chemical valences of chromium reagents testing by ESEM are shown in Figs. 5 and 6, respectively. In blank sample F (Fig. 5(A)), fly ash generates spongy gels with an alkali-activator, and enwraps those unreacted fly ash. There are many honeycomb micropores in the surface of geopolymer. Some fibrous C-S-H gels distribute in the surface or micropores of the sample. In Fig. 5(B), honeycomb micropores distribute more widely after dosing Cr(NO3)3, increasing internal specific surface energy of gels and promoting physical adsorption of Cr3+ by gropolymers. Besides, more and more unreacted fly ash reduces N-A-S-H gels and C-S-H gels. Cr(NO3)3 seriously affects dissolution rate of silicon aluminum phase, and slows down reaction process of geopolymers. It makes physical encapsulation and chemical bonding weak, and then affects the results of solidification/stabilization of Cr3+. It shows that heavy metal ions can slow down reaction process of geopolymers, which is corresponded to the results of our preliminary experiment [21]. In Fig. 5(C–E), it is obvious to find some big irregular substances. They may be Cr2O3, Cr and CrO3, which are enwrapped by N-A-S-H gels. Once Zhang et al. put element Pb into geopolymers, and cannot observe any element Pb in SEM figures. It represents those heavy metal compounds or elements that difficult to dissolve are mainly physically enwrapped by geopolymer gels [22]. In blank sample F (Fig. 6(A)), there are lots of fibrous C-S-H gels in the surface of the sample. However, after the addition of Cr (NO3)3, some small irregular substances was generate. It maybe shows that a portion of Cr(NO3)3 are enwrapped by N-A-S-H gels. The same phenomenon was observed in Fig. 6(C–E), but irregular substances were bigger in size, which is corresponded to the observation in Fig. 5(C–E). According to Fig. 2, compressive strengths of geopolymers have been improved after dosing Cr2O3, Cr and CrO3. The reasons may be that, on one hand, some heavy metals from Cr2O3, Cr and CrO3 participate in valence balance, on the other hand, Cr2O3, Cr and CrO3 physically fill into geopolymers. That decreases total pore volumes and makes structures more compact. In summary, chemical bonding and physical encapsulation affect heavy metal solidified bodies at the same time, but which effect plays the main role is up to the formation of heavy metals. Those heavy metal compounds which are easy to dissolve in alkali
Fig. 7. Compressive strength of Cr(NO3)3 solidified bodies that with chemical detoxification agents. (A) System of F-Cr(NO3)3-1.0. (B) System of F-Cr(NO3)3-3.0
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are mainly chemically bonded by geopolymers, and the others are mainly physically enwrapped by geopolymers. 3.3. Detoxification effectiveness of chemical agents for geopolymers 3.3.1. Detoxification effectiveness on compressive strength Fig. 7 shows compressive strength of Cr(NO3)3 solidified geopolymer bodies with four chemical detoxification agents, respectively. When dosage of Cr(NO3)3 is 1.0%, chemical
detoxification agents of Na2S, NaH2PO3 and TMT make no improvement to solidified bodies. But chemical detoxification agents of DTCR on the sample F-Cr(NO3)3-1.0-D is different from the others, and it is improved by 3.4%, 1.5% and 3.7% on 3 day, 7 day and 28 day, respectively. Sample F-Cr(NO3)3-1.0 possesses a good development of compressive strength, but F-Cr(NO3)3-3.0 shows worse than F-Cr (NO3)3-1.0. In Fig. 7(B), they are decreased by 45.9%, 44.4% and 32.3% on 3 day, 7 day and 28 day, respectively. It is easy to find that
Fig. 8. ESEM figures about Cr(NO3)3 solidified bodies with chemical detoxification agents before and after leaching. (A1) F-Cr(NO3)3-1.0 before leaching. (A2) F-Cr(NO3)3-1.0 after leaching. (B1) F-Cr(NO3)3-1.0-S before leaching. (B2) F-Cr(NO3)3-1.0-S after leaching. (C1) F-Cr(NO3)3-1.0-P before leaching. (C2) F-Cr(NO3)3-1.0-P after leaching. (D1) F-Cr (NO3)3-1.0-T before leaching. (D2) F-Cr(NO3)3-1.0-T after leaching. (E1) F-Cr(NO3)3-1.0-D before leaching. (E2) F-Cr(NO3)3-1.0-D after leaching.
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403
Fig. 8 (continued)
compressive strength at early age are seriously affected when dose 3.0% Cr(NO3)3. As the same as sample F-Cr(NO3)3-1.0, Na2S, NaH2PO3 and TMT give no advantage to compressive strength at early age and later age, except DTCR. It is worth noting that compressive strength of F-Cr(NO3)3-3.0-D is 47.2 MPa, which is higher than F-Cr (NO3)3-1.0-D. Compared with F-Cr(NO3)3-3.0, the compressive strength of F-Cr(NO3)3-3.0-D even has been improved by 59.5%. It shows that DTCR can improve compressive strength of solidified geopolymer bodies, and the more dosage of heavy metals, the more obvious the effect is. The reason for DTCR improving compressive strength may be that macromolecules can be physically adsorbed by C-S-H gels, and intercalate into the interlayer of the C-S-H gels, which make structures of C-S-H gels different [23]. In geopolymers, both of TMT and DTCR can make change of C-S-H gels through intercalation reaction, and DTCR makes solidified bodies more compact than TMT which is implied from compressive strength. TMT may not have good compatibility with C-S-H gels, and results in a bad performance even intercalate into C-S-H gels.
When the dosage of Cr(NO3)3 is up to 3.0%, leaching toxicity of F-Cr(NO3)3-3.0 is 1.93 mgL 1, which is 27.6 times as large as F-Cr (NO3)3-1.0. But it is still in the range of threshold limit of total chromium content. After adding four chemical agents, there are different improvements between them. It is obvious to find TMT has the best performance, then DTCR. Both of them are organic chemical agent, which indicate that organic chemical agents are better than inorganic chemical agents in geopolymers. Once Li et al. used TMT to deal with heavy metals in municipal solid waste incineration fly ash (MSWIFA), and found it is better than the other inorganic chemical agents [24], which is corresponded to our experiment results. Besides, Cr3+ is very easy to oxidize to Cr6+, and that will increase its harm to environment. The sulfur atom in the thiol group of TMT has an empty d orbit. It is easy to lose electrons, and capture heavy metals by negative electric field which generated from polarization distortion. It can make Cr6+ reduce as Cr3+, then forms stable precipitate through complexation reaction. DTCR also has the same sulfur atoms, so the results of leaching test are well, too.
3.3.2. Leaching tests Table 8 is the results of leaching tests of Cr(NO3)3 solidified bodies that with chemical agents. Threshold limit of total chromium content is 15 mgL 1. Leaching toxicity of blank sample F-Cr (NO3)3-1.0 is 0.07 mgL 1, which is far below threshold limit of total chromium content. It indicates that geopolymers have a great advantage in solidifying/stabilizing Cr(NO3)3.
3.3.3. Micrographs of geopolymers before and after leaching As is shown in Figs. 7 and 8, when dosage of Cr(NO3)3 is 1.0%, compressive strength of F-Cr(NO3)3-1.0 is high on 28 day, results of extraction toxicities is also good, solidified bodies of the sample F-Cr(NO3)3-1.0 was chosen to be observed by ESEM before leaching and after leaching. Fig. 8 shows the ESEM figures about Cr(NO3)3 solidified bodies with four chemical detoxification agents before leaching and after
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leaching, respectively. As is shown in Fig. 8(A1 and A2), there are some difference between two samples F-Cr(NO3)3-1.0 that before and after leaching. It is easy to find that the sizes of micropores are becoming larger, the number of micropores is increasing, and the surroundings of micropores become loose. After adding different chemical agents, small differences also appear in different samples. Especially for Na2S, there are much more crackles surround pores after leaching which are shown in Fig. 8(B2). According to Griffith theory, these pores and crackles will cause cracks due to stress concentration [25]. After leaching, the number of pores and crackles increases, which indicates that liquid will make more erosion to solidified bodies and leach more heavy metals. After leaching for 14 days, although the number of pores and cracks increases, it’s not obvious for geopolymers that with TMT or DTCR in Fig. 8(D2 and E2). The reasons can be listed as follows. (1) Maybe there is an intercalation reaction when TMT or DTCR capture Cr3+, so the samples are hard to be eroded. (2) There is some unreacted fly ash in geopolymers’ system before leaching. But when water comes into interior through capillary pores, the unreacted fly ash will generate N-A-S-H gels with water and alkali, furtherly filling solidified bodies. 4. 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 encapsulation played main role in geopolymers with Cr2O3, Cr and CrO3, respectively. 3. Chemical detoxification agent DTCR could effectively improve compressive strength, and the more dosage of Cr(NO3)3, the more improvements it will make. DTCR also could effectively capture Cr3+ of geopolymers, enhancing their abilities of antierosion.
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). References [1] J.G. Jiang, X.U. Xin, Y. Zhang, Investigation of leaching characteristics of heavy metals during cement stabilization of fly ash from municipal solid waste incinerator, Environ. Sci. 27 (12) (2007) 2564–2569.
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