Geopolymers produced from drinking water treatment residue and bottom ash for the immobilization of heavy metals

Accepted Manuscript Geopolymers produced from drinking water treatment residue and bottom ash for the immobilization of heavy metals Zehua Ji, Yuansheng Pei PII:

S0045-6535(19)30492-8

DOI:

https://doi.org/10.1016/j.chemosphere.2019.03.056

Reference:

CHEM 23370

To appear in:

ECSN

Received Date: 3 December 2018 Revised Date:

7 March 2019

Accepted Date: 10 March 2019

Please cite this article as: Ji, Z., Pei, Y., Geopolymers produced from drinking water treatment residue and bottom ash for the immobilization of heavy metals, Chemosphere (2019), doi: https:// doi.org/10.1016/j.chemosphere.2019.03.056. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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GRAPHICAL ABSTRACT

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Geopolymers produced from drinking water treatment residue and bottom ash for the

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immobilization of heavy metals

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Zehua Ji

Yuansheng Pei*

(The Key Laboratory of Water and Sediment Sciences, Ministry of Education, School of Environment, Beijing

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Normal University, No. 19, Xinjiekouwai Street, Beijing 100875, China)

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Abstract: Drinking water treatment residue (DWTR) and municipal waste incineration bottom ash (BA) have been

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traditionally considered as solid waste. With the development of urbanization, their subsequent treatment and

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resource regeneration need to be further researched. In this work, a composite geopolymer with BA and DWTR

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was successfully synthesized and applied in the immobilization of Cd, Pb and Zn. The analysis of the geopolymers

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with different ratios of BA and DWTR, curing times and heavy metals was performed through chemical analysis,

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SEM, FTIR, XRD, XPS, ICP-AES and compressive strength tests. The results show that the geopolymer samples

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based on BA and DWTR (BWG) presented higher compressive strength than the samples with single BA material.

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The sample with 20% DWTR and 80% BA (BWG20) possesses the highest compressive strength (24.10 MPa)

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among the materials ratios. Furthermore, the microstructure and characterization results indicate that the

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geopolymer matrix was successfully formed in BWG and was significantly changed by the ratio, curing time and

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addition of heavy metals. The immobilization efficiency for different categories and dosages of heavy metals by

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BWG20 were all higher than 99.43%. Moreover, the XPS results demonstrate that the heavy metals were

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immobilized in geopolymer mainly by divalent state forms.

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Keywords：drinking water treatment residue; bottom ash; composite geopolymer; immobilization; heavy

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metal

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1. Introduction

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Geopolymer is an alkali-activated aluminosilicate that is amorphous with semi-crystalline three-dimensional

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Si-O-Al polymeric networks (Liew et al., 2016). The word “geopolymer” was coined by Davidovits in 1989 (1989).

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Compared with Ordinary Portland Cement (OPC), this material has attracted worldwide interest in the past 40 years

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based on its low CO2 emission and low energy-resource consumption. Furthermore, geopolymer has advantages in

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the aspects of compressive strength, toughness, corrosion resistance and thermostability. These advantages indicate

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that geopolymer has potential applications, such as building materials processing, manufacturing, aerospace,

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nuclear waste encapsulation and heavy metal pollution control (Fernández Pereira et al., 2009; Tzanakos et al.,

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2014; Rashidian-Dezfouli et al., 2018; Yi et al., 2018). Initially, the manufacturing and research of geopolymer were mainly based on metakaolin. The uniqueness

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and boundedness of certain materials limit the application of geopolymer in different regions or fields (Guo et al.,

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2017a; Yakubu et al., 2017). During the past decades, industrial waste and by-products have been used in the

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preparation of geopolymer (Boca Santa et al., 2016; El-Eswed et al., 2017). Extension of the raw materials

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resources for geopolymers and further research on its application in pollution treatment are of great significance for

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energy saving, waste minimization, resource recovery and environmental protection (Al-Zboon et al., 2011; Lee et

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al., 2016; Qin et al., 2018).

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Drinking water treatment residue (DWTR) is considered as a safe by-product of drinking water treatment

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plants (Wang and Pei, 2012) and consists of the hydrolysate of Al- and Fe-flocculant. The main disposal method of

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DWTR is still placement in landfills, despite the high cost of this process and the lack of available land (Wang et al.,

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2012). With the development of urbanization and the growth of the population, drinking water treatment plants are

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producing more and more DWTR, which has led to the subsequent treatment and disposal of DWTR becoming an

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inevitable question that needs to be solved. A study of Waijarean et al. (2014) found that the major chemical

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components of DWTR are SiO2 (54.00%) and Al2O3 (29.30%), which can be used to synthesize geopolymers. This

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research has proven that WTR poses potential value during the manufacturing of this geopolymer.

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Incineration can greatly reduce municipal solid wastes by more than 80%, and thus it is widely applied as a

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treatment method for such waste management. Municipal waste incineration bottom ash (BA) accounts for nearly

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80% of the remaining ash generated in municipal solid waste incinerators and is deposited at the bottom of the

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boilers (Brück et al., 2018). The BA composition usually contains SiO2, Al2O3, CaO, Fe2O3, and Na2O (Li et al.,

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2018; Syc et al., 2018). Compared with fly ash, the composition of BA is more diverse and complex but less toxic;

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thus, BA can be more suitably used in resource recycling. Moreover, BA can generate hydraulic binders with water

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under the effect of an alkali activator, and these characteristics cause BA to be used as a raw material for

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geopolymers.

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In this paper, the BA and DWTR, which were acquired from a city environment and are traditionally

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considered as solid waste, were applied as replacement and active materials in geopolymer manufacturing. In this

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work, the geopolymerization of BA and DWTR was investigated to determine the appropriate proportioning on the

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compressive strength and microstructure of geopolymers. Afterwards, to research and evaluate the application of 2

ACCEPTED MANUSCRIPT BA & DWTR -based geopolymer (BWG) in heavy metal disposal and to explain the immobilization mechanism of

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metals, the heavy metal ions (Cd2+, Pb2+, and Zn2+) were added individually into BWG. The microstructures and

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characteristic changes of geopolymers under the influence of heavy metals were analysed by SEM, FTIR and XRD.

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The immobilization effects of heavy metals were tested by the Toxicity Characteristic Leaching Procedure (TCLP)

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method, and the immobilization mechanism was explored by XPS analysis. This research could provide valuable

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base data for the use of BA and DWTR in geopolymer preparation and heavy metal immobilization.

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2. Materials and methods

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2.1 Materials

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DWTR was obtained from the 9th water treatment plant in Beijing, PR China, where both iron (Fe) and

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aluminium (Al) salts are used as flocculants. BA was collected from the municipal waste incinerator plant in

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Zhuzhou, Hunan province, PR China. The DWTR and BA samples were air-dried and sieved to <0.075 mm. The

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chemical composition of raw materials were shown in Table 1, and the XRD patterns, FTIR spectra and particle

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size distribution is showed in Fig. S1 and S2, respectively.

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The solid mass ratio of sodium silicate is 34.0% (7.07% Na2O and 26.94% SiO2; the initial modulus is 3.3).

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NaOH was obtained from the Chinese Medicine Group Chemical Reagent Co., Ltd., and the purity was 96.0%.

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NaOH was used to adjust the sodium silicate to obtain a composite chemical activator whose modulus (M =

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n(SiO2)/n(Na2O)) was 1.5.

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Na2O

MgO

Al2O3

SiO2

DWTR

0.279

0.581

23.1

21.246

BA

0.879

0.618

23.238

43.508

SO3

K2O

CaO

TiO2

Fe2O3

SrO

ZrO2

Cd2+ (mg/kg)

Pb2+ (mg/kg)

Zn2+ (mg/kg)

0.206

1.231

0.384

9.358

0.232

42.004

0.022

0.016

0.85

10.0

64.0

0.215

3.662

1.052

10.52

2.772

9.749

0.034

0.04

1.65

55.0

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Materials

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Table 1 Chemical composition of raw materials (wt%)

2.2 Synthesis of geopolymers

Batches of different compositions of BA, DWTR, sodium silicate, NaOH and water were made as listed in

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Table 2. The municipal waste incineration BA-based geopolymer (BAG) and BA-DWTR-based geopolymer (BWG)

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were used for the geopolymer prepared with different compositions. Each geopolymer batch was prepared with

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mixed solid BA-DWTR and composite alkali activator in a mixer for 15 min at 300 rpm. Then, the slurry was cast

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into rubber moulds containing 6 moulds with a cube shape (the size was 20×20×20 mm). After eliminating the

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bubble with vibration, the geopolymer sample was cured at 80 ℃ for 12 h in an oven. Then, the sample was left at 3

ACCEPTED MANUSCRIPT room temperature for 7, 14, and 28 d for characterization tests and compressive strength tests. The preparation

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procedure of geopolymer was presented in Fig. 1. Furthermore, the addition of heavy metals (Cd, Pb and Zn) were

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by adding the heavy metal nitrate solutions into the GP slurry and the compositions were shown in Table 2.

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Fig. 1 Schematic representation of the procedure used for the geopolymer preparation.

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Table 2 The design of the prepared geopolymers.

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DWTR/g

BAG BWG10 BWG20 BWG30 BWG40

100 90 80 70 60

0 10 20 30 40

Sample with heavy metals

DWTR/ BA

BWG20-Cd-1.0 BWG20-Cd-2.0 BWG20-Cd-4.0 BWG20-Pb-1.0 BWG20-Pb-2.0 BWG20-Pb-4.0 BWG20-Zn-1.0 BWG20-Zn-2.0 BWG20-Zn-4.0

0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25

Sodium silicate/g 50.0 50.0 50.0 50.0 50.0

Sodium hydroxide/g 12.55 12.55 12.55 12.55 12.55

Water/ml

Si/Al

Na/Al

10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

1.598 1.598 1.598 1.598 1.598 1.598 1.598 1.598 1.598

0.514 0.514 0.514 0.514 0.514 0.514 0.514 0.514 0.514

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BA/g

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Composite activator/ DWTR-BA 0.6255 0.6255 0.6255 0.6255 0.6255 0.6255 0.6255 0.6255 0.6255

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Water/ml

Si/Al

Na/Al

10.0 10.0 10.0 10.0 10.0

1.759 1.678 1.598 1.517 1.436

0.518 0.516 0.514 0.512 0.511

Cd2+

Pb2+

Zn2+

1.0% 2.0% 4.0% -

1.0% 2.0% 4.0% -

1.0% 2.0% 4.0%

2.3 Leaching tests

Toxicity Characteristic Leaching Procedure (TCLP) was often used as an evaluation method for heavy metal

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immobilization. Based on the determination test of TCLP, the acetic acid solution with a 2.88±0.05 pH (fluid No. 2)

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and the liquid/solid ratio of 20 ml/g was used in this study. All of the samples were agitated in a horizontal

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agitation apparatus for 18 h and filtered through a 0.45 µm filter. The concentration of heavy metal ions (Cd2+, Pb2+, 4

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and Zn2+) in the leachate was measured by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES,

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PS-6, BAIRD). The immobilization efficiency (%) of a metal ion was calculated from Eq. (1):

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Immobilization efficiency (%)=(Ct –Cl)/Ct×100%

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where Cl is the leaching concentration of heavy metal cation in the geopolymer sample, Ct is the total concentration

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of heavy metal incorporated in the geopolymer sample.

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(1)

2.4 Methods of characterization

The X-ray fluorescence (XRF) method was used for the qualitative identification and quantitative analysis of

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the elements in BA and WTR. The particle size distribution of raw materials was measured by a Laser Particle Size

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Analyzer (S3500, Microtrac, USA). The microstructures of the geopolymer samples were analysed using a

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scanning electron microscope (SEM, X650, Hitachi, Japan), and the active group in the surface was analysed by

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Fourier transform infrared spectroscopy (FTIR, Nicolet iS5, Thermo Fisher, USA). The mineralogical

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characterization of the geopolymers was conducted by X-ray diffraction (XRD, X'Pert PRO MPD, PANalytical,

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Holland). The X-ray photoelectron spectroscopy (XPS) analysis was used for internal elemental analysis with an

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Axis Ultra Dld spectrometer (Shimadzu, Japan).

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3.Results and Discussion

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3.1 Compressive strength of the BA and DWTR-based geopolymer

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To investigate the feasibility and advantages of BA and DWTR-based geopolymer materials, the DWTR was

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mixed with incineration BA as the raw materials of the geopolymer. The DWTR contents were varied from 0% to

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40% (i.e., BAG, BWG10, BWG20, BWG30, and BWG40). The samples were treated for different curing times (7,

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14 and 28 d) and then tested for the unconfined compressive strength, and the results are presented in Fig. 2. The

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compressive strength of BAG was not developed at an early stage and was only 3.93 MPa after 28 days of curing.

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The compressive strength is an important evaluation parameter for the generation of geopolymer (Liew et al., 2016;

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Zhang et al., 2016). Compared with the geopolymer in other researches (Table S1), BAG doesn't have advantages

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on the mechanical strength. Furthermore, the XRD patterns were no significant change in BA (Fig. S1) and BAG

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(Fig. S3), indicating that the amorphous structure of geopolymer was not generated in the geopolymerization.

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These results indicated that the single BA was not suitable for the preparation of geopolymer, which may be due to

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the oligomeric silicate and aluminium by the depolymerization in BA cannot form a substantial -O-Si-O-Al-O-

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structure by the polycondensation. However, the geopolymer samples with DWTR show higher compressive strength compared to those with

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single BA material, especially in the range of DWTR:BA = 10:90 to DWTR:BA = 20:80. The proportion in the

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range of 10% to 20% was suitable for the generation of the monolithic construction of the geopolymer, which

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enhanced the compressive strength over a shorter time. Compared with the BAG, the compressive strength of

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BWG20 increased by 951.72% and 513.23% in 7 d and 28 d, respectively. Those changes indicated that adding of

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DWTR can significantly enhanced the mechanical strength of BA-based geopolymer. Moreover, BWG20 possesses

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the highest development of compressive strength among the different ratios of BA and DWTR, and its strength was

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enhanced from 12.2 MPa to 19.37 and 24.10 MPa in 14 d and 28 d, respectively. With the greater addition of

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DWTR, the compressive strength of BWG was rapidly diminished. This result may be due to the excess unreacted

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particles in DWTR caused structural damage to continuous structure of geopolymer matrix.

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DWTR:BA=20:80

DWTR:BA=10:90

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DWTR:BA=30:70 DWTR:BA=40:60

3 0

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7 14 28

7 14 28 7 14 28 Curing days

7 14 28

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Fig. 2 The unconfined compressive strength of a geopolymer with different proportioning and curing time

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The addition of DWTR caused the change of compressive strength, which may be due to the change of Si:Al

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and porosity. Generally, high Si:Al ratio increase the amount of -Si-O-Si- bonds to get a higher compressive

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strength. However, an appropriate Si/Al ratio is important for the generation of geopolymer matrix, and the

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changing of porosity and density also affect the compressive strength (Zhang et al., 2014; Wu et al., 2018). The

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proper raw material ratio makes the reaction more efficient and thorough, which promotes the depolymerization of 6

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the aluminosilicate glass phase. It accelerates formation of the colloid precipitation phase, and eventually enhances

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the strength of geopolymer (Huang et al., 2016). As the effect of aluminum dihydrogen triphosphate in the

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generation of geopolymer (Wu et al., 2018), the appropriate amount of unreacted granules in alkaline conditions

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can fill the intergranular voids of geopolymer matrix, which led to the enhancement of compressive strength. As an entirely waste material-based geopolymer, the compressive strength of BWG was not stronger than

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other metakaolin-based or fly ash-based geopolymers (Guo et al., 2017c; Phummiphan et al., 2018), but the

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strength after 28 d of curing (24.10 MPa) fully meet the requirement of the USEPA for stabilized materials to be

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disposed in a landfill (0.35 MPa) (USEPA, 1993), as well as the requirement for baked brick MU20 (20 MPa)

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(GB5101-2003, 2003). Furthermore, the compressive strength of BWG20 was well above that of the single BA

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geopolymer (3.93 MPa) or the single DWTR geopolymer (8.70 MPa) (Waijarean et al., 2014). In comparison with

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the geopolymers based on waste materials (Table S1), BWG possesses a great advantage in compressive strength

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and material acquirement, which indicates that BA and DWTR have the potential to be used as the raw materials

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for the composite geopolymer.

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3.2 Characterization of geopolymer with different proportioning and curing time

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3.2.1 SEM

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The geopolymer samples were collected after compressive strength test, and then the microstructures of the

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samples were observed through SEM. As presented in Fig. 1, most particles in raw materials were irregularly

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granular particles and spherical particles were seldom observed, which is different to the microstructure of fly ash

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(Zhu et al., 2019). As shown in Fig. 3a-c, the soluble silicate and aluminium ions in alkaline conditions can

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generate agglomerated structure of geopolymer. Fig. 3d-f presents the micrographs of BWG20, which possess the

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highest compressive strength among the different proportion of BA and DWTR. Compared with the micrographs of

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BAG (Fig. 3a-c) and BWG30 (Fig. 3g-l), a more compact structure was formed in BWG20 (Fig. 3d-f) at the early

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stage (7 d) and changed significantly with the increasing curing time. In addition, the geopolymer matrix of

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BWG20 tended to integrity with more curing time, which may affect the mechanical strength of the geopolymers at

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the end of the curing. In contrast, BWG30 displayed a microstructure characterized by a heterogeneous geopolymer,

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which was encircled by particles (Fig. 3g-l). With the increase of the DWTR, the unreacted particles of DWTR can

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agglomerate into smaller clusters, contributing to the geopolymer network and forming a looser structure. This is a

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reasonable explanation for the lower compressive strength of BWG30.

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3.2.2 FTIR To further analyse the structural change of the geopolymer with different proportioning and curing times,

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FTIR was used to obtain the infrared spectrum of the geopolymer (Fig. 3). As presented in Fig. 3 and Table S2,

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compared with the BAG, the FTIR spectra of BWG20 and BWG30 are more alike. The broad peaks approximately

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3400 cm-1 are due to the stretching vibration of –OH. The peaks approximately 1650 cm-1 are due to the bending

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vibration of H-O-H bonds in water molecules, which were in the surface or cavity of the geopolymer (Huang et al.,

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2016). The peaks between 950 cm-1 and 1000 cm-1 represent the asymmetrical stretching vibration of

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aluminosilicate (Si-O-Si or Si-O-Al) in the geopolymer. Those peaks exist in the spectra of BWG20 and BWG30

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and indicate that it is successful to synthesize the structure of the geopolymer by BA and DWTR and it is

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dominated by Si-O-Si and Si-O-Al bands. It is noteworthy that the peak at approximately 866 cm-1 represents the

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stretching vibration of six-coordinated Al-O (El-Eswed et al., 2017), which only existed in the BWG20 and

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indicates that this bond is an important link related to compressive strength.

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Moreover, the categories of the absorption peaks were increased with increasing DWTR, which was due to the

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Si and Al in the raw materials that can be dissolved in the alkaline solution, and generated oligomeric silicate ions

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and aluminium ions (which can be detected through FTIR) by the depolymerization reaction. However, due to

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variation of the Si/Al proportions, the fundamental structure (-O-Si-O-Al-O-) was hard to form by the

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polycondensation reaction and more nonbridging Si-O- and Al-O- were formed (these structures had been

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previously confirmed by Duxson et al (2007) and Melar et al (2015)). In this condition, the large numbers of

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nonbridging Si-O- and Al-O- in the matrix were hard to form into a geopolymer network gel by the

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polycondensation reaction, which led to decreasing of the compressive strength.

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3500

3000

2500

2000

1500

3

1000

453.03

760.34

866.49

988.84

BAG-14d BAG-28d BWG20-7d BWG20-14d

Transmittance 500

BAG-7d

566.94

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Wavenumber/cm

4

988.84 866.49 760.34 685.62 566.94 453.03

1455.10

EP

BWG20-7d BWG20-14d BWG20-28d

Transmittance 4000

BAG-7d BAG-14d BAG-28d

1644.86

3428.53

1 2

685.62

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BWG20-28d BWG30-7d BWG30-14d BWG30-28d

1000

900

800

700

600

500

400

Wavenumber/cm

Fig. 3 The SEM images and FTIR spectra of the geopolymer with different proportioning and curing times

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3.2.3 XRD

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ACCEPTED MANUSCRIPT The XRD diffractograms of the samples are presented in Fig. S3. Some mineralogy phases were observed in

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the geopolymer samples such as Anhydrite (CaSO4), Lamite (4CaO·3SiO2·H2O), Aragonite (CaCO3) and Rankinite

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(Ca6(Si6O17)(OH)2). Quartz (SiO2) and aluminium oxide (Al2O3) were detected in all of the samples, which are also

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the main phase of BA and DWTR (Fig. S1). The broad and amorphous hump from 20° to 40° (2θ) was observed in

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all of the samples, which provides evidence of the geopolymerization reaction (Novais et al., 2016) and has been

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described as the ‘geopolymer hump’ (Guo et al., 2017b). The phases of BAG (Fig. S3a), BWG20 (Fig. S3b) and

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BWG30 (Fig. S3c) have remained constant over time, which indicates that the crystal construction of BAG and

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BWG were stabilized in a short time and the main forms of geopolymer is amorphous. However, the mineralogy

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phases were increased with the increasing DWTR proportion in preparation (Fig. S3), which resulted from more

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impurity to the geopolymer provided by DWTR. DWTR contains few mineralogy phases (Fig. S1), and thus, some

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amorphous forms were converted to the crystalline form under the alkaline solution in the formation of

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geopolymeric matrices. Furthermore, no distinct heavy metal compounds were detected in the samples, which

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demonstrated that there were low crystalline heavy metals in the BA and DWTR.

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3.3 Heavy metal immobilization in BWG

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3.3.1 The compressive strength of BWG under the influence of heavy metals

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Nikolic et al. (Nikolic et al., 2018) proved that NO3- shows little influence (0.12%) on the compressive

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strength of geopolymers and demonstrates that the change of strength was based on cations rather than nitrate ions.

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Consequently, in this study, the heavy metals (cadmium, lead, and zinc) were added by the nitrate reagent and the

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addition of metal cations as shown in Table S1. As presented in Table 3, the addition of Cd and Zn decreased the

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compressive strength rather than Pb since the ionic radius of Zn2+(0.074 nm) and Cd2+(0.095 nm) is far less than

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that of Pb2+ (0.119 nm). Wang et al. (2017) demonstrated that the hydroxyl complex ions were formed by heavy

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metal ions and hydroxide ions, which caused the alkalinity to decrease and the viscosity to increase. Those changes

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hindered the dissolution of Si and Al. Meanwhile, hydroxyl complex ions containing heavy metals interfered with

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the condensation reaction of [SiO4] and [AlO4] tetrahedrons, which led to inhibition on the growth of the network

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skeleton and eventually reduced the compressive strength of geopolymer.

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With the increasing concentration of Cd and Zn, the diffusion of dissolved aluminosilicate and oligomers in

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the pore network of hardened matrix was limited by heavy metal cations, and thus, the geopolymerization reaction

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and binder hardening need more time for the formation of stable structure. Furthermore, the addition of heavy

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metals intensified the heterogeneity of the reaction products, which led to the formation of more structural defects, 10

ACCEPTED MANUSCRIPT and the strength decreased (Yunsheng et al., 2007; Andrejkovičová et al., 2016). Nikolic et al. (2018) found that the

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compressive strength of geopolymer (cured at 20 °C or 28 d) decreased 21% due to the addition of 4.0% Pb. In

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contrast, a small amount of Pb increased the compressive strength. The compressive strength grew 11.20% when

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the Pb dosage was 1.0 wt%. Guo et al. (2017b) considered that the dissolved Pb ions could modify the network of

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geopolymer, which would cause an increase of compressive strength with increasing Pb dosage. Zhang et al. (2008)

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also found that Pb2+ possessed a positive effect on the strength of the binder. Additionally, and in the present study,

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the samples containing DWTR showed significantly high compressive strength over the safe disposal of the

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hazardous waste standard value (0.35 MPa) (USEPA, 1993), which indicated that the BWG could be applied for

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the disposal of heavy metals.

Sample BWG20 BWG20-Cd

BWG20-Pb

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Compressive strength/MPa 12.20±0.73 12.30±1.76 5.00±0.41 5.03±0.25 13.57±1.61 10.27±0.61 8.23±0.42 10.03±0.41 10.33±1.43 4.77±0.54

Relative strength/% 100 100.82 40.98 41.26 111.20 84.15 67.49 82.24 84.70 39.07

Strength change/% 0.0 +0.82 -59.02 -58.74 +11.20 -15.85 -32.51 -17.76 -15.30 -60.93

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3.3.2 Leaching of heavy metals

Heavy metal wt/% 0.0 1.0 2.0 4.0 1.0 2.0 4.0 1.0 2.0 4.0

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Table 3 The compressive strength of the geopolymer with the addition of heavy metals

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The leaching concentration and immobilization efficiency of different heavy metals in geopolymer are

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provided in Fig. 4. A clear immobilization effect of the geopolymer on Zn compared to Cd and Pb was presented.

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For all of the samples, the immobilization efficiency was higher than 99.43%. Compared with the results in Table

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S3, the BWG shows a higher immobilization efficiency to heavy metals, even the addition of cations in this study is

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far higher than previous researches. It shows that the BWG is a suitable binder for the immobilization of heavy

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metals. Based on the regional criteria for hazardous waste (CEU, 2003; HJT299-2007, 2007), the leaching results in

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Fig. 4 indicate that the BWG-Pb and BWG-Zn samples (except BWG-Pb-4%) can be classified as non-hazardous

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waste. Even at high immobilization efficiency (99.55 - 99.74%), the Cd concentration exceed the criteria for

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hazardous waste (1 mg/L), demonstrating that the different criteria among metal categories should be taken into

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consideration in immobilization process. For the BWG-Pb-4% sample, the Pb was dissolved in an acetic acid solution, and a significantly higher

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concentration than Cd and Zn was shown. This phenomenon was due to the excess lead cations that cannot be

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effectively immobilized in geopolymeric matrices. This result is contrary to the change of compressive strength

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under the influence of heavy metals, which indicates that there was no clear correlation between the effectiveness

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of heavy metals and compressive strength. Furthermore, the Pb2+ ion was not completely immobilized in the

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matrices and can be partly dissolved in an acid solution. Guo et al. (2017b) concluded that there was a positive

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correlation between Pb immobilization and compressive strength, which is inconsistent with this study and perhaps

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due to the different curing situation in the synthesis process and the initial strength of the geopolymer. With regards

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to the results of Zn2+ immobilization, the immobilization efficiency of all of the samples was higher than 99.76%,

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and the leached concentrations are considerably lower than Cd and Pb. The ionic radius of Zn2+ (0.074 nm) is

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closer to Al3+ (0.0535 nm) and Si4+ (0.040 nm), which generates a stable formation in geopolymeric matrices and is

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hardly soluble in an acid solution. In addition, changes in the leached rate of heavy metals occurred slowly with the

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addition of reagents, which demonstrated that the BWG still have a potential immobilization capacity for higher

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heavy metal dosage.

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concentration(mg/L)

99.55%

Immobilization efficiency

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Leached concentration/(mg/L)

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99.43%

99.55%

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99.56%

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99.68% 99.74%

0

0

99.75%

99.88% 99.82% 99.76%

1 2 4 0 1 2 4 0 1 2 Pb Zn Cd Addition dosage of heavy metal/ wt%

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Fig. 4 The TCLP leaching concentration of heavy metals after 7 d of immobilization. 3.4 The mechanism of heavy metal immobilization

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ACCEPTED MANUSCRIPT Fig. 5 shows the micrographs of BWG20 containing Cd, Pb and Zn. BA and DWTR can generate smooth gels

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with an alkali activator and enwrapped the unreacted raw materials, which led to the smooth and homogeneous

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surface of BWG (Fig. 5a). Compared with BWG, the samples containing heavy metal (Fig. 5b-d) showed a quite

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different microstructure. The body of BWG with Cd2+ (Fig. 5b) and Pb2+ (Fig. 5c) was smaller and surrounded by

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fragments. The BWG with Zn (Fig. 5d) presented an integrated matrix but with a loose surface and more cracks,

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which effectively immobilized the Zn2+ ion in BWG. The new structures after the addition of heavy metals

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significantly decreased the compressive strength of the geopolymer. Zheng et al. (2016) have concluded that the

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substitution of heavy metals in the geopolymer was occurred in the condensation reaction between the

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aluminosilicate and hydrolyzed heavy metal. Furthermore, Nikolic et al. (2018) reported that the amorphous phases

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were formed in condensation reaction, which was also confirmed in our research. Thus, the surface changes of

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BWG20 under the effect of heavy metal is associated with the amorphous product in geopolymerization.

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Transmittance

Pb-4% Zn-2% Zn-4%

3500

3000

2500

2000

1500

1000

500

Wavenumber/cm

453.03

566.94

760.34

Pb-2% Pb-4% Zn-2% Zn-4%

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Cd-4%

Pb-2%

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2

685.62

Cd-2%

Cd-4%

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BWG20-7d

988.84

Cd-2%

1384.84

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760.34 685.62 566.94 453.03

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1384.84

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BWG20-7d

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1500 1400 1300 1200 1100 1000 900 800 700 600 500 400

Wavenumber/cm

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Fig. 5 The SEM micrographs and FTIR spectra of geopolymers with heavy metals.

The FTIR spectrums of BWG20 with Cd2+, Pb2+ and Zn2+ are also shown in Fig. 5. Since they are doped with

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heavy metals the peaks (988.84 cm-1) of Si-O-T (Si or Al) all moved to a higher wavenumber and the absorption

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peaks became wider. Under the influence of heavy metal ions, a complex cation layer was formed around Al-O,

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which led to the absorption peaks of Si-O-T being broadened (Huang et al., 2016). Furthermore, compared with the

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samples with Cd2+ or Pb2+, the peaks of symmetric stretching vibration of Si-O-T at 760 cm-1 almost vanished

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under the influence of Zn2+, which was important to the compressive strength of geopolymer and indicated that the

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evident decrease of compressive strength under effect of Zn2+ was based on this bond. Moreover, the peaks at 1384

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cm-1 (-NO2) appeared after the addition of heavy metals, which was caused by the high dosage of nitrates. It is

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noteworthy that the absorption bands in the FTIR spectrums at 1455.10 cm-1 were assigned to carbonate salts and

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indicated that Na2CO3 and heavy metal carbonates may be generated under the alkaline solution (El-Eswed et al.,

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2017).

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The XRD patterns of BWG20 with different heavy metals are presented in Fig. 6. With the addition of

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heavy metals, there was only one phase (lead silicate [PbSiO3]), which can be found in Fig. 6b. The crystalline

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Cd and Zn are not detected, which is because they were immobilized by amorphous states (El-Eswed et al.,

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2017). Furthermore, the Thermonatrite (Na2CO3·H2O) and Nitratine (NaNO3) were founded with the addition

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of heavy metals, which was probably due to the replaced Na+ and that generated the crystals containing

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sodium when the heavy metal cations were immobilized in the geopolymer network. The peaks of Nitratine in

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Fig. 6c were more significant than those in Fig. 6a and b, which was because the Zn(NO3)2 (formula weight =

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189.49) possesses a higher number of nitrate radicals than Cd(NO3)2 (formula weight = 236.42) and Pb(NO3)2

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(formula weight = 331.23) under the same mass fraction. To confirm the immobilization forms of heavy metals in BWG-20, the XPS analysis was conducted to address

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the interactions between Cd, Pb and Zn with geopolymer samples; the results are shown in Fig. 6 d, e and f,

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respectively. The binding energies of Cd3d5 and Cd3d3 in BWG20 were located at 405.85 eV and 412.62 eV,

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respectively, which correspond to the Cd2+ state (Barsbay et al., 2018). Then, the XPS data show that the two

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symmetric peaks of Pb4f5 and Pb4f7 were at 144.37 eV and 139.60 eV, respectively. The two peaks correspond to

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the divalent states of Pb (Xu et al., 2008). Furthermore, the XPS spectra of Zn2p display a doublet at 1022.75 eV

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(Zn2p3) and 1045.97 eV (Zn2p1), which indicates that the Zn element in BWG was mainly in the form of Zn2+

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(Morozov et al., 2015). Compared with the sample with no heavy metals (Fig. S4), the results of XPS analysis

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demonstrated that Cd, Pb and Zn were successfully immobilized on geopolymer. The peaks of binding energies

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indicate that the main forms of heavy metals were the divalent states (Morozov et al., 2015; Guo et al., 2017b),

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demonstrating that the Cd, Pb and Zn showed no chemical valence changes during the geopolymerization.

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Raw Intensity Peak Sum Background Peak1 Peak2

Cd3d3, 412.62eV

4500 4000

(e)

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Zn2p3,1022.75eV 9000

2100 1800 1500 1200

Pb4f5,144.37eV Raw Intensity Peak Sum Background Peak1 Peak2

8700 8400 8100

900

7800

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600

7500

2500 420

300 150

3500

415

410

B.E.(eV)

405

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(f)

Pb4f7,139.60eV

Count/s

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Cd3d5, 405.85eV

6000

Count/s

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Zn2p1,1045.97eV Raw Intensity Peak Sum Background Peak1 Peak2

7200 148

146

144

142

140

B.E.(eV)

138

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1060 1055 1050 1045 1040 1035 1030 1025 1020 1015

B.E.(eV)

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Fig. 6 The XRD pattern and XPS spectra of geopolymer sample with different heavy metals (a. XRD of BWG20-

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Cd, b. XRD of BWG20-Pb, c. XRD of BWG20-Zn, d. XPS of BWG20-Cd-4%, e. XPS of BWG20-Pb-4%, f. XPS

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different heavy metals. The binding energies of Si of BWG20-Cd, BWG20-Pb and BWG20-Zn declined to 0.309

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eV, 0.078 eV and 0.119 eV, respectively. This change was similar to the findings of compressive strength under

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effect of Cd, Pb and Zn, which indicated that heavy metals can impair the polymerization degree of Si-O (Guo et al.,

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2017c) and Al-O. The heavy metal ions were linked as Si-O-M and Al-O-M (M = heavy metal ions) in the

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geopolymer (Guo et al., 2017b). Under the effect of heavy metals, the changes in FTIR (Fig. 5) and XPS (Table S4)

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indicated the Cd, Pb and Zn were bonded into the geopolymeric structure by both Si-O-M and Al-O-M.

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In this work, the composite geopolymer with BA and DWTR was successfully prepared. The geopolymer

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samples with DWTR show higher compressive strength compared to single BA material. The BWG20 possesses

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the highest development of compressive strength among the different DWTR/BA ratios. The microstructural and

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characterization analysis results indicated that the cementation matrix was formed in BWG20, which changed

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significantly and tended towards integrity along with time. The absorption peaks of aluminosilicate (Si-O-Si or Si-

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O-Al) were found in BWG and demonstrated that the structure of geopolymer by BA and DWTR was successfully

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synthesized and it is dominated by Si-O-Si and Si-O-Al bands. In addition, the crystalline phases of geopolymer

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remain constant over time, which indicates that the crystal construction of BAG and BWG were stabilized in a

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short time and the main forms of geopolymer is amorphous.

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Then, the BWG20 was used in the immobilization of heavy metals. The addition of heavy metals degrades the

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compressive strength; however, the immobilization efficiency of heavy metals is higher than 99.43%. The BWG

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samples with heavy metals show a loose surface and more cracks. The FTIR analysis also indicated that the cations

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participated in geopolymerization mainly through the influence of the Si-O-T (Si or Al) bonds and decrease the

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compressive strength based on this structure. Furthermore, Pb was immobilized by lead silicate (PbSiO3) and Cd

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and Zn were immobilized by amorphous states. Finally, the XPS spectra results demonstrated that the heavy

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metals were successfully immobilized on geopolymer, and the main forms of heavy metals are the divalent states,

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which were linked as Si-O-M and Al-O-M (M = heavy metal ions) in the geopolymer. The present work suggests

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that BWG, as an entirely waste materials-based geopolymer, possesses great advantages in compressive strength,

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material acquirement, and potential value in heavy metal immobilization.

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Acknowledgments This work was financially supported by the Beijing municipal science and technology plan projects (Project no.

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Z181100005518005), the National Natural Science Foundation of China (Project no. 51579009, 51879012) and the

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Major Science and Technology Program for Water Pollution Control and Treatment (Project no.

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2018ZX07110004).

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Geopolymers produced from drinking water treatment residue and bottom ash for the immobilization of heavy metals Zehua Ji

Yuansheng Pei*

(The Key Laboratory of Water and Sediment Sciences, Ministry of Education, School of

Highlights: New geopolymer based on DWTR and BA was formed.

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Environment, Beijing Normal University, No. 19, Xinjiekouwai Street, Beijing 100875, China)

The compressive strength of geopolymer was enhanced by the level of DWTR replacement.

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Heavy metals exhibit significant influence on microstructural of geopolymer matrix.