*Article* **Experimental Study on Solidification of Pb2+ in Fly Ash-Based Geopolymers**

**Fang Liu 1, Ran Tang 1, Baomin Wang 2,\* and Xiaosa Yuan <sup>1</sup>**


**\*** Correspondence: wangbm@dlut.edu.cn; Tel.: +86-0411-8470-7101

**Abstract:** Fly ash from the incineration of domestic waste contains heavy metals, which is harmful to the environment. To reduce and prevent their contamination, heavy metal ions need to be sequestered. In this study, the geopolymer prepared by fly ash, a kind of power plant waste, is used to cure the heavy metal Pb2+, and to investigate the effect of different concentrations of Pb2+ on the compressive strength of the solidified body at different ages; the curing effect is judged by the toxic leaching concentration of heavy metals; the resistance of the solidified body to immersion is evaluated by comparing the change in strength before and after leaching; the fly ash-based geopolymer solidified body is compared with the cement solidified body in terms of curing effectiveness; the properties of the geopolymer and its mechanism of curing heavy metals are explored by microscopic tests. The results show that the fly ash-based geopolymer solidified body has good resistance to immersion; the optimum curing concentration of Pb2+ in fly ash-based geopolymers is 2.0%; compared to pure geopolymers, the strength of the solidified body at 28 d decreases by only 13.0%, and the leaching concentration of Pb2+ is 4.73 mg·L<sup>−</sup>1, which meets the specification requirements; the curing effect of the fly ash-based geopolymer is better than the cement solidified body; the microscopic test results indicate that the curing of Pb2+ by the fly ash-based geopolymer is a combination of both chemical bonding and physical fixation.

**Keywords:** fly ash; geopolymer; solidification; heavy metal; leaching concentration

#### **1. Introduction**

With the development of society, the discharge of domestic waste increases rapidly. Proper disposal of these wastes has become an important task. At present, there are three common methods: landfilling, composting, and incineration [1], of which incineration is becoming the main method because of its high efficiency, speed, significant volume reduction, and the possibility of converting waste into other energy sources; however, after waste incineration, most of the heavy metals such as Pb, Cr, and Zn will go into the fly ash that can cause irreversible damage to the environment and humans if handled improperly.

An important means of dealing with heavy metal waste is to cure them to reduce or even prevent their contamination. The solidified cement is commonly used for this purpose, but it has obvious drawbacks, including high concentrations of toxic leaching of heavy metals, poor durability, and poor resistance to acidic soils and rainfall [2,3].

The geopolymer [4] is a new type of inorganic polymeric material with high strength and durability. It is prepared by the polymerization of silica-aluminous raw materials, such as metakaolin [5,6], slag [7], kaolinite [8], and fly ash [9,10], under the solubilization of alkali activator to form an amorphous silica-aluminum compound. Andini, S. [11] investigated the effect of different curing temperatures on the performance of geopolymers synthesized from fly ash, and determined the optimum curing temperature. Duxson, P. [12] studied the mechanical properties of geopolymer with different types of alkali activators

**Citation:** Liu, F.; Tang, R.; Wang, B.; Yuan, X. Experimental Study on Solidification of Pb2+ in Fly Ash-Based Geopolymers. *Sustainability* **2021**, *13*, 12621. https:// doi.org/10.3390/su132212621

Academic Editors: Carlos Morón Fernández and Daniel Ferrández Vega

Received: 20 October 2021 Accepted: 12 November 2021 Published: 15 November 2021

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and different silica-aluminum ratios in relation to age, and found that the sodium alkali activator is more effective than the potassium alkali activator in stimulating geopolymers.

The internal structure of geopolymers exhibits a three-dimensional mesh-like structure similar to that of zeolites that makes it uniquely suited to the solidification of almost all heavy metal ions [13–16].

As reported in [17,18], geopolymers can efficiently bind heavy metals in their matrix structure; the reaction mechanism could be physical or chemical bonding. Studies by Van Jaarsveld et al. [19] suggested that Pb might be chemically bonded within the aluminosilicate matrix, but the exact form was unclear. In contrast, Palacios and Palomo [20] suggested that Pb was solidified to insoluble Pb3SiO5 in NaOH-activated fly ash cementitious materials.

Therefore, a geopolymer was prepared using waste fly ash as a matrix in this study to investigate its curing of heavy metals and the mechanism. Additionally, the heavy metal Pb2+ was solidified through the geopolymer to study the effect of its concentration on the compressive strength of the solidified body at different ages, and on the toxic leaching concentration of heavy metals, and to compare the curing effect of the fly ash-based geopolymer with that of the cement solidified body. Furthermore, the properties of the geopolymer and its mechanism of curing heavy metals were explored through microscopic tests (XRD, FT-IR, and SEM) to investigate the reaction process, structural morphological changes, and curing mechanisms at a microscopic level.

### **2. Raw Materials and Experimental Procedures**

*2.1. Raw Materials*

2.1.1. Fly Ash

The ultra-fine fly ash used in this study came from Guodian Zhuanghe power plant in Liaoning Province. The chemical composition of this fly ash is given in Table 1, and its main technical indicators are listed in Table 2.

**Table 1.** Chemical composition of fly ash.


**Table 2.** Summary of main technical indicators of fly ash.


#### 2.1.2. Water Glass

The sodium water glass (Na2O·nSiO2) was obtained from Usolf Chemical Technology Co., Ltd. in Shandong Province. The specific parameters are listed in Table 3.



#### 2.1.3. Other Raw Materials and Chemical Reagents

#### (1) NaOH

It was analytical pure level that was used to adjust the modulus of the water glass to configure a compound alkali activator required for the experiment.

(2) H2SO4, HNO3

They were analytical pure levels that were used to prepare leaching solutions in simulated acidic environments, and to leach fly ash-based geopolymer samples in preparation for subsequent heavy metal leaching concentration tests.

(3) Heavy metal salt

The aim of this study is to research the solidification of Pb2+; therefore, nitrate (Pb(NO3)2, analytical pure level) was chosen to avoid the interference of anions in heavy metal salts.

#### (4) Water

The deionized water used in this test was supplied by Bonuo Chemical Reagent Factory.

#### *2.2. Preparation of the Fly Ash-Based Geopolymer*

In this test, Pb2+ was introduced in the form of Pb(NO3)2, while its content (in terms of mass of Pb2+ as a percentage of solid mass) was set at six levels of 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%, respectively. The experimental ratios of materials used for the fly ash-based geopolymer are listed in Table 4.


**Table 4.** Material proportioning for fly ash-based geopolymer solidified body.

According to Table 4, fly ash, water glass, water, and Pb(NO3)2 were added to the mixing pot of the mechanical mixer in turn, and stirred for 3 min (30 s slow stirring and 1 min fast stirring, repeated, and then finished). Next, the mixture was poured into a 40 × 40 × 40 mm mold, which had been painted in advance with a release agent, and vibrated for 2 min on a vibrating table, before sealing the surface with polyethylene film to prevent moisture evaporation. After standing for 2 h at room temperature, the mold was placed in a high temperature curing box at 65 ◦C for 24 h before being taken out and demolded, and then it continued to be placed in a standard curing box for closed curing until the age of 3 d, 7 d, and 28 d.

#### *2.3. Analytical Test Methods*

2.3.1. Compressive Strength Test

The compressive strength test was performed according to *Method of testing cements— Determination of strength*, GB/T17671-1999.

#### 2.3.2. Leaching Concentration Test for Heavy Metal Ions

The test was performed according to GB5085.3-2007 and GB14569.1-2011. The preparation of the leaching solution required was carried out according to HJ/T299-2007 and HJ557-2010, simulating the acid rain environment and the normal environment, respectively; and then, the leaching concentration of heavy metals was measured by Inductive Coupled Plasma Emission Spectrometer (ICP) test according to GB/T23942-2009.

### 2.3.3. Microscopic Test

X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) were used to test and analyze the performance of samples of the fly ash-based geopolymer, and the heavy metal ion solidified body on a micromorphological basis.

#### **3. Results and Discussion**

#### *3.1. Compressive Strength*

The compressive strength and its trends in the curing of Pb2+ by the fly ash-based geopolymer are shown in Table 5 and Figure 1.


**Figure 1.** Comparison of the compressive strength of the geopolymer and the cement solidified body.

According to Table 5, the compressive strength of the fly ash-based geopolymer at each age decreases gradually with the increase in Pb2+ concentration. Additionally, at the same Pb2+ concentration, the internal reaction of the geopolymer is gradually complete, and the alkali activator fully acts with the increase of age, corresponding to the increasing strength of the solidified body. It is analyzed that, before Pb2+ concentration reaches 2.0%, the compressive strength of the solidified body at 3 d, 7 d, and 28 d can still be maintained at around 40 MPa, with good mechanical properties; after 2.0%, the strength decreases rapidly; when Pb2+ reaches 3.0%, the compressive strength at 3 d decreases by nearly 40%. The reason could be that the introduction of Pb2+ inhibits the polymerization reaction to some extent, or it has some effect on the internal structure of the solidified body.

#### *3.2. Toxic Leaching Concentration of Heavy Metals*

#### 3.2.1. Neutral Environment

According to Table 6, in a neutral environment, the toxic leaching concentration of heavy metals in the fly ash-based geopolymer solidified body increases with the increase of Pb2+ content. At the same heavy metal concentration, the polymerization reaction within the solidified body tends to be thorough with the increase in age; its internal cage-like sequestration structure is more complete; the sequestration effect on heavy metal ions is more effective; the leaching concentration of the heavy metal solidified body decreases accordingly. When the heavy metal content is 2.5%, the leaching concentrations of the solidified body at 3 d and 7 d exceed the specification maximum limit (5 mg·L−1), while the concentration at 28 d does not; when the heavy metal content reaches 3%, the leaching concentration of the solidified body at 28 d is only above the limit. Therefore, the optimum Pb2+ content ranges from 2% to 2.5% in a neutral environment.

**Table 6.** Results of toxic leaching concentration corresponding to different Pb2+ contents in a neutral environment.


#### 3.2.2. Acid Environment

As shown in Table 7, the toxic leaching pattern of heavy metals in an acid environment is generally the same as that in a neutral environment; however, the external and internal structure of the solidified body is more significantly attacked by the acid environment than the neutral environment, so that heavy metal ions are more likely to migrate out of the internal structure of the geopolymer, resulting in higher leaching concentrations for each Pb2+ content and each age of the solidified body in an acid environment than in a neutral environment.


5 2.5 6.31 6.02 5.47 6 3.0 8.36 7.97 7.61

**Table 7.** Results of toxic leaching concentrations corresponding to different Pb2+ contents in an acid environment.

When Pb2+ reaches 2%, the leaching concentrations of the solidified body at the ages of 3 d, 7 d, and 28 d are close to the normative limit, but does not yet exceed it, while at the Pb2+ content of 2.5%, the limit is exceeded at all ages. To sum up, the optimum solidifying concentration range for Pb2+ in the fly ash-based geopolymer in an acid environment is similarly between 2% and 2.5%.

#### *3.3. Immersion Resistance*

Two groups of fly ash-based geopolymer solidified body with different Pb2+ doping are set. One group is used as a control group to test the strength before immersion; the other is placed in a water tank at room temperature and is flooded over the surface. The specimens are removed and dried with a rag after being immersed for 90 days, and then, they are tested for the compressive strength. Test results are listed in Table 8.


**Table 8.** Changes of the strength before and after immersion.

According to Table 8, the strength loss of the solidified body does not exceed the limit of 25% for each Pb2+ content, except for the highest content (3.0%). Therefore, it can be concluded that the fly ash-based geopolymer solidified body has good resistance to soaking.

#### *3.4. Comparison between the Fly Ash-Based Geopolymer and the Cement Solidified Body*

To verify the effectiveness of the fly ash-based geopolymer in solidifying Pb2+, the cement solidified body is chosen for comparison in this study. Both are set to the same water to ash ratio, and the same amount of Pb2+ (0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%). The compressive strength of the solidified body at 7 d, and the toxic leaching concentrations at 28 d (acid environment) are used as indicators to compare the solidifying effect of these two. The experimental results are shown in Figures 1 and 2.

**Figure 2.** Comparison of heavy metal leaching concentration of geopolymer and cement solidified body.

As shown in Figure 1, the compressive strength of the fly ash-based geopolymer solidified body is significantly higher than that of the cement solidified body. It is analyzed that the compressive strengths of both tend to decrease with the increase in Pb2+ content. When Pb2+ concentration is 2%, the compressive strength of the geopolymer solidified body is still around 40 MPa; the strength decreases slowly until the Pb2+ concentration reaches 2%, and more rapidly after this. In contrast, the strength of the cement solidified body decreases at a relatively rapid rate all along. Therefore, the performance of the geopolymer solidified body is significantly better from the point of view of the compressive strength.

According to Figure 2, in an acid environment, the solidification of Pb2+ in the fly ash-based geopolymer solidified body is better than in the cement solidified body. At 2% Pb2+, the leaching concentration of the geopolymer solidified body is still below 5 mg·L<sup>−</sup>1, the limit value for Pb2+ is specified in GB5085.3-2007, whereas the cement solidified body is very close to the normative limit at 1.5% Pb2+.

This is mainly due to that in an acid environment, the hydration products of cement, such as C-S-H (calcium silicate hydrate) and Ca(OH)2, react with the acid to produce the corresponding calcium salts, resulting in an increase in pore space within the solidified body, which ultimately leads to the destruction of the solidified body and a reduction in its solidifying ability. By comparison, the silicon–oxygen (Si-O) and aluminum–oxygen (Al-O) bonded in the internal structure of the geopolymer are difficult to break in an acid environment, making it more resistant to acid attack.

To sum up, whether from the point of view of mechanical properties (compressive strength) or solidifying properties (leaching concentration), the performance of the fly ash-based geopolymer solidified body is better than that of the cement solidified body.
