Research paperA novel acidic phosphoric-based geopolymer binder for lead solidification/stabilization
Graphical Abstract
Introduction
When absorbed by human body, lead (Pb2+) is a potentially toxic element, accumulating in human blood, bones, liver, kidneys, brain, skin and other organs (Charkiewicz and Backstrand, 2020). Pb2+ is not only harmful to human health, but also pollutes soil and groundwater resources. Therefore, it is necessary to treat contaminated soil or solid waste using effective binders prior to the release of Pb2+. The bioavailability and toxicity of soluble Pb2+ in water and soil can be reduced by complexation with various substances (Miretzky and Fernandez-Cirelli, 2008). Solidification/stabilization (S/S) is a relatively rapid, economic, and convenient method for contaminated soil treatment (Li et al., 2014; Tajudin et al., 2016). Solidification involves, or may not, a chemical reaction between toxic pollutants and additives to transform liquid waste, semi-solid sludge or powder into bulk or granular materials, making it relatively easy to treat or transport to a landfill (Chen et al., 2009, Hunce et al., 2012, Wiles, 1987). Stabilization, which involves the chemical reaction, transforms toxic waste into stable chemical compounds (Chen et al., 2009; Hunce et al., 2012; Wiles, 1987). Solidification aims to improve the engineering properties (such as reducing permeability and increasing strength) of contaminated soil, while stabilization focuses on the chemical changes of pollutants, and its purpose is to eliminate or minimize its leachability, and thereby control the leaching of toxic substances to within the limits permitted by regulations (Wang et al., 2018). Additionally, S/S treatment can also recycle polluted solid waste into eco-friendly road building materials (Dubois et al., 2009, Pinto et al., 2011).
Cement stabilization is a practical method to treat Pb2+ contaminated soil (Du et al., 2014a, Du et al., 2014b). The mechanism of cement stabilization of heavy metals involves adsorption, surface complexation, precipitation, coprecipitation, micro or macro encapsulation, ion exchange and passivation (Chen et al., 2009, Li et al., 2001). The main mechanism of cement stabilization of heavy metals is the reaction of the heavy metals with the high alkaline environment produced by cement hydration, forming insoluble compounds precipitates (Xi et al., 2014). Although cement has been widely used in the S/S treatment of hazardous waste containing Pb2+, some problems still exist. For example, Pb2+ in the cement hydrate exists in the form of complex precipitation mixture phases (PbO, Pb(OH)2, PbOPb(OH)2, and others), and the compound precipitates forms an impermeable coating around the cement clinker, thus hindering the hydration reaction of cement (Cartledge et al., 1990, Wang et al., 2018). Moreover, it is difficult for cement to stabilize high concentration of Pb2+ effectively. A previous study indicated that when cement was used to treat 25,000 mg/kg of Pb2+ contaminated soil with a 33.33% cement dosage, the leaching of Pb2+ exceeded 5 mg/L and could not meet the S/S treatment requirements at 28 days of curing (Yin et al., 2006). Similarly, when 20% cement was used to treat 5000 mg/kg Pb2+ contaminated clay, the toxic leaching of Pb2+ was 13.6 and 11.8 mg/L at 7 and 28 days, respectively, the treatment still did not meet engineering requirements (Xiong et al., 2019). Additionally, the effective pH range of for Pb2+ S/S using cement is 8–12 (Xi et al., 2014). In the long-term or strong acidic environments, the C-S-H and Pb-C-S-H phases in the cement hydrates are dissolved due to decalcification (Du et al., 2014a, Du et al., 2014b, Li et al., 2001, Liu et al., 2013) and the hydroxide precipitates formed in the high alkaline environment are also dissolved in acidic environment (Du et al., 2014a, Du et al., 2014b, Li et al., 2001, Liu et al., 2013), causing heavy metals (Pb2+) to dissolve out again. Furthermore, a large quantity of energy and is consumed and large amounts of greenhouse gas (CO2) is produced in the cement production process (Albitar et al., 2017, Aliabdo et al., 2016, Shabhanasarmy and Sivakumar, 2019, Shahmansouri et al., 2020).
Because there are many problems for Pb2+ S/S using cement, alkali-activated geopolymer, as an environmentally friendly binder, has been widely paid attention to. Geopolymer has excellent physicochemical, thermal stability, mechanical properties, and fire and chemical resistance (Sakkas et al., 2014). Alkali-activated geopolymer is also used for the Pb2+ S/S(Ji and Pei, 2019; Xia et al., 2018). In alkali-activated geopolymer, Pb2+ does not exist as an exchange ion, but may play the charge balancing role and participates in the formation of the geopolymer structure, replacing the potential positions of Na+and K+ in the geopolymer (Jin et al., 2008, Jin et al., 2011), so Pb2+ can be effectively stabilized. In a neutral environment, geopolymer and cement exhibit a similar stabilization effect on Pb2+, but in strong acidic conditions, the alkali-activated geopolymer has a better stabilization effect on Pb2+ than cement (Meng and Zheng, 2010). When alkali-activated geopolymer is used for Pb2+ S/S treatment, the effective pH range is approximately 6–12. For example, a 1 wt%Pb2+ was stabilized using a geopolymer paste synthesized by NaOH and Na2SiO3 as the reactants, and blast furnace slag and fly ash (FA) as the raw materials (Fernández-Pereira et al., 2018). When the pH of the leached liquor is in the range of 6–12.5, the leaching of Pb2+ is lower than the limit of 5 mg/L. Beyond this pH range, the leaching of Pb2+ is higher than 5 mg/L and a lower pH increases the Pb2+ leaching (Fernández-Pereira et al., 2018). Furthermore, the activators used in alkali-activated geopolymer are strong alkaline NaOH and Na2SiO3 solutions, which are a strongly corrosive chemical substance and can corrode construction equipment in engineering applications, hindering the use of alkali-activated geopolymer in engineering applications. Due to a stimulated market demand, studies on Pb2+ S/S using phosphorus-based materials has attracted extensive attention. Common phosphorus-based materials include magnesium phosphate cement (MPC) (Du et al., 2014a, Du et al., 2014b), SPB binder synthesized from superphosphate, steel slag powder, and ordinary portland cement at a ratio of 5:4:1 and SPC binder synthesized from superphosphate and quicklime at a ratio of 3:1 (Feng et al., 2017), KMP synthesized from KH2PO4 and sintered magnesia at a ratio of 1:1 (Zhang et al., 2015), and others. Although these binders have a good stabilization effect on Pb2+, these phosphorus-based materials also belong to the medium to alkaline binder, and its strength is also affected by acid corrosion in acidic environments. However, the wastewater discharged from many mines is mostly strong acid wastewater. For example, mine wastewater containing sulfur is acid water and its pH is generally 4.5–6.5 (Wang, 2013). The pH of mine wastewater in the south of China is generally 3.0–3.5 with the pH sometimes reaching 2 (Hu and Gao, 1994). In such a strong acidic environment, conventional binders are obviously not suitable for the treatment of contaminated soil around the mine. Therefore, there is urgency to develop a new efficient and environmentally friendly binder for the treatment of heavy metal contaminated soil in acidic environment.
In view of this,a new acidic phosphoric-based geopolymer (APG) binder was synthesized in this study by using FA as the raw material and aluminum dihydrogen phosphate (ADP; Al(H2PO4)3) as the activator, and the prepared APG binder was used for Pb2+ S/S for the first time. The effect of the Pb2+ content, curing time on the compressive strength, pH, electrical conductivity (EC) of the AGP binder and on the leaching of Pb2+ are discussed in detail. The stabilization mechanism for Pb2+ S/S using am APG binder is investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS).
Section snippets
Materials
The APG binder was synthesized using ADP solution as the reactant and FA as the raw material. ADP reactant, purchased from HuiFeng Building Materials Co., Ltd., is an industrial grade product with a pH of 1.4 and content of 35%. Table 1 lists the basic physical and chemical characteristics of ADP. FA was also purchased from HuiFeng Building Materials Co., Ltd. The FA was sieved through a 1250 mesh (10 µm) sieve and the density was 2.25 g/cm3, placing it in the FA class F as per the ASTM C618-19
Compressive strength
Since the S/S matrix with a higher compressive strength can maintain its integrity better, compressive strength is a key parameter of S/S treatment because the binder with higher compressive strength usually has a lower permeability and porosity (Sörengård et al., 2019). Generally, the required compressive strength for solid waste landfill is > 0.35 MPa, and the required compressive strength for building materials is > 10 MPa (Faheem et al., 2018, Malviya and Chaudhary, 2006). According to the
Conclusions
This study synthesized an APG binder by using FA as the raw material and ADP as the reactant, and the synthesized APG binder was used for Pb2+ S/S for the first time. The pH range of the AGP binder was 2.56–4.19, placing it as an acidic material. Pb2+ had a significant effect on the APG binder compressive strength. When the Pb2+ content was < 0.6%, the APG binder compressive strength was improved due to the presence of Pb2+. When the Pb2+ content was 0.6%, the maximum compressive strength was
CRediT authorship contribution statement
Shaoyun Pu: Conceptualization, Methodology, Writing - original draft preparation. Zhiduo Zhu: Supervision. Weilong Song: Data curation. Hairong Wang: Data curation. Wangwen Huo: Data curation. Jie Zhang: experiment.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to acknowledgethe financial support from the National Natural Science Foundation of China (No. 42072297). The authors also express gratitude to the financial support from the Traffic Science and Technology Project in Nanjing, China: research and application of key technologies for green recycling and high-quality utilization of solid waste in highway reconstruction and extension project.
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