Development Of Phosphoric Acid-Based Geopolymer Binder And Its Application Of Solidification/Stabilization Of Lead Contaminated Soil

With the development of industry,the discharge of solid waste has been increasing,causing soil pollution to some extent,especially heavy metal pollution.Solidification/stabilization（S/S）is a common technology for heavy metal contaminated sites treatment.The use of appropriate binder is the key to the effective treatment of contaminated sites.Currently,the binders used in contaminated sites S/S treatment are mainly cement and cement-based materials,followed by alkali-activated and phosphorus-based materials.However,many contaminated sites are acidic,such as lead-acid battery plants,acid mine wastewater and other contaminated sites,in which lead(Pb²⁺)pollution is more common.Although the existing binders have good S/S effect on Pb²⁺,the binders basically are alkaline or slightly alkaline.If these alkaline binders are directly applied to the heavy metals S/S treatment in acidic or strongly acidic contaminated sites,the stabilization effect and material strength are bound to be affected by acid corrosion.Therefore,to make up for the deficiency of traditional or existing binders in acidic environment,this paper attempts to develop a new binder with high strength and efficiency in acidic environment and applies it to Pb²⁺contaminated soil S/S treatment.Due to the mechanical strength instability and poor water resistance of phosphoric acid（PA）-based geopolymer prepared with PA as reactant,in this work,the PA-based geopolymer was prepared with PA as activator and fly ash as raw material.Meanwhile,aluminum dihydrogen phosphate（ADP）was used to improve the strength and water resistance of PA-based geopolymer,namely,two new types of phosphate-based geopolymers（named ADP and PA-ADP based geopolymers）were proposed with ADP,PA and ADP mixture（PA-ADP）as activator.From the perspective of mechanical properties,water resistance,chemical and microscopic mechanism of the prepared new geopolymer,the acidic phosphorus-based geopolymer（APG）binder with excellent mechanical properties and water resistance and suitable for engineering application was deeply analyzed and selected,and the binder was used for Pb²⁺contaminated soil S/S treatment.The main research contents and conclusions are as follows:（1）The research status of Pb²⁺contaminated soil S/S treatment and phosphoric acid-based geopolymer were systematically summarized.Meanwhile,the S/S effects on Pb²⁺using cement,cement-based,alkali-activated,and phosphorus based materials were analyzed.Moreover,the application prospect of phosphoric acid-based geopolymers in Pb²⁺pollutants S/S treatment was clarified,and the research status of phosphoric acid-based geopolymers were summarized.Finally,the poor strength stability and water resistance of phosphoric acid-based geopolymers were found.（2）The activation effects of PA,ADP,and PA-ADP activators on fly ash were evaluated,and the effects of activator concentration and Liquid-to-solid ratio（L/S）on the mechanical strength,water resistance,electrical conductivity（EC）,and p H of the prepared geopolymer were clarified.Through strength,p H,and EC tests,the geopolymer strength,EC,and p H variation with L/S and curing time was investigated,and the correlation between strength and p H,and EC was established.The results show that ADP activator can improve the strength stability and water resistance of PA-based geopolymer,and the ADP-based geopolymer strength was higher than that of PA and PA-ADP based geopolymer.（3）The chemical and microcosmic mechanism of the prepared geopolymer were clarified.The chemical compositions of geopolymer were analyzed by the X-ray diffraction（XRD）,fourier transform infrared spectrum（FTIR）,nuclear magnetic resonance（NMR）technologies.The microcosmic characteristics and element content of geopolymer were studied by Scanning electron microscope（SEM）and Mercury intrusion porosimetry（MIP）.The strength weakening mechanism of geopolymer after soaking was revealed by XRD,SEM,mercury intrusion porosimetry（MIP）,and FTIR.The results show that quartz and mullite do not participate in the polymerization process,and the crystalline compounds formed in the prepared geopolymers matrix mainly included Ca PO₃（OH）·2H₂O,Ca HPO₄,Al PO₄,and Ca PO₃·OH,whereas the amorphous gels formed were composed of Si–O–Al–O–P,Al–O–P,Si–O–P,Si–O–Si,and Si–O–P–O–Si units.The reason for the strength weakening for the geopolymer after soaking in water was the Si–O–P units hydrolysis.（4）Based on the strength and water resistance indexes,a new high-strength and efficient APG binder was selected and used for Pb²⁺S/S treatment.The optimum material ratio of the APG binder was ADP concentration of 35%,L/S=0.317.The effects of curing time and Pb²⁺content on the APG binder strength were studied by strength tests.The stabilization effect of APG binder paste on Pb²⁺was studied by toxic leaching test.The chemical and microcosmic mechanism of Pb²⁺using APG binder were clarified by XRD,FTIR,X-ray photoelectron spectroscopy（XPS）,SEM and MIP tests;The relationship between the APG binder strength and EC,and p H was established,and the S/S effects on Pb²⁺were compared with cement and alkali-activated geopolymer paste.The results show that the stabilization effect of APG binder on Pb²⁺was better than that of cement and alkali-activated materials.The S/S mechanism of APG binder on Pb²⁺involved chemical precipitation,physical adsorption,and encapsulation.（5）The environmental impact and engineering characteristics of Pb²⁺contaminated soil treated with APG binder were systematically studied.The environmental safety and engineering stability of APG solidified contaminated soil were studied by strength and toxicity leaching tests.The leaching characteristics of Pb²⁺in contaminated soil solidified by APG binder under different p H environment were simulated by p H-dependent tests.Moreover,the S/S effects of cement,APG binder and alkali-activated Fly Ash geopolymer on Pb²⁺contaminated soil were compared.The results show that compared with cement and alkali-activated geopolymer,the developed APG binder had a better stabilization effect on Pb²⁺in contaminated soil in strong acid（p H=2）and strong alkali（p H=13）environment.