| Environmental problem is one of the important topics for human sustainable development in this century. With the continuous development of industrial production, increasing coal consumption, a large number of solid waste fly ash has not been timely and effectively disposed in our country every year, which is great harm to the atmosphere, water, soil and even human life. Meanwhile, the pollution of heavy metal was harmful for human health. Serious emissions of waste solid and pollution of heavy metal desperately needed a new kind of environmental material. As a New type of inorganic green building materials, geopolymer has overcome the disadvantage of Portland cement. It has both the advantages of inorganic minerals and polymer and unique zeolite-like cage structure, and gradually became a hotspot for researchers at home and abroad. This paper aimed at dealing with the practical problem of waste accumulation and heavy metal pollution. In order to high efficiently and comprehensively using fly ash from Neimeng province, fly ash based geopolymer and its related materials have been prepared. The study on immobilization and adsorption of heavy metal has been done.This paper has prepared fly ash based geopolymer through alkali-activated method by using Neimeng fly ash as raw materials, and has investigated the stability of structure and performance of products under high temperature or different chemical environment. The heavy metals were effectively fixed into geopolymer based on the zeolite-like and porous structur of geopolymer. The mechanism was explained through the change of structure and morphology during immobilization process. According to the relation between geopolymer and zeolite, fly ash based geopolymer has been successfully transformed into NaP and FAU zeolite via in situ hydrothermal method. The optimum preparation technology was chosen by analyzing the structure and morphology of hydrothermal products. The removal efficiency of heavy metal among fly ash, fly ash based geopolymer and hydrothermal products has been evaluated. The mechanism of transformation geopolymer into zeolite was illuminated through observing the variation of structure and morphology during hydrothermal process by using modern analysis determination technique. This work used waste to dispose waste and realized the objective of environmental protection.The innovations of this paper are shown as follow:(1) The zeolites block with excellent mechanical strength, high purity and regular morphology were transformed from geopolymer via in situ hydrothermal method. This method avoided the molding process of zeolite powders as compared with traditional method.(2) The type of zeolite which was transformed from fly ash based geopolymer could be controlled through adjusting the composition of geopolymer and the hydrothermal process. The NaP and FAU type zeolites were successfully prepared.(3) The geopolymer was preparaed by using synthetic chemical powders and kaolin ore resources as raw materials and then transformed into zeolite in current report, resulting in high cost and resource consumption. In order to avoid high cost and resource consumption, our research selected the industrial solid waste fly ash as as material and could turn "waste" into wealth.The main research results of this paper are shown as follow:(1) The optimal conditions for preparing fly ash based geopolymer (FA-GEO) and metakaolin based geopolymer (MK-GEO) by alkali-activated method were obtained, a) For FA-GEO. The alkali-activator with a modulus of 1.1 aged at room temperature for 24 h to 48h. The fly ash and alkali-activator was mixed by a liquid-solid ratio of 0.35 mixed at 1200-1500r/min for 15 min. Then the mixture was poured into the moulds, sealed and cured at 60 ℃ for 24 h. The compressive strength of product obtained after 28 days was 30 MPa. b) For MK-GEO. In addition to using alkali-activator with a modulus of 1.0, the preparation conditions of MK-geopolymer were the same as those of FA-GEO. MK-GEO sample exhibited a compressive strength of 75 MPa after curing for 28 days. XRD and SEM analysis indicated that samples FA-GEO and MK-GEO had unique amorphous phase and density gel structure. The FT-IR spectrum showed that both materials exhibited the characteristic functional group of geopolymer. The characteristic peak of air carbonation appeared in FA-GEO sample, but not found in MK-GEO sample. Research had revealed that FA-GEO and MK-GEO had been successfully prepared.(2) The structure, morphology and performance of FA-GEO and MK-GEO which was obtained optimal conditions were investigated before and after calcinations. The results of high temperature experiment among 200 to 1000℃ showed that the both samples had a certain temperature resistance. When the temperature of calcinations was below 600℃, sample FA-GEO kept the same phase composition and structure, and retained 70% of compressive strength of raw material. When the temperature of calcinations was improved above 800℃, the amorphous structure of FA-GEO sample began to transform into nepheline or vishnevite. The compressive strength strongly reduced due to the existing of pores in its structure. After being calcined at 1000℃, the FA-GEO block was obviously sintered and porous. The weight loss ratio and volume shrinkage rapidly increased. The compressive strength of FA-GEO sample was down to 2.91 MPa. For MK-GEO sample, only a small amount of nepheline appeared when calcined temperature reached to 1000 ℃. At this time, the weight loss ratio, volume shrinkage and compressive strength of MK-GEO sample were respectively 20.3%,7.9% and 10.3 MPa. The results had shown that the stability at high temperature of MK-GEO was better than that of FA-GEO. The FT-IR spectrum showed that the calcinations led to the changes of Si-O-T bond vibration peak and T-0 bending vibration peak and-OH vibration peak in the structure of geopolymer.(3) The stability difference of structure, morphology and performance between fly ash based geopolymer and metakaolin based geopolymer under different chemical environment was investigated. The results of chemical etching experiments in 5 wt% HCl,5 wt% H2SO4,10 wt% NaOH and simulated seawater solution among 1 day to 3 months showed that the corrosion resistance in H2SO4, HCl and NaOH solution for sample FA-GEO were excellent than those of sample MK-GEO, and both samples exhibited stabilization of structure and performance after Teaching in simulated seawater solution,(4) About 0.5～3.0 wt% of heavy metal (such as Cu2+, Pb2+, Cd2+, Cr3+, Ni2+) were immobilized by fly ash based geopolymer which prepared by the optimum conditions. The leaching test results showed that the curing rates of five heavy metals in FA-GEO sample were all above 99% when using deionized water, mixture of sulfuric acid and nitric acid solution and acetic acid buffer as extraction. A good correlation was discovered between the solidifying rates and the ion ratios of heavy metal. When adding the same amount of heavy metal into FA-GEO, the solidifying capacity was Pb2+>Cd2+>Ni2+>Cu2+>Cr3+. The results of curing time test revealed that adding 1.0wt% amount of Cu2+, Pb2+, Cd2+ into geopolymer could hinder the polycondensation of geopolymer at early age (< 1 day), and the strength of solidifying bodies and curing ability for heavy metals were reduced. When the curing time was prolonged, the compressive strength of solidifying bodies and the solidifying capacity of heavy metal in geopolymer could be promoted.(5) Various technologies were adopted to characterize the structure, morphology and valence state of heavy metal and the mechanism for immobilization of heavy metal was obtained. The phase compositions of solidifying bodies were not altered by adding heavy metal. The surface structure of solidifying bodies with Cu2+, Pb2+, Cd2+, Ni2+ were compact, while the loose gel structure and low density of sample contained Cr3+ exhibited loose gel structure confirmed that Cr3+ had a significant influence on the polycondensation process of FA-GEO. XPS analysis declared that heavy metal mainly existed in the form of precipitation or oxide in geopolymer and showed no changes in valence state. FT-IR spectrum found that the vibration frequencies of Si-O-T (Si/Al) asymmetric stretch bonds were affected after adding heavy metal. The heavy metal selectively entered into the frame structure of geopolymer and affected the surrounding environment of Si-O-T. The mechanism for immobilization of heavy metal was obtained based on above analysis and was shown as follow:Most of heavy metal existing in the form precipitation or oxide was fixed in geopolymer, and a few heavy metal ions entered into the frame of geopolymer by exchanging with Na+which balanced the negative charge of [AlO4]-.(6) Important parameters such as the composition of raw material, the concentration and volume of NaOH solution, and hydrothermal temperature and time were investigated through analyzing the variation of morphology and compressive strength of NaP zeolite which converted from geopolymer. The optimum synthesis conditions were found and listed as follow:The resulting geopolymer slurry with a Si/Al molar ratio of 1.67 was prepared by using modified activator with a modulus of 1.1, and then crystallized at 100℃ with 50ml 2.0 mol/L NaOH solutions for 24 h to obtain FAU zeolite. The compressive strength of NaP zeolite block could reach 23.21 MPa. The as-prepared NaP zeolite block was good crystallinity and exhibited a distinct diamond ball-like shape with uniform size and a very dense surface. In addition, the BET specific surface area, micropore surface area and theaverage mesopores diameter of NaP zeolite block were respectively 50.46 m2/g,7.51 m2/g and 11.06 nm. The FT-1R and TG showed that the product had the typical characteristics of NaP zeolite. The results of experiments on adsorbing Cu2+, Pb2+, Cr3-, Cd2+ and Ni2+ from waste water confirmed that FAU zeolite had a certain adsorption of heavy metals. When the pH of solution increased to 4.0, the adsorption capability of Cu2+, Pb2+, Cr3+,Cd2+ and Ni2+ were 19.85 mg/g,20.38 mg/g,17.52 mg/g,18.00 mg/g and 17.48 mg/g, respectively. In addition, the removal efficiency of those ions reached above 35%.(7) Through investigating structure, morphology and strength of FAU zeolite which prepared under different conditions, such as composition of raw materials, the concentration and volume of NaOH solution, and hydrothermal temperature and time, the optimum synthesis conditions for the preparation of FAU zeolite derived from geopolymers were as follow:The resulting geopolymer slurry with a Si/Al molar ratio of 1.65 was prepared by using modified activator with a modulus of 0.9, and then crystallized at 70℃ with 50ml 1.0 mol/L NaOH solutions for 24 h to get FAU zeolite. The compressive strength of FAU zeolite block could reach 15.8 MPa. The prepared FAU zeolite block had good crystallinity, regular octahedron shape and compact structure. The chemical composition of prepared sample was close to the theoretical value of FAU zeolite. The BET specific surface area, the micropore surface area and the average mesopores diameter of as-prepared NaP zeolite block were 174.35 m2/g,136.23 m2/g and 13.15 nm, respectively. The FT-IR and TG revealed that the product had the typical characteristics of FAU zeolite. The results of experiments on adsorbing Cu2+, Pb2+, Cr3+, Cd2+ and Ni2+ from waste water confirmed that FAU zeolite had good adsorption of heavy metals. When the pH of solution was 4.0. the adsorption capability of Cu2+, Pb2+, Cr3+, Cd2+ and Ni2+ were 30.74 mg/g,28.68 mg/g,28.39 mg/g,29.28 mg/g and 31.91 mg/g, respectively. In addition, the removal rate of those ions was above 55%. FAU zeolite showed more excellent adsorption property for heavy metal than NaP zeolite(8) The reaction mechanism of zeolite derived from geopolymer was obtained through investigating the structure, morphology and properties of fly ash, fly ash based geopolymer and hydrothermal products. A few ordered nanostructures was observed in the structure of amorphous geopolymer named FA-GEO-1.1 and FA-GEO-0.9 by HRTEM analysis, which could provide the basis for the nucleation of zeolite. The results of XRD, SEM-EDS and FT-IR showed that the relationship between fly ash based geopolymer and zeolites (NaP and FAU) was inheritance structures and mass transfer. Based on above results, the reaction mechanism of NaP and FAU zeolites derived from geopolymer was presented, which included three parts:depolymerization, condensation polymerization and hydrothermal crystallization processes. During depolymerization process, the Si-O and Al-O bond in the structure of geopolymer FA-GEO-1.1 and FA-GEO-0.9 was broken to from [Al(OH)4]- and [OSi(OH)3]- groups at the function of alkali-activator. The condensation of [Al(OH)4]- and [OSi(OH)3]- groups could form aluminosilicate intermidates with negative charges. The aluminosilicate intermidates formed the primary gel around Na+ in later stage of depolymerization process, which could improve the dissolution of Si and Al from fly ash powder. During the condensation polymerization process, the polycondensation of [Al(OH)4]- and [OSi(OH)3]- groups occurred to form polymer gels around the hydration layer of hydrated sodium ions which are located in primary gel and mesopores. These new polymer gels had lots of ordered nanostructures. The tetrahedral [AlO4] and [SiO4] units irregularly arranged to form the amorphous three-dimensional network structures of geopolymer around ordered nanostructures, which could be called secondary building units. In the hydrothermal crystallization process, these secondary building units in geopolymer gels tent to form crystal nucleus of zeolite under a certain temperature and pressure. Then the amorphous structures of geopolymer gels gradually became NaP and FAU type zeolite crystals on the surface of crystal nucleus. During the hydrothermal crystallization process, the mesoporous and pores in geopolymer played a key role in the transformation of inorganic gel to zeolite crystal, and they were the necessary channels for the transfer of moisture, free Na+, and seed crystal. |
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