Study on CO 2 Immobilization in the Environmentally Sound Treatment of Fly Ash

: To improve the carbonation performance of fly ash, waste incineration fly ash was subjected to high-temperature alkali fusion-hydrothermal and other treatments. The results showed that the material prepared with 8 g of sodium hydroxide, 10 h of maturation, 100 °C and 10 h of hydrothermal temperature had the best ability to isolate carbon dioxide, 521 g/kg, while the fly ash could only adsorb 160 g/kg of carbon dioxide, which improved the isolation ability by 212.5%.


Introduction
Excessive CO 2 emissions have caused many environmental and ecological problems [1] .The common disposal method for CO 2 sequestration is to achieve permanent isolation from the atmosphere through various sequestration techniques, of which mineralized sequestration is a hot research topic [2] . Solid waste has more active sites on the surface, better activity, so alkaline solid waste is generally used as the preferred raw material for mineralized sequestration. Alkaline solid waste contains alkaline metal oxides, which can react with CO 2 to produce stable carbonate substances, and solid waste emissions are large, so it is used to isolate CO 2 . The annual production of fly ash from waste incineration in China reaches more than 10 million tons and the fly ash contains heavy metals and dioxins, thus, fly ash is a hazardous waste, so the treatment of fly ash is a key area of concern [3] . The common cement curing stabilization is less costly but has a large volume increase ratio, which puts pressure on the land resources of our landfills and requires alternative ways to be sought [4,5] . Fly ash is an alkaline waste with the ability to absorb CO 2 , and the heavy metal leaching concentration is low after the fly ash is carbonated. At present, the mineralization performance of fly ash is low. In this paper, the silica and aluminum elements in fly ash are released by hydrothermal method and prepared as inorganic materials to isolate CO 2 . The concentration of NaOH, maturation time, and hydrothermal temperature as well as time were varied in order to prepare the material with the best mineralization properties and to consider the heavy metal leaching concentration.

Hydrothermal treatment
The liquid-solid ratio of fly ash to NaOH was 10:1 (mL/g) and stirred on a magnetic stirrer for maturation, and at the end of which the samples were subjected to hydrothermal reaction. After the reaction, the samples were washed with water by centrifugation, dried at 80℃ for 12 h, and ground and set aside. The experimental parameters are shown in Table1.

Mineralized Sequestration
The hydrothermally treated fly ash product is laid flat in the carbonization tank and passed through 100% carbon dioxide, with the gas entering through the bottom of the tank and the top connected to the tail gas absorption device.

Structural characterization
From Figure 1, it can be seen that a strong diffraction peak appears at 2θ=30°, which is known to be hydrated calcium silicate after comparison with the standard card。NaOH mixed with fly ash, OHwill destroy the glass phase in the fly ash, releasing AlO 2-, SiO 2-, free AlO 2-, SiO 2combined with Ca 2+ , finally forming hydrated calcium silicate. Maturation time is related to the number of silicaaluminate gels; hydrothermal conditions are related to the quality of the final hydrated calcium silicate. As can be seen from the figure, the best crystalline shape of wj-2 material was prepared by NaOH concentration 2M, maturation for 10 h, hydrothermal synthesis at temperature of 100°C for 10 h.

Mineralization properties
From Table 2, we can see that wj-1~wj-13 all have better mineralization performance than fly ash, so hydrothermal treatment can increase the ability of fly ash to mineralize CO2. wj-2 has the best performance in CO 2 adsorption with 500.1 g/kg, while fly ash can only separate CO 2 160 g/kg, an improvement of 212.5%.
The larger pore volume of fly ash makes the mass transfer enhanced, which facilitates the entry of carbon dioxide into the particles and better separates carbon dioxide. And the hydrothermal reaction increases the specific surface area of the fly ash, providing more contact area between carbon dioxide and fly ash and increasing the mineralization capacity of the end product. Table 3 shows that the specific surface area and pore volume of wj-2 are the largest with 26 m 2 /g and 0.134 cm 3 /g. As can be seen from the Figure 2, the nitrogen adsorption capacity of wj-2 was the largest among the 13 materials with 83 cm 3 /g STP, indicating that its adsorption capacity was the best. Therefore, combining the amount of CO 2 isolated, the pore structure and the adsorptiondesorption curve, it can be proved that the mineralization capacity of wj-2 is optimal.  Figure 3 show the SEM images of the original ash and wj-2, from which it can be seen that wj-2 has better dispersion and correspondingly larger specific surface area than the original ash. The surface of wj-2 shows crystallization products and the internal pore structure is relatively tight.

Microscopic characterization
The appearance of a regular mesh structure indicates that the product becomes more crystalline, while the regular mesh structure is hydrated calcium silicate aluminate. The well-developed pore structure increases the access and the possibility for CO 2 to enter inside the product, allowing CO 2 to be adsorbed inside the product to isolate CO 2 . Therefore, the mineralization capacity of the fly ash is enhanced.

Leaching concentration of heavy metals Pb
In this experiment, the leaching concentration of heavy metal Pb after hydrothermal treatment of fly ash was considered. As can be seen on Figure 4, the Pb concentration in the hydrothermally treated fly ash is characterized by significant reduction compared to the original fly ash and is lower than the landfill limit requirement. The reason is that on the one hand, Pb ions replace Ca 2+ in the original hydrated calcium silicate due to ion exchange, and under the influence of hydrothermal heat, Ca 4.6 Pb 0.322 Si6O 16 H 2 ꞏ4H 2 O is formed, Pb is immobilized in the lattice of the water-insoluble material phase, thus inhibiting the leaching of heavy metal Pb from the fly ash after hydrothermal treatment [6] ; On the other hand, the particle size of the porous material is obviously smaller than the original fly ash particles, and the structure is more dense, making it more difficult for Pb ions to be leached.

Conclusions
(1) Waste incineration fly ash can be mineralized and isolated from carbon dioxide through hydrothermal treatment, which can treat waste with waste and realize the high-value utilization of fly ash, providing a new direction for the resource utilization of fly ash.
(2) The optimal experimental conditions were that the fly ash was cooked with 2M sodium hydroxide at room temperature for 10 h, the temperature of hydrothermal synthesis was 100°C, and the temperature time of hydrothermal synthesis was 10 h. The product with the highest crystallinity and specific surface area after treatment.The amount of wj-2 mineralized carbon dioxide is 500.1 g/kg, which is an improvement of 212.5% compared to the original ash.
(3) According to the leaching concentrations of heavy metals, it can be seen that the leaching concentrations of Pb under different experimental conditions were below the standard limits, and Pb replaced Ca in the hydration products to produce calcium hydrated lead silicate, thus reducing the leaching concentrations of heavy metal Pb