Typical Gaseous Semi-Volatile Metals Adsorption by Meta-Kaolinite: A DFT Study

Abstract

Kaolinite can be used as in-furnace adsorbent to capture gaseous semi-volatile metals during combustion, incineration, or gasification processes for the purposes of toxic metals emission control, ash deposition/slagging/corrosion inhibition, ultrafine particulate matter emission control, and so on. In this work, the adsorptions of typical heavy metals (Pb and Cd) and typical alkali metals (Na and K) by meta-kaolinite were investigated by the DFT calculation. The adsorption energies followed the sequence of NaOH-Si surface > KOH-Si surface > PbO-Al surface ≈ CdO-Al surface ≈ NaOH-Al surface > KOH-Al surface > NaCl-Al surface ≈ Na-Si surface > Na-Al surface > KCl-Al surface > Pb-Al surface > PbCl2-Al surface > CdCl2-Al surface ≈ K-Si surface ≈ PbCl-Al surface > K-Al surface > CdCl-Al surface > NaCl-Si surface > KCl-Si surface > Cd-Al surface. Si surface was found available to the adsorptions of Na, K, and their compounds, although it was invalid to the adsorptions of Pb, Cd, and their compounds. The interactions between adsorbates and surfaces were revealed. Furthermore, the discussion of combining with the experimental data was applied to the subject validity of calculation results and the effect of chlorine on adsorption and the effect of reducing atmosphere on adsorption.

PbCl2 Capture by Kaolin and Metakaolin under Different Influencing Factors of Thermal Treatment

“…As shown in Figure , 700 °C was special that capture efficiency increased suddenly, which echoed the TG analysis: the end of mass loss peak for kaolin was at about 700 °C, indicating that most of the hydroxyls in Al–O-octahedra basically removed at 700 °C. In the previous work of our research group, the adsorption sites of kaolinite for PbCl 2 mainly appeared in the Al–O–Al bond, which can form the Pb–O and Cl–Al covalent bonds with PbCl 2 . Owing to the absence of hydroxyl groups, a new form of 5- and 4-coordinated Al observed in the analysis of NMR can promote the adsorption of PbCl 2 .…”

“…Owing to the rapid consumption of coal in China, the utilization of kaolin, which is one of the associated minerals in coal mining, has been proposed in national guidance on the comprehensive utilization of resources during the eleventh five year plan. Kaolin, a common layer of aluminosilicate based on uniting of Si–O tetrahedron sheet and Al–O octahedron sheet, had been widely used in many fields, e.g., in the manufacturing of paper, ceramics, and refractories, as well as for controlling the pollutant emission in combustion, incineration, or gasification as an additive recently . According to the literature, emission of metallic elements that are released during thermal processing of coal, biomass, and municipal solid waste (MSW), − like Na, Pb, Cr, Zn, etc., can not only cause severe fouling, slagging, and corrosion of the furnace but also damage the environment and human health seriously and therefore effective and inexpensive sorbents for capturing these toxic substances have gained great attention in recent years. − Kaolin has become a top choice for retention of particulate matter (PM), alkali and alkaline earth metals, and trace metals − at high temperature due to its great performance in physical and chemical adsorption.…”

Dehydroxylation and Structural Distortion of Kaolinite as a High-Temperature Sorbent in the Furnace

“…The temperature of municipal solid waste (MSW) incineration is usually 800-1200 • C, at which, semi-volatile heavy metals and alkali metals exist mainly in the form of chloride vapor, such as PbCl 2 , PbCl, CdCl 2 , CdCl, NaCl and KCl [40][41][42][43]. In the condition of in-furnace injection, the residence time of kaolinite in the furnace is usually less than 2 s, so 15-55% of hydroxyl groups or more would remain in kaolinite.…”

“…The temperature in the pulverized coal-fired furnace is usually 1200-1500 • C. The forms of semi-volatile heavy metals and alkali metals are mainly oxide vapors such as PbO, CdO, NaOH and KOH [4,40,42]. In the condition of in-furnace injection, the residence time of kaolinite in the furnace is usually less than 2 s, so around 90% of hydroxyl groups would be removed.…”

“…In the condition of in-furnace injection, the residence time of kaolinite in the furnace is usually less than 2 s, so around 90% of hydroxyl groups would be removed. The activity of Al atoms is the key to adsorption [9,10,40,44]. It could be described as Equation (14) and Figure 15.…”