Thermodynamic Study of the Synthesis of Zeolites from Coal Ash and Its Use as Sorbents for Heavy Metals

Overview on the Potential of Coal-Based Bottom Ash as Low-Cost Adsorbents

“…125 Depending on the operating conditions and chemical compositions of the bottom ash used in this process, various types of zeolites can be produced, which include zeolite A, zeolite P, zeolite X and Y, sodalite, chabazite, and many others. 7 Upon hydrothermal, the BET surface area of the synthesized zeolites from coal bottom ash is significantly higher than the corresponding precursors that are from 5.3 to 84.9 m 2 /g, which is due to crystallization during the hydrothermal process. 45 Chareonpanich et al 126 reported similar findings in which the surface area of synthesized zeolites and raw sub-bituminous coal bottom ash is 36.1 and 7.6 m 2 /g, respectively.…”

“…109 In fact, consumption of zeolites in heavy metal removal is effective due to their high stability, defined molecular structures, as well as ion exchange capacity. 7 Moreover, Salam et al 110 reported that zeolites that have exchangeable ions such as sodium, calcium, and potassium contribute to the heavy metal removal from the wastewater. Because of the high benefits in utilizing zeolites, a search for inexpensive feedstock rich in silica and alumina content is recommended.…”

“…In fact, the synthetic zeolites are reported to be highly marketable than natural zeolites, owing to its high purity and consistency of the particle sizes. 7,112 Not only that, Kikuchi et al 113 reported that high marketability of synthetic zeolites from coal ash is due to high pricing of the commercial zeolites that range from USD 200 to 400 per tonne.…”

“…In step ii, eluted silica and alumina species reacts with each other and produces an aluminosilicates gel (hydrogel) that will deposit onto the ash surface, whereas step iii is the formation of crystalline zeolites upon heating. 7,115,117 These synthesized zeolites vary from each other, depending on the operating condition, i.e., nature and concentration of reactant, temperature, pressure, reaction time, and liquid to solid ratio. 118−120 In the literature, typical operating conditions for the coal bottom ash conversion to zeolitic-based materials are at temperatures of 80 to 150 °C, reaction times of 24 to 36 h, and by using aqueous NaOH solution at concentrations of 1 to 5 M. Afterward, the hydrothermally treated solid products are filtered from the solution, washed to eliminate the excess NaOH and soluble impurity, and oven-dried for 12 h at temperatures of 100 to 110 °C.…”

“…Further, the isotherm should be the first step in order to evaluate the sorption mechanism, and the obtained data can be used to outline the operating conditions for the industrial equipment used in adsorption system. 7,141 By now, most of these isotherms studies focus on pollutants uptake in aqueous phase by using natural bottom ash, rather than the derived-adsorbent from these coal ashes, as shown in Table 3. The common isotherms that have been applied for the adsorption process are the Langmuir and Freundlich type, in which the findings of the suitable isotherms are translated to the mechanism or behavior of the adsorption process.…”