Sulfate and selenium (IV) adsorption by hydrothermal activated coal fly ash based synthesized zeolites

Abstract

Coal is generally composed of inorganic matters and minerals. The utilization in coal fired generating plant releases SO, NOx, and other heavy metals which produces severe problems to the environment. In coal combustion process, fly ash was considered as a major solid waste enriched of SiO2. There have been attempts on value adding these waste utilization, the applicability as an adsorbent has been interesting. However, the challenge is a high CaO composition of this fly ash which impacts on high crystallinity formation. This study was aimed to develop a functional zeolite material from alkali activated coal fly ash through hydrothermal treatment without purification. As being a porous and small feature size material of synthesized zeolite, it is subjected for sulfate adsorption. Since sulfate behaves highly toxic when overdosing in living organism and it is dissolved in high amount around coal mining area. The zeolite synthesis was carried out via hydrothermal method (105-180°C, 6-72 hours). Different instrumental techniques were used to characterize instance XRF, XRD, FTIR, BET, and zeta potential. The XRD patterns reveals the mineralogical phase of Na-P1, sodalite, and cancrinite. FTIR spectra confirm the present of functional group of aluminosilicates connected into tetrahedral/hexagonal, with the hydrate groups. In addition, particle size of synthesized zeolite was 0.3-0.15 um diameter. The BET specific surface area and pore volume were 65.38 ± 0.27 m2/g and 15.02 cm3/g, respectively. The zero point charges obtained between 6-8 of pH solution, a decreasing pH solution results higher zeta potential while an increasing pH solution results lower zeta potential. At higher temperature and longer hydrothermal time, cancrinite and sodalite phases were increased, however lower in specific surface area and pore volumes. Since the larger specific surface area and pore volume play an important role in influencing reactivity of zeolite. The optimal condition was selected based on the maximum surface area, pore volume, and product yields. Sulfate was adsorbed at pH 6 with 0.0005 min1 reaction rate, the efficiency of synthesized zeolite 80.43 % with adsorbent dosage 2 g/L., and 14.26 mg/g of uptake capacity at 27 °C. It is implied as monolayer adsorption fitted well by Langmuir model (R2 > 0.9). Lastly, the adsorption phenomena described by Pseudo second order reaction (R2 > 0.9). The removal of selenium ions is reported as 36.47% efficiency at pH 6 for 1 hour.