Insight into As2O3 adsorption characteristics by mineral oxide sorbents: Experimental and DFT study

Abstract As a highly toxic element of coal, arsenic (As) is mainly converted into As2O3 vapor during coal combustion. Mineral oxides are effective and abundant sorbents to control As2O3 emissions. In this paper, the characteristics of As2O3 adsorption by mineral oxides were studied in a two-stage reactor, and the density of states, adsorption sites, and adsorption energies for the adsorption reaction were investigated using molecular simulation. The results demonstrate that CaO is an excellent As2O3 sorbent. The As2O3 adsorption effect of CaO, MgO, and Al2O3 was found to become better with increasing temperature within the range of 300-900 ℃. The order of As2O3 adsorption at 300-700 ℃ follows CaO > Fe2O3 > Na2O > MgO > Al2O3 > SiO2, and SiO2 hardly adsorbs any As2O3. At 900 ℃, the melting of Na2O and Fe2O3 greatly reduces their As2O3 adsorption capacity. The O atoms in Na2O/CaO/MgO/Al2O3/SiO2 and the Fe atoms in Fe2O3 are the adsorption active sites for As2O3. The order of the calculated adsorption energy follows Fe2O3 > Na2O > CaO > Al2O3 > MgO > SiO2, and the As2O3 adsorption involves chemical adsorption. The adsorption products are Ca3(AsO4)2, FeAsO4, AlAsO4, etc., for which As exists in the form of As3+ and As5+, with As3+ transformed into As5+ with increasing adsorption temperature.