Speciation of Arsenic and Selenium in Coal Combustion Products

Abstract

Coal combustion is one of the main anthropogenic sources of toxic trace element emissions to the environment. Various species and oxidation states of the trace elements released from power stations may determine their ultimate environmental fate and health impacts. This study focuses on speciation of arsenic and selenium present in the coal combustion products. Speciation analysis in representative coal, bottom ash, and fly ash obtained from four different Australian power stations was carried out in this work. Laboratory ash and char were also produced by carrying out combustion and pyrolysis experiments in a laboratory based drop tube furnace. The synchrotron based nondestructive speciation analysis method X-ray absorption fine structure spectrometry (XAFS) was applied for arsenic and selenium speciation analysis of the selected coal, ash, and char samples. The semiquantitative analysis of arsenic revealed variations in arsenic species in the coal samples indicating the presence of As/pyrite, arsenite (As3+), and arsenate (As5+) with the latter as a dominant form. Arsenic in power station fly ash samples was found to be mainly in an arsenate form with little presence of arsenite (As3+). Selenium speciation in coal samples indicated organic/reduced or elemental forms as dominant selenium species along with presence of selenite (Se4+)/selenate (Se6+). Selenium in fly ash was mainly found to be selenite with a minor presence of selenate. Char produced by pyrolysis indicated different speciation behavior of arsenic and selenium compared to coal and ash samples, which might be due to their further reactions with other volatilized species produced during pyrolysis and/or retained mineral matter.