Mechanism of selenium sorption by activated carbon

Abstract

USAoal combustion, waste incineration as well as glass manufacturing generates hot flue gases containing selenium species. Selenium C is one of the most volatile trace elements in coal, and a signlficant portion of it escapes to atmosphere in vapour form (Linak and Wendt, 1993; Martinez-Tarazona and Spears, 1996). A substantial amount of selenium in the flue gas remains in the gas phase and its distribution between the vapour and condensed form depends on various factors such as initial concentration in the parent coal, design and operating conditions of the combustor facility, and the particulate control devices. Cermani and Zoller (1 988) reported an average of 59% of the in-stack Se to be in the form of vapour and its concentration in the vapour phase to be about 25 pg/m3. Mercury and the halogens are the only other elements that exhibit such high affinity for gas phase.Despite the fact that Se is an essential trace element for humans and other animals, selenium and its compounds have been linked to harmful toxicological impacts (Marshall, 1986; Oehm et al., 1991). The major sources of selenium and other trace metal emissions are the coal-fired power plants and utility boilers and these will most likely be required to reduce these emissions by impending EPA regulations. In an estimate, 62% of industrial selenium emission has beem linked to coal-fired power plant emissions (Sarquis and Mickey, 1980). Average selenium content of US. coal is reported as 4 ppm.The chemical state in which selenium exists in the flue gas has been a subject of intense controversy and speculation. Selenium exhibits -2, 0, +2, 4, and +6 as its possible oxidation states and is similar to sulfur in most of its chemistry. The elemental selenium and the oxide form (SeO,) have been postulated to be the two most probable species. Andren et al.(1 975) postulated that the SO, produced during coal combustion could act as a reducing agent to yield elemental Se from SeO, . Davison et al. (1974), on the other hand, concluded from the thermodynamic data that it is Se0, that escapes to the atmosphere, and at 25'C as much as 80 pg/m3 of Se can exist as SeO, . Later studies (Dismukes, 1994) indicate that selenium exists as SeO, for almost the entire course of its stay in the combustion environment, but at the lower temperature of the stack could be partially reduced to Se by the presence of SO, .Due to its high volatility and its existence in the form of vapour, selenium emission control is a very difficult and challenging task. A possible methodology of selenium control could involve reduction in the operating temperature of the stack in order to enhance condensation of the vapour and hence metal removal; however, the resulting loss In buoyancy of the stack emission eliminates this as a feasible technique. Improving performance of the particulate control device can lead to better capture of the finer metal aerosols, but it fails to control the emissions in vapour form. In the past few years, dry sorbent injection for 'Author towhom correspondme m...

Capture of Gas-Phase Arsenic Oxide by Lime: Kinetic and Mechanistic Studies

“…The details of the experimental procedure and differential reactor assembly used are given elsewhere (8). Arsenic oxide (As2O3) is used as the source of arsenic in all the experiments.…”

“…Wouterlood and Bowling (3) found that activated carbons were quite effective in reversibly trapping arsenic oxide at 200 °C. Jadhav et al (8) have shown that the mechanism of selenium dioxide capture by activated carbon involves a combination of physisorption and dissociative chemisorption. Fly ash has also been shown to remove trace elements such as As, Cu, Mo, Pb, and Zn from power station flue gas (9).…”

Retention of arsenic and selenium compounds present in coal combustion and gasification flue gases using activated carbons

“…Activated carbon offers an attractive option for use as a multifunctional sorbent because it has proved to have good retention characteristics for mercury species in gas phase (Krishnan et al, 1994;Ghorishi and Gullett, 1998). However, its effectiveness in controlling selenium, arsenic, and other volatile trace elements still needs to be ascertained (Jadhav et al, 2000). In order to obtain this knowledge, the aim of this work was to determine the capacity of different activated carbons for retaining the arsenic and selenium compounds present in gases from coal combustion and gasification.…”

Technologies to Remove Selenium from Water and Wastewater

“…In general, in a decontamination system, activated carbons are used in pre-treatment to remove as many pollutants as possible, including selenium. Therefore this is not a selective adsorption but rather a global adsorption (Jadhav et al 2000;Jeqadeesan et al 2003;Zhang et al 2008;Sandy and DiSante 2010;Dobrowolski and Otto 2013;Kwon et al 2015;Santos et al 2015;Bakather et al 2017;He et al 2018b). Granular activated carbons are the most promising adsorbents, particularly iron-coated carbons due to their applicability to continuous processes.…”