A surface chemistry approach to inhibition of particle-induced hydroxyl radical generation: a case study of respirable coal dust
Open Access
- Author:
- Eskanlou, Amir
- Graduate Program:
- Energy and Mineral Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 07, 2024
- Committee Members:
- Jeremy Gernand, Program Head/Chair
Shimin Liu, Major Field Member
Nelson Yaw Dzade, Major Field Member
Barbara Arnold, Co-Chair & Dissertation Advisor
Ismaila Dabo, Outside Unit, Field & Minor Member
Mohammad Rezaee, Co-Chair & Dissertation Advisor - Keywords:
- Coal
Pyrite
Quartz
Free radicals
Toxicity
Respirable coal dust
Diesel Particulate Matter - Abstract:
- Respirable coal dust includes ultrafine particles of coal, crystalline silica, pyrite, aluminosilicates, other minerals, and, in some cases, diesel particulate matter (DPM). With increases in lung diseases, such as Coal Worker’s Pneumoconiosis (CWP), related to respirable coal dust in certain regions, investigations in this focus area are warranted. These respirable particles all have an ultrafine size distribution in common. However, they have surface chemistry components in common as well, including the formation of reactive oxygen species (ROS), notably, hydroxyl radicals (•OH) on the surface, and, for some, the presence of iron. This research addressed these two components that have been frequently discussed in the literature when respirable dust toxicity is investigated. Coal of several ranks and associated quartz and pyrite along with the aging (potential oxidation) of the dust surfaces were studied. DPM was also studied as a potential contaminant of coal, quartz, and pyrite particles. Several additives along with several commercial dust suppressants were tested in deionized water, with some being tested in tap water, associated process plant water, and simulated lung fluid (SLF) to determine their effectiveness in reducing the hydroxyl radicals that in turn may reduce the toxicity of the dust. Ab initio molecular dynamics (AIMD) simulations using density functional theory (DFT) were performed to develop insights into atomic-level interactions on the pyrite and quartz surfaces. Key findings include: Coal particles generate remarkably more •OH radicals in acidic solutions than in neutral or alkaline conditions. Lower-rank coals with more oxygen species produce more •OH. Carboxymethyl cellulose (CMC) in the form of sodium salt effectively reduces •OH radical generation in coal particles by over 70% at a pH of 7 in deionized water. The ability of coal particles to generate •OH radicals and their surface chemistry characteristics are influenced by the composition of the solution. Surface aging in coal samples, except for anthracite, increases •OH radical production. Pyrites from various coal samples differ in •OH generation capability, which is affected by the presence of elements such as silicon, aluminum, and iron in pyrite. CMC was effective in reducing •OH production by targeting surface sulfide and silicon sites and affecting surface hydration and charge. Atmospheric aging was found to increase •OH production, especially in the pyrite with high iron and silicon and low calcium contents, relative to other samples. This highlights the role of pyrite surface properties and chemical composition and the solution pH and composition in •OH generation by coal-pyrites. The production of •OH by coal-quartz varies and is enhanced by iron contamination. Iron also increases the adsorption of hydroxamic acid on quartz, improving its interaction with the quartz surface, as confirmed by the atomistic simulations. Freshly cleaved quartz surfaces produce more •OH, particularly under alkaline conditions. CMC reduces •OH generation by quartz by about 91% at pH 7 in deionized water. The production of •OH by quartz particles is notably lower in SLF than in other tested solutions. The negative charge on the quartz surface in various aqueous solutions is directly linked to •OH production. Atmospheric surface aging of the quartz reduces its •OH generation, due to a decrease in surface siloxyl radicals. Quartz is the most toxic coal dust component in terms of •OH generation. It is followed by pyrite and then coal. Generally, the commercial dust suppressants were effective in reducing the •OH content for the coal dust components, though one substantially increased the generation of •OH for the coals and pyrites. These compounds were notably more effective in wetting the most hydrophobic low volatile bituminous coal dust. The impact of DPM contamination varied among different coal dust components. Anthracite coal showed no change in •OH production when exposed to DPM; whereas, bituminous coal, particularly the high-volatile sample (HVAb), exhibited a notable increase in •OH generation. Quartz experienced a slight decrease in •OH generation upon DPM exposure. The response of pyrite to DPM exposure varied; pyrite associated with the low-volatile bituminous coal (LVb) exhibited a notable increase in •OH generation, while the pyrite associated with the medium-volatile bituminous coal (MVb) and the HVAb coal showed minor increases. Future work should include optimizing the selection of CMC dosage and type as well as investigating combinations of CMC and commercial dust suppressants to both wet the dust and reduce the •OH content, producing a superior dust suppressant. Additionally, future research should examine quartz and other potentially hazardous mineral dusts from various sources, including metal and nonmetal mines. It is also advisable for upcoming studies to focus on environmentally persistent free radicals (EPFRs) from various coal ranks and associated minerals to assess their impact on the toxicity of coal dust.