Hazards and Usability of Coal Fly Ash

Abstract

Coal fly ash, a by-product of coal combustion in thermal power plants, is a fine powder made up of small, spherical, glass-like particles, the major matrix of which is ferro-alumino-silicate minerals. This review presents the detrimental and beneficial physical, chemical and biological properties of fly ash. These are a consequence of its particular chemical composition and can cause an environmental hazard, but also, under some conditions, enable it to be used in some branches of industry. Fly ash generally contains potentially dangerous and harmful heavy metals, organic pollutants and some radionuclides. Toxic concentrations of these in water, soil and air, as a consequence of ash scattering, can cause ecological problems (land degradation in the vicinity of the power plant), health problems (for human and animal respiration) and agricultural problems (harmful influence on soil, soil microorganisms and crops) in the ash dump surroundings and by leaching into watercourses and moving further into the food chain. The millions of tonnes of fly ash produced annually worldwide (about 800 Mt per year) are a major problem in many countries. Phytoremediation has been used to prevent the detrimental influences of fly ash on the environment. Grasses are suitable for cultivation on fly ash dumps as an initial, quick way to cover the ash dump and prevent ash from scattering. However, the best plant application is a mixture of grasses and legumes, due to the legumes’ ability to fix N2. When legumes are supplied with N2-fixing bacteria (fam. Rhizobiaceae), a highly effective bioinoculant, the net effect is the replacement of the mineral N fertiliser that would otherwise be required. The process of phytoremediation needs an enormous N2 content as fly ash contains little or no N. Symbiotic N2 fixation is discussed as an environmentally friendly, cost-effective method to supply crops with nitrogen. The root nodulation of the legumes can be used for an eco-toxicological evaluation of soils contaminated with fly ash heavy metals. Amendments for the effective establishment of remediation plants on ash dumps are reported in this review. Among them, the application of microorganisms (plant growth promoting rhizobacteria), alone or with the plants in this process, constitutes a more sustainable and cost-effective approach for removing heavy metals from fly ash dumps. However, nanotechnology could also help to remove heavy metals from fly ash dumps. Due to some desirable fly ash properties, primarily its alkaline pH, and some macro- and microelements, some of which are very rare, as well as the ash’s ability to transition into nanoparticles, fly ash has been used in many branches of industry, especially the construction industry and agriculture. However, around 40% of fly ash worldwide is not utilised, so the dumps this produces still require constant monitoring of their surrounding land, water and agricultural crops. Greater use of fly ash would improve the circular economy.