Fly ash releases from surface impoundments can be identified through spICP-TOF-MS fingerprinting

Coal fly ash is used in the construction industry and holds potential as a resource for the extraction of rare earth elements and high-value metals such as scandium. It is commonly stored in uncovered landfills or ash ponds close to the coal-fired power stations. Major releases from these surface impoundments into adjacent rivers pose an environmental concern due to the harmful effects of associated heavy metals. Future releases may occur more frequently as the globally impounded ash volume continues to grow and climate change threatens to further increase extreme rainfall events, which can cause overflows of surface impoundments and failure of their retention dams. Minor releases of fly ash from the storage sites could be early warning signs of their declining retention. When reaching a river, fly ash particles deposit in its sediments. Thereby, fly ash particles may leave a sedimentary record of minor releases over many years, but these particles are difficult to detect due to their small size and low concentration.

We used single particle inductively-coupled plasma time-of-flight mass spectrometry (spICP-TOF-MS) to identify element fractionation patterns in micrometer- and nanometer-sized fly ash particles. Some intermediately-volatile elements such as nickel, cobalt, and vanadium fractionate towards smaller particle sizes during coal combustion. These elements were enriched in fly ash up to three orders of magnitude above the natural sedimentary background levels. We found particles with this characteristic fly ash elemental fingerprint in four depths of a sediment core downstream of a fly ash landfill, indicating fly ash release into the river. Upstream river sediment reference samples, on the contrary, did not contain these particles. The detected releases of fly ash deposited only trace amounts of fly ash in the sediment, which were neither detected applying major and trace element analysis (ICP-OES, ICP-MS, and XRF), nor found through mineralogical (XRD) or morphological (SEM) investigations.

We demonstrated that spICP-TOF-MS can be used to identify fly ash particles in river sediment at previously undetectable concentrations. This technique can thus help assess surface impoundment integrity to prevent catastrophic future spills.