Measuring reactive pools of Cd, Pb and Zn in coal fly ash from the UK using isotopic dilution assays

Abstract

Large volumes of coal fly ash are continually being produced and stockpiled around the world and can be a source of environmentally sensitive trace elements. Whilst leaching tests are used for regulatory purposes, these provide little information about the true geochemical behaviour and 'reactivity' of trace elements in coal ash because they are poorly selective. Isotopic dilution (ID) assays are frequently used in soil geochemistry as a means of measuring the reactive pools of trace metals that are in equilibrium with soil pore waters. This paper examines the applicability of multi-element ID assays in measuring the labile or reactive pool of Cd, Pb and Zn in a range of fresh and weathered fly ash, where pH is generally much more alkaline than in soils. The method generally worked well using 0.0005M EDTA as a background electrolyte as it provided robust analytical ICP-MS measurements as well as fulfilling the important principle of ID that non-labile metal should not be solubilised. Reactive pools were equivalent to 0.5-3% of the total Pb pool and 4-13% of the total Cd pool. For Zn, where samples had pH<11.5, the reactive Zn pool varied between 0.3 and 2%; when fresh ash samples with pH>11.5 were tested, the method failed as the spiked isotope appeared to be sorbed or precipitated. Ash weathering was found to exert little impact on the lability of Cd, Pb and Zn. We compared isotope dilution results with 0.43M HNO 3 and 0.05M EDTA extractions, these commonly being used as analogues of the ID assay, and concluded that these can be used as fast, cost-effective and simple proxies for the ID assays. Results suggest that ID methods can be used to enhance our knowledge of trace element behaviour in fresh and weathered fly ash.

Measurement of isotopically‐exchangeable Zn in Zn‐deficient paddy soil

“…This is in sharp contrast to the E ‐value < 2 mg kg −1 obtained for short‐term equilibration of the control soil extracted and labelled after 18 days of incubation when the redox potential was stable and the conditions were not dissimilar to those of the pre‐incubated soil. Unrealistically large E ‐values caused by tracer‐derived artefacts (Hamon et al ., ) have been reported for Zn in alkaline media (Tiller et al ., ; Sinaj et al ., ; Izquierdo et al ., ) and attributed to the tracer undergoing irreversible sorption or precipitation.…”

In vitro assessment of oral and respiratory bioaccesibility of trace elements of environmental concern in Greek fly ashes: Assessing health risk via ingestion and inhalation

“…Coal-fired power plants are still predominant energy suppliers worldwide trying to fulfill the growing global energy demands (Izquierdo et al 2013). Fly ash (FA) as the primary by-product of coal-based energy supply, present increased production rates which reach 100 Mt in the European Union every year (ECOBA, 2016).…”