Trace element behavior during coal combustion: Insights from cadmium and zinc isotope analysis

Due to its low cost and abundance, coal has been pervasively used in recent decades to meet large energy demands worldwide. Coal combustion provides the dominant electrical source in the U.S and produces over 100 million tons of coal combustion products (CCPs) per year domestically. The disposal/beneficial use of CCPs has the potential to release toxic trace elements, including cadmium (Cd) and zinc (Zn), into the environment. High temperature combustion of coal changes both physical states and chemical properties of Cd and Zn compounds in CCPs. Related processes such as evaporation and condensation, can lead to mass-dependent fractionation of Cd and Zn isotopes. Hence, Cd and Zn isotope ratios can provide powerful tracers to identify anthropogenic metal inputs in the environment. In this study, CCPs [bottom ash (BA), economizer fly ash (EFA) and fly ash (FA)], parent feed (FC) and pulverized coal (PC) from two coal-fired power plants in New Mexico and Ohio were characterized systematically by a suite of geochemical and isotope techniques to understand trace element behavior during coal combustion and disposal/beneficial use of CCPs. Analysis by X-ray diffraction (XRD) showed that CCPs consists of ∼70% non crystalline (amorphous) material and ∼30% mineral phases. The major mineral composition of CCPs consists of silicates formed during high temperature combustion such as quartz (SiO2) and mullite (Ai6Si2O13). Metal distribution in CCPs was characterized by electron-microprobe (EMP) analysis. Enrichment of Pb, geochemically similar to Cd and Zn, on the outer surface of the CCP particles was observed. Leaching experiments showed that 5% HNO 3 mostly leaches Cd and Zn out of the particles. The Cd isotope composition (δ114Cd) observed in FA samples ( 0.39 to +0.47‰) is heavy relative to BA samples ( 0.75 to 0.52‰). The Zn isotope composition (δ66Zn) shows similarly that heavier Zn isotopes are enriched in FA samples (0.07 to 1.02‰) relative to BA samples ( 0.52 to 0.07‰). We suggest that isotopically heavy Cd and Zn preferentially condensate on the fine FA inside the boiler and continues as the FA moves downstream along with the Cd and Zn vapor. Transport of FA and isotopically heavy Cd and Zn out of the boiler causes depletion in heavy Cd and Zn isotopes on BA. Key parameters that control these isotope variations among CCPs include the condensation of Cd and Zn onto fine particulates on the various stages of power plant operations and the particle size distribution of CCPs. Mass balance calculations supported our explanation, revealing that Cd and Zn are mostly captured in FA (20 60% of the total) compared to BA (1-7% of the total). The remaining 35-70% Cd and Zn is assumed to be in the flue gas phase. Results from the mass balance model estimate that the vapor phase present prior to the flue gas desulfurization unit has light Cd and Zn isotope compositions ( 0.61 and 0.05‰ respectively) relative to those of CCPs. Coal combustion processes hence generate fractionated Cd and Zn isotopic signatures in CCPs. To determine the Cd and Zn isotopic signatures during the potential release of these elements from the CCPs to the environment, Cd and Zn isotope compositions in products from batch leaching experiments (DI water, acetic acid, hydroxyl ammonium chloride, hydrogen peroxide followed by ammonium acetate and 5% nitric acid), were also investigated. Low temperature ashed coal samples (FC and PC) show a narrow range of δ114Cd and δ 66Zn values after leaching with 5% HNO3 (0.26 to 1.17‰ and 0.06 to 0.52‰ respectively). Distinctly heavier Cd isotope compositions were observed in each leachate product in FA samples (1.1 to 7.05‰). In contrast, δ114Cd values of BA samples are depleted in heavier Cd ( 2.7 to +0.1‰). Similarly, enrichment of heavy Zn isotopes is observed in FA samples (1.57 to 3.31‰) relative to BA samples ( 0.25 to +0.09‰) after leaching by 5% HNO3. The Cd and Zn isotope systems provide a useful tool for tracing anthropogenic sources of toxic metals in the environment as our study reveals a strong enrichment in heavy Cd and Zn isotopes in fly ash samples, probably due to condensation of heavy Cd and Zn isotopes onto fine CCP particles during cooling of flue gas after coal combustion. Additionally, we estimated that the vapor phase is relatively depleted in heavy Cd and Zn isotopes. This observation shows that the Cd and Zn cycling in the environment has been altered due to high-temperature anthropogenic processes