Identification and Quantification of Chemical Forms of Cu and Zn in MSWI Ashes Using XANES

Jenny Rissler,Jonas Dahl,Mar Edo,Britt-Marie Steenari,Konstantin Klementiev

doi:10.1021/acs.energyfuels.0c02226

Abstract

Incineration is in many countries a common treatment method for municipal solid waste, and utilization of the ash residues has attracted significant interest. The bottom ash is best suited as a secondary construction material, whereas the fly ash is being investigated as a secondary raw material for recovery of, for example, Zn, Cu, and salts. For both types of application, knowledge about the chemical speciation of Zn and Cu in the ashes is valuable. The present work focuses on identifying and quantifying the chemical species of Zn and Cu in 12 samples of fly ash and bottom ash from three waste-to-energy plants using X-ray absorption near edge structure (XANES). The XANES spectra of the ash samples showed similar distinctive features, and both in the bottom and fly ash samples, the same chemical forms were identified but in various ratios. Cu and Zn occurred in several chemical forms, with typically 5-7 forms present in the same sample. For Cu, the XANES spectra of the fly ash samples were nearly identical, indicating very similar chemical speciation (same chemical forms and similar ratios). Cu was found to exist in various oxide, hydroxide, chloride, silicate, and metallic forms. The most commonly occurring Zn compounds were the aluminate, ferrite, silicate, and oxide along with chloride, basic carbonate (hydrozincite), and occasionally metallic forms, probably alloyed with Cu in brass. Cu occurred in different oxidation states from zero to +II, with a higher prevalence of the lower oxidation states in bottom ash than in fly ash. Zn occurred mainly in oxidation state +II in all ashes analyzed. Finally, we showed that during outdoor storage of bottom ash, levels of Cu and Zn hydroxycarbonates were increased compared to fresh bottom ash. This carbonate formation aims to make Cu and Zn less leachable.

Highlights

Incineration is a common method to treat solid waste
We investigated the chemical speciation of Cu and Zn in both bottom ashes (BAs) and fly ash (FA) from three MSW waste-toenergy (WtE) plants using X-ray absorption near edge structure (XANES)
The concentrations of the major elements as well as Cu and Zn are summarized in Table 2 together with other relevant parameters, such as mass loss on ignition (LOI) and moisture content (MC), for the 12 ash samples included in the study

Summary

Introduction

Incineration is a common method to treat solid waste. The benefits include reduction of the waste volume by 80−90%, destruction of harmful waste components, and recovery of energy as heat and electricity. Solid waste incineration bottom ashes (BAs) typically consist of noncombustible materials (e.g., slag, concrete, stones, glass, brick, and metal pieces) and are typically stored for several months in contact with air in order to react with CO2. This carbonation process aims at reducing the leachability of toxic metal compounds from the ash by the formation of minerals, such as aluminum hydroxides, iron hydroxides, and calcium carbonate, onto which the toxic metal ions are adsorbed and incorporated. The ash classification depends on the content of ecotoxic compounds.[8] there is an urgent need for methods to identify potentially toxic chemical forms of trace metals present in the ashes. Methods for analyzing the speciation of trace metals in ashes are important to understand ash formation processes and design methods to recover metals from fly ash (FA).[9−11]

Methods

Results

Conclusion