Amitava Roy, Danting Chen, Anna Bogush and Julia Stegemann
Summary
This study looks at how zinc behaves in air pollution control residues (APCR), a by-product produced by energy-from-waste facilities in the UK. These residues can create environmental challenges if they are not managed carefully. Using advanced X-ray techniques, we studied zinc speciation and leachability in seven APCR samples before and after a treatment that partially neutralises the material using acidic wastes, as is sometimes conducted before disposal.
We found that the same zinc species after neutralisation, and lower zinc leachability is attributed only to decreasing the pH to 10.0–10.7, rather than a true and lasting stabilisation of zinc. Because environmental conditions such as landfill leachate (typically pH 5–8) can gradually lower pH over time, this type of partial neutralisation treatment is unlikely to reduce the long-term environmental risk associated with zinc in APCR.
These findings highlight the importance of better strategies for managing APCR within a circular economy framework. Understanding the behaviour of metals like zinc can help scientists design smarter biological and chemical approaches to recover valuable metals from waste streams while minimising environmental impacts. Ultimately, this work supports more sustainable ways of turning waste into resources, aligning with the mission of ELEMENTAL Hub to advance resource recovery and circular solutions.
Abstract
Seven UK air pollution control residues (APCR) from municipal solid waste combustion were examined to understand the speciation of potentially ecotoxic zinc, before and after partial APCR neutralisation using acid wastes, as is sometimes conducted before disposal. Fe K-edge XAS showed Zn-containing magnetite (spinel) is an important phase in APCR, along with ferrihydrite. Linear combination fitting of X-ray absorption near edge structure (XANES) Zn K-edge spectra strongly concluded that the 2600-7300 mg/kg of zinc in the APCR is speciated mainly as spinel, hemimorphite, a glassy phase, zinc phosphate and hydrozincite, with statistically consistent findings both within and between APCRs. The extended X-ray absorption fine structure (EXAFS) spectra of the APCR mainly show the first Zn-O shell, which is consistent with Zn in solid solution, glass and poorly crystalline phases. Presented in the context of a full review of previous studies, these results suggest changes in Zn speciation under modern operating regimes. pH-dependent leaching behaviour of the raw APCRs was consistent with solubility control by secondary Zn(OH)2 over the alkaline range, and also Zn5(OH)8Cl and hydrozincite at mid-alkaline pH. Partial neutralisation of the APCRs with concentrated HCl formed secondary reaction products that agglomerated the APCRs, but the same zinc species were found after neutralisation, and lower zinc leachability is attributable only to decreasing the pH to 10.0-10.7. Since this pH is unlikely to be stable in the environment (e.g., in interaction with landfill leachate at pH 5-8), industrial “treatment” by partial neutralisation does not reduce the environmental risk associated with zinc in APCRs.