Immobilization of heavy metals in hazardous fly ashes

Abstract

This study investigated the possibility of enhancing immobilization of heavy metals in hazardous fly ashes by mineral carbonation. The research initially investigated five different fly ashes for their chemical composition and carbonation potential. The highest lime content resulted in the highest carbonation potential, but mineral carbonation occurred also in other fly ashes with very limited free lime. One of the municipal solid waste incineration (MSWI) fly ash (FA) with a high heavy metal content and the highest carbonation potential was selected for further carbonation and immobilization investigations.
The selected MSWI FA was carbonized following the first three out of four possible different routes:

1.Pre-carbonation of fly ash and casting with binder
2.Pre-carbonation of fly ash-binder mixture and casting after carbonation
3.Curing of the sample in the carbonation chamber
4.Carbonation of fresh mixture during mixing

The materials were tested for their crystalline composition before and after curing as well as for effective carbon-uptake. The second route resulted in the highest effective carbonation for the MSWI FA but also in the lowest compressive strength. The third route showed an increase in strength compared to the reference sample and effective reduction of final pH on the leachate and electrical conductivity. A fast shake leaching test was used to determine the leachability of relevant anions and cations from the mix designs. The results were compared with a reference sample prepared following a procedure used in industrial-scale applications and with the Dutch Soil Quality Decree and Regulation (Ministerie van Infrastructuur en Waterstaat, 2022).
The leaching tests resulted in an increased immobilization of lead, copper, and zinc for both a MSWI FA and a biomass (BM) FA also selected for its high carbonation potential. The highest total immobilization was reached for the MSWI FA through pre-carbonation of fly ash-binder mixture as the high reduction of pH of the sample cured in the carbonation chamber increased the solubility of other heavy metals. The immobilized BM FA through carbon-curing resulted in the optimal immobilization solution, followed by the pre-carbonation of the mixture of binder and fly ash. For both tested materials, the addition of carbonation outperforms the reference samples.
The comparison with the Dutch soil decree showed promising outcomes for future development of mineral carbonation for immobilization of hazardous materials. The carbonized specimens met most of the requirements for construction materials, which were expected to be more stringent than those for landfill use. The materials were evaluated under a worst-case scenario, showing the effective reduction of lead leachability. The carbonation of hazardous materials offered potential for application in highly lead-polluting waste streams to reduce carbon emissions and contribute to a cleaner environment.