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Permeable reactive barriers (PRBs) are a well-known technique for groundwater remediation using industrialized reactive media such as zero-valent iron and activated carbon. Permeable reactive concrete (PRC) is an alternative reactive medium composed of relatively inexpensive materials such as cement and aggregate. A variety of multimodal, simultaneous processes drive remediation of metals from contaminated groundwater within PRC systems due to the complex heterogeneous matrix formed during cement hydration. This research investigated the influence coarse aggregate, portland cement, fly ash, and various combinations had on the removal of lead, cadmium, and zinc in solution. Absorption, adsorption, precipitation, co-precipitation, and internal diffusion of the metals are common mechanisms of removal in the hydrated cement matrix and independent of the aggregate. Local aggregates can be used as the permeable structure also possessing high metal removal capabilities, however calcareous sources of aggregate are preferred due to improved removal with low leachability. Individual adsorption isotherms were linear or curvilinear up, indicating a preferred removal process. For PRC samples, metal saturation was not reached over the range of concentrations tested. Results were then used to compare removal against activated carbon and aggregate-based PRBs by estimating material costs for the remediation of an example heavy metal contaminated Superfund site located in the Midwestern United States, Joplin, Missouri.
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Role of Nanoscale Crystallinity on the Recovery of Rare Earth Elements (REEs) from Coal Fly Ash
Reclamation of coal fly ash, a legacy waste material, provides an alternative pathway for the recovery of rare earth elements (REEs) while reducing the environmental stresses that stem from traditional mining. The reactive transport processes underlying the recovery of REEs from ash wastes, however, are yet to be fully elucidated owing to the physicochemical complexity of the micro/nanoscale fly ash particles, including the crystallinity of the particulate matrix. In this work, we use transmission electron microscopy to characterize the material properties of ash particles and reveal the impact of crystallinity on the reactive transport processes governing access to and recovery of the encapsulated REEs. Our results show, for the first time, two distinct crystalline structures of REE-bearing aluminosilicate particles: dense amorphous matrices that facilitate the exchange of chemical species through their lattice interstices and porous polycrystalline matrices characterized by connected intraparticle pores and chemical inertness to leaching solutions. Notably, the presence of matrix crystallinity, or the lack thereof, governs the extent of reagents consumed parasitically by secondary reactions with the aluminosilicate matrix. Our work reveals how the variability of crystalline structures of the ash matrices hosting REEs defines the pathways for the recovery of REEs, providing key insights required for the development of targeted recovery processes.
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- Award ID(s):
- 2145374
- PAR ID:
- 10534102
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Environmental Science & Technology Letters
- Volume:
- 10
- Issue:
- 10
- ISSN:
- 2328-8930
- Page Range / eLocation ID:
- 943 to 948
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.estlett.3c00383
