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Zero-valent iron and organic carbon mixtures for remediation of acid mine drainage: Batch experiments

Authors
Journal
Applied Geochemistry
0883-2927
Publisher
Elsevier
Publication Date
Volume
23
Issue
8
Identifiers
DOI: 10.1016/j.apgeochem.2008.03.005

Abstract

Abstract A series of laboratory batch experiments was conducted to evaluate the potential for treatment of acid mine drainage (AMD) using organic C (OC) mixtures amended by zero-valent Fe (Fe 0). Modest increases in SO 4 reduction rates (SRRs) of up to 15% were achieved by augmenting OC materials with 5 and 10 dry wt% Fe 0. However, OC was essential for supporting SO 4 reducing bacteria (SRB) and therefore SO 4 reduction. This observation suggests a general absence of autotrophic SRB which can utilize H 2 as an electron donor. Sulfate reduction rates (SRRs), calculated using a mass-based approach, ranged from −12.9 to −14.9 nmol L −1 d −1 g −1 OC. Elevated populations of SRB, iron reducing bacteria (IRB), and acid producing (fermentative) bacteria (APB) were present in all mixtures containing OC. Effective removal of Fe (91.6–97.6%), Zn (>99.9%), Cd (>99.9%), Ni (>99.9%), Co (>99.9%), and Pb (>95%) was observed in all reactive mixtures containing OC. Abiotic metal removal was achieved with Fe 0 only, however Fe, Co and Mn removal was less effective in the absence of OC. Secondary disordered mackinawite [Fe 1+ x S] was observed in field-emission scanning electron microscopy (FE-SEM) backscatter electron micrographs of mixtures that generated SO 4 reduction. Energy dispersive X-ray (EDX) spectroscopy revealed that Fe–S precipitates were Fe-rich for mixtures containing OC and Fe 0, and S-rich in the absence of Fe 0 amendment. Sulfur K-edges determined by synchrotron-radiation based bulk X-ray absorption near-edge structure (XANES) spectroscopy indicate solid-phase S was in a reduced form in all mixtures containing OC. Pre-edge peaks on XANES spectra suggest tetragonal S coordination, which is consistent with the presence of an Fe–S phase such as mackinawite. The addition of Fe 0 enhanced AMD remediation over the duration of these experiments, however long-term evaluation is required to identify optimal Fe 0 and OC mixtures.

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