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

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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 (Fe0). Modest increases in SO4 reduction rates (SRRs) of up to 15% were achieved by augmenting OC materials with 5 and 10 dry wt% Fe0. However, OC was essential for supporting SO4 reducing bacteria (SRB) and therefore SO4 reduction. This observation suggests a general absence of autotrophic SRB which can utilize H2 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 Fe0 only, however Fe, Co and Mn removal was less effective in the absence of OC. Secondary disordered mackinawite [Fe1+xS] was observed in field-emission scanning electron microscopy (FE-SEM) backscatter electron micrographs of mixtures that generated SO4 reduction. Energy dispersive X-ray (EDX) spectroscopy revealed that Fe–S precipitates were Fe-rich for mixtures containing OC and Fe0, and S-rich in the absence of Fe0 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 Fe0 enhanced AMD remediation over the duration of these experiments, however long-term evaluation is required to identify optimal Fe0 and OC mixtures.