Abstract
Naturally occurring biogenic pyrite has been found in Holocene fluvial aquifers in the Uphapee watershed, Macon County, Alabama. The electron microprobe (EMP) analysis showed that the pyrite grains contain 0.20–0.92 weight% of arsenic (As). The scanning electron microscope and energy dispersive spectroscopy (SEM-EDS) analysis confirmed a similar concentration of As in the pyrite that was consistent with the EMP analysis. The SEM analysis also confirmed the presence of additional trace elements such as cobalt (0.19 wt.%), and nickel (0.15 wt.%), indicative of pyrite’s capacity to sequester As and other trace elements. Pyrite grains were naturally formed and developed as large (20–200 μm) euhedral (i.e., cube, octahedron) crystals and non-framboid aggregates. However, the inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the As concentration in the groundwater was not high, and it was within the EPA drinking water standard for As (10 µg/L). These results indicate that dissolved As is sequestered in naturally formed pyrite found in the fluvial sediments. The groundwater was moderately reducing to slightly oxidizing (Eh = 46 to173 mV), and nearly neutral to slightly acidic (pH = 5.53 to 6.51). Groundwater geochemistry data indicated a redox sequence of oxidation, denitrification, Mn(IV) reduction, Fe(III) reduction, and sulfate reduction along the flow path in the fluvial aquifer. The downgradient increases in dissolved Mn and then Fe concentrations reflect increased Mn(II) and Fe(II) production via microbial competition as the aquifer becomes progressively more reduced. Bacterial sulfate reduction seems to dominate near the end of the groundwater flow path, as the availability of Mn- and Fe-oxyhydroxides becomes limited in sediments rich in lignitic wood where increasing sulfate reduction leads to the formation of biogenic pyrite. The groundwater is a Ca-SO4 type and is not SO4 limited; thus, sulfate may serve as an electron acceptor for the bacterial sulfate-reducing reactions that sequester As into pyrite, which in turn results in very low groundwater As concentration (<2 µg/L).
Highlights
Occurring arsenic (As) is one of the most common metalloid contaminants found in groundwater, and the mode of occurrence and mobility of As in sedimentary aquifers are mainly influenced by local geology, geomorphology, hydrogeology, and geochemistry of sediments and water [1,2,3,4,5,6,7]
The results showed that the pyrite grains contain trace elements 0.17 wt.% of As, 0.19 wt.% of Co, 0.15 wt.% of Ni, and 0.18 wt.% of Al in oxide forms spectrumof ofbiogenic biogenic pyrite samples collected from the fluvial system
The results showed that the pyrite grains contain trace trace elements 0.17 wt.% of As, 0.19 wt.% of Co, 0.15 wt.% of Ni, and 0.18 wt.% of Al in oxide forms elements 0.17 wt.% of As, 0.19 wt.% of Co, 0.15 wt.% of Ni, and 0.18 wt.% of Al in oxide forms (Table 4)
Summary
Occurring arsenic (As) is one of the most common metalloid contaminants found in groundwater, and the mode of occurrence and mobility of As in sedimentary aquifers are mainly influenced by local geology, geomorphology, hydrogeology, and geochemistry of sediments and water [1,2,3,4,5,6,7]. Arsenic-bearing pyrite is considered the major solid arsenic phase formed under sulfate-reducing conditions in natural systems [17,19,23]. In the United States, As is considered as the second most common contaminant of groundwater [24]
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