Field Calibration of Stable Isotopic Fractionation Associated with Biotic and Abiotic Sulfate Removal Processes
A variety of natural processes, both biotic and abiotic, can remove sulfate from water. Consequently, many of these naturally occurring processes have been engineered or otherwise modified to form the basis of passive treatment systems for the remediation of acid-mine drainage. Removal typically occurs either through removal by as sulfide or as sulfate. Removal as sulfide typically involves sulfate-reducing bacteria. Sulfate-reducing bacteria play important roles in the geochemical behavior of sulfur in aqueous systems, especially related to topics such as (1) the global biogeochemical cycle of sulfur in the oceans, (2) the diagenesis of sedimentary rocks, and (3) the sequestration of metals and sulfur in natural settings, such as black shales, or in passive treatment systems designed for the remediation of acid-mine drainage. Removal as sulfate typically proceeds as an abiotic process and produces secondary sulfate minerals such as gypsum, jarosite, barite, or anglesite depending upon the geochemical environment.
Both bacterial sulfate reduction and the precipitation of secondary sulfate minerals produce distinctive oxygen and sulfur isotopic signatures. In fact, stable isotopes can be used to distinguish the relative importance of removal as sulfate or sulfide even for modest levels of removal. Sulfate-reducing bacteria produce distinctive sulfur-isotope signatures in both the resulting sulfide and in the residual aqueous sulfate. Sulfur-isotope fractionation is extremely sensitive to even small amounts of sulfate reduction, and thus represents a powerful tool in understanding the significance of this important biogeochemical process. However, interpretations on the basis of sulfur isotopes alone are commonly ambiguous because of isotopic differences among multiple sulfate sources in various settings. Oxygen isotopes in the residual sulfate offer an additional constraint to resolve the ambiguity associated with multiple sulfate sources, but there is considerable uncertainty in the oxygen isotopic fractionations associated with bacterial sulfate reduction. The research in this subtask is investigating sulfur and oxygen isotopic fractionations produced in a pilot acid-mine drainage treatment system in Pennsylvania that combines both bacterial sulfate reduction with acid neutralization by reaction with calcite and dolomite to field calibrate oxygen isotope fractionations in field settings. The study compares and contrasts the water chemistry and stable-isotope compositions of aqueous sulfate in two parallel treatment cells, one of which involves vertical flow through organic compost sandwiched between layers of limestone gravel, and the other with only limestone gravel. The potential isotopic effect resulting from the precipitation of gypsum on limestone particles is not anticipated to cause complications because the isotopic effects on the residual sulfate associated with gypsum precipitation are in the opposite direction of those associated with bacterial sulfate reduction.
Preliminary results from a pilot passive treatment system at the Bell Colliery, Pennsylvania, illustrate the usefulness of the technique. The pilot system at Bell is a successive alkalinity producing system (SAPS), also known as a "vertical flow" system in which flow is split between two cells where it then flows downward through the substrate and is discharged through a stand pipe from the base of the system. In the photograph below, mine drainage enters the system from the foreground, is split between the two cells, and exits from the far side of the cells. Photo 1 The cell on the left is filled only with limestone and dolomite gravel, whereas the one on the right also contains a layer of compost sandwiched between upper and lower gravel layers. The compost in the cell on the right is there primarily to reduce dissolved ferric iron to ferrous iron to help prevent clogging of the porosity, but also may serve as a suitable substrate for bacterial sulfate reduction. Both cells have the potential to remove sulfate as secondary precipitates of gypsum. Both cells increase pH from around 3.7 to above 6.0 and remove significant amounts of Fe and Al, but have only modest to no reduction in sulfate concentrations. The flow in the compost-free cell is approximately 4 times higher than the one containing compost. The sulfur isotope variations expected for bacterial sulfate reduction and gypsum precipitation can be modeled using Rayleigh equations. The two pathways yield divergent trajectories in terms of sulfur isotope composition as a function of reaction progress. Plot The isotopic composition of residual sulfate in the compost-bearing cell correlates well with the composition expected for gypsum precipitation, but differ dramatically from those expected for bacterial sulfate reduction. These results suggest that sulfate may be removed from the composted cell because of the longer residence time of water in that cell because of the reduced permeabilities imposed by the compost layer. In contrast, the lack of sulfate removal in the compost-free cell probably is due to the short residence time of water in that cell.
Passive treatment system at Bell Colliery, Pennsylvania [Large view of treatment system photo] Return to text above.
Plot of δ34S isotope variations vs. Reaction Progress (%) for test cells. Return to text location.
Robert Seal II
954 USGS National Center
12201 Sunrise Valley Drive
Reston, VA 20192
Phone: (703) 648-6290