Effects of substrate selection and inoculation on the performance of laboratory-scale sulfate-reducing bioreactor columns, The

Abstract

This thesis focuses on the geochemistry of two sets of laboratory-scale sulfate-reducing bioreactors (SRBRs) that examine the effects of substrate selection and inoculation on SRBR performance as it relates to zinc immobilization from actual mine-influenced waters (MIWs). Eight, 20 L down-flow columns that contained substrate permutations of alfalfa, woodchips, sawdust, and walnut shells were operated for more than 1.5 years. Analysis of the results demonstrated that alfalfa hay is an important, relatively-recalcitrant carbon source for the microorganisms within an SRBR. Metal precipitates occurring on the organic substrate were analyzed by energy-dispersive x-ray spectroscopy (EDX) using a random grid and running average. Water samples collected from ports along the length of the columns were analyzed with inductively coupled plasma atomic emission spectroscopy (ICP-AES). This combination of analytical techniques offers unique insights into metal removal and precipitation patterns in an SRBR. EDX determined zinc, sulfur, and calcium elemental abundance can be effectively organized on a ternary diagram to reveal spatial, temporal, and operational trends in these systems. In the active SRBR systems, this was manifested as a shift away from gypsum-like (CaSO4[middle dot]2H2O) toward sphalerite-like (ZnS) precipitates in the columns over an approximately 1/2 year time period. Images acquired through scanning electron microscopy (SEM) revealed crystal habits suggestive of the assigned minerals. In a subsequent set of experiments, results from eight up-flow, bench-scale columns all containing alfalfa hay and woodchips but seeded with various inoculation permutations (MIW, substrate, and anaerobic digester granules (ADGs)) and operated for 93 days highlight the importance of a viable inoculation source as a means of promoting rapid sulfate reduction and subsequent zinc removal as a zinc sulfide precipitate. Only columns inoculated with active ADGs demonstrated significant sulfide generation accompanied by the removal of Zn, Ni, and Co. Observations of Zn, Ni, and Co removal in columns not demonstrating significant sulfide generation reinforce the importance of sorption as a secondary metal removal mechanism whose importance compared to metal sulfide generation dwindles over time, as evidenced by diminishing metal removal rates in these columns with time (compared to steady and complete removal of Zn, Ni, and Co in the sulfide-generating columns). The dissolution of iron sulfides in the ADGs and subsequent precipitation of comparatively insoluble zinc sulfides and cadmium sulfides is another potential mechanism of metal removal in the columns. The geochemical data acquisition from these experiments is designed to accompany concurrent microecological studies to collectively enhance our understanding of the biogeochemistry of these systems and advance the field of SRBR technology. Independently, the data show that appropriate substrate and inoculation considerations are needed for rapid SRBR establishment.