BIOGEOCHEMICAL PROCESSES INVOLVED IN FORMATION OF SCHWERTMANNITE-RICH SCALE IN A PIPELINE CARRYING ACID MINE DRAINAGE AT IRON MOUNTAIN MINE, CALIFORNIA

A pipeline carrying acid mine drainage (AMD) to a lime-neutralization treatment plant at the Iron Mountain Mine Superfund Site, near Redding, California, has developed substantial scaling, requiring costly removal of scale every two to four years. The pipe scale and associated AMD were sampled at four locations along a 2.1 km reach of polypropylene (HDPE) pipeline with an internal diameter of 46 cm. Additional AMD samples were collected to evaluate seasonal changes in water chemistry. The mineralogy and microbial community (16S rDNA analysis of bacteria and archaea) of the scale samples were characterized. The scale was primarily schwertmannite (nominally Fe₈O₈(OH)₆SO₄) and goethite (FeOOH) with minor amounts of jarosite (KFe₃(OH)₆(SO₄)₂), as determined by powder XRD, wet chemical extractions, and SEM-EDS. The scale hosted a diverse group of acidophilic microorganisms, including several types of known iron-oxidizing microbes. The influent AMD to the pipeline had low pH (pH 2.5 to 3) and high ferrous iron (about 1,400 mg/L). A fraction (up to 20%) of the ferrous iron oxidized in the pipe leading to precipitation of the observed ferric iron phases. Although the bulk mineralogy of the scale remained relatively constant along the length of the pipeline, the concentration of some trace elements (Al, Cu, and Zn) in the scale decreased along the flow path. Biomass content in the scale, as indicated by concentrations of carbon, nitrogen, and phosphate-extractable cells, was highest at the most upstream site and decreased along the sampled pipeline. The macroscopic scale morphology also varied from loose textures upstream to coherent layered lamina downstream. Laboratory batch experiments with AMD demonstrated that iron oxidation was microbially mediated; the precipitate formed was similar in mineralogy to the pipe scale. Kinetics of microbial iron oxidation and schwertmannite precipitation in the laboratory batch experiments were modeled using the geochemical code PHREEQC. Information about the microbial community, rates of iron oxidation, and mineralogy of pipe scale facilitate construction of a biogeochemical model that will guide development of remedial strategies to decrease or eliminate the formation of scale in the pipeline.