Research Processes of Nickel and Cobalt Uptake by a Manganese Oxide Forming Sediment in Pinal Creek, Globe Mining District, Arizona JOHN T. KAY AND MARTHA H. CONKLIN* Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85721-0011 CHRISTOPHER C. FULLER Water Resources Division, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 PEGGY A. O’DAY Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287-1404
A series of column experiments was conducted using manganese oxide coated sediments collected from the hyporheic zone in Pinal Creek (AZ), a metal-contaminated stream, to study the uptake and retention of Mn, Ni, and Co. Experimental variables included the absence (abiotic) and presence (biotic) of active Mn-oxidizing bacteria, the absence and presence of dissolved Mn, and sediment manganese oxide content. Uptake of Mn under biotic conditions was between 8 and 39% higher than under abiotic conditions. Continuous uptake of Mn due to biotic oxidation was evident from extraction of column sediments. Manganese uptake is hypothesized to initially occur as adsorption, which led to subsequent surface and/or microbial oxidation. Complete breakthrough of Ni within 100 pore volumes indicated no process of continuous uptake and was modeled as an equilibrium adsorption process. Nickel uptake in the presence of dissolved Mn was 67-100% reversible. Sediment extractions suggest that Ni uptake occurred through weak and strong adsorption. Continuous uptake of cobalt increased with sediment manganese oxide content, and Co uptake was up to 75% greater under biotic than abiotic conditions. Cobalt uptake was controlled by both existing and newly formed manganese oxides. Only a small amount of Co uptake was reversible (1025%). XANES spectral analysis indicated that most Co(II) was oxidized to Co(III) and probably incorporated structurally into manganese oxides. Although manganese oxides were the primary phase controlling uptake and retention of Mn, Ni, and Co, the mechanisms varied among the metals.
Introduction Understanding the processes controlling the attenuation of metals by sediments in natural systems impacted by mining activities is critical to the management and design of remediation efforts. The transport and fate of metal con* Corresponding author phone: (520)621-5829; fax: (520)621-1422; e-mail:
[email protected]. 10.1021/es010514d CCC: $20.00 Published on Web 11/03/2001
2001 American Chemical Society
taminants from acid mine drainage are often largely affected by existing and newly formed iron, aluminum, and manganese oxide sediment coatings (1, 2). These oxides provide efficient adsorption surfaces, and iron and manganese oxides can participate in oxidation-reduction reactions that change the stability and bonding of adsorbed species, in some cases leading to incorporation within mineral structures (3, 4). Processes such as surface precipitation, solid solution formation, and microbially facilitated oxidation-reduction can substantially enhance trace metal accumulation at sediment surfaces (1, 5-7). The focus of this study was to characterize the interaction of dissolved Mn, Ni, and Co with manganese oxide surface coatings on sediments collected from the hyporheic zone at Pinal Creek (AZ), a perennial stream impacted by copper mining activities. The hyporheic zone is defined as the portion of the streambed where surface water and groundwater interact and that contains at least 10% surface water (8). Field investigations have observed that microbially enhanced manganese oxidation occurs in the hyporheic zone of Pinal Creek (9), resulting in continuous manganese oxide formation that enhances Mn, Ni, and Co uptake (10). Manganese, Ni, and Co uptake have also been found to increase with increasing sediment manganese oxide content in Pinal Creek (10). We present a series of metal uptake column experiments designed to simulate hyporheic zone conditions. Column breakthrough curves are used in conjunction with modeling results, sequential sediment extractions, and X-ray absorption near-edge structure (XANES) spectroscopy to investigate the uptake and retention of Mn, Ni, and Co by sediments at Pinal Creek. Site Description. Pinal Creek is located in central Arizona approximately 10 km north and hydrologically downgradient from the Globe-Miami copper mining district. Over the last century, low-pH leachate generated by mining operations has entered the aquifer in the Miami Wash area. Interaction between the low-pH leachate and aquifer material has resulted in a neutralized plume of groundwater with elevated concentrations of dissolved metals (e.g., Mn, 80 mg L-1; Co, 1.2 mg L-1; Ni, 1.5 mg L-1; Zn, 1.5 mg L-1) that discharges into the perennial reach of Pinal Creek as the aquifer pinches out and forces groundwater to the surface (11, 12). Pinal Creek has a sand and gravel streambed containing an active hyporheic zone, which is characterized by gradients in pH, dissolved oxygen, temperature, and water chemistry (9) and is host to numerous bacteria that facilitate Mn oxidation (13). Residence times of surface water in the hyporheic zone range from 1 to 80 min (9, 10), and hydrologic exchange of surface water into the streambed enhances metal attenuation by increasing contact of dissolved metals with potential reactive sites on sediment surfaces (10). The observed attenuation of Mn, Ni, and Co in Pinal Creek has been attributed to reactive uptake in the hyporheic zone (9, 10).
Methods Sediments. Streambed sediments were collected from the active stream channel at three locations along the 2-km study reach. Sediments were collected between 3 and 15 cm below the top of the streambed to avoid algal materials. Sediments were wet sieved in the field to
