Metal Removal in Wetland Treatment Systems

Abstract

Wetland treatment has successfully removed nickel, copper, cobalt, and zinc from neutral mine drainage in northeastern Minnesota. Pilot and full-scale overland flow wetlands have removed up to 90% of the incoming nickel and 50% to 90% of the other metals. Nickel is the major contaminant in the mine drainage, with average concentrations from 2 to 5 mg/L. The other metals were generally present at concentrations less than 0.1 mg/L. In short-term tests, using drainage that contained around 1 mg/L zinc and 0.4 mg/L of copper and cobalt, concentrations were reduced abont 90%. Input and output flow and water quality measurements were collected, and a mass balance was calculated. To determine the fate of the metals and the removal mechanisms, samples of vegetation and peat were collected to examine the distribution, form, and depth of metal removal. Although the overall metal concentrations in the predominant vegetation types (cattail, grass, and sedge) had increased by factors ranging from 3 for above ground vegetation to around 14 for cattail roots, the mass of metal removed by the vegetation was less than 1 % of the total mass removed. The remaining metals were associated• with the peat substrate. Core samples of the peat were collected and divided into 2 to 4 cm sections. Each section underwent a sequential extraction procedure and was analyzed for total concentration and metal forms. Nickel removal occurred to a depth of 20 cm, with about 60% of the nickel being organically bound.

Mine Closure - Can Passive Treatment Be Successful?

“…Nickel removal in the test cells and the WID system also decreased in the fall (Eger et al, 1994(Eger et al, , 1996. Winter operation data for other wetland systems · is limited, but year-round copper removal has been reported in British Columbia (Sobolewski et al, 1995) and metal removal was also observed throughout the winter in the Big 5 Tunnel pilot cells in Colorado (Wildeman et al, 1992).…”

“…Nickel removal was on the order of 90% and the cells were capable of producing water that met water quality standards (Eger et al, 1993(Eger et al, , 1994. Based on these results, LTV designed and constructed two wetland systems in 1992 (Eger et al, 1992, Frostmao, 1992.…”

“…The ability to achieve compliance is largely a function of the wetland' s size and the hydraulic and metal loading rate. The (Eger et al, 1994). With the exception of the system constructed in the fall of 1997 (EMS), the wetland systems were constructed with a series of berms and pools that ranged in depth from 0.3-0.6 m. This design increases the overall residence time in the systems but decreases the area where the drainage is in direct contact with the substrate.…”

Overview of Active and Passive Systems for Treating Acid Mine Drainage

“…Adding chemicals and energy-consuming treatment processes are nearly eliminated with passive treatment systems. Also, the operational and maintenance costs of passive systems like minimum inputs, low-skilled work such as technical manpower, chemicals, no external power required, reduce operational and maintenance cost [11,12] are significantly less than active treatment systems. The design of a passive treatment system for AMD requires the knowledge of mine water chemistry, available treatment techniques, and experience.…”

“…The metal adsorption capacity of wetlands depends on many factors like concentrations of dissolved metals, dissolved oxygen (DO), air, pH, the alkalinity of AMD, active microbial biomass and the retention time of AMD in the wetland [22].The area of the wetland depends on the influent load entering the wetland. Aerobic wetlands are a good option for long-term treatment in low maintenance and operational costs [12], but these wetlands require longer detention time and huge surface area [22] for acid mine water treatment. The major factors that should be taken into account for design of a suitable wetland system vary upon site characteristics, are following: biochemical processes, seasons variation, authoritative issues, substrate, sediment control, the influent acidity loads and retention time ,morphometry , redox state & pH, slope of wetland, vegetation density, rate of flow of water and the land available for a wetland [23].…”