Removing Sulfate from Mining-Impacted Waters on the Minnesota Iron Range

The Minnesota Iron Range produces about 70% of the iron used by U.S. steelmakers. Only in recent decades have those who depend on the region’s surface waters – for drinking water, for harvesting wild rice and for fishing -  become aware of the increasing concentrations of sulfate ( SO4-2 ) – that leach into abandoned mine-pit lakes, as they fill with rainwater, snow-melt and groundwater seepage.

According to a map-based study called The Lake Superior Iron District: Changing Landscapes of Water (https://www.industriallandscapes.org/story-map.html), the Mesabi Iron Range of Minnesota has 250 more lakes than it had in 1890. These lakes each resulted from the abandonment of an open pit when the ore in that pit was depleted and the mining company moved on to a new pit. Within the Mesabi Range, which is just south of the US-Canadian border, 87% of the the total lake acreage is made up of old mine-pits

Over time, many of these lakes have accumulated high concentrations of aqueous sulfate.  That happens because the exposed waste rock and the rock walls that form the perimeter of these lakes are laced with minerals, like iron pyrite (FeS2), that oxidize and produce sulfate.  As water from rain and snow seeps past these exposed surfaces, the sulfate leaches into the water.  The water in these sulfate-laden pit-lakes flows into the surrounding wetlands or enters the creeks and small streams that lead away from them and into surrounding Lake Superior watershed.

The native peoples who inhabit the Lake Superior basin traditionally harvest wild rice (a species of wild grass from the genus Zizania) and, for them, this grass has immense historical, cultural and economic significance. Studies have shown that wild rice cannot survive in waters with a significant concentration of sulfate. For humans and other mammals that depend on these affected surface waters for drinking water, higher concentrations of sulfate act as a laxative.  Some of these mine-pit lakes have concentrations of sulfate as high as 1,000 parts per million; the EPA has set a standard of 250 parts per million as the upper limit allowable.

Of equal concern is the fact that, as these sulfur-laden waters seep into the surrounding wetlands, they methylate the elemental mercury that is sequestered by wetland plants throughout the region (this sequestered elemental mercury owes its origin to acid rains produced by coal-fired power plants over many decades). Once it’s methylated, the methyl mercury passes into the fish and becomes a hazard for all who eat those fish.

Until recently, people concerned about these increasing concentrations of sulfate have been stumped over what to do. Historically, the only conventionally acceptable method for removing sulfate from affected waters has been reverse osmosis—which is prohibitively expensive.  This formidable expense has discouraged mining companies from taking a proactive approach to sulfate removal.

The prospect of having to spend US billions on mechanical sulfate removal processes like reverse osmosis is not attractive to an industry that produces a commodity that has its price determined, not by the producing company, but by global supply and demand and other market forces.

However, now that Clearwater BioLogic, LLC, has announced that it has developed a biological method for removing sulfate from waters with high sulfate concentrations for a fraction of the cost of reverse osmosis, some people on the Iron Range are hopeful that a practical solution may be at hand.

This breakthrough is being acknowledged by increasing numbers of stakeholders. In fact, on October 8, 2018, Clearwater BioLogic was awarded $10,000 by the Minnesota Department of Employment and Economic Development for being the best 2018 start-up company in Greater Minnesota.

In today’s energy-conscious world, the fact that Clearwater Biologic’s process requires very little external energy—in some applications it can run on gravity and solar power alone—makes it an extremely attractive alternative to reverse osmosis, which demands a great deal of electrical power and leaves behind a brine that must be evaporated and then landfilled. 

Clearwater BioLogic, LLC, is headquartered in Babbitt, Minnesota, the birthplace of taconite mining and the site for the proposed extraction of the confirmed copper-nickel-platinum group metals deposits in the region.

This material has been re-produced with permission from Clearwater BioLogic LLC.