Carbon capture and sequestration in mining pit lakes

The case for carbon capture

The developed world has spent the last century treating the atmosphere as a dumping ground for greenhouse gases. As a result, we will exceed 2°C of warming by 2050 without major changes in the energy, transportation, building, and manufacturing sectors. While drastic emissions reductions are crucial, they are not sufficient to ensure a safe climate. We could shut down every fossil power plant, every gas-powered vehicle, every refinery, every cement plant, and every smelter and the planet will continue to warm–albeit at a slower rate–from the carbon we’ve already put into the atmosphere. And we’re nowhere near eliminating emissions, especially in hard to decarbonize sectors like chemical manufacturing or steelmaking. Carbon capture and sequestration is no longer a choice: it’s a necessity.

Of course, pulling tens of billions of tons of carbon out of the atmosphere is an even greater challenge than not putting them there in the first place. Luckily, a huge array of carbon removal paths are beginning to come online, from direct air capture to enhanced weathering of minerals to sequestration in biomass. The large and growing number of approaches to our carbon problem is a good reason for optimism.

Ocean capture, and what’s wrong with it

One piece of the solution may lie in the ocean. Many organizations are looking there for permanent, relatively low-cost means of capturing and/or storing atmospheric carbon dioxide. Kelp farmers seek to grow massive amounts of macroalgae and sink them to the deep ocean, where their carbon will remain indefinitely. Sprinkling iron particles or bringing deep sea nutrients up to the surface can trigger the growth of carbon-sucking algae. Ocean alkalinity enhancement adds alkaline minerals or other bases to increase the ocean’s carrying capacity for aqueous carbon dioxide and carbonate and bicarbonate ions, collectively referred to as dissolved inorganic carbon (DIC). The key to ocean capture methods is the ocean’s size: over a quintillion tons of water means lots of capacity, and 360 billion km2 of surface area means you don’t need to use expensive air-liquid contactors or achieve terribly high rates of biological productivity per square meter.

But wait – I thought I was reading an article about mining… you are! Stick with me.

All ocean capture techniques suffer from some fundamental limitations:

⚖️ It’s really hard to measure how much carbon you’ve added to the ocean, and really hard to verify how long it stays there. That huge capacity mentioned earlier means the carbon gets diluted pretty rapidly. In the case of biological capture, that carbon is now part of the food chain and is incredibly difficult to track.

🦋 We don’t know what kinds of effects any of the ocean capture techniques will have on critical ecosystems, especially at scale. What if the carbon-sucking plants and algae we grow out-compete other organisms for vital nutrients? What if we tip the ocean’s chemistry toward one type of plankton at the expense of another? 

😰 Everything is more difficult (read: more expensive) at sea than on land. Need fresh water? Get an energy-hungry desalinator or you’re out of luck. Out of supplies? Take a long journey back to port. Want to use steel or aluminium construction? Hope you like dealing with corrosion.

So here’s where mining comes in. What if we could do ocean capture without the ocean? What if there were thousands of contained bodies of water, accessible by land, without any critical ecosystems, and big enough to store millions of tons of CO2 each?

Capture and sequestration in pit lakes

These bodies of water already exist. During a strip mining operation, megatons to gigatons of rock are removed from the Earth in pits that can be over a kilometer deep and a kilometer across. When the pit reaches the water table, or if there’s inflow from rain or streams, the water must be pumped out. When operations cease, the pumps are turned off and the pit fills with water. The problem is, many pits are formed in sulfide minerals which oxidize, creating sulfuric acid. They may also be full of dissolved toxic metals. As a result, these pits represent a major risk for local communities and nearby ecosystems, as well as a liability for mining companies.

We can use ocean capture techniques to verifiably draw carbon down into the pit lake. First, we’ll treat the water to neutralize the acid and precipitate toxic metals. When in-lake neutralization is done as part of a remediation process, it’s typically done by adding alkaline minerals until the pH reaches 7, precipitating metals along the way. This in itself increases the DIC in the water, but not by much, and not in a way that will maximize negative emissions. To do that, we’ll develop sources of materials that have low associated emissions and high carbonate solubility in water, combined with protocols which yield the most effective dispersal and measurable carbon capture. We will also continue to add alkalinity well beyond the point of neutralization and up to saturation, increasing the carbon stored by about two orders of magnitude. All told, we could capture a few million tons of CO2 in a lake 100 million m3 in volume just from alkalinity enhancement.

And there’s more! Once we reach saturation, we’ll continue to capture carbon. Lakes that are naturally alkaline–like Big Soda Lake in Nevada–are some of the most productive ecosystems on Earth. If we create an artificial alkaline lake, we’ll likely generate hundreds of tons of biomass per year. When a major obstacle to biofuels production is the high cost of growing the biomass, there’s enormous appeal in growing it as a side effect of a climate-positive process. We may be able to use the biomass as additional carbon sequestration, to aid in removal of toxic metals, or to realize additional revenue from its sale as fertilizer, animal feed, or chemical feedstock.

How you can get involved

We’re recruiting a co-founder to work alongside me (Dr. Kate Murphy) to further refine the concept and form a company in the next 6 months. If you have experience with mine remediation and mining-affected waters, with a strong background in geochemistry to match your strong drive to build a company, we’d like to hear from you.