Level Zero

Pallanza Bay, Italy. 2010. This is where this thinking started: how do you talk about science with people who aren’t trained in or maybe even focused on science? [This place – Pallanza Bay – also happens to be where Alessandro Volta discovered methane in the reed beds way back in the 1770s and where plumbing still goes whang-o now and again when his discovery bubbles up in the wrong places]. I was there for a chlor-alkali site characterization and the question of who did what when and how that might make different parties more or less liable in the apportioning of remediation costs. There were many parts of that site characterization that were interesting, but what caught my attention up there along the shoreline was this: The Museum of the Art of Hat-Making. Yeah. Mercury. And hat making. Which means there is the seed of a story there and that story contains, at least in a literary sense, a rabbit hole. But I am first, and foremost, an engineer, and a Southern girl, which means this story has to start with that run what you brung – meaning, with that sense of starting from what you know and building from there.

Level One

Over the span of its operation, a typical mercury cell chlor-alkali facility would have released somewhere in the neighborhood of 10 tons of elemental mercury into the aquatic environment. There are (or were) an unknown number of these facilities around the world, but a reasonable estimate might be 250 and includes locations everywhere: in the U.S., in Canada, in Latin America, in many of the countries of the European Union. To this day, nobody knows how many the Soviets built. Or where. They were built on rivers, lakes, fjords, in estuaries, in lagoons, and on harbors; by 2019 the vast majority have been abandoned, closed or converted to other technologies. The process itself begins with NaCl brine and ends with caustic soda and chlorine. Both caustic soda and chlorine have many uses; the chlorine, as example, to make pesticides, bleach for the pulp and paper industry, and PVC. 

In the period from roughly the middle of the 20th century until its end, chlor-alkali production relied globally on mercury and electricity to split that Na from the Cl. This process wasn’t a closed loop and elemental mercury was cheap: if the process was run efficiently, production might release ~10 lbs of mercury into wastewater a year. Depending on when you were on that 20th century arc, that wastewater could have been discharged into the river, lake, fjord, estuary, lagoon, or harbor. Or it could have been discharged into a treatment pond. Depending on when you were on that 20th century arc, the bottom of that pond might have been sealed with fine clay or a membrane liner. If the process was run inefficiently – although not necessarily in violation of any discharge regulations and as appeared to have happened in many locations in the initial years of facility operation – then over the life of the facility, production of chlor-alkali likely released somewhere in the neighborhood of 10 tons of elemental mercury into the aquatic environment. 

By volume, this much elemental mercury would overflow 3 × 55 gallon drums. Maybe this seems like a lot of mercury and maybe it doesn’t. The spectrum of ecological and human health impacts that result from this overflow depends on a long list of factors. Fundamentally, the type of ecosystem that that mercury was released into matters significantly. Even more fundamentally, the extent to which people living in the vicinity of that release have had to rely on that aquatic ecosystem for food and livelihood matters even more significantly. Critically, it is the type of environment – the river, lake, fjord, estuary, lagoon or harbor – that determines how far from the facility you have to be living for ‘in the vicinity’ to not include your community. 

Level Two

Energy baseball.

Think of a chlor-alkali cell like you might very generally think of a car battery. But in reverse.

In a typical car battery, there are plates and there is fluid. The plates are made of lead and are stacked or slotted into cells. In each cell, the plates alternate: solid lead (Pbs) and lead dioxide (PbO2). In a typical lead-acid battery, the fluid that circulates within and between these cells is sulfuric acid (H2SO4). Batteries work like a criss-cross game of baseball: pitchers and catchers and every time the ball is thrown energy changes hands. At the beginning of the game – in a fully charged battery – the plates alternate in cells and are different: solid lead and lead dioxide and the fluid is sulfuric acid. At the end of the game – in a fully discharged battery – the plates in each cell are the same: lead sulfate (PbSO4) – and the fluid is essentially water (H2O). Pitchers and catchers and the ball is made of electrons: to move the world, the ball goes pitcher to catcher and energy is made. Recharging the battery – the catcher there in their crouch throwing balls back to the pitcher – converting those plates from lead sulfate back to solid lead and lead dioxide and that fluid back from water to sulfuric energy – costs energy.

In a battery – in baseball – the goal is the game: pitchers to catchers and with the energy you generate maybe your team wins. In a chlor-alkali cell, the process is somewhat akin to recharging the battery: apply energy in the form of electricity to the plates and it’s the fluid between them you’re focused on. In chlor-alkali baseball, the pitcher (the solid lead plate in your car battery) is made of titanium; the catcher (the lead dioxide plate in your car battery) is a thin film of mercury flowing over steel. The fluid that fills the space between them is a brine 5-10x as salty as seawater (NaCl + H2O).There are several steps in this process, but, overall, the chemistry looks like this:


This reaction costs energy (there on the left hand side of the equation) but what it generates – caustic soda (NaOH) and chlorine (Cl2) there on the right – has value. Pitchers to catchers. Catchers to pitchers. Energy baseball.

Level Three

And here, maybe, we have to go all the way back to the source for some understanding of context. And so this part begins with a question:

How do you tell the story of something that is no longer there?

Maybe you start with this: It’s a bit less than 5 hours by train from Cadiz to Madrid. There are thirteen trains per day, most days of the year, and you’re on board and you’re thinking about work. Or the weekend. Or the trip you’ve just finished. Or the vacation time you’re about to begin. And ~ 400 km from Cadiz – at a crossing where you might not even see the sign – you’ll pass within 60 km of the Almaden mine. And the name might ring a bell or it might not – we all stare idly out the same window and notice different things – but the train has just passed within 60 km of the single largest deposit of mercury in the world. A deposit that – until mining ceased in 2004 – was in continuous operation for ~ 2000 years and in that time yielded approximately 1/3 of the world’s total mercury ore.

This one location.

And nobody really understands it – the distribution of that element. The geologists don’t understand it. The engineers don’t understand it. Why do elements concentrate like they do? In this case, in Spain. Slovenia. Italy. Peru. California. Why is the mercury where it is? And there at the top of that list – that first location – that one in Spain – that one called Almaden – is the largest of all deposits – the ore body there so rich in cinnabar that elemental mercury – the mercury in thermometers – sweats from the veins in the rock walls.

If you’re a scientist or an engineer and you’re thinking about mercury in the environment you’re thinking of numbers like this: in terms of concentration, mercury is somewhere around 1/20 of a part per million (ppm) as a “global average background” in soil. As it moves around the globe – in mining waste, in air, in water – that concentration enriches and it increases: it is around 1/10 of a part per million in soil on the otherwise uncontaminated downwind edge of a continent where everything carried on wind currents – in this case, specifically the mercury found in coal – ends up eventually sifting down and depositing; it is up to one part per million in that coal itself; it is up to 10 parts per million in contaminated sediment – maybe downstream of a facility that used elemental mercury in those room-sized, not-quite batteries to make bleach or PVC or DDT – downstream where the momentum of the river changes and whatever is in suspension settles to the bottom and is stored; and it is maybe 100 parts per million or higher in the soil in places where mercury is still mined and where environmental controls might not be what they are elsewhere on the planet.

These numbers are context. In the Almaden Mine, the mercury concentration in the ore body reaches 8%. That is, within the rock itself, the concentration of mercury reaches 80,000 parts per million. Working in this environment was – and remains – very hard on the miners.

When mining at Almaden began in earnest in the 1500s – after the discovery of silver in the Americas and the realization that – with the newly invented patio process – access to mercury meant access to silver – the Spanish Crown gave convicts a choice: the galleys or the mine. It is estimated that during the period in which the mines were worked principally by convicts, ~ 25% of those convicts died during their term of labor, mostly and most likely from mercury exposure. An unknown percentage of convicts were released from their term of labor insane. Later, the work force also included slaves and gypsies, arrested, in their case, for anything and everything including the crime of speaking Romany.  For slaves and for gypsies, the terms of their sentence were indefinite: for slaves, because they’d been purchased outright; for gypsies because for their term of sentence to end, the Crown required that they prove evidence of a settled home to return to. And they – the Romany – by confluence of culture, vocation and social caste, could never fulfill this criterion for legal release. For these individuals, working the mine at Almaden was a life sentence.

And you can’t really talk about the metal without also talking about the mineral: cinnabar. Or more correctly, the minerals: cinnabar and metacinnabar. The red and the black. Or in the scheme of the philosophers and the alchemists, the light and the heavy.  Even more correctly (though with no greater clarity), there are actually three forms of the mineral: cinnabar, metacinnabar and hypercinnabar. The red and the black and the other black. The transition between these minerals – if it can be thought of as a transition – if one form can actually change into the next – is still a conundrum. Temperature seems important: heat metacinnabar to > 400 C and it can be transformed to cinnabar; continue heating to > 500 C and hypercinnabar may appear. That you can create these transitions in the laboratory does not mean they occur in nature though. And while it is generally agreed that the three forms of HgS are distinct minerals with distinct crystal shapes, what happens outside of the laboratory when high temperature magmas cool and minerals begin to precipitate is anyone’s guess.

In all cases though, the mineral is HgS. And cinnabar – the red form – has been used by people for over 10,000 years – for as long as we’ve had that thing that we call culture, really – as a pigment. Vermillion. It has appeared throughout the world’s history in cosmetics, lacquers, wall paints, ceramics, and medicines. Yeah – I know – in medicines. And if you want to convert HgS to Hgyou simply roast the ore. The sulfur will be oxidized and the mercury will be liberated. And if you’ve roasted in a retort or some other vessel with a lid, the volatilized mercury will be captured and will cool and condense and dribble down as Hg0. Elemental Hg. The stuff of thermometers.

And here is where I go back to Almaden. And the richness of the cinnabar deposit there. And the fact that Hg0 occurs at ambient temperatures in the ore body itself. As a geologist or as an engineer, THAT is wild.