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.
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.