No Mercury Added
Starting about a year ago, I have seen the above slogan used increasingly in zinc-bearing goods, especially batteries made in China. It is one of those statements that is both technically correct, extremely misleading and dangerously deceptive.
The fact of the matter is that nobody ever intentionally added mercury to zinc products. This is why I have no doubt that the statement is correct. This is also what makes it misleading. The statement implies that Hg contamination is caused by an additive, kinda like lead in gasoline. But that implication misrepresents the most obvious source of mercury contamination.
A quick look at a periodic table will show that Zn, Cd, and Hg are all in column 2b, and one might surmise from this that they have broadly similar geochemistry. And indeed they do. In nature, they all occur primarily as +2 chalcophile cations, meaning that sulfur, and not oxygen, is their preferred anion in the Earth’s crust. Indeed, most zinc mined today is mined from sphalerite (ZnS) ore. But because of the similar chemistry of these three elements, Cd and Hg can, and generally do substitute for Zn in natural sphalerite to varying extents, depending on the particular mine and type of deposit. So in order to have zinc that is contaminated with Cd and Hg, you don’t need to add those toxic metals- they are often already in the orebody.
Therefore, the important health and safety question is not whether or not Hg has been added, it is whether or not the primordial Hg already present in the ore has been removed during the smelting and refining process. Most of the products I’ve seen with the “No mercury added” claim also say “Made in China” , a country not exactly famous for stringent environmental regulation of its heavy industry. So the fact that they put a true but irrelevant statement on their product does not exactly convince me that it is non-toxic.
In fact, it might be a fun undergraduate research project to figure out just how much Cd and Hg various zinc-based “no mercury added” products actually contain. Too bad I don’t work in a lab anymore…
5 comments:
Great explanation. I've noticed that many people tend to equate "natural" with "harmless," especially when it comes to geochemistry - and that often isn't the case. I've been working on setting up an intro geology class project observing what's dissolved in a local stream, to hopefully get the students thinking about what's in things naturally vs what's added by humans. The goal is to get away from student projects that rant about the dangers of, say, calcium and potassium pollution in groundwater.
So I've got an ICP-OES. And there is sphalerite in the mountains north of me. (In fact, I think that there's a sphalerite sample sitting outside my front door, though I'm not sure who picked it up.) So... what kind of sample prep would you use to get some of those "no mercury added" products into solution? Because you're right, that would make a very interesting class project.
(I guess I should add that there are two issues - one that students assume that anything dissolved in water has been added by humans, and the second that the natural stuff is inherently good, and the anthropogenic stuff is inherently bad.)
(Note to self: drink coffee first, then comment on blogs.)
I know a lab manager in the UK who has her students run their favorite brand of bottled water...
As for sphalerite disolution, my gut feeling is nitric, but a lit search for sphalerite trace elements should give you a useful methods section.
If your OES comes with a lab manager, she might be able to tip you off- I only occasionally ran solutions, and only prepared them a few times.
Alas, I kind of share the lab manager work with our petrologist. Small department. I co-wrote the grant, I've got to figure out how to do cool stuff with it.
Just be aware that you may have high backgrounds and lousy sensitivity with Hg...
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