An edited version of this piece has just gone up on the BBC Future site.
Some environmental activists have been called killjoys for seeking to ban firework displays. They are concerned that, as one campaigner put it, “fireworks shows spray out a toxic concoction that rains down quietly into lakes, rivers and bays.” But there may be a solution that doesn’t spoil the fun: green fireworks. A team of scientists at the US Army’s Pyrotechnics Technology and Prototyping Division at Picatinny Arsenal in New Jersey, USA, has found more eco-friendly replacements for one of the troublesome chemical components of fireworks, the so-called oxidizer that sets off the explosion.
As you might imagine, the researchers, led by Jared Moretti and Jesse Sabatini, are concerned less with the civilian pyrotechnics unleashed on 4 July in the US, 5 November in the UK, or at every conceivable opportunity in China, and more with military applications such as battlefield flares, which tend to use similar chemical formulations. But Moretti says that their new formulations also “have tremendous potential for civilian fireworks applications.”
Oxidizers are chemical compounds rich in oxygen, which they can relinquish to set the mixture burning. The most common types are nitrates and chlorates or perchlorates. Potassium nitrate is the ‘saltpetre’ used in old recipes for gunpowder, while sodium chlorate is a herbicide notorious for its use in homemade ‘sugar/weed-killer’ bombs. Many civilian and military pyrotechnic devices now use either potassium perchlorate or barium nitrate as the oxidizer.
Both of these have environmental drawbacks. The US Environmental Protection Agency (EPA) are scrutinizing the use of perchlorate because it can substitute for iodide in the thyroid gland, disrupting the production of hormones. It can also cause growth abnormalities in embryos. The strict limits placed on perchlorate levels in drinking water by the EPA has hampered military training in the US and threatens to cause problems for civilian firework displays too.
Barium is a health hazard too: it can interfere with heart function and cause constriction of the air passages in breathing. Aside from flares, both potassium perchlorate and barium nitrate are currently used by the US Army in an incendiary mixture called IM-28, which is added to armour-piercing bullets so that the impact creates a bright flash that marks the impact point. Finding a replacement incendiary oxidizer for this application was the immediate motivation for the research by Moretti and colleagues.
Among the alternatives considered already are nitrates that don’t contain barium, in particular sodium nitrate. However, that – as well as another candidate, strontium nitrate – has a different problem: it readily absorbs water vapour from the atmosphere (that is, it is hygroscopic), because the compound is quite soluble in water. This means that the substance is liable to become damp if the pyrotechnic device is stored for a long time, and so it won’t ignite.
Moretti and colleagues have now identified alternatives that don’t seem to have any of the health risks of current oxidizers nor suffers from moisture-sensitivity. This isn’t just a question of finding another compound that will cause ignition. It also has to produce a bright flash, ideally of white light (different metals, in particular, tend to generate different colours), and should not be so exotic as to be unaffordable. It had also better not be set off too easily: one doesn’t want flares and fireworks detonating in the box if they get too warm.
The researchers find that sodium and potassium periodate (pronounced “per-eye-oh-date”) seem to fulfil all these requirements. These are analogous to perchlorates, with the chlorine atoms replaced with iodine. That’s a crucial difference from the point of view of thyroid toxicity. It seems likely that perchlorate ions can nudge out iodide ions in the thyroid because they have a similar size. But periodate ions are considerably too big to substitute for iodide in the same manner.
Yet isn’t it a bit odd to talk at all of ‘green’ military technology – stuff that is used in combat, perhaps lethally, but doesn’t harm the environment? The apparent irony is not lost on the researchers engaged in such work. But it’s hardly cynical to say that, since armed conflicts do occur whether you like it nor not, one would rather not pollute the environment afterwards for civilians.
With that in mind, making military armaments greener has become a significant concern. The US Department of Defense issued a ‘statement of need’ last October calling for research proposals for ‘environmentally advantaged submunitions’ – basically, ‘green’ explosives. For example, the ‘primer’ that sets off the bullet-propelling explosive in small arms typically contains lead, which lingers in firing ranges and accumulates alarmingly in the blood of trainee soldiers and police officers.
High explosives are problematic too. TNT is a carcinogen, although rarely used now in military applications, while the most common alternatives, compounds called HMX and RDX, can cause neurological and reproductive problems. In 1984 a child was hospitalized with epileptic seizures after chewing on a piece of RDX plastic explosive stuck to the clothes of its mother, a munitions worker (and you thought your parenting was irresponsible?).
The army is worried about how much of this stuff is left lying around ranges and battlegrounds in unexploded dud shells, which constitute 3-4% of those supplied to troops. Hundreds of thousands of duds were dropped as cluster bombs in the 1991 Gulf War, for example.
The new green incendiary oxidizers represent another facet of this general trend – and they have the added appeal of benefitting peaceful pyrotechnics too.
Reference: J. D. Moretti, J. J. Sabatini & G. Chen, Angewandte Chemie advanced online publication, doi: 10.1002/anie.201202589.