It’s not just dogs and small children who are wary of firework displays. Some environmental activists have been labelled “killjoys” for seeking to ban them. Fireworks, as one campaigner put it, “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 and 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 chemicals have drawbacks for the environment. 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 also threatens to cause problems for civilian firework displays.
Barium is a health hazard too: it can interfere with heart function and constrict air passages to impair 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.
So Moretti and colleagues set out to find a replacement incendiary oxidizer for IM-28. Among the alternatives considered already are nitrates that do not 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 have neither the health risks of current oxidizers nor suffer from moisture-sensitivity. This isn’t simply a matter of finding another ignition-causing compound. 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.