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Rusting Away

Just because it is rust doesn't mean that it isn't interesting.

Dr Doug Sheppard

When I emptied the bathtub of the water we stored over the new year, I noticed some rust-coloured stuff on the bottom of the bath, as well as the usual ring. That set me thinking about why was it there, and that line of thought set me to thinking about the natural occurrence of such deposits and what role they play in our lives and in the rest of the natural world.

That deposit was, of course, oxidized iron, commonly known as rust. Rust is not the simple oxygen and iron compound which we might recall from the school chemistry lesson, but contains water as well, giving rise to compounds called iron oxy-hydroxides. These are incredibly common in nature and of quite some interest, for economic and environmental reasons.

So that deposit in the bath -- how did it get there? Our local water supply here in the Hutt Valley is derived from the Hutt Aquifer, the water in the river gravels under the city. That water contains dissolved minerals from the rocks and sand that it contacts.

Contrary to the common impression, rocks do dissolve in water, although at very slow rates (so much for the impression many people have that groundwater is "pure". What is natural is never pure and what is pure is never natural, but that is another story). Limestone is a rock that dissolves relatively fast, which gives rise to limestone caves, but normal rocks dissolve a bit too. One of the major components of rocks is iron, and that dissolves in the water.

The rock comprising the gravels under the Hutt Valley is greywacke, the hard grey or grey/green rock that forms the backbone of New Zealand. When greywacke is exposed to air, or air-impregnated water for a long time, it turns brown. The same happens to the water with the dissolved iron in it -- if left exposed to the oxygen, the dissolved iron turns brown, as it reacts with the oxygen. This brown iron is less soluble in water than the iron straight from the rock, so it precipitates to the bottom of the container it is in.

What puzzled me was that this normally would happen quite quickly as the water is pumped around in the pumping station, in pipes and in the local reservoir, and so we should not see such deposits in fresh water stored for a short time.

Then my wife reminded me that she had added some household bleach to the water to keep bugs from growing in it. Household bleach works by storing, in a chemical form, oxygen at very high concentrations, which basically burns organic material such as bugs (and in the process removes colour, which is why it is called bleach). It will also oxidise any remaining dissolved iron in the water, and this is what it had done in my bath. The iron then deposited.

This process, the deposition of iron oxy-hydroxides, is extremely common in nature. Our neighbours have a stream at the bottom of the bush-clad cliff they call a garden and this stream is polluted, according to them, because it has this nasty-looking brown rust deposit on its bottom. Not so -- much of the water in the stream comes from seeps from the surrounding hillsides and contains dissolved iron. When it emerges into the air and is aerated, the iron oxidises and falls out of solution, producing the brown deposit entirely naturally.

Of course, buried iron and steel, as in pipes, reinforcing iron or buried rubbish, dissolve comparatively rapidly and will deposit out as well. And if there is a lot of buried organic matter, iron dissolves even faster and deposits in seeps from the burial site; hence the association of such deposits with pollution.

Over time, huge deposits of iron oxy-hydroxides can form, such as the iron ore deposits at Onekaka and elsewhere in the world. These have had a significant use in many cultures apart from as sources of iron.

Maori made use of the ochre deposits such as those at Kokowai Springs on Mount Taranaki. According to an early report by Dr Ernst Dieffenbach in 1839, the local people would collect the ochre in baskets, carrying it out to bake dry and mix with shark oil to produce the vermilion "paint" used for buildings and canoes and personal adornment for tangi. It also had a use as a defence against sandflies and mosquitoes.

The chemical reactions are interesting: the ochre is comprised of minerals with names like ferrihydrite and schwertmanite, which are compounds of iron, oxygen and water with other stuff like sulphate, but mostly with iron oxyhydroxides. When heated, the water departs and hematite results, which has the red rather than rust colour.

The Kokowai (meaning earth from which red ochre is produced by burning, or more literally, red water) deposits form where they do and in such quantities because the volcanic rocks are fresh and fractured, so there is a lot of iron easily dissolved from them. The deposits at Kokowai Springs are quite extensive, and at least a metre deep, as some of us have found. The finely divided particles impregnate clothing and is impossible to get out -- as we found, and as anyone who use iron-rich waters for laundry without adequate removal finds.

Our encounter with these deposits, and smaller ones on Ruapehu, was for rather esoteric reasons. It has been suggested that the isotopic composition of the iron (i.e. the ratio between the lighter and the heavier iron atoms in the deposits) in the deposits differs if the reaction producing the deposit is purely inorganic or if it has been assisted by micro-biota. This difference in isotopic composition could provide a means of simply detecting if there has been life involved in the formation of iron deposits -- such as those on Mars or other planets, or even in subsurface environments on earth.

The reactions in these springs are almost exclusively inorganic, so Dr Tom Bullen of the US Geological Survey, Cyril Childs (formerly of the Soil Bureau and now at Victoria University) and I sampled the springs. It turns out that this method of detecting traces of life cannot be relied on, which is disappointing but by knowing this, much time and expense may have been saved in space programmes and other avenues of research.

The deposition of the iron has a considerable economic significance where groundwaters are required for agricultural or domestic use but have high iron content. The iron oxyhydroxides can deposit in groundwater wells, blocking them. It can deposit in pipes and tanks, or in baths and clothes, where it is very difficult (if not practically impossible) to remove. The iron deposits in nozzles and tubes to the extent that without treatment systems, it prevents the use of water that may be greatly needed.

Deeper waters tend to have more iron in them, mostly because they have had more time to dissolve the rocks, but it is the good shallow water which is the most vulnerable to over-exploitation and pollution and which is frequently rapidly exhausted in times of drought. Thus iron deposition is economically problematic.

The deposition of iron is useful to us as well, not just for the ochre that is produced. It is an intrinsic property of the iron oxy-hydroxides so readily formed that they soak up pollutants onto their surfaces. This property has been investigated for use in water treatment systems -- in removing arsenic from geothermal waters, for instance -- but it is also one of the important processes responsible for the ability of wetlands to remove contaminants from water.

The reducing (i.e. oxygen-deficient) water in wetlands dissolves iron which is then deposited where air has access and the contaminants are adsorbed with it.

Constructed wetlands have been used to treat contaminated waters -- even sewage pond effluent -- in New Zealand and overseas, and have been trialed even for removing toxic metals from acid mine drainages. One problem in the US has been that the plants die in very cold winters and the contaminants and the iron plus contaminants can move downstream, but this should not happen here in warmer climates.

So next time you see some iron staining in the bottom of the local creek, regard it as a friend and ally, an indicator of underground water seeps, cleaning up our environment rather than being a pollutant. In time, it might even be a valuable commodity.

Doug Sheppard is a geochemist working as a contract researcher and consultant for his company "Geochemical Solutions".