Trafigura case: toxic slop left behind by caustic washing

Waste from process used to clean fuel can pose deadly threat to human health
Ian Sample, science correspondent
guardian.co.uk, Wednesday 16 September 2009 17.55 BST
Caustic washing is used to remove corrosive and pungent sulphur compounds from poor quality crude oil so that it can be blended into petrol and diesel. The processed fuel smells better and is kinder to engines.
The procedure involves pumping caustic soda into tanks of dirty fuel and separating the two substances once they have reacted. If done properly the technique cleans the fuel but leaves a highly toxic waste "slop" of caustic soda that is contaminated with harmful sulphur compounds, including hydrogen sulphide, organic disulphides and volatile compounds called mercaptans.
In refineries, specialised equipment is used to turn the waste into chemicals that can either be reused or disposed of safely. One widely used procedure converts the highly acidic hydrogen sulphide into solid sulphur, a substance that can be stored or used again.
Modern refineries have made the process more environmentally friendly by adding a catalyst that converts mercaptans into less noxious compounds. If the procedure is done without the right equipment the waste slop will have high levels of the chemicals in it.
Waste from caustic washing dumped without being treated properly is a serious threat to human health and the environment. Much of the waste slop is caustic soda, which can be fatal if ingested and which causes burns on contact and possibly blindness.
Many of the sulphur compounds in the waste are highly volatile and will be blown downwind of dumped waste. Hydrogen sulphide can kill if inhaled, and a single breath of air that carries just 0.1% of the acid is enough to induce a coma in an adult. At low levels the substance damages the sense of smell, so vast numbers of people could be exposed to dangerous levels of the gas without realising it.
Mercaptans are highly volatile and are likely to be gradually released from dumped waste as it breaks down in the environment. The gases given off are severe irritants that are extremely toxic if inhaled. The substance can also leach into water courses, where it is fatal to aquatic life.
The slop contains a variety of disulphides, which cause severe irritation to the eyes and are extremely dangerous to inhale.