Lehigh University

“SCRAP” conjures up visions of rusting junkyards on the wrong side of the tracks. But this image may soon get a green makeover. A research project in China suggests that iron shavings from factories can be a cheap and efficient way to clean up polluted water. And because such scrap is widely available, the system could be particularly useful in developing countries.

The iron shavings are being used to treat wastewater in the Taopu Industrial District of Shanghai, which is home to many small pharmaceutical, petrochemical and textile factories that discharge water contaminated with dyes, phosphorus and nitrogen. The project, which began in August 2006, now treats about 60,000 cubic metres (about 13m gallons) a day of industrially contaminated water—which is about the volume of municipal wastewater a small town generates.

Wei-Xian Zhang of Lehigh University, Bethlehem, Pennsylvania, and Luming Ma of Tongji University, Shanghai, have been using the wastewater facility to test their methods of employing iron shavings to treat industrial wastewater. Iron powder (technically called zero-valent iron by chemists to show that it has not oxidised) has been used to treat groundwater for more than a decade, and to remove dangerous substances such as trichlorethene (used in paint strippers and adhesives) and arsenic. But no one had tried using iron shavings to treat water discharged from factories before.

The standard technique used to treat wastewater is to pass it through a series of tanks containing biological agents, such as biofilms, bacteria and other aerobic organisms, that break down the contaminants in a few days. But this often does not work with water from factories, especially as it may contain synthetic compounds that are toxic and not biodegradable.

Dr Zhang had previously invented a method to clean groundwater and contaminated soil using iron nanoparticles. While effective, such nanoparticles are expensive: about $100 a kilogram, which can prohibit their use in developing countries. Dr Zhang, who is from China and who went to college in Shanghai, thought iron shavings, which have a large surface area, may provide a cheap alternative. Scrap iron currently costs about 20 cents a kilogram in China. His idea was to treat industrial wastewater by passing it through the iron shavings, and then treat it as municipal wastewater. First, the non-biodegradable industrial chemicals are attracted to the surface of the iron shavings, where they react by sharing electrons with the iron and become degraded. (The iron gets oxidised in the process.) Then, any biodegradable contaminants that remain are neutralised by the second step.

Dr Zhang found that treating the iron shavings with a solution of copper chloride increases their effectiveness (but puts the cost up by only about five cents a kilogram). He teamed up with Dr Ma in Shanghai about five years ago. Using 40kg of scrap iron, they ran a prototype experiment which showed the method worked. Then the full-scale treatment facility came into operation. It consists of ten parallel cells containing a total of 914,000kg of iron shavings, all purchased locally. (The iron lasts about two years before it has to be replaced.) Some 80% of the water treated is industrial discharge.

Compared with biological treatment alone, big improvements have been recorded. The removal of nitrogen has gone from 13% to 85%; phosphorus from 44% to 64%; and colours and dyes from 52% to 80%. Given the success of the technique, Dr Zhang and Dr Ma have now been invited by several municipalities in China to help them build similar treatment centres. The team are also working on a much larger treatment centre in Shanghai that can handle 100,000 cubic metres of wastewater a day. Dr Zhang says he believes the scrap-iron method will open a new chapter in the treatment of industrial wastewater, not least because the vital ingredient is cheap and abundant.