Science / Molecule of the Month: Transformation to a greener cleaner: We blame them for polluting rivers and lakes, but phosphates are not all bad, reports John Emsley
Monday 18 April 1994
Phosphate is essential to all living cells. Indeed phosphate is part of DNA itself, although this accounts for only a small part. More phosphate is present as the energy molecule ATP (adenosine triphosphate), the messenger molecule GMP (guanosine metaphosphate) and as phospholipids in cell membranes. Plants store phosphate in their seeds so that these can germinate and put down roots without needing any phosphate from the soil. In animals most phosphate goes to making teeth and bones.
Calcium phosphate has been used as a fertiliser for more than 200 years, originally as bonemeal, later as superphosphate, a more soluble form made by treating calcium phosphate ore with sulphuric acid. Calcium phosphate bones resist the ravages of time, and this mineral also fossilises soft tissues and leaves.
The phosphate in detergents and dishwasher powders is sodium tripolyphosphate (STPP). This softens water by neutralising calcium and keeping dirt in suspension once it has been washed off clothes. The trouble starts when STPP enters the sewers to add to the phosphate from human excreta and industrial waste. Some of this phosphate is removed by conventional sewage treatments, but most ends up in rivers where it joins the phosphate that has drained from the land. Both organic farming and inorganic fertilisers cause a loss of phosphate.
Where rivers are long, or where they drain into lakes, the phosphate acts as a pollutant by encouraging algae to bloom and use up all the oxygen. In Britain such effects are rarely noticed because our rivers are short and drain to the sea. Phosphate in the oceans is quickly used up and falls to the deep as waste.
In the past long rivers such as the Rhine, and the Great Lakes of North America were the ones to suffer and detergents were seen as the primary cause since they contained up to a third of their weight as phosphate. This led to campaigns against the use of phosphates in detergents.
Yet detergent phosphate may not be as environmentally damaging as once believed. So says The Phosphate Report, written by Bryn Jones, a former director of Greenpeace, and Bob Wilson of the University of the West of England, Bristol. They carried out a full environmental audit of STPP and compared it with the alternative detergent component, zeolite, which is an aluminium silicate. The report, a cradle-to-grave analysis of these chemicals, found that there is little to choose between phosphates and zeolites. The recent developments in sewage treatment, however, might well tilt the balance in favour of phosphates.
New sewage technology allows phosphate to be recovered and treatment plants to reclaim waste phosphate as calcium phophate are already operating in the Netherlands - one of the countries, along with Germany, Italy and Switzerland, where phosphate-based laundry detergents have all but disappeared from supermarket shelves. Now Britain's largest producer of phosphate, Albright & Wilson, which funded the Jones and Wilson's survey, has shown that this can be recycled into STPP.
John Driver, Environmental Manager of Albright & Wilson says: 'Reclaiming phosphate has many benefits, not least because it also extracts heavy metal impurities from sewage, such as cadmium and chromium, leaving behind a sludge that can be used as fertiliser for farmland. At Albright & Wilson we have to remove these metals, and can dispose of them safely, as we clean up the phosphate - to food-grade quality if desired. Recycling phosphate could mean the ending of phosphate imports to the UK in the next century, while still allowing it to be used in detergents and food-processing.'
Some detergent phosphates have re- emerged under new guises. Trisodium phosphate (TSP) was for many years the miracle ingredient in the household cleaner Flash. Now this phosphate is being used in the US to remove food- poisoning germs from raw chicken. Surface bacteria, such as salmonella, are washed away when a carcass is sprayed with a solution of TSP, and it works by removing the surface fat that covers them.
Chemists at the Rhone-Poulenc labs in Cranbury, New Jersey, patented the TSP process, which got approval in 1992 from the US Food and Drugs Administration. According to Phil Coppen, Development Manager of Rhone-
Poulenc Food Ingredients, based at Watford: 'We have applied for the TSP treatment to be licensed in the EC, but this is a lengthy procedure which involves the approval of member countries. We do not expect this agreement before the end of this year.'
Meanwhile every day in the EC there are more than 5,000 cases of salmonella food poisoning, many of which can be traced to infected poultry meat and which could be eradicated with a little help from our phosphate friends.
Dr John Emsley is science writer in residence in the department of chemistry, Imperial College, London.
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