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MOPPING up river pollution might not always be a good idea. A little sewage
may protect wildlife by locking up toxic metals.

George Luther of the University of Delaware in Lewes and his colleagues
examined samples from 20 rivers on the eastern seaboard of the US. They found
that sulphides, which mostly come from sewage, can 鈥渕op up鈥� metals by
imprisoning them in sulphide clusters. The clusters trap common metals such as
iron, zinc and copper, as well as more toxic metals such as silver, cadmium and
mercury.

The complexes consist of hexagonal rings of sulphides around the
metals鈥攕tructures that haven鈥檛 been seen in natural systems before, the
researchers say. 鈥淭he high stability of these clusters suggests they will resist
further chemical degradation, and ultimately may help to protect aquatic life
from the poisonous metals,鈥� says Luther. Toxicity tests are now needed to see if
this is indeed the case. If so, it could be good news for water treatment
plants, which try to remove all traces of sulphides from raw sewage鈥攁
difficult task rarely accomplished.

In theory, you could use sewage to mop up toxic metals, agrees Alan Tappin of
the Natural Environment Research Council in Plymouth. 鈥淏ut the metal sulphides
need to be removed before they reach estuaries,鈥� he says, where they might break
apart. And stripping rivers of metals might deprive some organisms of essential
trace elements, he warns.