STALKS and leaves left over after hops are harvested could help decontaminate
watercourses and industrial waste water polluted with heavy metals.
Around 100,000 tonnes of hops are grown annually for the brewing industry
worldwide, but only the cones are used to make beer. Kirk Tiemann and his
colleagues at the University of Texas at El Paso took the stalks and leaves,
which usually go to waste, mashed them up, and mixed them into a silica-based
polymer.
They then packed granules of the polymer containing chunks of hop waste into
a glass column through which they pumped water containing several common metal
pollutants, including cadmium, chromium, zinc, copper and lead. The waste plant
matter turned out to be as effective at stripping metal ions out of water as the
commercial ion-exchange resins that normally do the job.
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As well as making use of material that would otherwise go to waste, Tiemann
says the hop system has other benefits. 鈥淭he plant matter seems to target
certain metals, so it won鈥檛 get fouled up by hard ions like calcium and
magnesium, which cause problems for typical ion-exchange resins.鈥�
Extracting metals from the water only solves half the problem, however. 鈥淚n
principle it鈥檚 a great idea,鈥� says Tom Stephenson, head of water sciences at
Cranfield University in Bedfordshire. 鈥淏ut always in these systems, it鈥檚 the
ultimate disposal of the metals that鈥檚 the issue: what are you going to do with
it once it鈥檚 stuck on the biomass?鈥� The El Paso researchers have come up with a
solution for this as well. Using mild acid, they have been able to recover over
90 per cent of the metals for reuse. This process also allows the plant matter
to be reused.