264 Environmental Biotechnology
metal pollutants from effluents. Something in the region of 100 000 tonnes of
hops are grown yearly worldwide, but only the cones are used, leaving the rest
as waste. When mashed, mixed into a silica-based polymer and then formed into
granules, the hop residual biomass was discovered to be as effective at removing
contaminating cadmium, chromium, copper, lead and zinc ions from wastewater
as conventional commercial ion-exchange resins. Moreover, preliminary evidence
suggests that the hop-residue system specifically targets certain metals, avoiding
the problems that resin-based exchangers typically have with ‘hard water’ ions
like magnesium and calcium. It is a common criticism of environmental biotech-
nology that all it ever achieves is to move a problem from one place to another
and there would be something in this if the story ended here, leaving the question
of the ultimate disposal of the metal/biomass combination unresolved. However,
using a mild acid treatment, over 90% of the metals collected can be recovered,
and the plant material remains suitable for reuse. Although commercial applica-
tion of this emergent technology is still some way away, it does illustrate the
potential power of integration to tackle multiple environmental concerns, in this
case addressing issues of waste, effluent and recycling.
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