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Effects of heavy metals as stress factors on anaerobic digestion processes and biogas production from biomass
Mudhoo, A. & Kumar, S.
Abstract
Heavy metals affect the biochemical reactions
that take place during anaerobic digestion processes of
organic matter. In this review, the different effects
observed in anaerobic digestion processes and during the
production of biomethane and biohydrogen from several
substrates contaminated with and/or inheriting heavy metals from the substrates themselves were discussed. It has
been found that heavy metals exert important roles in
biochemical reactions. Heavy metals like copper, nickel,
zinc, cadmium, chromium and lead have been overwhelmingly reported to be inhibitory and under certain
conditions toxic in biochemical reactions depending on
their concentrations. Heavy metals like iron may also
exhibit stimulatory effects, but these effects have been
scantily observed. This review also concludes that the
severity of heavy metal inhibition depends upon factors
like metal concentration in a soluble, ionic form in the
solution, type of metal species, and amount and distribution
of biomass in the digester or chain of biochemical reactions
which constitute the anaerobic digestion process. A
majority of studies have demonstrated that the toxic effect
of heavy metals like chromium, cadmium and nickel is
attributable to a disruption of enzyme function and structure by binding of the metal ions with thiol and other groups on protein molecules or by replacing naturally
occurring metals in enzyme prosthetic groups. This review
has not found published data on the effects of heavy metals
on the hydrolysis stage of anaerobic digestion process
chemistry, and hence further studies are required to depict
any changes.
Keywords
Inhibition; Acidogenesis; Methanogenesis; Biomethane; Biohydrogen
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