H2 g1 dry biomass here vs. 31e153 mL H2 g1 dry biomass for
other feedstocks). This might be the result of the uncontrolled
fermentation conditions, and as shown in the next section,
can be optimized by further investigation. Throughout the
course of fermentation, methane production was negligible.
This indicates that heat treatment was effective in inhibiting
methanogens in the inoculum. Although alkaline pretreatment
resulted in the highest solid loss, the fermentation of
alkaline-pretreated sample did not give the highest biogas or
biohydrogen production (Fig. 3a, b). This is possibly due to the
inhibition of fermenting microbes by the lignin-rich liquid
(black liquor). Acid pretreatment resulted in the highest
biogas and biohydrogen production, 33% and 42% higher than
with alkaline and thermal-pretreated duckweed, respectively.
Acid pretreatment has been reported to effectively hydrolyze
algae and agricultural residues, and render it suitable for
biohydrogen production. However, because of the recalcitrant
structure of these substrates, higher temperatures (>120 C) or
elevated acid concentrations (up to 3%) are normally applied
to achieve reasonable yields and rates during fermentation,
which inevitably increases conversion cost [25,44e46].