Fig. 1 illustrates the effect of treating beech wood with HNO3
and CH3COOH for 2 and 4 h at two different acid concentrations (0.1
and 1 wt.%) and temperatures (20 and 50 C). In general, much
higher ash removal was achieved when biomass was treated with
an acidic solution, as opposed to pure water. It is well established in
literature that acid treatment is more effective for the demineralization
of biomass [14,16,18,21]. Scott et al. [27] suggested that inorganics
are present in the biomass as water-soluble salts and
cations that are bound to reactive sites. Treatment with water is
adequate for the removal of the water-soluble metal salts but the
cations bound to reactive sites require ion exchange and thus, the
presence of an acid is essential for higher levels of ash removal. It
has also been suggested that acid treatment opens up or loosens
the structure of the biomass, which facilitates mass transfer and higher levels of ash removal [21]