In comparison to the separate acid production and leaching
step experiment, the single step leaching experiments showed several
advantages. Higher leaching efficiencies (100% of Cd, 30% of
Pb and 19% of Zn) occurred in the one-step process than in the
two-step process (84.4%, 25% and 14.4% for Cd, Pb and Zn). The
result may be due to two reasons. Firstly, bioaccumulation occurs
in the fungal bioleaching and enhances metal leaching via altering
the equilibrium metal concentration in the suspension [13].
Secondly, a different metabolite was involved in two-step leaching
(gluconic > citric > oxalic >malic) compared with one-step leaching
(gluconic >malic > citric). However, the leaching efficiency of Cu in
the one-step processwas lower than in the two-step process which
may be due to the weaker selective biosorption of Cu by mycelia
of the A. niger than for the other metals. Intact microbial cells, live
or dead, and their products can be highly efficient bioaccumulators
of both soluble and particulate forms of heavy metals [28]. Onestep
processes can reduce capital and operating costs; the two-step
process producing organic acids in a separate step can facilitate the
production ofmetal leaching acids, and avoid the difficulties related
to: (a) maintaining optimum fungal culture conditions in the field;
and (b) the toxicity of soil involved in the treatment process. However,
capital costs would increase due to the requirement for extra
bioreactor tanks to produce the acids [29].