3.3. Impact of ZVI on microorganisms in the ZVI-sludge combined
system
As previously analyzed, appropriate ZVI dosage was helpful
to microbial activity, but excessive stimulation from the external
source might disrupt the balance of this system, thereby causing
damage to microorganisms. LDH is a stable and sensitive indicator
of the cell membrane integrity and cell activity, which is often used
to measure the cytotoxicity of metal particles [28]. Fig. 3A presents
the LDH comparison analysis of the different rZVI/VSS system. There
was no measurable discrepancy of LDH leakage between the sludge
control and the combined system at the rZVI/VSS of 0.32, 0.74 and
1.58. Then the value increased slightly with the rZVI/VSS increasing
to 2.63 and 5.26. These results demonstrated that the cells were vulnerable
to be damaged when exposed to high concentration of ZVI.
As reported in the literatures [18,19], there were the potential risks
of inhibition for bacterial activity with the addition of ZVI. That was
attributed to accumulation of solid iron species at the cell surface
or over-production of free radical species, and thereby causing the
cellular injury [29–31]. The results of ATP in Fig. 3B were in general
accordance with the performance assays at the same rZVI/VSS stage.
The presence of ZVI was able to elevate the intracellular ATP level
in sludge, but it was decreased rapidly thereafter at high concenFig.
trations such as 50 g ZVI/L. ATP was contained and used by all living
organisms as their main energy source, and hence it was often used
as an indicator of microbial metabolic activity [32]. It was conjectured
that the enhancement of hydrogenotrophic methanogenesis
induced by corrosion-induced H2 might make some contribution
to the increased ATP, which was considered to release more energy
per methane formation than aceticlastic methanogenesis [22]