Table 1 The ZVI corrosion condition in various systems with
different rZVI/VSS. However, increasing the rZVI/VSS also companied
with the presence of increasing amount of minerals within the
sludge. At the rZVI/VSS of 5.26, the iron content in sludge was
up to 826.32 mg/gVSS (Table 1). The large amounts of solid iron
species were likely to hinder the metabolism of the anaerobic
microorganisms and even to damage the cellular structure through
accumulating at cell surfaces as ferrous precipitate or encapsulating
the microbial cells. The investigation of Joshua et al. [18]
confirmed that the interaction of solid iron species with bacteria
was the main reason for the slower rates of perchlorate reduction
in the ZVI-mixed bacterial culture. Therefore, in order to alleviate
the negative impact of solid iron species on microorganism, the
amount of ZVI addition should not be too large, as the excessive
ZVI would weaken the ZVI-facilitated effect. In the individual ZVI
systems, it was found that the COD removal efficiency gradually
increased from 5.8% to 27.6% and the phosphate removal efficiency
rapidly increased from 9.5% to 70.9% (see Table 4 in Supplementary
material), indicating that the most corrosion-induced Fe2+ reacted
to form precipitates with phosphates (KspFe3(PO4)2 = 1.0 × 10−36)
[16]. In addition, some Fe2+ could react with hydroxyl groups
and residual dissolved oxygen of inflow to form colloids, such as
Fe(OH)2, Fe(OH)3, Fe(H2O)x
2+, and Fe(H2O)x
3+ [27]. The aging processes
of these colloids would inevitably make the contaminants
adsorption on to and co-precipitation with the corrosion products
occur, resulting in the removal of partial suspended COD. So the
presence of ZVI created a physico-chemical removal way of COD for
the anaerobic system in addition to stimulating the biodegradation
[15].