Zn(II) sequestration by fungal biogenic manganese oxide through enzymatic and abiotic processes

Abstract

Here, we examined the ability of biogenic manganese oxide (BMO), formed in the cultures of a Mn(II) oxidizing fungus Acremonium strictum strain KR21-2, to sequester Zn(II) in the presence and absence of extra Mn(II) and found that BMOs sequester Zn(II) through various paths involving enzymatic and abiotic processes. Newly formed BMOs were observed to effectively sequester Zn(II), with virtually no release of Mn(II), when treated with Zn(II) in 4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid (HEPES) buffer (pH7.0) under aerobic conditions. Under anaerobic conditions, smaller amounts of Zn(II) were sequestered, whereas a significant amount of Mn(II) was released. Similar trends were observed when the BMOs were heated at 85°C for 1h to inactivate the Mn(II) oxidase associated with the BMOs. Combined with a two-step extraction of the resulting solid phase, these results demonstrated that the Mn(II) oxidase associated with BMOs can oxidize Mn(II) which is released through an ion exchange at the BMO surface and can subsequently reduce the competition for sorption with released Mn(II). Assays using the concentrated Mn(II) oxidase crude solution indicated that coexisting Zn(II) had a decreased inhibitory effect on enzymatic Mn(II) oxidation if the preformed Mn oxide phase was present. XRD revealed that no alteration of the bulk structure occurred after Zn(II) treatment under either aerobic or anaerobic conditions. When the newly formed BMOs were repeatedly treated with mixed solutions of Zn(II) and Mn(II), the Mn(II) was effectively oxidized to an oxide phase, thereby sequestering Zn(II). The XRD of the resulting solid phases indicated the formation of woodruffite (ZnMn3IVO7·2H2O) in addition to the original vernadite (birnessite) structure, which suggested that ongoing Mn(II) oxidation by the associated Mn(II) oxidase in the presence of Zn(II) may participate in the formation of mixed Zn–Mn oxide phases. Following repeated treatment of the heated BMOs in the mixed Zn(II)/Mn(II) solution at a Mn(II)/Zn(II) ratio of ~4.0, the resulting solid phase possessed well-defined XRD peaks consistent with hetaerolite (ZnMn2IIIO4) formation through an abiotic comproportionation reaction between added Mn(II) and structural Mn(IV). With a decrease in the Mn(II)/Zn(II) ratio in the bathing solution, the XRD peaks weakened and shifted toward a pattern that was identified as hydrohetaerolite rather than hetaerolite. Thus, the higher concentration of Zn(II) relative to Mn(II) in the bathing solution may have inhibited the proper formation of crystalline hetaerolite. No comproportionation reaction occurred in the absence of Zn(II) under the evaluated experimental conditions (pH7.0, ~1mM added Mn(II)), indicating that Zn(II) triggered the comproportionation reaction at the BMO surface. The results presented here increase our understanding of the role of BMO in Zn(II) sequestration through microbial (enzymatic) and abiotic processes in natural environments and provide new insights into the application of enzymatically active BMOs for Zn(II) removal processes.