Nitrate removal and Fe(III) recovery through Fe(II)-driven denitrification with different microbial cultures
Ferrous iron mediated autotrophic denitrification is an innovative bioprocess for nitrate removal, simultaneously with iron oxidation in wastewaters. Chemoautotrophic denitrifiers convert nitrate to nitrogen gas and Fe(II) oxidation results in the production of ferric iron precipitates that can be subsequently removed and recovered. Both mixed enrichments and pure cultures isolated from various habitats have been reported to perform nitrate-dependent Fe(II) oxidation, especially at circumneutral pH. Under these conditions, Fe(II) is not stable and a chelating agent, commonly ethylenediaminetetraacetic acid (EDTA), needs to be employed in order to promote a higher Fe(II) solubilization. Different biogenic Fe(III) (hydro)oxide mineral phases can be formed, depending to many factors such as microorganisms, medium composition and incubation conditions (Kiskira et al., 2017a).
The feasibility of maintaining Fe(II)-mediated autotrophic denitrification with a Thiobacillus mixed culture, an activated sludge inoculum and pure cultures of Pseudogulbenkiania strain 2002 and T. denitrificans under different pH and EDTA:Fe(II) conditions was initially investigated in batch bioassays. Lower EDTA: Fe(II) ratios resulted in higher nitrate removal efficiency and rates. After a longer acclimation to Fe(II) and stimulation with S2O32-, the Thiobacillus mixed culture resulted in the highest specific nitrate removal rate, equal to 10.5 mg/(g VSS·d) (Kiskira et al., 2017b).
Subsequently, the Thiobacillus mixed culture was seeded in two identical up-flow packed bed reactors (PBRs) in order to optimize the operating parameters such as the hydraulic retention time (HRT) and the nitrate loading rate and evaluate their influence on nitrate removal and Fe(II) oxidation during 153 d of operation. The HRT was shortened from 31 to 20 h. During the steady state the maximum nitrate volumetric loading rate (VLR) and volumetric removal rate (VRR) were 280 and 240 mg NO3-/(L·d) for PBR1 and 210 and 160 mg NO3-/(L·d) for PBR2, respectively. Results showed that the nitrate removal rate increased at increasing feed nitrate concentrations. At the steady state, nitrate removal and Fe(II) oxidation were 86±2% and 95.5±2.5%, respectively, in PBR1, whereas 65.5±3.5% and 70.0±2%, in PBR2. The lower initial feed NO3- concentration resulted in lower efficiencies in PBR2.
Moreover, the influence of heavy metals (Ni, Cu, Zn) was assessed in batch bioassays. The highest nitrate removal efficiency and rates were achieved with the Thiobacillus-dominated mixed culture, whereas Pseudogulbenkiania strain 2002 was the least effective. At initial 20.0 – 40.0 mg Me/L, Cu showed to be the most inhibitory metal for mixed cultures, resulting in an inhibition in the range 33-66%. A lower impact was observed when Zn was supplemented, leading to a 17-41% inhibition of nitrate removal. Ni showed the lowest inhibitory effect, with nitrate removal that was affected by 5-34% at Ni concentrations from 20.0 to 40.0 mg Ni/L. A higher sensitivity to metal toxicity was observed for the pure cultures (Kiskira et al., 2018).
Finally, the mineral characterization of the precipitates obtained in the experiments with Cu, Ni and Zn was investigated by different methods, i.e. scanning electron microscopy (SEM), Fourier transformation infrared spectroscopy (FTIR), Raman spectroscopy and X-ray fluorescence (XRF). All samples contained ferrihydrite. In abiotic controls, the chemical Fe(II) oxidation resulted in hematite formation and goethite or akaganeite. Precipitates of the experiments carried out with the Thiobacillus-dominated mixed culture were a mixture of hematite and akaganeite. When T.denitrificans was used, hematite and maghemite were observed. The use of pure culture of Pseudogulbenkiania strain 2002 resulted in hematite and maghemite formation and the activated sludge enrichment allowed the production of hematite and maghemite and/or magnetite. No difference in the mineralogy of the precipitates was observed with the addition of Cu, whereas the addition of Ni and Zn markedly stimulated the formation of maghemite. Heavy metals in the precipitates were identified by chemical analysis, indicating co-precipitation and/or absorption in Fe(III) (hydr)oxides.