Biogenic iron oxides for phosphate removal.

Biogenic iron oxides (BioFeO) formed by Leptothrix sp. and Gallionella sp. were compared with chemically formed iron oxides (ChFeO) for their suitability to remove and recover phosphate from solutions. The ChFeO used for comparison included a commercial iron-based adsorbent (GEH) and chemically oxidized iron precipitates from groundwater. Despite contrary observations in earlier studies, the batch experiments showed that BioFeO do not have superior phosphate adsorption capacities compared to ChFeO. However, it seems multiple mechanisms are involved in phosphate removal by BioFeO which make their overall phosphate removal capacity higher than that of ChFeO. The overall phosphate removal capacity of Leptothrix sp. deposits was 26.3 mg P/g d.s., which could be attributed to multiple mechanisms. This included adsorption on the solid phase (6.4 mg P/g d.s.) as well as removal via precipitation and/or adsorption onto suspended complexes released from the BioFeO of Leptothrix sp. (19.6 mg P/g d.s.). Only a very small part of phosphorus (0.3 mg P/g d.s.) was retained in the Leptothrix sp. sheats during bacterial growth. Deposits of Gallionella sp. had an overall phosphate removal capacity of 39.6 mg P/g d.s. Significant amounts of phosphate were apparently incorporated into the Gallionella sp. stalks during their growth (31.0 mg P/g d.s.) and only one-fifth of the total phosphate removal can be related to adsorption (8.6 mg P/g d.s.). Their overall ability to immobilize large quantities of phosphate from solutions indicates that BioFeO could play an important role in environmental and engineered systems for removal of contaminants.

Reduced iron induced nitric oxide and nitrous oxide emission.

Formation of the greenhouse gas nitrous oxide in water treatment systems is predominantly studied as a biological phenomenon. There are indications that also chemical processes contribute to these emissions. Here we studied the formation of nitric oxide (NO) and nitrous oxide (N(2)O) due to chemical nitrite reduction by ferrous iron (Fe(II)). Reduction of nitrite and NO coupled to Fe(II) oxidation was studied in laboratory-scale chemical experiments at different pH, nitrite and iron concentrations. The continuous measurement of both NO and N(2)O emission showed that nitrite reduction and NO reduction have different kinetics. Nitrite reduction shows a linear dependency on the nitrite concentration, implying first order kinetics in nitrite. The nitrite reduction seems to be an equilibrium based reaction, leading to a constant NO concentration in the liquid. The NO reduction rate is suggested to be most dependent on reactive surface availability and the sorption of Fe(II) to the reactive surface. The importance of emission of NO and N(2)O coupled to iron oxidation is exemplified by iron reduction experiments and several examples of environments where this pathway can play a role.