Ferrous iron mediated autotrophic denitrification: a new bioprocess for oxidation of iron and nitrate removal from wastewaters

This study focuses on the biorecovery of iron and other metals, simultaneously with nitrogen removal from wastewaters, by means of an innovative bioprocess which is ferrous iron-mediated autotrophic denitrification. Biological oxidation with metal recovery is a promising and cost-effective alternative to the conventional chemical processes of neutralization followed by precipitation/sedimentation for metal-containing water treatment. Nowadays, the need of metals is increasing but the natural high-grade ores are rapidly decreasing (Hoque et al., 2011). In addition, heavy metal pollution is one of the biggest environmental issues (Fu et al., 2011) mainly due to the discharge of contaminated industrial and mining wastewaters into the hydrosphere (Jadhav et al., 2013). As an example, an environmental catastrophe recently occurred and is still taking place in Brazil’s mineral-rich state of Minas Gerais, due to the collapse of two mining dams having serious consequences for the population such as the contamination of the water supplies. Environmental regulations have become more stringent and require eco-friendly technologies, with low cost and low energy requirements while giving high removal yields (Hoque et al., 2011; Jadhav et al., 2013). Therefore, biorecovery of metals from wastewater is an economic and profitable solution for the high demand of metals and the reduction of the mining impact. Among metals, iron is one of the most important and the processing of iron for industrial purposes accounts for 95% of worldwide metal production (Ilbert et al., 2013). The discharge of nitrogen compounds into the water resources is another serious environmental problem. Metal and nitrogen contamination can be simultaneously present in mining processing waters and acid mine drainage (AMD). Denitrification can be used for nitrogen removal, as it converts nitrate to nitrogen gas (Papirio et al., 2014) and, recently, chemoautotrophic denitrifiers capable of coupling ferrous iron oxidation to nitrate reduction have gained scientific interest (Di Capua et al., 2015). Fe(II) oxidation results in the production of ferric iron precipitates or co-precipitates with other metals that can be subsequently removed and recovered. Within this study, the feasibility of maintaining denitrification with different microbial cultures was initially investigated by using batch bioassays. A Thiobacillus denitrificans-dominated mixed culture and an activated sludge inoculum were used as their capability in reducing nitrate with Fe(II) as electron donor had been previously demonstrated (Straub et al., 1996; Nielsen and Nielsen, 1998). For both inocula, Fe/NO3- ratio was maintained at 5, with feed Fe(II) and NO3- ranging between 8-16 and 2-5 mM, respectively. Thiosulfate (S2O32-) in concentration of 0.5 mM was occasionally used as an additional electron donor to stimulate the process. Besides the influence of different inocula, experiments were carried out in order to evaluate the optimal values of pH and concentration of EDTA, used to keep Fe(II) in solution. Batch bioassays were performed at room temperature (22 ± 2°C) at feed pH of 7 and 8 using 125 mL serum bottles, flushed with He, aseptically sealed and placed on a gyratory shaker. Microcosms were prepared in triplicate and, in addition, more bottles were prepared for electron-donor free and abiotic controls. The results showed that the process was strongly pH dependent with the highest efficiency achieved at pH 7. An initial chemical Fe(II) oxidation of approximately 30% was observed in all the tests in the first 6 h of incubation. When Thiobacillus denitrificans-dominated cultures were used, denitrification coupled to Fe(II) oxidation was maintained only when thiosulfate was supplemented as an additional electron donor. After 7 d, ferrous iron oxidation was complete with a maximum nitrate removal of 70% demonstrating that S2O32- supplementation was only needed in order to start-up the process. Fe(II) oxidation rate averagely remained at 1mM/d. In the bottles seeded with the activated sludge inoculum, nitrate reduction occurred with Fe(II) as sole electron donor. Under these conditions, denitrification was slower resulting in 50% of Fe(II) oxidation and nitrate reduction in 10 d of incubation. Finally, the microbial activity of the two previously described mixed cultures will be compared with that of a pure culture of Pseudogulbenkiania sp. strain 2002. Subsequently, the most performing microbial incubation will be seeded in two moving bed biological reactors (MBBRs) in order to optimize the operating parameters of the process under continuous-flow and steady-state conditions.