Nitrate removal through Fe(II)-mediated denitrification: effect of copper, nickel and zinc

Nitrate (NO3-) is commonly found in municipal and industrial wastewaters. In mining environments, NO3- contamination is often accompanied by metal (e.g. iron) and heavy metal (e.g. copper, nickel and zinc) pollution, leading to a higher impact of the generated wastewaters. Fe(II)-mediated autotrophic denitrification is a promising biotechnology, suitable for simultaneous NO3- removal and iron biorecovery. However, the activity of denitrifying and Fe(II)-oxidizing microorganisms can be considerably hindered by the presence of heavy metals, resulting in a not complete denitrification. This study investigated the efficiency of Fe(II)-driven denitrification in the presence of copper (Cu), nickel (Ni) and zinc (Zn) with four different microbial cultures in batch bioassays. Fe(II) and NO3- were added at 600 and 120 mg/L, whereas Cu, Ni and Zn were fed at 5, 10, 20 and 40 mg/L in order to evaluate the effect of different metal concentrations. Each bottle was inoculated with one of the following biomasses: i) a Thiobacillus-dominated mixed culture, ii) an activated sludge enrichment, iii) a pure culture of T. denitrificans and iv) a pure culture of Pseudogulbenkiania sp. 2002. A complete NO3- removal was achieved with the Thiobacillus-dominated mixed culture and the pure culture of Pseudogulbenkiania in the absence of Cu, Ni and Zn. Under the same conditions, a 96 and 91% NO3- removal efficiency was obtained with the pure culture of T. denitrificans and the activated sludge enrichment, respectively. When Cu, Ni and Zn were supplemented, the Thiobacillus-dominated mixed culture and activated sludge enrichment better tolerated the presence of metals. Differently, a significantly higher sensitivity to metal toxicity was observed for the pure cultures, especially for Pseudogulbenkiania sp. 2002 that only resulted in a 6÷8% NO3- removal at the highest metal concentration. Cu averagely led to the highest inhibition of denitrification among the three heavy metals investigated.