Iron and nickel nanoparticles role in volatile fatty acids production enhancement: Functional genes and bacterial taxonomy in an anaerobic fluidized bed reactor

This study is the first report that describes Iron (16.7 mg Fe⁰ g^-1) and Nickel (0.04 mg Ni⁰ g^-1) nanoparticles (NPs) immobilization on the support material of an Anaerobic Fluidized Bed Reactor (AFBR-NP) and the assessment of its influence on fermentation pathway when compared with a reactor without NPs (AFBR-Control). AFBRs were operated at initial pH 6.8, 37 °C, 3.0 h Hydraulic Retention Time. Natural fermentation along with bioaugmentation with Clostridium butyricum were used as inoculation strategy. Both reactors were fed with glucose as main substrate (56 ± 6 and 21 ± 2 g COD L^-1 d^-1 at operational Stages I and II, respectively). 78–98% higher Volatile Fatty Acid (VFA) was observed on AFBR-NP. Metals higher availability specifically favored butyric (1.04 ± 0.37 g L^-1) and acetic (1.22 ± 0.33 g L^-1) acid production metabolic route on AFBR-NP. Under the same operational conditions, lactic acid was observed as main metabolite on AFBR-Control (1.23 ± 0.25 g L^-1). Enzymes related to butyric (butyryl-CoA dehydrogenase, phosphate butyryltransferase, butyrate kinase) and propionic acid (pyruvate-ferredoxin oxidoreductase) metabolic routes were identified on AFBR-NP reactor biofilm. Through these results, it is possible to infer that Fe⁰ and Ni⁰ NPs could effectively improve VFA production and the fermentation process.