Investigating the Molecular Mechanism of Optimized Bio Denitrification by Thiobacillus denitrificans in the Presence of Metal Nanostructures in Bioreactor

Nitrate contamination of groundwater and surface water has become a serious problem. Zero-capacity iron (Fe⁰) nanoparticles are of great interest for the purification of toxic compounds from water. The aim of this study was to synthesis and optimize starch-coated “St” and carbon quantum dot-coated “CQD” Fe⁰ nanoparticles (St-Fe⁰ and CQD-Fe⁰) and investigate their role in bio denitrification and expression of nirS, narH and recA genes using the q-PCR technique. This study has a non-treatment control. The structural and morphological characteristics of nanoparticles were determined by XRD, TEM, FE-SEM and FTIR. Two important operating parameters including temperature (̊C) and nanoparticles concentration (g l⁻1) influencing the bio denitrification process of Thiobacillus denitrificans in the presence of nanoparticles were optimized by the response surface method (RSM). The FTIR spectrum confirmed the formation of CQD-Fe⁰ bonds. The average diameter of CQD-Fe⁰ nanoparticles was in the range of 29.31 to 38.32 nm. Denitrification in the presence of St-Fe⁰ and CQD-Fe⁰, with increasing temperature (35 C°) and nanoparticle concentration (1 g l⁻1), was 71.42% and 73.43%, respectively. After determining the optimal conditions, bio denitrification in the presence of nanoparticles at different concentrations of nitrate was investigated in a moving bed bioreactor. According to the results, in the discontinuous conditions, bio denitrification efficacy of Thiobacillus denitrificans in the presence of St-Fe⁰ and CQD-Fe⁰ nanoparticles was 80.10% and 79.83%, respectively. While in the bioreactor, denitrification efficacy in the presence of St-Fe⁰ and CQD-Fe⁰ was increased to 94.07% and 95%, respectively. In other words, bio denitrification of microorganisms in the moving bed bioreactor in the presence of CQD-Fe⁰ was higher compared to St-Fe⁰. The results of this study showed that the expression of all three genes nirS, narH and recA in cells treated with St-Fe⁰ and CQD-Fe⁰ significantly increased compared to the control group, which indicates the high impact of these nanoparticles, especially CQD-Fe⁰ on bio denitrification of this microorganism.