Enhanced biological nitrate removal by gC₃N₄/TiO₂ composite and role of extracellular polymeric substances

The biological denitrification in the presence of gC₃N₄ doped TiO₂ composite was investigated through series of batch experiment. gC₃N₄ doped TiO₂ was synthesized and characterized by FT-IR, XRD, SEM-EDAX and the prepared composite used as electron donor for the enhancement biological denitrification. The role of extracellular polymeric substances in the biological nitrate reduction and electron transfer process has been elucidated. The XRD result confirms that TiO₂ nanoparticle has 80% anatase and 20% rutile phase. The gC₃N₄ shows the diffraction peaks at 27.57°, corresponds to the diffraction planes of (002) the hexagonal graphitic carbon nitride. The SEM image of modified gC₃N₄/TiO₂ nanocomposites showed agglomerated small spherical TiO₂ nanoparticles attached on the surface of gC3N4. The highest level of nitrate removal was 90% (from 100 mg/L to 10 mg/L nitrate) in gC3N4/TiO2 nanocomposite in the 15% wt TiO₂ doped gC3N4. The nitrate reduction in the biofilm with gC₃N₄ doped TiO₂ composite have significantly enhanced the nitrate reduction than the control. Photoexcited electrons were generated from gC₃N₄ doped TiO₂ photocatalyst act as excellent electron donor to the microbial communities. Extracellular polymeric substances acted as a passing media for microbial extracellular electron transfer and protective barrier for microbes. The electroactive microbes were harvested electrons from the gC₃N₄ doped TiO₂ composite under irradiation and enhancing the biological nitrate reduction. Overall, the present study suggests that insight into the mechanism of photoexcited electron facilitated biological nitrate reduction and role of extracellular polymeric substances. The successful integration of gC₃N₄ doped TiO₂ photocatalyst and biofilm is a promising technology for nitrate removal.