Removal characteristics of a composite active medium for remediation of nitrogen-contaminated groundwater and metagenomic analysis of degrading bacteria

Abstract

To investigate the removal characteristics of ammonium-nitrogen (NH₄⁺-N), nitrite-nitrogen (NO₂^--N), nitrate-nitrogen (NO₃^--N), and total nitrogen from groundwater by a degradable composite active medium, kinetics, thermodynamics, and equilibrium adsorption, experiments were performed using scoria and degrading bacteria immobilized on scoria. Removal of NH₄⁺-N, NO₂^--N, and NO₃^--N was conducted in adsorption experiments using different times, initial concentrations, pH values, and groundwater chemical compositions (Ca²⁺, Mg²⁺, HCO₃^-, CO₃^2-, Fe²⁺, Mn²⁺, and SO₄^2-). The results showed that the removal of nitrogen by the composite active medium was obviously better than that of scoria alone. The removal rates of NH₄⁺-N (C₀ = 5 mg/L), NO₂^--N (C₀ = 5 mg/L), and NO₃^--N (C₀ = 100 mg/L) by the composite active medium within 1 h were 96.05%, 82.40%, and 83.16%, respectively. The adsorption kinetics were well fitted to a pseudo-second order model, whereas the equilibrium adsorption agreed with the Freundlich model. With changes in the pH, variation in the removal could be attributed to the combined effect of hydrolysis and competitive ion adsorption, and the optimum pH was 7. Different concentration conditions, hardness, alkalinity, anions, and cations showed different promoting and inhibiting effects on the removal of nitrogen. A careful examination of ionic concentrations in adsorption batch experiments suggested that the sorption behavior of nitrogen onto the immobilized medium was mainly controlled by ion exchange. The degrading bacteria on the scoria surface were eluted and analyzed by metagenomic sequencing. There were significant differences in the number of operational taxons, relative abundances, and community diversity among degrading bacteria after adsorption of the three forms of nitrogen. The relative abundance of degrading bacteria was highest after NO₃^--N removal, and the diversity was highest after NO₂^--N removal. Pseudomonas and Serratia were the dominant genera that could efficiently remove NH₄⁺-N and NO₂^--N.

Keywords: Degrading bacteria; Groundwater; Metagenomic analysis; Nitrogen pollution; Scoria.