Characteristics and metabolic pathway of the bacteria for heterotrophic nitrification and aerobic denitrification in aquatic ecosystems.

The present study investigated the nitrogen removal characteristics and metabolic pathway of bacteria in aquatic ecosystem, with a focus on heterotrophic nitrification and aerobic denitrification. The bacteria demonstrated significant heterotrophic nitrification and aerobic denitrification capacity. The highest ammonium-N, nitrate-N, and nitrite-N removal efficiencies were 95.31 ± 0.11%, 98.91 ± 0.05%, and 98.79 ± 0.09%, respectively. The Monod model was used to estimate the maximum rate of substrate utilization (Rmo) and the half-saturation concentration (Ks) for the two substrates, i.e., ammonium and nitrate. The kinetic coefficients were 3.34 mg/L/d (Rmo) and 30.59 mg/L (Ks) for ammonium-N, respectively, and 14.23 mg/L/d (Rmo) and 215.24 mg/L (Ks) for nitrate-N, respectively. The effects of initial nitrogen (ammonium-N or nitrate-N) concentration, temperature, and dissolved oxygen (DO) on nitrogen removal rate were investigated using response surface methodology (RSM), and the optimal conditions for nitrogen removal were determined. The principal nitrogen removal pathway of the bacteria was proposed as complete heterotrophic nitrification and aerobic denitrification, which was performed by six key genera: Arthrobacter, Pseudomonas, Rhodococcus, Bacillus, Massilia, and Rhizobium. Chryseobacterium and other denitrifying species may also reduce nitrification products (NOX-) via aerobic denitrification.

A lab-scale study on heterotrophic nitrification-aerobic denitrification for nitrogen control in aquatic ecosystem.

Nitrogen (N) loss is generally caused by denitrification under anaerobic conditions and the N loss in the heterotrophic nitrification_aerobic denitrification (HN_AD) system is of recent research interest. However, previous studies are generally focused on pure cultures-based system and the information on HN_AD in the complex aquatic ecosystem is limited. In this study, HN-AD system was established in the mixed cultures of the sediments and the performances of HN-AD were evaluated under different conditions. Further, the N loss mechanism in HN_AD system was explored. The study found that the N was lost in the sediment cultures with ammonium-N (NH4+_N) (or) and nitrate-N (NO3-_N) as N source under aerobic conditions. The highest N loss rate was achieved under the TOC/TN mass ratio of 10 with citrate as the carbon source. Under this condition, the N loss percentages of NH4+_N (201.91 mg/L) and NO3-_N (130.00 mg/L) reached 99.61% and 100.00%, respectively, which were higher than those in the pure HN_AD strains reported in the literature. High NH4+_N removal efficiencies were also achieved at low C/N mass ratio and high NH4+_N concentration (493.12 mg L-1). The N loss pathway in the system was investigated by adding Na2WO4 as the nitrate reductase inhibitor. The study found that the N was not lost via partial nitrification/denitrification pathway, i.e., NH4+ → NH2OH → NO2- → N2O (N2), instead via full nitrification/denitrification pathway, i.e., NH4+ → NH2OH → NO2- → NO3- → NO2- → N2O (N2), since nitrate was a key intermediate. The variation in NH4+_N, NO3-_N, and NO2-_N concentrations in the HN_AD processes further confirmed the N transformation pathway. Therefore, HN_AD may occur and cause N loss in natural aquatic ecosystems. The results of this study demonstrate that N was lost through HN-AD and that the well-cultured HN-AD sediments could be useful biological tool to remediate eutrophic water bodies.

Changes in the fluorescence intensity, degradability, and aromaticity of organic carbon in ammonium and phenanthrene-polluted aquatic ecosystems.

Mixed cultures were established by a sediment to investigate the changes in organic carbon (C) in a combined ammonium and phenanthrene biotransformation process in aquatic ecosystems. The microorganisms in the sediment demonstrated significant ammonium-N and phenanthrene biotransformation capacity with removal efficiencies of 99.96% and 99.99%, respectively. The changes in the organic C characteristics were evaluated by the fluorescence intensity, degradability (humification index (HIX) and UV absorbance at 254 nm (A 254)), aromaticity (specific UV absorbance at 254 nm (SUVA254) and fluorescence index (FI)). Compared with C2 (the second control), the lower values of fluorescence intensity (after the 15th d), HIX (after the 8th d), A 254 (after the 11th d), and SUVA254 (after the 8th d) and the higher FI value (after the 8th d) in ammonium and phenanthrene-fed mixed cultures (N_PHE) suggest that aromatic structures and some condensed molecules were easier to break down in N_PHE. Similar results were obtained from Fourier transformation infrared spectroscopy (FTIR) and nuclear magnetic resonance (1H NMR) spectra. Changes in organic C characteristics may be due to two key organisms Massilia and Azohydromonas. The biodiversity also suggested that the selective pressure of ammonium and phenanthrene is the decisive factor for changes in organic C characteristics. This study will shed light on theoretical insights into the interaction of N and aromatic compounds in aquatic ecosystems.

Batch Adsorption and Column Transport Studies of 2,4,6-Trinitrotoluene in Chinese Loess.

In present study, batch and column tests were conducted to investigate the kinetic and thermodynamic characteristics of the adsorption and transport of 2,4,6-trinitrotoluene (TNT) in Chinese loess with specific focus on the role of inherent colloid particles. Batch tests showed that a lot of TNT was absorbed in suspended colloid particles, and its adsorption reached equilibrium after about 10 h, the adsorption process can be best-fit by the pseudo-second order kinetic and Freundlich model. The adsorption was spontaneous, endothermic process, implying the adsorbed TNT is likely to release from soil matrix. These portend that the adsorbed TNT has a potential to co-transport with inherent colloid particles in loess. The column tests identified the potential, and showed TNT transport had obvious retardation effect, which may be ascribed to the release and transport of inherent colloidal particles as a key carrier. These findings are helpful to evaluate the loess interception and antifouling performance.