Distribution fractions and potential ecological risk assessment of heavy metals in mangrove sediments of the Greater Bay Area.

The restoration of mangrove in coastal wetlands of China has been started since the 1990s. However, various pollutants, especially for heavy metals (HMs), contained in wastewater might present a significant risk to mangrove forests during the restoration. In this study, sediments of five typical mangrove wetlands with varying restoration years and management measures in the Greater Bay Area were collected to evaluate the distribution fractions and potential ecological risk of HMs. Cd (0.2-1.6 mg/kg) was found in high concentrations in the exchangeable fraction (37.8-71.5%), whereas Cu (54.2-94.8 mg/kg), Zn (157.6-332.6 mg/kg), Cr (57.7-113.6 mg/kg), Pb (36.5-89.9 mg/kg), and Ni (29.7-69.5 mg/kg) primarily presented in residual fraction (30.8-91.9%). According to the geo-accumulation index (Igeo) analysis, sediment Cd presented a high level of pollution (3 ≤ Igeo ≤ 4), while Zn and Cu were associated with moderately pollution (1 ≤ Igeo ≤ 2). Besides, high ecological risk of Cd was found in sediments of five mangroves, with risk assessment code (RAC) ranging from 45.9 to 84.2. Redundancy analysis revealed that the content of NO3--N was closely related to that of HMs in sediments and, pH value and NO3--N concentration affected the distribution of HMs geochemical fractions. High concentration of HMs in QA and NS sampling sites was caused by the formerly pollutants discharge, resulting in these sediments still with a higher HM pollution level after the plant of mangrove for a long period. Fortunately, strict drainage standards for industrial activities in Shenzhen significantly availed for decreasing HMs contents in mangrove sediments. Therefore, future works on mangrove conversion and restoration should be linked to the water purification in the GBA.

Reducing the inhibitive effect of fluorine and heavy metals on nitrate reduction by hydroxyapatite substrate in constructed wetlands.

Bio-toxic inorganic pollutants, e.g., fluorine (F) and heavy metals (HMs), in wastewaters are the potential threats to nitrate (NO3--N) reduction by microorganisms in constructed wetlands (CWs). Selection of suitable substrate with high F and HMs adsorption efficiency and capacity is a potential alternative for simultaneous removal of these pollutants in CWs. Herein, this study investigated the feasibility of applying hydroxyapatite (HA)-gravel media for F and HMs adsorption and its effect on NO3--N reduction in CWs (HA CWs) by comparing the CWs filled with gravel substrate (CK CWs). The results indicated that the removal efficiency of F, Cr, As, and NO3--N in HA CWs increased by 113.6-, 3.3-, 2.7-, and 0.6-folds, respectively, compared to CK CWs. The NO3--N reduction rate decreased by 11-46% in CK CWs after the presence of F and HMs in influent, while for HA CWs, it was only 13-22%. Excellent F and HMs adsorption capacity of HA substrate availed for wetland plants resisting F/HMs toxicity and making catalase activity lower. The HA substrate in CWs resulted in the certain succession of nitrogen-transforming bacteria, e.g., nitrifiers (Nitrospira) and denitrifiers (Thiobacillus and Desulfobacterium). More importantly, key functional genes, including nirK/nirS, korA/korB, ChrA/ChrD, arsA/arsB, catalyzing the processes of nitrogen biotransformation, energy metabolism, NO3--N and metal ions reduction were also enriched in HA CWs. This study highlights HA substrate reduce the inhibitive effect of F and HMs on NO3--N reduction, and provides new insights into how microbiota structurally and functionally respond to different substrates in CWs.

Interactive effects of aquatic nitrogen and plant biomass on nitrous oxide emission from constructed wetlands.

Understanding of mechanisms in nitrous oxide (N2O) emission from constructed wetland (CW) is particularly important for the establishment of related strategies to reduce greenhouse gas (GHG) production during its wastewater treatment. However, plant biomass accumulation, microbial communities and nitrogen transformation genes distribution and their effects on N2O emission from CW as affected by different nitrogen forms in aquatic environment have not been reported. This study investigated the interactive effects of aquatic nitrogen and plant biomass on N2O emission from subsurface CW with NH4+-N (CW-A) or NO3--N (CW-B) wastewater. The experimental results show that NH4+-N and NO3--N removal efficiencies from CW mesocosms were 49.4% and 87.6%, which indirectly lead to N2O emission fluxes of CW-A and CW-B maintained at 213 ± 67 and 462 ± 71 µg-N/(m2·h), respectively. Correlation analysis of nitrogen conversion dynamic indicated that NO2--N accumulation closely related to N2O emission from CW. Aquatic NH4+-N could up-regulate plant biomass accumulation by intensifying citric acid cycle, glycine-serine-threonine metabolism etc., resulting in more nitrogen uptake and lower N2O emission/total nitrogen (TN) removal ratio of CW-A compared to CW-B. Although the abundance of denitrifying bacteria and N2O reductase nosZ in CW-B were significantly higher than that of CW-A, after fed with mixed NH4+-N and NO3--N influent, N2O fluxes and N2O emission/TN removal ratio in CW-A were extremely close to that of CW-B, suggesting that nitrogen form rather than nitrogen transformation microbial communities and N2O reductase nosZ determines N2O emission from CW. Hence, the selection of nitrate-loving plants will play an important role in inhibiting N2O emission from CW.

Integration of CW-MFC and anaerobic granular sludge to explore the intensified ammonification-nitrification-denitrification processes for nitrogen removal.

The integration of constructed wetland-microbial fuel cell (CW-MFC) and anaerobic granular sludge (AGS) is an important way to promote its ammonification efficiency and decrease the land use scale. This study explored the integration of CW-MFC and AGS for nitrogen removal via the intensified ammonification-nitrification-denitrification processes with initial NH3-N, NO3-N, Org-N and total nitrogen (TN) concentrations of 10.5, 13.8, 21.4, and 45.7 mg L-1 in wastewater. Two reactors with AGS inoculated with a separated area (R1) and directly inoculated into gravel substrate (R2) were designed, respectively. Results showed that chemical oxygen demand (COD) removal efficiency could reach 85% in R1 and 81% in R2, and the conversion of Org-N to NH3-N and NO3-N to gaseous nitrogen were 80% and 90%, respectively. Although the conversion efficiency of NH3-N to NO2-N/NO3-N via nitrification process was only 18%, it could reach 45%, 94%, and 98% with the aeration rates of 50-, 100-, and 200-mL min-1. According to microstructural property and microbial community analyses, the separation gravel substrate and AGS areas in R1 availed for stable particle size of AGS, archaeal diversity, and metabolic activity even with a 1.5 times daily wastewater treatment capacity than that of R2. Overall, although the intensified ammonification-nitrification-denitrification processes for nitrogen removal could be achieved with supplementary aeration, further investigation is still needed to explore other substrate materials and high CW-MFC/AGS volume ratio for intensified nitrification process in CW-MFC associated with AGS.