The sulfur cycle is an important part of constructed wetland biogeochemistry because it is intimately intertwined with the carbon, nitrogen, and iron cycles. However, to date, no quantitative investigation has been conducted on the sulfur cycle in constructed wetlands because of the complexity of wetland systems and the deficiencies in experimental methodology. In this study, 34 S-stable isotope analysis was extended in terms of the calculation for the enrichment factor and the kinetic analysis for bacterial sulfate reduction. With this extended method, we attempted for the first time to assess the true rate of bacterial sulfate reduction when sulfide oxidation co-occurs. The joint application of the extended 34 S-stable isotope and mass balance analyses made it possible to quantitatively investigate the primary sulfur transformation in a wetland microcosm. Accordingly, a sulfur cycle model for constructed wetlands was quantified and validated. Approximately 75% of the input sulfur was discharged. The remainder was mainly removed through deposition as acid volatile sulfide, pyrite, and elemental sulfur. Plant uptake was negligible. These findings improve our understanding of the physical, chemical, and biological transformations of sulfur among plants, sediments, and microorganisms, and their interactions with carbon, nitrogen, and iron cycles, in constructed wetlands and similar systems.
In this study, a self-supplying carbon source constructed wetland (CW) was developed and evaluated. Both the effects of plants (Typha latifolia) and plant fermentation broth on nitrate removal were measured. The results showed that the addition of plant fermentation broth greatly improved the nitrate removal rate. As the ratio of added chemical oxygen demand to influent nitrate (COD Add / NO 3-N ratio) increased from 0 to 3, the nitrate removal rate attributed to the plants increased from 0.09 to 0.29 g N m-3 d-1 , but the proportion of total nitrate removal decreased from 27.3% to 10.7%, and denitrification was always the dominant nitrate removal mechanism. Furthermore, there were strong positive correlations between the COD Add / NO 3-N ratio and the nitrate removal rate, both in unplanted (R 2 = 0.977) and planted (R 2 = 0.996) microcosms. Plant biomass could potentially support a nitrate removal rate of 0.05-0.54 g N m-2 d-1 in self-supplying carbon source CWs.
The volatile fatty acids (VFAs) produced in the process of wetland plant litter (WPL) anaerobic digestion could be used as external carbon sources to enhance the removal efficiencies of oxidized contaminants in constructed wetlands. In this study, the effects of temperature on WPL hydrolysis and VFAs accumulation under neutral and strongly alkaline conditions were explored. In neutral (pH 7.0) fermentation, biotic factors were the leading reasons for WPL hydrolysis, and the maximal SCOD accumulation (2467 mg L-1) occurred at 35 °C with a fermentation time of 20 days. In strongly alkaline (pH 12.0) fermentation, abiotic factors were the leading reasons for WPL hydrolysis, and SCOD concentrations increased with temperature at a given fermentation time. Further investigation showed that biotic release of carbohydrate was more sensitive to temperature change than abiotic release. 25 °C was the optimal temperature for biotic release of carbohydrate, while abiotic release of carbohydrate slightly increased with temperature. From the results of linear regression, strong positive correlation was observed between VFAs production and the total release of carbohydrate. The optimal temperatures for VFAs accumulation under neutral and strongly alkaline conditions were respectively 35 and 25 °C, both with a fermentation time of 20 days, and the VFAs concentrations were respectively 1890.1 mg COD L-1 and 1276.4 mg COD L-1. VFAs produced in all fermentations consisted of acetic, propionic, iso-butyric, n-butyric, iso-valeric and n-valeric acids, with acetic acid being the most prevalent product. The fermentation broth fermented at 35 °C with a fermentation time of 20 days has the highest biological utilizability.
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