Distribution And Photooxidation Of Typical Reduced Phosphorus In Lake Taihu

It is critical to understand phosphorus (P) behavior since P chemistry controls key aspects of eutrophication, microbial nutrition, corrosion and other environmental processes. Traditionally, P (+5) is assumed to be the dominant form in the environment. The traditional P cycle was believed to be restricted to P species that are transported in the liquid or solid phase. Little is known about the presence and transfer of the reduced P forms (any P species with an oxidation state lower than+5) in P biogeochemical cycle in lake ecosystems. Recent evidences have identified the presence of reduced P such as phosphine, phosphite or hypophosphite, which are important parts of P cycle. Reduced P could be oxidized to phosphate and then be utilized; moreover, some reduced P could be utilized directly by some microbe. Increasing concerns have recently been given to a new pattern of P biochemical cycle since some reduced P species were identified in natural environment. In this work, the spatial and temporal distribution of phosphine and phosphite in different matrices including atmosphere, water and sediment from Lake Taihu has been discussed in eutrophic lake ecosystems. The main results are as follows:(1) The quantitative evidence of phosphite (HPO32-,+3) from the freshwater matrix correspondent to the typically eutrophic Lake Taihu in China was presented.By ion chromatography coupled with gradient elution procedure, efficient separation of μM levels of phosphite is possible in the presence of mM levels of interfering ions, such as chloride, sulfate and hydrogen carbonate in freshwater lakes. Optimal suppressed ion chromatography conditions include the use of 500 μL injection volumes and an AS11 HC analytical column heated to 30℃. The method detection limit of 0.002 μM (0.062 μg P/L) was successfully applied for phosphite determination in natural water samples with recoveries ranging from 90.7 ± 3.2% to 108 ± 1.5%. Phosphite in the freshwater matrix was also verified using a two-dimensional capillary ion chromatography and ion chromatography coupled with mass spectrometry.(2) The seasonal occurrence and distribution of phosphite in sedimentary interstitial water from Lake Taihu was performed. Phosphite were detected ranging from 0 to 14.32 ± 0.19 μg P/Kg with a mean concentration of 1.58 ± 0.33 μg P/Kg, which accounts for 5.51% total soluble P (TSPs) in surficial sediments (0-20 cm). Spatially, the concentrations of sedimentary phosphite in north areaswere relatively was higher than those in south areas. Generally, phosphite in the deeper layers (20-40 cm and 40-60 cm) showed minor fluctuations, whereas phosphite in the surficial sediments showed more variations. Phosphite concentrations in surficial or core sediments were exhibited as:spring> autumn> summer> winter. Higher phosphite levels occurred in the areas with lower redox (Eh), higher P contents, and particularly metal bonded P such as Al-P and Ca-P (p<0.05), Phosphite might be an important media in P biogeochemical cycle in Lake Taihu. In addition, a significant negative correlation existed between phosphite concentrations and Eh (p<0.05), indicating that the phosphite levels could be related to the lower redox potential of the local environment, which mediated the production and elimination of phosphite in sediments.(3) In the bottom waters, phosphite concentrations ranged from 0 to 18.54 ± 1.14 μg P/L, with a mean value of 2.72 ± 3.40 μg P/L. Meanwhile, the concentrations of phosphite in overlying waters ranged from 0 to 15.5 ± 0.48 μg P/L, with a mean concentration of 4.16 ± 2.67 μg P/L. Phosphite in waters accounted for 8.01% of TSPw, suggesting that phosphite was an important part of P cycle. Generally, higher levels of phosphite were always found in seriously polluted sites, which suggested that the spatial distribution was associated with the environmental background. The concentrations of phosphite in overlying waters were relatively higher than those in bottom waters. Seasonally, the variations of phosphite in waters decreased as:spring> winter> autumn> summer. Light and temperature play an important role in phosphite distribution in waters in Lake Taihu.(4) Indoor imitated experiments showed that phosphite in water samples is affected by multiple factors including microbes, temperature, and light. Samples stored at cold temperatures (4℃ or-6℃) were relatively stable for at least 14 days. Under light, phosphite in waters decreased sharply after 12 hours, with a photooxidation rate of 0.30 mM/(m3·h). By adding biostatic agents, phosphite in samples would keep stable for at least 30 days. In addition, the water samples were acidified or basified presenting a possible phosphite storage alternative in water. Phosphite in interstitial water samples extracted immediately after sample collection remained stable for at least three days.(5) Simulated experiments were carried out to study the influence of major factors on the photooxidation of phosphite in the solutions. The photooxidation of 1 βM phosphite in nitrate solutions followed a pseudo-first-order kinetics. The lower pH values of solution would promote the photooxidation rate of phosphite. The higher concentration of chloride, hydrogen-carbonate and sulfate in the solutions would reduce the phosphite photooxidation process. As for the metal ions, iron would promote the photooxidation process while manganese ion would reduce the phosphite photooxidation. It was demonstrated that phosphate was the main generated product in the phosphite photooxidation reactions. Hydroxyl radical was determined to be formed during the photolysis process of phosphite using isopropanol as molecular probe.(6) The diurnal atmospheric phosphine (PH3) concentrations and fluxes at the water-air interface in Lake Taihu were reported. Both the diurnal and seasonal fluxes of PH3 in Lake Taihu fluctuated significantly. PH3 flux at the water-air interface ranged from -69.9 ± 29.7 to 121 ± 42 ng/(m2-h), with a mean flux of 14.4 ± 22.5 ng m-2h-1. The seasonal PH3 flux (ng/(m2-h)) occurred in the order of summer (35.5 ± 55.6)> autumn (29.6 ± 6.3)> winter (5.38 ± 14.5)> spring (-13.1 ± 35.0). In addition, the PH3 fluxes were positively correlated with water temperature, total soluble phosphorus and soluble reactive phosphorus, while they were negatively correlated with water redox potential. The concentrations of PH3 ranged from 0.15 ± 0.23 ng/m3 to 139 ± 34 ng/m3, with the mean value 33.1 ± 12.9 ng/m3. A similar diurnal variation curve of atmospheric PH3 concentrations was observed during all four seasons, with the maximum level occurring in early morning and the minimum appearing around midday. These findings suggest that light plays an important role in the elimination of atmospheric PH3. The average diurnal atmospheric PH3 concentration (ng/m3) occurred in the following order:summer (45.3 ± 42.7)> autumn (42.6 ± 18.2)> spring (25.8 ± 22.3)> winter (18.6 ± 9.4). A significant positive correlation was found between air temperature and atmospheric PH3 concentration. The mean flux of PH3 in Lake Taihu was estimated at 2.94×105 g/y.