| Ecological engineering is one of the main methods to repair the eutrophic lake ecosystem. Using submerged plant to control the water eutrophication and to restore the eutrophic water body has the vital significance. High concentrations of nitrogen, phosphorus and other nutrients cause the eutrophication of water bodies, and phosphorus is the major limiting factor to cause water eutrophication. Calcium is an important nutrient in water and an important element to decide the stability of lake ecological system. The mainly chemical precipitation of phosphorus is aluminum phosphate, calcium phosphate and iron phosphate. Calcium carbonate-phosphorus coprecipitation in water body can maintain phosphorus concentrations at a relatively low level and control the eutrophication of water effectively.In this study, biological pot experiment with Potamogeton crispus and the indoor coprecipitation simulation experiment were carried to investigate the mechanism of the formation of calcium carbonate-phosphorus coprecipitation in leafysphere of Potamogeton crispus. Through the pot experiment, the changes of physico-chemical properties of overlying water and characteristics of coprecipitates on leaf surfaces of P. crispus were undertaken. Under different Ca:P ratio the simulation of CaCO3-P coprecipitates was uesd to find the optimal conditions of coprecipitation and to character these coprecipitates. The main conclusions are as follows:(1) Under adding phosphorus (P0, P1) in overlying water over the sediment of Lake Nanhu with P. crispus, the pH value and Ca+concentrations of overlying water increased, and TP and SRP decreased. After P. crispus photosynthesis in the daytime the pH value in overlying water increased significantly up to9.47; due to the respiration of P. crispus in the nighttime the pH value decreased smally and it was down to8.93. No matter day and night the pH value under adding phosphorus (PI) in overlying water was higher than that under adding phosphorus (PO) in overlying water. Planting P. crispus made P concentration close finally to zero and Ca2+concentrations increase up to22.87mg/L in overlying water. P. crispus provided appropriate conditions for CaCO3-P coprecipitation. Ion product calculated by pH value, concentration of calcium and phosphorus in overlying water showed that the coprecipitates on leaf surface of P. crispus may be Ca4H(PO4)3, Ca5(PO4)3·OH and Ca3(PO4)2.(2) Under adding phosphorus (P0, P0.1and P0.5) in overlying water over the sediment of Lake Nanhu, Lake Donghu, Lake Yuehu and Lake Tangxunhu with P. crispus, the pH value and Ca2+concentrations had a wavy change and TP and SRP decreased. The pH value was high as well as Ca2+concentration was low. Calcium concentration increasd up to30mg/L in overlying water in20d and phosphorus concentration closed finally to zero. The relationship between calcium and phosphorus was not the same in different sediment or different P treatments. Ca2+and TP had very significant positive correlation under different P treatments by using the sediment of Lake Nanhu as the substrate. Under adding phosphorus (P0, P0.1and P0.5) in overlying water, Ca2+and TP had very significant positive correlation in D-PO and D-P0.1treatments to Lake Donghu sediment and Y-PO and Y-P0.1treatments to Lake Yuehu sediment. Ca2+and SRP had very significant positive correlation in Y-PO.5treatments to Lake Yuehu sediment. Ca2+and TP had very significant negative correlation in T-P0.1treatments to Lake Tangxunhu sediment.(3) Under adding phosphorus (P0, P1and P5) in overlying water over the sediment of Lake Nanhu, Lake Donghu, Lake Yuehu and Lake Tangxunhu with P. crispus, the pH value and Ca2+concentrations had a wavy change, too, and TP and SRP decreased. The pH value increased much more under all of adding phosphorus in overlying water and all were above9.0. TP and SRP concentration in overlying water closed finally to zero. At the same time, the pH value was high as well as Ca2+concentration was low. Due to adding tap water (Ca2+concentration was37.5mg/L) as overlying water, Ca2+concentration in the overlying water was all above30mg/L. In case of sediment of Lake Nanhu and Lake Yuehu, Ca2+concentration in overlying water was all above40mg/L. Ca2+concentration increased up to59.44mg/L in the treatment of Y-P1in60d. Comparing with the corresponding Ksp the Ca-P ion product showed that the coprecipitates on leaf surface of P. crispus may be Ca4H(PO4)3, Ca5(PO4)3OH, and Ca3(PO4)2in the pot experiment.(4) Chemical analysis of the CaCO3-P coprecipitates found that phosphorus content in coprecipitates under constant temperature condition was higher than that under normal temperature conditions and phosphorus content in field was as high as1.14g/kg. XRD characterization of the coprecipitates on leaf surface of P. crispus found that the main component in the coprecipitates was calcium carbonate. FTIR characterization found that the absorption peaks of1421cm-1,876cm-1and713cm-1were the specific absorption peaks of calcite. Although it showed that the main component of the coprecipitates was calcium carbonate,564cm-1was the keys asymmetric stretching vibration peak of P-O in phosphoric acid root. This showed there was phosphoric acid root in the coprecipitates. SEM-EDX characterization found that calcium and phosphorus content in the coprecipitates under constant temperature condition were higher than that under normal temperature conditions. The observation of coprecipitates on leaf surface of P. crispus in-situ found that the main component of the coprecipitates was calcium carbonate and phosphorus content is higher. The coprecipitates in normal temperature conditions appeared like dumbbell shape, and then grew in a different direction like a bud. Under constant temperature condition the coprecipitates formed a circle of flowers surrounded by tiny granule. EDX characterization showed that calcium content is very high in the crystal and phosphorus content in some parts increased obviously. All information reflected that the coprecipitates on leaf surface of P. crispus not only contained a lot of calcium carbonate but also had a certain amount of phosphorus. The coprecipitates on leaf surface of P. crispus could be thought as mixed crystals in which calcium carbonate combined with hydroxyapatite.(5) The optimal calcium to phosphorus ratio of the indoor calcium carbonate-phosphate coprecipitation was50mgCa/1mgP and the pH value was9.7at25℃. It was consistent with pH range8-10in overlying water when P. crispus grew in the actual water body. When pH>9.5, partition coefficient was greater than1, PO43-coprecipitated with CO32-obviously, and coprecipitation conformed to logarithmic distribution law. Through XRD and FTIR characterization the coprecipitates of the indoor carbonate-phosphate coprecipitation simulation experiment found that trace components was hydroxyapatite. |
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