Effect Of Exotic Sulfate On Phosphorus Mobilization In The Sediment Of Nanhu Lake In Wuhan

Eutrophication is the most important problem concerning water environment. The water-sediment interface is the transformation-zone between overlying water and sediments, a special and important area of aquatic environment. Sediment is becoming a major source of phosphorus releasing to overlying water, after the external nutrient loadings have been controlled. The input of sulfate in lakes is growing in intensify, and sulfate-reducing bacteria may play an important role in the mineralization of phosphorus in sediments. In this research, Nanhu Lake was selected as the sampling field under indoor simulating condition to investigate the effect of sulfate reduction to sediments on the circulation of phosphorus of eutrophic lakes. From the results obtained, the following conclusions can be drawn:(1) Sulfate reduction could change the physical and chemical properties of Nanhu Lake sediments significantly, and especially had an impact on the pH and Eh that affect the phosphorus release from sediments. In the process of sulfate reduction, the pH of sediments became higher significantly. There was a negative correlation between pH and Eh, The higher the sulfate concentration input into the overlying water, the lower Eh was.(2) Acid volatile sulfide (AVS) and FeS2 acted as the formation of end-products during sulfate reduction in sediments, and the sulfide remained in the sediments as the deposition form. In the early weeks of the experiment stage, the main form of sulfide was AVS. With the chemical reaction happened:FeS+S0→FeS2, AVS was turned to FeS2 which was more stable than AVS in the sediments. A significant correlation was observed between AVS and Eh. AVS decreased with the increase of Eh during the last three weeks.(3) Sulfate reduction index (SRI) increased with higher SO42- concentration. Meanwhile, sulfate reduction bacteria (SRB) play an important role in the organic matter degradation under anaerobic conditions. There was a negative correlation between SRI and organic matter, with sulfate reduction proceeded, and organic matter was consumed and decreased gradually.(4) Sulfate reduction played a critical influence on the phosphorus cycle in water-sediment interface. On the one hand, organic phosphorus was mineralized into inorganic phosphorus due to sulfate reduction and released into the overlying water subsequently. It was worth noticing that sulfate reduction could promote the mineralization of labile organic phosphorus (LOP) significantly. On the other hand, the reaction of Fe (III) reduced to Fe (II) happened as the Eh decreased. The ratio of Fe:P decreased and iron-bound inorganic phosphate released, meanwhile, the formation of AVS and FeS2 made the release of iron-bound inorganic phosphate. Besides, the increased pH of sediments resulted in the competitive adsorption between OH- and phosphate anion and phosphorus existing in PO43- which was hard to be fixed in sediments by adsorption. The content of Ca2-P increased, which was probably caused by the calcium phytate mineralizing into Ca2-P in the sediments.(5) 31P-NMR was used to characterize the molecular structure and relative quantity of phosphorus compounds in the sediment extracts by NaOH-EDTA. The components of phosphorus in Nanhu Lake sediment were mainly orthophosphate, phosphate monoester, phosphodiester and pyrophosphate. Orthophosphate occupied absolute predominance, the proportion of orthophosphate was 84.1-95.5%, and the rank order of biogenic phosphorus species was phosphate monoester>phosphodiester>pyrophosphate. Sulfate reduction significantly promoted orthophosphate of the sediments to release into the overlying water.