Research On The Process Of Sulfur And Iron Reduction And Phosphorus Release From Sediments Driven By Algal Degradation

The ecological environment effect caused by algae bloom in eutrophic lakes has been widely concerned.The deposition and decomposition of high density algae could cause the sediment water interface hypoxia quickly,form and maintain a long period of anaerobic and strong reduction environment,and promote the anaerobic biochemical process of water body.Sulfate reduction and iron reduction are two important and interrelated anaerobic reduction processes in lake ecosystem.The increase of organic matter and the change of dissolved oxygen condition will break the original oxidation-reduction balance of iron sulfur after algae bloom outbreak.In addition,the continuous increase of sulfate concentration in fresh water lakes in recent years highlights the influence of algal bloom on the sulfur iron cycle of lakes.In addition,the iron cycle in lakes generally controls the fixation and release of iron bound phosphorus in sediment.Therefore,it was of great theoretical and practical significance to study the effect of algae decay on the process of sulfur,iron reduction and sediment phosphorus release in fresh water lakes.In this study,the dynamic process of sulfur and iron reduction and endogenous phosphorus release in sediment during algae decomposition was systematically studied through indoor simulation experiments.（1）Algae decomposition provided sufficient organic carbon source and anaerobic conditions for sulfate reducing bacteria（SRB）,which significantly promoted the dissimilatory reduction process of sulfate in water.The concentration of sulfate in overlying water decreased rapidly and was reduced to dissolved sulfide(（?）S^2-),followed by（?）S^2-accumulates from overlying water to sediments,or combined with metal ions,which led to the rapid decrease of its concentration in overlying water.At the same time,with the accumulation and decomposition of algae,sulfate reduction in the sediments increased significantly,and mainly occurred in the surface layer（0～2 cm）.（2）Algae decomposition also stimulated the process of iron reduction in water,which was reflected in the rapid increase of free ferrous ion（Fe（II））concentration and the rapid decrease of Fe（III）concentration.It should be noted that the concentration of Fe³⁺in the overlying water of low algae group and high algae group increased from 0.21±0.02 mg/L to 0.18±0.07 mg/L and 0.14±0.07 mg/L,respectively,which indicated the transformation of free iron from overlying water to solid iron form in sediments.On the one hand,Fe（III）can be dissimilated to Fe（II）by iron reducing bacteria（Fe RB）under strong anaerobicreduction conditions（?）S^2-binding;On the other hand,it is likely to be directly related to the redox reaction of（?）S^2-results in the formation of solid-phase iron sulfide,which reduced the free iron storage in water.（3）Algae decomposition promoted the accumulation of solid sulfur,iron forms（acid volatile sulfide（AVS）,Fe S₂and S⁰）in surface sediments,and the content of Fe S₂continued to increase;In accordance with this,the content of solid Fe（II）in sediments also increased with the decomposition of algae,and its average value increased from291.99±28.08 mg/L to 852.79±18 mg/L,from the beginning to the end of the experiment.It was found that 32%of Fe（II）in the sediments existed in the form of Fe S₂at the end of the experiment,indicating that an irreversible iron reduction process occurred in the sediments during the experiment period,which inhibited the reoxidation of Fe（II）and blocked the iron cycle.（4）In the strong anaerobic reduction environment formed by algae decomposition,the blocking of iron cycle resulted in the release of iron bound phosphorus（Fe-P）from sediments to overlying water.The results showed that the concentration of orthophosphate in overlying water and sediment interstitial water increased continuously during the process of algal decomposition,and the content of Fe-P in sediment decreased by 35.4%on the 21st day of algal decomposition,which fully proved that algal decomposition promoted the release of Fe-P in sediment.The results showed that there was a coupling effect between the reduction of sulfur and iron and the release of iron bound phosphorus in the anaerobic sediment environment caused by high-density algae accumulation and decomposition（?）S^2-combines with Fe（II）,or directly reacts with Fe oxides to form AVS and Fe S₂,which were stored in the sediment as solid phase and block the iron cycle.On the one hand,the reduction of sulfur and iron caused the desorption of iron bound phosphorus,on the other hand,it reduced the stock of free Fe（II）in the water body,greatly reducing the ability of re fixation of phosphate,resulting in the release of iron bound phosphorus from the sediment to the overlying water.The results of this study could effectively promote the research on phosphorus release mechanism of algae eutrophic lake sediments,and provide innovative theory and scientific basis for the prevention and control of Lake endogenous phosphorus pollution.