Effect of environmental changes on nitrogen release from sediments in polluted urban river

Water and sediment samples were collected from a polluted river in the old town of Suzhou City.The water in the river is classified as poor quality V according to the National Surface Water Quality Standard of China.We examine the regularity of nitrogen release from sediments under different environmental conditions such as the dissolved oxygen(DO) level,the redox potential status measured as Eh level,the pH value,the temperature,and the dredging depth,using the indoor experimental method.The results show that the contents of ammonia-nitrogen and nitrate materials in sediment samples are 277.03 mg/kg and 1.22 mg/kg,respectively,and these contents are 6.743 mg/L and 1.500 mg/L in the overlying water.The experiments reveal that the favorable conditions for the ammonia-nitrogen release from sediments include the anaerobic environment of DO < 0.5 mg/L,the strong reducing environment of Eh <-108 mV,and the acidic environment of pH < 6.0,as well as the high temperatures; while the opposite behaviors in these conditions are observed for the migration and transformation of NO₃^--N.The analysis of sediment samples collected at different dredging depths of 5 cm,10 cm,15 cm,and 20 cm indicates that the minimum release of nitrogen nutrient is found from the sample of 15 cm depth.The effects of dredging depth,DO,and Eh on the ammonia-nitrogen release in sediments are found to be greater than that of pH and temperatures.Our experimental study show that the ammonia-nitrogen release from sediments will be effectively inhibited if high dissolved oxygen levels in the overlying water as well as strong oxidizing potential status in the sediment can be properly maintained.