Migration And Transformation Of Inorganic Nitrogen In The Process Of Contaminated Sediment Remediation In Using In-situ Injection Of Calcium Nitrate

Contaminated sediment remediation is an important part for river restoration because it’s an inner source of pollutants. Amongst various sediment remediation technologies, in-situ biological/chemical remediation has attracted wide application and attention due to its good efficiency and less engineering work. In in-situ biological/chemical remediation methods, in-situ injection of calcium nitrate is the most used one. Nevertheless, previous studies have shown that the implementation of calcium nitrate injection could cause rapid release of nitrate and ammonia from treated contaminated sediment. It might have adverse influences on ecosystem of river environment. Thus, it is necessary to study the migration and transformation of inorganic nitrogen in sediment during in-situ calcium nitrate injection for providing theoretical reference for improving this technology and monitoring the asssociated environmental impact.In this study, the diffusion of calcium nitrate after dosing by rake injecting device was investigated in sediments with different particle size distribution and the chemical dosing efficiency was examined. In addition, the transformation and migration of nitrate in the treated sediment was investigated in column setup. The migration of inorganic nitrogen, including nitrate, nitrite, and ammonia, from the treated sediment to the overlying water was further studied in columns with varying the conditions of water level, sediment particle size distribution, and AVS content in sediment.Results showed that the diffusion of calcium nitrate in sediment right after dosing by rake injecting device depended on sediment particle size distrubition. It was observed in 24 to 48 h after the injection operation in coarse sand sediment, calcium nitrate was mainly present in the upper layer; lateral diffusion occurred in a range of 10 cm radius; and chemical dosing efficiency only was 43%. In siltly sand sediment, calcium nitrate was also mainly present in the upper layer; lateral diffusion occurred in a range of 10 cm radius; and chemical dosing efficiency was 78%. In silt sediment, nitrate also mainly concentrated in the middle layer; lateral diffusion occurred in a range of 5cm radius; and chemical dosing efficiency was above 80%.After adding calcium nitrate, the sulfide-driven autotrophic denitrification are the predominant functioning process which contributed 90% consumption of the reduced nitrate. The occurrence of dissimilatory nitrate reduction to ammonium process was also observed accounting 5%-10% comsumption of the reduced nitrate. In this process, higher sediment AVS concentration resulted in more driving force by N2 gas cavities which was generated in sulfide-driven autotrophic denitrification. Thus, nitrate in sediment show the tendency of moving up. Further research showed that when sediment AVS concentration was higher and sediment particle size was larger, it would cause more release of nitrate, nitrate, and ammonia from the treated sediment to the overlying water. Especially, comparing to the scenario with flowing water, much higher inorganic nitrogen concentration was experimenced in the scenario with stagnant water.