Background: Achieving nitrogen (N) and phosphorus (P) control in sediment remediation projects by adding natural materials is receiving increasing attention. In this study, different natural materials, including iron-rich clay minerals, calcite, kaoline, sugarcane bagasse and Phragmites detritus, were applied to test their effects on N and P in a mesocosm experiment. Results: Iron-rich clay minerals and Phragmites detritus had obvious advantages for P control; they resulted in almost undetectable levels of soluble reactive phosphorus (SRP) in the water column throughout the experiment. This finding can be explained by their strong P adsorption ability. The high level of available organic carbon in the sediment and water column after sugarcane bagasse addition provided enough electron donors for denitrification and dissimilatory nitrate reduction to ammonium (DNRA), which caused nitrate (NO 3 −-N) deficiency and ammonium (NH 4 +-N) accumulation in the water column. Additionally, this resulted in anaerobic conditions, further fuelling the release of P from iron-bound P induced by anoxia. Thus, the amount of sugarcane bagasse applied should be considered carefully in order to balance denitrification and DNRA as well as P releases. Calcite and Phragmites detritus are recommended as remediation materials for N removal based on their significant promotion of denitrification and limitation of DNRA, which resulted in low NO 3 −-N and NH 4 +-N levels. These results can be explained by the shifts in the functional microbial community composition and abundance after the addition of natural materials. Conclusions: Iron-rich clay minerals and Phragmites detritus are promising sediment remediation materials for P immobilization due to their strong P adsorption ability. Taken together, the selection of sediment remediation materials should consider the N and P coupling relationship, to prevent NO 3 −-N removal from causing P leaching or NH 4 +-N accumulation.

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Purpose Nitrogen (N) and phosphorus (P) are the key elements leading to eutrophication, and it is important to jointly control N and P release from sediments into the water column. Methods Different mixed materials including P sorbent, natural organic carbon (C), and an oxidizing agent were applied in a 1-year pilot-scale experiment. Results The addition of iron-rich (IR) clay and Phoslock agent promoted the formation of iron bound P (Fe(OOH)~P) and calcium bound P (CaCO 3~P ) in sediments, respectively. IR clay offered more advantages in immobilization of phosphorus as refractory P, and the Phoslock agent more effectively reduced the risk of P release into water, which was expressed as a low equilibrium P concentration (EPC 0 ). Mixtures of sugarcane (SU) detritus and IR clay exhibited high carbohydrate (CHO) contents, which further fuelled both denitrification and dissimilatory nitrate reduction to ammonium (DNRA). This indicated that the SU dosage should be controlled to avoid DNRA over denitrification. Attention should be given to the fact that SU introduction significantly promoted the generation of an anaerobic state, leading to the desorption and release of Fe(OOH)~P, which could be alleviated by using Oxone. Multienzyme activity analysis showed that P and N transformation shifted from P desorption to organic P hydrolysis and from ammonification to denitrification and DNRA, respectively. Conclusion We recommend the use of P sorbent and organic C combined with oxidizing agents as effective mixed materials for sediment remediation, which could enhance P adsorption and provide electron donors for denitrification, while also avoiding the generation of anoxia.

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Background Nitrogen (N) and phosphorus (P) control in sediment remediation through adding natural material has been paid more and more attention. In this study, different natural material including iron-rich clay mineral, calcite, kaoline, sugarcane bagasse and Phragmites detritus was applied to test the effect on N and P control in a mesocosm experiment. Results Iron-rich clay mineral and Phragmites detritus had an obvious advantage on P control in terms of almost undetectable soluble reactive phosphorus (SRP) in water column throughout the experiment, which could be explained by strong P adsorption ability. The high available organic carbon in sediment and water column after sugarcane bagasse addition provided enough electron donors for denitrification and dissimilatory nitrate reduction to ammonium (DNRA), which was responsible for nitrate (NO 3 - -N) deficiency and ammonium (NH 4 + -N) accumulation in water column. Also this resulted in anaerobic status, further fuelling P release from iron-bound P induced by anoxia. Thus, sugarcane bagasse application should be considered its dosage in order to balance the denitrification and DNRA as well as P release. Calcite and Phragmites detritus should be recommended as remediation material for N removal according to the significant promotion of denitrification and limitation of DNRA, finally resulting in low NO 3 - -N and NH 4 + -N. These results could be explained by the shift of functional microbial community composition and abundance after natural material addition. Conclusions Iron-rich clay mineral and Phragmites detritus should be regarded as the promising sediment remediation material for P immobilization due to the increase of P adsorption ability. Taken together, the selection of sediment remediation material should combine the N and P coupling relationship, avoiding that NO 3 - -N removal caused the P leaching or NH 4 + -N accumulation.