Impact of application mode on the control of phosphorus release from sediments using zirconium-modified bentonite as geo-engineering material

In this study, the influence of zirconium-modified bentonite (ZMBT) addition, capping, and addition/capping on the transport and transformation of phosphorus (P) in sediments were comparatively investigated using incubation experiments to determine the effect of ZMBT application mode on the controlling efficiency. Results showed that the release of soluble reactive P (SRP) from sediment to the overlying water was effectively intercepted by all the ZMBT treatments. The inactivation of pore-water SRP, diffusive gradients in thin films-labile P (DGT-LP) and mobile P (Mob-IP) in sediment played a pivotal role in the regulation of SRP liberation from the sediment to the overlying water by ZMBT. An application mode change from capping and addition/capping to addition resulted in a decline of the reduction efficiency of overlying water SRP by the ZMBT treatment to some extent. The variation in the reduction efficiency of pore-water SRP and DGT-LP in the uppermost sediment were responsible for the change of the reduction efficiency of overlying water SRP by the ZMBT treatment. A change in application mode from capping to addition/capping and addition caused an obvious increase in the immobilization efficiency of pore-water SRP, DGT-LP and Mob-IP in the lower sediment by the ZMBT treatment. Results of this work indicate that, when the ZMBT capping layer on the top of sediment was completely mixed with the sediment, although the stability of P in the lower sediment obviously increases, the controlling efficiency of SRP liberating from the sediment to the overlying water decreases to some extent. Thus, the repeated addition of ZMBT to form a covering layer on the ZMBT-amended sediment is very necessary for the effective control of sediment-P release to the overlying water. (C) 2019 Elsevier B.V. All rights reserved.