Synergistic adsorption of phosphorus by iron in lanthanum modified bentonite (Phoslock®): New insight into sediment phosphorus immobilization

Shiming Ding, Qin Sun, Xiang Chen, Qing Liu, Dan Wang, Juan Lin, Chaosheng Zhang, Daniel C.W. Tsang

Research output: Journal article publicationJournal articleAcademic researchpeer-review

106 Citations (Scopus)


Iron redox cycle plays a primary role in controlling the mobility of P in sediments. It is crucial to better understand how lanthanum (La) modified bentonite (LMB, Phoslock ®), an increasingly employed capping agent, immobilizes P from sediments by altering Fe redox-coupled P cycling. Batch adsorption experiments found that LMB effectively adsorbed Fe(II) with a capacity of 8.51 mg g −1. Fe(II)-preloaded LMB effectively retained P during a 518-hour equilibration, while up to 16.7% of adsorbed P was release-sensitive in LMB without Fe(II) preloading. A 60-day incubation experiment was performed using sediment cores, with an LMB amendment dosage of up to 200 LMB/P mob (w/w, P mob denotes the amount of mobile P in the surface 40 mm sediment layer). The concentrations of pore water soluble reactive P (SRP) and labile P were measured by high resolution dialysis (HR-Peeper) and by diffusive gradient in thin films (DGT), respectively, at a vertical millimeter scale. They stratified into static layers with extremely low concentration distribution in the top 16–22 mm sediments (mean SRP ≤ 0.28 mg L −1 and mean DGT-labile P ≤ 0.051 mg L −1) and active layers with decreased upward diffusion potential (≤5.85 for SRP and ≤12.7 for DGT-labile P) below the static layer, when the applied dosage reached 60 LMB/P mob. The LMB amendment reduced the pore water Fe and DGT-labile Fe in sediments, while considerable amounts of Fe and Fe-bound P existed in the LMB binding layer (25% of the total P in 200 LMB/P mob treatment). These findings show that the adsorption of Fe by LMB plays a significant role in the stabilization of LMB-bound P, possibly by adsorbing release-sensitive P initially bound to the rhabdophane surface. LMB adsorbed Fe and P were not released until the redox potential decreased to extremely reductive conditions (−150 mV to −300 mV), possibly due to the re-adsorption of Fe and P by LMB. This study reveals synergistic effects of Fe adsorption and provides new insight into the immobilization mechanisms of P by LMB application.

Original languageEnglish
Pages (from-to)32-43
Number of pages12
JournalWater Research
Publication statusPublished - 1 May 2018


  • Capping
  • Diffusive gradients in thin films
  • Iron redox
  • Lanthanum modified bentonite (Phoslock )
  • Phosphorous
  • Sediment

ASJC Scopus subject areas

  • Ecological Modelling
  • Water Science and Technology
  • Waste Management and Disposal
  • Pollution


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