Role of sand capping in phosphorus release from sediment

Abstract

Sand is the simplest capping material proposed to date to contain phosphorus in sediment in situ. This research was carried out to understand the role of sand capping in the speciation and release of phosphorus from sediment. Clean sand was laid on sediment sampled at a eutrophicated lake in one dimensional columns with different capping thicknesses. It could be understood from the experiments that sand capping with a thickness of less than 50 mm reduced the phosphorus concentration by over 85% and was effective in reducing the amount of phosphorus released from the sediment. Thicker sand capping reduces the amount of phosphorus released from the sediment to a greater extent. Since sand capping retards the release of phosphorus from the sediment, a higher fraction of non-apatite P (NA-P) was able to be converted into the less soluble Apatite-P (AP).

Reduction of Internal Phosphorus Load in New Lakes by Pretreatment of the Former Agricultural Soil—Methods, Ecological Results and Costs

“…The sand was added directly on top of the undisturbed soil, without removing the groundcover. A sand-cap of approximately 6 cm, corresponding to 99 kg•m −2 , was applied since P release has been found not to decrease significantly by further increasing cap thickness [49].…”

“…The sand used in this study had a low content of Fe but may have contained free adsorption sites able to sequester P from e.g., Al or Ca [53], which was supported by the high proportion of P-HCl found in our study. If the P adsorbing capacity of the sand was too low to sequester the P mobilized from the soil beneath, P would at some point be expected to diffuse through the sand-cap [49,88]. The effect of depth-plowing was expected to depend on the distribution of P through the soil profile and to work best on layered soils with P-rich topsoil and deeper layers of mineral soils with low P content and high content of metals with P binding capabilities, e.g., Fe.…”

“…Physical and chemical barriers of capping materials can reduce P release from lake sediments, which is demonstrated with various materials such as sand, P-adsorbing natural soils, calcite, modified clays, and zeolites [46][47][48][49][50]. Physical barriers reduce P release by separating chemical and microbial processes in the sediment and water column by decreasing the diffusive flux of dissolved compounds, preventing resuspension and elevated exchange through bioturbation [38,44].…”

“…The diffusive P release through the sand-cap layer decreases as the cap thickness increases and as grain size decreases. Changes from mobile P to non-mobile apatite associated P have been found in sand-capped sediments, due to extended diffusion time and increased sorption to Ca species [46,49,[52][53][54]. The P binding capacity of sand depends on free adsorption sites of metal compounds and is generally not regarded as high, but specialized materials that immobilize P through specific chemical reactions can decrease the amount of product needed [44,47,53].…”

Suppression of phosphorus release from sediments using water clarifier sludge as capping material

“…Among these approaches, subaqueous capping is regarded as a costeffective method for stabilizing sediments and decreasing the dissolved nutrients in overlying waters. Sea sand is the most commonly used material for subaqueous capping [2]. In recent years, new agents such as modified zeolites [3,4], calcite [5,6], and iron slag [7][8][9][10] have been employed as alternative capping materials.…”