Controls on solid-solution partitioning of radium in saturated marine sands

Abstract

Solid-solution partitioning of Ra determines the dissolved Ra composition of porewater in marine sands. Therefore, sorption controls also influence the endmember concentration of Ra in submarine groundwater discharge (SGD). Ra is widely used as a tracer of SGD, and constraining sorption controls in permeable sands is necessary to evaluate spatial and temporal variation in Ra groundwater activities. This work presents Ra distribution coefficients measured in seawater (salinity 35) for some common solid sorbents as well as different solution compositions relevant to permeable marine sands and the subterranean estuary. There was a strong correlation of Ra distribution coefficient (Kd=solid-phase Ra/solution Ra) with surface area for size-fractionated sediments (log Kd (L/g)=0.77 [log S.A. (m2/g)]+0.73; r2=0.76). Ra sorption showed no direct relationship with solid-phase Fe or Mn content of the sands, although removal of visible surficial oxide coatings with dilute acid reduced Kd by a factor of 2 to 3. Synthetic Fe-oxides showed nearly two orders of magnitude difference in Ra sorption. Ferrihydrite had the highest Ra sorption coefficient at 1535±410Lkg−1, followed by lepidocrocite (174±21Lkg−1), hematite (75±17Lkg−1), and goethite (20±8Lkg−1). A marked increase in Ra adsorption was observed with increasing pH, with the sorption edge of natural sands falling within the pH range of 5–8. The extent of Ra sorption at a given pH varied among different substrates. No effect of dissolved Fe was observed on Ra partitioning. A large increase in Ra Kd was evident with increasing Ba concentration when seawater contained sulfate, opposite the effect that would be expected for sorption competition. No effect of Ba concentration was observed when sulfate was excluded from the ASW, indicating that barite precipitation caused the Kd increase. There was no clear effect of temperature on Ra sorption between 2 and 60°C. Results of this study show that minor solid-phase components increase the Ra sorption capacity of bulk sands and buffer the dissolved Ra concentration (i.e., the SGD endmember). Solution controls on Ra sorption have the potential to greatly alter the Ra composition of discharging groundwater. Given that high-salinity, high-pH conditions probably prevail in porewater below the sediment–water interface, the actual SGD Ra endmember may be less variable than suggested by compilations that include groundwater from deep and fresh groundwater.