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

机译：饱和海沙中镭固溶分配的控制

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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 (SCD). Ra is widely used as a tracer of SCD, 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 (K_d = solid-phase Ra/solution Ra) with surface area for size-fractionated sediments (log K_d (L/g)=0.77 [log SA (m~2/g)] + 0.73; r~2 = 0.76). Ra sorption showed no direct relationship with solid-phase Fe or Mn content of the sands, although removal of visible surfkial oxide coatings with dilute acid reduced K_d 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± 410 L kg~1, followed by lepidocrocite (174±21 L kg~1), hematite (75±17 L kg~1), and goethite (20± 8 L kg~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 K_d 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 K_d 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 SCD 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.
机译：Ra的固溶分配确定了海洋砂中孔隙水的溶解Ra组成。因此，吸附控制也影响海底地下水排放（SCD）中Ra的末端成员浓度。 Ra被广泛用作SCD的示踪剂，必须限制渗透性砂土中的吸附控制，以评估Ra地下水活动的时空变化。这项工作提出了在海水（盐度为35）中测量的一些常见的固体吸附剂以及与渗透性海砂和地下河口有关的不同溶液组成的Ra分布系数。粒度分级沉积物的Ra分布系数（K_d =固相Ra /溶液Ra）与表面积具有很强的相关性（log K_d（L / g）= 0.77 [log SA（m〜2 / g）] + 0.73； r〜2 = 0.76）。尽管用稀酸去除了可见的表面氧化物涂层使K_d降低了2到3倍，但Ra吸附与砂中固相Fe或Mn含量没有直接关系。 Ra吸附。水铁矿的Ra吸附系数最高，为1535±410 L kg〜1，其次是纤铁矿（174±21 L kg〜1），赤铁矿（75±17 L kg〜1）和针铁矿（20±8 L kg〜1）。 ）。随着pH值的增加，Ra吸附量显着增加，天然砂的吸附边缘在5-8的pH范围内。在给定pH下，Ra吸附的程度在不同的底物之间有所不同。未观察到溶解的Fe对Ra分配的影响。当海水中含有硫酸盐时，随着Ba浓度的增加，Ra K_d明显增加。与吸附竞争所预期的效果相反。当从ASW中排除硫酸盐时，未观察到Ba浓度的影响，表明重晶石沉淀导致K_d增加。在2至60°C之间，温度对Ra吸附没有明显影响。这项研究的结果表明，少量的固相成分会增加散装砂的Ra吸附能力，并缓冲溶解的Ra浓度（即SCD端基）。对Ra吸附的溶液控制有可能极大地改变排放地下水的Ra组成。考虑到高盐度，高pH条件可能在沉积物-水界面以下的孔隙水中普遍存在，实际的SGD Ra末端成员可能不如包括深层和新鲜地下水在内的汇编所建议的那样可变。

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