Anoxic sediments as a source of persistent organic pollutants: the role of organic carbon and iron reduction

波罗的海的污染是一个重大的环境问题，有必要了解持久性有机污染物在这一环境中的运输和转化过程。沉积物是持久性有机污染物的重要储存库，但也可能成为这些化学物质进入水柱的来源。持久性有机污染物与有机碳有很强的相互作用，包括固相和溶相。有机碳是沉积物中对持久性有机污染物最有效的吸附剂之一，在溶解阶段，持久性有机污染物可以随溶解性有机碳（DOC）一起运移。因此，调节沉积物中有机碳动态的过程可能是更好地解释持久性有机污染物命运的关键。通常，缺氧沉积物被认为是高有机碳埋藏区。然而，铁结合有机碳可以在缺氧条件下在还原性铁溶解过程中被动员，并从沉积物孔隙水扩散到水柱中。由于持久性有机污染物与有机碳的高度关联，与有机碳同时存在的持久性有机污染物很可能从沉积物中释放到孔隙水中，并与DOC共同扩散到水柱中。因此，污染物的命运和动态与水生系统中氧状态的变化密切相关。这一过程在水生系统中的相关性，特别是在据报道缺氧增加的波罗的海，目前尚不清楚。我们假设沉积物中氧浓度的变化会影响持久性有机污染物在固相和溶相之间的分配。具体来说，在缺氧条件下，与铁结合的有机碳相关的持久性有机污染物将被重新活化到孔隙水中，从孔隙水中化合物可以扩散到水柱中。在这项研究中，我们将研究缺氧对波罗的海哥特兰深海沉积物中POPs和DOC动态的影响。哥特兰深海通常在数年内缺氧。但目前，富氧盐水的侵入使缺氧沉积层暴露在氧气中。这一现象为研究氧气条件变化对有机污染物命运的影响提供了巨大的机会。在这个项目中，计划进行几次活动，对哥特兰深水的沉积物和水柱进行DOC、铁和持久性有机污染物浓度的采样，包括在缺氧条件下的时间和何时将重新建立缺氧条件。此外，通过不同氧条件下的培养实验，定量分析了缺氧沉积物中POPs和DOC的通量。这些结果将有助于(i)了解持久性有机污染物与有机碳的复杂相互作用，以及（ii）量化持久性有机污染物在不断变化的环境条件下的再活化。

Pollution of the Baltic Sea is of great environmental concern and there is a need to understand processes of transport and transformation of persistent organic pollutants (POPs) in this environment. Sediments constitute a significant reservoir for POPs but can also act as a source of these chemicals to the water column. POPs strongly interact with organic carbon, both the solid and dissolved phase. In sediments organic carbon is one of the most efficient sorbent for POPs and in the dissolved phase, POPs can be transported along with dissolved organic carbon (DOC). Therefore, processes that are regulating organic carbon dynamics in the sediments might be a key to better explain the fate of POPs. Conventionally, anoxic sediments are perceived as zones of high organic carbon burial. However, iron-bound organic carbon can be mobilized under anoxic conditions during reductive iron dissolution and diffuse from the sediment pore water into the water column. Due to the high association of POPs with organic carbon, it is likely that concomitant with organic carbon also POPs are released from the sediment to pore water and consecutively co-diffuse with DOC into the water column. Thus, pollutant fate and dynamics are tightly related to change in oxygen regime in aquatic systems. The relevance of this process in aquatic systems, especially in the Baltic Sea where an increase in anoxia has been reported, is currently not known. We hypothesize that the change in oxygen concentration in sediments will affect the partitioning of POPs between solid and dissolved phase. Specifically, at anoxic conditions, POPs that are associated with iron-bound organic carbon will be remobilized into pore water from where the compounds can diffuse into the water column. For this study we will investigate the effect of oxygen depletion on POPs and DOC dynamics in sediments of the Gotland Deep in the Baltic Sea. The Gotland deep is usually anoxic over several years. But currently, an intrusion of oxygen-rich salt water exposes anoxic sediment layers to oxygen. This phenomenon opens great opportunities to study the effect of changing oxygen conditions on the fate of organic pollutants. In this project, several campaigns are planned to sample sediment and the water column at the Gotland Deep for DOC, iron and POPs concentrations, covering the time during oxic conditions and when anoxic conditions will be re-established. Additionally, fluxes of POPs and DOC from anoxic sediments will be quantified by incubation experiments under different oxygen conditions. The results will contribute (i) to understand the complex interactions of POPs and organic carbon and (ii) to quantify the remobilization of POPs under changing environmental conditions.