1. Background

Eutrophication is occurring across the world and is especially harmful in lakes, coastal areas, estuaries, and inland seas where excess nutrients boost primary production and cause algal blooms, that can be hazardous to aquatic life and a nuisance for human recreation and health. Eutrophication often causes oxygen depletion in large areas, so called “dead zones”, that occur when surplus biomass settles on the seafloor and is degraded by microbes in a process that consumes oxygen. The oxygen is supplied by the overlying water, and sometimes by photosynthesizing organisms in the surface sediment. But microbes in sediment can use other chemical agents than oxygen to fuel their metabolism. These include nitrate, manganese, iron, and sulfate (Froelich et al. 1979). Oxygen depleted bottoms, therefore, generate hydrogen sulfide, a toxic gas, that is produced from microbial sulfate reduction. Such anoxic and sulfidic bottoms are wide-spread in the Baltic Sea, that houses the largest anthropogenic dead zone in the world (Diaz and Rosenberg 2008). In these conditions, multicellular life cannot survive. Anoxic sediments also release methane – a greenhouse gas more potent than carbon dioxide – as an end-product of organic matter degradation. In shallow coastal areas, produced methane by microorganisms easily diffuses to the atmosphere where it contributes to global warming (Humborg et al. 2019).

In contrast with oxygenated sediments, anoxic bottoms have an impaired ability to retain phosphate (PO₄^3-, the free ionic and directly bioavailable form of phosphorus). This is a direct effect of oxygen conditions; when oxygen is lacking, formed sulfide can compete with phosphate for binding to iron and manganese (Ingri, Löfvendahl, and Boström 1991; Smolders et al. 2006). Free phosphate can then re-enter the nutrient cycle and be available for primary production. In severe cases, this can generate a feedback loop that supplies itself with phosphate and proliferates when anoxic areas expand, as is the case for the Baltic Sea. This internal loading of phosphate can be the main driver of eutrophication even if nutrient input from land has stopped.

Many techniques to remediate eutrophic ecosystems have therefore focused on stopping this internal cycling of phosphate, preferably by immobilizing it in the sediments using a strong sorbent. The single most utilized sorbent to remediate eutrophic waters is treatment with aluminum. Various forms of aluminum salts have been used for many decades, often successfully, to react with and bind legacy phosphate, primarily in lakes (Huser et al. 2016). A Baltic Sea bay in the Stockholm archipelago was successfully treated with aluminum injection into the sediment, along with land-based efforts to stop external nutrient loading, which together decreased phosphorus and nitrogen concentrations, and, ultimately, plankton biomass (Rydin et al. 2017).

Another method based on phosphate sorption is sediment treatment with Phoslock™ - a bentonite clay enriched with Lanthanum (a rare earth element) that can form insoluble minerals with phosphate (LaPO₄) (Zamparas et al. 2015). Phoslock™ has been used in hundreds of water bodies all over the world, often successfully (Wang et al. 2017).

Sorbents containing calcium are also of interest since insoluble calcium-phosphates such as apatite can be formed in the reaction with phosphate. Our study focused on the new calcium-based sorbent activated limestone (in Swedish aktiverad kalksten (AK)). The AK is a bright gravel-like mineral that is produced by heat treating of so-called marl – a by-product from limestone mining – that is ubiquitous. Our material was collected and heat treated at concrete plants conveniently located on Gotland in the Baltic Sea. The AK has several attractive attributes as a phosphate sorbent for nutrient rich sediments. First, it has a very high sorption capacity for phosphate: 10 g phosphorus/kg_sorbent (Blomqvist et al. 2023) and recently even a higher capacity has been measured; ca 50 g phosphorus /kg_sorbent (Björkman, 2023). This can be put into perspective by comparing it with the sorption capacity of Aluminum chloride solution at ca 10 g phosphorus/kg_solution (Rydin et al. 2016; Schütz, Rydin, and Huser 2017) and Phoslock, also at ca 10 g phosphorus/kg_sorbent (Haghseresht, Wang, and Do 2009). Second, the AK particles have a high settling velocity which makes it suitable for treating deep areas (100 m) that are often out of bounds for powdered materials or sorbents in solution. The particle size can also be modified to change its velocity to reach different depths without losing material through dissolution. Once on the bottom, the AK will disintegrate to many small microparticles with a high surface area available for phosphate sorption. Finally, for the Baltic Sea, AK produced on Gotland is a potentially cheap and sustainable material due to the vicinity of the gigantic amount of feedstock marl that sits on dumps on the island.

Importantly, the AK has already been applied in the field in a Swedish remediation project in 2022. With the aim to study phosphorus retention by AK under real conditions, 30 tons of the sorbent was spread over 10 hectares by helicopter in the Baltic Sea bay Kyrkviken in Gryts archipelago, Sweden, in September 2022. The application method was quick and efficient, and the results are currently being evaluated. Preliminary findings show that the previous phosphate concentration in the bottom water was reduced by 45% following remediation with AK. A final report from the pilot field study is to be published in 2024 (Björkman, 2023).

On top of the sorbents described here, a rapidly increasing number of materials for in situ (on site) immobilization of various contaminants have been developed and tested in laboratory studies. Unfortunately, there is a disconnect between this developmental engineering and real-world application (Chiang et al. 2012). Therefore, the present study aimed to experiment under field-like conditions using intact samples. We tested the AK on nutrient rich sediment sampled at a fish farm on Åland, with the aim to provide the knowledge needed to conduct future on-site studies in a safe, effective, and sustainable manner.