The field experiment with treatment of sulfide-rich sediment started on September 29, 2020 by dredging sediment from Toby River (Laihianjoki) in Mustasaari, Finland.
The dredging was done with an excavator and about 3 m3 of sediment was lifted from the bottom of the river. The estimated sulfur content in the sediment was 0.65%. Samples for microbiological and geochemical analyses were taken from the fresh, untreated sediment.
Sampling of dredged sediment for microbiological and geochemical analyses.
The sediment was treated on-site with seven different treatments:
- Untreated reference (only mixed)
- Sediment layered with three layers of agricultural limestone (CaCO3) according to Finnish recommendations (10 kg/m3)
- Agricultural limestone (CaCO3) according to Finnish recommendations (10 kg/m3)
- Agricultural limestone (CaCO3) according to Swedish recommendations (%S x 62 kg/m3 à 40 kg/m3)
- Ultra-fine grained CaCO3 (C2 quality by Nordkalk) according to Finnish recommendations (10 kg/m3)
- Ultra-fine grained CaCO3 (C2 quality by Nordkalk) according to Swedish recommendations (%S x 62 kg/m3 à 40 kg/m3)
- Combination of C2 (10 kg/m3) and fine-grained peat (10 kg/m3)
The sediment was mixed with a DIEM concrete mixer and for treatments 3-7, the chemicals were added to the mixture before the treated sediment was put into boxes specifically designed for the experiment. All treatments were made in three replicates.
Mixing of dredged sediment. The mixing was done with a DIEM concrete mixer.
The sediment boxes were moved to the Risöfladan experimental field. The boxes were not covered, and rainwater could irrigate the sediment and the drainage waters were collected at the bottom of the boxes. The drainage waters were sampled on November 5, 2020, for pH, EC, redox, Fe(II) and acidity measurements as well as for multielement and anion analyses. After sampling, the boxes were completely drained and covered to avoid any accumulation of precipitation that could cause freeze damage to the boxes over winter. In the spring of 2021, the boxes will be uncovered, the sediments allowed to be irrigated, and drainage waters will be sampled again.
The experiment will continue for at least a year and at the end of the experiment, soil will be sampled for microbiological and geochemical analyses.
Sediment boxes at the Risöfladan experimental field.
Experimental assessment of the soil characteristics
Sulfide soil is mostly excavated for constructions purposes and replaced by granular soils which have better workability and geotechnical characteristics. The excavated sulfide soils need to deposit in landfills which need maintenance to minimize the environmental impact of sulfide soil waste. One way to reduce the maintenance cost of these soils is to reuse them in different applications especially geotechnical applications. Improvement of the geotechnical and environmental properties of these soils is therefore necessary in order to reuse them in other applications. However, the mechanical properties of sulfide soils such as low bearing capacity and workability are combined with the soil’s acid producing capacity.
The characteristic of sulfide-rich soil from two different sites (Umeå and Skellefteå in Västerbotten, Sweden) is examined (Figure 1).
The soil characteristics include water content, particle density, specific weight, consistency limits (plastic and liquid limit using fall cone test) and particle size distribution (Figure 2).
Figure 1. Sulfide soil sample.
Figure 2. Dried sulfide soil sample for measuring water content.
Figure 3. Uniaxial test sample preparation (1) layers of compaction (2) hammer.
The soils are treated with two different type of binders (lime and mesa-kalk a waste from the pulp and paper industry) to improve geotechnical characteristics of the soil including strength and workability. Three different dosages of amendment (4, 8, 16 %) are used and the samples are cured for 7, 14, 28 days after compaction in 3 layers (Figure 3).
The uniaxial compression test is used to derive the unconfined compressive stress (UCS) of soil specimens, i.e. the maximum amount of stress that on sample can bear when the confiding stress is zero (Figure 4).
The results are evaluated to analyze the effect of type of binders, amount of binders and curing time on geotechnical characteristic of the different sulfide soils.
Stabilized sulfide soil which meets the standard mechanical characteristics, i.e. most importantly strength needed for construction material, can be utilized in road construction, geotechnical applications like noise barriers alongside of the roads etc.
Figure 4. Uniaxial compression test.
_____________________
WP3 Laboratory experiment - Mitigation of the oxidation of sulfide rich soil
June 24, 2020, Ida Kronsell (LTU)
Amendment of sulfide rich soil with lime and charcoal – laboratory experiment started in May 2020.
Sulfide rich soil collected in Umeå from a bridge construction site, was treated with different types of charcoals and limes with the objective to mitigate the effect of sulfide oxidation i.e. to neutralize the acid leachate and/or reduce the amount of trace elements leached. Changes in pH, electrical conductivity, redox and trace element content are monitored in leachate from treated as well as untreated soil.
Start-up of WP3 laboratory experiment - Mitigation of the oxidation of sulfide rich soil. A) Ultra-fine grained CaCO3 (C2) being thoroughly mixed with sulfide soil. B) A lysimeter being filled with sulfide soil treated with biochar.
The treatments used are:
- Untreated reference
- Garden limestone (CaCO3)
- Ultra-fine grained CaCO3 (C2 quality by Nordkalk)
- Mesa lime
- Biochar (<8 mm)
- Charcoal (<8 mm)
The amendment was mixed thoroughly into the soil by hand and then the soil was put into small lysimeters specifically designed to drain and collect the leachate at the base. All treatments were made in triplicates. The soil mixtures are irrigated once a week with one liter of ultrapure water and the leachate that is formed is collected at the bottom of each lysimeter. The leachate is sampled once a month for pH, EC, redox and metal concentrations. The experiment will continue for at least a year. At the end of the experiment the stabilizing effect of the different treatments will be evaluated.
Treated sulfide soil from Umeå. Treatments from left to right: biochar, mesa lime, charcoal.
_____________________
WP5 - Laboratory experiment with dredged sulfide-rich sediment
June 12, 2020, Eva Högfors-Rönnholm (Novia UAS)
The laboratory experiment with treatment of sulfide-rich sediment started on December 2, 2019 by dredging sediment from Toby River (Laihianjoki) in Mustasaari, Finland.
The dredging was done with an excavator and about 400 liters of sediment was lifted from the bottom of the river. The sediment was transported to Novia for treatment. The estimated sulfur content in the sediment was 0.65%. Samples for microbiological and geochemical analyses were taken from the fresh, untreated sediment.
The sediment was treated with six different treatments:
- Untreated reference (only mixed)
- Agricultural limestone (CaCO3) according to Finnish recommendations (10 kg/m3)
- Agricultural limestone (CaCO3) according to Swedish recommendations (%S x 62 kg/m3 → 40 kg/m3)
- Ultra-fine grained CaCO3 (C2 quality by Nordkalk) according to Finnish recommendations (10 kg/m3)
- Ultra-fine grained CaCO3 (C2 quality by Nordkalk) according to Swedish recommendations (%S x 62 kg/m3 → 40 kg/m3)
- Combination of C2 (10 kg/m3) and fine-grained peat (10 kg/m3)
The treatments were mixed in thoroughly into the sediments with an electrical mixer before the treated sediment was put into boxes specifically designed for the experiment. All treatments were made in three replicates. The sediment boxes are irrigated once a week with one liter of ultrapure water and the drainage water that is formed is collected at the bottom of the box. The drainage water is sampled once a month for pH, EC, redox, Fe(II) and acidity measurements as well as for multielement and anion analyses. The experiment will continue for at least a year and at the end of the experiment, soil will be sampled for microbiological and geochemical analyses.
The sediment boxes are stored in a rack in the laboratory in Technobothnia and the box placements were randomized over the treatments. Two boxes without sediment are used as water controls.
_____________________
The meeting was divided over two days starting off with the first steering group meeting where the participants were informed about the project funding and the funders instructions as well as discussed project activities, partner roles, project communication and equality integration, project risk assessment, partnership agreement and the stakeholder representatives for the focus group.
After the steering group meeting, the participants got a tour in the laboratories that are shared by Novia UAS and VAMK in the Technobothnia building. Sten, Eva (Novia UAS) and Pekka (VAMK) also introduced the participants to the laboratory study in WP 5 with treatment of dredged sulfide-rich sediments that started in December 2019 and experimental setup and microbiological, chemical, and geochemical analyses were discussed.
After the tour, the kickoff meeting continued with further discussions about project equality integration and stakeholder representatives for the focus group. A first focus group meeting was decided to be held at Dåva DAC in Umeå on 3–4 June 2020 with a project presentation, excursion, and discussion with the stakeholder representatives. The rest of the kickoff meeting was dedicated to planning the activities in WP 1–5 in more detail for reporting periods 1 to 3 (November 2019 to February 2021).
Participants from all five partners at the kickoff meeting dinner at Restaurant HEJM. From left to right: Åsa Tykesson-Nilsson (Dåva DAC), Christian Maurice (LTU), Eva Högfors-Rönnholm (Novia UAS), Gustaf Sjölund (Dåva DAC), Raili Häggblom (Novia UAS), Sten Engblom (Novia UAS), Mats Åström (LNU), Qi Jia (LTU), Pekka Stén (VAMK) and Ida Kronsell (LTU).
_____________________