Experiments with undisturbed soil samples 2016-2017
Laboratory experiments with undisturbed soil samples 2016–2017
Column leaching experiments with undisturbed soil samples continued in the PRECIKEM II project during the years 2015–2018. Samples were taken in the same way as in the previous PRECIKEM project in the years 2011–2014 and the experimental setup consisted, as earlier, largely of the Concell equipment made by the Geo-Petech company. Experimental conditions and some peripheral equipment were new, and new answers were also sought in the experiments. Two duplicate experiments were performed simultaneously, and the equipment as well as the treatment suspensions were kept in a refrigerator at 10 °C which reflects the mean temperature in the ground at 70–85 cm below ground level. Instead of hydrostatic pressure, membrane pumps (Grundfoss DME or DDA) were used to pump rinsing water and treatment suspensions through the soil samples. A constant flow could thereby be maintained, and the flow could be adjusted during the experiment if needed. YSI Professional Plus meters together with Quatro cables and flow-through cells were used to continuously measure pH, conductivity, and redox potential in the leachates during the whole experiment. Measurement values were immediately transferred to a computer. In the experiments, effects on microbe populations were of special interest, and the suspensions were therefore prepared with autoclaved water (steam sterilisation at 120 °C) to minimise the risk for contamination.
In the treatments, suspensions of ultrafine-grained limestone (Nordkalk’s product C2 with a median particle diameter of 2.5 µm), precipitated calcium carbonate (Nordkalk’s product Enrich Bio with a median particle diameter of 0.3 µm and treated with a biodegradable dispersing agent to stabilise the suspension), and ultrafine-grained peat (median particle diameter of 20 µm), were used. Of these, the C2 limestone was also used in the previous PRECIKEM project. For the C2 and Enrich Bio suspensions, a concentration of 8 grams/litre of suspended substance was used, while the concentration of suspended peat was 4 grams/litre. Combination treatments were also performed where the soil sample was first treated with a limestone suspension (4 grams/litre) and then with a peat suspension (1 gram/litre).
The leachate was analysed with respect to pH, conductivity, and redox potential in the same way as in the previous PRECIKEM project. Results from treatments with limestone suspensions were also comparable with corresponding results from the PRECIKEM project. Additionally, samples were taken with regular intervals for the analysis of Cl-, NO3-, SO42- and Br- with ion chromatography in the laboratories of the Vaasa and Novia Universities of Applied Sciences. Metal analyses with ICP-OES technique were subcontracted.
Treatment effects on the leachate were favourable. With a C2 or Enrich Bio treatment the pH values rose to near neutral, while the metal concentrations and redox potential came down. In the combination treatments with suspensions of limestone and peat, the pH value did not rise as much, but the metal concentrations and redox potential were low. The combination treatment had a very favourable effect on the microbe populations with an increased diversity. A pure peat treatment had a weak effect on pH value, metal concentrations and redox potential.
The quality of the leachate was of great interest, but so was also the geochemical and microbiological processes taking place a) on surfaces of cracks and other macropores in the soil, and b) in the inner parts of the structure consisting of compact clay. After each column leaching experiment, the soil samples were placed (incubated) in a refrigerator at 10 °C for four weeks, after which samples were taken of the surfaces as well as of the inner compact parts for further geochemical and microbiological analyses.
Geochemical and X-ray analyses (XRF and XAS) showed that sulfur and, to a lesser degree, iron accumulated on the macropore surfaces in the form of the sulfur- and iron-containing minerals jarosite and schwertmannite. As a result of C2 and Enrich Bio treatments, jarosite dissolved and iron reprecipitated as iron hydroxide.
The results showed that the quality of the leachate from a column leaching experiment to a very high degree is determined by processes occurring at the macropore surfaces. Therefore, it can be concluded that methods for mitigating the negative effects of acid sulfate soils should aim at treating these surfaces in the soil. The experiments showed that the surfaces can be treated by pumping suspensions of limestone, precipitated calcium carbonate, and peat through the macropores of soil samples. In the analyses of the composition of the microbe populations, it was shown that the proportion of the acidophilic microbes was reduced at the surfaces while treatments had very little effects in the inner compact parts of the structure.
Detailed results are presented in the article Iron-sulfur geochemistry and acidity retention in hydrologically active macropores of boreal acid sulfate soils: effects of mitigation suspensions of fine-grained calcite and peat published in the journal Science of the Total Environment in 2022, as well as in the article Targeting oxidation sites on boreal acid sulfate soil macropore surfaces mitigates acid and metal release to recipient water streams published in the journal Applied Geochemistry in 2023.
