Experiments with undisturbed soil samples 2011-2014
Laboratory experiments with undisturbed soil samples 2011–2014
In the laboratory experiments 2011–2014, performed in the laboratories of the Vaasa UAS and the Novia UAS, the purpose was to choose chemicals and treatment conditions prior to the large-scale field experiments. In the experiments, undisturbed soil samples in the shape of cylinders with a volume of about 2.4 litres were used. The experiments were also used to uncover the mechanisms behind the effects of the treatment chemicals.
The experiments were done as column leaching experiments with specialised equipment (see description below) where the soil samples were leached with solutions and suspensions of treatment chemicals and the leachates were recovered and analysed. The chemicals used were ultrafine-grained limestone and slaked lime. The limestone used, consisting largely of calcium carbonate (CaCO3), was Nordkalk’s product FC2,5 with a median particle diameter of 2.5 µm. This product was later renamed C2. The slaked lime used, consisting of calcium hydroxide (Ca(OH)2), was Nordkalk’s product SL 90 T where 96.3 % of the particles have a diameter < 90 µm. The concentrations used, and chosen based on results in preliminary experiments, were for the limestone a suspension of 10 grams/litre, and for slaked lime a saturated solution (lime water with a concentration of about 1.7 grams/litre of calcium hydroxide). The soil samples were taken next to the experimental field at Risöfladan.
Soil sampling in the field
Sampling of the undisturbed soil samples used in the experiments was done by pressing plastic tubes with an outer diameter of 16 cm (inner diameter 14.2 cm) with an excavator into the soil after first removing the plough layer (Fig. 1). The tubes were retrieved by first removing the soil besides them, after which the filled tubes could be lifted (Fig. 2). This type of sampling was successful in the spring or autumn when there was enough water in the ground. Water functions as a lubricant and so a sampling during a dry period fails as the friction is too large and soil material is pressed away instead of entering the tube. In the laboratory, a length of 15 cm was cut from the tube at the desired depth, usually 70–85 cm below ground level, and the cylindrical soil sample was carefully pressed out of the tube and was ready for use in the experiments (Fig. 3). The undisturbed soil sample was not mixed or homogenized but retained its original structure in the form of cracks, pores, and more compact inner parts of the structure.
The equipment used in the leaching experiments
In the laboratory experiments, a Concell apparatus originally intended for geotechnical measurements of water permeability made by the Geo-Petech company (Naantali, Finland) was used. In the bottom piece of the equipment, tube connections are available that was used for ingoing treatment solutions/suspensions as well as outgoing leachate via the corresponding piece on top of the sample. The soil sample was enclosed in a rubber membrane that was pressed against the soil sample by pressurized water (maximum 1.5 bars) to prevent bypass flow on the side of the sample (Fig. 4). The treatment solution/suspension flowed using hydrostatic pressure through the cracks and pores in the soil, from the bottom to the top (Fig. 5). The outgoing leachate passed a flow-through cell for immediate measurement of pH, conductivity and redox potential using an YSI Professional Plus meter with a Quatro cable. Samples were also taken for further analyses.
Detailed results from the column leaching experiments have been described by Wu et al. in the article Impact of mitigation strategies on acid sulfate soil chemistry and microbial community published in the journal Science of the Total Environment in 2015. Here only a few of the results from the experiments with ultrafine-grained limestone will be mentioned.
The experiments were always initiated by rinsing the soil sample with several litres of water. The pH value of the leachate stabilised around 4 and the conductivity sank as ions in the hydrologically active macropores were flushed out. After the initial rinsing, the treatment phase with a limestone suspension commenced. The pH value in the leachate rose to 6 and remained stable there even though the ingoing liquid was switched from suspension to water. The fact that a treatment with a limestone suspension could increase the pH of the leachate by two units gave reason to be optimistic regarding the method’s usefulness in the treatment of acid sulfate soils in a larger scale. The conductivity and the sulfate concentration in the leachate rose as the treatment commenced but started to sink back soon.
Based on the results of chemical analyses of the leachate from column leaching experiments with pure water and thermodynamic models, the hypothesis was presented in the article mentioned above that the pH value in the leachate (and in the drainage water under field conditions) was buffered around pH 4 by the equilibrium between jarosite (KFe3(SO4)2(OH)6) and schwertmannite (Fe8O8(OH)6SO4) (Reaction 1). These minerals are both commonly found on crack and pore surfaces in acid sulfate soils.
When a soil is treated with a limestone suspension, the jarosite starts to decompose leading to the observed increase in conductivity and sulfate concentration.
In cooperation with a group at Linnaeus University, the soil microbe populations were characterised using 16S rRNA gene sequencing, and the effects of the treatments in laboratory-scale experiments could be studied with respect to microbiology. Central findings were: a) bacteria in an acid sulfate soils could be characterised using these methods, b) acidophilic bacteria catalyse the acidification reactions, and c) their activity could be diminished by raising the pH values.
