In order to study the impact of biochar application into the soil for the emanation coefficient and radon exhalation rate, biochar was applied to the soil in various doses. Then, after the soil stabilization period, samples were taken for laboratory tests and field measurements of exhalation arte were carried out. Based on the obtained results, a discussion was conducted focused on the perspective of environmental changes in the context of radon emission from soil depending on the dose of biochar. A detailed description of the materials and measurement methods used to conduct the research is presented in the following subsections.
Biochar
The biochar incorporated in the field experiment was produced from sunflower husk in the pyrolysis process in the temperature range of 450–550 °C and consist on grains with diameters from 50 μm up to 10 mm. The biochar was characterized by specific surface area of 2.0 m2 g−1 in occupancy of Cu ions and 5.1 m2 g−1 for Ag ions23.
Field experiment
The field experiment was conducted in ten plots, each with a dimension of 1.1 m × 1.1 m, located in Lublin/Felin at the Institute of Agrophysics of Polish Academy of Science. In addition one plot was left without biochar as a reference. The biochar was applied into the soil in April 2018. The soil presented on the fields were classified as Haplic Luvisol with 66% of sand, 23% of silt 11% of clay and 0.91% of organic matter (data for 0–15 cm layer)32. The following doses of biochar were applied for the following fields: 1, 5, 10, 20, 30, 40, 50, 60, 80, and 100 Mg ha−1 (which corresponded to the percentage of biochar per unit mass of soil: 0.05, 0.24, 0.48, 0.95, 1.43, 1.90, 2.28, 2.86, 3.81, and 4.76%, respectively). The fields were kept without vegetation by application of herbicide (Roundup 360 PLUS at 2.5 L ha−1). For the purpose of presented work six fields were investigated: with 0 (control), 20, 40, 60, 80 and 100 Mg ha−1 biochar doses.
Soil sample collection and preparation
The soil samples were collected from five experimental plots, where the biochar was applied into the soil at the doses of 20, 40, 60, 80, and 100 Mg ha−1, and from a control field, where no biochar was applied (denoted as 0 Mg ha−1). Soil samples were collected from each field at five statistically chosen points to get about 2 kg of soil. After collection, the soil was mixed and dried at room temperature for two weeks. One sample for laboratory examinations was prepared from each part of the soil. The sample was a 4.7 cm high and 5.2 cm diameter steel cylinder, as presented in Fig. 1. The volume of each sample was 100 cm3. The bulk density of each sample was evaluated. The soil net weight of each sample was measured by measurements of each sample and subtraction of the weight of the steel cylinder. Next, the cylinders were closed on the bottom with a rubber cap to reduce the radon emanation surface to the size of 19.62 cm2 at the top of the sample.
The example of sample collected from the experimental field and prepared for measurements within the small accumulation chamber in the process of radon emanation assessment.
Total porosity measurements
The total porosity was measured with the weight method after water saturation. First, the samples were dried at 105 °C for 1 h and weighed to measure the total mass of the soil-biochar samples. To assess the total pore volume, the samples were placed in a tray filled with water for 24 h to reach saturation and the weight measurements were repeated. The total porosity η was calculated according to the equation:
$$eta =frac{{V}_{p}}{{V}_{s}}=frac{{m}_{w}}{{rho }_{w}cdot {V}_{s}}=frac{left({m}_{s}-{m}_{d}right)}{{rho }_{w}cdot {V}_{s}}$$
where ms is the mass of a saturated sample, md is the mass of a dried sample, Vs and Vp represent the sample volume (100 cm3) and pore volume, respectively, and ρw is water density. The weight measurements were realized by electronic laboratory balance with the accuracy of 0.1 mg. The total uncertainty of the method was assessed for 5%.
The uncertainty for radon in air concentration measurements were assessed for 18% basing on the data provided by the instrument.
Emanation coefficient assessment
The radon emission from samples in the laboratory environment were measured using an AlphaGUARD instrument equipped with a measuring chamber made of stainless steel, as presented on the left in Fig. 2. The chamber was 11 cm in diameter and 12 cm in height, giving 0.00114 m3 of volume. The measurement of the radon concentration in air was made setting a 1 dm3 min−1 flow rate and 10 min reading cycles. Data were next averaged for 1 h, resulting to one data point represents the average values from six originally measured data points. Measurement for one sample took 20 h. Values of radon concentration in air was registered with the uncertainty for each data point assessed directly by the instrument.
Set up for radon accumulation measurements for assessment of the emanation coefficient in the laboratory environment. Set include the small accumulation chamber with the total volume of 0.00114 m3 (present on the left side) operating with the AlphaGuard instrument (presented on the right).
The emanation coefficient ε was determined based on the Ra-226 activity concentration and radon potential Ω according to the methodology proposed by33 and making an additional assumption that the samples were dried to zero humidity, which implied that no pores filled with water were present within the samples during measurements. The assumption reduces the equation for calculating the time bound exhalation constant.
The emanation coefficient expressed in % was evaluated according to the equation:
$$varepsilon =frac{{Omega }}{{Ac}_{Ra}}cdot 100{%}$$
(2)
where AcRa represents the Ra-226 activity concentration in a soil sample expressed in Bq kg−1.
The AcRa was assessed using gamma spectrometry and a high purity germanium detector according to the methodology described by25. For properly Ra-226 activity concentration in soil assessment the impact of 185.7 keV gammas from U-235 was subtracted after its evaluation basing on the 63.3 keV peak of Th-234.
The main advantage of the proposed method is the ability to assess the emanation coefficient based on short-time measurements (below 24 h) with a cumulative chamber method. Originally, in the methodology developed by33, the assessment requires evaluation of Ω according to the formula:
$$Omega = frac{{a + lambda _{{eff}} cdot C_{{Rn}}^{0} }}{{lambda _{{Rn}} }}~frac{{V_{e} }}{m}$$
(3)
where a (Bq m−3 s−1) represents the slope of the linear fit of radon exhalation rate data series measured in the cumulative chamber, λeff is an effective time constant (s−1), CRn0 is the initial Rn-222 concentration in the accumulation chamber (Bq m−3), λRn denotes the Rn-222 decay constant, Ve describes the effective accumulation volume in the experimental setup (m3), and m (kg) is the mass of the sample.
The effective time constant describes the effective time of the presence of radon exhaled within the experimental set up and is the sum of the Rn-222 decay constant λRn (s−1), the bound exhalation constant λb (s−1) characterizing the sample, and the leakage constant characterizing the accumulation chamber equipment λl (s−1):
$${lambda }_{eff}={lambda }_{Rn}+{lambda }_{b}+{lambda }_{l}$$
(4)
The λb coefficient is dependent on the soil sample porosity according to the simplified equation:
$${lambda }_{b}={lambda }_{Rn}eta frac{{V}_{0}}{{V}_{e}}$$
(5)
where η represents the total porosity of the sample, as the assumption of zero humidity of samples during measurements was made, and V0 represents the volume of the sample.
λl was evaluated experimentally by measuring the Rn-222 decay in the empty accumulation chamber system with a natural radon concentration at the starting point. The most significant compound of total uncertainty for the emanation coefficient was associated with estimation for slope a, CRn0 and with assessment of the total porosity for the samples η. The uncertainty was evaluated using differentiation method.
Radon exhalation rate assessment
The radon exhalation rate (ERn) was assessed according to the methodology presented in25. The field radon exhalation rate was assessed indirectly by measurement of radon concentration in air using an AlphaGUARD instrument equipped with accumulation open-wall chamber placed on the ground as presented on the Fig. 3. The chamber volume was 0.024 m3. The setup measuring parameters were the same as in the case of the laboratory measurements with the small closed chamber but the measuring time was 70 min giving seven data points for each field. Values of radon concentration in air was registered with the uncertainty for each data point assessed directly by the instrument.
Setup for assessment of the radon exhalation rate in the field measurements. The set consist of the accumulation chamber with 0.024 m3 in volume and the AlphaGuard instrument.
The increase of radon concentration in air (CRn) in accumulation box were registered and the linear function was interpolated basing on seven measuring points registered for each experimental field. Basing on the measurement of radon concentration in air the radon exhalation rate could be assessed according to the equation:
$${E}_{Rn}=frac{V}{A}frac{partial {C}_{Rn}}{partial t}$$
(6)
where V/A are the ratio of accumulation chamber volume to area covered by the chamber and is a constant value of 0.2, (frac{partial {C}_{Rn}}{partial t}) is a change of radon concentration in air registered in accumulation chamber in time t and was represented as a linear fit into the experimental data as:
$${C}_{Rn}=acdot t+b$$
(7)
After differentiation we can compute the radon exhalation rate as slope, a of linear fit scaled by 0.2:
$${E}_{Rn}=0.2a$$
(8)
The uncertainty for radon exhalation rate assessment was associated mainly with the assessment of the slope of linear fit ad was calculated as a standard deviation for data point used for linear fits.
Source: Ecology - nature.com
