00.About the size of the model
Hg is liquid at room temperature. Hg is easy to form Hg vapor and release in soil. In soil, Hg can only exist in shallow layer. It can only migrate about 10cm in vertical direction for 60d, and horizontal migration after 10cm. Therefore, after referring to "master's thesis: Research on migration characteristics of heavy metals in topsoil (Chen Manli, Zhengzhou University, April 2019)", the basic model is cylindrical, with a diameter of 10cm and a height of 10cm (5cm is set below to place high concentration Hg contaminated soil). The schematic diagram is as follows:
Fig. 1. Schematic diagram of Hg vertical diffusion model in soil
According to Chen Manli's model, the tested soil is brown soil (organic matter is about 16.57g/kg, clay is about 26.11%), bulk density is about 1.47g/cm3, pH > 7.5, water content is about 23%. The test temperature is 20 ℃, and pH of the test soil is about 8.16.
1.Establishment of permeability model
Below 10 cm underground (defined as "0 cm ~ - 5 cm" for the convenience of description in the model): mercury ions are uniformly distributed with a concentration of 40 mg / g (prepared by mixing HgCl2 or methylmercury with soil), and ideally, it is not involved in the calculation of Hg fixation [Bacillus subtilis (BS) and earthworm are not considered]. 10 CM BELOW GROUND: Mercury ion diffuses to the ground. According to the Hg ion concentration C1 of each soil layer (1 ~ 10 cm, calculated by 1cm per layer) measured in real time, and the time of Hg diffusion to 10 cm (set as 60d), the Hg migration amount of each layer was calculated according to the formula. Migration quantity (MQ): the mass of heavy metals passing through unit area in unit time, unit: g/(m2·a). The calculation formula is as follows:
In the formula：MQ——Migration of heavy metals，g/(m2·a)； C1——Migration concentration of heavy metals，mg/kg（Real time measurement）； M——soil quality （0.5cm soil layer is taken for calculation at each point），g； A——Cross sectional area，cm2； T——time，d。
Bs (The size of a single cell of Bacillus subtilis is 0.7～0.8×2～3µm. For the convenience of calculation, the size is set to 0.6×3µm.) was uniformly distributed in the tested soil, and the concentration of BS in each soil layer was C2(5×10^7 cells/cm soil) . The measure is based on the following: The weight of soil =cylinder volume × soil bulk density = =ρ×πr2h=1.47g/cm3×3.14×52 cm2×10 cm=1153.95g According to previous experimental experience, 0.5 L of bacterial liquid (1×108 cells/L) can be evenly distributed into these soils. 0.5 L of bacterial liquid was evenly distributed into 10 layers of soil, and the bacterial content of each layer (cm) of soil was 5×10^7 cells.
Single cell surface area of Bs. S1 = 2πr2+πdh=2×3.14×0.32+3.14×0.6×3=6.22µm2 The total surface area of Bs per cm of soil . S2= 6.22µm2×5×10^7=3.11×10^8µm2=3.11×10-4m2 Mercury adsorption by Bs in each centimeter of soil = soil migration（g/m2·a）×S2
The calculation based on the above formula is shown in Table 1:
The organic mercury content in soil is very small. Taking Wanshan mining area in Guizhou Province as an example, the organic mercury content only accounts for 0.74% of the total mercury content. Therefore, in the modeling, we do not consider the mercury ions released from the decomposition of organic mercury by merb, but only consider the fixation of soil original mercury ions by atlcd. The results showed that the rate of H2S production by atlcd was 0.6 nmol / mg · min, and that of DCD was 6.2 nmol / mg · min. Referring to "master's thesis: atlcd and atdcd can enhance the tolerance of Escherichia coli to Cd2 + (Shen jiejie Shanxi University, June 2013)", the immobilization rate of H2S producing enzyme to Cd2 + is about 97%. Considering that mercury ion and cadmium ion have strong affinity for sulfur ion, we assume that atlcd has a fixation rate of 97% for mercury ion. Based on the above settings, the following calculations can be made:
The Hg adsorption capacity of Bacillus in the specified days = ∑ Hg adsorption capacity of BS in each cm soil layer × Hg migration time in the soil layer
Fixed amount of Bacillus subtilis Hg in designated days = Hg adsorption capacity of Bacillus in specified days × 97%
In the model, the total amount of Hg fixation in soil was about 0.5172mg when 0.5L concentration of 1 × 10^7 Bacillus subtilis was applied on 60 days. Because the weight of soil in each layer (CM) in the model was 115g, after converted into 1kg soil, the fixed total amount of Hg in soil was about 4.4970 mg / kg when the concentration of Bacillus subtilis was 1 × 10^7 on 60 days.
According to the above formula and setting, conversion table 2
2.Construction of infiltration + earthworm model
Permeation model Ibid.. Just add an earthworm (set at 0.5 × 5 cm) to the model. Compare the amount of mercury immobilizeed in soil with and without earthworms
The model soil volume =πr2h=3.14×52×10=785cm3
Earthworm volume =πr2h=3.14×0.52×5=3.92cm3
Number of runways that earthworms need to move in the model: 785/3.92 = 200
The speed at which earthworms advance: About 3 cm/s
The time needed for an earthworm to complete a running track (in vertical direction) is 10cm/3 = 3.3s, and the time needed to complete 200 runways is 3.3 × 200 = 660s < 1d. So the activity time of the earthworm in the model can be neglected.
According to Zeng Lingtao et al. (2016, effects of earthworm composting combined with probiotics on soil fertility and microbial characteristics) reported that the amount of probiotics (Bacillus amyloliquefaciens, Pseudomonas fluorescens) in soil can be increased by about 1.4 times when earthworm is composted with probiotics.
So earthworms working with BS could probably increase the number of Bs in the soil by 1.4 times. That is, the immobilizeation ability of mercury in soil is 1.4 times of that of the osmotic model.
In 60 days, a single earthworm and Bacillus (1×108cells/l) could cumulatively immobilize 4.50*1.4=6.30mg of Hg from every kg soil.
Source: Characteristics of heavy metal transport in surface soil (Chen Man-li, Zhengzhou University 2019.04)
Fig2. the concentration of Hg at different time and distance (removing background value)
The depth of vertical migration was 1 cm, 1 cm, 1 cm, 2 cm, 2 cm, 5 cm, 9 cm and 10 cm at 1 D, 3 D, 5 D, 7 D, 14 D, 30 D and 60 D. The migration rate changed little with the increase of migration time from 1D to 7d, increased from 7d to 30D, and decreased from 30d to 60d. Figure 3.6(d) shows a similar migration trend and gives a more accurate picture of the concentration of heavy metals.
As shown in Figure 3, with the increase of time, the migration amount of heavy metal Hg gradually decreases, and with the increase of time, the migration rate is slow. As shown in Fig. 3, at the distance of 1cm, except that the migration amount on 1d is lower than 3d. With the increase of time, the heavy metal Hg migrates 2cm at 1d and 12cm at 60d; as shown in Fig. 3, Hg migrates 1cm vertically at 1d and 10cm at 60d. It can be seen from Fig. 3 that there is migration phenomenon at 12cm before horizontal migration on 60d, and the migration amount of Hg at 1cm is 0.425g / (m 2·a), and that at 12cm is 0.0013g ((m 2·a); according to figure 3, there is migration phenomenon at 10cm before 600 vertical migration, at this time, Hg migration at 1cm is 0.220g/(m 2·a), and Hg migration at 10m is 0.0029g / (m 2·a) As shown in Fig. 3, the migration of heavy metals is unstable from 1d to 7d, and increases and decreases at 1cm and 2cm, and tends to be stable from 14d to 60d, and decreases with the increase of migration distance at the same time. The results showed that there was no significant difference between horizontal and vertical migration rates of Hg, and horizontal migration was higher than vertical migration.
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