1 materials and methods
1. 1 test soil
The subsoil of paddy soil and topsoil of vegetable fields in Taihu Lake area are taken from Mudu Town, Wuxian City, Jiangsu Province, and their basic properties are shown in Table 1.
1.2 experimental method
Effect of 1.2. 1 on soil physical properties
In the (1) dispersion experiment, 8.55g of air-dried yellow soil sample (equivalent to 0. 18 mmol exchangeable cation) sieved by 1mm sieve was weighed to saturate Na+, then washed with dilute NaCl, and 50mlLAS solution was added to make its concentration series be 0 ~1.
(2) Experimental extraction of critical coagulation concentration (CFC)
(3) Prepare LAS solution with concentration ranging from 0 to 500 mg/L in capillary rising height experiment, and measure its capillary rising height in water system with a capillary with diameter of 65438±0mm, and repeat it for 8 times; 1∶ 1 preparation of subsoil suspension, shaking for 4h, and pumping.
Filter and collect soil extract, and the rest steps are the same as above, and measure the capillary rising height of soil solution.
Effect of 1.2.2 on adsorption of toxic substances in soil
(1) In the Cu2+adsorption experiment, 5g (40 ~ 60cm) soil sample under 60 mesh yellow soil was taken, and a certain amount of CuSO4 solution and different amounts of LAS solution were added, and the constant volume was 100ml, so that the water-soil ratio was 10: 1. At the same time, the concentration series of LAS was established. 50? 100mg/L, shaken at 25℃ for 2 hours, centrifuged at 2000r/min for 20 minutes, filtered, and the concentration of Cu2+ in the filtrate was determined by atomic absorption spectrometry.
(2) The adsorption experiment of phenol is the same as above. Put it in an oscillating bottle and add a certain concentration of phenol and LAS standard solution. After constant volume 100ml, make LAS concentration series 0~ 100mg/kg, phenol concentration 20mg/kg, water-soil ratio 1: 10, fully shake and centrifuge.
Effects of 1.2 .3 on soil microorganisms
(1) nitrification and ammoniation experiment: put 30ml nitrate bacteria culture medium into 150ml triangular flask, and then inoculate1:10ml vegetable field underground soil suspension after sterilization. The LAS concentration series is 0~ 150mg/kg. At 28℃, 20 grams of fresh vegetable surface soil samples were weighed and placed in a 500 ml sterilized triangular flask, and 5 ml sterilized 0.2% peptone and 2 ml sterilized ammoniated bacteria liquid culture medium were added to make the concentration series of LAS 0 ~ 50 mg/kg. Adjust the soil moisture to 50%~60% of the maximum water holding capacity with sterilized distilled water, plug it tightly with a cotton plug, and cultivate it in an incubator at 28℃ for 7 ~ 10. Add 1mol/LKCl solution according to the amount of soil: water = 1∶4, oscillate 1h, filter and absorb a certain amount of distilled nitrogen from the filtrate, and measure the ammoniation strength.
(2) Denitrification experiment: Take 10g vegetable subsoil that has passed the 2mm sieve, put it in a 150ml triangular flask, and add a certain amount of KNO3 _ 3 to make the concentration series of LAS be 0~50mg/kg. Then put the triangle bottle into a vacuum dryer, vacuumize, inject nitrogen, repeat several times, put alkaline gallic acid at the bottom of the dryer to absorb residual oxygen, and use methylene blue as an indicator.
1 2.4 Effects on soil microflora and respiratory activities
(1) In the experiment of microbial flora, a certain amount of subsurface soil of vegetable field was taken, and a certain amount of LAS solution was added to make the LAS concentration series of soil samples be 0~ 100mg/kg, and then cultured at 28℃ for 10 day, and then counted and determined by the plate conventional counting method.
Composition of microbial flora.
(2) Microbial respiration experiment: 500g of the same soil samples as above were placed in polypropylene sample bottles with stoppers, so that the concentration of LAS in each bottle was 0~ 100mg/kg series, and absorption bottles containing 0.2mol/LNaOH4ml were placed respectively, and internal caps were added. The outer cover shall be sealed with adhesive tape. The absorption bottle should be replaced regularly and the absorption liquid in it should be poured into the triangular bottle. Add 2 ~ 3ml 1.5mol/L bacl 2 solution and 3~4 drops of phenolphthalein indicator, titrate with 0.2 mol l-1HCl, and use quartz sand equal to soil as blank.
2 Results and discussion
2. Effects of1las on soil physical and chemical properties
Effect of (1)LAS on capillary rising ability of water system
LAS is a kind of surfactant, and its chemical function is first manifested in its influence on the surface properties of water system. This adverse effect is often manifested in its impact on aquatic organisms. It is generally believed that when the surface tension of water drops below 5 0× 10-4N/L, it will affect the breathing of fish gills and fish cannot survive [9]. Figure 1 shows that LAS can significantly reduce the surface properties of aqueous solution within the selected LAS concentration treatment range. In the range of 1~5mg/L, the increase of LAS concentration significantly reduces the capillary rising height of water system, and when LAS is added to 1~5mg/L, the relative reduction range is 1 1.89%. However, in soil solution system, its influence is milder than in water. When LAS reached 10mg/kg, the capillary height decreased significantly, but it did not decrease significantly until 50mg/kg, which may be related to the complex chemical composition of soil system and the buffering performance of soil solution.
(2) Dispersion of LAS on soil colloid Figure 2 shows that at low concentration, with the increase of LAS concentration, the relative dispersibility of soil colloid increases sharply, reaching about 50mg/L, which has the greatest influence on dispersibility, while the further increase of LAS concentration has little effect on turbidity. The dispersion of low concentration LAS on soil colloid may be repelled by its hydrophobic group (active alkyl chain). Colloids between alkyl chains repel each other due to hydrophobic interaction and strong van der Waals force, which is related to the spatial stereo hiding caused by the linear structure of alkyl chains. In addition, after LAS is adsorbed by the positive charge sites of clay particles, the surface electrostatic negative charge increases, especially after LAS forms a bimolecular structure. The electrostatic repulsion between soil clay particles is further enhanced. However, because LAS is essentially an organic salt ion, with the increase of LAS concentration to 50mg/L, the electrolyte effect gradually becomes dominant and the flocculation effect is enhanced, so LAS can reduce the soil dispersion at high concentration?
The influence of LAS shown in Figure 3 on the critical flocculation concentration of soil colloid shows that with the increase of LAS concentration, the critical flocculation concentration of soil particles increases sharply, which proves that the above low concentration LAS promotes the dispersion of soil clay particles. Piccolo also reported similar results, but some studies think that the * * effect of SAS and humic acid can improve the stability of water-stable aggregates, and LAS in soil environment can promote water permeability and soil permeability. This shows that the flocculation effect of LAS on soil particles is related to soil properties.
(3) Effect of LAS on soil adsorption of environmental poisons As can be seen from Figure 4, the effect of adding LAS on soil adsorption of phenol is not obvious. In the range of LAS concentration of 0 ~ 30 mg/L, the adsorption capacity of phenol decreases with the increase of concentration. Therefore, the pollution of LAS in soil will inhibit the adsorption of polar organic molecules, which may reduce the environmental capacity of soil to polar organic poisons. However, when the concentration of LAS is greater than a certain value, the adsorption capacity of phenol in soil remains basically unchanged, which may be due to the fact that after LAS forms a double micelle structure on soil, the outermost layer of soil colloid is hydrophilic benzene sulfonic acid group, and the hydrophilicity of soil colloid is enhanced, which slows down this effect.
The experimental results of the effect of LAS on Cu2+ adsorption by soil are shown in Figure 5. 1.5 mg/L? 50mg/L？ Three concentrations of LAS (100mg/L) did not change the adsorption capacity of Cu2+ in soil, so LAS had no obvious effect on the adsorption of inorganic heavy metal ions in soil.
2.2 Effect of LAS on the Activity of Soil Biological System
Effect of (1)LAS on soil nitrification and ammoniation As can be seen from Figure 6, soil nitrification and ammoniation are quite sensitive to the reaction of LAS. LAS treatment significantly reduced the nitrification activity of soil, but the nitrification intensity did not increase continuously after 5mg/kg. On the contrary, LAS has a continuous inhibitory effect on soil ammoniation, indicating that soil nitrifying bacteria and ammonifying bacteria have different tolerance to LAS. The results showed that the low concentration of LAS (5mg/kg) in the tested soil had a great inhibitory effect on the soil biochemical activity, which was similar to that of Tu et al. (10mg/kg), but eight herbicides had little effect on the soil nitrification at the kg level, and profenofos had a certain effect on the soil nitrification at 10~300mg/kg.
(2) Effect of LAS on soil denitrification Some data show that with the increase of LAS concentration, soil Eh decreases, which makes paddy soil in an unfavorable redox state, thus intensifying the soil anaerobic process and promoting the soil denitrification process.
Figure 7 shows that with the increase of LAS concentration, the intensity of soil denitrification tends to increase, and it becomes obvious when the concentration is as high as 30mg/kg. Therefore, LAS has little effect on soil denitrifying bacteria.
Fig. 7 Effect of LAS on soil denitrification
(3) Effect of LAS on soil microbial flora and quantity As can be seen from Table 2, the addition of LAS has a significant effect on soil microbial flora, which stimulates the growth of bacteria and has a certain effect on fungi in the range of 0~ 100mg/kg.
Inhibition, but no obvious effect on actinomycetes. This shows that there are bacterial populations in the soil that can tolerate high concentration of LAS or use it as carbon source or energy source. The research results of Martimez-Tokedo et al. on profenofos show that it can obviously promote the growth of bacteria in the range of 10~300mg/kg. In the range of100 ~ 300 mg/kg, the number of fungi first decreased and then returned to the control level, while 100 mg/kg LAS inhibited the growth of fungi in the tested soil.
Table 2 Effect of LAS on soil microbial flora and quantity
(The effect of LAS on soil respiration. The effects of different concentrations of LAS on soil respiration intensity are shown in Figure 8. The effects of different concentrations of LAS on soil respiration were all stimulated within one week, and increased with the increase of LAS concentration. This may be related to the growth of some microbial populations stimulated by LAS. However, after the culture period exceeded 65,438+0 weeks, except for the low concentration of 5mg/kg, the respiratory intensity of other treatments was lower than that of the control.
Fig. 8 Effect of LAS on soil respiration
3 Conclusion
LAS reduced the capillary height of soil solution, promoted soil dispersion at low concentration, and showed electrolyte effect at high concentration. For the tested yellow soil, LAS affects the adsorption capacity of polar toxic organic matter in the soil environment, but the adsorption effect on heavy metals is not obvious. LAS has a strong influence on soil organisms, and soil nitrification and ammoniation are very sensitive to LAS stress. LAS stimulates bacteria but inhibits fungi. The effect of LAS on soil respiration is different from that of pesticides and other organic pollutants. Although there may be microbial communities tolerant to LAS in soil, the impact of LAS on soil environment is different from other organic pollutants and needs further study.
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