Key words: potassium; Simulated acid rain; Leaching; Leaching amount
China Library Classification Number: TF826. 1 Document Identification Number: A Document Number:
Preface to chapter 1
Acid rain is the combination of acidic gases in the air (usually sulfur dioxide and nitrogen oxides) and water phase in the air, which falls to the ground in the form of rain (snow, fog, particulate matter and fine particulate matter). Because its pH is between 0 and 5, it is acidic, so it is called acid rain. Due to the difference of acid gases, acid rain is generally divided into sulfuric acid type and nitric acid type. Usually, the pH value of clean rainwater is 5.6, which is slightly acidic. Besides chemical acid, acid rain also contains many pathogenic and carcinogenic factors, which can induce many diseases and cancers and endanger human beings and the environment.
Chapter II Experimental Part
2. 1 soil sample collection and preparation
Sawtooth sampling method was used for soil samples, and the sampling depth was 1-20cm. Soil samples collected from the field are placed in a cool, dry and ventilated room without special gases (such as chlorine, ammonia and sulfur dioxide). ) and dust pollution. After the sample is crushed, it is spread flat on clean kraft paper, spread into a thin layer, and frequently turned to speed up drying. After the soil sample is slightly dry, the large clods should be crushed to avoid being difficult to grind after forming lumps. After the sample is air-dried, the litter, plant roots, stubble, worms, iron-manganese nodules, lime nodules or stones in the soil are picked out. Take air-dried soil samples and grind them to pass through a sieve with a pore size of 3 mm Put the treated samples into plastic bags and store them on a sample rack to avoid the influence of sunlight, high temperature, humidity or acid-base gas.
2.2 Main reagents
① 48% hydrofluoric acid ② 70-72% perchloric acid ③ 6N hydrochloric acid ④ 70% nitric acid ⑤ 1% hydrochloric acid solution ⑤ NaCl solution: 50.0? G/ml ⑦ potassium standard solution: 25.0? Gram/ml
Accurately weigh 1.9070g of potassium chloride (high-grade pure) dried at10℃ for 2h, and dissolve it in secondary distilled water with constant volume of 1000 ml, that is, the mass concentration of potassium is 1.000 mg/ml.
2.3 Main tools
Z-2000 Atomic Absorption Spectrophotometer (Hitachi, Japan)
Potassium hollow cathode lamp (Shanghai electro-optic)
Self-made leaching device (as shown in Figure 2-2)
2.4 Working conditions of atomic absorption spectrophotometer instrument
According to the measured elements, choose the best instrument working conditions. The working conditions of the potassium instrument are shown in the following table:
Table 2- 1 Working conditions for determination of potassium by atomic absorption spectrophotometer
2.5 Acid Rain Leaching Experiment
2.5. 1 preparation of simulated acid rain
Due to the particularity and imbalance of natural landscape conditions and social and economic development, there are great differences in air pollution and corresponding acid rain components in different regions. In this way, in the process of configuring simulated acid rain, it is necessary to determine which acid rain components are used as reference standards. In order to make the experimental results relatively comparable, acid rain was prepared according to the average concentration of common ions in acid rain from 2007 to 20 12. The average concentrations of SO42-, NO3-, PO43-, Cl-, K+, Na+, Ca2+, Mg2+ and F- ions are 0. 150, 0.045, 0.002, 0. 163, 0.058 and 0.058, respectively.
In the preparation process of simulated acid rain, the first principle to be followed is that other impurities should not be introduced, and the interaction and reaction characteristics of various added components should also be considered. The concentration of each component in simulated acid rain is adjusted by analytical chemicals with analytical purity or above, and F-, Mg2+, K+, NH4+, Ca2+, Cl- and Na+ are replaced by NaF, MgSO4 7H2O, K2SO4, (NH4)2SO4, Ca (NO3) 2 4H2O and CaCl2 respectively.
When preparing acid rain, the concentrations of the above seven components (NH4++, Na+, K+, Ca2+, Mg2+, Cl- and F-) are prepared into high-concentration standby mother liquor in advance, which is diluted to the required concentration when used, and the mixed solution of sulfuric acid and nitric acid is added dropwise to adjust the pH value of acid rain.
2.5.2 Preparation of leaching device
Chapter 3 Results and discussion
3. 1 Leaching law of potassium in acid rain with different pH values and leaching amounts.
In the experiment, the preparation method of acid rain remained unchanged. We adjust the pH value of acid rain by dropping the mixed solution of sulfuric acid and nitric acid (3: 1), and the leaching volume will affect the leaching concentration of potassium. The results are shown in table 3- 1:
Figure 3- 1 Total leaching concentration of potassium in acid rain under different leaching amounts (mg/L)
The leaching concentration of potassium is closely related to the leaching amount of acid rain. As shown in Table 3- 1, when the same soil is leached with acid rain with different pH values, the greater the acidity of the simulated acid rain, the greater the leaching concentration of potassium in the soil. Moreover, when pH≥4.5, the leaching of potassium in soil is slow, but when pH≤3.5, the leaching of potassium in soil will be accelerated. At the same pH value, the leaching concentration of potassium in 200mL volume of leachate is greater than that in 100mL volume of leachate, and at the same leaching volume, the lower the pH value, the greater the leaching concentration of potassium (as shown in Figure 3- 1). In the first sampling, the leaching concentration of potassium in 200mL leaching solution and 100mL leaching solution is the biggest difference, and the leaching concentration of potassium in acid rain with other pH values is not obvious. In addition, the leaching concentration of potassium in 200mL acid rain leaching solution with the same pH value in the same sample is greater than that in 100mL leaching solution.
3.2 Variation of potassium leaching concentration of acid rain with different pH values at the same time.
In the experiment, the preparation method of acid rain remained unchanged. We adjust the pH value of acid rain by dropping the mixed solution of sulfuric acid and nitric acid (3: 1). See Figure 3-2 for the difference of leaching concentration of potassium in the same time interval:
Fig. 3-2 Leaching concentration of potassium by acid rain with different pH values at the same time.
As can be seen from the figure, when the interval is not long, except for acid rain with pH = 4.5, the leaching amount of potassium with other pH values tends to decrease, and the trend is not obvious. This situation is related to the buffering effect of soil: (1) The buffering effect of carbonate minerals: H++CaCO3→HCO3-+Ca2+.
As the reaction progresses, bicarbonate will leach from the soil surface. In this way, the carbonate buffering capacity of the soil may be gradually exhausted, and finally the pH of the soil will also decrease. (2) Buffering effect of exchange alkali: 2H++Ca2+ ... clay =2H+ ... clay +Ca2+
The buffering mechanism of soil exchangeable base on acid is not to neutralize acid and make it disappear in soil, but to store the exchanged acid in soil acid bank. If soil is leached, the ability and value of this mechanism to resist acid erosion on soil will be weakened with the consumption of soil exchangeable base.
When the interval is 20 hours, we can see that the order of potassium concentration difference from big to small is pH 2.5 > pH 3.5 > pH 4.5 > pH 5.5 > pH 6.5.
3.3 Effect of acid rain with different pH values on potassium concentration in leachate with time
The preparation method of acid rain in the experiment is the same as 3.2. For the same soil, the cumulative concentration of potassium increases with time, but when the same soil is leached by acid rain with different pH values, the cumulative leaching concentration of potassium in the soil is different. The relationship between time and cumulative leaching concentration of potassium is shown in the following figure:
Fig. 3-3 Variation of cumulative leaching concentration of potassium with time under acid rain with different pH values.
The above figure shows that when the initial time is about 3 hours, the leaching concentration of potassium in acid rain with pH = 2.5, pH = 3.5, pH=4.5, pH = 5.5 and pH = 6.5 is about 4 mg/L, and the average leaching concentration of potassium increases by 2mg/L with the passage of time. Under other pH conditions, the increase of potassium is very small, and the cumulative leaching concentration of potassium is the largest when pH = 2.5. Under other pH acid rain conditions, the cumulative leaching concentration of potassium in soil has no obvious difference. In real life, acid rain with pH≤2.5 does not exist, and it is in the range of 3.5 to 5.5. Only from the observation of pH acid rain in this range, the cumulative leaching concentration of potassium has an upward trend after 126 hours, but it is not clear how long it will take for potassium in soil to basically leach out. The continuous leaching of potassium from soil will lead to the barrenness and weathering of soil, thus affecting the normal growth of plants. Because the absorption of potassium by plants decreases, the intake of potassium by vegetables decreases, which is harmful to our health. Therefore, the leaching effect of acid rain on soil potassium can not be ignored.
conclusion
(1) The leaching amount of potassium in soil is affected by the pH value of acid rain. The research work in this paper shows that when acid rain with different pH values washes the same soil, the greater the acidity of simulated acid rain, the greater the leaching amount of potassium. The experimental results also show that when the pH of acid rain is ≥ 4.5, the leaching of potassium in soil is slow, but when the pH of acid rain is ≤ 3.5, the leaching of potassium in soil will be accelerated.
(2) The same soil was leached by acid rain at the same pH, and the larger the volume of leaching solution, the greater the concentration of potassium in leaching solution, indicating that the leaching concentration of potassium in soil increased with the increase of acid rain leaching time.
(3) During the 0 ~ 20 hours of simulated acid rain leaching, the leaching amount of potassium acid rain with different pH values did not change regularly. When the interval is 20 hours, the concentration difference of potassium leaching is pH=2.5 > pH 3.5 > pH 4.5 > pH 5.5 > pH 6.5. Further research shows that when the pH value of acid rain is 2.5, the cumulative leaching concentration of potassium changes the most with time, which is significantly different from other pH values. There is no significant difference in the cumulative leaching concentration of pH = 3.5, pH = 4.5, pH = 5.5 and pH = 6.5.
refer to
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About the author: Fang Guozhi, male, Han nationality,1born in August 1984, is an assistant engineer in Xining City, Qinghai Province. Graduated from the Department of Applied Chemistry of Chengdu University of Technology in 2007, majoring in chemical analysis (water quality detection, rock and mineral detection, soil detection).
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