Soil Respiration Characteristics and Its Relationship with Environmental Factors of Different Vegetation Types during Growing Season in Dongting Lake

Yan YANG Jie TANG Yongjin LI Yuxi TANG Changzhu LI Lijuan JIANG

AbstractWith the contamination of soil heavy metals becoming more and more serious, remediation has become a hot issue, and it is of great significance to study the remediation effects of heavy metal contaminated soil. The heavy metal contaminated soil was leached with water, acetic acid and citric acid, and the pH value of the soil after leaching was adjusted with 2% lime water. Finally, the pH value and conductivity of the soil leacheate were determined. The results showed that the best leacheate was citric acid for the remediation of soil contaminated with heavy metals Cu, Cd and Pb, and the optimum leaching concentration was 1.0, 0.4, 0.7 mol/L, respectively. For the remediation of soil contaminated with Zn, the best leacheate was acetic acid, and the optimal leaching concentration was 1 mol/L. The leaching effect of lime water on heavy metal contaminated soil was positively correlated with the concentration of organic acid, that is, the leaching effect on heavy metals was better at stronger acidic condition. The conductivity of the soil sample treated with lime water increased with the increase of the concentration of organic acid, and at the same concentration, the conductivity was in the order of citric acid>acetic acid.

Key wordsSoil; Heavy metal pollution; Organic acid; Leaching; Remediation effect; Conductivity

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Received: June 20, 2018Accepted: November 22, 2018

Supported by the Special Fund for AgroScientific Research in the Public Interest of the Ministry of Natural Resources of the Peoples Republic of China (20151108202).

Bing YU (1994-), female, P. R. China, master, devoted to research about land resource evaluation and utilization planning, Email: yubing921@sina.cn.

* Corresponding author, doctoral tutor, Ph.D., devoted to research about land resource evaluation and utilization planning, Email: wukening@cugb.edu.cn.

With the rapid development of industrialization and urbanization, heavy metal pollution incidents occur frequently in China, and the remediation of heavy metal contaminated soil has become an environmental problem that cannot be underestimated. The cultivated land contaminated by heavy metals is nearly 20 million hm2 in China, accounting for about 20% of the total cultivated land［1］. Heavy metal pollution of soil directly causes economic losses to China. The annual reduction of food output caused by heavy metal pollution is more than 10 million t, and the grain contaminated by heavy metals is as high as 12 million t. The total economic loss is at least 20 billion Yuan. At the same time, soil heavy metal pollution results in the decrease of food quality, and the production of cadmium rice in China is increasing day by day, with an annual output of hundreds of millions. In addition, soil heavy metal pollution is also a serious threat to human health.

In China, the soil in many regions is polluted by heavy metals, and emissions from the mining and smelting of "three wastes" (waste gas, waste water and industrial residue) have caused serious heavy metal pollution in the Xiangjiang River Basin and Zijiang River Basin in Hunan Province, posing serious ecological and environmental risks［2-3］. In Zhangshi Irrigation Area of Shenyang, Liaoning Province, more than 30 years of industrial sewage irrigation has caused serious Cd pollution in the soil, and the mobility and bioavailability of Cd in the soil remain high after 10 years of cessation of tillage［4］. The contents of Cd, Ni, Zn and Cu seriously exceed the standards in sewageirrigated area in Sizhuangding, Xinxiang City of Henan Province［5］. The "13th FiveYear Plan" clearly states that the pollution prevention and control action plan should be taken as the key link to implement the classification and prevention of soil pollution, priority should be given to the protection of soil environmental quality and safety of agricultural land, and the supervision of construction land soil environment should be effectively strengthened. According to the results of soil condition survey of the Ministry of Land and Resources of the Peoples Republic of China, the moderately and heavily polluted cultivated land is roughly 33 000 hm2 in area. The Cultivated Geochemical Survey Report in China (2015) issued by the Ministry of Land and Resources of PRC carried out macroplanning of land resources. The theme of the 2015 International Year of Soils was "healthy soil to serve healthy life", meaning that only good soil can truly guarantee food security. Although the soil does not sound, it should be highly valued. The introduction of the Soil Pollution Prevention and Control Action Plan also shows that heavy metal pollution can not be ignored. Therefore, the remediation of heavy metal pollution in soil system cannot be delayed.

Soil leaching can be divided into in situ leaching and ectopic leaching［6］. The leaching mechanism is to combine the leacheate or chemical auxiliaries with the contaminants in the soil, and achieve the aim of contaminated soil remediation through the chemical actions of desorption, chelation, dissolution or fixation of the leacheate［7］. The soil leaching remediation method is economical and practical, which can effectively remove various forms of pollutants, posing no destruction to the basic physical and chemical properties of the soil, and also cause no secondary pollution and environmental pollution［8］. Many studies have been done on soil leaching remediation of heavy metal pollution at home and abroad. Ni et al.［9］ proved that the leaching effects of oxalic acid on heavy metals was in the order of Cu > Zn > Pb. Ke et al.［10］ found that the reaction of EDTA (ethylenediaminetetraacetic acid) with heavy metals was in the fast stage for the first 2 h. Yang et al.［11］ pointed out that the composite solution of citric acidsodium citrate was an ideal Cd and Cu leaching solution. Zhao et al.［12］ pointed out that the leaching effect on heavy metal Cd was in the order of EDTA > EDDS (ethylenediamine disuccinic acid). Ji et al.［13］ proved that multistep leaching had a better leaching effect on heavy metals than in one leaching. The remediation work of heavy metal contaminated soil has been carried out in the Nanjing Institute of Soil Science of the Chinese Academy of Sciences, the Institute of Applied Ecology of the Chinese Academy of Sciences, Sun Yatsen University, and Zhejiang University［14］. JeanSoro et al.［15-16］ believed that biodegradable chelating agents, organic acids, etc. which had a great damage to soil structure should be used instead of EDTA and other leacheates. About 2 billion Euros a year is used for the remediation of contaminated sites in Europe［17］. In the United States, the NPL (Super Fund Project) has been established specially for the remediation of contaminated sites［18-21］.

In this study, the remediation effects of 2 organic acids of the same concentrations on Cu, Zn, Pb, Cd contaminated soil were compared by using organic acids (citric acid and acetic acid) to leach the heavy metal contaminated soil around the Sixian Industrial Zone in Xihuan Road Office, Renqiu City, Hebei Province, so as to find out the optimal concentration of citric acid and acetic acid to get the best remediation of soil contaminated with Cu, Zn, Pb, Cd. In addition, lime water was used to treat the soil samples leached by citric acid and acetic acid, which could make the pH values reach the ranges which were favorable for the growth of crops, so as to provide theoretical guidance for the research on the remediation of heavy metal contaminated soil using organic acid leaching, thereby providing effective and reasonable suggestions for the comprehensive control of cultivated land soil contaminated with heavy metals.

Materials and Methods

Test materials

In 2016, heavy metal contaminated soil samples were collected around the Sixian Industrial Zone in Xihuan Road Office of Renqiu City. The site had had an electroplating plant that discharged a large amount of untreated wastewater, which made the soil heavy metals exceed the standard, making it hard for plants to grow. According to the monitoring results of the environmental monitoring station of Renqiu Environmental Protection Bureau, the hexavalent chromium content in the wastewater discharged from the site was 49.9 mg/L, which was 248.5 times of the national surface water environmental quality standard, and the total zinc content was 140 mg/L, which was 92.3 times of the national standard, both of which led to extremely larger content of heavy metals that had gone far exceeded the standards when going through the ditches.

Test treatment

Collecting soil samples

During the sampling process, soil samples are collected using shoves, pickaxes and bamboo chips with a sampling depth of 0-20 cm. Samples were avoided being taken from the foreign soil and the newly disturbed soil layer, and surface impurities and gravels were removed from the soil samples. The fivepoint sampling method was taken to collect the soil, the soil samples were collected using the quartering method to keep 1 kg of soil in the polyethylene ziplock bag, and the "field sampling site information table" was recorded on the spot［22］. The samples were naturally airdried in the laboratory, and after picking out the stones and other impurities, the soil samples were sieved (1 mm). Then, 24 soil samples were taken using the balance, each of 250 g, and then the soil samples were sent to test the pH value and the contents of Cu, Zn and Cd.

Leaching treatment

(1) The 24 copies of soil samples were leached once using 750 ml of water. The soil samples were put into the centrifuge tube, which was oscillated for 12 h on the oscillator at 150 r/min at constant temperature (25 ℃) after adding water, and the obtained samples were centrifuged for 15 min at 3 000 r/min［23］. The leacheate was filtered with a funnel, and then the filtrate was collected and the soil sample was transferred to a beaker. Each treatment was repeated 3 times, and then marked with labels of number 1-24.

(2) Soil samples of number 1-3, 4-6, 7-9 and 10-12 were leached once using 750 ml of citric acid solution with the concentration of 0.1, 0.4, 0.7, 1.0 mol/L respectively (the molar mass of citric acid was 192 g/mol, and the citric acid solution with different concentrations could be prepared simply by dissolving the pure citric acid into water).

(3) Soil samples of number 13-15, 16-18, 19-21 and 22-24 were leached twice using 750 ml of acetic acid solution with the concentration of 0.1, 0.4, 0.7, 1.0 mol/L respectively (the molar mass of acetic acid was 17 g/mol, and the acetic acid solution with different concentrations could be prepared simply by dissolving the pure acetic acid into water).

(4) Soil samples of number 1-12 were rinsed trice using 0.2% lime aqueous solution, and the soil samples of number 13-24 were rinsed once.

Measuring instrument

The instruments used for the measurement of soil samples were as follows: SHZ digital display water bath constant temperature oscillator, TD5Z desktop low speed centrifuge, atomic absorption spectrophotometer, pH meter with composite glass electrode, DDP210 portable conductivity meter.

Determination of heavy metal content of soil samples

Soil samples of number1-24 were placed in a drying oven for indoor drying, and the temperature was controlled at (35 ± 5) ℃ until drying out. Then, the samples were ground and sieved (0.25 mm). The mixed solution of 10 ml of nitric acid, 8 ml of perchloric acid and 2 ml of concentrated sulfuric acid was prepared for the digestion［24］. Afterwards, distilled water was added to the volumetric flask before oscillation and setting to the constant volume. Finally, the contents of heavy metals in the soil samples were measured using the atomic absorption spectrophotometer.

Determination of pH and conductivity of leacheate

The pH and conductivity of the leacheate were measured by the pH meter with composite glass electrode and DDP210 portable conductivity meter.

Results and Analysis

Remediation effects of organic acid solution on heavy metals Cu, Zn, Pb and Cd

The chemical leaching method of the soil is a remediation technology for heavy metal contaminated soil by using leacheate to leach the heavy metals in the oil into the filtrate, and transferred the solid heavy metals into liquid phase, thereby reducing the content of heavy metals in the soil, and proceeding corresponding treatments to the leacheate. The type of leacheate, the leaching concentration, and soil texture were all factors affecting the leaching effect, and the choice of the type and concentration of the leacheate was particularly important.

When the organic acid was at the concentration of 0.1, 0.4, 0.7, 1.0 mol/L, the leaching effect on heavy metal Cu was in the order of citric acid > acetic acid, and the leaching effect enhanced with the increase of the concentration of organic acid (Fig. 1). Therefore, the best leacheate for the remediation of soil contaminated with heavy metal Cu was citric acid, and the best leaching concentration was 1.0 mol/L.



Fig. 1Cu content in the soil samples after organic acid leaching

After leaching with citric acid and acetic acid once, the leaching effects on heavy metal Zn were as follows: when the organic acid concentration was 0.1, 0.4, 0.7 mol/L, the leaching effect was in the order of citric acid < acetic acid; when the organic acid concentration was 1.0 mol/L, the leaching effect was citric acid > acetic acid, and the leaching effect of acetic acid was the best when the concentration of organic acid was 0.4 mol/L. However, the leaching effect of acetic acid showed an overall complex changing trend, and thus when the concentration was 1.0 mol/L, the content of Zn reached the peak in the filtrate after leaching twice with the acetic acid. When the concentration of organic acid was 1.0 mol/L, there was no significant difference in the content of Zn in the filtrate leached once with citric acid and acetic acid, but the content of Zn in the filtrate with twice acetic acid leaching was much more than that with citric acid leaching (Fig. 2). Therefore, the best leacheate for the remediation of soil contaminated with heavy metal Zn was acetic acid, and the best leaching concentration was 1.0 mol/L.



Fig. 2Zn content in the soil samples after organic acid leaching

Agricultural Biotechnology2019

After leaching with citric acid and acetic acid once, the leaching effects on heavy metal Pb were as follows: when the organic acid concentration was 0.1, 0.7 mol/L, the leaching effect was in the order of citric acid > acetic acid; when the organic acid concentration was 0.4, 1.0 mol/L, the leaching effect was citric acid < acetic acid, and the leaching effect of acetic acid was the best when the concentration of organic acid was 1.0 mol/L. However, citric acid only leached once, and the content of Pb first increased and then decreased, but the difference was not significant, making it hard to select the best leaching concentration, so the content of Pb of the filtrate after twice leaching of acetic acid was analyzed. When the concentration of organic acid was 0.7 mol/L, there was no significant difference in the content of Pb in the filtrate with once acetic acid leaching and citric acid leaching, but the content of Pb in the filtrate with twice acetic acid leaching showed significant differences from that after citric acid leaching. Moreover, the concentration of Pb was the lowest after twice leaching with 0.7 mol/L acetic acid. When the concentration of organic acid was 1.0 mol/L, the difference in the content of Pb was not significant between the filtrate with once, twice acetic acid leaching and citric acid leaching. Thus, when selecting the best leacheate, consideration should be given to the effect of leaching of multiple times, and citric was more proper as the best leacheate (Fig. 3).



Fig. 3Pb content in the soil samples after organic acid leaching

Therefore, the best leacheate for the remediation of soil contaminated with heavy metal Pb was citric acid, and the best leaching concentration was 0.7 mol/L.

When the organic acid was 0.1, 0.4, 0.7, 1.0 mol/L, the leaching effect on heavy metal Cd was in the order of citric acid > acetic acid (Fig. 4). Therefore, the best leacheate for the remediation of soil contaminated with heavy metal Cd was citric acid, and the best leaching concentration was 0.4 mol/L.



Fig. 4Cd content in the soil samples after organic acid leaching

Remediation effects of lime water on heavy metal contaminated soil after organic acid leaching

Soil samples of number 1-12 were leached twice using the 0.2% lime water, and the soil samples of number 13-24 were leached once. Then, the leacheate was filtered with a funnel, and the filtrate was collected and transferred to a beaker. The purpose of treating the soil sample leached with organic acid by lime water was to achieve the acidbase neutralization and adjust the pH value of the soil samples to make it within the pH range which could maintain the normal growth of crops. After treating the leached soil samples with lime water, the content of heavy metals in the filtrate was determined, and the releaching effect on heavy metals was analyzed, making it possible to compare the remediation effects of lime water treatment on heavy metal contaminated soil under different acidic conditions.

The content of heavy metals increased with the increase of citric acid concentration in the filtrate of the oncetwice lime water leached soil samples after citric acid leaching. After lime water leaching, the content of heavy metals in the filtrate increased, suggesting that the leaching effect of lime water on the heavy metal contaminated soil was positively correlated with the concentration of citric acid. The leaching effect on Pb was the most significant, followed by Zn, Cu, Cd, which showed stable changes, and the Cd was at a relative stable stage. After the first leaching of lime water, the content of Cd in the filtrate was close to 0, and after the second leaching of lime water, the content of Cd was 0 (Fig. 5 & 6).

In the filtrate of the first lime water leached soil sample after acetic acid leaching, the content of heavy metals basically increased with the increase of acetic acid concentration, indicating that the leaching effect of lime water on heavy metal contaminated soil was positively correlated with the concentration of acetic acid. The leaching effect of Zn was the most significant, followed by Pb, Cu, which showed stable changes, and the content of Cd in the filtrate after leaching was 0 (Fig. 7).



Fig. 5Heavy metal content in the filtrate of the first lime water leached soil sample after citric acid leaching



Fig. 6Heavy metal content in the filtrate of the second lime water leached soil sample after citric acid leaching



Fig. 7Heavy metal content in the filtrate of the first lime water leached soil sample after acetic acid leaching

As shown in Fig. 5-7, after leaching, the content of heavy metal Cd in the filtrate was almost 0, which was correlated with the solubility of lime water, but the main factor was that the Cd content had been greatly reduced after organic acid leaching, so the content of heavy metal Cd was close to 0, or even was 0, in the filtrate with lime water treatment. Therefore, the leaching effect of lime water on heavy metal contaminated soil was positively correlated with the concentration of organic acid, that is, the stronger the acidic condition, the better the leaching effect on heavy metals.

Effects of lime water treatment on soil pH and conductivity

It is difficult to for the crops to maintain the growth at the soil pH below 4.5. Therefore, it was necessary to adjust the pH value of the soil sample after the organic acid leaching, and it was of important significance for the normal growth and development of crops to determine the pH and conductivity in the filtrate of soil sample after lime water treatment following organic acid leaching.

The pH and conductivity of the soil sample filtrate after the first water washing was determined using a pH meter with a composite glass electrode and a DDP210 portable conductivity meter (Table 1). As shown in Table 1, the pH of soil samples without lime water treatment was mostly in the range of 3.3 to 3.6, all smaller than 4.5 (the critical value for maintaining crop growth)［19］, which was unable to maintain the growth of crops.

Table 1pH value and conductivity of soil sample filtrate after the first water washing

Soil No.pH valueConductivity∥mS/cm

1~33.39.456

4~63.39.903

7～93.39.440

10～123.310.400

13～153.37.866

16～183.37.834

19～213.67.856

22～243.58.278

As shown in Table 2, most of the soil samples without lime water treatment had the pH ranged from 2.3 to 2.4, all smaller than 4.5, and the soil pH decreased with the increase of the concentration of organic acid, in stable changing trend. The analysis on the conductivity of the soil samples with organic acid leaching showed that the conductivity increased with the increase of organic acid concentration, and at the same concentration, the conductivity of the soil sample after organic acid leaching was in the order of citric acid > acetic acid.

Table 2pH value and conductivity of soil sample filtrate after organic acid leaching

Organic acidconcentration∥mol/L

Citrate acid

pHvalueConductivitymS/cm

Acetic acid

pHvalueConductivitymS/cm

0.12.416.5332.45.329

0.42.424.6772.46.234

0.72.325.4662.46.600

1.02.326.8662.47.160

As shown in Table 3, the pH values were ideal in the soil samples with twice lime water leaching following citric acid leaching and once lime water leaching following acetic acid leaching, most above 5.0 (>4.5), which were in the range for the normal growth of crops (5.0-8.0). Moreover, the pH values of the soil samples decreased with the increase of organic acid concentration. The analysis on the conductivity of the soil samples with organic acid leaching showed that the conductivity increased with the increase of organic acid concentration, and at the same concentration, the conductivity of the soil sample after organic acid leaching was in the order of citric acid > acetic acid.

Table 3pH value and conductivity of soil sample filtrate after lime water treatment

Organic acid concentration∥mol/L

Citrate acid

1st leaching2nd leaching

pH valueConductivity∥mS/cmpH valueConductivity∥mS/cm

Acetic acid

pH valueConductivity∥mS/cm

0.15.45.3738.03.0107.64.207

0.43.37.4657.93.3027.54.473

0.72.98.1807.84.0676.75.403

1.02.811.3834.03.7554.37.500

Conclusion

In the remediation of soil contaminated with heavy metals Cu, Cd and Pb, the best leacheate is citric acid, and the best leaching concentration is 1.0, 0.4, 0.7 mol/L respectively. In the remediation of soil contaminated with heavy metal Zn, the best leacheate is acetic acid with the best leaching concentration of 1.0 mol/L. The leaching effect of lime water on the heavy metal contaminated soil is positively correlated with the concentration of organic acid, that is, the leaching effect on heavy metals is better at stronger acidic conditions. Lime water treatment can increase soil pH to maintain the normal growth of crops. The results show that the pH values are ideal in the soil samples with twice lime water leaching following citric acid leaching and once lime water leaching following acetic acid leaching, most above 5.0 (>4.5). The conductivity of the soil samples with lime water treatment increases with the increase of organic acid concentration, and at the same concentration, the conductivity of the soil sample is in the order of citric acid>acetic acid. 

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