Lead contamination of soil and biological evaluation of the effectiveness of progress

Study:

Abstract Starting from the nature of the metal lead, review the distribution of lead in soil, the source of lead pollution of soil, soil lead focuses on the validity of evaluation methods and the effects of plant soil lead bioavailability factors that lead with the full amount of the soil as a soil Environmental quality evaluation standard lead shortcomings, and to carry out the evaluation of soil lead bioavailability study will lead the improvement of soil environmental quality standards provide a more practical basis.

Keywords: lead; soil pollution; bioavailability; evaluation

AbstractStarting from the character of metallic lead, the distribution of lead in soil, the sources of soil lead pollution were reviewed. The evaluation of lead bioavailability and its impact factors were introduced.It also indicated that the use of soil total amount of lead as a soil environmental quality evaluation criteria was in deficiencies, and it would provide a more effective basis for improving soil environmental quality standards to carry out the study on evaluation of soil lead bioavailability.

Keywords:lead; soil pollution; bioavailability; evaluation

1, the nature of the metal lead
Lead is one of the periodic table family IVA pollution elements, atomic weight 207.2, and the nuclear charge of 82 electrons, electron shell structure [Xe] 4f145d106s26p2, electronegativity is 1.55, ionic radius 1.21? Di (Pb2 +) and 0.84? Di (Pb4 +). can be seen as the ions can lead +2 and +4 exist. the +2 oxidation state stability, +4 oxidation state of instability. +4 oxidation state has a strong lead in the oxidation , in the soil environment is not stable. Therefore, lead in soil chemical properties of lead and its compounds involved +2. Lead is not high strength metal, high density (11.34 g/cm3). new lead cut metal luster, but soon become dark gray, due to the oxygen in the air, water and carbon dioxide effect, the surface quickly generate a dense protective layer of alkaline carbonate samples, the metal surface passivation. Lead is used in ancient China first metals, widely used in daily life.

2, the distribution of lead in soil and accumulation of
The average crustal abundance of lead in 16 mg / kg, lead 200 kinds of minerals are the main form of the mineral galena (PbS, 87% by weight), marcasite (PbCO3) and lead sulfate (PbSO4 .) Lead often associated with zinc, copper co [1]. world-wide soil lead levels ranged in the 2 ~ 200 mg / kg, between the value of 35 mg / kg. national soil background values of basic statistics show that China's soil The maximum lead content to 1 143 mg / kg, a minimum of 0.68 mg / kg, an average of 26 mg / kg [2].

Lead content in soil and soil parent materials related. According to Johnson et al (1985) to provide information, gneiss, granite, limestone, sandstone, shale and other lead 10 ~ 50 mg / kg with an average of 16 mg / kg. igneous rocks is generally higher than the lead content of sandstone and limestone and other sedimentary rock, acid rock than basic rock and ultra-basic rocks. developed in the ice water sediment, moraine material, buried loess soil parent materials such as lead content than high. ancient river sediments in the activity of the lead content higher than modern sediments. has been suggested that a change of heavy metals in soil depends more on the rock type, rock parent material differences, rather than the zonal distribution of soil [3].

3 sources of lead contamination in soil
Soil accumulation of lead can be divided into "natural source" and "unnatural source" 2. The natural environment of lead through the crust erosion, volcanic eruptions, tsunamis and other natural phenomenon of forest fires release into the atmosphere. Precipitation of lead The average concentration of 34 ��g / kg, far from the highway and not subject to lead contamination of lead in the new land of snow 0.034 ~ 0.056 ��g / kg between the [4]. constitute the most substantial environmental pollution, the most frequent sources of pollution are human activities , called non-natural source, the total annual global emissions of lead up to the unnatural 349 250 t [5]. Compared with the natural sources, non-natural source of lead emissions of the absolute superiority, they include the following:
3.1 Atmospheric Deposition
Be used as a gasoline explosion agent to join the millions of tons of lead so far as many. The first is a direct consequence of elevated lead levels in air, especially in the industrial area and population, the accumulation area, busy cities and suburbs. The United States An EPA survey showed that 70 of the 20th century city in Los Angeles in the air lead levels 10 times higher than the remote town of [6]. into the atmosphere in the lead the final destination of the ocean and soil. 70% of vehicle exhaust Lead deposition in the road on both sides of the soil [7-9]. Many survey studies have shown that road on both sides of the concentration of lead in surface soil and increased traffic flow is closely related to the global environment caused by leaded petrol is the most important factor in lead contamination [ 10-11]. Xinjiang of China, Beijing, Shanghai, Liaoning, Hunan, Jilin, Guangdong, Shanxi, Shaanxi, Ningxia [12-14], etc. Many of the roads near the soil survey shows that many of the roads on both sides of the soil has been contaminated, The accumulation time and the traffic density and traffic was positively correlated [15]. lead accumulation in the road on both sides of 8 ~ 50 m distance [16], and upwind of the downwind location of the accumulation of more than [17]. into the lead in the atmosphere can be spread to far away places, even in the Greenland ice year-round place, you can still find traces of lead pollution [18].

3.2 sludge, municipal solid waste use
Urban garbage and other solid waste contains many essential plant growth nutrients to give a certain amount of crop nutrients, but contains large amounts of waste pollution of heavy metals [19]. Harrison (1981) pointed out that in the municipal solid waste Lead content in 1 000 ~ 50 000 mg / kg between. Berrow and Webber (1972) collected and analyzed from 42 premises across the UK representative of the sludge taken and found that lead levels in 120 ~ 3 000 mg / kg, mean 820 mg / kg. 16 U.S. cities in the lead content of sludge is 136 ~ 7 630 mg / kg, a mean of 1 450 mg / kg [20]. Patterson (1989) reported in England Longmosete park land Application of sludge 8 t/hm2 for 30 years, the soil contains acetic acid extractable lead is 1500 mg / kg. As the use of solid waste, Beijing, part of the city park lead pollution there, and with the time the park was built garden growing trend [21-22] In addition, fly ash contains a variety of pollution elements in some parts of the Application also led to the accumulation of lead in soil [23 ].

Links to Free Download Center http://www.hi138.com paper sludge also contain more heavy metals, Tiller (1989) summarizes the different countries in the heavy metal content of sludge was found close to the scope of their content, lead The content of roughly 20 ~ 5 300 mg / kg. long-term Application of sludge, will inevitably lead to increased heavy metal content in soil [24].

3.3 Irrigation
Direct use of urban industrial wastewater irrigation can also lead into the soil a lot. Our sample survey showed that in 1991, 6.67% of irrigation water levels of lead [25]. Zhangnai Ming [26-27] wastewater irrigation in Taiyuan years of Research come to sewage irrigation Soil contamination with heavy metals lead to the accumulation over time filling an increasing trend, the annual accumulation of lead increased to 0.67 mg / kg. It is reported that many cities such as Beijing, Taiyuan, silver City, Guixi (Jiangxi), Xi'an, Chengdu, Tianjin, Shenyang and so subject to different degrees due to sewage contamination of irrigation [28-29].

3.4 Mining and metals processing industry
Including the mining of lead and other heavy metals [30], smelting [31], battery industry, glass manufacturing, metallurgy and related businesses generate waste, fuel oil, fuel, combustion of coal, paint, paint, glaze, pharmaceutical, cosmetic chemicals, chemical reagents and other lead products production and use [32-33], but the main source for the fuel and lead smelting, batteries and other industrial pollution. Taolin Lead-Zinc District of Hunan Province in the lead content of rice (1 601 + -106) mg / kg [34]. smelter Tongling Nonferrous metal companies 4.46% of lead in dust, pollution, 2 000 m radius of the soil [35]. rooftop area of Zhejiang Province for a lead-zinc silver tail Mine survey of the surrounding soil pollution is serious lead in the soil [36]. Shaoxing Pb in soil around a battery factory is clean, the control area from 2.2 to 5.8 times [37].

3.5 pesticide and fertilizer use
Some pesticides and pesticide use lead, such as lead arsenate used in the orchard, resulting in the accumulation of lead in soil. By the chemical fertilizer, especially in the industrial by-products into the lead can not be ignored. According to the determination of certain trace lead content of fertilizer elements in 1% up to or even higher [38].

Human activities to increase the soil lead sources, and now has 200 million t per year did not lead to a variety of recycling non-uniform way into the water, air and soil, causing many of the lead content of anomalies [39]. In addition According to F. Pin-amonti results suggest that the use of organic fertilizer also increased the possibility of lead contamination [40].

4 forms of lead in soil and its bioavailability
Bioavailability of soil lead and lead in soil on species distribution. At present, the morphology of lead in soil classification of most of the methods used Tessler, will lead into the soil: water soluble, exchangeable, carbonate, iron manganese oxides, organic matter and residual forms of sulfide [41-42]. on some of our species distribution of lead in soil studies showed that the residue of lead in soil for 35% of the total lead to 75%. China 10 natural soil the distribution of different forms of lead to iron and manganese oxidation state are the highest, accounting for 40% of non-residual 80%; followed by organic sulfides percarbonates state; exchangeable and water soluble lowest Hezhen Li et al [43], Mo fight, etc. [44] of exogenous soluble forms of heavy metals into the soil after the distribution and morphological transformation over time, results showed that lead, after entering the soil mainly in the iron and manganese oxides bound, organic and residual form, and also to the accumulation of certain carbonate phase. Elsokkary et al [45] to lead in soil is divided into exchangeable (EXCH), carbonate bound (CARB), easily reducible (EASR) more difficult to restore the state (MODR), organic and sulfide (ORGS), and residual (RESD). and that lead contamination of soil with low lead content of each part of the order of: RESE> ORGS> CARB> MODR> RESD. high Lead contaminated soil lead content of each part of the order of: ORGS> MODR> CARB> EASR> RESD, or ORGS> CARB> MODR> EASR> RESD.J.pichtel et al [46] studies have shown that the main form of plant uptake of lead Lead for the exchange of state (including water soluble), carbonate phase lead and manganese oxides form of lead under certain conditions, can be absorbed by plants, organic sulfides and residual lead plant lead difficult to use, which Wang Lianping, etc. findings were consistent. bioavailable lead include water soluble, exchangeable and carbonate part of state and manganese oxides form. Liu Xia [47] The results showed that the greatest contribution to the absorption in the form of rape is a carbonate and manganese bound fraction.

5 Evaluation of bioavailability of soil lead
Davies et al [48] for a long time around the mining of lead ores soil studies have shown that the total amount of lead in soil and lead content in radish leaves with a good linear correlation, indicating that under certain circumstances, lead in soil the total amount you can assess their bioavailability in soil. But more Research showed that soil total lead content and crop yield and lower correlation between lead content of lead effectively. Therefore, to clarify the biological effects of lead, in soil lead more meaningful and effective. And the key to the effectiveness of lead in lead selection of extractant effective state, the current Research in this area for many, but the nature of the soil, planting crops, different extraction conditions of the different conclusions, and always did not form a unified standard, originally suggested by 0.1 mol / L hydrochloric acid as the effective extraction of heavy metals in general agents, but on soil not suitable, because it can many non-exchangeable metals are dissolved down. Jiaoli Zhen [ 49] in acid soil compared the 0.05 mol / L HCl, 0.5 mol / L acetic acid, 2% citric acid and 1 mol / L ammonium acetate extraction, and found that they were Baptist's lead content and lead content in brown rice was significantly related, making it difficult to determine the merits of these extraction agents. Jones et al [50] found that with 0.5 mol / L barium chloride extraction of lead in soil have a good effect, and pointed out that this may be due to the radius of Ba2 + and Pb2 + radius similar reason; Liuyun Hui et al [51] compared different extraction agents concluded that NH4Ac, CaCl2, EDTA are brown, cinnamon the better tide of lead in soil extraction solvent, and NaNO3 and 0.1 mol / L HCl acid only soil (brown); He Feng et al [52] in the purple soil of 2.5% HAc extraction is that basically lead to different threshold levels of lead in purple soil to achieve unity. Davies et al [48] correlation analysis shows that 0.05 mg / L EDTA or 5% acetic acid extraction of lead could predict the absorption of lead by crops, the correlation coefficients were 0.89 and 0.83, in recent years by many Researchers use. in neutral and calcareous soil , Lindsay and effective extraction method for trace elements - DTPA method has been widely used, but there are different views on the effect of extraction [53-54].

In summary, the use of lead in soil total amount of soil environmental quality standards as the only national assessment based on different regions of China is difficult to lead in the soil to make an objective assessment of pollution levels, and through evaluation of the effectiveness of soil lead plants, to find a effective broad-spectrum soil-plant species of the extraction of lead agent, with the soil plant available contents of lead as the evaluation index, will lead our country's soil environmental quality standards provide a more practical basis for improvement.

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