J. Cent. South Univ. (2012) 19: 2620-2626
DOI: 10.1007/s11771-012-1319-4
Chemical fractions and potential mobility of lead in soils irrigated by sewage in Pearl River Delta, South China
HUANG Guan-xing(黄冠星), CHEN Zong-yu(陈宗宇), SUN Ji-chao(孙继朝),
LIU Jing-tao(刘景涛), ZHANG Yu-xi(张玉玺), WANG Jin-cui(王金翠)
Institute of Hydrogeology and Environmental Geology,Chinese Academy of Geological Sciences, Shijiazhuang 050061, China
? Central South University Press and Springer-Verlag Berlin Heidelberg 2012
Abstract: The chemical fractions, i.e., water soluble (WS), exchangeable (Ex), carbonate (Car), weakly organic (WO), Fe-Mn oxide (FMO), strongly organic (SO), residual (Res) fraction, of Pb in irrigated soils in South China were investigated by a modified Tessier sequential extraction technique. The results show that the chemical fraction of Pb in soil is mainly the Res fraction and followed by FMO fraction, and the WS, WO, FMO, and SO fractions in topsoils (0-10 cm) are higher than those in subsoils (30-40 cm). The sum of contents of WS and Ex fractions (SWE) in topsoils is significantly positively related with that in subsoils, indicating the strong mobility of Pb in WS and Ex fractions in soils, and the SWE in soils is higher than the German trigger value for the transfer path soil-plant, indicating the high bioavailability of Pb in soils of this area. Fortunately, SWE and the ratio of WS and Ex fractions (RWE) to the sum of all fractions generally decrease with the soil depth in soil profile and the RWE in soil profile is lower than 0.5%, indicating the low pollution risk for Pb in groundwater. In addition, soil particles, pH and Fe2O3 play an important role in the impact of mobility and chemical fractions of Pb in soils.
Key words: chemical fractions; mobility; lead; irrigated soil; soil profile
1 Introduction
Water demand for industrial, commercial, and domestic uses is steadily increasing. The growing industrialization and urbanization increase water use while supplying wastewater can be used for non-potable purposes, such as agricultural irrigation. The wastewater is becoming a preferred marginal water source, since its supply has the beneficial aspects of adding valuable plant nutrients to soil and the costs associated to this water source are low compared with those of other water sources [1]. In developed countries, the predominant trend in agricultural wastewater reuse is to irrigate treated wastewater [1-2]. On the contrary, most developing countries such as Mexico rely on raw wastewater for agricultural irrigation [3]. However, agricultural irrigation with wastewater can cause heavy metals accumulation in soil [4] and result in heavy metals pollution of groundwater [5].
In recent years, Pearl River Delta has become one of the largest economic zones in China due to the deepening reform and opening policies. With the rapid development of industrialization and urbanization, a lot of wastewater with lead (Pb) has been produced and discharged into surface water in Pearl River Delta. As a consequence, the total Pb concentrations in soils in plenty of agricultural areas are higher than the threshold of natural background in China due to the wastewater irrigation in this region [6]. However, total Pb concentrations give insufficient information on the bioavailability and mobility of Pb in soils, and speciation of Pb as complementary information can help assess the bioavailability and mobility of Pb in soils. Until now, only limited studies utilized the speciation of Pb for accessing the potential mobility of Pb in agricultural soils of the Pearl River Delta [7], and fewer studies utilized the speciation of Pb for accessing the potential mobility of Pb in soils irrigated with sewage in this region.
At present, different sequential extraction techniques, such as the five-step procedure of TESSIER et al [8] and the three-step procedure of BCR [9], are commonly applied [7, 10] to access the potential mobility of Pb in soils. And the modified versions of Tessier sequential extraction procedure [11-12] have become very popular during recent years because they have good inter laboratory reproducibility.
Using a modified Tessier method, the objective of the present work was to 1) determine chemical fractions of Pb in a soil irrigated with sewage in the Pearl River Delta, South China, 2) access the potential mobility of Pb in this soil, and 3) investigate the relationship between soil properties and chemical fractions of Pb.
2 Materials and methods
2.1 Study area and sampling
The study area is located in the south central Pearl River Delta area and it is surrounded by mountains in the north, west and east, and by the South China Sea in the south. Its total area is about 0.2 km2, and it belongs to the Xibei River alluvial plain where river network is relatively well developed. It is reported that the annual average temperature is in the range of 21.7-22.6 °C and precipitation is in the range of 1 714-1 944 mm in this area. The land-use types of the study area consist of agricultural areas, living area, and industrial zone, of which the industrial area is mainly dominated with metal powder treatment plants (Fig. 1), and the agricultural areas are mainly dominated with flowers and vegetables. Part of industrial waste water is directly discharged into surface water, and the agricultural areas are long-term irrigated by this surface water. The upper stratum of the study area consists of Quaternary alluvial sediments and belongs to marine-terrigenous facies. According to our investigation, it is shown that the media of unsaturated zone in research area are dominated with silty clay and mucky silty clay [13].
All the soil samples used in this study area were collected from 14 sampling sites in an irrigation area of the Pearl River Delta (Fig. 1), in which 13 points have two sets of soil samples, respectively (0-10 cm, 30-40 cm), one point is characterized with vertical profile samples in an unsaturated zone (0-60 cm, each sample was collected at continuous intervals of 10 cm), and the total number of soil samples was 32. The soil samples were collected by diagonal method with 0.5 m2. Each soil sample of about 1 kg was collected by stainless steel sampler and loaded in self-locking polythene bags. They were air-dried for 2 d, crushed, passed through a 2 mm sieve, mixed, and stored in the polythene bags before analysis.
2.2 Sample analysis
Soil samples were analyzed for the chemical fraction of Pb, pH, organic carbon and Fe2O3. pH was measured at soil-to-water mass ratio of 1:5. Total organic carbon was determined using dichromate oxidation technique [14], and the organic matter (OM) content was calculated by multiplying the total organic carbon content by 1.72 for soil. Fe2O3 was extracted twice by the HCl-HNO3-HClO4-HF method, and the content of Fe2O3 in the extract was determined using a sequential plasma spectrometer. The chemical fraction of Pb in soil samples was extracted by the sequential extraction method described by TESSIER et al [8] with some revisions, and the reagents employed and shaking periods for the extraction of the fractions of Pb in soil are summarized in Table 1. Furthermore, for the determination of total content of Pb, the soil samples were digested in a mixture of HNO3 and HF (9:1) by microwave heating, and the content of extracted Pb was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) on a Perkin-Elmer 2100DV apparatus. The sum of the seven fractions is 80%-120% of the total content.
Fig. 1 Location of sampling sites
3 Results and discussion
3.1 Soil characteristics
The characteristics of the different statistical parameters (minimum, maximum and mean) of soil properties such as pH, particle size, organic matter (OM), and Fe2O3 contents were reported in the previous study of HUANG et al [12].
3.2 Total contents and fractions of Pb in soils
As it can be seen in Fig. 2, the total contents of Pb in topsoils are above the natural background value (35.9 mg/kg) of soil in Guangdong Province [15], and the mean of total Pb content is 128.6 mg/kg, which is significantly higher than that of agricultural soils in this area [7]. In conclusion, the topsoils of the study area appear to be polluted by Pb to some extent.
From Table 1, Pb in soils is categorized in the seven chemical fractions: Water soluble (WS), exchangeable (Ex), carbonate (Car), weakly organic (WO), Fe-Mn oxide (FMO), strongly organic (SO) and residual fraction (Res). The distribution of Pb in the seven fractions varies greatly among the topsoils (Fig. 2). In general, the order of decrease in the various fractions is Res>FMO>WO>Car>SO>WS>Ex, and a similar order is observed in urban soils of Nanjing City, China [10]. The WS and Ex fractions are considered to be the most mobile and bioavailable phases present in the soils [8], and in topsoils of study area, the WS and Ex fractions account for 0.41% and 0.06% of the total Pb, respectively, which is consistent with the results of other authors [7, 16] who also observed that the WS and Ex fractions of Pb in paddy soils and urban soils of this region were very low. The Car fraction is also considered to be the bioavailable phase present in the soils [8], and the Car fraction is 5.95% of the total Pb in topsoils of study area, which is far lower than the result (about 20%) of other authors [7] who investigated paddy soils and urban soils of this region. Compared with the bioavailable phases, WO, FMO and SO fractions are relatively stable phases in nature, but they can be mobilized under strong oxidizing conditions or reducing conditions due to organic matter degradation and Fe-Mn oxides reduction [8, 12]. In topsoils of study area, the WO, FMO and SO fractions account for 9.3%, 16.49% and 1.92% of the total Pb, respectively. The dominating chemical fraction for Pb in topsoils of study area is Res fraction, accounting for about 65.87% of the total Pb, which is in broad agreement with most previous studies in other irrigation areas [17]. In contrast to most studies, WONG et al [7] reported that the dominant fraction of Pb was the FMO fraction in crop soils of this region.
Table 1 Sequential extraction scheme for chemical fractions of Pb
Fig. 2 Total contents and chemical fractions of Pb in topsoil
As it can be seen in Fig. 3, the total contents of Pb in subsoils are also higher than the natural background value (35.9 mg/kg) of soil in Guangdong Province [15], and the mean of total Pb content is 96.9 mg/kg, which is significantly lower than that in topsoils. Similar to topsoils, the distribution of Pb in the seven fractions also varies greatly among the subsoils (Fig. 3). The dominating chemical fraction for Pb in subsoils of study area is also Res fraction, accounting for 66.2% of the total Pb, and the order of decrease in the various fractions is Res>FMO(13.28% of total)>Car(11.63%)> WO(6.72%)>SO(1.79%)>WS(0.29%)>Ex(0.08%). It is worth mentioning that the proportion of most of non-Res fractions (except Car and Ex fractions) in subsoils is lower than that in topsoils, which is in accordance with the results reported by other authors [18] who also observed that the proportion of non-Res fractions decreased with soil depth. The proportion of Car fraction in subsoils is significantly higher than that in topsoils maybe due to the higher pH value of subsoils (Table 2), because the formation of Car fraction will be enhanced if the soil pH declines [8].
S09 soil sampling site in the study area is long-term irrigated by the surface wastewater [19], which is considered as a representative sewage irrigated point. The total contents and chemical fractions of Pb in soil profile of S09 soil sampling site are presented in Fig. 4. In the soil profile, total contents of Pb decrease with the soil depth, which indicates that more Pb is enriched in surface soil than in subsurface soil under the condition of external Pb input such as sewage irrigation, which is consistent with the results of other authors [20]. It is worth highlighting that decrease of total Pb contents with soil depth does not appear to be uniform, and the discontinuity in the slope of the total content line from 0-20 cm to 20-40 cm and 40-60 cm can be attributed to the degree of soil disturbance. The soil at 0-20 cm belongs to cultivated horizon with strong disturbance, and soil at 20-40 cm belongs to plow pan with weak disturbance, while soil at 40-60 cm hardly has any disturbance [19] in the soil profile. Therefore, the total contents of Pb from 0-10 cm to 10-20 cm have little decrease while those from 10-20 cm to 20-30 cm have much decrease. Similarly, the total contents of Pb from 20-30 cm to 30-40 cm have little decrease while those from 30-40 cm to 40-50 cm have much decrease (Fig. 4). The changes of seven chemical fractions of Pb in the soil profile are different. WS and SO fractions fluctuate throughout the soil profile with slight change, Ex fraction in 0-10 cm is far higher than that in 10- 60 cm, Car fraction in 30-50cm is far higher than that in 0-30 cm and 50-60 cm, WO and FMO fractions in 0- 30 cm are far higher than those in 30-60 cm, and the Res fraction in 0-40 cm is far higher than that in 40-60 cm. The distribution of seven chemical fractions of Pb in 0-30 cm has the same order: Res>FMO>WO>Car>SO> WS>Ex, while in 30-60 cm it has an order of Car>WO.
Fig. 3 Total contents and chemical fractions of Pb in subsoil
Fig. 4 Total contents and chemical fractions of Pb in soil profile
3.3 Potential mobility of Pb in soils
Generally, heavy metals in the WS and Ex fractions are considered readily and potentially mobile, therefore, the uptake by plants for Pb in soils mainly depends on the content of WS and Ex fractions, but not the total content. In the study area, the sums of WS and Ex fractions in topsoils and subsoils are 0.3-1.45 and 0.14-0.88 mg/kg (Figs. 2 and 3), respectively, and both of them are higher than the German trigger value of 0.1 mg/kg for the transfer path soil-plant [21], which indicates that plants in the study area may be polluted with Pb and the bioavailability of Pb in soils of the study area may be high. This is in accordance with the result of other authors [13] who observed that the Pb contents in both of leaf and stem of pakchoi in the study area were 0.34 and 4.25 mg/kg, respectively, and both of them are higher than the maximum Pb limit for human health which has been established for edible parts of leaf vegetables (0.3 mg/kg) [22]. On the other hand, the relationship between Pb fractions in topsoils and in subsoils can reflect the mobility of Pb fractions in soils. As it can be seen in Fig. 5, the correlation coefficient (R) between sum of WS and Ex fractions (SWE) in topsoils and in subsoils is 0.769 (R2=0.591, P<0.01), which shows that SWE in topsoils is significantly positively correlated with that in subsoils, while other fractions in topsoils and in subsoils have no significant correlation, which confirms that the mobility of Pb in soils of this area is mainly dependent on the WS and Ex fractions. Therefore, in this work, the ratio of WS and Ex fractions (RWE) to the sum of all fractions and the SWE are used for the pollution risk assessment of Pb-contaminated soil to groundwater. Both of RWE and SWE generally decrease with the soil depth in soil profile (Fig. 4), and the RWE in soil profile is in the range of 0.25%-0.42% and far lower than that in other irrigation soils [17], which can be concluded that the groundwater of Pb pollution may have low risk in the study area. This is proved by the fact [19] that Pb contents of groundwater samples in the study area are lower than the upper limit of Pb for drinking water recommended by the World Health Organization [23].
Fig. 5 Correlation between SWE in topsoils and in subsoils
3.4 Correlations between Pb fractions and soil properties
Generally, the migration and transformation mechanism of Pb in soils not only contains the distribution of Pb fractions but also includes the relationship between Pb fractions and soil properties. As it can be seen in Table 2, soil pH is negatively related with the WS fraction, which indicates that the WS fraction decreases with the increase of soil pH, mainly due to the adsorption and precipitation. For instance, increasing soil pH increases Pb retention to soil surfaces via adsorption [24-25]. Therefore, soil pH plays an important role in the impact of mobility and chemical fractions of Pb. OM content has no significant correlation with Pb fractions, which indicates that the extraction of WO or SO fraction cannot truly represent the fraction associated with soil organic matter. Fe2O3 content is negatively related with the WO and SO fractions, which would be mainly owing to the contrary redox conditions for Fe2O3 and WO/SO fractions, because Fe2O3 will be reduced in reducing environment while WO and SO fractions will be decreased under oxidizing environment due to the oxidation of organic matter [8]. Fine sand and silt contents show significantly positive correlation with WO, FMO, SO and Res fractions. On the contrary, clay content shows significantly negative correlation with WO, FMO and Res fractions, which may be partly due to the following hypothesis that clay particles wrap outside of soil granular structure and block off the Pb in nonresidual fractions [11].
Table 2 Correlation coefficients between Pb fractions and soil properties (N=28)
4 Conclusions
1) Total contents of Pb in soils are above the natural background value in the study area, and the mean of total Pb content in topsoil is higher than that in subsoil, and total content of Pb decreases with the depth in soil profile.
2) The modified Tessier sequential extraction technique shows that the chemical fraction of soil Pb is mainly the residual fraction and followed by Fe-Mn oxide fraction, and most nonresidual fractions in topsoils are higher than those in subsoils except the carbonate and exchangeable fractions, while the Car fraction in subsoils is higher than that in topsoils maybe due to the higher pH value of subsoils.
3) The mobility of Pb in soils of this area is mainly dependent on the WS and Ex fractions, and SWE in soils is higher than the German trigger value for the transfer path soil-plant, which is in agreement with the fact that plants have been contaminated with Pb in the study area. Fortunately, both of RWE and SWE generally decrease with the soil depth in soil profile and the RWE in soil profile is lower than 0.5%, indicating the low pollution risk for Pb in groundwater. Furthermore, soil particles, pH and Fe2O3 play an important role in the impact of mobility and chemical fractions of Pb in soils.
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(Edited by YANG Bing)
Foundation item: Project(41103059) supported by the National Natural Science Foundation of China; Project(2010CB428806-2) supported by the National Basic Research Program of China
Received date: 2011-09-21; Accepted date: 2011-12-26
Corresponding author: HUANG Guan-xing, PhD Candidate; Tel: +86-13833398747; E-mail: huangguanxing2004@126.com
