Environ Sci Pollut Res DOI 10.1007/s11356-015-4565-9
RESEARCH ARTICLE
Sediment metal bioavailability in Lake Taihu, China: evaluation of sequential extraction, DGT, and PBET techniques Jinghua Ren 1,2 & Paul N. Williams 3 & Jun Luo 1 & Hongrui Ma 4 & Xiaorong Wang 1
Received: 3 October 2014 / Accepted: 19 April 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract The European BCommunity Bureau of Reference^ (BCR) sequential extraction procedure, diffusive gradient in thin-films technique (DGT), and physiologically based extraction test were applied to assess metal bioavailability in sediments of Lake Taihu (n=13). Findings from the three methods showed that Cd was a significant problem in the western lake whereas Cu, Zn, and Ni pollution was most severe in the northern lake. Results from the sequential extraction revealed that more than 50 % of the Cu and Zn were highly mobile and defined within the extractable fraction (AS1+FM2+OS3) in the majority of the sediments, in contrast extractable fractions of Ni and Cd were lower than 50 % in most of the sampling sites. Average Cu, Zn, Ni, and Cd bioaccessibilities were Meiliang Bay (MB)>the east bank (EB)>the south bank (SB) > the central Lake (CL) > the west bank (WB). The highest metal concentrations among these sampling sites appeared at ZB5, which is in Zhushan Bay and was ∼8 times higher than the lowest metal concentration from SB8. Similar reports regarding the seriousness of pollution in Zhushan Bay have been documented, implying that there might be local metal pollution source discharging a large amount of wastewater from Wuxi and Changzhou (Yin et al. 2013). The highest concentration (3.37 mg/kg) of Cd in sediment was found at WB7, in an area of the west bank near to Yixing City. This is similar to the findings of earlier studies with high Cd concentrations in this region and was probably due to effluents of nonferrous metal smelting, electroplating, and chemical factories (Tao et al. 2012). To evaluate possible environmental consequences of these metals, the numerical sediment quality guidelines of effects
range-low (ERL) and effects range-median (ERM) were adopted (Long et al. 1998). ERL values represent chemical concentrations below which adverse biological effects are rarely observed. Results of total metal concentrations showed that 31, 23, 92, and 15 % of the sampling sites exceeded the ERL values (34, 150, 20.9, and 1.2 mg/kg) for Cu, Zn, Ni, and Cd, respectively (Zahra et al. 2014). They were from MB (MB2 andMB3), ZB (ZB4, ZB5, and ZB6), and WB (WB7). ERM values represent chemical concentrations above which effects were more frequently expected. The ERM values for Cu, Zn, Ni, and Cd were 270, 410, 51.6, and 9.6 mg/kg, respectively (Zahra et al. 2014). Ni concentrations at MB2, MB3, and ZB6 exceeded the ERM value that can cause living organisms potential adverse effects. Ni was the only metal, in this study, which was deemed harmful to aquatic organisms and should be more closely monitored in the future. Ma et al. (2013) reported that Ni pollution is the most serious in Chinese lakes including Lake Taihu. The Ni contamination is likely related to the influence of numerous electroplating companies in the watershed of Lake Taihu. Lake Taihu is the second most polluted lake of the six major Chinese lakes (Ma et al. 2013). Compared with other lakes listed in Table 2, the pollution of Lake Taihu was considered to be of a medium hazard level and should be related to local pollution sources in Lake Taihu drainage basin. MB and ZB were high-risk areas and overall Ni pollution was found to be the most serious problem in Lake Taihu. TOC concentrations of all the sampling sites were in a range of 0.42–1.54 %. Higher TOC contents in ZB5 and MB3 might be due to the input of sewage (rich in organic matter) from the MB and ZB, which were consistent with the spatial distributions of heavy metal except Cd. As for TOC, coefficients (r) of correlation between the TOC contents and Zn, Cu, and Ni concentrations were 0.83, 0.80, and 0.86, respectively, suggesting consistent distribution of metals and TOC. Dou et al. (2013) found a strong correlation between the contents of TOC and metals in sediments of the eastern Beihu Bay. Generally, the surface of organic debris contains a number of functional groups (e.g., –OH, –COOH), which have a high affinity for metals by adsorption, ion exchange, or chelation (Li et al. 2012). Geochemical forms of metals assessed by the BCR procedure Geochemical forms of Cu, Zn, Ni, and Cd in the sediment samples are summarized in Fig. 2. They varied largely with different sampling sites. Metal geochemical species in sediment control solubility and thereby availability. AS1, FM2, and OS3 fractions have greater potential to mobilize from sediments and become bioavailable to benthic organisms (Yin et al. 2013). More than 50 % of the Cu and Zn was present within the extractable fractions (AS1 + FM2 + OS3) (accounting for
Environ Sci Pollut Res Table 1
Total metal concentrations, TOC contents and pH in surface sediments of Lake Taihu
No.
Sampling sites
MB1 MB2 MB3 ZB4 ZB5 ZB6 WB7 SB8 SB9 EB10 EB11 CL12 CL13
Tuoshan Lujiangkou Xiaowanli Jiaoshan Baidukou Shatanggang Dapukou Xintanggang Xiaomeikou Manshan Xukou Pingtaishan Daleishan
pH
6.81 6.78 6.82 6.85 6.79 6.82 6.96 6.87 6.81 6.81 6.87 6.84 6.88
TOC (%)
0.78 0.68 1.15 0.80 1.54 0.94 0.55 0.44 0.58 0.58 0.97 0.42 0.71
Concentrations (mg/kg) (mean±standard deviation) Fe
Mn
Cu
Zn
Ni
Cd
27,402±680 27,179±115 34,105±307 27,301±429 35,448±797 22,205±1145 23,970±285 22,800±1347 24,624±3480 31,521±629 28,608±132 25,227±530 26,579±525
624±23 846±15 1,264±14 705±32 2,136±88 546±39 717±4 664±46 743±27 943±49 936±1.2 538±45 664±10
24.5±1.6 65.2±3.3 47.0±2.5 21.6±1.2 117±5.5 36.3±2.8 17.6±1.9 13.8±0.9 17.0±1.9 20.3±1.5 24.4±1.2 17.7±1.7 19.4±1.3
92.6±2.3 176±0.8 161±4.8 93.6±1.9 344±11 126±4.4 76.2±1.2 67.7±6.1 88.9±12 81.7±4.5 81.5±3.9 76.7±2.5 83.8±5.8
34.6±1.6 52.1±2.4 62.0±4.9 34.2±2.3 106±6.2 32.4±4.6 23.6±2.8 18.8±2.4 22.6±1.4 29.6±1.9 28.5±3.7 25.9±3.4 25.6±2.4
0.839±0.021 0.788±0.044 0.784±0.056 1.13±0.01 1.40±0.14 1.03±0.04 3.37±0.07 0.484±0.040 0.726±0.040 0.570±0.019 0.562±0.027 0.659±0.075 0.613±0.016
62 %) in most of sampling sites. Especially in MB2, ZB5, and ZB6, approximately 90 % of Zn was in this extractable fraction, implying high potential toxicity. As for Ni and Cd, the percentage of extractable fractions was lower than 50 % for most of the sampling sites, but MB1, MB2, MB3, and ZB5 for Ni and CL13, SB8, and SB9 for Cd showed higher percentage (>60 %) of extractable fractions. For Zn, the AS1 concentration was 6.16–166 mg/kg (accounting for 9–56 % of the total Zn concentration). Less than 20 % of Zn existed in the AS1 fraction for most sampling sites except MB2, MB3, ZB5, and ZB6. For other metals, percentages of AS1 were 9–35 % (Cu), 7–36 % (Ni), and 20–33 % (Cd). Similarly, the AS1 concentrations and percentages were higher in MB2, MB3, ZB5, and ZB6 for both Cu and Ni. For Cd, AS1 did not differ greatly among sampling sites, with ranges of 20 to 33 %. The FM2 fraction accounted for 11–24 % (Cu), 15–37 % (Zn), 10–32 % (Ni), and 12–34 % (Cd) of total metal Table 2
concentrations. The percentages of FM2 for Zn and Ni were higher in these sampling sites (MB1, MB2, MB3, and ZB5) located in the MB and ZB, whereas percentages of Cu and Cd seemed similar in most of sampling sites. Coefficients (r) of correlation between concentrations of FM2 fraction and total Fe and Mn concentrations are shown in Table 3. Concentrations of Cu, Zn, and Ni in FM2 correlated well with total Fe and Mn contents, while Cd had no significant relationship with Fe (r= 0.034) and Mn (r=0.25). The hydrous Mn oxides exhibit more extensive isomorphic substitution than amorphous Fe oxides and have greater conditional equilibrium constants for heavy metals than Fe oxides (Shen et al. 2007), with the implications of this being that Mn oxides tend to adsorb more metals than Fe oxides. That could account for the higher coefficient of correlation between heavy metals and Mn was higher than Fe. Percentages of the OS3 fraction were 18–31 % (Cu), 10– 20 % (Zn), 14–19 % (Ni), and 0.4–9 % (Cd) of total metal concentrations in sediments. The percentages of OS3 for Cu
Comparison of total metal concentrations in sediments from some important lagoons and lakes of China and the world (mg/kg)
Regions
Cu
Zn
Ni
Cd
References
Hazar Lake (Turkey)
10–64
46–210
38–130
–
Ozmen et al. (2004)
Venice Lagoon (Italy)
4.4–1.7
48–96
10–15
0.2–0.94
Rigollet et al. (2004)
Akyatan Lagoon (Turkey)
15–31
54–102
80–219
ND
Davutluoglu et al. (2010)
San Jose Lagoon (Puerto Roco)
29–211
48–1,530
–
0.2–4.8
Acevedo-Figueroa et al. (2006)
Lake Dongting (China)
51
140
40
2.7
Yao (2008)
Lake Datong (China) Lake Taihu (China)
67 21–143
120 75–432
49 26–113
0.6 0.40–2.1
Yao (2008) Yin et al. (2013)
ERL
34
150
20.9
1.2
Zahra et al. (2014)
ERM
270
410
51.6
9.6
Zahra et al. (2014)
(–) not available, ND not detected, ERL effects range-low sediment quality criteria, ERM effects range-median sediment quality criteria
Environ Sci Pollut Res 400
[Zn] BCR (mg/kg)
350
150
A
RS4 OS3 FM2 AS1
300 250 200 150 100
[Cu] BCR (mg/kg)
Fig. 2 Geochemical forms of Zn (a), Cu (b), Ni (c), and Cd (d) in sediment samples
B
RS4 OS3 FM2 AS1
120 90 60 30
50 0 M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 1 EB 0 1 CL 1 1 CL 2 13
M B M 1 B M 2 B3 ZB 4 ZB 5 ZB W 6 B7 SB 8 SB EB 9 1 EB 0 1 CL 1 1 CL 2 13
0
Sampling sites
Sampling sites RS4 OS3 FM2 AS1
C
80 60 40 20
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 1 EB 0 1 CL 1 1 CL 2 13
and Zn were higher in these sampling sites (MB3, ZB5, and SB9) located in the MB and ZB, whereas percentages of Ni seemed similar in most of the sampling sites. Coefficients (r) of correlations between TOC contents and the OS3 fractions of Cu, Zn, Ni, and Cd were 0.86, 0.89, 0.90, and 0.20, respectively, indicating that the OS3 fraction of Cu, Zn, and Ni in this study was influenced mainly by the organic contents in sediments, but Cd was not so affected. Other studies have observed similar trends. Cd concentrations in four ornithogenic coral-sand sedimentary profiles had no correlation with plant-originated organic matter in the top sediments, but displayed a strong positive correlation with phosphate fertilizers (Dou et al. 2013; Liu et al. 2012). Irrigation water discharge might be an important source for Cd. Labile metal concentrations measured by DGT DGT can measure labile species of metals, such as free metal ions, inorganic complexes, and a small proportion of organic Table 3 Coefficients (r) of correlations of the metals in different geochemical forms with total Fe, Mn, and TOC contents Correlation coefficients
Cu
Zn
Ni
Cd
r1 r2 r3
0.60* 0.80** 0.86**
0.68* 0.88** 0.89**
0.77** 0.88** 0.90**
0.034 0.25 0.20
r1 coefficients of correlation of metal FM2 concentrations with total Fe, r2 coefficients of correlation of metal FM2 concentrations with total Mn, r3 coefficients of correlation of metal OS3 concentrations with TOC **
RS4 OS3 FM2 AS1
D
3.0 2.5 2.0 1.5 1.0
0.0
Sampling sites
Correlation is significant at 0.05 level; 0.01 level
3.5
0.5
0
*
4.0
Correlation is significant at
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 1 EB 0 1 CL 1 1 CL 2 13
100
[Cd] BCR (mg/kg)
[Ni] BCR (mg/kg)
120
Sampling sites
complexes, which can be taken up by organisms (Ernstberger et al. 2005). The magnitude of the DGT-measured concentration depends on the concentration in porewater, diffusion process, and resupply process from the solid phase. Metal concentrations measured by DGT in the surface sediments are shown in Fig. 3. The overall DGT-measured concentration ranges in all sampling sites were 0.04–0.63, 6.4–18.3, 0.19– 6.32, and 0.03–0.12 μg/l for Cu, Zn, Ni, and Cd, respectively. The highest level of Cu, Zn, and Ni was found in the northern most sites (MB2, ZB5, and ZB6), where the highest total metal contents and extractable metal concentrations by BCR were found. SB8 showed the highest DGT-measured concentration of Cd (0.117 μg/l), indicating local pollution sources in this area, which was suggested by the highest total content and extractable concentration of Cd as well. Yin et al. (2013) reported similar DGT results which were 0.3–0.7, 20–60, 1.0– 3.5, and 0.004–0.011 μg/l for Cu, Zn, Ni, and Cd in Lake Taihu. Fan et al. (2008, 2007) found the concentrations of Cu, Zn, Ni, and Cd measured by DGT were 0.07–9.01, 11.9–65.1, 0.04–9.15, and 0.20–0.57 μg/l, respectively, in surface sediments of Daliao River (China), which was higher than those in Lake Taihu in this study. Overall, DGT-measured concentrations in this study showed spatial variation for metals with serious pollution in ZB and MB, which were consistent with the BCR assessment trends. Metal bioaccessibility evaluated by PBET PBET was applied to evaluate the bioavailability of metals in river sediments to study potential risk of metals (Devesa-Rey et al. 2010). Figure 4 summarizes Zn, Cu, Ni, and Cd bioaccessibility in the gastric and intestinal phases for the 13
Environ Sci Pollut Res 8
1.0
Fig. 3 DGT-measured concentrations of Cu, Zn, Ni, and Cd in 13 sediment samples from Lake Taihu
A
B NiDGT (µg/L)
CuDGT (µ g/L)
0.8 0.6 0.4
6
4
2
0.2
0
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 EB10 1 CL 1 CL12 13
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 EB 1 0 1 CL 1 CL 1 2 13
0.0
Sampling sites
Sampling sites 0.20
30 25
C
D CdDGT (µg/L)
ZnDGT (µ g/L)
0.15 20 15 10
0.10
0.05 5
0.00
M B M 1 B M 2 B3 ZB 4 ZB 5 ZB W 6 B7 SB 8 SB EB 9 1 EB 0 1 CL 1 1 CL 2 13
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 EB10 1 CL 1 CL12 13
0
Sampling sites
Sampling sites
samples, showing obvious variation among the different samples. Metal bioaccessibilities from MB (MB1, MB2, and MB3) and ZB (ZB4 and ZB6) were higher than those from other sampling sites. Average accessibilities of Zn and Cd in the intestinal phase are lower than the gastric phase while a contrary result for Cu and Ni. For example, mean Zn bioaccessibility in the intestinal phase (12.9 %) was lower than 24.9 % in the gastric phase,
70
50 40 30 20 10 0
B
50 40 30 20 10
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 EB10 1 CL 1 1 CL 2 13
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 1 EB 0 11 CL 1 CL 2 13
0
Sampling sites
Sampling sites
70
70
C
Cd bioaccessibility (%)
60
60
50 40 30 20 10
60
D
50 40 30 20 10 0
MB 1 MB 2 MB 3 ZB 4 ZB 5 ZB 6 WB 7 SB 8 SB 9 EB 10 EB 11 CL 12 CL 13
0
Sampling sites
MB 1 MB 2 MB 3 ZB 4 ZB 5 ZB 6 WB 7 SB 8 SB 9 EB 10 EB 11 CL 12 CL 13
60
A
Ni bioaccessibility (%)
Cu bioaccessibility (%)
70
Zn bioaccessibility (%)
Fig. 4 Gastric and intestinal bioaccessibility of Cu (a), Ni (b), Zn (c), and Cd (d) in surface sediments of Lake Taihu
similar to the results observed in 14 mildly acidic and alkali (pH 5.87–8.30) soils using PBET by Li and Zhang (2013). Zn and Cd bioaccessibilities largely depended on their solubility in the gastrointestinal phase. When pH rises from gastric phase into intestinal phase, Zn and Cd solubility decreased. In a higher pH, carbonate-rich environment of the intestine, metals may be stabilized in solution by complexation, undergo readsorption to preexistent or altered sites at the particle
Sampling sites
Environ Sci Pollut Res Table 4 Coefficients (r) of determination between DGTmeasured concentrations, geochemical forms concentrations, and bioaccessibility in both the gastric (G) and intestinal (I) phases for Cu, Zn, Ni, and Cd
Elements
AS1
FM2
AS1+FM2
OS3
AS1+FM2+OS3
G
I
Cu-DGT Cu-G Cu-I Zn-DGT Zn-G
0.81** 0.50* 0.48* 0.79** 0.81**
0.76** 0.46 0.44 0.71** 0.80**
0.80** 0.49* 0.47 0.77** 0.81**
0.77** 0.11 0.10 0.51* 0.66**
0.81** 0.35 0.34 0.75** 0.80**
0.27 – – 0.75** –
0.23 – – 0.74** –
Zn-I Ni-DGT Ni-G Ni-I Cd-DGT Cd-G Cd-I
0.62* 0.93** 0.85** 0.70** 0.82** 0.91** 0.88**
0.53* 0.87** 0.95** 0.84** 0.85** 0.75** 0.71**
0.59* 0.92** 0.90** 0.77** 0.85** 0.88** 0.85**
0.29 0.85** 0.90** 0.76** 0.66* 0.47 0.38
0.56* 0.91** 0.90** 0.77** 0.86** 0.87** 0.83**
– 0.75** – – 0.75** – –
– 0.68** – – 0.70** – –
*
Correlation is significant at 0.05 level; ** Correlation is significant at 0.01 level
surface, or precipitate as relatively insoluble compounds. But from the gastric phase into the intestinal phase, Cu and Ni are mainly stabilized by complexation with organic ligands, such as malate, chenodeoxycholate, hyodeoxycholate, etc. With respect to bile acids, Cu complexes and aggregates are more stable than Zn and Cd by polarographic measurements (Turner and Ip 2007). It was consistent with the BCR extraction results that geochemical forms of Cu were related with TOC (Table 3). During digestion, it has been shown that the absorption process mostly takes place in the epithelium of the intestine (Turner and Ip 2007). So their toxicity to organisms increases because Cu and Ni bioaccessibilities in the intestinal phase were higher than gastric phase.
Relationship between BCR, DGT, and PBET measurements To gain a mechanistic understanding of different bioavailability assessment methods and how the measurements varied when applied to the different sediment samples, sequentially extracted fractions (AS1, FM2, OS3, and AS1+FM2+OS3) were compared with the DGT-measured concentrations and PBET (the gastric and intestinal phases) using the Pearson correlation analysis (Table 4). For Cu, Zn, Ni, and Cd, the sequentially extractable fractions (AS1, FM2, AS1+FM2, OS3, AS1+ FM2+OS3) had significant correlations with DGT-measured concentrations (pthe central lake>the western bank, suggesting that metal pollution in the northern Lake Taihu was the most serious according to the numerical sediment quality guidelines of ERL and ERM. However, the most Cdpolluted area was the western bank, up to 3.37 mg/kg Cd in the surface sediment. Similarly, labile metal concentrations measured by the sequential extraction, DGT, and PBET
Environ Sci Pollut Res
confirmed that the northern Lake Taihu has the greatest hazard risk for Cu, Zn, and Ni, whereas the west bank is heavily polluted by Cd. The sequential extraction data showed that >50 % of Cu and Zn were mainly in the extractable fractions (AS1+FM2+OS3) in most of the sampling sites while the extractable fractions of Ni and Cd were lower than 50 % in most of sampling sites. Higher extractable concentrations of metals pose a greater potential toxicity risk to the aquatic environment. Cu, Zn, and Ni in FM2 and OS3 correlated well with total Fe/Mn and TOC, respectively, in sediments, but Cd did not show any relationship with them. Pollution of Cu, Zn, and Ni in the northern Lake Taihu may be a result of sewage discharge while Cd pollution in the western bank is due to industrial effluents containing Cd. Linear regression analysis suggested that the sequentially extractable fractions (AS1, FM2, AS1+FM2, OS3, AS1+ FM2+OS3) correlated well with DGT-measured concentrations (p
RESEARCH ARTICLE
Sediment metal bioavailability in Lake Taihu, China: evaluation of sequential extraction, DGT, and PBET techniques Jinghua Ren 1,2 & Paul N. Williams 3 & Jun Luo 1 & Hongrui Ma 4 & Xiaorong Wang 1
Received: 3 October 2014 / Accepted: 19 April 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract The European BCommunity Bureau of Reference^ (BCR) sequential extraction procedure, diffusive gradient in thin-films technique (DGT), and physiologically based extraction test were applied to assess metal bioavailability in sediments of Lake Taihu (n=13). Findings from the three methods showed that Cd was a significant problem in the western lake whereas Cu, Zn, and Ni pollution was most severe in the northern lake. Results from the sequential extraction revealed that more than 50 % of the Cu and Zn were highly mobile and defined within the extractable fraction (AS1+FM2+OS3) in the majority of the sediments, in contrast extractable fractions of Ni and Cd were lower than 50 % in most of the sampling sites. Average Cu, Zn, Ni, and Cd bioaccessibilities were Meiliang Bay (MB)>the east bank (EB)>the south bank (SB) > the central Lake (CL) > the west bank (WB). The highest metal concentrations among these sampling sites appeared at ZB5, which is in Zhushan Bay and was ∼8 times higher than the lowest metal concentration from SB8. Similar reports regarding the seriousness of pollution in Zhushan Bay have been documented, implying that there might be local metal pollution source discharging a large amount of wastewater from Wuxi and Changzhou (Yin et al. 2013). The highest concentration (3.37 mg/kg) of Cd in sediment was found at WB7, in an area of the west bank near to Yixing City. This is similar to the findings of earlier studies with high Cd concentrations in this region and was probably due to effluents of nonferrous metal smelting, electroplating, and chemical factories (Tao et al. 2012). To evaluate possible environmental consequences of these metals, the numerical sediment quality guidelines of effects
range-low (ERL) and effects range-median (ERM) were adopted (Long et al. 1998). ERL values represent chemical concentrations below which adverse biological effects are rarely observed. Results of total metal concentrations showed that 31, 23, 92, and 15 % of the sampling sites exceeded the ERL values (34, 150, 20.9, and 1.2 mg/kg) for Cu, Zn, Ni, and Cd, respectively (Zahra et al. 2014). They were from MB (MB2 andMB3), ZB (ZB4, ZB5, and ZB6), and WB (WB7). ERM values represent chemical concentrations above which effects were more frequently expected. The ERM values for Cu, Zn, Ni, and Cd were 270, 410, 51.6, and 9.6 mg/kg, respectively (Zahra et al. 2014). Ni concentrations at MB2, MB3, and ZB6 exceeded the ERM value that can cause living organisms potential adverse effects. Ni was the only metal, in this study, which was deemed harmful to aquatic organisms and should be more closely monitored in the future. Ma et al. (2013) reported that Ni pollution is the most serious in Chinese lakes including Lake Taihu. The Ni contamination is likely related to the influence of numerous electroplating companies in the watershed of Lake Taihu. Lake Taihu is the second most polluted lake of the six major Chinese lakes (Ma et al. 2013). Compared with other lakes listed in Table 2, the pollution of Lake Taihu was considered to be of a medium hazard level and should be related to local pollution sources in Lake Taihu drainage basin. MB and ZB were high-risk areas and overall Ni pollution was found to be the most serious problem in Lake Taihu. TOC concentrations of all the sampling sites were in a range of 0.42–1.54 %. Higher TOC contents in ZB5 and MB3 might be due to the input of sewage (rich in organic matter) from the MB and ZB, which were consistent with the spatial distributions of heavy metal except Cd. As for TOC, coefficients (r) of correlation between the TOC contents and Zn, Cu, and Ni concentrations were 0.83, 0.80, and 0.86, respectively, suggesting consistent distribution of metals and TOC. Dou et al. (2013) found a strong correlation between the contents of TOC and metals in sediments of the eastern Beihu Bay. Generally, the surface of organic debris contains a number of functional groups (e.g., –OH, –COOH), which have a high affinity for metals by adsorption, ion exchange, or chelation (Li et al. 2012). Geochemical forms of metals assessed by the BCR procedure Geochemical forms of Cu, Zn, Ni, and Cd in the sediment samples are summarized in Fig. 2. They varied largely with different sampling sites. Metal geochemical species in sediment control solubility and thereby availability. AS1, FM2, and OS3 fractions have greater potential to mobilize from sediments and become bioavailable to benthic organisms (Yin et al. 2013). More than 50 % of the Cu and Zn was present within the extractable fractions (AS1 + FM2 + OS3) (accounting for
Environ Sci Pollut Res Table 1
Total metal concentrations, TOC contents and pH in surface sediments of Lake Taihu
No.
Sampling sites
MB1 MB2 MB3 ZB4 ZB5 ZB6 WB7 SB8 SB9 EB10 EB11 CL12 CL13
Tuoshan Lujiangkou Xiaowanli Jiaoshan Baidukou Shatanggang Dapukou Xintanggang Xiaomeikou Manshan Xukou Pingtaishan Daleishan
pH
6.81 6.78 6.82 6.85 6.79 6.82 6.96 6.87 6.81 6.81 6.87 6.84 6.88
TOC (%)
0.78 0.68 1.15 0.80 1.54 0.94 0.55 0.44 0.58 0.58 0.97 0.42 0.71
Concentrations (mg/kg) (mean±standard deviation) Fe
Mn
Cu
Zn
Ni
Cd
27,402±680 27,179±115 34,105±307 27,301±429 35,448±797 22,205±1145 23,970±285 22,800±1347 24,624±3480 31,521±629 28,608±132 25,227±530 26,579±525
624±23 846±15 1,264±14 705±32 2,136±88 546±39 717±4 664±46 743±27 943±49 936±1.2 538±45 664±10
24.5±1.6 65.2±3.3 47.0±2.5 21.6±1.2 117±5.5 36.3±2.8 17.6±1.9 13.8±0.9 17.0±1.9 20.3±1.5 24.4±1.2 17.7±1.7 19.4±1.3
92.6±2.3 176±0.8 161±4.8 93.6±1.9 344±11 126±4.4 76.2±1.2 67.7±6.1 88.9±12 81.7±4.5 81.5±3.9 76.7±2.5 83.8±5.8
34.6±1.6 52.1±2.4 62.0±4.9 34.2±2.3 106±6.2 32.4±4.6 23.6±2.8 18.8±2.4 22.6±1.4 29.6±1.9 28.5±3.7 25.9±3.4 25.6±2.4
0.839±0.021 0.788±0.044 0.784±0.056 1.13±0.01 1.40±0.14 1.03±0.04 3.37±0.07 0.484±0.040 0.726±0.040 0.570±0.019 0.562±0.027 0.659±0.075 0.613±0.016
62 %) in most of sampling sites. Especially in MB2, ZB5, and ZB6, approximately 90 % of Zn was in this extractable fraction, implying high potential toxicity. As for Ni and Cd, the percentage of extractable fractions was lower than 50 % for most of the sampling sites, but MB1, MB2, MB3, and ZB5 for Ni and CL13, SB8, and SB9 for Cd showed higher percentage (>60 %) of extractable fractions. For Zn, the AS1 concentration was 6.16–166 mg/kg (accounting for 9–56 % of the total Zn concentration). Less than 20 % of Zn existed in the AS1 fraction for most sampling sites except MB2, MB3, ZB5, and ZB6. For other metals, percentages of AS1 were 9–35 % (Cu), 7–36 % (Ni), and 20–33 % (Cd). Similarly, the AS1 concentrations and percentages were higher in MB2, MB3, ZB5, and ZB6 for both Cu and Ni. For Cd, AS1 did not differ greatly among sampling sites, with ranges of 20 to 33 %. The FM2 fraction accounted for 11–24 % (Cu), 15–37 % (Zn), 10–32 % (Ni), and 12–34 % (Cd) of total metal Table 2
concentrations. The percentages of FM2 for Zn and Ni were higher in these sampling sites (MB1, MB2, MB3, and ZB5) located in the MB and ZB, whereas percentages of Cu and Cd seemed similar in most of sampling sites. Coefficients (r) of correlation between concentrations of FM2 fraction and total Fe and Mn concentrations are shown in Table 3. Concentrations of Cu, Zn, and Ni in FM2 correlated well with total Fe and Mn contents, while Cd had no significant relationship with Fe (r= 0.034) and Mn (r=0.25). The hydrous Mn oxides exhibit more extensive isomorphic substitution than amorphous Fe oxides and have greater conditional equilibrium constants for heavy metals than Fe oxides (Shen et al. 2007), with the implications of this being that Mn oxides tend to adsorb more metals than Fe oxides. That could account for the higher coefficient of correlation between heavy metals and Mn was higher than Fe. Percentages of the OS3 fraction were 18–31 % (Cu), 10– 20 % (Zn), 14–19 % (Ni), and 0.4–9 % (Cd) of total metal concentrations in sediments. The percentages of OS3 for Cu
Comparison of total metal concentrations in sediments from some important lagoons and lakes of China and the world (mg/kg)
Regions
Cu
Zn
Ni
Cd
References
Hazar Lake (Turkey)
10–64
46–210
38–130
–
Ozmen et al. (2004)
Venice Lagoon (Italy)
4.4–1.7
48–96
10–15
0.2–0.94
Rigollet et al. (2004)
Akyatan Lagoon (Turkey)
15–31
54–102
80–219
ND
Davutluoglu et al. (2010)
San Jose Lagoon (Puerto Roco)
29–211
48–1,530
–
0.2–4.8
Acevedo-Figueroa et al. (2006)
Lake Dongting (China)
51
140
40
2.7
Yao (2008)
Lake Datong (China) Lake Taihu (China)
67 21–143
120 75–432
49 26–113
0.6 0.40–2.1
Yao (2008) Yin et al. (2013)
ERL
34
150
20.9
1.2
Zahra et al. (2014)
ERM
270
410
51.6
9.6
Zahra et al. (2014)
(–) not available, ND not detected, ERL effects range-low sediment quality criteria, ERM effects range-median sediment quality criteria
Environ Sci Pollut Res 400
[Zn] BCR (mg/kg)
350
150
A
RS4 OS3 FM2 AS1
300 250 200 150 100
[Cu] BCR (mg/kg)
Fig. 2 Geochemical forms of Zn (a), Cu (b), Ni (c), and Cd (d) in sediment samples
B
RS4 OS3 FM2 AS1
120 90 60 30
50 0 M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 1 EB 0 1 CL 1 1 CL 2 13
M B M 1 B M 2 B3 ZB 4 ZB 5 ZB W 6 B7 SB 8 SB EB 9 1 EB 0 1 CL 1 1 CL 2 13
0
Sampling sites
Sampling sites RS4 OS3 FM2 AS1
C
80 60 40 20
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 1 EB 0 1 CL 1 1 CL 2 13
and Zn were higher in these sampling sites (MB3, ZB5, and SB9) located in the MB and ZB, whereas percentages of Ni seemed similar in most of the sampling sites. Coefficients (r) of correlations between TOC contents and the OS3 fractions of Cu, Zn, Ni, and Cd were 0.86, 0.89, 0.90, and 0.20, respectively, indicating that the OS3 fraction of Cu, Zn, and Ni in this study was influenced mainly by the organic contents in sediments, but Cd was not so affected. Other studies have observed similar trends. Cd concentrations in four ornithogenic coral-sand sedimentary profiles had no correlation with plant-originated organic matter in the top sediments, but displayed a strong positive correlation with phosphate fertilizers (Dou et al. 2013; Liu et al. 2012). Irrigation water discharge might be an important source for Cd. Labile metal concentrations measured by DGT DGT can measure labile species of metals, such as free metal ions, inorganic complexes, and a small proportion of organic Table 3 Coefficients (r) of correlations of the metals in different geochemical forms with total Fe, Mn, and TOC contents Correlation coefficients
Cu
Zn
Ni
Cd
r1 r2 r3
0.60* 0.80** 0.86**
0.68* 0.88** 0.89**
0.77** 0.88** 0.90**
0.034 0.25 0.20
r1 coefficients of correlation of metal FM2 concentrations with total Fe, r2 coefficients of correlation of metal FM2 concentrations with total Mn, r3 coefficients of correlation of metal OS3 concentrations with TOC **
RS4 OS3 FM2 AS1
D
3.0 2.5 2.0 1.5 1.0
0.0
Sampling sites
Correlation is significant at 0.05 level; 0.01 level
3.5
0.5
0
*
4.0
Correlation is significant at
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 1 EB 0 1 CL 1 1 CL 2 13
100
[Cd] BCR (mg/kg)
[Ni] BCR (mg/kg)
120
Sampling sites
complexes, which can be taken up by organisms (Ernstberger et al. 2005). The magnitude of the DGT-measured concentration depends on the concentration in porewater, diffusion process, and resupply process from the solid phase. Metal concentrations measured by DGT in the surface sediments are shown in Fig. 3. The overall DGT-measured concentration ranges in all sampling sites were 0.04–0.63, 6.4–18.3, 0.19– 6.32, and 0.03–0.12 μg/l for Cu, Zn, Ni, and Cd, respectively. The highest level of Cu, Zn, and Ni was found in the northern most sites (MB2, ZB5, and ZB6), where the highest total metal contents and extractable metal concentrations by BCR were found. SB8 showed the highest DGT-measured concentration of Cd (0.117 μg/l), indicating local pollution sources in this area, which was suggested by the highest total content and extractable concentration of Cd as well. Yin et al. (2013) reported similar DGT results which were 0.3–0.7, 20–60, 1.0– 3.5, and 0.004–0.011 μg/l for Cu, Zn, Ni, and Cd in Lake Taihu. Fan et al. (2008, 2007) found the concentrations of Cu, Zn, Ni, and Cd measured by DGT were 0.07–9.01, 11.9–65.1, 0.04–9.15, and 0.20–0.57 μg/l, respectively, in surface sediments of Daliao River (China), which was higher than those in Lake Taihu in this study. Overall, DGT-measured concentrations in this study showed spatial variation for metals with serious pollution in ZB and MB, which were consistent with the BCR assessment trends. Metal bioaccessibility evaluated by PBET PBET was applied to evaluate the bioavailability of metals in river sediments to study potential risk of metals (Devesa-Rey et al. 2010). Figure 4 summarizes Zn, Cu, Ni, and Cd bioaccessibility in the gastric and intestinal phases for the 13
Environ Sci Pollut Res 8
1.0
Fig. 3 DGT-measured concentrations of Cu, Zn, Ni, and Cd in 13 sediment samples from Lake Taihu
A
B NiDGT (µg/L)
CuDGT (µ g/L)
0.8 0.6 0.4
6
4
2
0.2
0
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 EB10 1 CL 1 CL12 13
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 EB 1 0 1 CL 1 CL 1 2 13
0.0
Sampling sites
Sampling sites 0.20
30 25
C
D CdDGT (µg/L)
ZnDGT (µ g/L)
0.15 20 15 10
0.10
0.05 5
0.00
M B M 1 B M 2 B3 ZB 4 ZB 5 ZB W 6 B7 SB 8 SB EB 9 1 EB 0 1 CL 1 1 CL 2 13
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 EB10 1 CL 1 CL12 13
0
Sampling sites
Sampling sites
samples, showing obvious variation among the different samples. Metal bioaccessibilities from MB (MB1, MB2, and MB3) and ZB (ZB4 and ZB6) were higher than those from other sampling sites. Average accessibilities of Zn and Cd in the intestinal phase are lower than the gastric phase while a contrary result for Cu and Ni. For example, mean Zn bioaccessibility in the intestinal phase (12.9 %) was lower than 24.9 % in the gastric phase,
70
50 40 30 20 10 0
B
50 40 30 20 10
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 EB10 1 CL 1 1 CL 2 13
M B M 1 B M 2 B3 ZB ZB4 5 ZB W 6 B7 SB 8 SB EB 9 1 EB 0 11 CL 1 CL 2 13
0
Sampling sites
Sampling sites
70
70
C
Cd bioaccessibility (%)
60
60
50 40 30 20 10
60
D
50 40 30 20 10 0
MB 1 MB 2 MB 3 ZB 4 ZB 5 ZB 6 WB 7 SB 8 SB 9 EB 10 EB 11 CL 12 CL 13
0
Sampling sites
MB 1 MB 2 MB 3 ZB 4 ZB 5 ZB 6 WB 7 SB 8 SB 9 EB 10 EB 11 CL 12 CL 13
60
A
Ni bioaccessibility (%)
Cu bioaccessibility (%)
70
Zn bioaccessibility (%)
Fig. 4 Gastric and intestinal bioaccessibility of Cu (a), Ni (b), Zn (c), and Cd (d) in surface sediments of Lake Taihu
similar to the results observed in 14 mildly acidic and alkali (pH 5.87–8.30) soils using PBET by Li and Zhang (2013). Zn and Cd bioaccessibilities largely depended on their solubility in the gastrointestinal phase. When pH rises from gastric phase into intestinal phase, Zn and Cd solubility decreased. In a higher pH, carbonate-rich environment of the intestine, metals may be stabilized in solution by complexation, undergo readsorption to preexistent or altered sites at the particle
Sampling sites
Environ Sci Pollut Res Table 4 Coefficients (r) of determination between DGTmeasured concentrations, geochemical forms concentrations, and bioaccessibility in both the gastric (G) and intestinal (I) phases for Cu, Zn, Ni, and Cd
Elements
AS1
FM2
AS1+FM2
OS3
AS1+FM2+OS3
G
I
Cu-DGT Cu-G Cu-I Zn-DGT Zn-G
0.81** 0.50* 0.48* 0.79** 0.81**
0.76** 0.46 0.44 0.71** 0.80**
0.80** 0.49* 0.47 0.77** 0.81**
0.77** 0.11 0.10 0.51* 0.66**
0.81** 0.35 0.34 0.75** 0.80**
0.27 – – 0.75** –
0.23 – – 0.74** –
Zn-I Ni-DGT Ni-G Ni-I Cd-DGT Cd-G Cd-I
0.62* 0.93** 0.85** 0.70** 0.82** 0.91** 0.88**
0.53* 0.87** 0.95** 0.84** 0.85** 0.75** 0.71**
0.59* 0.92** 0.90** 0.77** 0.85** 0.88** 0.85**
0.29 0.85** 0.90** 0.76** 0.66* 0.47 0.38
0.56* 0.91** 0.90** 0.77** 0.86** 0.87** 0.83**
– 0.75** – – 0.75** – –
– 0.68** – – 0.70** – –
*
Correlation is significant at 0.05 level; ** Correlation is significant at 0.01 level
surface, or precipitate as relatively insoluble compounds. But from the gastric phase into the intestinal phase, Cu and Ni are mainly stabilized by complexation with organic ligands, such as malate, chenodeoxycholate, hyodeoxycholate, etc. With respect to bile acids, Cu complexes and aggregates are more stable than Zn and Cd by polarographic measurements (Turner and Ip 2007). It was consistent with the BCR extraction results that geochemical forms of Cu were related with TOC (Table 3). During digestion, it has been shown that the absorption process mostly takes place in the epithelium of the intestine (Turner and Ip 2007). So their toxicity to organisms increases because Cu and Ni bioaccessibilities in the intestinal phase were higher than gastric phase.
Relationship between BCR, DGT, and PBET measurements To gain a mechanistic understanding of different bioavailability assessment methods and how the measurements varied when applied to the different sediment samples, sequentially extracted fractions (AS1, FM2, OS3, and AS1+FM2+OS3) were compared with the DGT-measured concentrations and PBET (the gastric and intestinal phases) using the Pearson correlation analysis (Table 4). For Cu, Zn, Ni, and Cd, the sequentially extractable fractions (AS1, FM2, AS1+FM2, OS3, AS1+ FM2+OS3) had significant correlations with DGT-measured concentrations (pthe central lake>the western bank, suggesting that metal pollution in the northern Lake Taihu was the most serious according to the numerical sediment quality guidelines of ERL and ERM. However, the most Cdpolluted area was the western bank, up to 3.37 mg/kg Cd in the surface sediment. Similarly, labile metal concentrations measured by the sequential extraction, DGT, and PBET
Environ Sci Pollut Res
confirmed that the northern Lake Taihu has the greatest hazard risk for Cu, Zn, and Ni, whereas the west bank is heavily polluted by Cd. The sequential extraction data showed that >50 % of Cu and Zn were mainly in the extractable fractions (AS1+FM2+OS3) in most of the sampling sites while the extractable fractions of Ni and Cd were lower than 50 % in most of sampling sites. Higher extractable concentrations of metals pose a greater potential toxicity risk to the aquatic environment. Cu, Zn, and Ni in FM2 and OS3 correlated well with total Fe/Mn and TOC, respectively, in sediments, but Cd did not show any relationship with them. Pollution of Cu, Zn, and Ni in the northern Lake Taihu may be a result of sewage discharge while Cd pollution in the western bank is due to industrial effluents containing Cd. Linear regression analysis suggested that the sequentially extractable fractions (AS1, FM2, AS1+FM2, OS3, AS1+ FM2+OS3) correlated well with DGT-measured concentrations (p
