The concentrations and the amount of cumulative release of heavy metals (Zn, Cu, Cd, Cr, and Pb) in contaminated sediments collected from combined sewer system were investigated in order to provide a complete overview of the key variables affecting the heavy metals release from storm sewer sediments. The heavy metals release rates were affected to a much greater extent in the low pH (4–7) condition than in high pH (8–10) condition. At higher temperature (30–35°C) the release rates of metals were increased more rapidly than at low temperature. The release of Zn, Cu, Cr, and Pb appeared to increase under the aerobic condition during the first 150 min and then the concentration kept stable. Moreover, the adsorption of these metals and the release of Cd occurred in the anaerobic condition. The flow rate significantly affected the release amount of Zn, Pb, and Cr, while it slightly affected the concentration of Cu and Cd. This study reflects that any change of pH, temperature, dissolved oxygen, and flow rate of overlying water all cause the different variations of the concentrations of heavy metals.
Combined sewer overflows (CSOs) are major sources of intermittent pollution affecting the receiving waters in the natural water areas as well as in many other urban areas serviced by combined sewers [
When overflow events occur, large volumes of water, charged with high concentrations of sediments, suspended solids, and dissolved and particulate-associated contaminants, such as metals, pesticides, polycyclic aromatic hydrocarbons, and nutrients, are released from the sewer system into the receiving water [
The objective of this study is to evaluate the influence of the pH, temperature, dissolved oxygen, and flow rate of overlying water on the release of heavy metals from storm sewer sediments. More precisely, metals including Pb, Zn, Cu, Cr, and Cd were investigated and the results were discussed in terms of how the factors may modify the release rate, the cumulative release amount, and the concentration of heavy metals.
Metal contaminated sediments were collected from an old neighborhood’s sewer system of North Lishi Road, located in Xicheng District of Beijing, China. This area is poor that was surrounded by traders’ stalls. The sediments were sampled using a shovel on August 3rd, 2011. The date was chosen because it had been kept dry for 5 days.
Sampling site (128 mm depth, 30–100 mm wide) was in the sewer (30 cm diameter), which was 0.3–0.5 m close to the sewer inspection pit. Once dredged, samples were immediately collected into the preparation bottles and then taken to the laboratory after the removal of large debris on-site. The water samples taken from the runoff were also collected by using the plastic containers. Then all sediments were kept in the plastic bags and stored at 4°C [
All samples were transferred to the beakers (1000 mL) and soaked (6 cm depth) with the filtered runoff sewer water. To avoid photosynthesis, all experimental devices were wrapped with black plastic bags. Samples were run duplicates for each procedure. The concentrations of each heavy metal speciations and the main characteristics of the sediments are summarized in Table
Concentration of heavy metals (mg·Kg−1) in test sediments.
Metal | Water-soluble metals | Acetic acid-extractable metals | Oxidizable metals | Reducible metals | Residual metals | Total metals |
---|---|---|---|---|---|---|
Pb | 0.191 | 1.69 | 63.0 | 1.62 | 23.5 | 131 |
Cu | 0.220 | 5.72 | 126 | 1.30 | 16.6 | 166 |
Zn | 2.07 | 9.51 | 18.4 | 21.6 | 149 | 210 |
Cr | 0.582 | 4.73 | 30.9 | 8.81 | 116 | 184 |
Cd | 0.0139 | 0.564 | 0.603 | 0.120 | 0.246 | 2.18 |
The effect of overlying water pH on metal release from sediments was investigated at high (pH 10), mid (pH
Different studies based on combined sewer sediments have shown that heavy metals with different speciations are generally associated with sewer water [
The concentration of Zn in the overlying water changed with times.
At low-pH (pH = 4), the release rate of Zn in solution was much larger than at high-pH or mid-pH. As a matter of fact, the same discrepancy could be observed for the Zn concentrations in the overlying waters at pH 6, 7, and 8. These observations confirmed that the release of Zn to the overlying waters makes no significant difference under any pH conditions. The release of Zn remained low at pH 4 during the first 110 min, but the Zn concentration increased later. In contrast, under pH 6, 7, and 8 conditions, the release of Zn in the overlying waters was observed to be the easiest. Moreover, the lowest Zn concentration was found to appear at 100 min and then remained stable.
At all pH conditions, the Zn concentrations in the overlying waters changed significantly during 100–110 min and then remained stable, which indicated a good correlation between the maximum amount of accumulated Zn release from sediments and the pH (Figure
The relationship between the maximum amount of accumulated Zn release from sediment in experimental period and pH.
The concentration of Cr, Cu, Cd, and Pb in the overlying water changed with times.
Coefficient of determination:
Compared to the experiments of Zn, Figure
There are many combined forms of heavy metals in sediments, including organic matter bound, which could play a regulatory role in the process of heavy metal release from the sediment into the overlying water. While the bound metals release into the water when the decomposition of organic compound occurred [
The amount of cumulative release of Cu, Cr, Zn, and Pb from sediments changed with times.
The results of all the heavy metal releases indicated that the higher DO in the overlying water facilitated more the release of the Zn, Cu, Pb, and Cr than the lower one. It could possibly be explained by the following reasons: (i) the samples were collected in the old neighborhood area and the organic matter contents of them were high; (ii) under the aerobic condition, the organic compound oxidation rate is higher than that under the anaerobic condition and then the release appeared to be higher with higher DO.
With DO > 7 mg/L, there was a significant difference among the release amounts of Cu, Pb, and Cr (i.e., 26.4
As a consequence, the release of Cu, Pb, and Cr was much faster at DO > 5 mg/L than DO < 5 mg/L, especially at the first 50 minutes. Furthermore, the adsorptions of Cu, Pb, and Cr were easily measured under DO < 5 mg/L, while the concentrations of these metals were kept at the steady level. These results indicated that DO = 5 mg/L might be the critical state. Contrary to Pb and Cr, the release of Cu was quite different under anaerobic condition. The small release rate of Cu was detected, which might be due to the different contents and percentages of the different forms of Cu. The results concerning Cd (DO = 9 mg/L) are not shown, since there was no release activity detected during the whole experiments (Figure
The amount of cumulative release of Cd from sediments changed with times.
Under anoxic and anaerobic conditions, the adsorptions of Zn, Cu, Pb, and Cr might be due to ionic bond, formed by the reducible metals and iron and manganese oxides integrating, ruptured, the Fe2+, Mn2+ were released and these ions formed into iron and manganese (hydr) oxide solid phases, which could absorb heavy metals in the overlying water under those conditions. Furthermore, the sulphides might compound with the heavy metals in the overlying water and might be deposited to the sewer sediment under anoxic and anaerobic conditions [
As a matter of fact, there was little Cd in the overlying water at DO = 7 mg/L. Compared to the experiments conducted at DO < 7 mg/L, Figure
Metals of sediments have been equilibrated at temperatures ranging from 4 to 25°C [
The concentration of Zn, Cd, Cu, Pb, and Cr from sediments changed with times.
During the whole experiments, the release of Zn was significant at high-temperature with Zn concentrations of 0.381–0.483 (
The variations in concentrations of Pb were smaller than Cu and Cr at the same temperature. In addition, the changes in concentration of Pb were within 1
The concentrations Cu, Pb, Zn, Cr, and Cd increased rapidly during the first 10 mins and then the rate appeared to be gradually slow, which could be explained as the main effect factor affecting heavy metal release was the disturbance and then was the temperature. Moreover, dissolved Cu, Pb, Cr, and Cd concentrations decreased dramatically after 50 minutes. The results are of great importance and may indicate that the release rate of different metals could be increased at high temperature. However, the final metal concentrations in the overlying water generally reached to the initial concentrations, indicating that the release of metals to the overlying water was only temporary. The present results are consistent with the conclusions drawn from the previous studies [
Moreover, during the whole experiments, the results indicated that the average concentrations of Pb, Cu, and Cr in the overlying water were very well correlated to the temperature evolution. According to Figure
The relationship between the maximum amount of accumulated release of Cu, Pb, and Cr from sediment in experimental period and temperature.
The effect of flow rate was investigated by varying the rotating speed of agitator to simulate the different flow rates. With the flow of the sewer water increasing, the sediment could be exposed to the aerobic environment more easily and the oxidation rate of the organic compound and sulfide fraction might increase. Therefore the organic phase and sulfide fraction metals released more rapidly. Additionally, the flow rate also contributed to the physical disturbance of sediments and the disturbance may change the physical environment, such as pH and DO. The effects of DO and pH on the release of heavy metal mainly affected by the heavy metal speciation, and the contents of different species were different. Therefore the results of the metals release were different during different flow rate conditions and the results shown in Figures
Figure
The concentration of Zn, Cr, and Pb in the overlying water changed with times.
The concentration of Cu and Cd in the overlying water changed with times.
The relationship between the average concentration of metals in overlying water in experimental period and flow rates: (a) Cu, Cr, and Pb; (b) Zn.
The effect of flow rate on the release of Cd was similar to Cu (Figure
All the results during different flow rates appeared to be the same diversification tendency that the concentrations of the metals increased firstly and then decreased again. The same reason could be summed up among the pH, DO, and temperature experiment results. Furthermore, the significant increase of the concentration at the first 10 mins also confirmed the previous research achievement [
Preliminary results about the effect of flow rate may be obtained by deducing eventual correlations between the metal content in the overlying water and the flow rate. Therefore, the parabolic relationships between the average concentration of Cu, Pb, Cr, and flow rates (Figure
A good correlation between Zn contents and flow rates was shown in Figure
The present study focused on examining the effect of pH, temperature, DO, and flow rate of overlying water on the release of heavy metals and has indicated the major roles played by each factor. The effect of pH indicated that (i) the release of Zn, Cu, Cr, Cd, and Pb increased under both the acidic and alkaline condition; (ii) the higher release rate occurred at lower pH; (iii) the maximum release amounts of the metals followed in an order: Zn ≥ Cu > Pb > Cr > Cd at the same pH; (iv) there was a good correlation between the average concentration/the maximum release amounts of each metal and the pH value.
The effect of temperature on the release of Zinc is not significant. Although the concentration of Cd was relatively low, the concentration varied considerably with the changing of the temperature. Actually, at higher temperature the release rates of metal increased with the temperature increasing. At the same temperature, the release rate followed in the order: Zn > Cu > Pb > Cr > Cd. The linear relationship between the average concentration/the maximum release amount of metal and the temperature was deduced. Equilibration time was observed at about 100 min, which was likely extended since the temperature became higher.
The influence of DO on the release of metals was quite different from other factors. The effect of DO was varied from chemical species and proportions of heavy metals in the oxidizable and reducible level. As a matter of fact, the release behavior of Zn, Cu, Cr, and Pb was similar. The release of the metal increased rapidly under the aerobic condition and the adsorption of the metal in the overlying water occurred under the anaerobic condition. In contrast, the release of Cd occurred in the anaerobic condition, and the release became more significant when the concentration of DO was low. The relationship between the average concentration of Zn, Cu, Pb and Cr and the concentration of DO was shown to be a parabolic curve. In addition, logarithmic relationship between the maximum release and the concentration of DO was observed. Nevertheless, the relationship between the average concentration of Cd and the concentration of DO followed an index correlation well.
High flow rate greatly promoted the release of metal from the sewer sediment, but it could also change the conditions such as pH, DO, and others. Therefore, the release behavior of the metals is different at different flow rates. The results suggested that (i) the flow rate significantly affected the release amount of Zn, Pb, and Cr, while it did not significantly affect the concentration of Cu and Cd; (ii) a good relationship between the average concentration/the maximum release amount of Zn, Cu, Pb, and Cr and the flow rate was deduced, and Cu was followed with a linear relationship. This research is a first step towards a better understanding of heavy metal pollution in the combined sewer system.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was supported by the Beijing Academic Innovation Group in Urban Stormwater System and Water Environmental Eco-Technologies (PHR201106124). The authors are also grateful to Beijing Climate Change Response Research and Education Center, BCCRC (PXM 2013-014210-000122).