Evaluation of the As, Cu and Pb Immobilizing Efficiency by Tessier, TCLP and SBET Method

Abstract:

Utilize of property evaluation method for an immobilizing procedure is very important. In this paper, an immobilizing procedure was used to stabilize heavy metals in tested soils. Tessier sequence extract procedure (SEP), toxicity leaching procedure (TCLP) and simple bioaccessibility extraction test (SBET) were used to evaluate the efficiency of the stabilization procedure. The leachability, bioaccessibility and speciation of As, Cu and Pb in the tested soils were analyzed using Tessier SEP, TCLP and SBET method separately. The results indicated that the most active part of the heavy metals could be extracted out by TCLP method and it could be used to evaluate the risk of the tested soils. Tessier method divided heavy metals in soils to different fractions. Monitoring the changes of different fractions during the immobilizing procedure could help fully understand the mechanism of stabilization. The SBET method simulated the human digest system and evaluated the risk changes to human during the immobilizing procedure. And it helped to reveal the potential risk of chemical stability. In our project, these three evaluation method should be utilized properly to help accessing the risk, electing suitable immobilizing method and evaluating the efficiency of stabilization.

• Periodical: Advanced Materials Research (Volume 878)
• Pages: 520-531
• DOI: https://doi.org/10.4028/www.scientific.net/AMR.878.520
• Online since: January 2014
• Authors: Dan Dan Liu, De Ren Miao, Fei Liu
• Keywords: Heavy Metal, Immobilisation, SBET, TCLP, Tessier

References

[19] cation-exchange capacity (CEC) was measured by ammonium acetate saturation method; clay and sand was distinguished by X-ray diffraction analysis (XRD); CaO, Fe2O3 and P2O5 were determined by X-ray fluorescence. All chemicals used in this study were AR or higher grade. All solutions were prepared with deionized water (Milli-Q water system). All batch experiments were done in triplicates. Results and discussion Heavy metals in the soil samples. Soil samples used in this study were seriously polluted by heavy metal (Table 3). The leaching concentration of Pb was about 1. 6-2. 6 times than the regulatory limitation (5. 0mg·L-1) in EPA standards.

DOI: 10.7554/elife.03604.008

[20] And Cu in S2 and S3 were also exceeded the limitation(100mg·L-1). Figure 2. The fraction distribution of As, Cu and Pb (F1: exchangeable fraction; F2, carbonate fraction; F3, Fe-Mn oxides fraction; F4, Organic fraction; F5, residual associated) In the tested soils, the main form of As were residual (89. 95~91. 84%) and Fe-Mn oxides (7. 30~9. 34%). Exchangeable and organic associated were the main speciation of Cu, taken 18. 56~34. 26% and 13. 78~29. 96% separately. And 38. 71~46. 98% of residual and 32. 92~35. 81% of carbonate were observed for Pb. Expect As, bioavailability and to be easily bioavailability fractions took the main parts of the total heavy metal (53. 02~81. 44%), which mean that that heavy metals present in these soils have a high risk for human and plants. Gleyzes.

[21] reported that Tessier scheme overestimated residual As when used to characterize As in iron-rich industrial or mining site soils. In out tested soil, the Fe2O3 was as high as 13. 41%. Arsenic preferred to combine with iron forming iron oxides, which was hard to be extracted by 0. 04M NH2OH·HCl. And this part was took as residual fraction as misleading. Thus, the Tessier results showed that the residual fraction of As in the tested soil took as much as 80-90% of the total As in soils. Fractions changes of heavy metals analyzed by Tessier method during the stabilization process. The fractions of heavy metals changed when different reagents were added during the stabilization process. After incubated with H3PO4 for one week, the fraction of As, Cu and Pb were changed greatly. The exchangeable and carbonate fractions of Pb decreased by 12. 95%, 11. 15% and 9. 76% while the residual fraction increased to 67. 09%, 81. 61% and 87. 78%. The residual fraction of Cu changed no more than 3. 18%. In theory, phosphate ions can format phosphate precipitation not only with Pb but also with Cu in pure water.

[22] Nervertheless, soil system is much more complicate compared with the water system.

[23] Some researchers found that Cu was hard to form phosphate minerals in soils [24-26]. The fifth week's samples, compared with the third week, Pb decreased in the iron and manganese oxides and increased in the residual fraction. Cu increased in the organic fraction, decreased in the fraction of exchangeable, carbonate, iron and manganese oxides fraction. In this process, the organic fractions of all three heavy metals were increased, especially for Cu. This meant that Cu might formatted sulfides mineral in the tested soil. After the first immobilization step, the pH value of tested soils decreased because of increasing of H+. In this step, the soils' pH value increased greatly. This was because S2- hydrolyzed as following: S2-aq+H2Ol→HS-aq+OH-aq (3) And all metal sulfides could be hydrolyzed; even they had the low solubility. The hydrolysis process was as following: MSs+H2Ol→M2++OH-aq+HS-aq (4) Where M is metals The hydrolysis of S2- and metal sulfides increased the concentration of OH- in soil solutions. After the seventh week, the carbonate fraction of Pb and Cu increased due to the effect of CaCO3. When CaCO3 was added into the elevated moisture content soils, it increased the pH of the solution and accelerated the precipitation of heavy metals [27-29]. The chemical reactions between CaCO3 and heavy metals might happen as flowing: CaCO3+H2O→Ca2++CO32- (5) CO32-+H2O→HCO3-+OH- (6) Mn++n(OH)-→M(OH)n (7) Where M is Metal. Ion exchange and surface adsorption(i. e. CuCO3 could generate by cation exchange).

[30] played an important role when CaCO3 present in the tested soil. Ion exchange was achieved by the following equation.

[31] CaCO3+Cd2+→CdCO3+Ca2+ (8) CaCO3+Pb2+→PbCO3+Ca2+ (9) Carbonate was not a very stable fraction. Heavy metal could be transferred to exchange fraction in acidic conditions. In immobilizing procedure, it should be avoided formatting considerable amount of carbonate fraction. In this step, the increase of carbonate fraction was no more than 2. 08%. It was because the exchangeable heavy metal ions had been transported to more stabile fractions in the former process. Available ions that could be transported to carbonate fraction were limited.

[32] Figure 3. The fraction distribution of As, Cu and Pb (F1: exchangeable fraction; F2, carbonate fraction; F3, Fe-Mn oxides fraction; F4, Organic fraction; F5, residual associated. The 0 week was the initial chemical speciation of heavy metals, the first week was simples after added H3PO4 and incubating for a week, the 5th week was samples after added Na2S and incubating for four weeks, the 7th week was samples after added CaCO3 and sampled two weeks later, the 20th week was sampling at the end of the experiment process). Concentration changes of heavy metals analyzed by TCLP during the stabilized process. After the first week incubated with H3PO4, concentration of Pb in leachate decreased rapidly. The leachability of Pb in S1, S2, and S3 were reduced from 3. 23%, 4. 94%, and 1. 06% to 0. 18%, 0. 24%, and 0. 04%, respectively. This agreed with Tessier's results. P was extremely efficient for Pb stabilization due to the generation of phosphate precipitation or minerals, and it was very stable in various environmental conditions.

[3] The leachability of Cu in S1 and S3 also decreased at the first week, but it increased in S2. H3PO4 have different affections on Cu stabilization. On the one hand, it decreased the soil pH which inhabited the immobilization of Cu. On the other hand, H3PO4 was a good adsorbent. Cu was absorbed by extra H3PO4. The amount of H3PO4 added in S1 and S3 was larger than in S2. Accordingly, the adsorption effect of it on S1 and S3 were more significantly than S2. In tested soils, As was activated by H3PO4. The leachability in S1, S2, and S3 increased from 0. 44%, 1. 15%, and 0. 89% to 4. 97%, 3. 82%, and 3. 85%, respectively. This was due to the similar chemistry characteristic of P and As. AsO43- was exchanged into soil by PO43.

DOI: 10.1158/2326-6066.22546742.v1

[3] When the Na2S was added after two weeks (fifth week), the leachate concentration of Pb increased little but still under the regular limits. And the concentration of Cu in S1, S2, and S3 reduced from 15. 5%, 43. 36%, and 19. 20% to 2. 34%, 2. 07%, and 4. 34%, respectively; The effect of S2- on As stabilization was much more complex, which depend on the speciation of sulfarsenite.

[33] Former study has reported that in sulfide reduction environment, the trio-arsenate (arsenic trioxide) takes more than 50% of the total As[34, 35]. Moreover, the increasing of SO42- from sodium sulfide can inhibit the adsorption of As by iron oxides, sulfate.

[36] So immobilizing efficiency of Na2S on As was vary from soil to soil. lg K=8. 69 (10) lg K=3. 54 (11) lg K=5. 06 (12) lg K=11. 78 (13) lg K=3. 89 (14) lg K=16. 16 (15) As showed in figure 4, CaCO3 have no effects on Pb and Cu, but significantly reduced the leaching concentration of As and the final concentration at the twentieth week was considerably lower (<5. 00 mg/L). Figure 4. Concentrations of Pb in TCLP leachate of tested soils at different sampling times during the whole incubative process. ■S1: [As]0=0. 58 mg·L-1 , [Pb]0=15. 83 mg·L-1, Cu]0=81. 28 mg·L-1, ●S2: [As]0= 0. 55mg·L-1, [Pb]0=8. 41mg·L-1, [Cu]0=, 88. 64mg·L-1 ▲S3: [As]0= 2. 41mg·L-1 [Pb]0=15. 27mg·L-1 [Cu]0=196. 76mg·L-1. SBET analysis for the heavy metals after the stabilization process. Oomen.

[11] used SBET method to assess the bioaccessibility of As, Cd and Pb in soils with distinct physical properties. He found that the bioaccessibility of As in three different kind of soils was between 6-95%, 1-19% and 10-59%; As for Pb, they were between 4-91%, 1-56%, and 3-90%. In this study, the bioaccessibility of As in S1, S2, and S3 were 0. 52%, 1. 24%, and 0. 21% receptivity. Values for Pb were 3. 51%, 2. 96%, and 1. 15% respectively. The bioaccessibility of the tested soils were significantly lower compared with the former results reported. The bioaccessibility of Pb decreased greatly after the immobilization process. There are two reasons: first, phosphorus in the stabilization process increased the generation of phosphate precipitation or minerals, which induced the decrease of the bioaccessibility of Pb; second, amino acid used as the extraction in SBET method could react with Pb2+ (lgk1=5. 05, lgk2=5. 58) which also led to the underestimation of the bioaccessibility of Pb. An increasing trend was observed for As, which means the competition between the phosphate ions and the arsenate ion was existed even in the mammalian digestive system. The decrease in carbonate fraction had nothing to help on the reducing of bioaccessibility. Figure 5. Bioaccessibility of As and Pb in tested soils Conclusion In this study, TCLP could directly response to changes the most active part heavy metal in soils. In the whole immobilization procedure, the variation of heavy metal concentration in leachate was monitored. It could help assessing the risk of soil samples. In the experiment, both H3PO4 and Na2S reduced the leachate concentration, however, the mechanism of the two immobilizing reagents on Cu were different. And the stability of precipitations generated in the two steps was also different. Tessier could display the changes between various fractions. It could help reflect some of the problems that cannot observe by TCLP results. From the Tessier analysis, it was easy to evaluate what kind of mineral formed in the stabilizing procedure. This method could not only be used to evaluate the stabilization efficiency but help analyze the mechanism of stabilization. The SBET method simulated the human digest system and evaluated the risk changes to human during the immobilizing procedure. It helped to reveal the potential risk of chemical stability. During the experimental procedure, As was first disturbed by H3PO4 during our experimental procedure. At the end of the procedure, both TCLP and Tessier results showed that the activated As has been re-immobilized. However, the SBET results found that the bioavailability of As increased 2 to 5 times than originally tested soils. In order to relevant the problems during the immobilizing procedure and overall evaluate the stabilizing efficiency different evaluate method should combine utilize in the further work. Acknowledgements This work was funded by the project from China Ministry of Science and Technology (2007AA06A410), the Fundamental Research Funds for the Central Universities, 2010ZD13, and the National Natural Science Foundation of China (40972162). The analytical data were supplied by the Lab of Water Resources and Environmental Engineering in the China University of Geosciences (Beijing). Referances.

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