«Dissertation zur Erlangung des Grades “Doktor der Naturwissenschaften” Im Promotionsfach Geowissenschaften Am Fachbereich Chemie, Pharmazie und ...»
Figure (28) and (29) showed the total thallium concentration of top and sub-soils at the sampling location 2. The results suggested that highest total thallium concentrations were identified at the top-soils with the concentration range between 0.8-3.2 ppm in comparison with 0.4-1.4 ppm concentration range in the sub-soils. The sub-soils total Tl content (figure 29) was in similar range with sampling location 1(figure 24).
The maximum total thallium content in the top-soil was 33% higher in this case compared with location 1(figure 23). The majority of the area showed total thallium concentration was in above 1 mg/kg limit (FRG) and the area which above this threshold value was larger than in sample location 1. The result was over 3-folds of the common occurrence of soil Tl concentration. A Tl hot spots (sub-sample 5) in which exceeding the 1 mg/kg value was found in the figure 29. Tl content exceeding the safety limit was mainly located at sub-samples 1-6 and the surrounding area. Sub-sampling points in above figure 28 and 29 which demonstrated above 1 mg/kg thallium content should require careful monitoring by the corresponding authority.
By comparing the above results from top and sub-soils at sampling location 2, the found excessive Tl contents on the surface soils indicated that the excessive Tl was from the external source but not from the parental origin. This could be revealed by both locations 1 and 2 subsoils total thallium were in similar concentration (figure 24 and 29) however the top-soils suggested rather various maximum total Tl concentrations (figure 23 and 28).
Results and discussions
In between of the isoline plot of figure 21, 22, 26 and 27, all of these concentration maps demonstrated that the top soils at location 2 was more Tl contaminated than at the location 1.
Previous studies in the past suggested the source of Tl from cement factory was found to be residues of pyrite roasting added as a ferric oxide additive to powdered limestone and fly-dust with majority occurring at water soluble Tl(I) chloride compound. The major source of thallium in air was found to be emission of fly-ash (LIS, 1980). Installation of filter was efficient in reducing cement dust (99%) but not the Tl-containing particles (50%) (Pielow 1979, Prinz et al.
1979 and Weisweiler et al. 1985). Alternation of production method could reduce the thallium emission by more than 99% ( 25µg/m3) (Pielow 1979, Prinz et al. 1979 and LIS 1980).
However, the above results suggested the current soil thallium concentration at location 1 and 2 still showed different degrees of thallium contamination. Thallium itself is a non-volatile heavy metal. In anthropogenic sources thallium exists as an oxide (Tl2O), hydroxide (TlOH), sulphate (Tl2SO4) and sulfide (Tl2S).
In the cement production calcination process heating a mixture of limestone and shale or clay up to 1000 oC is usually required. Thallium compounds are volatile at high temperatures and are not efficiently retained by most emission control facilities. Thallium sulphate and hydroxide could be precipitated out from the atmosphere and the thallium oxides could be removed by atmosphere dispersion and gravitational settling (EPA, 1988). These were possible speculative anthropogenic sources and mechanisms causing soil thallium contamination at the above two sampling locations (location 1 and 2).
Overall, the mobile and total thallium contents in the top and sub-soils at the vicinity of cement factory in Lengerich, Germany were demonstrated.
(6.0) Conclusions (6.1) Arsenic The physicochemical behaviour of arsenic presents in limestone oxide as impurities had been thoroughly investigated. The observed results suggested that arsenic could be partitioned in different metal-bearing phases by the applied sequential extraction scheme and hence arsenic in different forms in the soil system could be identified.
The arsenic contents in five different extraction steps were successfully measured with hydride generation atomic absorption spectroscopy (HG-AAS) under standard addition method to the sub-samples. The accuracy of the measurement was confirmed with the matching As content of the reference material and in addition the total As content of the summation of the 5 extraction steps measured by HG-AAS was in resemblance with the X-ray fluorescence analysis (XRF) of the total arsenic (bulk) in each sub-samples.
All of the measured sub-samples demonstrated much higher As content than the above global average content of arsenic being found in limestone, pellet limestone for soil pH amendment and MAP/DAP phosphate fertilizers. The matrix effect observed in the HG-AAS measurement could be minimized with addition of 1% cysteine as masking agent and the inference mechanism could involve capture and decomposition of the gaseous hydride at the freshly precipitated metal. A significant amount of As was being mobilized by small and high charge density phosphate and one third of the total As managed to be mobilized under exchangeable to mildly acid-reducing conditions. Although the majority of As showed strong binding with crystalline Fe phases, however, uncertainty arouse due to possible repartitioning of As between dissolved Mn and residual Fe oxides in the first three extractions steps. Direct X-ray spectroscopic analysis was important for future analysis.
(6.2) Copper The physicochemical behaviour of copper presents in limestone oxide as impurities had been thoroughly investigated. The observed results suggested that copper could be partitioned in different metal-bearing phases by the applied sequential extraction scheme and hence copper in different forms in the soil system could be identified.
The copper contents in four different extraction steps were successfully measured with atomic absorption spectroscopy (AAS) to the sub-samples. The accuracy of the measurement was confirmed with the matching Cu content of the reference material and in addition the total Cu content of the summation of the 4 extraction steps measured by AAS was in resemblance with the X-ray fluorescence analysis (XRF) of the total copper (bulk) in each sub-samples.
All of the measured sub-samples demonstrated much higher Cu content than Cu content of 2-8 mg/kg limestone commonly used for soil pH amendment (McBride and Spiers, 2001). The recovered copper was partitioned almost evenly among the four extraction steps. The results suggested that copper was mobilized under both acidic and reducing conditions but not possible to identify Cu associated with different specific metal-bearing solid phases (e.g. carbonate matrix or the oxide dendrite). Direct X-ray spectroscopic speciation analysis is required to resolve these uncertainties.
(6.3) Thallium The investigation of soils with thallium exposure in the vicinity of a cement plant at Lengerich, Germany was successfully carried out. The results showed mobile and total thallium concentration at top and sub-soils at two sampling location (location 1 and 2) with both sampling sites contained different level of thallium pollution. Statistical analysis suggested that there were significantly statistic differences of the thallium concentrations between top and sub-soils in the two locations. The variance was potentially due to anthropogenic emission sources.
Semivariance analysis of top-soils ammonium nitrate extracts targeting exchangeable thallium at sampling location 1 had also been performed and the outcome suggested that exchangeable thallium in top-soils was classified as moderately spatially dependent.
Suspect pathways of leading to the soil contamination on the sampling locations were predicted from the possible water soluble thallium compounds being precipitated out from the atmosphere. Thallium containing fly-ash could be volatized and escaped through chimneys and under atmosphere dispersion and gravitational settling with eventually polluting proximity of the soils.
The top and sub-soils thallium concentrations at two sampling sites were demonstrated with different area sizes in where exceeding the safety limit of mobile thallium set by the German Federal Minster of Justice and total thallium established by North-Rhine Westphalia of Federal Republic of Germany (FRG). Remediation control by the corresponding authority was recommended.
(7.0) Acknowledgments This section has been deleted in conformity with the rules stipulated by the internet publisher (ArchiMeD). I am grateful to all the people for their help, comments, discussions and support. Their names are listed in the print-version of this thesis.
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