Sequential Extraction Procedure

Sequential extraction is an analytical method that sequentially chemically leaches elements out of the various mineralogical fractions present in soil, sludge or sediments. It has been recently applied to solid waste such as municipal solid waste incinerator (MWSI) bottom ash and fly ash and coal fly ash (Bruder-Hubscher et al, 2002). Sequential extraction results can give detailed information about origin, mode

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of occurrence, bioavailability, potential mobilizability, and transport of the metals in natural environments. It is therefore widely used as a tool for the study of the source and fate of elements in many solid materials such as soil, sludge, sediment and solid waste. Sequential extraction procedures involve subjecting a given solid sample to a sequence of increasingly strong reagents possessing different chemical properties (acidity, redox potential, or complexing properties) under specified conditions. These extractions are intended to replicate the different possible natural and anthropogenic modifications of environmental conditions. The five-step sequential extraction applied in this study is described below. The extraction method was a modified form of the method proposed by Tessier et al, (1979), (steps 2, 3, 4 and 5). The modification consisted of a water soluble extraction step which was added in this study. First, a water soluble fraction extraction was executed followed by exchangeable fraction extraction, carbonate fraction extraction, iron manganese fraction extraction and residual fraction extraction. The metals extracted were analysed for using inductively coupled plasma - optical emission spectrometer (ICP-OES). The detailed procedures are as follows:

Step 1: Water soluble fractions

1 g of the ash samples taken directly from the ash core samples at various intervals in the ash horizon was weighed into 50 mL centrifuge tubes and 45 mL of ultra-pure water (H2O) was added. The samples were then shaken at room temperature for 1 hour with a mechanical shaker. The procedure was repeated to give triplicate samples. The solution was allowed to settle down for 1 hour. The mixture was centrifuged at 6000 rpm for 20 minutes and the supernatant filtered through a 42 µm pore nucleopore membrane. The remaining solid portion was carefully decanted into a 100 mL plastic clear bottle to reduce weight loss. 42 mL of filtered supernatant was recovered. 10 mL of the supernatant solution was measured into a standard volumetric flask and made up to 100 mL with ultra-pure water (ELGA Pure lab UHQ). The solution obtained was set aside for analysis of major and trace species with ICP-MS. The solid residue remaining after the extraction was quantitatively recovered and kept in a refrigerated condition for the next extraction method.

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Step 2: Exchangeable fractions (Extraction at pH 7)

45 mL of the 1M ammonium acetate buffer solution at pH 7 was added to the solid residue recovered from the water soluble fraction. The solution was shaken for 1 hour at room temperature, and then allowed to settle for 1 hour. The mixture was then centrifuged at 6000 rpm for 20 minutes and filtered through a 42 µm pore nucleopore membrane. The solid portion was decanted into a 100 mL plastic clear bottle quantitatively to avoid weight loss. 44 mL of filtered supernatant was recovered. 10 mL of the supernatant solution was measured into a standard volumetric flask and made up to 100 mL with ultra-pure water (ELGA Pure lab UHQ). The solution obtained was set aside for analysis of major and trace species with ICP-OES. The residue recovered from this step was kept in a refrigerated condition to avoid contamination.

Step 3: Carbonate fractions (Extraction at pH 5)

45 mL of 1M ammonium acetate buffer solution at pH 5 was added to the 0.97 g of solid residue recovered from step 2. The solution was shaken for 1 hour at room temperature. The solution was allowed to settle for 1 hour, and then centrifuged at 6000 rpm for 20 minutes and then filtered through a 42 µm pore nucleopore membrane. The solid portion was quantitatively decanted into 100 mL plastic clear bottle to avoid weight loss. 45 mL of supernatant was recovered. 10 mL of the supernatant solution was measured into a standard volumetric flask and make up to 100 mL with deionised water (ultra-pure water generated with ELGA Pure lab UHQ instrument). The solution obtained was set aside for analysis major and trace species with ICP-OES. The residue recovered from this step was kept in a refrigerated condition.

Step 4: Fe and Mn fractions

45 mL of hydroxylamine hydrochloride (0.25M) in nitric acid (0.025 M) solution was added to the 0.94 g of solid residue recovered from step 3. The solution was shaken for 1 hour at room temperature. The solution was allowed to settle down for 1 hour,

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centrifuged at 6000 rpm for 20 minutes and then filtered through a 42 µm nucleopore membrane. The solid portion was quantitatively decanted into a 100 mL plastic clear bottle carefully to avoid weight loss. 44 mL of filtered supernatant were recovered. 10 mL of the supernatant solution was measured into a standard volumetric flask and made up to 100 mL with ultra-pure water (ELGA Pure lab UHQ). The solution obtained was set aside for analysis of major and trace species with ICP-OES. The solid residue recovered from this extraction step was kept in a refrigerated condition

Step 5: Residual fractions

The solid residue recovered from step 4 was rinsed with ultra-pure water and quantitatively transferred to a crucible, oven dried at 105 ºC and weighed. 0.90 g of dried sample was carefully transferred into the Teflon cup of a Parr bomb. 14 mL of the combined acid (HClO4: HF: HNO3) mixed in the ratio of 3:3:1 respectively was

added and the Teflon cup placed in a Parr bomb, sealed and heated to 180 °C for 3 hours in an oven. It was removed from the oven and allowed to cool down. After cooling, the solution was diluted with 40 mL of 1% HCl and then filtered through a 42 µm pore nucleopore membrane. 10 mL of the solution was measured into standard volumetric flask and made up to 100 mL with ultra-pure water (ELGA Pure lab UHQ). The solution obtained was set aside for analysis of major and trace species with ICP-OES. Dry weight concentration of the analytes for this residual fraction was calculated using the dilution factor and the weight of the solid residue used in the digestion.