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CASESTUDY AREA

The sediment samples used in this example study where taken from Uddevalla harbor seen in the image to the left, an area with a long histiry of industral activity. The local is situated on what is defined as "fill material" by the SGU soilmap to the right. 

BCR-three step secuential extraction

Earlier assesments of the Uddevalla Bay area

Tyrens investigation on the site described the sediment as fillmaterial for the top 1-4meters consisting of sand, gravel and dredged material that was mostly clay. This fillmaterial is underlain by 20-30 meters of clay. The phase 2 MIFO assesment they made of the site, before using sequential extraction was that the ground classified as less sensitive. After further investigation and more visits to the site, the area was changed to a risk 1 area, meaning it poses a threat to both humans and environment if not dealt with.

Four sample was taken from the site. The method to take the sample was a tracked vehicle and also a 100 mm screw auger used, the sample was taken on both grassland and asphalt down 1.1 meter in the soil. From the screw auger took the sample with a spatula and placed in plastic bags. To let the same metal concentration stay in the sample they were placed in a cooler bo.    

In the lab XRF and PID was used together with sequential extraction.

After the samples has been dried the sequential extraction started on material < 2mm. The material > 2 mm was put through sieving.

To do sequential extraction you have to measure the metals flexibility, that is how strong and type of bonding the metals are having in the soil. When the flexibility is decided, the influence of the metal on the surrounding environment, that is to say the vegetation, groundwater, animal life and streams, can be estimated.    

The purpose of BCR optimized three-step sequential extraction procedure is to identify metals that are acid extractable, reducible and oxidizable in the ground.

The result shows the total amount of metal in the sample, divided into three different fractions

  • First fraction 1 is the acid-soluble fraction. This step needs 0,11 M acetic acid (CH3COOH). Of which 40 ml is added to the sample and shaking for 16 h at room temperature. The metals that are released are exchangeable and contain weak electrostatic charge in metal bond.   

  • In fraction 2 needs 0.5 M hydroxyl ammonium chloride (NH2OH.HCl) + 2 M nitric acid (HNO3). Of which 40 ml is added to the sample and shaking for 16 h at room temperature.  Here are the metals going through reducible fraction. In the soil these metals mainly bond to iron (Fe) and manganese (Mn) oxides. Metals are released in this fraction under reducing conditions.

  • Fraction 3 needs 8.8 M hydrogen peroxide (H2O2). 10 ml is added, occasionally manual shaking for 1 hour, placed in water bath (85±2 °C) in 1 h with occasional manual shaking for first ½ h and then reduce volume to less than 3 ml. The procedure is repeated, but the volume is reduced to about 1 ml. After cooling 50 ml of 1 M ammonium acetate (NH4OAc) is added with pH 2.0 ±0.1 (adjusted with concentrated HNO3) and shaking for 16 h at room temperature. Fraction 3 is the metals oxidizable fraction, which is bond to sulphides and organic material in the soil. In this fraction are metals under a long period adsorbed in the soil. Decomposition of the organic material happened under oxidizing conditions.   

  • Residual.  Aqua regia (conc. 1:1 HNO3: HCl). To the residue from Fraction 3 Aqua regia is added (modified to conc. 1:4 HF: HNO3 in this study). The residual fraction is just the metal as is bond inside the crystal lattice of the minerals.

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