A detailed description of soil locations, textures and physio-chemical properties can be found in Sections 2.3.1(page 41) and 3.3.1(page 56). We selected four types of soils out of seven types of soils, namely MGA, KBA, TAA and DUA. Key soil characterisation of these four soils is summarised in Table 4.2. The reason why these four types of soils were chosen for this chapter was that they had distinct soil parameters in terms of total organic carbon (TOC), Fe oxide, Al oxide, pH which have been observed as important controllers for bioaccessibility of As, Cd and Pb reported in previous chapters. MGA soils contained a high amount of TOC (8.37 %), Fe oxide (12.49 g kg-1), Al oxide (12.6 g kg-1) whilst few TOC (1.48 %), Fe oxide (0.64 g kg-1) and
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Bioaccessibility in soils reached a steady state within 30-90 days of aging (Table 4.3). This was in accordance with previous researchers who have reported that bioaccessibility of As, Cd and Pb levelled off when soils were aged for 1 to 3 months (Yang et al., 2002; Tang et al., 2006c; Tang et al., 2007b; Tang et al., 2008). In addition, it is noted that the bioaccessibility of As, Cd and Pb measured in soils which were aged for 90 days were similar to those aged for one year using the same soils reported in Chapters 2 and 3. The uniformity in bioaccessibility between 90 days and one year indicates no pronounced metal/metalloid-soil interaction that may lead to a significant decrease in bioaccessibility after 90 days under the aging conditions in this current study. Also, in the previous chapters, it is demonstrated that TOC, Fe oxide and Al oxide can decrease the bioaccessibility of As, Cd and Pb as measured by UBM. These relationships were also observed here with MGA soils which had the highest amount of TOC, Fe oxide and Al oxide among the four types of soils showing the lowest bioaccessibility of As, Cd and Pb. Moreover, within the four types of soils in this study, Fe and Al oxides seemed to play a more important role than TOC. Bioaccessibility of As, Cd and Pb in TAA soils was found to be lower than those in KBA soils. When comparing their soil properties, it is worthy of notice that TAA soils possessed higher contents of Fe and Al oxides but lower organic carbon than KBA soils. Arsenic, Cd and Pb in DUA soils were most bioaccessible since it contained apparently fewer binding sites (i.e. lower organic carbon, Fe and Al oxides). Gastric bioaccessibility of Cd and Pb were generally observed to be much higher than those of As because under acidic gastric condition (pH = 1.2 – 1.5), the soil surfaces would be more positively charged than at ambient soil
Bioaccessibility in soils reached a steady state within 30-90 days of aging (Table 4.3). This was in accordance with previous researchers who have reported that bioaccessibility of As, Cd and Pb levelled off when soils were aged for 1 to 3 months (Yang et al., 2002; Tang et al., 2006c; Tang et al., 2007b; Tang et al., 2008). In addition, it is noted that the bioaccessibility of As, Cd and Pb measured in soils which were aged for 90 days were similar to those aged for one year using the same soils reported in Chapters 2 and 3. The uniformity in bioaccessibility between 90 days and one year indicates no pronounced metal/metalloid-soil interaction that may lead to a significant decrease in bioaccessibility after 90 days under the aging conditions in this current study. Also, in the previous chapters, it is demonstrated that TOC, Fe oxide and Al oxide can decrease the bioaccessibility of As, Cd and Pb as measured by UBM. These relationships were also observed here with MGA soils which had the highest amount of TOC, Fe oxide and Al oxide among the four types of soils showing the lowest bioaccessibility of As, Cd and Pb. Moreover, within the four types of soils in this study, Fe and Al oxides seemed to play a more important role than TOC. Bioaccessibility of As, Cd and Pb in TAA soils was found to be lower than those in KBA soils. When comparing their soil properties, it is worthy of notice that TAA soils possessed higher contents of Fe and Al oxides but lower organic carbon than KBA soils. Arsenic, Cd and Pb in DUA soils were most bioaccessible since it contained apparently fewer binding sites (i.e. lower organic carbon, Fe and Al oxides). Gastric bioaccessibility of Cd and Pb were generally observed to be much higher than those of As because under acidic gastric condition (pH = 1.2 – 1.5), the soil surfaces would be more positively charged than at ambient soil