Metal mobility in the soil
The amount of Cd in different speciation fractions is presented in Table 2. The F1 fraction representing the exchangeable fraction of metals is widely recognized as the most mobile and readily available for microorganisms and plants [30]. Hence, in the present paper, F1 fraction is used to evaluate the variation in Cd mobility in different treatments. The results on F2, F3 and F4 fractions, corre- sponding to metals bound to iron and manganese oxides, to organic matter, and the residual fraction of metals, respectively, are not discussed in detail.
The extractable Cd in F1 fraction was quite similar in the S and SB soils, suggesting that no significant effect on the exchangeable
Cd with single bioaugmentation of B38. In the soil of SN and SNB treatments, the Cd in F1 was obviously lower than that in the control (S). On average, Cd content in F1 decreased by 77% and 94% in the SN and SNB treatment, respectively, as compared to the control (Table 2). A slight increase in pH was observed, i.e., 0.5 units in SN and 0.4 units in SNB. This is because the pH of NG is as high as 11.5 due to the amendment of CaCO3 during treatment. The slight elevation in pH could not totally explain the decrease in exchangeable Cd (F1). We concluded that other interactions might account for the immobilization of Cd by the amendment of NG (SN) and co-amendment of NG and B38 (SNB). As revealed by the FT-IR spectra, both NG and B38 contained active functional groups that can bind Cd, leading to high adsorption capacity for Cd in solution [19]. Hence, the BAB technology was most efficient for metal immobilization. Moreover, in a previous study, the biosorption ability of Cd by both living B38 and nonliving B38 has been investigated. The results revealed that both living and nonliving B38 biomass had high affinity for Cd ions. Moreover, the nonliving biomass showed similar adsorptive capacities as compared with the living biomass and was not likely to be affected by the solution parameters [31]. This implied that the remediation strategy had a long-term effect, even when the B38 died. However, long-term experiment is still needed in field.