Evaluation of Cadmium Behavior in a Calcareous Soil as Affected by Walnut-Shell Residues Biochars Coated by Nanoscale Zero-Valent Iron

Using biochar surfaces as a suitable substrate for placing nano-scale zero-valent iron (nZVI), in addition to increase the stability and reduce the agglomeration of these nanoparticles, could improve the biochar sorption mechanisms for various pollutants in environment. In this research, walnut-shell residues (R), walnut-shell residues biochar(B), and walnut-shell residues biochar coated by nZNI (BN) were applied to a Cd-spiked (200 mg Cd kg-1 soil) calcareous soil at three levels (0.5, 2, and 4%); and after incubation process (90 days), soil Cd behavior was examined using desorption kinetic and chemical fractionation experiments. The results showed that application of BN compared with two other amendments significantly reduced Cd desorption (59.69%, 80.16% and 80.5%, respectively, at 0.5, 2 and 4% levels) compared to the control sample. The low values ​​of the Q1 parameter (labile form of Cd) in two-step first-order reactions model fitted on Cd two-phase desorption data in treated soils indicated the positive effect of amendments (in particular BN) on reducing the available Cd compared to the control sample. Application of all three amendments had reduced the relative percentage of carbonate and exchangeable forms, as these reductions in BN-treated soils (2 and 4% levels) were obviously more than the other treatments. Reduction of Cd mobility factor from 68.2% (in control sample) to 35.51, 43.83 and 54.1% in BN-, B-, and R-treated samples, respectively, showed the high effectiveness of BN as compared to B and R treated soils. Based on the results of this study, the biochars coated with nZVI, due to the integration of stabilization mechanisms of biochar (processes of ionic exchange, superficial complexes and surface precipitation or co-perception) and nZVI (processes of sorption and complex formation), showed a higher efficiency on Cd stabilization in soil samples compared to the none-coated biochar and raw organic residues.