The Interfacial Behavior between Biochar and Soil Minerals and Its Effect on the Char Stability.

In this study, walnut shell derived biochar was incubated with 4 representative soil minerals for 3 months to simulate the combined process in real soil, and the incubated biochar was then studied for the interfacial behavior with surrounding minerals by XRD and SEM-EDS. In addition, XPS and H₂O₂ oxidation as well as TGA were used to evaluate the effect of minerals on the short-term stability of biochar surface and long-term stability of entire biochar particles, respectively. Fe₈O₈(OH)₈Cl_1.35 and AlCl₃·6H₂O were formed and attached tightly to biochar surface or inserted into biochar inner pores. The Al/Si mole ratio of a small mineral phase in a biochar pore was about 0.75, much less than the ratio (1) of kaolinite, and at the interface between biochar and minerals, the Fe/O mole ratio both increased greatly while C mole ratio decreased but still remained at high levels (over 10%), which implied the formation of organometallic Al-O-C and Fe-O-C complexes. The Al, Ca, Fe compounds and kaolinite enhanced the short-term stability of biochar surface by decreasing the relative content of C-O and C=O from 34.2% to 6.37%~21.3%, while antagonistic effects occurred when kaolinite and these metal minerals acted together. However, the long-term stability of entire biochar particles was consistently increased for both these minerals alone and the minerals combined with kaolinite. The mechanisms of short-term and long-term stability are likely attributed to the physical isolation by soil minerals and the formation of organometallic complex, respectively. Results implied that mineral-rich soil seemed to be a beneficial environment for biochar as the combination with soil minerals could increase its stability, which displays an important environmental significance for carbon sequestration.