Interactive effects of rice straw biochar and γ-Al2O3 on immobilization of Zn

Biochar system technology has been proved as a sustainable remediation method for metal contaminated soils. However, little attention has been paid to the interaction between biochar and oxide minerals and their influence on metal immobilization in soils. In this study, batch-type Zn sorption experiments were conducted using the mixture of gamma-Al2O3 and rice straw biochar as a model binary geosorbent systems. In addition, advanced spectroscopic technics such as EXAFS, FTIR and XRD were performed to reveal the mechanism. EXAFS spectroscopy revealed that 62% of Zn existed as Zn-Al layered double hydroxide (LDH) on gamma-Al2O3 at pH 7.5 (for 2 mM Zn loading) within 24 h, which was 19% in the mixture. The Zn in biochar samples mainly existed as Zn-OM (53%-76%) and Zn2SiO4 (21%-47%), while the proportion of Zn2SiO4 (0-6%) was negligible compared with Zn-Al silicate (26-48%) in the mixtures. The overall findings confirmed that Al released from gamma-Al2O3 was sorbed in parallel with Zn on biochar to form Zn-Al silicate, rather than Zn-Al LDH on the gamma-Al2O3 surface. These results unveiled the dynamic interactions between amended biochar and soil oxide minerals which can significantly affect the immobilization pathways of metals in contaminated soils.