Effects of Production Temperature on Suitability of Environmental Applications of Rice Husk Biochars

The environmental application of biochar in soil or water treatment depends on the surface chemistry of biochar and its interaction mechanisms with heavy metals. In this study, rice husk biochars were produced at 300, 500, and 700 °C (RH300, RH500, and RH700). The surface chemistry of these rice husk biochars and their interaction mechanisms with lead (Pb) were investigated. The surface area (from 0.632 to 193.149 m²/g) and pH (from 7.13 to 9.80) of the rice husk biochars significantly increased as production temperature rose from 300 to 700 °C, while the number of functional groups (e.g., carboxyl) decreased. The maximum adsorption capacity (Q_max) values for Pb are in the order of RH300<RH500<RH700 (14.1, 21.7, and 26.7 mg/g respectively). Although RH300 has the smallest Q_max value, its exchangeable Pb amount is the largest (2.61 versus 0.223-0.377 mg/g), suggesting RH300 may be suitable for water treatment due to the easy separation of immobilized Pb and better recycling usage. The Pb immobilized on RH500 and RH700 was mainly acidic soluble and generally stable. Hydrocerussite (Pb₃(CO₃)₂(OH)₂) is one important form within the acidic soluble fraction. Within the generally stable formation, pyromorphite (Pb₅(PO₄)₃X (X=Cl, F, OH)) is a form for the immobilized Pb on the rice husk biochars, particularly for RH500 and RH700. These findings suggest RH500 and RH700 are of huge potential to be applied in soil remediation to immobilize Pb and reduce its environmental risks.