Contribution of different mechanisms to Pb2+ and Cd2+ sorption on magnetic wheat straw biochars: Impact of pyrolysis temperature and DOM in biochar

This work quantitatively studied the sorption of Pb2+ and Cd2+ on magnetic biochars (MBCs) pyrolyzed at temperatures of 300, 500 and 700 °C and the effect of biochar-derived dissolved organic matter (BDOM) on Pb2+ and Cd2+ sorption. Surface characteristics of the MBCs before and after sorption were analyzed using Boehm titration and various spectroscopic techniques. For Pb2+, with increasing pyrolysis temperature, the sorption amounts by precipitation and coordination with π electrons increased from 63.5 to 109.3 mg/g and from 27.0 to 84.9 mg/g, respectively. While the sorption amount by complexation decreased from 34.5 to 16.5 mg/g. The contribution of precipitation (45.9–52.5%) and coordination with π electrons (19.5–39.1%) increased with increasing pyrolysis temperature, while contribution of complexation (7.6–24.9%) decreased. Increased ash content and aromaticity with pyrolysis temperature played the key roles in the change of Pb2+ sorption. The ion exchange and precipitation mechanisms dominated Cd2+ sorption on MBCs. Their respective sorption amounts were in the ranges of 5.08–10.36 and 1.31–9.82 mg/g, accounting for 48.8–72.5% and 18.7–46.3% of the total sorption, respectively. The change of ion-exchangeable sites on minerals with pyrolysis temperature is the main reason influencing the sorption via ion exchange for Cd2+. Unlike MBC500 and MBC700, removing BDOM from pristine biochar of MBC300 enhanced the total amounts of Pb2+ and Cd2+ sorption by 1.2 and 3.6 times, respectively, which was attributed to the enhanced sorption via ion exchange, precipitation and complexation for Pb2+ and enhanced sorption via ion exchange and precipitation for Cd2+. Therefore, the DOM-removed low-temperature MBC is a promising sorbent for treating heavy metal polluted waters.