Optimizing pyrolysis temperature of contaminated rice straw biochar: Heavy metal(loid) deportment,properties evolution,and Pb adsorption/immobilization

Pyrolysis of rice straw (RS), a popular method for producing biochar, effectively treats heavy metal(loid)-contaminated RS. Here, we carried out this process at different temperatures and investigated the deportment of heavy metal(loid)s and the property evolution of biochars. Also, the optimal pyrolysis temperature for Pb adsorption and immobilization was studied. We observed that increasing the temperature could volatilize the heavy metal(loid)s. Cd was the most volatile metal therein, followed by As, while Ni, Cu, and Pb were relatively refractory. More than 75% of the remaining heavy metal(loid)s were non-exchangeable fractions at 700 °C, significantly reducing the environmental risk during subsequent application. Meanwhile, higher pyrolysis temperature resulted in higher pH values, higher surface areas, and stronger Pb adsorption capacity of RS biochars. The maximum adsorption capacity (Q_m) of biochars was in the order of BC300 (77.2 mg·g^?1) < BC500 (137.2 mg·g^?1) < BC700 (222.6 mg·g^?1). Besides, high-temperature biochar could significantly reduce the vertical Pb migration. And BC700 increased the fraction of residual Pb from 39.7% to 44.0% in the soil under the acid rain leaching condition. Therefore, we propose that the heavy metal(loid)-contaminated RS biochar produced at 700 °C might be more suitable for the remediation of soil heavily polluted in the Pb-smelting area.