Component Properties and Heavy Metal Accumulation Characteristics of Contaminated Rice Straw Biochar During Oxygen-controlled Pyrolysis

Abstract

Pyrolysis is an important technology to achieve the harmlessness and recycling of contaminated biomass. In this study, the effects of oxygen-controlled atmosphere on the component properties and heavy metal accumulation characteristics of contaminated rice straw biochar were studied. The results showed that low-oxygen pyrolysis could effectively produce biochar using contaminated rice straw and improve the stability of heavy metals in biochar. Under the nitrogen atmosphere, the yield of rice straw biochar was 29.4%-34.9%. The aromatization index (SUVA254) of dissolved organic matter (DOM) increased first and then decreased with the increase in pyrolysis temperature, whereas the fluorescent components were mainly humic-like acid substances. Meanwhile, Ca mainly existed in the form of CaCO3 in biochar. Compared with the pure nitrogen condition, the biochar yield was reduced by 5.6%-13.5% and 14.9%-15.7% under the pyrolysis atmosphere containing 10% and 20% oxygen content, respectively. Ca existed in the form of CaO in biochar, which increased the pH value of the biochar by more than 0.5 units. The oxygen of the pyrolysis atmosphere accelerated the degradation of the lignin component, resulting in the gradual decrease in SUVA254 of DOM. With the increase in oxygen content in the pyrolysis atmosphere, humic-like acid substances in DOM were transformed into fulvic-like acid substances. Under the conditions of 400℃ and a 10% oxygen-containing atmosphere, the exchangeable fractions of Cu, Cd, Pb, Ni, and As in biochar were decreased by 5.2%, 3.7%, 1.7%, 0.8%, and 0.7%, respectively, indicating that heavy metals are transformed into more stable states. The results suggested that the higher biochar yield and heavy metal stability could be obtained by introducing a proper amount of nitrogen into the air (controlling the oxygen content of approximately 10%) for pyrolysis treatment of contaminated rice straw, providing an economic and feasible technology for the achievement of harmlessness and recovery of contaminated rice straw.