Formation and regulation of environmentally persistent free radicals in biochar and removal of diethyl-phthalate in water

Abstract

Biochar contains environmentally persistent free radicals (EPFRs), a class of pollutants whose regulation and influencing factors need to be studied. In this study, electron paramagnetic resonance (EPR) signals were observed for biochar samples obtained by the pyrolysis of wheat straw, corn straw, peanut shell, and cotton stalk. The effects of pyrolysis temperature, residence time, and alkali/alkaline earth metals (AAEMs) on EPFRs in biochar were investigated. The adsorption effect of diethyl phthalate (DEP) on biochar obtained from the pyrolysis of wheat straw impregnated with K, Na, Ca, and Mg was also studied. The concentration of EPFRs increased first and then decreased upon increasing the pyrolysis temperature within the rage of 300–700 ℃, conforming to a Gaussian fit. Residence time experiments showed that the concentration of EPFRs was the highest at a residence time of 10 min. The effect of AAEMs on the EPFRs concentration followed the order Mg > K > Ca > Na due to the catalysis of a secondary reaction. Because of hole-filling and electrostatic attraction, biochar obtained by pyrolysis of wheat straw impregnated with MgCl2 (WSMg) had the highest DEP removal rate of 95.969%. DEP removal efficiency of the five adsorbents followed the order WSMg > WSCa (88.701%) > WS (68.867%) > WSNa (53.852%) > WSK (50.845%). Hydroxyl radicals (•OH) signals were detected in these five adsorption systems, showing that EPFRs could activated H2O2 to produce •OH. However, the five •OH signals had similar intensities, indicating that AAEMs had not significantly effects on the process of activation of H2O2 to produce •OH by EPFRs. The results of this study are of great significance for understanding the formation and regulation of EPFRs and the utilization of biochar.