Effects of Biological and Enzymatic Ageing on the Surface Properties of Mg-modified Biochar

Abstract

The excellent surface properties of Mg-modified biochar have highlighted a broad application prospect for soil heavy metal mitigation. However, the soil biotic aging process can have an impact on the physical and chemical properties of biochar as well as the heavy metal removal performance. Therefore, MgCl2-modified biochar was subjected to a six-month microbial and enzymatically aged test to explore the effects of different biotic aging processes on the physical and chemical properties. The Pb2+ adsorption performance of Mg-modified biochar that experienced different aging processes was compared using isothermal adsorption experiments. The results showed that after microbial aging, the Mg-modified biochar pyrolyzed at 700 ℃ displayed a low persistent free radical signal, with the biofilm secreted by microorganisms wrapped on the surface of the biochar, leading to the increase in the oxygen-containing functional groups on the biochar surface and the increase in the maximum adsorption capacity of Pb2+ from 55.31 mg·g-1 to 86.27 mg·g-1. In contrast, a higher free radical signal of 500 ℃ pyrolyzed biochar suggested that the free radical inhibited the growth and reproduction of microorganisms on the biochar surface, resulting in an insignificant enhancement in maximum adsorption capacity to Pb2+ compared to that with unmodified biochar. The pores of the Mg-modified biochar were severely damaged during the enzymatically aged process, resulting in a decrease in the adsorption capacity of Pb2+ to 22.18 mg·g-1, which was significantly lower than that of the unmodified biochar. Therefore, the adsorption capacity of microbially aged Mg-modified biochar is superior to that of enzymatically aged Mg-modified biochar, mainly because the oxygen-containing functional groups exhibited on the surface of the biochar covered by microorganisms come from the extracellular secretions of microorganisms, which promote the removal of Pb2+.