Comparison of Pb2+ Adsorption Properties of Biochars Modified Through CO2 Atmosphere Pyrolysis and Nitric Acid

Abstract

Acid modification has been widely used to modify the structural properties of biochars. However, acid modification led to the large consumption of acid, increased difficulty of waste effluent disposal, and a high application cost. To evaluate the advantages and application potential of biochars prepared under CO2, utilizing pyrolysis to directly modify biochars to improve heavy metal removal efficiency and reduce production cost, would be an important prerequisite for the broad application of biochars. The sorption performance of Pb2+ with CO2-modified biochars was compared with that of HNO3-modified biochar. The elemental compositions and structural properties of biochars were characterized through elemental analysis, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results revealed that for biochars produced at 500℃, HNO3 modification produced abundant carboxylic groups and -NO2 (asy) and -NO2 (sym) groups, promoting the surface activities and complexing abilities of biochars. The CO2-modified biochars contained abundant carbonate minerals, which could remove Pb2+ by electrostatic ion exchange and coprecipitation or complex. In addition, compared to that of HNO3-modified biochars, CO2-modified biochars had the larger specific surface area and better microporous structures, which were beneficial to the diffusion of Pb2+ and further promoted surface sorption. CO2 modification increased the maximum Pb2+ sorption capacity of W500CO2 and W700CO2, which were 60.14 mg·g-1 and 71.69 mg·g-1. By contrast, HNO3-modified biochars W500N2-A and W700N2-A showed the lower Pb2+ sorption capacities, which were 42.26 mg·g-1 and 68.3 mg·g-1, respectively. The increasing of the specific surface area and functional groups simultaneously promoted the sorption capacity of CO2-modified biochars. Consequently, the CO2-modified biochar had the advantages of low cost, environmental friendliness, and high heavy metal removal efficiency, which is a modification method worthy of promotion and application.