Effect of nitric acid pre-oxidation concentration on pore structure and nitrogen/oxygen active decoration sites of ethylenediamine -modified biochar for mercury(II) adsorption and the possible mechanism

Abstract

Controlling of pre-oxidation conditions can effectively enhance the aimed active functional groups via promoting the oxidation and grafting reaction on biochar's surface. Here, the effect of different nitric acid pre-oxidation concentration (NAPOC) was investigated on the type and content of active oxygen-containing functional sites during the pre-oxidation stage, as well as the active nitrogen-containing binding sites for the following grafting process. And the possible reaction mechanisms for introducing nitrogen/oxygen-containing functional groups such as amide, pyridinic, carbonyl, carboxyl, etc., into the surface by ethylenediamine (EDA) were proposed. The samples were characterized by various analyses including N2 adsorption/desorption, Boehm titration, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Results showed that the NAPOC played a crucial role in promoting the formation of oxygen-containing initiators, and difference of NAPOC resulted in different reaction principles. At higher NAPOC, more carbonyl, carboxyl and hydroxyl functional groups were formed, which facilitated the decoration of nitrogen binding active sites of amide and pyridinic for mercury ions adsorption into the carbon lattice of mesoporous biomass-derived biochar (MBB). The proportions of micropore and mesopore remained basically unchanged, indicating that the decorated nitrogen/oxygen sites were highly uniformly dispersed in MBB's frame and thus resulted in high activity. The comparison of adsorption properties of MBB showed that MBB-25-EDA had the highest adsorption capacity of 153 mg g−1 at pH 6, confirming that the 25% was the optimum NAPOC for introducing nitrogen/oxygen functional binding sites for effectively anchoring mercury.