Enhanced adsorption complexation of biochar by nitrogen-containing functional groups

Biochar has abundant oxygen-containing functional groups and a unique surface structure, which endow it with properties that are desirable for various environmental protection applications. However, to our knowledge, the potential applicability of biochar comprising non–‍oxygen-containing functional groups has not yet been systematically studied. This work comprehensively investigated the adsorption capacity of grafted biochars modified with various N-, O-, and S-containing functional groups. We also analyzed the mechanisms governing, pollutant removal processes, and their recycling properties of biochars. The degree of adsorption of the biochars increased with the increasing electron-withdrawing ability of the grafted functional groups (i.e., -NO2 > -NH2 > -CN > -SO3H > -OH), and the optimal temperature for preparing biochars with grafting modifications was 300°C. The -NO2 group was successfully grafted onto the cellulose of bagasse-based biochar. The maximum adsorption capacity of -NO2 grafted biochar (NBC300) was 158.73 mg/g, and the desorption rate was 88.35% for 0.1 M HCl. Nitro grafting modifications of internal molecular compositions led to biochars with good stability and regeneration properties in acidic, alkaline, and oxidizing environments. Thermodynamic studies suggested that Cd(II) could be effectively removed by NBC300 at low temperatures through a spontaneous and feasible process. Spectroscopic analysis and batch adsorption experiments showed that the modified biochar removed Cd(II) via a mechanism involving strong adsorption-complexation of the π-conjugated system on the -NO2 groups. The present study supports biochar applications for the recycling and reuse of agricultural waste and describes a novel technology for addressing the key issue of metal pollution.