Mechanisms of cadmium adsorption by ramie nano-biochar with different aged treatments

Abstract

Biochar (BC) has proven to be very effective against the remediation of cadmium contamination. However, aged treatment is inevitable and variations in the adsorption mechanism caused by ageing remain unclear. In this study, different treatments (i.e., photo-aged (nano-PBC) and chemical treatment (nano-NBC)) were used on the nano-BC. The nano-BC as well as the two aged nano-BCs were characterized separately to examine their surface morphology, functional groups, and variations in phase composition. The aged nano-BCs exhibited larger specific surface area and mesopore volume. The nano-NBC produced more surface functional groups (i.e., carboxyl groups, lactone groups, and phenolic hydroxyl groups) compared to the nano-BC and nano-PBC. In addition, the study revealed the effect of aged treatment on the adsorption mechanism of the nano-BCs by adsorption kinetic model and adsorption isotherm model, respectively. The fitted consequences revealed that the adsorption of Cd by the three nano-BCs was consistent with the pseudo-second order kinetic model and the Langmuir adsorption isotherm model. The aged nano-BCs have higher Cd adsorption capacity, which was more effective for the nano-NBC. In addition, different adsorption conditions (i.e., pH, ionic strength, coexisting cations, and initial nano-BCs addition concentration) were performed to explore the optimal adsorption capacity. The results showed the optimal adsorption efficiency of 93.24 % was achieved for nano-NBC at pH 7, a background solution of 0.001 M Na+, and an initial nano-BC addition concentration of 50 mg L−1. The different aged treatments did not significantly affect the adsorption mechanism of the nano-BCs, but changed the chemisorption mode and enhanced the physical adsorption (pore size filling and hydrogen bonding). Namely, aged treatment significantly enhanced the adsorption capacity of the nano-BCs on Cd.