Characteristics and quantification of mechanisms of Cd2+ adsorption by biochars derived from three different plant-based biomass

Abstract

The physicochemical properties of biochars derived from rice straw, eucalyptus leaves, and vetiver grass pyrolyzed at 400 °C were investigated. It was found that rice straw biochar (RSB) may possess more acidic functional groups than eucalyptus leaves biochar (ELB) and vetiver grass biochar (VGB) while the specific surface area of VGB was the largest. Cd2+ adsorption characteristics by the three biochars were established under the effects of time, initial pH and concentration of solution. Maximum adsorption capacities of RSB, ELB, and VGB calculated by Langmuir model were 57.87 mg/g, 50.21 mg/g, and 44.09 mg/g, respectively, reflecting adsorption mechanisms may be dominated by chemical reaction than physical interaction. SEM-EDS, XRD, FTIR and XPS were employed before and after adsorption to qualitatively explore possible adsorption mechanisms of the three biochar. It was found that precipitation, ion exchange, complexation and cation π interaction occurred in all the biochars. The relative contribution of the four main mechanisms to total adsorption capacity was quantitatively determined, suggesting precipitation, ion exchange, and cation π interaction were dominated for the three biochar, while complexation accounted for an insignificant part. This study clearly demonstrated mineral constituents of biochar had significant effects on Cd2+ adsorption by plant-based biochars, shedding some light on their future application.

Keywords

• Feedstocks
• Biochar
• Cadmium
• Adsorption mechanisms