Capacity and mechanisms of ammonium and cadmium sorption on different wetland-plant derived biochars

Abstract

The objective of this study was to investigate the relationship between Cd2+/NH4+ sorption and physicochemical properties of biochars produced from different wetland plants. Biochars from six species of wetland plants (i.e., Canna indica, Pennisetum purpureum Schum, Thalia dealbata, Zizania caduciflora, Phragmites australis and Vetiveria zizanioides) were obtained at 500°C and characterized, and their sorption for ammonium and cadmium was determined. There were significant differences in elemental composition, functional groups and specific surface area among the biochars derived from different wetland plant species. Sorption of ammonium and cadmium on the biochars could be described by a pseudo second order kinetic model, and the simple Langmuir model fits the isotherm data better than the Freundlich or Temkin model. The C. indica derived biochar had the largest sorption capacity for NH4+ and Cd2+, with a maximum sorption of 13.35 and 125.8mgg−1, respectively. P. purpureum Schum derived biochar had a similar maximum sorption (119.3mgg−1) for Cd2+. Ammonium sorption was mainly controlled by cation exchange, surface complexation with oxygen-containing functional groups and the formation of magnesium ammonium phosphate compounds, whereas for Cd2+ sorption, the formation of cadmium phosphate precipitates, cation exchange and binding to oxygen-containing groups were the major possible mechanisms. In addition, the sorption of ammonium and cadmium was not affected by surface area and microporosity of the biochars.