Modelling cadmium binding to plant-based biochars using the three-site NICA-Donnan model

Abstract

In this study, generic three-site NICA model parameters for cadmium (Cd) binding to biochars were derived. For this purpose, five biochars prepared from two mixtures of pure cellulose and lignin and maize straw at different pyrolysis temperatures were selected from our previous work to measure Cd binding in batch experiments. This dataset was complemented with data on Cd binding from five other plant-based biochars taken from literature. The three-site NICA model in combination with an electrostatic Donnan model using a constant Donnan volume $V_D$ of 0.1 L kg⁻¹ successfully described the data, covering a wide range in pH and free Cd²⁺ concentration in solution. The model provided quantitative insights into the surface speciation of Cd and the electrostatic interactions between Cd and biochar. Carboxylic- and phenolic-type reactive groups exhibited a lower Cd affinity compared to protons, leading to a strong pH dependency of Cd binding. Cadmium ions bind predominantly as a monodentate complex with carboxylic groups and as mono- and bidentate complexes with phenolic groups on biochar. Although phenolic groups have a higher affinity to bind Cd, carboxylic groups dominate Cd binding due to the smaller difference in the affinity of the latter groups to bind Cd and protons. In addition, the small ionic strength effect on Cd adsorption can be ascribed to the low charge density of biochar and the presence of basic groups (i.e., the third type of sites) on biochar. The basic groups generate positive charge, neutralizing to a certain extent the negative charge generated by the acidic groups, thus weakening the ionic strength effect. Due to this electrostatic effect, ignoring the presence of the basic groups will lead to an overestimation of Cd binding by biochar. In addition to these quantitative mechanistic insights, the derived generic model parameters for the three-site NICA-Donnan model can help to predict Cd binding to unknown biochars over a wide range in pH and ionic strength to support the design of in-situ soil remediation operations.