Generic Proton Binding Parameters of a Three-Site NICA-Donnan Model for Plant-Based Biochars

The charging behavior of biochars resulting from (de)protonation is important for assessing their potential as adsorbents for in situ immobilization of trace metals in polluted soils. Here, the pH-charge curves of 20 newly produced biochars were measured, whereas curves of 16 other biochars were collected from literature. All 36 biochars were produced from different plant-based materials at various pyrolysis temperatures. The pH-charging data were used to derive proton binding parameters for a novel three-site NICA-Donnan model, accounting for proton binding to carboxylic (sitea1) and phenolic (sitea2) acidic groups as well as to a basic (siteb) group. This model successfully described the proton binding behavior of the 36 biochars at different ionic strengths for the first time, by extending the classical two-site NICA model with a basic group in combination with an electrostatic Donnan model using a fixed volume of 0.1 L kg–1. A set of generic proton binding parameters derived from the data resulted in a relative error of ∼6% for proton adsorption by all 36 biochars. Recommended parameters are Qa1 = 0.39 mol kg–1, log K̃a1 = 4.55, ma1 = 0.70, Qa2 = 0.59 mol kg–1, log K̃a2 = 8.10, ma2 = 0.80, Qb = 0.39 mol kg–1, log K̃b = 4.78, and mb = 0.76.