Three-site NICA-Donnan
Extended NICA-Donnan with three binding site types — carboxylic, phenolic, and pyridinic nitrogen — optimized for biochar and pyrogenic carbon materials.
Site Parameters
Defaults: Huang et al. (2024) generic plant-biochar set — a1 carboxylic, a2 phenolic, b basic N.
These are log KH (intrinsic median affinities), not thermodynamic pKa values. The Donnan potential shifts the apparent half-protonation pH; at VD = 0.1 L/kg the shift can exceed one pH unit.
Experimental Conditions
Advanced: competitive metal parameters (required when Total Metal > 0)
No verified Cu/Pb/Zn three-site metal parameters exist in this repository. Huang et al. (2025) Cd values are not transcribed; any metal output is UNVERIFIED. Proton-only runs (Total Metal = 0) use the literature Huang (2024) set.
Site Types
- a1 — carboxylic: Qmax = 0.39 mol/kg, log KH = 4.55, m = 0.70 (Huang et al. 2024 generic biochar).
- a2 — phenolic: Qmax = 0.59 mol/kg, log KH = 8.10, m = 0.80.
- b — basic N (pyridine/graphitic): Qmax = 0.39 mol/kg, log KH = 4.78, m = 0.76. Protonation gives a positive charge; the published Cd model infers that this site does not bind Cd.
- Donnan phase: fixed VD = 0.1 L/kg in biochar mode; the Donnan potential follows from electroneutrality. log KH is not the apparent half-protonation pH — the Donnan potential shifts it by up to ~1.8 pH units at this VD.
- Metal scope: no external Cu/Pb/Cd/Zn dataset is bundled. Metal competition is implemented, but it runs only with user-supplied log KM, nM, nH and p; its output is marked UNVERIFIED.
References
- Huang et al. (2024) ACS Sustain. Resour. Manage. 1(4):707–720 — three-site NICA-Donnan generic biochar parameters, VD = 0.1 L/kg.
- Huang et al. (2025) Appl. Geochem. 186:106404 — Cd binding structure (mono/bidentate on a1/a2).
- Kinniburgh et al. (1999) Colloids Surf. A 151:147–166; Milne et al. (2001) Environ. Sci. Technol. 35:2049–2059 — classical two-site NICA-Donnan.
- Scope note: the Huang Cd parameter table is not transcribed in this repository. Limits, monotonicity and electroneutrality are consistency checks, not predictive validation.