Table 1.

Model VI parameters for humic acid (Tipping, 1998).

Parameter	Description	Values
nA	Amount of type A sites (mol g−1)	3.3 10−3
nB	Amount of type B sites (mol g−1)	0.5 × nA
pKA	Intrinsic proton dissociation constant for type A sites	4.1
pKB	Intrinsic proton dissociation constant for type B sites	8.8
ΔpKA	Distribution terms that modifies pKA	2.1
ΔpKB	Distribution terms that modifies pKB	3.6
log KMA	Intrinsic equilibrium constant for metal binding at type A sites	Variable (Marsac et al., 2010)
log KMB	Intrinsic equilibrium constant for metal binding at type B sites	3.39 log KMA−1.15
ΔLK1	Distribution term that modifies log KMA	2.8 (REE)
ΔLK2	Distribution term that modifies the strengths of bidentate and tridentate sites	0.55 log KNH3 = 0.29 (REE)
P	Electrostatic parameter	−330
Ksel	Selectivity coefficient for counterion accumulation	1
M	Molecular weight	15000 Da
r	Molecular radius	1.72 nm