Application of the NICA-Donnan approach to calculate equilibrium between proton and metal ions with lignocellulosic materials.

ABSTRACT

The NICA (nonideal competitive adsorption)-Donnan model is employed to describe the interactions between Cu2+, Pb2+, Cd2+, Mn2+, and Fe3+ ions and the lignins extracted from wheat bran (lignocellulosic substrate, LS) and from kraft pulp (residual kraft lignin, RKL), and between Cu2+, Mn2+, and Fe3+ ions and wood fibers from kraft pulps. The charge of the LS and the fiber charge need to be obtained from potentiometric titration data for the LS, and by use of Donnan equilibrium, mass balance, and electroneutrality equations for the kraft fiber. The proton binding parameters for the LS and the kraft fiber, the total site densities (Qmax,1 and Qmax,2), the median protonation constants (K1 and K2), and nonideality-generic heterogeneity parameters (m1 and m2) (subscripts 1 and 2 refer to the carboxyl and phenolic functional groups) are obtained by fitting these charge data. With the above proton parameters, the interactions between metal ions and the lignins (LS and RKL)/kraft fibers are calculated, and the metal binding parameters are obtained. These parameters are the binding constants of metal i (K(i,1) and K(i,2)), ion-specific nonideality parameters (n(i,1) and n(i,2)), and intrinsic heterogeneity parameters (p1 and p2). p1 and p2 are the same for all metal ions binding to a specific sorbent. Here, p1 and p2 values obtained by fitting the binding data of a specific metal ion are used directly in binding calculations for other metal ions, and do not need to be fitted. By use of the above parameters for single metal ion binding, the binding relationship between a mixed metal ion and lignocellulosic substrate/kraft fiber can be predicted.