A quantum mechanics/molecular mechanics (QM/MM) investigation on the mechanism of adsorptive removal of heavy metal ions by lignin: single and competitive ion adsorption

Abstract

In this paper, an adsorption removal mechanism of heavy metal ions (Pb, Cu, Cd, Zn, and Ni) by lignin is investigated by molecular and quantum chemical modeling. First, the lowest energy sites of lignin for heavy metal ions were investigated using a Metropolis Monte Carlo search and simulated annealing. Then, equilibrium adsorption capacities of lignin for heavy metal ions were calculated with conductor-like screening models with a segmented activity coefficients, together with generalized gradient approximation and Volsko–Wilk–Nusair density functional functional theory. These calculations followed the local pseudo-thermodynamic equilibrium at the interface of lignin and ion-containing effluent. Several kinetic Monte Carlo simulations were performed to analyze the surface kinetics of ion adsorption. The affinity of lignin for metal ions follows the order: Pb > Cu > Cd > Zn > Ni which is in agreement with experimental observations. The stability of ions follows the order: Pb > Cd > Zn > Ni > Cu, indicating that the adsorption affinity does not demand the same order of stability on ions. In addition, it was found that while the adsorption of heavy metal ions on the lignin is accessible, the adsorbed heavy metal ions, however, are less stable than the adsorbed water molecules. As such, the used lignin must be replaced by fresh lignin in a cyclic manner. While lignin provides desirable adsorption performance for single ion removal, it failed in processing of practical heavy metal ion solutions expected in environmental issues.

Appendix

The mole fractions (z) can be converted to concentrations (c) using the following expression:

$$\left\{ \begin{aligned} c_{i} = cz_{i} \hfill \\ c = \frac{\rho }{M} \hfill \\ \end{aligned} \right. \to c_{i} = \frac{\rho }{M}z_{i}$$

(3)

where M denotes molecular weight ($M = \sum {z_{i} M_{i} }$) and ρ is density ($\rho = \sum {\rho_{i} }$).

The mole fractions (z) can be related to the amount of adsorbed adsorbate (q _A ) as z = ε $q_{A}$ using the following expression:

$$z_{1} = \frac{{n_{1} }}{n} = \frac{{n_{1} }}{n}\frac{M}{M} = \frac{{n_{1} M}}{m} = \frac{{n_{1} M}}{{m_{1} + m_{2} }}$$

(4)

$$\begin{array}{*{20}c} {\frac{1}{{z_{1} }} = \frac{{m_{1} + m_{2} }}{{n_{1} M}} = \frac{{m_{1} }}{{n_{1} M}} + \frac{{m_{2} }}{{n_{1} M}}} & \to & {\frac{1}{{z_{1} }} - \frac{{m_{1} }}{{n_{1} M}} = \frac{{m_{2} }}{{n_{1} M}}} \\ \end{array}$$

(5)

$$\frac{{n_{1} }}{{m_{2} }} = \frac{{\frac{1}{M}}}{{\frac{1}{{z_{1} }} - \frac{{m_{1} }}{{n_{1} M}}}} = \frac{{\frac{1}{M}}}{{\frac{1}{{z_{1} }} - \frac{{M_{1} }}{M}}}$$

(6)

The unit of $\frac{{n_{1} }}{{m_{2} }}$ is mol/gr; thus, to give mmol/gr, both sides of Eq. 6 are multiplied by 10⁻³.