Molecular Dynamics Simulations of EDTA-modified Graphene Oxide for Pb(Ⅱ) and Na(Ⅰ) Removal†

doi:10.7503/cjcu20160658

Abstract

Abstract:

The chelating groups N-(trimethoxysilylpropyl) ethylenediamine triacetic acid(EDTA-silane) were linked to graphene oxide(GO) surfaces through a silanization reaction, and this modified GO(GO-EDTA) was used to remove the heavy metal in the solution. By using molecular dynamics simulation at the molecular level, the absorption distribution, dynamic properties of Pb²⁺ on the GO-EDTA surface were investigated, the different absorption behaviors of Pb²⁺ and Na⁺ between GO and GO-EDTA systems were compared. At the same time, the interaction between GO-EDTA and Ca²⁺ was simulated, which was comparable to the adsorption behavior of Pb²⁺. The simulation result shows that: (1) the adsorption sites of Pb²⁺ and Na⁺ are the carboxyl groups in the GO-EDTA system, rather than the hydroxyl groups on graphene oxide surface; (2) the absorption conformations of Pb²⁺ and Na⁺ to carboxyl groups are different. For Pb²⁺ ions, they form the 2:1 type with carboxyl group, i.e. one Pb²⁺ ion combines with two carboxyl groups; while for Na⁺ ions, it is the 1:1 type, i.e. one Na⁺ ion combines with one carboxyl group; (3) comparison with Ca²⁺ and Na⁺ ion, the energy barrier of forming COO^--Pb²⁺ ion pair conformation is lowest while the energy barrier to break this conformation is higher. This result indicates that the Pb²⁺ ions are adsorbed well on GO and GO-EDTA films.

Key words: Graphene oxide, Heavy metal, Adsorption capacity, Molecular dynamic

CLC Number:

O641

TrendMD:

LIU Shasha, ZHANG Heng, WANG Hua, YUAN Shiling, HOU Shifeng. Molecular Dynamics Simulations of EDTA-modified Graphene Oxide for Pb(Ⅱ) and Na(Ⅰ) Removal^†[J]. Chem. J. Chinese Universities, 2017, 38(1): 63.

Figures/Tables 12

Table 1 Forced field parameters for GO-EDTA and metal ions used in this worka

Site	σ^b/nm	ε^b/(kJ·mol^-1)	q/e	Site	σ^b/nm	ε/(kJ·mol^-1)	q/e
C(graphene)	0.340	0.359	0.000	N	0.298	0.879	-0.210
C(—COO^-)	0.375	0.455	0.700	C(—CH₃)	0.340	0.115	0.034
O(—COO^-)	0.296	0.879	-0.900	H(—CH₃)	0.250	0.052	0.071
O(—OH)	0.307	0.649	-0.513	Pb²⁺	0.300	0.799	2.000^c
H(—OH)	0.000	0.000	0.330	Na⁺	0.189	0.281	1.000^d
Si	0.339	2.450	1.310	Ca²⁺	0.24	0.502	2.000^e

Fig.1 Schematic diagram of the GO(A) sheet and grafting N-(trimethoxysilylpropyl) ethylenediamine triacetic acid(EDTA-silane)(GO-EDTA3, B)

Fig.2 Adsorption of Na+ and Pb2+ on GO(A—C) and CO-EDTA3(D—F)(A) and (D) Initial structures; (B) and (E) final snapshots; (C) and (F) two local amplifications of absorbed layers. The GO and GO-EDTA3 are represented by sticks and balls, respectively. Red and gray balls represent oxygen atoms and carbon atoms, respectively. Sodium ions and lead ions are shown in pink and dark gold balls. Green balls represent chloride ions. Water molecules are represented by red lines.

Fig.3 Number density profiles of Na+ and Pb2+ in GO and GO-EDTA3 systems, respectivelya. GO-Na+; b. GO-Pb2+; c. GO-EDTA3-Na+; d. GO-EDTA3-Pb2+.

Fig.4 Microstructures of metal ions around carboxyl groups of GO-EDTA3 oxide membrane(A) RDFs of Na+ around carboxyl groups and hydroxyl groups; (B) RDFs of Pb2+ around carboxyl groups and hydroxyl groups; (C) SDFs of Pb2+, Na+ and water molecules around carboxyl groups; (D) schematic diagram of metal ion in absorption layer. Sodium ions are shown in pink ribbon. Lead ions and water molecules are represented in dark gold and blue-green ribbons, respectively.

Fig.5 Adsorption conformations of metal ions and carboxyl groups and the proportion of different conformations (A) Na+; (B) Pb2+.

Fig.6 Absorption percentages of Na+ and Pb2+ in considered systems(A) a. GO-EDTA1; b. GO-EDTA1'. (B) a. GO; b. GO-EDTA1; c. GO-EDTA2; d. GO-EDTA3; e. GO-EDTA4.

Fig.7 Average electrostatic potential energy between carboxyl groups and each metal ion for GO-EDTA3 oxide membranea. Na+; b. Pb2+.

Fig.8 PMF between carboxyl oxygen atoms and metal ions(A) and the binding energy of carboxyl groups and metal ions, and schematic diagram of combining structures at minimum points for clarity(B)a. Na+; b. Pb2+.

Table 2 Binding and dissociation energies between ions and carboxyl oxygen atoms

Ion	ΔE⁺/(kJ·mol^-1)	ΔE^-/(kJ·mol^-1)
Na⁺	7.5	14.7
Pb²⁺	5.9	15.4

Fig.9 Adsorption of Na+ and Ca2+ on GO-EDTA3(A) Initial structures; (B) final snapshots; (C) local amplifications of absorbed layers. The GO-EDTA3 is represented by sticks and balls. Sodium ions and calcium ions are shown in pink and purple balls. Green balls represent chloride ions.

Fig.10 PMF between carboxyl oxygen atoms and metal ions(A) and the binding energy of carboxyl groups and metal ions, and schematic diagram of combining structures at minimum points for clarity(B)a. Na+; b. Ca2+.

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