华中科技大学硕士学位论文
Abstract
Arsenic is a toxic heavy metal which was considered as one of the priority controlled contaminates. The removal of As(V) from aqueous environment through adsorption has drew much attention due to its high efficiency and low-cost. In this work, iron modified attapulgite (Fe III/A TP and Fe III,II/A TP) were prepared and applied to As III and As V removal. We used SEM, XRD and XPS in order to map surface elemental distributions, crystalline situation and valence state distribution in relation to particles embedded in the A TP matrix. The processes of adsorption kinetics and thermodynamic of different materials were discussed. We preliminarily investigated the adsorption process of As species on the Fe/ATP with surface complexation model together with FTIR, XPS and etc. At last, the mechanism of adsorbing As(III) onto Fe(III)/ATP were studied by DFT calculation. The main conclusions were summarized as followed:
(1) Natural ATP showed a short-rod like particles. After co-precipitation with iron, those rod-shapes were loaded with iron clearly and formed obvious crystal structure. Fe III/ATP and Fe II,III/A TP contains Fe2O3 and Fe3O4 structure, both of them had higher porous structure and decreased pore size the iron content of the Fe III/A TP and Fe III,II/A TP were 16.7% and 17.1%, respectively.
(2) Fe/A TP had good adsorption capacity for As and the adsorption follows pseudo-second-order rate mechanism, indicating the adsorption is a chemisorption process. Fe III/A TP had better adsorption capacity for As than Fe III,II/A TP, The maximum sorption capacities for As III were 21.8 , 16.3 mg/g and for As V were 29.8 and 26.3mg/g, respectively. The adsorption process were greatly affected by initial pH, which stated the electronegativity of the adsorber and arsenic form.
(3) XPS、FTIR analysis indicated that the Fe connected with the Al-O plane of ATP is the main site for As. Loaded Fe oxide is the key factor for the sorption of As, which retain As by virtues ligand exchange between As and –OH groups on iron (hydro) oxides.Surface hydroxyl of Fe/ATP surface seemed to be formed a 1:1 complex with As(V), which indicates the formation of monodentate complexes.
(4) The growth pattern of iron oxide on ATP surface is cross growth, which comfirmed
华中科技大学硕士学位论文
by DFT caculation. Monodentate complexes is more easily formed than bidentate complexes. The energy of As atom on bidentate form is lower than on monodentate form through PDOS analysis, indicating monodentate complexes will transform into bidentate one turn to be stable.
(5) Orbital(s, p) of loaded iron on ATP contracted to Fermi energy level, therefore, the iron turns to be more active than the previous one. Active iron on Fe/ATP contains the bonding styles of Fe2O3 and also unique one, which is the key factor for its good adsorption capacity. PDOS of Al atom on ATP skeleton changed after the location of iron oxide indicated the Al-O plane of ATP greatly impacted the iron activity of ATP surface.
Keywords: Arsenic, Fe/ATP, Surface Complexation Model, Density Functional Theory, Adsorption mechanism