Density Functional Theory Study on Aqueous Aluminum−Fluoride Complexes: Exploration of the Intrinsic Relationship between Water-Exchange Rate Constants and Structural Parameters for Monomer Aluminum Complexes

Abstract

Density functional theory (DFT) calculation is carried out to investigate the structures, (19)F and (27)Al NMR chemical shifts of aqueous Al-F complexes and their water-exchange reactions. The following investigations are performed in this paper: (1) the microscopic properties of typical aqueous Al-F complexes are obtained at the level of B3LYP/6-311+G**. Al-OH(2) bond lengths increase with F(-) replacing inner-sphere H(2)O progressively, indicating labilizing effect of F(-) ligand. The Al-OH(2) distance trans to fluoride is longer than other Al-OH(2) distance, accounting for trans effect of F(-) ligand. (19)F and (27)Al NMR chemical shifts are calculated using GIAO method at the HF/6-311+G** level relative to F(H(2)O)(6)(-) and Al(H(2)O)(6)(3+) references, respectively. The results are consistent with available experimental values; (2) the dissociative (D) activated mechanism is observed by modeling water-exchange reaction for [Al(H(2)O)(6-i)F(i)]((3-i)+) (i = 1-4). The activation energy barriers are found to decrease with increasing F(-) substitution, which is in line with experimental rate constants (k(ex)). The log k(ex) of AlF(3)(H(2)O)(3)(0) and AlF(4)(H(2)O)(2)(-) are predicted by three ways. The results indicate that the correlation between log k(ex) and Al-O bond length as well as the given transmission coefficient allows experimental rate constants to be predicted, whereas the correlation between log k(ex) and activation free energy is poor; (3) the environmental significance of this work is elucidated by the extension toward three fields, that is, polyaluminum system, monomer Al-organic system and other metal ions system with high charge-to-radius ratio.

Density Functional Theory Studies on the Structures and Water-Exchange Reactions of Aqueous Al(III)–Oxalate Complexes

“…As shown in , contrary to trans effect of the aqueous AlÀF complexes observed previously. 12,32,33 This is in line with the previous simulation of Al(III)Àoxalate complexes. 34 Considering that the structural property is also found in the nonchelate Al(III)Àacetate complexes, 35,36 we speculate that the chelation is not responsible for this phenomenon and the appearance of carboxylate ligand might be the major reason.…”

Simultaneous removal of arsenic and fluoride by freshly-prepared aluminum hydroxide

“…The formation of aluminum fluoride complexes (Al-F complexes) occurs at acidic condition [7][8][9], and this effect facilitates fluoride removal as compared to its direct adsorption onto Al(OH) 3 precipitates [7]. Additionally, Al-F complexes formation also affects the species distribution of aluminum ions and their hydrolysis products [9], and assumedly impacts the coagulation behaviors of Al salt towards arsenic thereafter.…”

“…The formation of aluminum fluoride complexes (Al-F complexes) occurs at acidic condition [7][8][9], and this effect facilitates fluoride removal as compared to its direct adsorption onto Al(OH) 3 precipitates [7]. Additionally, Al-F complexes formation also affects the species distribution of aluminum ions and their hydrolysis products [9], and assumedly impacts the coagulation behaviors of Al salt towards arsenic thereafter. As for the removal of arsenic, the Fe-based coagulants and adsorbents showed priority due to their strong affinity towards arsenic [10,11], and the formation of bi-nuclear bidentate and monodentate complexes on the iron hydro (oxide) surfaces has been proposed [10,13,14].…”

Modelling the adsorption of fluoride onto activated alumina in the presence of other ions

“…, and AlOH 3 -n n (Nordin et al, 1999;George et al, 2010;Jin et al, 2010). This factor has been ignored in the present work.…”