Mechanistic study of Hg(II) interaction with three different α-aminophosphonate adsorbents: Insights from batch experiments and theoretical calculations
Herein, efficient and potential chelating α-aminophosphonate based sorbents (AP-) derived 20
from three different amine origins (aniline/anthranilic acid/O-phenylenediamine) to form AP- 21
H, carboxylated and aminated enhanced aminophosphonate as AP-H, AP-COOH, and AP-NH2 22
were synthesized via a facile method. The structure of the synthesized sorbents was elucidated 23
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using different techniques; elemental analysis (CHNP/O), FT-IR, NMR (1H-, 13C- and 31P- 24
NMR), TGA and BET. The fabricated sorbents were exploited for Hg(II) removal from 25
aqueous solution via sorption properties. Isotherm fitted by Langmuir equation: the maximum 26
sorption capacities at optimum pH 5.5, and T:25 ± 1oC, were found to be 1.33, 1.23, and 1.15 27
mmol Hg g-1 for AP-COOH, AP-NH2, AP-H, respectively, which is roughly correlated with 28
the active sites density and the hard/soft characteristics of adsorbents’ reactive groups. Metal- 29
ligand binding affinities are qualitatively rationalized in terms of hard and soft acids and bases 30
(HSAB) theory. The interaction of Hg(II) (soft) has a stronger affinity to AP-COOH can be 31
considered a softer base compared with reference material (AP-H) over than AP-NH2 (hard). 32
This sequence result showed opposite trends consistent with their reciprocal properties 33
according to the steric effect modulates and the specific surface area. Thermodynamics analysis 34
for absolute values of ΔH°, ΔS° and ΔG° afford the selectivity towards Hg(II) sorption with 35
the following order: AP-COOH >AP-NH2 >AP-H. Elution and regeneration was carried out by 36
HCl solution and recycled for a minimum of five cycles, the sorption and desorption 37
efficiencies are greater than 91%. Such sorbents exhibit good durability, stability and 38
promising potential for Hg(II) removal. Finally, a new modelling technique for quantitative 39
non-linear description and comparison of equivalent geographical positions in 3D space of 40
extended relationships. Exothermic and spontaneous behavior were observed using a proposed 41
Floatotherm that included the Van't Hoff parameters model.