Molecular design of a novel ligand for Menshutkin complexation of Bi(iii) from aqueous acidic copper sulfate electrolyte solutions and experimental investigations

Article abstract of DOI:10.1039/c6ra01960a

5-Amino-2,9-diphenoxy-1,10-phenanthroline functionalized silica is designed to reversibly adsorb bismuth impurities from strongly acidic aqueous copper sulfate solutions. The ligand interacts with Bi(iii) ions through weak non-covalent interactions due to the presence of the arene-π electron cloud of the phenyl moiety. The stronger complexation of Bi(iii) as compared to Sb(iii), As(iii) and Cu(ii) ions with the ligand was explored using Density Functional Theory (DFT) under solvated conditions. The theoretical predictions were validated by experimental studies with Bi(iii)/Cu(ii) mixtures in aqueous acidic solutions. The ligand shows very high selectivity (α = 978) towards Bi(iii) in the presence of a high concentration of Cu(ii) in the acidic solutions.

Full text of DOI:10.1039/c6ra01960a

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Molecular design of a novel ligand for Menshutkin
complexation of Bi(III) from aqueous acidic copper
sulfate electrolyte solutions and experimental
investigations†
Cite this: RSC Adv., 2016, 6, 39663
*
Jyotsna S. Arora and Vilas G. Gaikar
5-Amino-2,9-diphenoxy-1,10-phenanthroline functionalized silica is designed to reversibly adsorb bismuth
impurities from strongly acidic aqueous copper sulfate solutions. The ligand interacts with Bi(III) ions through
weak non-covalent interactions due to the presence of the arene–p electron cloud of the phenyl moiety.
The stronger complexation of Bi(III) as compared to Sb(III), As(III) and Cu(II) ions with the ligand was explored
using Density Functional Theory (DFT) under solvated conditions. The theoretical predictions were validated
by experimental studies with Bi(III)/Cu(II) mixtures in aqueous acidic solutions. The ligand shows very high
selectivity (a ¼ 978) towards Bi(III) in the presence of a high concentration of Cu(II) in the acidic solutions.
Received 22nd January 2016
Accepted 13th April 2016
DOI: 10.1039/c6ra01960a
www.rsc.org/advances
carbonates to the electrolytic solutions is also suggested by
Hyvarinen et al.,7 because of poor solubility characteristics of
1 Introduction
sulfates. However, precipitation is not advisable taking into
consideration the environmental concerns associated with
these heavy metal sludge generated in the process.3
Copper is one of the most important strategic metals manu-
factured by electrolytic rening of blister copper anodes. The
electrorening process consists of electrochemically dissolving
copper from impure anodes in aqueous sulfuric acid solutions
and selectively plating the dissolved copper in a pure form onto
copper cathodes. The principal interfering impurities in copper
anodes are As, Bi, Fe, Ni, Pb, Sb, Se and Te.1,2 Of these impuri-
ties, most parts of Sb, As and Bi are electrochemically dissolved
into the acidic solutions. Hence, the concentrations of these
impurities in the electrolyte solutions increase in the course of
the continuous electro-rening process.1 In addition, the elec-
trode potential of Bi(III) is very close to that of Cu(II) making it
probable to plate on the Cu cathode, thus affecting the quality
of copper.3 Hence, to accomplish the ASTM standards of Cu
electrode, the contamination by Bi(III) ion needs to be
controlled.2
Several studies have demonstrated the separation of heavy
metals by adsorption.8 Wang et al.9 reported the selective
adsorption of Bi(III) (98%) from copper electrolyte solutions at
pH 2 (percentage adsorption (% R) of Cu(II) ¼ 0.02) using
bayberry tannin (ortho-phenolic hydroxyl groups)10 loaded
collagen ber. Though the collagen ber is selective, it will not
be able to withstand the highly acidic conditions of a typical
copper electrolyte solution; viz. >18% H2SO4. The other reported
adsorbents for the removal of contaminants from acidic copper
solutions are chelating resins and activated carbons.8 Nagai
et al.11 reported the chelation of Bi(III) and Sb(III) by imino-bis-
methylene phosphonic acid groups bound to a phenolic
matrix. However, this chelating resin suffered from the strong
co-extraction of Fe(III) ion as well as its subsequent slow elution
with 6 M HCl resulted in a poor quality cathode copper and loss
of precious metal. Ogata et al.12 disclosed the use of a phos-
phomethylamine chelating resin for the selective removal of
bismuth and/or antimony from the sulfuric acid solutions.
However, batch adsorption experiments exhibited only ꢀ49%
and ꢀ77% adsorption of bismuth and antimony, respectively,
with signicant copper uptake (ꢀ7%). Sasaki et al.13 used Epo-
rous MX-2 ion exchange resin and Dreisinger et al.14 reported
the use of DUOLITE C-467 and UR-3300 resins, all functional-
ized with aminomethylene phosphonic acid groups on a poly-
styrene–divinylbenzene matrix for separation of Bi(III).
Various authors have proposed the use of organophos-
phorus,1,4 organothiophosphorus1,5 compounds and hydroxa-
mic acids1,4 as ligands for removal of Bi(III) by liquid–liquid
extraction. Even the use supported liquid membranes (SLM) for
recovery of bismuth from aqueous solutions has been re-
ported.2,6 Co-precipitation of bismuth by addition of Ba-, Sr-/Pb-
Department of Chemical Engineering, Institute of Chemical Technology, Nathalal
Parekh Marg, Matunga, Mumbai-19, India. E-mail: vg.gaikar@ictmumbai.edu.in;
Fax: +91-22-33612020; Tel: +91-22-33612013
† Electronic supplementary information (ESI) available: The characterization data
of the intermediates (IV and V) and ligand (VI) using different spectroscopic
techniques, the 1H NMR spectra (IV to VI) (Fig. S1 to S3) and the comparative
FTIR spectra of silica and CPTES functionalized silica (Fig. S4). See DOI:
10.1039/c6ra01960a
For eluting and recovering Bi, most of these resins require
a large excess of a strongly acidic stripping solution, typically 4–
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6 M concentration.14 Further, for the regeneration of the strip- computational feasibility of the calculations with signicant
ped resin, a large volume of base would be required to treat the accuracy while LANL2DZ was employed for the metal ions32–34 as
residual acid, making the entire process laborious, expensive as the computed geometries of metal complexes show good
well as it generates a large amount of salt solution that must be agreement with the experimental data. In order to account for
disposed off.15,16
relativistic effects, the inner electrons were substituted by an
To overcome these problems, specic ligands need to be effective-core potential (ECP).35 Natural Bond Order (NBO)
designed for highly selective separation of Bi(III) ions. Recent charge analysis was performed using the NBO package as
reports have explained the non-covalent interactions of arenes implemented in Gaussian 09 in order to compute the quantum
with heavier group 15 elements (pnictogens). These pnic- chemical properties. The geometry optimization of the free
togens–arene interactions play an important role in various ligand as well as that of the complexes was performed under
chemical and biological processes including molecular recog- solvated conditions using the Integral Equation Formalism
nition, crystal engineering, enzymatic mechanisms and supra- Polarized Continuum Model (IEFPCM), which includes implicit
molecular assembly design.17–21 Watt et al.22 described bismuth long-range hydration, without imposing any initial symmetry
ion (Lewis acid) and aromatic ring (p-base) complex as “Men- restriction.36 The solvent dielectric constant (3) was taken as
shutkin complexes”, where donor orbitals on the arene interact 78.4, viz. the same as bulk water. The absence of imaginary
with the acceptor orbitals on the metal center. Further, 1,10- frequencies during the Hessian calculations characterized the
phenanthroline and its derivatives exhibit interesting metal optimised structures as stationary points.
complexation properties because they possess two nitrogen
donor atoms situated in the same plane.23 The phenanthroline
moiety is locked by integrated benzo rings, which benets
3 Materials and methods
3.1 Materials
thermodynamic stabilization by avoiding s-bond rotation in
their bipyridine analogues.24 1,10-Phenanthroline, therefore,
oen acts as a chelating ligand exhibiting high affinity towards
metal ions.23,24 Bi(III) ion forms a stable 5-membered chelate
complex with 1,10-phenanthroline-2,9-dicarboxylic acid as
Bi(III) ion possesses the right size to t in the cle of the diacid.25
The present study involves density functional theory (DFT)
assisted design of a ligand which is highly selective towards
bismuth even in the presence of large concentration of Cu(II) in
aqueous H2SO4 solutions. These theoretical studies were also
performed with Sb(III) and As(III) ions26 in order to explore the
competitive complexation behaviour of Bi(III) in presence of
other pnictogens and Cu(II) ions with the ligand. These studies
provide a deeper insight of metal–ligand interaction and thus
assist in the design of highly Bi-specic ligands. The complete
synthesis of the ligand is described along with its covalent
attachment to the silica backbone that exhibits low interaction/
binding of the target analytes27,28 as well as provides chemical,
mechanical and operational stability. The adsorption–desorp-
tion characteristics of the functionalised adsorbent for Bi(III)
uptake in the presence of large concentration of copper ions are
explored by batch adsorption experiments to support the
theoretical predictions.
Special chemicals, such as 1,10-phenanthroline monohydrate,
1,3-dibromopropane, potassium t-butoxide, phosphorus oxy-
chloride, phosphorus pentachloride, hydrazine hydrate (99%),
10% Pd/C, sodium hydride, phenol (all AR grade), solvents and
bulk chemicals like 98% H2SO4 and 68% HNO3 solutions, were
used as received from s.d. Fine Chemicals, Mumbai. (1-Chlor-
opropyl)triethoxysilane (CPTES) (>99%) and silica having pore
volume of 0.8 cm3 gꢁ1, specic surface area of 550 m2 gꢁ1, and
ꢀ
an average pore diameter of 60 A, were obtained from Alfa-
Aesar, Mumbai. Bi(III) and Cu(II) stock solutions were prepared
using bismuth sulfate (Thomas Baker, >99%) and copper
sulfate (s. d. Fine Chemicals, >99%), respectively, by dissolving
in aqueous H2SO4 solutions of appropriate strength. Standard
solutions of bismuth(III) and copper(II) (ICP grade, 1000 mg
dmꢁ3) were obtained from Sigma-Aldrich, Mumbai. Deionized
(DI) water obtained from Millipore, was used for all
experiments.
3.2 Experimental methodology
3.2.1 Synthesis of the designed ligand. The synthesis of
silica based functionalized adsorbent was investigated by the
“Retrosynthetic approach” as described in Scheme 1, while the
synthetic method for the phen–arene functionalized adsorbent
is described in Scheme 2.
2 Computational methodology
All DFT calculations were conducted using Gaussian 09
The rst 3 intermediates, starting from 1,10-phenanthroline
program.29 For investigating the structural and electronic to 2,9-dichloro-1,10-phenanthroline (III), were synthesized
properties of the designed ligand and its complexes, the according to the procedure reported by Guo and co-workers.37
dispersion corrected DFT-D3 viz. BP86-D3 level of theory was 1,10-Phenanthroline was initially protected by reacting with
chosen as dispersion is important to explain the pnictogen–p excess (1 : 5 molar ratio) of 1,3-dibromopropane in toluene
complexes.19 The copper(II) (possessing one unpaired electron) resulting in the formation of (I) (97% yield). The oxidation of (I)
has doublet multiplicity, while in pnictogen(III) ions there are no was achieved with air in the presence of excess t-BuOK in t-
unpaired electrons. Hence, for the simulation studies of Cu(II)– BuOH producing the brown solid (II), which was used directly
phen–arene complex surrounded by anions (HSO4ꢁ), unre- for the next step without any purication while the ltrate was
stricted formalism (viz. UBP86-D3) was considered.30 The basis evaporated to dryness and the solid residue obtained was
sets used for H, C, O, N, S was 6-31+G(d)31 considering the puried using silica column. The total yield of (II) was 73%
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Scheme 1 Retrosynthetic approach for phen–arene synthesis.
which is comparable to the reported value (74%).37 The reaction temperature. To the resulting dark-brown solution, the nitrat-
of (II) with POCl3 and PCl5 produced (III) in 82% yield.
ing mixture (HNO3 : H2SO4 ¼ 1 : 1) (3.5 cm3) was added over
Synthesis of 5-nitro-2,9-dichloro-1,10-phenanthroline (IV). a period of 10 min with vigorous stirring. The reaction mass was
Sulfuric acid (20 cm3; 98%) was added over 5 min to (III) (3.3 g, then stirred for another 45 min at the same temperature, and
13.3 mmol) at 0–5 ꢂC and stirred for 30 min at the same then the temperature of the reaction mass was raised to 60 C
ꢂ
Scheme 2 Synthetic route of phen–arene formation and its covalent functionalisation on silica.
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with continuous stirring for 8 h. The reaction mass was cooled aqueous acidic bismuth solution (free H2SO4 concentration ¼
to room temperature and then carefully poured over crushed ice 180 g Lꢁ1; [Bi] ¼ 96.9 ppm). The asks were kept on an orbital
to afford a bright yellow solid. The solid was ltered off, washed shaker with an agitation rate of 100 strokes per min at an
ꢂ
with cold water and recrystallized from hot aqueous THF ambient temperature of 60 ꢄ 2 C (ref. 6 and 8) for 24 h. The
(THF : water ¼ 4 : 0.25, 60 ꢂC) to give (IV) as a golden yellow residual metal-ion concentration was measured by inductively
ꢂ
solid. The crystallized solid was dried at 80 C for 8 h (2.53 g, coupled plasma optical emission spectroscopy (ICP-OES) as
yield 65%).
described later in this report.
Synthesis of 5-amino-2,9-dichloro-1,10-phenanthroline (V). The
The kinetics of Bi(III) uptake was studied separately at 60 ꢄ 2
nitro compound (IV) (2.5 g, 8.5 mmol) was dissolved in meth- ꢂC using 100 cm3 (V0) of aqueous acidic Bi(III) solution in 2 M
anol (60 cm3) under reuxing for 30 min. The solution was then H2SO4 and 1.5 g of silica–phen–arene. The samples (Vt ¼ 1 cm3)
cooled and purged with N2 for 30 min to remove oxygen, as its were withdrawn at regular intervals from the solution and to
presence may result in the deactivation of the catalyst. Hydra- maintain the volume of the solution constant during the
zine hydrate (4 cm3, 8.8 mmol) and 10% Pd/C (0.5 g) were added experiment, an equal amount of 2 M aqueous H2SO4 solution
to this solution. The reaction mixture was then reuxed for 5 h was added to the solution.
under N2 atmosphere aer which it was ltered under hot
For competitive batch sorption of Bi(III) in the presence of
conditions (50 ꢂC). The lter cake was washed well with hot Cu(II), the adsorbent was equilibrated with 20 cm3 of copper
methanol (45 ꢂC). The combined ltrates were then concen- electrolyte solution ([H2SO4] ¼ 180 g Lꢁ1; [Cu] ¼ 36 000 ppm,
trated under reduced pressure to afford the golden brown [Bi] ¼ 96.9 ppm). The asks were kept on an orbital shaker with
ꢂ
amine (V). The obtained solid is vacuum dried at 80 C for 5 h an agitation rate of 100 strokes per min at an ambient
(1.84 g, 82%).
temperature of 60 ꢄ 2 ꢂC for 24 h to reach equilibrium. The
Synthesis of 5-amino-2,9-diphenoxy-1,10-phenanthroline (VI). residual metal-ion concentrations were measured by ICP-OES.
Alcoholic sodium hydroxide (0.64 g, 16 mmol) solution (5 cm3)
The elution of Bi(III) was performed by using 0.02 M aqueous
was added to the mixture of phenol (1.5 g, 16 mmol) and EDTA solutions as an eluent.9 The Bi(III) loaded phen–arene
methanol (10 cm3). To this mixture, the amino compound (V) functionalized silica was equilibrated with the eluent for 3 h at
(2.0 g, 7.6 mmol), dissolved in methanol (10 cm3), was added 30 ꢂC. The mixture was then ltered and the resulting solution
over a period of 15 min. The reaction mixture was reuxed for 4 was analysed for Bi(III) and Cu(II) concentration on ICP-OES. The
h. Aer this, the reaction mass was cooled and poured into ice adsorbent was washed thoroughly with DI water, dried at 80 ꢂC
cold water and stirred. The solid was ltered, then washed with and was recycled in the subsequent four-cycles of adsorption–
water (20 cm3 ꢃ 2) and recrystallized using chloroform. The desorption experiments.
crystallized solid was vacuum dried at 80 ꢂC for 8 h (2.18 g,
76%).
Synthesis
3.3 Characterization methods
of
(g-chloropropyl)triethoxysilane
(CPTES)-
functionalized silica (silica-Cl) (VII). Silica-Cl (VII) synthesis was The infrared spectra of the intermediates, the ligand and the
performed according to the procedure reported by Wang et al.38 ligand functionalised silica were recorded using Bruker-
A total of 4 g of silica was suspended in 100 cm3 of dry toluene VERTEX 80V vacuum FTIR spectrophotometer aligned with
containing 4 g of (g-chloropropyl)triethoxysilane (CPTES) as Ultra-Scan interferometer in KBr pellets. The Thermo-Finnigan
silylation reagent. The reaction mixture was stirred for 24 h at Mass Spectrometer was used to perform mass spectrometry
ꢂ
90 C under N2 atmosphere. The resulting powder, denoted as electrospray ionization (MS/EI) of the organic compounds by
silica-Cl (VII), was ltered and washed with toluene (30 cm3 ꢃ dissolving the samples in methanol. D2O and CDCl3 were used
2), ethanol (30 cm3 ꢃ 2), and diethyl ether (30 cm3 ꢃ 2), as solvents to record the H NMR spectra using a Bruker DPX-
1
ꢂ
respectively. The solid was dried in vacuum at 80 C for 12 h.
300 (300 MHz) spectrometer at 25 ꢂC. All of the chemical
Synthesis of silica bound phen–arene ligand (VIII). Hybrid shis were reported in the standard d notation of parts per
mesoporous material (silica–phen–arene) was synthesized by an million. The solvent proton signals are observed at: D2O 4.66
elimination reaction of silica-Cl (VII) with phen–arene ligand ppm and CDCl3 7.27 ppm. The CHN(O) contents in the graed
(VI). 2.0 g of silica-Cl was dispersed in 70 cm3 toluene under N2 adsorbent were determined on the Perkin-Elmer 240B
atmosphere. To this mixture, a solution of (VI) (1.52 g, 8 mmol) Elemental Analyser. The surface area of the graed adsorbent
in toluene was added over 10 min. The reaction mass was was obtained from nitrogen adsorption–desorption isotherms
reuxed for 24 h in N2 atmosphere. The resulting product was using the Micrometrics ASAP2020 instrument. Prior to the
ltered and washed with toluene (30 ꢃ 2 cm3), methanol (20 testing, the samples were degassed at 100 C overnight in the
ꢂ
cm3), and diethyl ether (20 cm3), in sequence, to obtain (VIII) as degassing port of the adsorption analyzer. The Barrett–Joyner–
yellowish solid. The solid was dried in vacuum at 80 ꢂC for 10 h Halenda (BJH) algorithm on the adsorption branch was used to
(yield ¼ 2.86 g).
calculate the pore size distribution while the pore volume was
3.2.2 Batch adsorption studies. The rst set of adsorption investigated at the P/P0 0.973 single point.
experiment was conducted with aqueous acidic Bi(III) solutions
The metal ion concentrations during the adsorption studies
without Cu(II). Batch sorption studies were conducted by sus- were determined using ICP-OES (Thermo Fisher Scientic,
pending varying amounts (0.1–0.5 g) of the silica–phene–arene iCAP, 6000). The charge-coupled device (CCD) was used along
in stoppered conical asks, each containing initially 20 cm3 of with a radiofrequency (RF) generator exhibiting power and
39666 | RSC Adv., 2016, 6, 39663–39674
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