Removal of toxic tellurium (IV) compounds via bioreduction using flucloxacillin in aqueous acidic medium: A kinetic and mechanistic approach

Article abstract of DOI:10.1016/j.molliq.2019.111436

This paper describes a novel method for the removal of potassium tellurite (Te^IV), a toxic tellurium (IV) compound, via its bioreduction using the drug flucloxacillin (Flx) in an aqueous sulfuric acid solution. The kinetics of the bioreduction process were monitored using UV–Vis absorption spectra at an ionic strength of 2.0 mol dm^?3 and 298 K. The reaction between Te^IV and Flx was set at a 1:1 stoichiometry. The reduction reaction followed first-order kinetics for [Flx] and fractional-first-order kinetics for [Te^IV] and [H⁺]. The effects of ionic strength and relative permittivity of the reaction medium were also explored. Supplementation with divalent transition metal ions enhanced the reduction rate. The reaction products were identified, in order of their stoichiometric results, spot tests and FT-IR spectra as 3-(2-chloro-6-fluorophenyl)-5-methylisoxazol-4-carbocylic acid, 5,5-dimethyl-thiazolidine-2,4-dicarboxlic acid, ammonium ion, carbon dioxide and elemental tellurium (Te⁰). The reaction rate dependence on temperature was studied, and the activation and thermodynamic parameters were assessed and discussed. The derived rate-law expression was found to be in excellent accordance with the acquired investigational outcomes. A conceivable reaction mechanism has been provided, which includes a reaction between the protonated flucloxacillin (Flx⁺) and tellurous acid (H₂TeO₃) as the essential reactive species, resulting in the construction of an intermediate complex. Such complex decays in the rate-determining step to yield the final reaction products.

Full text of DOI:10.1016/j.molliq.2019.111436

Journal of Molecular Liquids 292 (2019) 111436
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Removal of toxic tellurium (IV) compounds via bioreduction using
flucloxacillin in aqueous acidic medium: A kinetic and
mechanistic approach
Ahmed Fawzy
Chemistry Department, Faculty of Applied Sciences, Umm Al-Qura University, Makkah, Saudi Arabia
Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt
a r t i c l e i n f o
a b s t r a c t
This paper describes a novel method for the removal of potassium tellurite (TeIV), a toxic tellurium (IV) com-
pound, via its bioreduction using the drug flucloxacillin (Flx) in an aqueous sulfuric acid solution. The kinetics
of the bioreduction process were monitored using UV–Vis absorption spectra at an ionic strength of
2.0 mol dm−3 and 298 K. The reaction between TeIV and Flx was set at a 1:1 stoichiometry. The reduction reaction
followed first-order kinetics for [Flx] and fractional-first-order kinetics for [TeIV] and [H+]. The effects of ionic
strength and relative permittivity of the reaction medium were also explored. Supplementation with divalent
transition metal ions enhanced the reduction rate. The reaction products were identified, in order of their stoi-
chiometric results, spot tests and FT-IR spectra as 3-(2-chloro-6-fluorophenyl)-5-methylisoxazol-4-carbocylic
acid, 5,5-dimethyl-thiazolidine-2,4-dicarboxlic acid, ammonium ion, carbon dioxide and elemental tellurium
(Te0). The reaction rate dependence on temperature was studied, and the activation and thermodynamic param-
eters were assessed and discussed. The derived rate-law expression was found to be in excellent accordance with
the acquired investigational outcomes. A conceivable reaction mechanism has been provided, which includes a
reaction between the protonated flucloxacillin (Flx+) and tellurous acid (H2TeO3) as the essential reactive spe-
cies, resulting in the construction of an intermediate complex. Such complex decays in the rate-determining
step to yield the final reaction products.
Article history:
Received 1 June 2019
Received in revised form 20 July 2019
Accepted 23 July 2019
Available online 29 July 2019
Keywords:
Removal
Tellurium (VI)
Flucloxacillin
Bioreduction
Kinetics
Mechanism
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
important due to the increasing employment of tellurium in several in-
dustries. Additionally, this process could be a treatment for the removal
Tellurium is a harmful and essential rare metalloid present in a trace
amounts in the earth's crust [1]. It exists in nature in several forms, in-
cluding the nontoxic, elemental state (Te0), telluride (Te2−), and as
the oxyanions tellurite (TeO32−) and tellurate (TeO24−), which are toxic
for a variety of life forms [2,3]. In humans, tellurium is one of the most
abundant trace elements in bone. It is a critical element utilized in en-
ergy and defence applications [4]. Tellurium compounds have several
applications in the manufacture of ceramics, glass, semiconductors,
and metals [3]. Tellurium oxyanions are strong oxidants that can be pro-
duced by their reductive precipitation to form insoluble elemental tellu-
rium (Te0) [5]. Tellurium oxyanions have also been investigated as
potential antibacterial agents [6,7]. Tellurite (TeIVO32−) is highly toxic
to a variety of microorganisms [4] and is an extremely stable compound,
although it can be reduced to Te0 by electrolysis, by using a powerful re-
ducing agent [2,4,8], by living cells [9] or by some bacteria [10]. The re-
duction of highly toxic soluble tellurite, which has detrimental impacts
on the environment and human health, to the nontoxic insoluble Te0 is
of toxic tellurite from polluted areas to address serious pollution prob-
lems. Therefore, the development of a biochemical reduction method
for toxic tellurite for environmental clean-up purposes is of interest.
Antibiotics are a group of pharmaceutical drugs used to treat bacte-
rial and fungal infections in both humans and animals. They are not only
used in medicine but also in food industries and in scientific research ac-
tivities [11]. However, antibiotics are chemical substances that are for-
eign to the human body; hence, the body eliminates them through
drug metabolism processes, which may result in pharmacologically ac-
tive, inactive, or toxic metabolites. Antibiotics are introduced into the
environment through many routes, including human or animal excreta,
wastewater effluent and industrial wastes and processes [12,13]. The
presence of such chemicals, which contain complex organic com-
pounds, even at low environmental concentrations may negatively af-
fect the ecosystem and human health and have toxic impacts on the
soil, water resources and organisms. Therefore, they are designated as
a dangerous environmental pollutant [13]. There has been increasing
concern for the removal of these compounds to protect the human
health and the environment [14]. Several well-known methods or
E-mail addresses: afsaad13@yahoo.com, afsayed@uqu.edu.sa.
https://doi.org/10.1016/j.molliq.2019.111436
0167-7322/© 2019 Elsevier B.V. All rights reserved.

2
A. Fawzy / Journal of Molecular Liquids 292 (2019) 111436
techniques have previously been utilized for the removal of different
pollutants, including adsorption techniques using various adsorbents
[15–17]. However, conventional biological treatment methods still do
not satisfy all requirements and are not effective as a stand-alone
method due to the complicated chemical structures of pharmaceuticals
[18]. However, new treatment methods have been proposed [18–21].
One of the most significant methods is the chemical oxidation of antibi-
otics, which is a broad pathway for their degradation and plays a major
role in water treatment processes and in understanding the metabolism
of medical drugs in pharmacokinetics studies [22–26]. During the oxida-
tion process, oxidizing agents transform toxic substances to less harmful
compounds that can be safely discharged to the environment [25,26].
Despite the pharmaceutical significance of antibiotics, a careful liter-
ature review revealed very few kinetic investigations on the oxidative
degradation of antibiotics in aqueous media, and the reported investiga-
tions gave poor mechanistic information [22–26]. The current study was
undertaken in view of the limited knowledge of the oxidation kinetics
and mechanisms of such biologically and medically serious molecules.
This investigation deals with the kinetic, mechanistic and thermody-
namic aspects of the redox reaction between the well-known
bioreductant flucloxacillin (Flx) and the highly toxic tellurite ion
TeIVO23− in aqueous acidic medium (sulfuric acid). Flucloxacillin is a
narrow-spectrum beta-lactam antibiotic of the penicillin class and is
commonly employed to treat bacterial infections. This study presents
an unprecedented treatment method, with a dual benefit for the
human health and the environment which can be applied for removal
or degradation of flucloxacillin antibiotic and conversion of the highly
toxic tellurite to a non-toxic valuable tellurium metalloid. This method
is expected to be more convenient, simple, rapid, inexpensive and safe
procedure that provides more benefit for the environment and human
health than previously reported methods. The study elucidates the reac-
tive species in such system, explores the selectivity of flucloxacillin
bioreductant towards tellurite and identifies the reaction products.
This study also delineates the redox reaction mechanism and estab-
lishes a rate-law expression that is based on the kinetic outcomes.
2. Experimental
2.1. Materials
All chemicals were purchased from Sigma or Merck in spectro-
scopic grade and were employed as received without further pro-
cessing. Double-distilled water was used to prepare all solutions. A
stock solution of flucloxacillin sodium (Sigma, purity N95%) was pre-
pared by dissolving it in water. The solution of potassium tellurite
(Sigma, purity = 95%) was freshly prepared in water and was nor-
malized using spectrophotometry. The stock solutions of the acidic
medium (H2SO4) and the sulfate salts of zinc, cobalt and cadmium
were from Merck (Germany). Sodium sulfate and acetic acid were
employed to maintain and study the impact of the ionic strength
(I) and to explore the impact of the relative permittivity of the me-
dium, respectively.
2.2. Kinetic measurements
Kinetic measurements were ascertained in pseudo–first-order
circumstances, whereas the concentration of potassium tellurite
oxidant was in large excess with respect to flucloxacillin
bioreductant. The ionic strength, I, of the reaction medium was
attained at 2.0 mol dm−3. The absorbance readings were per-
formed with a Shimadzu UV-VIS-NIR-3600 double-beam spec-
trophotometer. The advance of the redox reaction was
monitored by assaying the decay of flucloxacillin absorbance
over time at λ = 342 nm, because potassium tellurite does
not absorb at this wavelength. First-order graphs of ln Abs. vs.
time were set to be linear (up to N80% of the reaction perfor-
mance), and the observed pseudo-first-order rate constant
values (kobs) were obtained as the mean value of three runs.
The reaction order (n) of the tellurite oxidant and sulfuric acid
medium was obtained from the slopes of log kobs vs. log
Consn. graphs.
3. Results
3.1. Time-resolved spectra
Fig. 1 presents the time-resolved spectra for the reduction of tellurite ion (TeIV) by flucloxacillin (Flx) in H2SO4. The recorded spectra of the
reacting mixture exhibited a decreasing Flx band at its absorption maximum (λ = 342 nm) as the reaction advanced due to its oxidation by TeIV,
with a noticeable bathochromic shift of 6 nm. Furthermore, the presence of two isosbestic points at (λ = 303 and 265 nm) suggests that an equilib-
rium was established between Flx and the probable constructed Flx - TeIV intermediate complex.
1.0
Flx
Mixture
0.8
0.6
0.4
0.2
TeIV
0.0
240
280
320
360
400
Wavelength, nm
Fig. 1. Time-resolved spectra for the reduction of tellurite ion by flucloxacillin in H2SO4 solution. [Flx] = 5.0 × 10−4, [TeIV] = 6.0 × 10−3, [H+] = 1.0 and I = 2.0 mol dm−3 at 298 K.

A. Fawzy / Journal of Molecular Liquids 292 (2019) 111436
3
3.2. Stoichiometry and product description
A variety of reaction mixtures containing several ratios of [Flx] / [TeIV] at fixed [H+] = 1.0 and I = 2.0 mol dm−3 at room temperature were
allowed to react for 24 h until reaction completion in all mixtures. A spectrophotometric estimation of the unconsumed [Flx] atλ = 348 nm identified
a 1:1 stoichiometry, as presented in the following equation:
ð1Þ
This stoichiometric equation is in full agreement with the product description, where the compounds (I), (II) and (III) are flucloxacillin
and its oxidation products 3-(2-chloro-6-fluorophenyl)-5-methylisoxazol-4-carbocylic acid and 5,5-dimethyl-thiazolidine-2,4-dicarboxlic
acid, respectively. The oxidation products of flucloxacillin drug were identified by spot tests and FT-IR spectra. FT-IR spectra of
flucloxacillin (1) and its oxidation products (2) are illustrated in Fig. 2 which showed different fingerprints in both. The spectrum of
the oxidation products (2) showed disappearance of the sharp absorption peak which was located at 1768 cm−1 (due to amidic ketone
C_O stretch) in the spectrum of flucloxacillin (1). Appearance of a broad peak at 3375 cm−1 (due to carboxylic OH with N\\H stretching)
is in consistent with the product 5,5-dimethyl-thiazolidine-2,4-dicarboxlic acid. In addition, appearance of a peak at 1652 cm−1 is due to
carboxylic C_O stretch. The by-products were characterized by spot tests as ammonium ion and CO2. The formation of elemental tellu-
rium, Te0, was indicated by the dense black precipitate that formed by the reaction, which was soluble in an aqueous bromine solution
[9,10].
3.3. Effect of [Flx] on the reduction rate
The impact of the flucloxacillin reductant concentration, [Flx], on the reduction rate was explored at [TeIV] = 6.0 × 10−3, [H+] = 1.0, ionic
strength, I = 2.0 mol dm−3, T = 298 K and at several [Flx] ranging from 1.0 × 10−4 to 9.0 × 10−4 mol dm−3. The graphs of ln Abs. vs. time yielded
straight lines for approximately 80% of the reactions, and the values of kobs were found to be unaffected when the concentration of Flx was varied, as
shown in Table 1. The results indicated that the reduction rate was independent of the reductant concentration and that there was a first-order re-
action regarding the reductant.
40
1
30
3521
2
20
10
0
3364
3375
1768
1652
1603
4000
3500
3000
2500
2000
1500
1000
500
Wave number, cm-1
Fig. 2. FT-IR spectra of flucloxacillin (1) and its oxidation products (2).

4
A. Fawzy / Journal of Molecular Liquids 292 (2019) 111436
200
288 K
298 K
150
308 K
318 K
100
50
0
0
2
4
6
8
10
103 [TeIV] (mol dm-3)
Fig. 3. Effect of [TeIV] on the value of kobs in the reduction of tellurite ion by flucloxacillin in H2SO4 solution. [Flx] = 5.0 × 10−4, [H+] = 1.0 and I = 2.0 mol dm−3 at several temperatures.
3.4. Effect of [TeIV] on the reduction rate
The reduction rate was assessed at several concentrations of tellurite ion oxidant (TeIV), 2.0 × 10−3 b [TeIV] b 10.0 × 10−3 mol dm−3, with fixed
[Flx], [H+] and I, and at four temperatures (288, 298, 308 and 318 K). Increasing the tellurite ion concentration increased the reduction rate, as shown
in Table 1. Graphs of kobs vs. [TeIV] at various temperatures were obviously linear with positive intercepts (Fig. 3) demonstrating that the reduction
reaction was less than unit order (n) in [TeIV]T (0 b n b 1).
3.5. Effect of [H+] on the reduction rate
The consequence of varying [H+] on the reduction rate was explored to further investigate the reduction mechanism. Kinetic experiments were
carried out at a variety of [H2SO4] from 0.4 to 1.8 mol dm−3 while maintaining the other constituents and the four studied temperatures. Increasing
the sulfuric acid concentration enhanced the reduction rate (Table 1), indicating that the redox reaction was acid-catalyzed. Graphs of kobs vs. [H+] at
several temperatures were linear, with presumed positive slopes designating fractional-first order reliance with reverence to [H+] (Fig. 4).
3.6. Effect of ionic strength and relative permittivity of the reaction medium
The impact of the reaction medium's ionic strength on the reduction rate was studied to better understand the nature of the reactive species in the
rate-limiting step of the redox reaction. This was accomplished by the addition of known concentrations of Na2SO4 solution to fixed concentrations of
the other reaction ingredients. The results showed that varying the ionic strength did not significantly alter the reduction rate.
Additionally, the influence of the relative permittivity of the reaction medium was explored by altering the acetic acid–water content (vol%) in the
reaction medium (up to 40% acetic acid) while maintaining all other conditions. The reduction rate was unaffected by altering the relative permit-
tivities of the acidic medium.
160
288 K
298 K
308 K
318 K
120
80
40
0
0.0
0.5
1.0
1.5
2.0
[H+] (mol dm-3)
Fig. 4. Effect of [H+] on the value of kobs in the reduction of tellurite ion by flucloxacillin in H2SO4 solution. [Flx] = 5.0 × 10−4, [TeIV] = 6.0 × 10−3 and I = 2.0 mol dm−3 at several
temperatures.

A. Fawzy / Journal of Molecular Liquids 292 (2019) 111436
5
Table 1
Effect of [Flx], [TeIV], [H+] and I on kobs in the reduction of tellurite ion by flucloxacillin in H2SO4 solution at 298 K.
104 [Flx]
103 [TeIV
]
[H+
]
I
103kobs
mol dm−3
mol dm−3
mol dm−3
mol dm−3
s−1
1.0
3.0
5.0
7.0
9.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
6.0
6.0
6.0
6.0
6.0
2.0
4.0
6.0
8.0
10.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.4
0.7
1.0
1.4
1.8
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.5
3.0
3.5
4.0
49.9
51.3
51.0
52.1
50.6
23.2
33.9
51.0
61.3
70.0
29.8
41.2
51.0
59.9
67.3
51.0
51.5
53.4
52.2
52.5
Experimental error 4%.
3.7. Effect of divalent transition metal ions on the reduction rate
The rate of the reduction reaction was assayed at several concentrations of certain divalent metal ions, namely, CoII, ZnII and CdII, in the range of
2.0–10.0 × 10−4 mol dm−3, with the other reactants being fixed, and at 298 K. The resulting reduction rates increased with increasing concentrations
of the supplemented metal ions, as presented in Fig. 5.
3.8. Effect of temperature
To estimate the activation and thermodynamic parameters, the reduction rate was recorded at four temperatures ranging between 283 and 313 K
with various concentrations of tellurite ion and H2SO4 and at [Flx] = 5.0 × 10−4 and I = 2.0 mol dm−3. The results showed that raising the temper-
ature increased the reduction rate. The rate constant activation parameters of the slow step (k1) and the thermodynamic parameters of the equilib-
rium constants (K1 and K2) included in the reaction mechanism were assessed and are presented in Tables 2 and 3, respectively.
3.9. Polymerization study
The appearance of free radicals throughout the redox reaction was assessed by supplementing the reaction mixture with acrylonitrile for approx-
imately 4 h in an inert atmosphere, followed by diluting the mixtures with methanol. The test was negative, i.e., no white precipitate was formed,
signifying the absence of free radical production in the present redox reaction.
80
75
ZnII
70
CoII
CdII
65
60
55
50
0
2
4
6
8
10
104 [MII] (mol dm-3)
Fig. 5. Effect of divalent transition metal ions on kobs in the reduction of tellurite ion by flucloxacillin in H2SO4 solution. [Flx] = 5.0 × 10−4, [TeIV] = 6.0 × 10−3, [H+] = 1.0 and I =
2.0 mol dm−3 at 298 K.

6
A. Fawzy / Journal of Molecular Liquids 292 (2019) 111436
4. Discussion
Additionally, the acquired less-than unit order of the reduction
rate with respect to the tellurite ion concentration can be ascribed
to the formation of complexes among the reactive species (Flx+
and H2TeO3) prior to the rate-limiting step. Additionally, complex
formation was confirmed by the acquired non-zero intercepts of 1/
] at all the studied temperatures, similar to
Michaelis–Menten kinetics for complex formation, as shown in
Fig. 7.
It has been stated [27] that tellurite ion (TeO23−) is present in solu-
tions as hydrotellurite, also termed hydrogen tellurite ion HTeO−3 . In
slightly acidic solutions, HTeO−3 can be present, but in more acidic solu-
tions tellurous acid, H2TeO3, is predominant [10,28]. In aqueous solu-
tions, tellurous acid behaves as a weak acid, as exemplified in the
following equilibria [29]:
kobs vs. 1/[TeIV
Based on the acquired kinetic outcomes and product characteriza-
tion, a likely reduction mechanism was suggested and is illustrated in
Scheme I. The first step of the anticipated mechanism is the protonation
−
H2TeO3 þ H2O⇌H3Oþ þ HTeO3 Ka1 ¼ 2:0 ꢀ 10−3
ð2Þ
ð3Þ
of flucloxacillin bioreductant to form the more reactive species, Flx+
,
HTeO3 þ H2O⇌H3Oþ þ TeO3 Ka2 ¼ 1:0 ꢀ 10−8
−
2−
(Eq. (4)), which reacts with tellurous acid, H2TeO3, in the second step
to form an intermediate complex (C), as represented by the following
equation:
The dissociation constant (Ka1 and Ka2) values specify that depro-
tonation occurs first, to a presumed extent, in aqueous acidic me-
dium. However, due to the very low value of Ka2, the second
deprotonation does not occur under the acidic condition of the pres-
ent study. Furthermore, the observed increasing reduction rate upon
raising [H+] also signifies that tellerous acid is the predominant reac-
tive species.
K2
Flxþ þ H2TeO3
½Flx–H2TeO3ꢁþ ðCÞ
ð5Þ
This intermediate promptly disintegrates in the rate-limiting
step, giving rise to an intermediate oxidation product of
flucloxacillin and the very reactive tellurium (II) ion, HTeO−2 . Fi-
nally, the latter two intermediate products rapidly interact,
forming the final redox products. This can be depicted by the fol-
lowing equation:
On the other hand, flucloxacillin tends to become protonated in
acidic medium, as represented by the following equilibrium [30]:
Flx þ Hþ Flxþ
ð4Þ
Therefore, the fractional first-order rate constant with reference to
[H+] can be interpreted as the protolytic process of flucloxacillin in an
acidic medium, i.e., the protonated species of flucloxacillin may be the
reactive species, playing the principle role in the present reaction
kinetics.
k1
½Flx–H2TeO3ꢁþ → Flx intermediate product
slow
H2
O
−
þ HTeO3 → Flx oxidation products þ Te0
ð6Þ
fast
Flucloxacillin has a good ability to form complexes with transition
metal ions [31]. The findings of the current work showed that adding
certain divalent transition metal ions (CoII, ZnII and CdII) increased the
rate of the redox reaction. These findings can be interpreted on the
basis of the complexation of transition metal ions with flucloxacillin in
such medium. In view of the results found here and in previous studies,
the anticipated structure of flucloxacillin-metal complexes (shown in
Fig. 6) are of the general formula MII(Flx)2(H2O)2, where M is the tran-
sition metal ion and Flx− is the mono anion of flucloxacillin. It is sug-
gested that flucloxacillin coordinates with the transition metal ion
through a carbonyl oxygen and forms a covalent bond with the
deprotonated nitrogen atom.
The negligible impact of the ionic strength and the relative permit-
tivity of the reaction medium are in agreement with the reaction occur-
ring between an ion and a neutral molecule [32], i.e., between Flx+ and
H2TeO3 (as in the anticipated mechanism).
According to the anticipated mechanism, the following rate-law ex-
pression was deduced (see Appendix A),
h
i
ꢀ
ꢁ
k1K1K2½Flxꢁ TeIV Hþ
h
i
Rate ¼
ð7Þ
ꢀ
ꢁ
ꢀ
ꢁ
1 þ K1 Hþ þ K1K2 TeIV Hþ
This equation is in a good agreement with all of the observed orders
regarding the existing reactants.
Under pseudo-first order conditions:
−d½Flxꢁ
Rate ¼
¼ kobs½Flxꢁ
ð8Þ
dt
COOH
S
O
N
Comparison of Eqs. (7) and (8) gives:
N
h
i
F
ꢀ
ꢁ
k1K1K2 TeIV Hþ
h
i
kobs
¼
ð9Þ
ꢀ
ꢁ
ꢀ
ꢁ
O
N
1 þ K1 Hþ þ K1K2 TeIV Hþ
OH2
MII
Cl
O
Cl
and with rearrangement of Eq. (9) results in the following equations:
H2O
N
O
N
!
ꢀ
1 þ K1 Hþ
ꢀ
ꢁ
1
kobs
1
TeIV
1
F
h
i
ꢁ
¼
¼
þ
þ
ð10Þ
ð11Þ
N
k1K1K2 Hþ
k1
S
HOOC
0
@
1
1
kobs
1
1
Hþ
1
1
k1
A
h
i
h
i
ꢀ
ꢁ
þ
k1K1K2 TeIV
k1K2 TeIV
Fig. 6. Suggested molecular structure of flucloxacillin complexes with divalent transition
metal ions.

A. Fawzy / Journal of Molecular Liquids 292 (2019) 111436
7
O
O
H2
N
N
H
N
+
N
S
S
K1
+
+ H
O
O
N
N
O
O
Cl
Cl
COOH
COOH
(I)
(Flx)
(Flx+)
O
O
H2
H2
N
N
O
+ HO Te OH
COOH
+
+
N
N
S
S
N
K2
O
O
N
..
+
O
O
Cl
Cl
COOH
Te
(II)
HO
O
(C)
-
OH
k1
slow
O
O
N
H2
N
+
N
S
HOOC
-
COOH
HTeIIO2
S
O
HN
H2O
O
HN
Cl
Cl
+
O
COOH
+
fast
COOH
+
+ Te0 + CO2 + NH+4
-
HTeIIO2
(III)
Scheme 1. Mechanism of reduction of tellurite ion by flucloxacillin in H2SO4 solution.
As stated in Eqs.(10) and (11), graphs of 1/kobs vs. 1/[TeIV] at a fixed
[H+] and 1/kobs vs. 1/[H+] at fixed [TeIV] (at several temperatures)
should form straight lines with positive intercepts on the 1/kobs axes.
This was shown in the investigational outcomes (Figs. 7 and 8, respec-
tively), confirming the validity of the anticipated mechanism. The ac-
quired values of k1 at several temperatures are listed in Table 2. The
activation parameters of k1 were estimated from Arrhenius and Eyring
graphs (Figs. 9a and b, respectively) and are also inserted in Table 2.
Similarly, on the basis of Eq. (11), graphs of 1/kobs vs. 1/[H+] yielded
straight lines. Using the slopes and intercepts of such graphs, the values
of K1 and K2 at several temperatures were acquired and are presented in
Table 3. The Van't Hoff graphs (Fig. 10) were constructed to show the
variation of K1 and K2 with temperature. The values for enthalpy, en-
tropy, and free energy of the reaction were evaluated and are also pre-
sented in Table 3.
The anticipated mechanism and the rate-law expressions are sup-
ported by the acquired activation parameters. The high positive values
of both enthalpy (ΔH≠) and free energy (ΔG≠) signified that the forma-
tion of the intermediate complex is endothermic and non-spontaneous,
while the acquired negative entropy (ΔS≠) indicated the establishment
60
80
288 K
298 K
45
288 K
298 K
308 K
318 K
60
308 K
318 K
30
40
15
0
20
0
0.0
0.6
1.2
1.8
2.4
3.0
0
100
200
300
400
500
600
1/[H+] (dm3 mol-1)
1/[TeIV] (dm3 mol-1)
Fig. 8. Graphs of 1/kobs vs. 1/[H+] at several temperatures for the reduction of tellurite ion
by flucloxacillin in H2SO4 solution. [Flx] = 5.0 × 10−4, [TeIV] = 6.0 × 10−3 and I =
Fig. 7. Graphs of 1/kobs vs. 1/[TeIV] at several temperatures for the reduction of tellurite ion
by flucloxacillin in H2SO4 solution. [Flx] = 5.0 × 10−4, [H+] = 1.0 and I = 2.0 mol dm−3
.
2.0 mol dm−3
.

8
A. Fawzy / Journal of Molecular Liquids 292 (2019) 111436
(a)
-1.4
-1.6
-1.8
-2.0
-2.2
(b)
-31.0
-31.2
-31.4
-31.6
3.1
3.2
3.3
3.4
3.5
3.1
3.2
3.3
3.4
3.5
103 (1/T) K
-1
-1
103 (1/T) K
Fig. 9. (a) Arrhenius and (b) Eyring graphs of k1 in the reduction of tellurite ion by flucloxacillin in H2SO4 solution. [Flx] = 5.0 × 10−4, [TeIV] = 6.0 × 10−3 and I = 2.0 mol dm−3
.
activation energy (E≠a) indicates that the slow (rate-limiting) step was
due to the intermediate complex decaying into the reaction products
[33].
6
4
5. Conclusions
K1
K2
1) A novel treatment method for the removal of toxic tellurite ion was
described via its bioreduction using flucloxacillin antibiotic in H2SO4
solution.
2
2) This treatment method has a dual benefit for the human health and
the environment due to the degradation of flucloxacillin antibiotic
and the conversion of highly toxic tellurite to a nontoxic valuable tel-
lurium metalloid.
0
3) The suggested method is expected to be more convenient, simple,
rapid, inexpensive and safe and provides more benefit for the
human health and the environment than previously reported
methods.
-2
3.1
3.2
3.3
3.4
3.5
103 (1/T) K
-1
4) The kinetics of the bioreduction process were monitored
spectrophotometrically.
5) Supplementation with certain divalent transition metal ions aug-
mented the reaction rate.
Fig. 10. Van't Hoff graphs of K1 and K2 in the reduction of tellurite ion by flucloxacillin in
H2SO4 solution. [Flx] = 5.0 × 10−4, [TeIV] = 6.0 × 10−3 and I = 2.0 mol dm−3
.
of a rigid associative intermediate complex with diminishing degrees of
freedom of the reactant molecules. The values of ΔH≠ and ΔS≠ were both
consistent with the electron-transfer process. Furthermore, the higher
6) The redox products were identified as 3-(2-chloro-6-fluorophenyl)-
5-methylisoxazol-4-carbocylic acid and 5,5-dimethyl-thiazolidine-
2,4-dicarboxlic acid, NH+4 , CO2 and Te0.
Table 2
Values of k1 at several temperatures and the related activation parameters in the reduction of tellurite ion by flucloxacillin in H2SO4 solution. [Flx] = 5.0 × 10−4, [TeIV] = 6.0 × 10−3 and I
= 2.0 mol dm−3
.
Rate constant (s−1
)
Temperature (K)
Activation parameters
288
298
308
318
ΔS≠ 8(Jmol−1 K−1
)
ΔH≠ (kJ mol−1
)
ΔG≠298 (kJ mol−1
)
E≠a (kJ mol−1
)
102 k1
12.11
14.95
18.50
22.53
−37.83
13.26
24.53
15.96
Experimental error 4%.
Table 3
Values of K1 and K2 at several temperatures and their thermodynamic quantities in the reduction of tellurite ion by flucloxacillin in H2SO4 solution. [Flx] = 5.0 × 10−4, [TeIV] = 6.0 × 10−3
and I = 2.0 mol dm−3
.
Equilibrium constant (dm3 mol−1
)
Temperature (K)
Thermodynamic parameters
288
298
308
318
ΔHo (kJ mol−1
)
ΔGo298 (kJ mol−1
)
ΔSo (Jmol−1 K−1
)
102 K1
16.02
5.76
28.79
4.92
46.05
3.81
66.43
3.04
36.17
−16.51
3.07
−15.36
−111.07
3.86
10−2 K2
Experimental error 5%.

A. Fawzy / Journal of Molecular Liquids 292 (2019) 111436
9
7) The activation and thermodynamic parameters were estimated and
Then,
½FlxꢁF
confirmed.
½FlxꢁT
8) The probable reaction mechanism presents the protonated
flucloxacillin and tellurous acid as the reactive species.
9) The derived rate-law expression was found to be in good agreement
with the acquired investigational outcomes.
h
i
¼
ðA:12Þ
ꢀ
ꢁ
ꢀ
ꢁ
1 þ þK1 Hþ þ K1K2 TeIV Hþ
Because of the low [TeIV] used, the term K1K2[TeIV][H+] in the de-
nominator can be neglected. So,
Appendix A. Derivation of the rate-law expression
½FlxꢁT
ꢀ
ꢁ
½FlxꢁF
¼
ðA:13Þ
As stated in Scheme 1:
1 þ K1 Hþ
ꢀ
ꢁ
Flxþ
ꢀ
þꢁ
ꢀ
ꢁ
Based on high [H+], we can write:
ꢀ
ꢁ
K1
¼
; Flx ¼ K1½Flxꢁ Hþ
ðA:1Þ
ðA:2Þ
½Flxꢁ Hþ
ꢀ
ꢁ
ꢀ
ꢁ
Hþ ¼ Hþ
ðA:14Þ
F
T
and
Substituting Eqs. (A.9), (A.13) and (A.14) into Eq. (A.4) gives:
½Cꢁ
h
ꢁ
i
K2
¼
ꢀ
h
i
Flxþ TeIV
ꢀ
ꢁ
k1K1K2½Flxꢁ TeIV Hþ
h
i
Rate ¼
ðA:15Þ
ꢀ
ꢁ
ꢀ
ꢁ
1 þ K1 Hþ þ K1K2 TeIV Hþ
h
i
h
i
ꢀ
ꢁ
ꢀ
ꢁ
½Cꢁ ¼ K2 Flxþ TeIV ¼ K1K2½Flxꢁ TeIV Hþ
Under pseudo-first-order circumstances:
From step (III), the rate-law expression can be written as in
Eq. (A.3):
−d½Flxꢁ
Rate ¼
¼ kobs½Flxꢁ
ðA:16Þ
dt
−d½Flxꢁ
Rate ¼
¼ k1½Cꢁ
ðA:3Þ
ðA:4Þ
ðA:5Þ
Comparing Eqs. (A.15) and (A.16), the following relationship is ac-
quired:
dt
Substituting Eq. (A.2) into Eq. (A.3) results in:
h
i
ꢀ
ꢁ
k1K1K2 TeIV Hþ
h
i
ꢀ
ꢁ
h
i
Rate ¼ k1K1K2½Flxꢁ TeIV Hþ
kobs
¼
ðA:17Þ
ꢀ
ꢁ
ꢀ
ꢁ
1 þ K1 Hþ þ K1K2 TeIV Hþ
Total concentration of TeIV is given by:
With rearrangement, the following essential equations are obtained:
h
i
h
i
!
ꢀ
1 þ K1 Hþ
ꢀ
ꢁ
TeIV
¼
TeIV þ ½Cꢁ
1
kobs
1
TeIV
1
k1
T
F
h
i
ꢁ
¼
¼
þ
þ
ðA:18Þ
ðA:19Þ
k1K1K2 Hþ
where ‘T’ and ‘F’ indicate total and free.
0
@
1
A
Substituting Eq. (A.1) into Eq. (A.5) leads to:
1
kobs
1
1
1
1
k1
h
h
i
i
h
h
i
h
i
h
i
h
i
ꢀ
ꢁ
þ
ꢀ
ꢁ
Hþ
TeIV
TeIV
¼
¼
TeIV þ K1K2½Flxꢁ TeIV Hþ
ðA:6Þ
ðA:7Þ
k1K1K2 TeIV
k1K2 TeIV
T
T
F
i
ꢂ
ꢀ
ꢁꢃ
TeIV 1 þ K1K2½Flxꢁ Hþ
F
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