Polymerisation occurrence in the anodic oxidation of phosphite on a boron-doped diamond electrode

Article abstract of DOI:10.1016/j.electacta.2008.02.043

The electrogeneration of polymeric phosphorus compounds during the anodic oxidation of aqueous solutions of phosphites on a boron-doped diamond electrode has been studied. Although the main oxidation product is orthophosphate, the results indicate the simultaneous generation of short-chain and cyclic compounds containing two and three phosphorus atoms whose evolution has been followed by ion chromatography. The effect on the reaction yield of several operative parameters such as current density, pH, temperature and initial phosphite concentration has been investigated. Consistently with the data presented, a new process for the generation of polymeric phosphates is obtained.

Full text of DOI:10.1016/j.electacta.2008.02.043

Electrochimica Acta 53 (2008) 4952–4957
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Polymerisation occurrence in the anodic oxidation of phosphite on
a boron-doped diamond electrode
Elisabetta Petrucci∗, Daniele Montanaro, Carlo Merli
Department of Chemical Engineering Materials Environment, University of Rome “La Sapienza”, Via Eudossiana 18, 00184 Roma, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrogeneration of polymeric phosphorus compounds during the anodic oxidation of aqueous solu-
tions of phosphites on a boron-doped diamond electrode has been studied. Although the main oxidation
product is orthophosphate, the results indicate the simultaneous generation of short-chain and cyclic
compounds containing two and three phosphorus atoms whose evolution has been followed by ion
chromatography.
Received 26 November 2007
Received in revised form 12 February 2008
Accepted 14 February 2008
Available online 21 February 2008
The effect on the reaction yield of several operative parameters such as current density, pH, temperature
and initial phosphite concentration has been investigated.
Consistently with the data presented, a new process for the generation of polymeric phosphates is
Keywords:
Anodic oxidation
Boron-doped diamond electrode
Polymerisation
obtained.
© 2008 Elsevier Ltd. All rights reserved.
Phosphite
Polyphosphates
1. Introduction
Apart from phosphates, oxidised forms of phosphorus include
[27] polymeric compounds resulting from the condensation of
Great attention has recently been given to the study and devel-
opment of boron-doped diamond (BDD) as an electrode material
in several electrochemical applications [1,2]. The interest in BDD is
due to its outstanding properties such as high anodic stability and
low background current, together with a wide potential window of
water discharge [3–5]. The oxidation reaction has been proved to
take place through different mechanisms depending on the applied
potential [6]. In the range of water stability, direct electron trans-
fer occurs, while at higher potential, the hydroxyl radical, acting as
a very strong oxidising agent (E◦ = +2.8 V vs. NHE), is generated by
water discharge.
These unique characteristics have allowed wide use of BDD elec-
trodes as the anode in the oxidation of a broad range of organic
compounds [7–11], in the treatment of a variety of industrial
wastewater pollutant [12–14], water disinfection [15–17] and elec-
trosynthesis [18–22].
A recent study, focused on treating an effluent from the man-
ufacture of phosphorus-based flame retardants [23], confirmed
the expected performance of the BDD electrode. Compared to
Fenton’s treatment, either chemically or electrochemically gen-
erated [24–26], BDD led to a faster and quantitative oxidation
of a notable amount of hypophosphite and phosphite ions to
phosphate.
two or more hydrogenophosphate units to form condensed lin-
ear polyphosphates with the generic formula PnO3n+1(n+2)−, or
metaphosphates PnO3nn− characterised by a cyclic structure. Major
applications of these polyphosphates are in detergent and cleaner
products, the food industry and water treatment as soften-
ing agents. Another class of phosphorus compounds contains a
peroxide group. The most important species in this group, per-
oxomonophosphate and peroxodiphosphate, are mainly used as
oxidising agents.
Although analytical determination of these chemicals is a critical
issue [28], recent developments in the field of ion chromatography,
such as electrolytic eluent generators, have allowed the detection
of phosphorus compounds in aqueous media in the range of con-
centrations required for the present study [29].
The aim of this paper is to investigate the electrogeneration of
oxidised phosphorus compounds other than phosphate when the
anodic oxidation of phosphite on a boron-doped diamond electrode
occurs. The influence of certain variables on the process perfor-
mance is also investigated.
2. Experimental
2.1. Electrochemical equipment
All the electrolyses were performed under galvanostatic condi-
tions using a potentiostat AMEL 2051. An undivided thermostated
100 ml cell was used stirred with a magnetic bar. A boron-doped
∗
Corresponding author. Tel.: +39 06 44585569; fax: +39 06 44585622.
E-mail address: elisabetta.petrucci@uniroma1.it (E. Petrucci).
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.02.043

E. Petrucci et al. / Electrochimica Acta 53 (2008) 4952–4957
4953
diamond electrode of a geometric area of 5 cm2 constituted the
anode and the counter electrode was a platinum wire. Unless oth-
erwise specified, all the experiments were conducted on solutions
containing 16 mM of sodium phosphite, T = 20 ◦C and j = 100 A m−2
.
Since the initial pH of this solution was 8.3, the addition of 1N
sulphuric acid was needed for tests carried out at pH 2 and pH 7,
while a pH value of 12 was achieved by the addition of 1N sodium
hydroxide.
Experiments were replicated at least three times.
2.2. Analysis
pH was measured with a Crison GLP 421 pH meter. The
concentration of total phosphorus was determined by a UV–vis
spectrophotometer according to standard methods [30]. The ionic
species concentration was determined by a Dionex 120 ionic chro-
matograph equipped with an IONPAC AS16 anionic column and an
ASRS ULTRA II suppressor. This system was updated with an RFC
30 eluent generator that provided for electrolytic production of
high-purity hydroxide eluent with gradient capabilities.
Fig. 1. Elution order of compounds (—) from electrolysis of 16 mM phosphite
(j = 100 A m−2, V = 100 ml, pH 2, T = 20 ◦C, time of electrolysis = 2 h) including spiked
compounds (- - -). Peak identification: (1) hypophosphite, (2) phosphite, (3) sul-
phate, (4) phosphate, (5) unknown peak, (6) trimetaphosphate, (7) hypophosphate,
(8) pyrophosphate, (9) peroxodiphosphate, (10) unknown peak, (11) tripolyphos-
phate, (12) tetrametaphosphate, (13) tetrapolyphosphate.
The method of analysis developed for this work utilized an elu-
ent gradient from 15 mM at 0 min to 65 mM at 30 min, a flow rate
of 1.5 ml min−1 and an injection volume of 25 ␮l.
The inorganic ionic species were identified through com-
parison to commercial and prepared standards. The phosphate,
phosphite and hypophosphite standards were prepared from the
corresponding sodium salts (Na3PO4, Na2HPO2 and NaH2PO2) sup-
plied by Merck. Sodium pyrophosphate (Na4P2O7·10H2O), sodium
the peaks deriving from a 2-h electrolysis of 16 mM phosphite
(j = 100 A m−2, pH 2) are shown as solid lines, while the spiked com-
pounds hypophosphite, hypophosphate, tetrametaphosphate, and
tetrapolyphosphate are shown as dotted lines. The elution order,
except for trimetaphosphate, is consistent with the general rule
according to which the retention time directly depends on charge
and molar mass. The electrogeneration of polymeric compounds
was unequivocally confirmed by means of electrospray mass spec-
trometry where traces of P compounds with n > 4 were also found.
At the end of the same electrolysis, when the overall dis-
appearance of phosphite was observed, the concentration of
phosphate was 12.8 mM, trimetaphosphate 2.3 mM and pyrophos-
phate 0.9 mM.
To better understand how the operating conditions affect
the electrogeneration of polymeric compounds, a study focus-
ing on peroxodiphosphate (P2O84−), pyrophosphate (P2O74−) and
trimetaphosphate (P3O93−) was conducted. The relevance of these
compounds lies both in their significant concentration and in their
being representative of three different classes of phosphorus com-
pounds: diperoxide, linear and cyclic polyphosphates.
trimetaphosphate (Na3P3O9), sodium tripolyphosphate (Na5P3O10
were supplied by Sigma–Aldrich.
)
Sodium tetrametaphosphate (Na4P4O12) was prepared by low
temperature hydration of phosphorus pentoxide [31] while per-
oxodiphosphate was electrochemically synthesized starting from
aqueous solutions of phosphate as described by Can˜izares et al.
[21]. Hypophosphate was obtained through oxidation of red phos-
phorus with hydrogen peroxide under alkaline conditions [32]
and tetrapolyphosphate from hydrolysis of tetrametaphosphate
[33]. Since the reaction yield in the synthesis of tetrametaphos-
phate, peroxodiphosphate and hypophosphate was not exactly
calculated, only semiquantitative analyses of these compounds
were conducted. The identification of all the examined phosphorus
compounds was confirmed by means of electrospray mass spec-
trometry.
3. Results and discussion
3.1. Electrolyses
The influence of current density on the production of poly-
meric compounds was studied in the range from 50 to 200 A m−2
in galvanostatic electrolyses of 16 mM phosphite solutions at
pH 2. The data reported in Fig. 2, where the temporal evolu-
tions of all the main products are presented, indicate that an
increase in the current density results in an enhanced produc-
tion of phosphate. On the contrary, trimetaphosphate was found
to be notably sensitive to the current applied, doubling its max-
imum concentration when current density was halved. This is
despite an initial trend, in which trimetaphosphate was appar-
ently independent of the current density especially when j = 50
and 100 A m−2. In general, after a rapid increase, the curves tend
toward a quasi-steady state concentration. A similar trend is also
observed for peroxodiphosphate, even though its concentration
reaches comparable values at higher current density and signifi-
cantly rises only at 50 A m−2. A considerably different behaviour
is seen for pyrophosphate, whose concentration is nearly unaf-
fected by current and grows continuously, although at a very
low rate.
During the study of the anodic oxidation of phosphite using
a BDD electrode, in a few of the experiments, when sulphuric
acid instead of hydrochloric acid was used as the supporting elec-
trolyte, unexpected behaviour was observed. At the end of the
oxidation treatment, although spectrophotometric determinations
had verified the total phosphorus content, the concentration of
orthophosphate, determined by means of ionic chromatography,
was lower than that stoichiometrically expected. This result pro-
vides evidence for the generation of soluble oxidised phosphorus
compounds other than phosphate that are not detectable with
the used chromatographic technique. After updating the DX120 as
described in Section 2.2, it was possible to verify that, although
phosphate represents the main product, significant amounts of
polymeric phosphorus compounds were also observed. The order
of elution of phosphorus compounds obtained with the developed
chromatographic method is shown in Fig. 1, where IC signal inten-
sity is expressed as a function of retention time. In particular, all

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E. Petrucci et al. / Electrochimica Acta 53 (2008) 4952–4957
Fig. 2. Effect of current density on temporal evolution of (a) phosphate, (b) trimetaphosphate, (c) peroxodiphosphate and (d) pyrophosphate. j = 50 A m−2 (ꢀ), j = 100 A m−2
(ꢀ), j = 200 A m−2 (ꢁ). [P]i = 16 mM, V = 100 ml, pH 2, T = 20 ◦C.
Given that an increasing current density is expected to enhance
the generation of hydroxyl radicals, the higher production of phos-
phates observed in the tests suggests that their formation occurs
through a reaction mediated by hydroxyl radicals. Moreover the use
of different anode materials (i.e. graphite), known to be unable to
produce hydroxyl radicals, led to very little phosphate production.
This result likewise inidicates that the oxidation process of phos-
phite using a BDD anode has to be attributed to hydroxyl radicals
as follows:
This result can be explained considering that, under the same
conditions of current density, i.e. in the presence of the same
amount of hydroxyl radicals, and increasing the concentration of
phosphite, that means reducing the ratio between hydroxyl radicals
and phosphite, a reaction pathway different from the hydroxylation
to phosphate (Eq. (2)) seems to be favoured.
To study the influence of pH on the concentration of phospho-
rus compounds, a series of tests were then performed on 16 mM
phosphite solution at 100 A m−2. Data referring to runs conducted
at pH 2, 7 and 12 are reported in Fig. 4. As shown, by increasing
the working pH, not only a higher initial rate of phosphate pro-
duction was observed but also a different final concentration was
found. The difference in the amount of final phosphate is attributed
to the electrogeneration of compounds other than phosphate that
reach a maximum under acidic conditions. In particular, passing
from pH 2 to pH 7, a significant effect on peroxodiphosphate and
pyrophosphate production was observed while trimetaphosphate,
in the same range, shows high and comparable concentrations.
Working at pH 12, apart from phosphate, very scarce concentra-
tions of polymeric phosphates are produced. Data not reported here
indicate that the phosphite removal rate is pH independent, sug-
gesting that the generation mechanism of all the products occurs
through a first common step that transforms phosphite into an
intermediate species via a reaction not influenced by pH. Never-
theless, pH appears to be crucial when the reaction evolves into
the final compounds.
•
OH
2−
2−
HPO3 −→ •PO3
(1)
(2)
•
OH
•PO32−−→ PO4
3−
On the other hand, the electro-generation of the other oxidised
compounds, being favoured by a lower current density, suggests the
occurrence of a different mechanism involving hydroxyl radicals as
well, in competition with the oxidation to phosphate. According
to these results, the production of polymeric compounds seems
to be favoured when the ratio between the hydroxyl radicals and
the phosphite concentration is lower. To support this hypothesis
a series of tests were thus performed at different initial concen-
trations of phosphite in electrolyses conducted under the same
conditions of current density.
The temporal evolution of the phosphorus compounds gener-
ated during electrolyses of solutions at initial phosphorus contents
ranging from 9.6 to 29 mM is illustrated in Fig. 3. As can be
seen, the amount of pyrophosphate, trimetaphosphate and perox-
odiphosphate increases with the initial concentration of phosphite.
Different formation yields (calculated as the ratio of the final
concentration of each compound to initial phosphite) were also
observed. In particular, the production yield varied from 3% to 29%
for trimetaphosphate, from 3% to 15% for peroxodiphosphate and
from 1% to 8.5% for pyrophosphate at the expense of phosphate,
whose production yield dropped from 100% to 58%.
A beneficial effect on the rate of the reactions, which is expected
with increasing temperature, is confirmed by the evolution of phos-
phite in a range varying from 10 to 40 ◦C (data not shown for clarity).
On the other hand, an almost constant production is found for phos-
phates under the same conditions, as shown in Fig. 5. This implies
that temperature positively affects a reaction pathway different
from that leading to phosphate (Eq. (2)), favouring in particular the
cyclization into trimetaphosphate. As illustrated in the same figure,

E. Petrucci et al. / Electrochimica Acta 53 (2008) 4952–4957
4955
Fig. 3. Effect of initial phosphite concentration on (a) phosphate, (b) trimetaphosphate, (c) peroxodiphosphate and (d) pyrophosphate. [P]i = 9.7 mM (ꢀ), [P]i = 16 mM (ꢀ),
[P]i = 29 mM (ꢁ). V = 100 ml, j = 100 A m−2, pH 2, T = 20 ◦C.
Fig. 4. Effect of pH on temporal evolution of (a) phosphate, (b) trimetaphosphate, (c) peroxodiphosphate and (d) pyrophosphate. pH 2 (ꢀ), pH 7 (ꢀ), pH 12 (ꢁ). [P]i = 16 mM,
V = 100 ml, j = 100 A m−2, T = 20 ◦C.

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E. Petrucci et al. / Electrochimica Acta 53 (2008) 4952–4957
Fig. 5. Effect of temperature on temporal evolution of (a) phosphate, (b) trimetaphosphate, (c) peroxodiphosphate and (d) pyrophosphate. T = 10 ◦C (ꢀ), T = 20 ◦C (ꢀ), T = 40 ◦
(ꢁ). [P]i = 16 mM, V = 100 ml, j = 100 A m−2, pH 2.
C
pyrophosphate appears to be insensitive to temperature even if the
low concentration produced does not allow a good evaluation of
the effect. Similar behaviour is presented by peroxodiphosphate,
which shows different concentrations, after the initial almost con-
stant trend. This is due to the concomitant occurrence of thermal
decomposition, most likely to phosphate.
4. Conclusions
The anodic oxidation of phosphite solutions on a boron-
doped diamond electrode results in the electrogeneration of
not only phosphate but also peroxodiphosphate and other poly-
meric compounds, mainly pyrophosphate and trimetaphosphate,
whose production depends on the operative conditions. Low cur-
rent density and high concentration have been proven to be
beneficial while increasing pH strongly affects the formation of
these products. Increasing the temperature tends to suppress
peroxodiphosphate, does not affect pyrophosphate, and notably
enhances only trimetaphosphate production.
3.2. Discussion
The data presented above show that a variety of oxidised phos-
phorus polymeric compounds besides phosphates can be produced
in acidic or circum-neutral media on a BDD electrode at a current
density of below 200 A m−2 also at room temperature. Although
further investigation is needed to ascertain the oxidation and
polymerisation mechanisms and also the actual radical species
involved, a few remarks can be made in accordance with the exper-
imental evidence.
For a better understanding of the mechanism involved, further
studies are in progress also considering hypophosphite as the oxi-
dizable species.
Acknowledgements
Since phosphite has a readily abstractable proton, the oxida-
tion process can cause cleavage of the P–H bond, leading to the
formation of a phosphorus-centred radical (Eq. (1)), which is sub-
sequently oxidized to the main final product, phosphate (Eq. (2)).
Although possible, the dimerization via phosphite radical to form
hypophosphate was not evidenced. To explain the formation of the
phosphorus polymeric compounds, characterised by a sequence
of oxygen and phosphorus intermolecular bonds, a further oxi-
dation reaction has to be assumed involving the activation of an
oxygen atom. This likely occurs through the formation of an oxygen-
centred radical, which seems possible at the conditions used here
only starting from phosphite ions. In fact, unlike the findings of pre-
vious work [21], the electro-generation of polymeric compounds
from phosphate has to be excluded since, preliminary experimental
tests, proved the total unreactivity of phosphate under the milder
operative conditions ([P]i = 16 mM, pH 2, j = 100 A m−2, undivided
cell) adopted for this work.
The authors are grateful to Prof. C. Furlani for his helpful dis-
cussion on reaction mechanisms and Dr. L. Salvini for analyses by
electrospray mass spectrometry.
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