# Photocatalyzed Degradation of a Pesticide Derivative Glyphosate in Aqueous Suspensions of Titanium Dioxide

Authors: Mohammad Muneer; Colin Boxall
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

The photocatalytic degradation of a herbicide derivative, glyphosate [(N-phosphonomethyl) glycine] has been investigated in aqueous suspensions of titanium dioxide at different pH values. This compound was found to degrade more efficiently under alkaline pH, where no adsorption takes place on the surface of the catalyst in the dark. The main degradation route involves the cleavage of the P–C bond giving rise to sarcosine and glycine as the intermediate products formed during the photooxidation process.


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## Body

## 1. INTRODUCTION

A wide variety of organic pollutants especially pesticides are introduced into the water system from
various sources such as industrial effluents, agricultural runoff, and chemical
spills [1, 2]. Their toxicity, stability to natural decomposition, and
persistence in the environment have been the cause of much
concern to the societies and regulatory authorities around the world [3, 4].The control of organic pollutants in water is an important measure in environmental protection. Among many processes proposed
and/or being developed for the destruction of the organic contaminants, biodegradation
has received the greatest attention. However, many organic chemicals,
especially those that are toxic or refractory, are not amendable to microbial
degradation [5]. Recently, considerable attention has been focused on the use
of semiconductor photocatalysis as a means to oxidise toxic organic chemicals [6–18]. The
mechanism constituting heterogeneous photocatalytic oxidation processes has
been discussed extensively in the literature (see, inter alia, [19, 20]). In many
of these studies, although the initial disappearance of the pollutant is rapid,
a number of by products are formed which can also be potentially harmful to the environment.The organophosphates exhibit considerable
persistence in groundwaters and are highly hydrophilic. These compounds are the most prevalent pesticides and
remain the choice of most farmers because of their effectiveness against large
numbers of insect species. They are potent acetylcholinesterase inhibitors and
are utilised to control chewing and sucking insects and spider mites on
ornamental plants, citrus fruits, stone fruits, and other agriculture crops.
For example, the pesticide derivative, glyphosate is a nonselective systemic herbicide that can
control most annual and perennial plants [21]. It controls weeds by inhibiting
the synthesis of aromatic amino acids necessary for protein formation in
susceptible plants. Glyphosate is strongly adsorbed to soil particles, which prevents
it from excessive leaching or from being taken up from the soil by nontarget
plants [22–26]. It is degraded primarily by microbial metabolism, but strong
adsorption to soil can inhibit microbial metabolism and slow degradation. The
half-life of glyphosate ranges from several weeks to years, but averages are two months. In water,
glyphosate is rapidly dissipated through adsorption to suspended and bottom
sediments and has a half-life of 10–12 days [22–24, 27–29].Although originally thought to be unaffected by sunlight [24], later studies found
glyphosate to be susceptible to photodegradation. A half-life of 4 days in deionised
water under UV light has been reported [30]. Recently, the photocatalytic
degradation of glyphosate in the presence of TiO2 has been reported
by Shifu and Yunzhang [31]. However, no effort has been made to look into the
degradation products. Therefore, we have studied the degradation of glyphosate
in aqueous suspension of TiO2 with an aim to identify the products
formed during the photooxidation process.


## 2. EXPERIMENTAL METHODS

### 2.1. Reagent and chemicals

Analytical grade glyphosate was obtained from
Fluka and used without further purification. The water employed in this study
was purified by a Millipore system. The photocatalyst titanium dioxide Degussa
P25 was used in all the experiments reported here. Degussa P25 contains 75%
anatase and 25% rutile with a specific BET surface area of 50 m2 g-1 and a primary particle size of 20 nm [32].


### 2.2. Procedure

A solution of glyphosate at the
desired concentration was prepared in water. For the irradiation experiments,
80 cm3 of this desired solution was pipetted into a photochemical
reactor. The pH of the reaction mixture was adjusted by adding a dilute aqueous
solution of HNO3 or NaOH. The photoreactor was comprised of a quartz
glass reaction vessel equipped with a magnetic stirring bar, a water circulating
jacket, and opening for gas supplies, placed within a bespoke
circular photoirradiation system (photochemical reactors LTD Buckingham, UK)
comprised of 12×15 watt Blacklight UVA lamps (λmax=312 nm), 42 cm
long.The required amount of the particulate photocatalyst was added to the glyphosate solution, and the
solution was stirred in the dark for 10 minutes to allow equilibration of the
system so that the loss of compound from the solution phase due to to-particle
adsorption can be taken into account. The zero time reading was obtained from
blank solution kept in the dark but otherwise treated similarly to the
irradiated solution. Photodegradation experiments were conducted by irradiating
the sample solutions with 312 nm light, the irradiated solutions being
continuously purged with air throughout each experiment. Samples (5 cm3)
were collected before and at regular intervals during the irradiation. These
were centrifuged before being subjected to analysis.


### 2.3. Sample analysis

Elucidation of the mechanism and kinetics of
photocatalytically driven glyphosate degradation requires a means to follow the
concentration of glyphosate as a function of time. Secondary amines such as
glyphosate and sarcosine are most easily determined by spectroscopic analysis
of their nitro derivative. All secondary alkyl or aryl amines yield
N-nitrosoamines with an absorbance at or aboutλ = 243 nm. Scheme 1 shows the route by which
secondary amines are transformed to the N-nitroso compounds.Scheme 1
Scheme showing the conversion of secondary amine to its N-nitroso derivative on treatment with
nitrous acid in the presence of acid.The procedure for both calibration measurements
and the analysis of experimental samples, whether derived from dark adsorption
experiments (vide infra) or photodegradation experiments, involves the
shaking of centrifuged glyphosate adsorbed or irradiated solutions (1 cm3 of expt. sample or 1 cm3 of 1×10−3 mol dm-3calibration solution) in the presence of water (5 cm3), H2SO4(1 : 1,
0.5 cm3), KBr (25 %, 0.1 cm3), and
NaNO2 (0.2 N, 0.5 cm3) in a 25 cm3 red
volumetric flask for 30 minutes followed by dilution with water to 25 cm3.
Absorbance was measured at 243 nm versus blank reagent containing all above
reagents except glyphosate.The product analysis was carried out using HPLC
(Perkin Elmer 410 Bio) fitted with a nitrile column (cyanopropyl 5μm). The
eluent consisted of pure water; the compounds were detected employing a
UV-detector at 254 nm.


## 2.1. Reagent and chemicals

Analytical grade glyphosate was obtained from
Fluka and used without further purification. The water employed in this study
was purified by a Millipore system. The photocatalyst titanium dioxide Degussa
P25 was used in all the experiments reported here. Degussa P25 contains 75%
anatase and 25% rutile with a specific BET surface area of 50 m2 g-1 and a primary particle size of 20 nm [32].


## 2.2. Procedure

A solution of glyphosate at the
desired concentration was prepared in water. For the irradiation experiments,
80 cm3 of this desired solution was pipetted into a photochemical
reactor. The pH of the reaction mixture was adjusted by adding a dilute aqueous
solution of HNO3 or NaOH. The photoreactor was comprised of a quartz
glass reaction vessel equipped with a magnetic stirring bar, a water circulating
jacket, and opening for gas supplies, placed within a bespoke
circular photoirradiation system (photochemical reactors LTD Buckingham, UK)
comprised of 12×15 watt Blacklight UVA lamps (λmax=312 nm), 42 cm
long.The required amount of the particulate photocatalyst was added to the glyphosate solution, and the
solution was stirred in the dark for 10 minutes to allow equilibration of the
system so that the loss of compound from the solution phase due to to-particle
adsorption can be taken into account. The zero time reading was obtained from
blank solution kept in the dark but otherwise treated similarly to the
irradiated solution. Photodegradation experiments were conducted by irradiating
the sample solutions with 312 nm light, the irradiated solutions being
continuously purged with air throughout each experiment. Samples (5 cm3)
were collected before and at regular intervals during the irradiation. These
were centrifuged before being subjected to analysis.


## 2.3. Sample analysis

Elucidation of the mechanism and kinetics of
photocatalytically driven glyphosate degradation requires a means to follow the
concentration of glyphosate as a function of time. Secondary amines such as
glyphosate and sarcosine are most easily determined by spectroscopic analysis
of their nitro derivative. All secondary alkyl or aryl amines yield
N-nitrosoamines with an absorbance at or aboutλ = 243 nm. Scheme 1 shows the route by which
secondary amines are transformed to the N-nitroso compounds.Scheme 1
Scheme showing the conversion of secondary amine to its N-nitroso derivative on treatment with
nitrous acid in the presence of acid.The procedure for both calibration measurements
and the analysis of experimental samples, whether derived from dark adsorption
experiments (vide infra) or photodegradation experiments, involves the
shaking of centrifuged glyphosate adsorbed or irradiated solutions (1 cm3 of expt. sample or 1 cm3 of 1×10−3 mol dm-3calibration solution) in the presence of water (5 cm3), H2SO4(1 : 1,
0.5 cm3), KBr (25 %, 0.1 cm3), and
NaNO2 (0.2 N, 0.5 cm3) in a 25 cm3 red
volumetric flask for 30 minutes followed by dilution with water to 25 cm3.
Absorbance was measured at 243 nm versus blank reagent containing all above
reagents except glyphosate.The product analysis was carried out using HPLC
(Perkin Elmer 410 Bio) fitted with a nitrile column (cyanopropyl 5μm). The
eluent consisted of pure water; the compounds were detected employing a
UV-detector at 254 nm.


## 3. RESULTS AND DISCUSSION

### 3.1. Dark adsorption of Glyphosate onto TiO2

The possible dark adsorption of glyphosate  on the surface
of the photocatalyst was investigated by stirring aqueous solutions of
glyphosate (1×10−3 mol dm-3, 35 cm3) at a range of pH in the absence of
illumination for 4 hours at different catalyst loadings (2–10 g dm3). Analysis of samples after centrifugation
using the N-nitroso method (vide supra) showed that greatest adsorption occurs
at lower pH values and that the extent of adsorption decreases as pH increases,
as can be seen in Figure 1.Figure 1
Glyphosate adsorption on the surface of
Degussa P25 TiO2 photocatalyst in the dark at different pH, assessed
by measuring the concentration of free solution glyphosate as a function of solution loading of the catalyst.This behaviour could be attributed to glyphosate containing three functional groups—phosphate, amino,
and carboxylic—all of which can be protonated and deprotonated depending on individual functional group pKa
values.The point of zero charge (pzc) of TiO2 (Degussa P25) is widely reported as pH ~6.25
[33]. With this in mind, Table 1
shows the ionic structure of glyphosate indicating the net charge on the
molecule and TiO2 surface as different pH values.Table 1
Table showing different forms of
glyphosate and the net charge on glyphosate and the surface of TiO2 as a function of pH.At low pH values (~pH 3), the
TiO2 surface will be positively charged, while the phosphate group
of glyphosate will be negatively charged leading to the expectation that the
compound will adsorb the surface of TiO2. In contrast, at higher pH
values, the catalyst surface as well as the compound will be negatively charged,
and hence adsorption in dark would not be expected, as evident from the
experimental results shown in Figure 1. The Langmuir constants at pH 3
(glyphosate uncharged, where phosphate is −1 and the TiO2 surface has a net positive charge) and pH 5 (when glyphosate is −1 (phosphate and
carboxlate are both −1) and TiO2 is slightly
positive/near neutral) have been calculated as being 88610 and 1087 dm3 mol-1, respectively. At pH 7 (when glyphosate is −2 (phosphate −2,
carboxylate −1) and the TiO2 surface has a net negative charge), there is little evidence of any adsorption.
Results at pH 9 and pH 11 are similar to those at pH 7. Therefore, the Langmuir adsorption pattern is
congruent with that expected from columbic arguments. The magnitude of the
adsorption coefficient at pH 3 compared with that at pH 5 suggests that the
primary locus of adsorption is through phosphate group.


### 3.2. Photocatalytic destruction of Glyphosate on TiO2-loss of Glyphosate

Irradiation of an aqueous suspension of glyphosate (80 cm3,
1×10−3 mol dm3) in the presence of TiO2 (Degussa P25, 1 g dm3) using 312 nm light in a
tubular photochemical reactor with constant stirring and bubbling of air led to
a decrease in glyphosate concentration as a function of time. The
photodegradation was investigated at pH values 3, 5.7, 7, 9, and 11. The
decrease in secondary amine concentration (as determined by the N-nitroso
method, vide supra) as a function of irradiation time at different pH values is
shown in Figure 2. The net loss and rate of loss of secondary amine was found
to increase with the increase in pH, the highest rate being observed at pH 11.
Further, for data recorded at pH 3 and pH 5.7, there appears to be an induction period during which no loss of
secondary amine is observed for a time immediately after the onset of
illumination. We will return to this point below.Figure 2
Photocatalytically induced loss of secondary amine functionality (measured using the N-nitroso method, see text)
in glyphosate as a function of pH.


### 3.3. Photocatalytic destruction of Glyphosate on TiO2-product analysis

Figure3 shows the HPLC trace of an
aqueous suspension of glyphosate and TiO2 at zero time irradiation
indicating a strong peak at retention time Rt= 1.184 minutes and a
medium peak at Rt=0.941  minute. Irradiation of the solution for 90
minutes at pH 2 led to the formation of a product which is indicated by
appearance of a strong peak at Rt= 1.651 minutes and a shoulder at
Rt= 1.599 minutes in addition to the unchanged starting material
(see Table 2). Said product exhibited an HPLC retention time and peak shape
typical of a carboxylic acid (determined through comparison with HPLC analysis
of methanoic and ethanoic acids
on same instrument, not shown). However, during this period, no concomitant
loss of secondary amine was observed leading us to conclude that the C–P bond had
been broken in the formation of this product and that the product itself was
most likely the amino acid sarcosine-N-methyl-2-aminoethanoic
acid. The HPLC trace of authentic sarcosine is shown in Figure 4.Table 2
Table showing HPLC peaks retention
times,Rt, for glyphosate, sarcosine, glycine (sample rows 1–3, resp.),
irradiated mixtures of glyphosate at pH 3 and pH 11 (sample rows 4 and 6,
resp.), and irradiated mixtures of glyphosate at pH 3 and pH 11 with authentic
samples of sarcosine and glycine (sample rows 5 and 7, resp.). Peaks
associated with glyphosate, sarcosine, and glycine are shown as bold,
underlined, and double underlined
text, respectively.
SampleHPLC peak retention
time (Rt)/minutes1. Glyphosate (from Figure 3)0.941 (min), 1.184 (s)2. Sarcosine (from Figure 4)1.105 (w), 1.531 (s)3. Glycine (from Figure 5)0.834 (w),1.558 (s)4. Glyphosate, 90 min Irradiation, pH30.948 (min), 1.061 (min), 1.13 (s), 1.599 (min), 1.651 (s)5. Glyphosate (90 min irradiation, pH3) + authentic Sarcocine0.948 (min), 1.039 (min), 1.117 (s), 1.523 (min), 1.651 (s),6. Glyphosate (60 min irradiation, pH
11)1.161 (s), 1.548 (min), 1.662 (min)7. Glyphosate (60 min irradiation, pH 11) + authentic Glycine1.108 (s), 1.543 (min), 1.665 (min)s-strong, m-medium, w-weak.Figure 3
HPLC analysis of glyphosate in aqueous suspension of TiO2 in
the dark (zero irradiation time).Figure 4
HPLC analysis of authentic sarcosine in aqueous solution.Primary product assignment was confirmed
by adding authentic sarcosine to the photodegraded reaction mixture. Peaks in
the HPLC trace recorded from this degraded sample/authentic sarcosine mix were
found to be coincident with peaks in the HPLC trace recorded from the degraded
reaction mixture alone (compare sample row 5 with sample row 4 in Table2).At longer times, both the concentration of secondary amine as
determined by the N-nitroso method and the area of the peak atRt=
1.523 minutes, associated with the secondary amine sarcosine, were seen to decrease with
illumination time. The latter was accompanied by the concomitant formation of a
second product peak at Rt= 1.56 minutes, in the vicinity of but not
coincident with the sarcosine peak in the HPLC trace. Again, by addition of
authentic material to photodegraded reaction mixtures, this second product was
identified as glycine—2-aminoethanoic
acid. The HPLC trace of authentic glycine is shown in Figure 5.Figure 5
HPLC analysis of authentic glycine in aqueous solution.HPLC analysis of photodegraded reaction mixtures at pH 11 also indicates the formation
of product (Rt= 1.548 and 1.662 minutes) in the vicinity of but not
coincident with the sarcosine peak in the HPLC trace. No sarcosine peak was
observed at any irradiation time at this pH. Following the methodology employed
at pH 3, HPLC analyses of the photodegraded reaction mixture and samples of
authentic materials shows that the primary reaction
product is glycine (compare sample row 7 with sample row 6 in Table 2 and note
coincidence of peaks at Rt= 1.548 minutes). This is most likely
derived from C–N bond cleavage in nonadsorbed glyphosate (Figure 6).Figure 6
The interaction of glyphosate with the
surface of TiO2 in dark and the subsequent interaction of
photogenerated electrons and hydroxyl radicals with glyphosate upon
illumination of the TiO2 with
ultraband gap irradiation.The N-nitroso-based secondary amine
analysis of Figure2 indicates that at low pH, secondary amine is retained but
glyphosate is lost due to loss of phosphate group and appearance of a peak in
the carboxylic acid region of the HPLC trace. In conjunction with the product
identification and the fact that adsorption occurs at low pH, the results
suggest that at low pH the photocatalytic reaction is most likely initiated by
from-particle electron transfer. In contrast, under alkaline pH where little
adsorption is seen to occur (Figure 1) solution-phase hydroxyl radical attack
plays an important role, said hydroxide radicals having been generated in the
reaction medium by photogenerated holes.Figure6 provides a schematic summary of
these processes, wherein it is assumed that, at low pH, the photogenerated
electron on irradiated TiO2 attacks through
-phosphate-adsorbed-glyphosate leading to the formation of carbon centered radical
which then reacts to form nonadsorbed sarcosine. Readsorption of sarcosine onto
the positively charged TiO2 surface is then inhibited by the
anticipated net positive charge on the molecule, arising from the protonation
of the secondary amino nitrogen at this pH. At high pH, photoelectrochemically
generated hydroxyl radicals attack
nonadsorbed glyphosate, causing C–N bond fracture and ultimately forming
glycine. It is pertinent to mention here that photocatalysed degradation of
glycine has a well-known mechanism and has been reported earlier [34].
Returning to pH 3, it can now be seen that sarcosine which is not adsorbed at
the TiO2 surface due to the loss of the phosphate group is then
susceptible to similar solution-phase OH radical attack as nonadsorbed
glyphosate at pH 11, so generating the same product, glycine. A mechanism
consistent with these observations is shown in Scheme 2.Scheme 2
Scheme showing the probable route for the degradation of glyphosate on irradiated
TiO2 at
low and high pH.


## 3.1. Dark adsorption of Glyphosate onto TiO2

The possible dark adsorption of glyphosate  on the surface
of the photocatalyst was investigated by stirring aqueous solutions of
glyphosate (1×10−3 mol dm-3, 35 cm3) at a range of pH in the absence of
illumination for 4 hours at different catalyst loadings (2–10 g dm3). Analysis of samples after centrifugation
using the N-nitroso method (vide supra) showed that greatest adsorption occurs
at lower pH values and that the extent of adsorption decreases as pH increases,
as can be seen in Figure 1.Figure 1
Glyphosate adsorption on the surface of
Degussa P25 TiO2 photocatalyst in the dark at different pH, assessed
by measuring the concentration of free solution glyphosate as a function of solution loading of the catalyst.This behaviour could be attributed to glyphosate containing three functional groups—phosphate, amino,
and carboxylic—all of which can be protonated and deprotonated depending on individual functional group pKa
values.The point of zero charge (pzc) of TiO2 (Degussa P25) is widely reported as pH ~6.25
[33]. With this in mind, Table 1
shows the ionic structure of glyphosate indicating the net charge on the
molecule and TiO2 surface as different pH values.Table 1
Table showing different forms of
glyphosate and the net charge on glyphosate and the surface of TiO2 as a function of pH.At low pH values (~pH 3), the
TiO2 surface will be positively charged, while the phosphate group
of glyphosate will be negatively charged leading to the expectation that the
compound will adsorb the surface of TiO2. In contrast, at higher pH
values, the catalyst surface as well as the compound will be negatively charged,
and hence adsorption in dark would not be expected, as evident from the
experimental results shown in Figure 1. The Langmuir constants at pH 3
(glyphosate uncharged, where phosphate is −1 and the TiO2 surface has a net positive charge) and pH 5 (when glyphosate is −1 (phosphate and
carboxlate are both −1) and TiO2 is slightly
positive/near neutral) have been calculated as being 88610 and 1087 dm3 mol-1, respectively. At pH 7 (when glyphosate is −2 (phosphate −2,
carboxylate −1) and the TiO2 surface has a net negative charge), there is little evidence of any adsorption.
Results at pH 9 and pH 11 are similar to those at pH 7. Therefore, the Langmuir adsorption pattern is
congruent with that expected from columbic arguments. The magnitude of the
adsorption coefficient at pH 3 compared with that at pH 5 suggests that the
primary locus of adsorption is through phosphate group.


## 3.2. Photocatalytic destruction of Glyphosate on TiO2-loss of Glyphosate

Irradiation of an aqueous suspension of glyphosate (80 cm3,
1×10−3 mol dm3) in the presence of TiO2 (Degussa P25, 1 g dm3) using 312 nm light in a
tubular photochemical reactor with constant stirring and bubbling of air led to
a decrease in glyphosate concentration as a function of time. The
photodegradation was investigated at pH values 3, 5.7, 7, 9, and 11. The
decrease in secondary amine concentration (as determined by the N-nitroso
method, vide supra) as a function of irradiation time at different pH values is
shown in Figure 2. The net loss and rate of loss of secondary amine was found
to increase with the increase in pH, the highest rate being observed at pH 11.
Further, for data recorded at pH 3 and pH 5.7, there appears to be an induction period during which no loss of
secondary amine is observed for a time immediately after the onset of
illumination. We will return to this point below.Figure 2
Photocatalytically induced loss of secondary amine functionality (measured using the N-nitroso method, see text)
in glyphosate as a function of pH.


## 3.3. Photocatalytic destruction of Glyphosate on TiO2-product analysis

Figure3 shows the HPLC trace of an
aqueous suspension of glyphosate and TiO2 at zero time irradiation
indicating a strong peak at retention time Rt= 1.184 minutes and a
medium peak at Rt=0.941  minute. Irradiation of the solution for 90
minutes at pH 2 led to the formation of a product which is indicated by
appearance of a strong peak at Rt= 1.651 minutes and a shoulder at
Rt= 1.599 minutes in addition to the unchanged starting material
(see Table 2). Said product exhibited an HPLC retention time and peak shape
typical of a carboxylic acid (determined through comparison with HPLC analysis
of methanoic and ethanoic acids
on same instrument, not shown). However, during this period, no concomitant
loss of secondary amine was observed leading us to conclude that the C–P bond had
been broken in the formation of this product and that the product itself was
most likely the amino acid sarcosine-N-methyl-2-aminoethanoic
acid. The HPLC trace of authentic sarcosine is shown in Figure 4.Table 2
Table showing HPLC peaks retention
times,Rt, for glyphosate, sarcosine, glycine (sample rows 1–3, resp.),
irradiated mixtures of glyphosate at pH 3 and pH 11 (sample rows 4 and 6,
resp.), and irradiated mixtures of glyphosate at pH 3 and pH 11 with authentic
samples of sarcosine and glycine (sample rows 5 and 7, resp.). Peaks
associated with glyphosate, sarcosine, and glycine are shown as bold,
underlined, and double underlined
text, respectively.
SampleHPLC peak retention
time (Rt)/minutes1. Glyphosate (from Figure 3)0.941 (min), 1.184 (s)2. Sarcosine (from Figure 4)1.105 (w), 1.531 (s)3. Glycine (from Figure 5)0.834 (w),1.558 (s)4. Glyphosate, 90 min Irradiation, pH30.948 (min), 1.061 (min), 1.13 (s), 1.599 (min), 1.651 (s)5. Glyphosate (90 min irradiation, pH3) + authentic Sarcocine0.948 (min), 1.039 (min), 1.117 (s), 1.523 (min), 1.651 (s),6. Glyphosate (60 min irradiation, pH
11)1.161 (s), 1.548 (min), 1.662 (min)7. Glyphosate (60 min irradiation, pH 11) + authentic Glycine1.108 (s), 1.543 (min), 1.665 (min)s-strong, m-medium, w-weak.Figure 3
HPLC analysis of glyphosate in aqueous suspension of TiO2 in
the dark (zero irradiation time).Figure 4
HPLC analysis of authentic sarcosine in aqueous solution.Primary product assignment was confirmed
by adding authentic sarcosine to the photodegraded reaction mixture. Peaks in
the HPLC trace recorded from this degraded sample/authentic sarcosine mix were
found to be coincident with peaks in the HPLC trace recorded from the degraded
reaction mixture alone (compare sample row 5 with sample row 4 in Table2).At longer times, both the concentration of secondary amine as
determined by the N-nitroso method and the area of the peak atRt=
1.523 minutes, associated with the secondary amine sarcosine, were seen to decrease with
illumination time. The latter was accompanied by the concomitant formation of a
second product peak at Rt= 1.56 minutes, in the vicinity of but not
coincident with the sarcosine peak in the HPLC trace. Again, by addition of
authentic material to photodegraded reaction mixtures, this second product was
identified as glycine—2-aminoethanoic
acid. The HPLC trace of authentic glycine is shown in Figure 5.Figure 5
HPLC analysis of authentic glycine in aqueous solution.HPLC analysis of photodegraded reaction mixtures at pH 11 also indicates the formation
of product (Rt= 1.548 and 1.662 minutes) in the vicinity of but not
coincident with the sarcosine peak in the HPLC trace. No sarcosine peak was
observed at any irradiation time at this pH. Following the methodology employed
at pH 3, HPLC analyses of the photodegraded reaction mixture and samples of
authentic materials shows that the primary reaction
product is glycine (compare sample row 7 with sample row 6 in Table 2 and note
coincidence of peaks at Rt= 1.548 minutes). This is most likely
derived from C–N bond cleavage in nonadsorbed glyphosate (Figure 6).Figure 6
The interaction of glyphosate with the
surface of TiO2 in dark and the subsequent interaction of
photogenerated electrons and hydroxyl radicals with glyphosate upon
illumination of the TiO2 with
ultraband gap irradiation.The N-nitroso-based secondary amine
analysis of Figure2 indicates that at low pH, secondary amine is retained but
glyphosate is lost due to loss of phosphate group and appearance of a peak in
the carboxylic acid region of the HPLC trace. In conjunction with the product
identification and the fact that adsorption occurs at low pH, the results
suggest that at low pH the photocatalytic reaction is most likely initiated by
from-particle electron transfer. In contrast, under alkaline pH where little
adsorption is seen to occur (Figure 1) solution-phase hydroxyl radical attack
plays an important role, said hydroxide radicals having been generated in the
reaction medium by photogenerated holes.Figure6 provides a schematic summary of
these processes, wherein it is assumed that, at low pH, the photogenerated
electron on irradiated TiO2 attacks through
-phosphate-adsorbed-glyphosate leading to the formation of carbon centered radical
which then reacts to form nonadsorbed sarcosine. Readsorption of sarcosine onto
the positively charged TiO2 surface is then inhibited by the
anticipated net positive charge on the molecule, arising from the protonation
of the secondary amino nitrogen at this pH. At high pH, photoelectrochemically
generated hydroxyl radicals attack
nonadsorbed glyphosate, causing C–N bond fracture and ultimately forming
glycine. It is pertinent to mention here that photocatalysed degradation of
glycine has a well-known mechanism and has been reported earlier [34].
Returning to pH 3, it can now be seen that sarcosine which is not adsorbed at
the TiO2 surface due to the loss of the phosphate group is then
susceptible to similar solution-phase OH radical attack as nonadsorbed
glyphosate at pH 11, so generating the same product, glycine. A mechanism
consistent with these observations is shown in Scheme 2.Scheme 2
Scheme showing the probable route for the degradation of glyphosate on irradiated
TiO2 at
low and high pH.


## 4. CONCLUSIONS

The pesticide derivative glyphosate has
been found to degrade efficiently under alkaline conditions. Under acid
conditions, it exhibits a strong, primarily coulombically driven dark
adsorption onto the surface of the TiO2 photocatalyst. Again, under
acid conditions, the compound has been found to undergo efficiently photocatalytically promoted P–C
bond cleavage leading to the formation of sarcosine and glycine like the main
intermediate products. A probable degradation route involving a direct
interfacial electron transfer reaction at low pH and hydroxyl radical
/superoxide radical anion-mediated solution-phase oxidation at high pH on
irradiated semiconductor particles has been proposed.



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*Source: 197346-2008-06-15.xml*