Degradation of a Model Naphthenic Acid
J. Phys. Chem. A, Vol. 114, No. 45, 2010 12069
TABLE 1: Chemical Formulae and Proposed Structure(s)
of Major By-Products Formed by VUV and UV/H2O2 at m/z
141 and 159
2.3. UV Irradiation in Presence of H2O2. The UV/H2O2
experiments were carried out using a collimated beam
apparatus from Calgon Carbon Corporation (Pittsburgh, PA,
USA). A 10-W low pressure UV lamp (Calgon Carbon,
Pittsburgh, PA, USA) was used for the UV/H2O2 experiments.
The main light emission of the low pressure lamp is at 254
nm. A radiometer (International Light Inc. Model IL 1400A)
equipped with a UV detector (International Light Inc. Model
SED240) and a neutral density filter (Model QNDS2),
calibrated at 5 nm intervals in the range of 200-400 nm,
was used to measure the irradiance at the surface of the water
samples. The average fluence rate in the solution and the
delivered fluence (UV dose) were calculated based on the
Bolton and Linden protocol26 and the spreadsheets available
cm) solutions. The fluence rate of the lamp was calculated
to be 13.69 mW/cm2 and the yield of •OH generation
8.2 × 10-6 mol s-1 (Φ ) 1, Petri dish area ) 28.26 cm2,
radiant energy of 1 mol of 254 nm photon ) 470.971 kJ
mol-1).
2.4. Analytical Methods. The analytical method was de-
signed for separation of CHA and two hypothesized degradation
products, cis-4- and trans-4-hydroxycyclohexanoic acid. Ana-
lytical high-performance liquid chromatography (HPLC)
columns (Luna C8, 5 µm, 150 × 3 mm, and 250 × 3 mm;
Phenomenex) were used for reversed-phase chromatography.
For routine analysis, an HPLC coupled to an ion trap mass
spectrometer (Varian 500-MS) with unit mass resolution was
used. For confirmation of empirical formulas, an HPLC coupled
with a time-of-flight mass spectrometer (Agilent 6220, 10,000
mass resolution) was used. Both mass spectrometers were
equipped with an electrospray interface operating in negative
ion mode. Chromatography was performed at 40 °C at a flow
rate 200 µL/min, and injection volumes were 20 µL. The mobile
phase consisted of 100% methanol and 4 mM ammonium
acetate with 0.1% acetic acid in aqueous solution. The concen-
tration of methanol was ramped from 40 to 80% over 20 min.
Chromatograms of carboxylic acid standards showed that
retention times were generally in order of decreasing polarity.
To achieve better resolution of more polar byproduct and their
isomers, a longer column (250 mm, otherwise identical to above)
was also applied. The same mobile phases were used, but the
initial concentration of methanol was 30%, and after 10 min it
was slowly ramped to 70% over 30 min. Heptanoic and octanoic
acid both eluted after 80 min; thus 80 min was the total run
time.
Ion chromatography with conductivity detection was used
to detect any short-chain carboxylic acids. Chromatography
was performed at 40 °C and injection volumes were 200 µL.
The mobile phase consisted sodium hydroxide solution at
flow rate 1 mL/min. The concentration of NaOH was ramped
from 2.5 to 5 mM over 15 min and then from 5 mM to 38.25
mM. The electrochemical suppressor current was 100 mA.
The AS11-HC (4 mm ×250 mm) with a guard column AG-
11 was used (Dionex, Sunnyvale, CA, USA). To remove
carbonate residue from the eluent, an anion trap column
(ATC-HC, Dionex, Sunnyvale, CA, USA) was used.
of concentrations used here, the kinetics of degradation were
examined. Direct photolysis follows zero-order kinetics,
whereas degradation processes based on generation of
hydroxyl radical should follow pseudofirst order kinetics.11,14,24
It was determined that at up to 20 mg/L of CHA (highest
concentration used in the experiments) in aerated aqueous
solutions, the degradation followed pseudo-first-order kinetic
decay. Under these conditions, the hydroxyl radicals are
present in solution in excess, and the process is independent
•
of the rate of OH generation from photolysis of water.
Furthermore, we observed that the addition of 80 µM tert-
butanol, a hydroxyl radical scavenger, significantly decreased
the effectiveness of CHA decomposition (40 µM). Taken
together, these results indicated that decomposition of CHA
•
in the investigated range was induced primarily by OH.
At an initial concentration of 20 mg/L CHA, 90%
degradation was achieved after 15 total minutes of irradiation.
In a study of the reaction products, HPLC high resolution
mass spectrometry indicated two major byproduct at m/z )
159.0737 and 141.06287 (the calculated m/z of CHA is
128.1690) (Table 1). These same products were also found
using the ion trap in autoscanning MS/MS mode, and we
examined the product ions of m/z ) 159.0737 and 141.06287
for structural elucidation, but the m/z 159 signal intensity
was too low to collect MS/MS spectra. However, retention
times of the m/z 159 degradation products identified here were
the same as those observed during UV/H2O2; thus MS/MS
spectra of m/z 159 were collected on the UV/H2O2 samples
(discussed in the next section).
The ion at m/z ) 141.06287 is characteristic of an oxo-
cyclohexanoic acid (oxo-CHA, Table 1), and in MS/MS the
product ion at m/z ) 97, representing a neutral loss of 44,
suggested the presence of a carboxylate group (Figure 1). The
formation of such a product suggested that after hydrogen
abstraction by hydroxyl radical (reaction 13) the resulting radical
reacted with oxygen to form peroxyl-CHA radical (reaction 14)
which may subsequently decay by recombination reactions
(reactions 15-17).
3. Decomposition of CHA by VUV
The study of hydroxyl radical decomposition of organic
compounds using VUV must be conducted at relatively low
concentration. This is because, at high concentration, direct
photolysis of organic compounds can interfere.6 To ensure
that direct photolysis of CHA was not occurring in the range