Spectroelectrochemistry is a multi-response technique that provides both electrochemical and spectroscopic information about a chemical system in a single experiment, i.e., it offers information from two different points of view. Spectroelectrochemistry focused on the UV/VIS region is one of the most important combinations because this allows us to obtain not only valuable qualitative information, but also outstanding quantitative results. In this application note, the degradation kinetics for 4-nitrophenol, a known pollutant, were determined using SPELEC.

In-situ spectroelectrochemistry provides both electrochemical and real-time spectroscopic information while a redox reaction takes place. In contrast, ex-situ methods usually require one or more samples and external instruments for data analysis in order to better understand the different processes occurring on the electrode surface.

Although 4-nitrophenol (4-NP) is included in the U.S. Environmental Protection Agency List of Priority Pollutants [1,2], this compound (which is hazardous for humans, plants, and animals) has been widely used in the fabrication of drugs, leather, or pesticides [3], in agriculture, as well as in domestic activities [4]. Therefore, the study of its degradation is currently an important issue.

In this work, the combination of electrochemistry and UV/VIS spectroscopy in a single experiment has allowed monitoring the degradation of 4-NP from an aqueous solution. Furthermore, spectroelectrochemistry displays the quantitative information needed to calculate the efficiency of the degradation process.

Methods

Electrochemical degradation of 4-nitrophenol is based on its irreversible reduction in Na₂SO₄ aqueous solution. UV/VIS spectra were simultaneously recorded along with the electrochemical signal, obtaining additional information of the electrode surface during the whole experiment.

Spectroelectrochemical behavior of 4-NP was studied by linear sweep voltammetry (Fig. 2a), scanning the potential from -0.30 V to -1.00 V at a rate of 0.01 V s^-1 in a solution composed of 1 × 10^-4 M 4-NP and 0.5 M Na₂SO₄. UV/VIS spectra were simultaneously recorded to the electrochemical experiment and the spectrum of the initial solution (4-NP) was taken as reference for the UV/VIS absorption spectra. As can be observed in Fig. 2b, UV/VIS spectra show two absorption bands, centered at 320 and 400 nm, related to the consumption of 4-NP and the generation of degradation products, respectively.

As a proof of concept, the electrochemical degradation of 4-NP was performed by chronoamperometry, applying -1.00 V over a period of 150 s (blue line in Fig. 3a) in a mixed solution of 2 × 10^-5 M 4-NP and 0.5 M Na₂SO₄. Although 750 UV/VIS spectra were simultaneously recorded to the chronoamperometry, only a few spectra from the experiment are represented in Fig. 3b.

Time-resolved spectroelectrochemistry utilizes short integration times and  therefore it provides a large number of spectra during an electrochemical experiment. It allows us to represent the evolution of the absorption band at 400 nm with time, and as can be seen in Fig. 3a (red line), the absorbance at 400 nm increases during the chronoamperometry.

However, spectroelectrochemistry not only provides qualitative information but also quantitative information that can be used to calculate the efficiency of the degradation process. The theoretical maximum absorbance value is calculated according to the Lambert-Beer’s law:

where ε is the molar absorption coefficient, b is the optical pathway length, and C is the concentration (in moles per liter, M). The experimental maximum absorbance value is extracted from the UV/VIS spectra and the efficiency (r) is calculated as:

According to the Lambert-Beer’s law (eq. 1) with ε = 17357 L mol^-1 cm^-1 at 400 nm [5], b = 0.36 cm, and C = 2 × 10^-5 M, the A_theoretical value was 0.125 a.u. On the other hand, the experimental maximum value of absorbance obtained is 0.095 a.u., so the efficiency of this simple electrochemical degradation process (eq.2) was r = 76.0%.

The efficiency of this procedure was evaluated with respect to the time while the potential was applied:

1. US Environmental Protection Agency, US Environmental Protection Agency, Federal Register, 1979, 44, 233, Fed. Regist. 44 (1979) 23.
2. US Environmental Protection Agency, US Environmental Protection Agency, Fed. Regist. 1989, 52, 131, Fed. Regist. 52 (1989) 131.
3. D. Chaara, I. Pavlovic, F. Bruna, M.A. Ulibarri, K. Draoui, C. Barriga, Removal of nitrophenol pesticides from aqueous solutions by layered double hydroxides and their calcined products., Appl. Clay Sci. 50 (2010) 292–298.
4. S. Laha, K.P. Petrova, Biodegradation of 4- nitrophenol by indigenous microbial populations in Everglades soils, Biodegradation. 8 (1998) 349–356.
5. D. Ibañez, E. Gomez, E. Valles, A. Colina, A. Heras, Spectroelectrochemical monitoring of contaminants during the electrochemical filtration process using free-standing carbon nanotube filters, Electrochim. Acta. 280 (2018) 17–24.