Fluorescence Spectroscopy and Chemometrics: A Simple and Easy Way for the Monitoring of Fluoroquinolone Mixture Degradation

Abstract

In this work, fluorescence excitation–emission matrices (EEMs), in combination with the chemometric tool and parallel factor analysis (PARAFAC), have been proposed as an unexplored methodology to follow the removal of the fluorescent contaminants of emerging concern, fluoroquinolones (FQs). Ofloxacin, enrofloxacin, and sarafloxacin were degraded by different advanced oxidation processes employing simulated sunlight (hν): photolysis, H2O2/hν, and photo-Fenton. All experiments were performed in ultrapure water at three different pH values: 2.8, 5.0, and 7.0. With the obvious advantage of multivariate analysis methods, EEM-PARAFAC allowed the monitoring of degradation from the overall substances (original and formed ones) through simultaneous, rapid, and cost-efficient fluorescence spectroscopy determinations. A five-component model was found to best fit the experimental data, allowing us to (i) describe the decay of the fluorescence signals of the three parent pollutants, (ii) follow the kinetics profile of FQ-like byproducts with similar EEM fingerprints than the original FQs, and (iii) observe the formation of two families of reaction intermediates with completely different EEMs. Results were finally correlated with high pressure liquid chromatography, total organic carbon, and toxicity tests on Escherichia coli, showing good agreement with all the studied techniques.

Emerging Photochemical Processes Involving Iron for Wastewater Treatment

“…According to the results above reported, three interesting tasks should be addressed in the next future for a better understanding of the process: a) investigate the nature of the interaction between Fe and the FQs to explain the good performance of photo-Fenton at mild pH, b) to combine EEM-PARAFAC with smart HPLC-MS analysis to correlate the components obtained in the mathematical model with a compound or group of compounds, c) to explore the real applicability of the methodology here developed with lower pollutant concentrations (e.g μg/L), using different aqueous matrices (role of salinity and dissolved organic matter) and scaling it up to higher volumes using real sunlight as irradiation source. As an extension from previous works [119,137], we have followed with EEM-PARAFAC the degradation of a mixture of 5 quinolone-antibiotics: OFL, CIP, ENR, OA and FLU, in different water matrices: MQ, SW and simulated wastewater (SWW). Solar-photo-Fenton at pilot plant scale was used as water treatment.…”

“…Therefore, inner filter effect corrections or the separation/elimination of, for instance, metals, as well as the use of buffer solutions to analyse the whole dataset at fixed pH, should always be evaluated. This is even more important when dealing with AOPs, such as Fenton processes, where abrupt pH changes occur and there is a significant amount of iron that can be chelated by the fluorophore(s) [118,119]. Further EEM-PARAFAC data processing details are thoroughly described in section 2.4.2 from the Experimental chapter.…”

“…EEM-PARAFAC is still in constant improvement, with its application with real-time continuous monitoring [133], machine learning [134], or even for laboratory-scale fluorescence of CECs simultaneous degradation analysis employing AOPs: from heavy polycyclic aromatic hydrocarbons [135], polyphenols [136] to FQs antibiotics [119,137], the latter shown in the Results and Discussion chapter. Therefore, besides from the simultaneous monitoring of the several parent pollutants that can be obtained in a simple, economical and fast way, it can also be elucidated the plausible molecular changes when observing new deconvoluted components corresponding to oxidation by-products.…”

“…This section shows the information from the publication referenced as [119] FQs are fluorescent molecules, thus being an advantage for their determination at low concentrations in complex samples [210]. However, without a separation method such as HPLC, when measuring a single emission fluorescence scan from a solution containing a mixture of these compounds, signals will likely overlap.…”

“…Since (F)Qs fluorescence can be quenched by Fe 3+ [119], comparison of EEM-PARAFAC with and without iron removal was performed. For those samples without iron, they were previously adjusted to pH 11 with NaOH 1 M and filtered with PTFE 0.45 μm filters.…”