How Raman Spectroscopy Method Can Improve PAH Detection in Oily Sludge

The petroleum industry is essential to the global economy. Apart from the amount of jobs the industry creates for working class people, the petroleum industry is an important sector in energy production that help power society. As a result of this industry’s importance, numerous studies have explored how spectroscopy can help analyze mixed-crude oil to help improve operational efficiency and environmental responsibility (1).

A recent study from Northwest University in Xi’an, China, explored this topic further. In this study, Hua Li and Hongsheng Tang’s team of researchers introduced a new portable Raman spectroscopy method for quantifying polycyclic aromatic hydrocarbons (PAHs) in oily sludge. Published in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, the research addresses a critical environmental challenge posed by petroleum and petrochemical waste (2).

Close-up of an old, rusty pipeline spilled oil over the ground, reflecting light and creating a messy, environmentally concerning scene. | Image Credit: © VK Studio - stock.adobe.com.

Oily sludge is a byproduct of petroleum production, and it can have deleterious effects on the environment. PAHs, such as phenanthrene (Phe), fluoranthene (Flt), fluorene (Flu), and naphthalene (Nap) are some of the most significant pollutants found in oily sludge (2). Accurately measuring these compounds is vital for assessing environmental impact and ensuring compliance with environmental regulations. To measure the PAH compounds, the research team developed a novel calibration model that uses Raman spectroscopy, hybrid spectral preprocessing, and variable selection strategies to enhance the precision of PAH analysis (2).

As part of the experimental procedure, the research team extracted PAHs from oily sludge using solid–liquid extraction (SLE) with methanol. Raman spectra were collected from 21 samples using a portable Raman spectrometer, a critical step that demonstrates the feasibility of using compact, field-deployable instruments for environmental monitoring (2).

Chemometrics was also used by the team to improve prediction accuracy. First, hybrid spectral preprocessing methods, including first derivative (D1st) and wavelet transform (WT), were applied to the Raman spectral data (2). These preprocessing steps are essential for minimizing noise and highlighting relevant features within the spectral data (2).

Next, the study implemented backward interval partial least squares (biPLS), a variable selection method, to optimize the calibration model. By selecting the most relevant spectral variables and combining them with preprocessing strategies, the team improved the model's ability to predict PAH concentrations accurately (2).

The calibration models, which were optimized for the analysis, showed how all of them were fairly accurate. For Flt, the biPLS-D1st-WT-PLS model achieved a coefficient of determination prediction (R²P) of 0.9987 and a mean relative error of prediction (MREP) of 0.0606 (2). Meanwhile, the WT-biPLS-PLS model delivered good predictions for Phe, Flu, and Nap, with R²P values of 0.9995, 0.9996, and 0.9983, and MREP values of 0.0426, 0.0719, and 0.0497, respectively (2).

This method of first selecting characteristic variables using biPLS, followed by hybrid spectral preprocessing, represents a novel approach in quantitative PAH analysis (2). The hybrid strategy effectively removes redundant variables while enhancing the robustness of the calibration models (2).

The accurate detection and quantification of PAHs in oily sludge are essential for environmental protection and petroleum geochemistry. Traditional analytical methods often require time-consuming sample preparation and expensive laboratory equipment (2,3). In contrast, the portable Raman spectroscopy approach is rapid, cost-effective, and field-compatible, making it an attractive option for industrial applications and regulatory compliance (2).

The findings from Northwest University pave the way for further research into portable spectroscopy tools for environmental and industrial challenges. By refining preprocessing and variable selection methods, researchers can extend the applicability of this technique to other complex matrices, such as contaminated soil or water (2).

The study underscores the importance of interdisciplinary approaches that combine analytical chemistry, environmental science, and advanced data analysis techniques. As the demand for sustainable industrial practices grows, spectroscopy will continue to play a role in helping to build new methods to reduce the environmental footprints and ensure ecological balance (2).

References

1. Qi, H.; Li, H.; Wang, Q.; Li, H.; Zhang, X. Optical Constants of Mixed Crude Oil in Visible Waveband Based on the Double-Thickness Transmittance Method. Spectroscopy 2024, ASAP. https://www.spectroscopyonline.com/view/optical-constants-of-mixed-crude-oil-in-visible-waveband-based-on-the-double-thickness-transmittance-method
2. Ma, C.; Zhai, L.; Ding, J.; et al. Raman Spectroscopy Combined with Partial Least Squares (PLS) based on Hybrid Spectral Preprocessing and Backward Interval PLS (biPLS) for Quantitative Analysis of Four PAHs in Oil Sludge. Spectrochimica Acta Part A: Mol. Biomol. Spectrosc. 2024, 310, 123953. DOI: 10.1016/j.saa.2024.123953
3. Niu, A.; Sun, X.; Lin, C. Trend in Research on Characterization, Environmental Impacts and Treatment of Oily Sludge: A Systematic Review. Molecules 2022, 27 (22), 7795. DOI: 10.3390/molecules27227795