Correction method of effect of soil moisture on the fluorescence intensity of polycyclic aromatic hydrocarbons based on near-infrared diffuse reflection spectroscopy

Highlights

• A correction method to reduce the effect of soil moisture on fluorescence intensity of polycyclic aromatic hydrocarbons.

• NIR diffuse reflectance spectroscopy was used to correct the fluorescence spectroscopy of polycyclic aromatic hydrocarbons.

• It is feasible to detect polycyclic aromatic hydrocarbons in soil by fluorescence and NIR diffuse reflectance spectroscopy.

• After correction fluorescence spectra showed better performance than before correction.

Abstract

Soil moisture has a strong impact on the fluorescence intensity of PAHs, which is undoubtedly posing a challenge for the development of rapid real-time fluorescence detection technology of PAHs in soil. In this work, NIR diffuse reflectance spectroscopy was used to correct the fluorescence spectra of PAHs in order to reduce the effect of the soil moisture. To establish the correction method, eight soil samples with different moisture contents and a given phenanthrene concentration (8 mg/g) were prepared. The fluorescence and NIR diffuse reflectance spectra were collected for of all samples. It was found that the fluorescence spectra of the soil samples that vary with the moisture content together with the NIR diffuse reflectance spectra were considered for the correction of the fluorescence intensity of phenanthrene related to the moisture content. The results showed that the ratio of the fluorescence intensity at 384 nm to the NIR diffuse reflectance spectrum absorbance at 5184 cm⁻¹ can be used as a correction factor to reduce the effect of the soil moisture on the fluorescence intensity of phenanthrene in the soil. The validity of the correction method was verified by the quantitative analysis of PAHs with different concentrations and soil moisture contents. The results showed better linearity between the fluorescence intensity and the concentration of PAHs after the correction (with a correlation coefficient R of 0.99) than before the correction (with R of 0.86). The relative prediction errors for three unknown samples decreased from 19%, 51% and 40% before the correction to 5%, 13% and 0.44% after the correction, respectively, indicating the feasibility of the detection of PAHs in the soil by the combination of fluorescence and NIR diffuse reflectance spectroscopy.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are important organic pollutants due to their carcinogenicity, mutagenicity and teratogenicity. As an important environmental medium, soil bears at least 90% of the environmental load of PAHs (Song et al., 2018; Xiao et al., 2006). Therefore, PAHs-contaminated soil is particularly prominent. Soil contamination by PAHs not only affects the quality of agricultural products and harms human health (Juhasz and Naidu, 2000; Wilcke, 2000; Dugay et al., 2002; Liao et al., 2013) but also restricts the sustainable development of the soil. Anthropogenic and natural sources of PAHs in combination with global transport phenomena contributed to their worldwide distribution (Lawal, 2017). Many governments and public authorities have found themselves under increased pressure to ensure that any discovery of contaminated soils is conveyed and remediated quickly and effectively (Bray et al., 2010). Therefore, the development of a rapid and convenient method for analysis of PAHs in the soil is of great significance.

Due to the complexity of soil properties and structure, and the low content of PAHs in the soil, accurate detection and analysis of PAHs in the soil is difficult (Khodadoust et al., 2000; Viglianti et al., 2006). In previous research, several techniques have been developed for the analysis of PAHs in the soil such as the immunoassay (IMA) techniques (Zhou et al., 2009; Wei et al., 2009), general gravimetry (Villalobos et al., 2008), gas chromatography with mass spectrometry (GC/MS) (Lorenzi et al., 2010; Yang et al., 2011), infrared (IR) spectroscopy (Forrester et al., 2010), Vis-NIR Spectroscopy (Bray et al., 2010), Raman spectroscopy (Li and Dai, 2012; Pfannkuche et al., 2012), and fluorescence spectroscopy. Reuben and Abdul (2013) reviewed these techniques and evaluated their performance from the perspectives of their cost, portability, operational time, accuracy, and occupational health and safety considerations, and pointed out that the fluorescence spectroscopy technique had the best performance in comparison to the other techniques. Therefore, fluorescence spectroscopy is as an important trace analysis method that is widely used in the detection of PAHs due to its advantages of high sensitivity, good selectivity, and reproducibility.

In fact, optical fibre and laser induced fluorescence (LIF) spectroscopy have been used for on-line and in situ monitoring of PAHs in the soil since 1990 (Alarie et al., 1995; Baumann et al., 2000; Lieberman et al., 1992; Schultze and Lewitzka 2005). The variation of fluorescence intensity due to the heterogeneity and complexity of the soil media was investigated. It was pointed out that soil properties such as particle size, soil type, moisture, and organic matter content exert a strong effect on the fluorescence intensity of PAHs (Sinfield et al., 1999; Boas 2004; Lee et al., 2004; Ko et al., 2004). Ko et al. (2004) investigated the variation in the LIF intensity related to the moisture content and soil particle size distribution under different soil conditions. It was found that diffuse reflectance was related to the moisture content and the particle size distribution. It was also found that diffuse reflectance increases with increasing sand content and decreases with increasing moisture content. Lee et al. (2004) performed quantitative analysis for pyrene and phenanthrene in soil using LIF spectroscopy combined with the partial least squares method. It was demonstrated that the effect of the soil physical composition on the PAHs fluorescence intensities was reduced by dividing the LIF intensity by the diffuse reflectance at 532 nm. Yang et al. (2016; 2017) studied two-dimensional correlation fluorescence spectral characteristics of PAHs under the external perturbation of soil particle size and moisture content respectively, and proposed that the correction of the effect of the above soil properties on PAHs fluorescence intensity may be realized through the establishment of the relationship among the PAHs fluorescence intensity, Rayleigh scattering intensity and soil particle size and moisture content.

The above studies showed that soil moisture has a strong effect on the fluorescence intensity of PAHs, which undoubtedly presents a challenge for the development of rapid real-time fluorescence detection of PAHs in the soil. In the present work, based on the study of the influence of soil moisture on the fluorescence and NIR diffuse reflectance spectra, a correction method combined with fluorescence and NIR diffuse reflectance spectroscopy for reducing the effect of soil moisture on the PAHs fluorescence intensity was proposed and its effectiveness was verified. This work lays a theoretical and experimental foundation for the on-site and rapid detection of PAHs in the soil.