Near-Infrared Spectroscopic Study of Heavy-Metal-Contaminated Loess Soils in Tongguan Gold Area, Central China

Min Yang,San Liu,Weiliang Li,Youning Xu,Jianghua Zhang,Ying Hao,Huaqing Chen

doi:10.3390/min10020089

Min Yang, San Liu + Show 5 more

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https://doi.org/10.3390/min10020089

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Abstract

Loess soil is a kind of widespread soil type in northwest China. Human engineering activities such as mining have caused numerous problems related to heavy metal pollution in soils, which threaten people’s health. The band formation mechanism of the near-infrared (NIR) spectral features in loess soils forms the theoretical basis for the study of the soil environment by hyperspectral remote sensing. Some NIR features of loess soils will shift because of the variations of the soil composition and microstructure after they adsorb heavy metal cations. In this study, we focused on the heavy metal adsorption of the illite, smectite, and illite–smectite (I/S) mixed layer in loess soils; evaluated the pollution by Nemerow indexing; applied X-ray diffraction (XRD), mid-infrared (MIR) spectral analysis, and inductively coupled plasma mass spectrometry (ICP-MS); and carefully observed the shift behavior of the MIR and NIR features. Then, the NIR bands were assigned to MIR bands according to the vibration behavior. Furthermore, the relationships between the NIR band positions and the six heavy metal cations as well as the Nemerow index were investigated via multiregression and simple linear correlation methods. Finally, the relationship obtained from the experiments was analyzed using the physical and chemical mechanisms of the heavy metal cations in the clay minerals. These findings may benefit the application of NIR and remote sensing techniques for detecting heavy-metal-polluted soils.

Highlights

Soil is a loose material layer on the Earth’s surface that provides habitats for green plants, animals, and microorganisms [1]
X-ray diffraction (XRD) peak near 30◦ indicated a minute quantity of calcite, and the NIR bands contained negligible carbonate influence
This study confirmed the assignment of NIR bands in loess soils with abundant clay minerals, and the key factors of NIR spectra were achieved by comparing them with MIR absorption features and the Nemerow index

Summary

Introduction

Soil is a loose material layer on the Earth’s surface that provides habitats for green plants, animals, and microorganisms [1]. The soil environment is a critical factor related to the ecological balance and the safety of human health. During the last three decades in China, mineral exploration, especially metallic mineral exploitation, has added various inorganic and organic substances into the soil. Heavy metal pollution of the soil environment has become one of the most serious problems influencing the ecological environment of the mining areas or even entire mining cities [2,3]. Numerous studies of soil with heavy metal pollution have been reported using traditional chemical analyses such as colorimetric analysis, flame emission spectrometry, atomic absorption spectrometry, and so Minerals 2020, 10, 89; doi:10.3390/min10020089 www.mdpi.com/journal/minerals. These laboratory methods show high accuracy, the sample preparation and analysis processes are time and labor consuming [4]

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