Copper Distribution and Binding Affinity of Size-Fractioned Humic Substances Taken from Paddy Soil and Correlation with Optical Characteristics

Abstract

Soil organic matter (SOM) is heterogeneous and a complex mixture, whose concentration, chemical composition, and structure are strongly associated with the binding behavior of heavy metals (HMs) in soil. The HM-SOM binding affinity affects the HM biotoxicity and bioavailability in the environment. This study investigated Cu’s distribution and binding affinity on five size-fractioned alkaline-extracted soil organic matters (AEOMs) taken from paddy fields. The fractioned AEOMs were M-A (100 kDa −0.45 μm), M-B (3–100 kDa), M-C (1–3 kDa), M-D (0.3–1.0 kDa), and M-E (<0.3 kDa). The average organic carbon (OC) mass percentages were 10.0, 40.3, 6.3, 5.0, and 38.4%, and Cu mass percentages were 9.8, 66.7, 4.7, 4.7, and 14.1% for fractioned M-A, M-B, M-C, M-D, and M-E solutions, respectively. The Cu and AEOM binding affinity, [Cu]/[DOC] ratios, ranged from 3.1 to 127.6 μmol/g-C with 41-fold variation. The ratio order was 63.5 ± 32.3 (M-B) > 32.8 ± 12.8 (M-A) > 28.0 ± 10.5 (M-D) > 24.25 ± 10.7 (M-C) > 12.5 ± 6.0 (M-E) μmol g-C−1. Cu preferred binding with size-fractioned AEOMs ranging from 3 to 100 kDa. The specific ultraviolet absorbance at 254 nm (SUVA254), the fluorescence index (FI), and the biological index (BIX) were significantly correlated with the [Cu]/[DOC] ratios. In each site, the combined bulk and the five size-fractioned AEOMs, the selected optical indicators had a significant correlation with the corresponding [Cu]/[DOC] ratios. Cu-AEOM binding affinity was enhanced by AEOM rich in aromaticity. However, high microbial and autochthonous AEOM origins decreased the binding affinity.

Copper Distribution and Binding Affinity to Size-Fractioned Dissolved and Particulate Organic Matter in River Sediment

“…The CuBA AEOM ratios at site-1 and site-2 were 111.2 ± 10.2 and 108.7 ± 8.7 µmol g-C −1 , respectively, which were higher than the CuBA ratios of the soil solution (ranging from 3.7 to 20.2 µmol g-C −1 ), water, and CaCl 2 extracted soil organic matter (ranging from BLD to 34.0 µmol g-C −1 ), and NaOH extracted soil and sediment organic matter (ranging from 13.2 to 56.2 µmol g-C −1 ) [21,22,37]. The CuBA DOM ratios showed a wide variation, as observed during in-field measurements.…”

“…This correlation suggested the complex sources of the DOM in the present study. In a soil NaOH extracted study, the CuBA AEOM ratios of size-fractioned AEOM exhibited a significantly negative correlation with indicators FI and BIX [37]. Hence, high CuBA ratios followed high aromaticity, terrestrial and allochthonous sources in soil and sediment solutions.…”

“…In site-1, the Cu-DOM binding affinity exhibited a significantly positive correlation with the FI value, which indicated that the Cu binding affinity increased along with a great extent of microbial sources. Previous studies have reported that the CuBA ratios had a significantly positive correlation with the aromaticity of natural water DOM [23,25] and with the aromaticity of soil solution [21,22,37]. However, when the DOM had complex sources, such as input by wastewater treatment plant effluent [25] and anthropogenic ligand [23], the CuBA DOM ratios had a poor correlation with SUVA 254 .…”

“…Research has studied the affinity of Cu for DOM and POM in various media. For instance, Cu binding to soil organic matter extracted using different solvents, like water, CaCl 2 , and NaOH, has been studied [21,22,36,37]. Similarly, in different aquatic environments, Cu binding to DOM has been studied [23][24][25][26].…”

Humic Substances: Importance for Agriculture, Affinity and Interactions with Soil Amendments and Pollutants

“…Copper ions' distribution and affinity in five size-fractioned alkaline-extracted soil organic matters from paddy fields containing humic substances were investigated by W.-H. Huang, T.-C. Lin, C.-M. Huang, T.-C. Chen, and Y.-L. Yeh in their article on the "Copper Distribution and Binding Affinity of Size-Fractioned Humic Substances Taken from Paddy Soil and Correlation with Optical Characteristics" [5]. The results showed that Cu preferably associated with fractions of high aromaticity.…”