Abstract
Soil humic substances (SHS) are heterogeneous, complex mixtures, whose concentration, chemical composition, and structure affect the transport and distribution of heavy metals. This study investigated the distribution behavior of two heavy metals [cadmium (Cd) and nickel (Ni)] in high molecular weight SHS (HMHS, 1 kDa–0.45 μm) and low molecular weight SHS (LMHS, <1 kDa) extracted from agricultural soils. The HMHS mass fractions were 45.1 ± 19.3%, 17.1 ± 6.7%, and 57.7 ± 18.5% for dissolved organic carbon (DOC), Cd, and Ni, respectively. The metal binding affinity, unit organic carbon binding with heavy metal ratios ([Me]/[DOC]), were between 0.41 ± 0.09 μmol/g-C and 7.29 ± 2.27 μmol/g-C. Cd preferred binding with LMHS (p < 0.001), while Ni preferred binding with HMHS (p < 0.001). The optical indicators SUVA254, SR, and FI were 3.16 ± 1.62 L/mg-C/m, 0.54 ± 0.18 and 1.57 ± 0.15, respectively for HMHS and 2.65 ± 1.25 L/mg-C/m, 0.40 ± 0.17, and 1.68 ± 0.12, respectively for LMHS. The HMHS contained more aromatic and lower FI values than LMHS. Multilinear regression showed a significant positive correlation between the measured predicted [Me]/[DOC] ratios (r = 0.52–0.72, p < 0.001). The results show that the optical indices can distinguish the chemical composition and structure of different size SHS and predict the binding ability of Me-SHS.
Highlights
Soil organic matter (SOM) is an important component of soil [1,2,3,4]
This study investigated the distribution behavior of two heavy metals [cadmium (Cd) and nickel (Ni)] in high molecular weight soil humic substances (SHS) (HMHS, 1 kDa–0.45 μm) and low molecular weight SHS (LMHS,
The results show that the optical indices can distinguish the chemical composition and structure of different size SHS and predict the binding ability of Me-SHS
Summary
Soil organic matter (SOM) is an important component of soil [1,2,3,4]. Heavy metals in soil are mainly associated with SOM and fine particles, such as oxide-type components and clay minerals [1,5,6,7].The SOM’s major components are soil humic substances (SHS). The SHS is a heterogeneous and complex organic substance comprised of various functional groups such as phenol, carboxyl, and hydroxyl functional groups [1,5,7,8,9,10], which have a strong ability to bind heavy metals. When heavy metals bind with SHS, the complex affects the heavy metal mobility, biotoxicity, and fate in the soil environment [1,4,5,6,7,9,10]. The SHS concentration (represented by C content); does not fully predict the mobility potential and binding strength of metals. In strongly-binding Me-SHS complexes, metals are stable and will not readily release; this reduces the metal mobile potential and biological toxicity. In a weakly-binding Me-SHS complex, metals may release into the
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