Radionuclide uptake by colloidal and particulate humic acids obtained from 14 soils collected worldwide

Peng Lin,Wei Xing,Chen Xu,Luni Sun,Kathleen A Schwehr,Daniel I Kaplan,Peter H Santschi,Nobuhide Fujitake,Chris M Yeager

doi:10.1038/s41598-018-23270-0

Abstract

Uptake of six particle-reactive and/or redox-sensitive radionuclides (210Pb, 234Th, 7Be, 59Fe, 237Np and 233Pa) by 14 humic acids (HAs) was investigated in artificial groundwater under mildly acidic conditions (pH~5.5). In HA-groundwater slurry, Pb, Be, Fe and Pa bound strongly to particulate HA (>0.45 µm), supporting their application as tracers of soil erosion. Th bound strongly to the colloidal HA (3 kDa-0.45 µm) and as such, would not be a good candidate as a tracer for monitoring soil erosion. HAs likely reduced the oxidized neptunyl form (Np(V)O4+) to Np(IV) based on its enhanced particle-reactivity and Np uptake by particulate HAs, partially retarding the movement of anthropogenic 237Np in field polluted environments. Particulate/colloidal carbonyl/O-aryl (likely through hydroquinone/quinone) functionalities in the HA correlated to Np and Pa uptake, but only particulate O-aryl functionalities was responsible for Fe uptake. The carboxylate- and carbonyl/O-aryl-containing organic functionalities in the HA correlated strongly with Th uptake. In contrast, no significant correlations between organic parameters and Pb or Be uptake implied their predominance of uniform surface adsorption onto particles. This study provides novel insight into the binding of six radionuclides with different organic functionalities of three size fractions, as well as its possible impact on their application in the soil-tracing research.

Highlights

Pa) by 14 humic acids (HAs) was investigated in artificial groundwater under mildly acidic conditions
The results showed the proportion of released colloidal organic matter (COM) varied among different HAs (Table S2), ranging from 6% to 77% for organic carbon and from 0%
The organic carbon and organic nitrogen of the initial HA had similar tendencies to be associated with the colloidal fraction, i.e., 29 ± 22% of organic carbon and 31 ± 23% of organic nitrogen were released from initial HAs into the colloidal fraction (ANOVA, p > 0.05, n = 14)

Summary

Introduction

Pa) by 14 humic acids (HAs) was investigated in artificial groundwater under mildly acidic conditions (pH~5.5). In HA-groundwater slurry, Pb, Be, Fe and Pa bound strongly to particulate HA (>0.45 μm), supporting their application as tracers of soil erosion. Strong particle-reactivity could result in strong affinity to natural colloidal organic matter (COM). A potential artifact associated with using tracers to estimate soil particulate movement is that tracers associated with the mobile colloidal fraction may indicate greater mobility than occurs with the soil particles. This compromises the use of tracers for their intended purpose of estimating such geochemical processes as soil/sediment erosion. NOM in the colloidal form, may promote movement of radioisotopes, while NOM in the particulate form or bound to soils, may promote radioisotope immobilization and thereby are useful for the intended purpose of soil-tracing studies

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