An investigation of soil actinide-humic interactions using electrophoresis

Soils in the UK contain both naturally occurring and anthropogenic actinides. The latter originate mainly from (i) atmospheric deposition from weapons testing and, in coastal areas, (ii) onshore transfer of contaminated particulates arising from industrial discharges to marine systems. Following deposition, the speciation of these highly radiotoxic elements within soils is important with respect to subsequent transport and potential availability. This study focused on the associations of U and Pu with humic substances and the implications for migration in deciduous broadleaf and coniferous forest soils. Humic substances were isolated from soil samples by (i) solvent extraction (0.045 M Trisborate and 0.1 N NaOH) and (ii) novel gel electrophoretic extraction (pH 8.5; 0.045 M Trisborate as running buffer). Gel electrophoresis was also used to extract humic substances from soil that had been 'pre-equilibrated' with a solution containing 236 or 242Pu (pH 4.2). The relative concentrations of humic substances in the solvent and electrophoretic extracts were obtained by UV-visible spectrophotometry (A 254 ) whilst the 238U concentrations in the extracts were determined by inductively coupled plasma-mass spectrometry (ICP-MS). In addition, the molecular size and functional group content of the humic molecules in selected fractions of the gel electrophoretic extracts were investigated using gel filtration chromatography and FTIR spectroscopy, respectively. The methodology developed in this study enabled identification of several operationally defined soil actinide pools: (i) labile (< 0.2 pm, at pH 4.2), (ii) hydrophilic-mobile (<0.15 pm, negatively-charged at pH 8.5) and (iii) immobile (>0.15 Pm and/or uncharged at pH 8.5). The adopted procedures avoid the extremes of pH used in the traditional methods of extraction (pH 13) and fractionation (pH < 2) which may alter humic substances. It was found that the 238U content of the hydrophilic-mobile pool (i.e. the gel electrophoretic extract) accounted for 14-41 % and 15-34 % of the total 233U in the deciduous broadleaf and coniferous forest soils, respectively. In general, these values were greater than those obtained by 0.045 M Tris-borate solvent extraction but less than those obtained using 0.1 N NaOH as the solvent. A further advantage of the new methodology was that the distribution of 238U within the humic extract could be investigated. Two patterns were observed for the distribution of 238U amongst the gel electrophoretic fractions. The first was characterised by the association of 238U with predominantly less mobile fractions and by a strong correlation between 238U content and humic concentration. In the second, 238U was more evenly distributed amongst the fractions. In all cases, however, the largest 238U pool was the immobile pool (67-81 %), i.e. the 238U remaining in the soil residue after gel electrophoretic extraction. The results for humic concentration and associated 238U of the solvent and electrophoretic extracts revealed that humic substances (preferentially the higher molecular weight fractions) and actinides were removed from the soil solution to a greater extent in the coniferous relative to the deciduous broadleaf soil profile. This was interpreted as due to a chemical difference in the humic ligands, as influenced by the lignin constituents of the two types of forest vegetation. From the soil-spiking experiment, the percentage of the total added 236 or 242Pu in the hydrophilic-mobile pool generally decreased with increasing soil depth. The 236 content decreased from 13 to 10 % and 16 to 5 % and the 242Pu from 8 to 4 % and from 8 to 2 % in the deciduous broadleaf and coniferous soils, respectively. This was attributed to the decrease in concentration of extractable humic substances with increasing soil depth. Characterisation of the gel electrophoretic fractions revealed only the first pattern for the distribution of introduced actinides, i.e. association with the less mobile fractions. As for 238U, however, the greatest proportion of added 23U was found in the immobile pool (63-84 %). In contrast, 242Pu was found to a greater extent in the labile pool (55-88 %), in accord with previous observation of the greater mobility of Pu relative to that of u in forest soils. Characterisation of the less mobile molecules within the hydrophilic-mobile pool showed that they comprise a polydisperse component (with molecular weights in the 1000's100,000's D range) of the total huniic substances. Also identified were small, highly electrophoretically mobile molecules that are likely to be very mobile upon release to natural receiving waters. 238U was associated with these molecules whilst added 236 or 242Pu was associated with the less mobile humic molecules.