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Influence of the Nature of Clay Minerals on the Fixation of Radiocaesium

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Influence of the Nature of Clay Minerals on the Fixation of Radiocaesium R European Journal of Soil Science, March 1999, 50, 117±125 In¯uence of the nature of clay minerals on the ®xation of radiocaesium traces in an acid brown earth±podzol weathering sequence E. MAESa,A.ISERENTANTa, J. HERBAUTSb & B. DELVAUXa aUniversite Catholique de Louvain (UCL), unite Sciences du Sol, Place Croix du Sud 2/10, 1348 Louvain-La-Neuve, Belgium, and bUniversite Libre de Bruxelles (ULB), Laboratoire de GeÂneÂtique et d'Ecologie veÂgeÂtales, ChausseÂe de Wavre 1850, 1160 Bruxelles, Belgium Summary The magnitude of radiocaesium ®xation by micaceous clay minerals is affected by their transformation, which depends on weathering in soil. The net retention of radiocaesium traces was quanti®ed by sorption±desorption experiments in the various horizons of four sandy soils forming an acid brown earth± podzol weathering sequence derived from sandy sediments and characterized by marked changes in mineral composition. The features of the 2:1 minerals of the four soils, resulting from an aluminization process in depth and a desaluminization process towards the surface, had a strong in¯uence on Cs+ ®xation. Beneath the desaluminization front, which deepens from the acid brown earth to the podzol, hydroxy interlayered vermiculite was dominant and the 137Cs+ ®xation was the weakest. At the desaluminization front depth, vermiculite was responsible for the strongest 137Cs+ ®xation. In the upper layers, smectite appeared in the podzolized soils and the 137Cs+ ®xation decreased. The magnitude in Cs+ ®xation therefore appeared as a tracer of the transformation process affecting the 2:1 clay minerals in the acid brown earth±podzol weathering sequence. This magnitude was positively correlated with the vermiculite content of the studied soil materials estimated by the rubidium saturation method. Introduction vermiculite retained much more radiocaesium in acid conditions, because of its dioctahedral character or a Soils retain radiocaesium because they contain a few highly greater resistance to acid weathering or both. It was selective sites associated with the presence of micaceous concluded that Cs+ ®xation in the studied weathering minerals (Sawhney, 1972; Eberl, 1980; Cremers et al., sequence occurred on vermiculitic sites associated with 1988). Micas are common in some soils derived from micaceous wedge zones. sediments and weathered rocks, particularly of the sialic Such observations suggest that weathering in soil strongly type. In soils where the transformation of micas is active in¯uences the ®xation of Cs+. Large differences may be these minerals undergo changes in their structure and expected not only in the different pro®les of a soil weathering properties. The K+ depletion, gain in hydrated exchangeable sequence but also in the various horizons at each site. In acid cations and oxidation of structural Fe2+ cause the forest soils strong mineralogical variations are characteristic. transformation of biotite into vermiculite and smectite They are related both to horizon differentiation and to the (Fanning et al., 1989) in which Al interlayering may occur distribution of plant roots, which are dynamic weathering (Barnhisel & Bertsch, 1989). The retention of 137Cs+ traces agents in soils (Courchesne & Gobran, 1997). is strongly in¯uenced by these changes, as reported by This paper describes the 137Cs+ retention properties of the A Maes et al. (1999) in a laboratory weathering sequence and B horizons in a soil weathering sequence acid brown biotite ® vermiculite ® oxidized vermiculite ® hydroxy earth±podzol developed in sandy sediments. The soil sequence interlayered vermiculite (HIV). These authors found that the is represented by four distinct pro®les in which vermiculite, interlayer occupancy by potassium in biotite and hydroxy- smectite and hydroxy interlayered 2:1 clay minerals are Al groups in HIV strongly limited Cs+ ®xation. Vermiculite derived from the weathering of mica and chlorite. The ®xed large amounts of radiocaesium, but the oxidized 137Cs+ retention properties are assessed in relation to the Correspondence: B. Delvaux. E-mail: [email protected] mineralogical variations and the quanti®cation of vermiculitic Received 20 January 1998; revised version accepted 27 October 1998 minerals. # 1999 Blackwell Science Ltd 117 L 118 E. Maes et al. Figure 1 Schematic diagram of the evolution of the clay mineralogy in the acid brown earth±podzol weathering sequence (adapted from Herbauts, 1982). Materials and methods hydroxy-Al intergrades are the dominant clay minerals. In the more acidic surface horizons, aluminium is complexed by The acid brown earth±podzol weathering sequence organic compounds and removal of Al interlayers occurs. The Four soil pro®les studied earlier by Herbauts (1982) were latter process has been called `dechloritization' by Frink sampled on the Lower Lias outcrop in southeast Belgium. The (1969), `de-aluminization' by Vicente et al. (1977) and altitude ranges between 320 and 335 m, the annual rainfall `desaluminization' by Herbauts (1982). In the surface amounts to 1100 mm, and the mean annual temperature is horizons, desaluminization occurs with increasing intensity 7.7°C. All the soils were sampled in deciduous forest with towards the more weathered soils, resulting in the dominance Fagus sylvatica, Quercus petraea and Quercus robur as of smectite-like minerals. These smectite-like minerals present dominant species. a 1.7±1.8-nm re¯ection in their X-ray diffraction after The bedrock (calcareous sandstones of Lower Lias age) is saturation with Mg2+ and ethylene-glycol solvation, but covered by a two-layered sheet: an autochthonous sandy layer collapse to 1.0 nm when heated (200°C) after saturation with formed by the dissolution of the calcareous bedrock is overlain K+. These minerals are identi®ed as degradation smectites by a mixture of this sandy material with loessic silt-sized derived from micaceous minerals. From the ochreous brown particles. The soil sequence developed in the upper material is earth to the podzol, the deepening of the desaluminization as follows: acid brown earth ® ochreous brown earth ® front is therefore associated with the advent of smectitic clays. brown podzolic soil ® podzol, i.e. Dystric Cambisol ® Haplic Whatever the soil weathering stage, the sand and silt fractions Podzol according to the World Reference Base for Soil have homogeneous mineralogical compositions dominated by Resources (WRB) classi®cation (FAO, 1998). Following a quartz (Herbauts, 1982). gradient of increasing podzolization, this sequence of soil coincides with the evolution of the humus from mull to Soil characterization moder-mor. This evolution coincides with the morphological development of the pro®le from Ah-Bw to Ah-Bh and The following horizons were sampled in the four soil pro®les eventually Ah-E-Bh. The increasing podzolization is well of the sequence: the Ah and B horizons of the acid brown illustrated by (i) selective chemical extraction of Fe and Al and earth, the Ah, AB and Bw1 horizons of the ochreous (ii) the mineralogical composition of the clay fraction (< 2 mm) brown earth, the Ah, Bh, Bw1 and Bw2 horizons of the (Herbauts, 1982). brown podzolic soil, and the Ah, E, Bhs, Bs and C horizons of The general evolution of the clay minerals in the sequence is the podzol. The samples from mineral horizons were air-dried illustrated in Figure 1 (Herbauts, 1982). In the moderately acid and sieved to < 2 mm. The samples from organic-rich Ah B horizons, Al interlayering affects the 2:1 clay minerals horizons were sieved to < 2 mm without drying and kept in produced by the weathering of mica and chlorite: 2:1±2:2 closed bags at 4°C to avoid their becoming hydrophobic. The # 1999 Blackwell Science Ltd, European Journal of Soil Science, 50, 117±125 R 137Cs+ ®xation in an acid brown earth±podzol weathering sequence 119 particle size analysis was done after dispersion using an methanol. The sample was heated at 110°C overnight, cooled + ultrasonic probe (50 W, 15 min) and Na resins as a dispersing in a desiccator and then washed four times with 10 ml of 0.5 M agent (Rouiller et al., 1972). Total carbon (Walkley-Black), NH4Cl. Supernatants were discarded. The sample was then exchangeable bases and cation exchange capacity (CEC) (1 M digested in HF±HNO3±HClO4 (Lim & Jackson, 1982). The Rb NH4OAc at pH 7), and KCl-extractable Al and H were content was determined by atomic absorption spectrophoto- determined using the procedures outlined by Page et al. metry. The vermiculite content in g kg±1 was evaluated by Rb ±1 (1982, pp. 570, 160, 163, respectively); pH was measured in content (cmolc kg )/0.154: the evaluation is based on a CEC ±1 both H2O and 1 M KCl (10 g:25 ml). Elemental analyses were of the vermiculite of 154 cmolc kg (Ross et al., 1989). made in HF±H2SO4 digests of calcined < 2 mm soil samples (Page et al., 1982, p. 7) by atomic absorption spectropho- tometry. Results and discussion Soil characteristics Radiocaesium sorption±desorption experiments Table 1 presents major characteristics of each horizon of the Soil samples (air-dried in the case of mineral horizons, sieved four soils. All the horizons are acidic and depleted of to pass 2 mm) were equilibrated with a mixed KCl±CaCl2 exchangeable bases, with base saturation less than 5% except ±3 solution with a total chloride concentration of 10 M,a in the case of the Ah horizon in the acid brown earth (16%). 1/2 potassium adsorption ratio (PAR) value of 0.057 mM (PAR Aluminium extractable in KCl represents 30±60% of the is de®ned as (K)/Ö(Ca) where () refers to ion concentration in effective cation exchange capacity (ECEC) in the organic-rich (mM) and a K:Ca molar ratio of 0.082. Dialysis bags horizons and 65±94% in the mineral horizons. containing 1 g of soil and 5 ml of the mixed KCl±CaCl2 The depth distribution of organic carbon reveals a regular 137 solution were placed in 95 ml of CsCl-labelled KCl±CaCl2 decrease in the pro®le of the acid brown soil (59), the ochreous 137 137 + ±10 solution (carrier free Cs, Cs concentration 1 3 10 M brown soil (57) and the brown podzolic soil (29).
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