Bruno Delvaux

Halloysite clay minerals - A review

Abstract Halloysite clay minerals are ubiquitous in soils and weathered rocks where they occur in a variety of particle shapes and hydration states. Diversity also characterizes their chemical composition, cation exchange capacity and potassium selectivity. This review summarizes the extensive but scattered literature on halloysite, from its natural occurrence, through its crystal structure, chemical and morphological diversity, to its reactivity toward organic compounds, ions and salts, involving the various methods of differentiating halloysite from kaolinite. No unique test seems to be ideal to distinguish these 1:1 clay minerals, especially in soils. The occurrence of 2:1 phyllosilicate contaminants appears, so far, to provide the best explanation for the high charge and potassium selectivity of halloysite. Yet, hydration properties of the mineral probably play a major role in ion sorption. Clear trends seem to relate particle morphology and structural Fe. However, future work is required to understand the possible mechanisms linking chemical, morphological, hydration and charge properties of halloysite.

Soil organic horizons as a major source for radiocesium biorecycling in forest ecosystems.

Here we review some of the main processes and key parameters affecting the mobility of radiocesium in soils of semi-natural areas. We further illustrate them in a collection of soil surface horizons which largely differ in their organic matter contents. In soils, specific retention of radiocesium occurs in a very small number of sorbing sites, which are the frayed edge sites (FES) born out of weathered micaceous minerals. The FES abundance directly governs the mobility of trace Cs in the rhizosphere and thus its transfer from soil to plant. Here, we show that the accumulation of organic matter in topsoils can exert a dilution of FES-bearing minerals in the thick humus of some forest soils. Consequently, such accumulation significantly contributes to increasing 137Cs soil-to-plant transfer. Potassium depletion and extensive exploration of the organic horizons by plant roots can further enhance the contamination hazard. As humus thickness depends on both ecological conditions and forest management. our observations support the following ideas: (1) forest ecosystems can be classified according to their sensitivity to radiocesium bio-recycling, (2) specific forest management could be searched to decrease such bio-recycling.

Characterization of a Weathering Sequence of Soils Derived From Volcanic Ash in Cameroon - Taxonomic, Mineralogical and Agronomic Implications

The characteristics of eight pedons developed in Quaternary basaltic pyroclasts in Western Cameroon under humid tropical conditions are reported. The soil weathering sequence consists of an Andept-Tropept-Udalf-Udult association. Andic properties, physico-chemical characteristics and weathering stage clearly distinguish the taxonomic units. The total reserve in bases (TRB) and the clay content measured after adequate H2O dispersion with Na+-resins both appear to be suitable weathering indices. Clay mineralogy of non andic soils shows direct relationships between interstratified halloysite-smectite content, CEC of the clay fraction and weathering stage. The weathering stage of the investigated soils is reflected not only in their components and classification, but also in their nutrient status and banana crop performance.

Coexistence of allophane, gibbsite, kaolinite and hydroxy-Al-interlayered 2:1 clay minerals in a perudic Andosol

Allophane and halloysite are the most common secondary aluminosilicate minerals in Andosols developed in pyroclasts in humid tropical areas. We studied the composition and the charge properties of the clays extracted from the weathered B horizon of a perhydrated Andosol developed in andesitic ash in Guadeloupe, French West Indies, by selective dissolution, X-ray diffraction (XRD), thermal analysis and characterization of ion exchange properties. The Bw horizon contained weatherable minerals and exhibited an allophane content of 25%. Its clay fraction contained gibbsite, allophane, kaolinite and hydroxy-Al interlayered 2:1 clay minerals. The occurrence of gibbsite, allophane and kaolinite was linked with intense leaching of silica. After oxalate dissolution of allophane, the cation exchange capacity (CEC) was 11.8 cmol(c) kg(-1) clay, i.e. a typical value for low-activity clays. Subsequent citrate treatment completely dissolved gibbsite, but partly extracted Al interlayers from the hydroxy-Al-interlayered phase. This extraction led to a 5.6-fold increase of the cation exchange capacity (62.3 cmol(c) kg(-1) clay) and a 2.3-fold increase in K+ selectivity. The 2:1 clay minerals involved a combination or continuum of high-charge beidellite and vermiculite. From our data, we believe that the 2:1 clay minerals first acted as a sink for aluminium and might have expressed an anti-gibbsitic effect. After substantial Al interlayering of 2:1 clay minerals mimicking low-activity clay, gibbsite might have formed in a second stage

Sulfate, chloride and fluoride retention in Andosols exposed to volcanic acid emissions.

The continuous emissions of SO(2), HCl and HF by Masaya volcano, Nicaragua, represent a substantial source of atmospheric S-, Cl- and F-containing acid inputs for local ecosystems. We report on the effects of such acid depositions on the sulfate, chloride and fluoride contents in soils (0-40 cm) from two distinct transects located downwind from the volcano. The first transect corresponds to relatively undifferentiated Vitric Andosols, and the second transect to more weathered Eutric Andosols. These soils are exposed to various rates of volcanogenic acid addition, with the Vitric sites being generally more affected. Prolonged acid inputs have led to a general pH decrease and reduced exchangeable base cation concentrations in the Andosols. The concentrations of 0.5 M NH(4)F- and 0.016 M KH(2)PO(4)-extractable sulfate (NH(4)F-S and KH(2)PO(4)-S, respectively) indicate that volcanic S addition has increased the inorganic sulfate content of the Vitric and Eutric soils at all depths. In this process, the rate of sulfate accumulation is also dependent on soil allophane contents. For all soils, NH(4)F extracted systematically more (up to 40 times) sulfate than KH(2)PO(4). This difference suggests sulfate incorporation into an aluminum hydroxy sulfate phase, whose contribution to total inorganic sulfate in the Vitric and Eutric Andosols is estimated from approximately 34 to 95% and approximately 65 to 98%, respectively. The distribution of KH(2)PO(4)-extractable chloride in the Vitric and Eutric Andosols exposed to volcanic Cl inputs reveals that added chloride readily migrates through the soil profiles. In contrast, reaction of fluoride with Al and Fe oxyhydroxides and allophanes is an important sink mechanism in the Masaya Andosols exposed to airborne volcanic F. Fluoride dominates the anion distribution in all soil horizons, although F is the least concentrated element in the volcanic emissions and depositions. The soil anion distribution reflects preferential retention of fluoride over sulfate and chloride, and of sulfate over chloride. The primary acidifying agent of the Andosols subject to the volcanic acid inputs is HCl.

Iron toxicity and other chemical soil constraints to rice in highland swamps of Burundi

Iron toxicity is suspected to be a major nutritional disorder in rice cropping systems established on flooded organic soils that contain reductible iron. A pot trial was carried out to assess Fe toxicity to rice in flooded Burundi highland swamp soils with a wide range of organic carbon contents. Soil and leaf analyses were performed and total grain weight was determined. Clear Fe toxicity was diagnosed, based on leaf Fe content at panicle differentiation. Leaf Fe contents higher than 250 μg g−1 dry matter induced lower Mg (and probably Mn) uptake, and a 50% total grain weight reduction. These features were associated with exchangeable Fe equivalent fractions higher than 86%. Besides, several non-Fe toxic soils exhibited an Mg-Mn imbalance.

Greenhouse response of micropropagated bananas inoculated with in vitro monoxenically produced arbuscular mycorrhizal fungi

The effect of an in vitro monoxenically produced arbuscular mycorrhizal fungus (AMF), i.e. Glomus intraradices on the growth and phosphorus content of micropropagated banana (Musa spp. c.v. Grand Naine) plantlets was investigated in a pasteurized and non-pasteurized Vertisol. The experiment was conducted as a 2 x 2 randomly factorial design with the Vertisol pasteurized or non-pasteurized and the plants inoculated or non-inoculated with the AMF. The plants colonized by AMF had larger shoot and root dry weight and P content than the non-mycorrhizal plants. The largest growth increment was observed in the plants inoculated during the weaning phase with the monoxenic AMF inoculum and subsequently grown in the pasteurized soil. The native AMF population also induced a marked increase in plant growth supporting further the efficiency of this natural population, whereas both inoculum sources in combination did not yield the largest growth increment. The latter suggests a competitive phenomenon between the introduced AMF and the other soil micro-organisms including the native AMF population. Our results demonstrate, for the first time, the potential benefits of a monoxenically produced AMF in micropropagated banana plants

Distinct silicon and germanium pathways in the soil-plant system: Evidence from banana and horsetail

Plants strongly impact the continental silicon cycle by taking up Si and precipitating opal phytoliths which are recycled into the soil. Studying Ge incorporation, a chemical analog of Si, relative to Si may provide a useful tracer of Si pathways. However, Ge uptake and transport through plants and the impact on Ge/Si of phytoliths remain poorly understood. Here, we report Ge uptake and accumulation and Ge/Si fractionation in all plant parts and solutions from: (1) hydroponic banana, (2) in situ sampled banana, and (3) horsetails. We further combine these data with delta Si-29 from banana plants. Our data reconcile opposite conclusions drawn from previous studies on Ge uptake and pathways. No discrimination of Ge occurred at the root-solution interface. Banana and horsetails were shown to accumulate Ge in roots: a previous study provided evidence of low Ge/Si ratios in root phytoliths which contrasts with high bulk Ge/Si ratios in roots we report here. This suggests that Ge is organically trapped in roots. Consequently, shoots display lower Ge/Si ratios, without fractionation between shoot parts since Ge would follow transpiration stream as silicon, and is not discriminated between shoot parts. This contrasts with the two-step discrimination against heavy Si isotopes, at the root-solution interface, and then within the shoots. The soil composition (clays versus Fe oxides) has a leading role on the Ge/Si signatures of plants which may in turn impact on the Si and Ge fluxes to the global ocean.

Morphology, Texture, and Microstructure of Halloysitic Soil Clays As Related To Weathering and Exchangeable Cation

This paper aims at characterizing the morphology, texture, and microstructure of three hydrated kaolin rich clays (f > 0.2 μm) from volcanic soils. These clays represent a weathering sequence in which CEC, halloysite content with respect to kaolinite, as well as smectite content in the halloysite-smectite mixed-layer clays decrease with increased weathering. The clay samples were made homoionic (K+ or Mg2+) and hydrated under a low suction pressure (3.2 kPa). After replacing water by a resin, ultrathin sections were cut and examined by TEM. Particle shape varies with increased weathering, as follows: spheroids → tubes → platelets. Higher aggregation and dispersion are observed by TEM after Mg2+ and K+ saturation, respectively, at two levels of the clay-water system organization: intraparticle and interparticle. The microstructure variations induced by the nature of the exchangeable cation become less pronounced with decreasing layer charge of the 2:1 layers. They are thus related here to the presence of smectite layers localized in the halloysite habitus, mostly at the particle periphery. These results show that small amounts of smectite largely affect the organization of clays rich in kaolins at a high water content, and that K+ behaves here as a dispersing ion.

Relevance of Radiocaesium Interception Potential (RIP) on a worldwide scale to assess soil vulnerability to 137Cs contamination

The extent of radiocaesium retention in soil is important to quantify the risk of further foodchain contamination. The Radiocaesium Interception Potential (RIP -Cremers et al., 1988, Nature 335, 247-249) is an intrinsic soil parameter which can be used to categorize soils or minerals in terms of their capacity to selectively adsorb radiocaesium. In this study, we measured RIP for a large soil collection (88 soil samples) representative of major FAO soil reference groups on a worldwide scale and tested the possibility to predict the RIP on the basis of other easily accessible or measurable soil data. We also compared RIP values with those obtained from separate chemical extraction experiments. The range of measured RIP values (1.8-13300 mmol kg(-1)) was shown to include nearly all possible cases of agricultural soil contamination. Only Podzols, Andosols and Ferralsols were clearly characterized by a very low RIP (<2000 mmol kg(-1)). On a worldwide scale, RIP was in fact slightly related to soil reference type or other simple major physicochemical parameters such as clay percentage or organic matter. Conversely our results indicated a link between the RIP and radiocaesium extractability across very different soils. We showed that, with the proposed scale of RIP values, a simple acid extraction method can provide an operational result highly predictive of potential RIP despite very contrasting soil properties. The RIP could be estimated from the empirical equation: RIP = (-31.701 ∗ log(AER) + 58.886)(2) where AER is the fraction of acid-extractable radiocaesium.