Enregistrement scientifique n": 386
Symposium n": 6
Presentation: oral
The Dynamics of Metal Adsorption and Surface Precipitate
Formation on Soil Materials as Monitored by Surface Molecular
Techniques
Dynamique de ('adsorption des métaux et de leur precipitation ä la
surface des constituants des sols, suivie par des techniques
moléculaires de surface
SCHEIDEGGER A.M., STRAWN D C , ROBERTS DR.. SCHECKEL K.G., SPARKS Donald L.
Department of Plant and Soil Sciences, 147 Townsend Hall, University of Delaware, Newark, Delaware
19717-1303 USA
An understanding of the dynamics and mechanisms of metal sorption on soil materials is fundamental in
assessing the fate, speciation and bioavailability of metals in natural systems [1,2]. Traditionally, sorption
has been studied using macroscopic equilibrium approaches It is now well recognized that the kinetics of
metal sorption/release must be understood if accurate predictions are to be made about metal fate/mobility.
With some metals (e.g., Co, Cd, Cu) residence time (the time over which the soil material and metal react)
affects the rate of metal desorption while with other metals (e.g., Pb), there is little effect residence time To
definitively understand the dynamics of metal sorption mechanisms, one must employ in-silu molecular
approaches X-ray adsorption fine structure (XAFS) spectroscopy and scanning force microscopy (SFM) are
two such techniques that can be used to study metal reaction mechanisms at the mineral/water interface This
paper will discuss the dynamics of metal adsorption and surface precipitate formation on soil mineral
surfaces using XAFS and SFM. It will be shown that with metals such as Ni, polynuclear surface complexes
occur on an array of mineral surfaces that are common in the soil environment.
The surface precipitates often form on rapid time scales, at metal surface loadings far below a theoretical
monolayer coverage, and in a pH range well below that at which metal hydroxide precipitates would be
expected to form according to the thermodynamic solubility product XAFS studies indicate that the
polynuclear complexes are mixed metal Al/hydroxide phases and form when a metal such as Ni(ll), Co(ll),
Zn(H), or Mn(ll> is introduced into an environment in which there is a source of hydrolyzed species of
Al(lll), Fe(lll), and Cr(lll)[3] With metals such as Pb, little if any, polynuclear complexes form with time
Scanning force microscopy and batch kinetic studies show that the mixed metal/Al hydroxide phases
extensively cover mineralsurfaces and they are much more resistant to dissolution than the pure divalent
metal hydroxide compounds The dynamics of surface precipitate and adsorption complex formation should
be considered in modelling sorption processes.
Enregistrement scientifique n° : 2006
Symposium n° : 06
Presentation : poster
Adsorption of phosphate on kaolinite : Effect of iron oxides and pH
Adsorption du phosphate sur la kaolinite : Effet des oxydes de fer et
du pH
SEI Joseph (1), BERTRAND lsabelle (2), JUMAS Jein-Cliude (1), OLIVIER-FOURCADE Josette (1)
& STAUNTON Siobhan (2)
(1) LPMC.Univ. Montpellier II,pi. E Bataillon,CC 03, 34095 Montpellier Cedex 5
(2) UFR de Science du Sol. INRA, place P. Viala. 34060 Montpellier Cedex 1, France
Kaolinite is the major mineral component of tropical soils and so phosphate adsorption on this mineral may
be a determinant factor for soil fertility. In addition, the phosphate adsorption properties of minerals might
be exploited for the depollution of waste water, We have investigated the adsorption of phosphate on three
samples taken from different mineral deposits in the Ivory Coast
Various techniques, including X-ray diffraction, "Fe Mössbauer spectroscopy and surface area
measurement, have been used to characterise the samples The clay sized fraction (< 2 urn) was extracted by
sedimentation Free iron oxides were removed from subsamples of each clay using the dithionite-citratebicarbonate (DCB) method All clays were saturated in either Na or Mg. Phosphate adsorption was
measured in suspension under controlled conditions of ionic strength and pH
The Langmuir adsorption isotherm gave a good description to adsorption data for all samples The
Langmuir adsorption maximum, Q™„ was about 10 mmol kg'1 which is higher than that typically observed
for reference minerals and much closer to the values reported for Australian kaolinitic soils (Singh & Gilkes.
1992) Adsorption was always slightly higher samples saturated in Mg than in Na, due to bridging
complexes The removal of free iron oxides caused the adsorption maximum to be halved, however Qmx
was still higher that usually observed for reference kaolinites.
The pH dependence profiles of phosphate adsorption did not exhibit the characteristic marked decrease
in adsorption at acid and at alkaline pH (Manning & Goldberg, 1996). The phosphate adsorption pH
dependence of the samples varied between the three clays and was influenced by the saturating cation and
the removal of free iron oxides.
Keywords ; phosphate, adsorption, pH, iron oxides, kaolinite, clay minerals
Mots clés : phosphate, adsorption. pH, oxydes de fer, kaolinite, argiles
References
1 Sparks, D.L. 1989 Kinetics of soil chemical processes. Academic Press, San Diego
2 Sparks. D.L. 1995. Environmental soil chemistry. Academic Press, San Diego.
3 Scheidegger, A.M., GM. Lamble and D.L. Sparks. 1997. Spectroscopic evidence for the formation of
mixed-cation hydroxide phases upon metal sorption on clays and aluminium oxides
Keywords : XAFS, AFM, kinetics, metal sorption, surface precipitation.
Mots dés : XAFS, AFM. cinélique. sorption des métaux, precipitation de surface
Enregistrement scientifique n": 607
Symposium n": 6
Presentation poster
Farming system type as a determinant factor in soil and water P
enrichment in a concentrated livestock area
If type d'exploitation agricole comme facteur determinant de
l'enrichissement en P des sols et des eaux de surface en zone de
concentration d'élevages
SIMARD Regis R- BEAUCHEMIN Suzanne et NOL1N Michel C.
Agriculture et Agroalimentaire Canada, 2560. boul Hochelaga, Sainte-Foy, Quebec. Canada, G1V 2J3. Emailsimardr(ö<em.agr.ca
The type of animal production and plants grown impact on the amounts and sources of P added to soils.
Forage production influence the distribution of soil P and pore network continuity by limiting disruption and
incorporation of added nutrients Thus, the transport of P by surface runoff and preferential flow in
subsurface drainage to surface waters may also be affected The objective of this study was to assess the
impact of three farming systems on P content of soils and waters in three soil series from the Boyer
watershed Province of Quebec, Canada The three fanning systems investigated are: D. dairy with solid
manure handling and forage-spring cereal crops, M hog-dairy mixed farms with liquid manure and foragespring cereals and H hog farms with liquid manure and com cropping. Soil series are: Kamouraska clay
(Humaquept), Saint-Nicolas and Saint-André sandy loams (Haplorthods) The A, B and C soil horizons and
ditch and tile drainage waters were sampled at three sites per farm. Soil P was characterised by Mehlich 3
(M3P) and water (P.) extractions Filtered (045 urn) and unfiltered water from ditches and drains was
analysed for molybdate reactive and total forms (Pi). The M3P was significantly larger in the 0-40 cm layer
of M and H than in D farms on the sandy loams but not on clay. The M3P content of the C horizon was not
affected by farm types The P„ content of all soil layers was affected by soil types and farm systems and was
1 5 to 5 times larger in the 60-80 cm layer of soils from the H farms than in any other types The Quebec
water quality standard of 30 ug P, L"1 was exceeded in most samples The P, content of ditch waters was the
largest on M farms on all soil types and exceeded those drainage Pt These results suggest that M farms are
particularly at risk of contributing P to surface waters Soil tillage, by breaking pore continuity and
incorporating nutrients, probably reduced ditch and drainage water P contents in hog farms
Keywords : phosphorus, drainage water, runoff, particulate P
Mots clés : phosphore, eau de drainage, ruissellement. P particulaire
Scientific registration n° : 908
Symposium n° : 6
Presentation ; oral
Assessment of isotopically exchangeable zinc in polluted and
nonpolluted soils
Determination du zinc isotopiquement ('changeable dans des sols
pollués et non pollués
SINAJ Sokrat. FROSSARD Emmanuel and MÄCHLER FÉIlx
Group of Plant Nutrition. Institute of Plant Sciences. Swiss Federal Institute of Technology, ETHZ,
Versuchsstation Eschikon, Eschikon 33, CH-8315 Lindau, Switzerland
A method for assessing the availability of trace metals for plants in soils is needed both to identify and
correct the trace metal deficiencies which are limiting crop production in many agrosystcms worldwide, and
to assess the risk of transfer of these metals in the food chain In this paper the interest of the isotope
exchange kinetic method to quantify the amount of exchangeable soil Zn was studied The study was carried
out on samples from the surface horizons (0-20 cm) of a Calcic Gleysol sampled in a permanent pasture, and
of an Haplic Luvisol sampled in the neighbourhood of a smelter According to the swiss ordinance on
dangerous substances the Haplic Luvisol can be considered as polluted Isotope exchange kinetics
experiments were carried out on soil/solution systems at a steady slate equilibrium for free Zn. using carrier
free 6^2^n2+ ^ tracer Our results showed that the concentration of free Zn in the soil solution, the ratio of
radioactivity remaining in the soil solution after 1 minute over the total added radioactivity, and the amount
of isotopically exchangeable Zn increased as the total Zn content of the soil increased The amount of
isotopically exchangeable Zn increased as the soil/solution ratio decreased probably because of the larger
accessibility of sites by 6 ^Zn2 + in the more dilute suspensions Finally the kinetic of exchange of
radioactive Zn was exactly modelled assuming the presence of an infinity of exchange sites for this ion in
the soils. This approach could be used to characterize the availability of other trace metals in soils, and could
be also used to further understand and quantify the effect of various soil conditions or plant growth on trace
metals availability
Key words: zinc, isotope exchange kinetics, steady state equilibrium, soil solution ratio
Mots clés: zinc, cinétiques d'échange isotopique, équilibre ä 1'etat stationnaire, rapport sol/solution