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Meccanismi di biodisponibilità nel suolo
Gianniantonio Petruzzelli
CNR Istituto per lo Studio degli Ecosistemi,
Chimica del suolo Pisa
Soil characteristics that determine contaminant distribution in the soil phases
• pH• Clay minerals• Organic matter• Cation exchange capacity• Fe and Mn oxides and
hydroxides• Redox potential• Temperature• Ionic strength
• The living phase(1g soil=10 billion organisms)
Clay minerals
• Ion exchange and specific adsorption are the mechanisms by which clay minerals adsorb metal ions through the adsorption of hydroxyl ions followed by attachment of the metal ion to the clay by linking to the adsorbed hydroxyl ions or directly to sites created by proton removal.
•• Highly selective sorption occurs at
the mineral edges, but noteworthy differences exist among clay minerals in the ability to retain heavy metals which are more strongly adsorbed by kaolinite than montmorillonite probably due to a higher amount of weakly acidic edge sites on kaolinite surfaces
Clay mineralClay mineral
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--
-- --
-- --
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MeMe
OOHH
OO
HHHH
OOHH
VAN DER WAALSVAN DER WAALS
Electrostatic Electrostatic linkageslinkages
Complexes between organic matter and heavy metals
• a) organic matter of high molecular weight containing aromatic nucleicondensed in polymer complexes,which have a very high affinity for heavy metals and are for the most part insoluble
• b) organic matter of low molecular weight of recent origin, essentially deriving from the break-up ofmicrobial cells, roots, etc.. They represent the primary units for theformation of humic composts of ahigher molecular weight andgenerally exhibit high solubility
• c) soluble organic matter, which forms insoluble salts reacting withthe metals.
Cation Exchange Capacity(CEC)
The total number of exchangeable cations a soil can hold(amount of its negative charge)
Negatively Charged ColloidsAttract Cations
K+
Pb++
Na+
Ca++
H+
Mg++
-
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- -
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Soil Colloid
Pools of heavy metals in soil
v Simple or complexed ions in soil solution
v Exchangeable ions
v Complexed ions (i.e. organic matter)
v Occluded or cooprecipitated with oxides, carbonates and phosphates or other secondary minerals
v Ions in the crystalline lattice of primary minerals
La biodisponibilità nel suolo
I complessi processi di trasferimento di massa e di uptake dei contaminanti negli organismi viventi del suolo sono determinati da:
Proprietà chimico fisiche dei contaminanti
Proprietà del suolo
Biologia dell’organismo interessato
conclusioni• L’influenza che le caratteristiche del suolo hanno sulla
distribuzione del contaminante tra le fasi, il trasporto verso gli organismi viventi, l’accumulo nell’organismo, ed i possibili effetti tossici determinano i “ processi di biodisponibilità”
• Comprendere questi processi, è la chiave di volta per chiarire il legame tra la qualità del suolo e la salute, migliorare le procedure di risk assessment, e scegliere le migliori strategie di bonifica.
Criticità associate ai processi di biodisponibilità nel suolo
• Ridotta conoscenza di come i processi chimici fisici e biologici agiscono nel suolo sulle reazioni di assorbimento e rilascio dei contaminanti
• Variabilità spaziale del suolo• Mancanza di conoscenza di come gli organismi
viventi possono modificare la biodisponibilità• Possibile variazione della forma chimica dei
contaminanti in seguito a potenziali cambiamenti che possono avvenire nel suolo
• Determinazione della biodisponibilità
Measuring bioavailabilityThe only direct way of measuring bioavailability is the use
of the target organism (Bioavailabilitytarget organism).
Bioavailabilitytarget organism can be approximated by:• a biological test bioavailability bio
• by chemical test bioavailability chem
in which the chemical or biological processes are allowed to work in the soil for a specified period of time
• Bioavailability t.o = f1(bioavailability bio) = f2 (bioavailability chem)
Biological tests and chemical tests have to be considered as a tool to give indices of bioavailability to a target organism, both Bioavailabilitybio and Bioavailabilitychem should not be taken literally as ‘bioavailability’.
Bioavailability as a tool in remediation strategies
•Improving the risk assessment procedures
Inserting bioavailable values instead of total•Selecting appropriate remediation technologies,
Bioavailability evaluation is an essential step in the treatability tests. •Risk associated with residual contamination
More attention can be given to impacts on soil quality
Bioavailability at contaminated sites
Bioavailability describes the complex processes of mass transfer and uptake of contaminants into soil-living organisms which are determined by
substance properties,
soil properties,
the biology of organisms
The bioavailable contaminant fraction in soil represents therelevant exposure concentration for soil organisms.
Contributo percentuale di diversi alimenti all’asssunzione di metalli pesanti
1211473170450Pb
2132603050Cd
6165281700015000Zn
Acqua potabile
%
Pesce%
Carne%
Vegetali%
Valori osservati µg/d
Dose massima
µg/d
elemento
Concluding remarksConcluding remarks• While the term “bioavailability” is relatively new, bioavailability
as a concept has a long history in toxicology, pharmacology, crop science, and nutritional science. Common to all of these contexts is uptake by living organisms
• In contrast, the consideration of bioavailability process in theenvironmental field has occurred much more recently, largely within the last decade, and it involves such contextual issues as solubility, mass transfer, mobility, and reaction in addition to uptake by living organisms
• The influences that soils have on contaminant interactions between phases, the transport of contaminants to organisms, the entry of contaminants into living cells, and contaminant accumulation within organisms and possible toxic effects determine the “bioavailability processes.”
• Understanding these processes is central to improving risk assessment, prioritizing among various remediation strategies, and using resources to achieve the greatest benefit.