Meccanismi di biodisponibilità nel suolo

Gianniantonio Petruzzelli

CNR Istituto per lo Studio degli Ecosistemi,

Chimica del suolo Pisa

Bioavailability processesCommittee on Bioavailability of Contaminants in Soils and Sediments

(2003)

Il suolo

Composizione del suoloFase vivente

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)

Heavy metal reactions in soil

pH

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++

-

---

- -

---

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

Bioavailability processesCommittee on Bioavailability of Contaminants in Soils and Sediments

(2003)

Relazione tra la concentrazione di Cd nel terreno e nei vegetali

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

•Grazie per l’attenzione

Processi di adsorbimento

biodisponibilità

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.

Arsenico nella dieta di ragazzi tra 1 e 6 anni

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

Pathways for metal plant uptake

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.

Importanza delle differenti vie di esposizione(Abrahams 2002)

Trasferimento dei contaminanti dal suolo all’uomo

• Ingestione diretta di suolo• Inalazione di polveri• Contatto dermico

• Catena alimentare

10%

90%