Fe and associated As(V) reactivity in wetland soil : Kinetic modelling approach Mélanie Davranche, Aline Dia, Mohamad Fakih, Bernd Nowack, Guillaume Morin, and Gérard Gruau EMEC 2010

Why focusing effort on Fe (III)-oxyhydroxides behaviour understanding? Key factor on (bio)geochemical process controlling trace metal distribution in soils and waters Ubiquitous in soils, sediments, geological materials Major trace elements carrier Fe Redox behaviour controls trace element mobility Study focus : Reductive dissolution in wetland soil system

Fe oxide reactivity upon dissolution depend on: crystal structure crystal size distribution. All modifications  changes of the reaction kinetic and subsequent reactivity Kinetic : Alternative to the classical chemical extractions or thermodynamic methods.  Kinetic : Alternative to the classical chemical extractions or thermodynamic methods. Why studying Fe reactivity with a kinetics approaches?

Methodological procedure Fe-oxides fixed on a slide system  Quantitative dissolution analyses (XRF)  Mineralogical analysis (DRX, EXAFS) Fe-oxides : Fe-oxides : ferrihydrite and lepidocrocite  Different crystallinity  Contrasted surface areas Associated element : Arsenic (V)  Potentially toxic  Redox sensitive 2 cm

Methodological procedure Experimental insight :Anoxic Incubation in equilibrium soil column - Soil sample : organo-minral horizon of the Naizin Kervidy wetland soil - Soil Solution analysis : pH, Eh, As(V)*, As(III)*, Fe(II), NO3, SO4, acetate Peristaltic pump Synthetic solution Soil 0.6 mol.L -1 Slide+ Fe-oxides Stirrer

References experiments As-ferrihydrite: As-Fh (Bacteria) (Autochtonous bacteria), (Burnol at al., 2007) As-Ferrihydrite : As-Fh(Ascorbate) (Ascorbic acid) Ferrihydrite : Fh(Ascorbic acid and S. Putrefaciens) (Roden, 2006) As-lepidocrocite: As-Lep (Ascorbic acid) Lepidocrocite : Lep (S. Putrefaciens)

Kinetic framework Non linear least-square regression γ and k’ (Postma, 1993) Generalized rate law As-Fh As-Lp Applied to mineral dissolution As-Lp:As-Fh: γ Undissolved mineral fraction Rate of dissolution Depends on morphology, size distribution and reactive site density of the oxide during dissolution Rate constant γ =γ = k’ =

NO 3 - , Fe(II) , SO 4 2- , and Acetate  Typical redox evolution of waterlogged soils Time (hours) Acetate (mg L -1 ) Fe(II), Fe(tot), NO 3 - and SO 4 2- (mg L -1 ) Reductive dissolution 

‘Slide’ system macroscopic observations Progressive dissolution of Fe-oxides stuck onto slides Reduction 1 week months Time

SEM observations Bacterial colonization Thick biofilms Surface alteration Diversity of newly formed minerals

not total Fe dissolution with bacteria same initial dissolution rate with bacteria Intermediary Reductive dissolution in soil  limited Fe decreasing reactivity Ferrihydrite kinetical modeling

Lepidocrocite kinetical modeling Total As dissolution whatever the reducing agent lower initial dissolution rate in soil Intermediary Reductive dissolution in soil  limited As decreasing reactivity

As kinetical modeling

Discussion Ferrihydrite: Fe decreasing dissolution rate with bacteria: - Fe(II) readsorption - Secondary mineral precipitation In soil : Fe(II) complexed by dissolved organic matter and soil mineral  Limited Fe(II) readsorption and subsequent newly mineral formation

Fe(II) bound to dissolved organic matter

Discussion Lepidocrocite : Lower Fe decreasing dissolution rate than Fe from ferrihydrite: - stronger solubility - lower Fe(II) readsorption (lower surface aera) and subsequent secondary mineral precipitation In soil : lower initial rate dissolution : soil bacteria consortium

Discussion As: As and Fe have closed dissolution rate :  Fe reactivity control in part As reactivity As stronger solubilized from lepidocrocite than Ferrihydrite Ferrihydrite : As lesser solubilized than Fe As readsorbed on newly formed mineral

Conclusion Kinetic modeling relaible to predict coprecipitated As reactivity from Fe-oxide dissolution In wetland soil : organic matter controlled Fe reactivity from ferrihydrite dissolution As reactivity from ferrihydrite dissolution In wetland soil : As readsorption on secondary phases (hygher for ferrihydrite than lepidocrocite)  Are Wetland soils source of As for hydrosystems ?

Newly formed phases New Fe- and S-rich minerals evidenced: iron sulphides? None other minerals formed in simple experimental system. - Ex. green rusts, magnetite, vivianite or siderite Blocking Fe-oxide surface sites by adsorption Complexing Fe(II)  preventing re-adsorption and re-precipitation onto Fe-oxides Key control of organic oxy-anions (such as acetate >240 mg L -1 )

Conclusions Important bacterial colonization and biofilms occurrence  biologically-mediated processes Two dissolution ways : Fh -2D & Lp-3D Dissolution rates remained fairly constant through time : Fe(II)-MO complexes  Prevention of Fe(II) adsorption and hygh Fe phases precipitation Secondary minerals (Iron sulphides ?), Arsenic behaviour: As(V)  As(III) => Bacterial reduction  Fh  Re-adsorption  Lp  Destruction of adsorption sites  Release or adsorption onto other soil minerals