Talal Almeelbi

Surface Complexations of Phosphate Adsorption by Iron Oxide

Outline

Introduction Surface Complexation Reactions Surface Complexation Model Principles Case Study Phosphate-NZVI Modeling Summary

Why P and Fe?

Iron Oxides present in soils, Sediments, aquatic systems, and minerals.

Phosphate resources are rapidly depleting Excess phosphate in water is undesirable Need statement: An efficient method for

phosphate removal and recovery.

Introduction

Distribution Coefficient

Limitations : Fails to describe reactive transport Need for a new concept to describe the chemical

interaction between solid-liquid interface.

Surface Complexation Reactions

2+ 2+

2+ + +

2+ 0 +2

SOH + (M ) SOH(M )

SOH + (M ) SOM H

2 SOH + (M ) ( SO) M 2H

aq aq

aq

aq

outer-sphere complex

inner-sphere complex

bidentate inner-sphere complex

Pierre Glynn, USGS, March 2003

Surface Complexation Reactions

For all surface reactions:0 0 0 0

exp

total intrinsic coulombic intrinsic

app int

G G G G ZF

ZFK K

RT

is variable and represents the electrostatic work needed to transport

species through the interfacial potential gradient.

Kint strictly represents the chemical bonding reaction.

0coulombicG

Electrostatic or coulombic correction factor

Surface Complexation Model Principles

Sorption on oxides takes place at specific sites.

Sorption reactions on oxides can be described quantitatively

via mass law equations.

Surface charge results from the sorption reaction

themselves.

The effect of surface charge on sorption can be taken into

account by applying a correction factor derived from EDL

theory to mass law constants for surface reactions.David A. Dzombak, François Morel,(1990), Surface complexation modeling: hydrous ferric oxide, Wiley-Interscience. 

Why SCM?

To determine the chemical and electrostatic forces involved in ion retention

To provide a framework that allows such processes to be modeled

To improve problem solving

Case Study

Spiteri et al., (2008), Surface complexation effects on phosphate adsorption to ferric iron oxyhydroxides along pH and salinity gradients in estuaries and coastal aquifers, Geochimica et Cosmochimica Acta 72: 3431–3445

Case Study

SCM - to describe the adsorption of phosphate on the iron oxide goethite, along the transition from freshwater to seawater in surface and subterranean mixing regimes.

The SCM is coupled with a 2D groundwater flow model to explore the effect of saltwater intrusion on phosphate mobilization in a coastal aquifer setting

Case Study – Modeling

The SCM describes the adsorption of phosphate on goethite (FeO(OH)), the most common and stable crystalline iron (hydr)oxide in soils and sediments

Case Study – Modeling

Total phosphorus

Total number of surface cites

Case Study- Modeling

Case Study – Result

Conclusion

Phosphate adsorption on minerals in aquatic environments reflects the interaction

the mineral surfaces and in solution, and the chemical interactions leading to the

formation of aqueous and surface complexes.

(SCM) describing phosphate binding to goethite is the first step in unraveling how

this interplay controls the dissolved phosphate levels in surface and subsurface

estuaries

Phosphate adsorption and desorption behavior in surface and subterranean

estuaries is different, due to difference in salinity-pH relationships in both settings,

but also because the sorbing phase, which is transported with the flow in surface

estuaries, is part of the solid matrix in a groundwater system.

SCM for Fe- PO4-3 Adsorption

PO4-3 Recovery using NZVI

99% removal of PO4-3

80% recovery Idea: to use SCM to describe NZVI-phosphate

sorption reactions n aqueous solutions using data from my research.

The Model – Input

The Model- Output

Fe3(PO4)2:8H2OFe2O3

Summary

The concept of SCM was applied to Fe- PO4-3

reactions. PHREEQC modeling results: ERROR! Problem:

References Arai and Sparks, (2001), Journal of Colloid and Interface Science

241: 317–326 Elzinga and Sparks, (2007), Journal of Colloid and Interface

Science 308: 53–70 David A. Dzombak, François Morel,(1990), hydrous ferric oxide,

Wiley-Interscience.  Spiteri et al., (2008), Surface complexation effects on phosphate

adsorption to ferric iron oxyhydroxides along pH and salinity gradients in estuaries and coastal aquifers, Geochimica et Cosmochimica Acta 72: 3431–3445

Pierre Glynn, (2003) USGS, Available online, http://www.ndsu.edu/pubweb/~sainieid/geochem/PHREEQCi-course-notes/phreeqci-sorption&kinetics/( accessed Dec. 2010. )

Thank you

Q&A