Surface Complexations of Phosphate
Adsorption by Iron Oxide
Talal Almeelbi
Outline
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Introduction
Surface Complexation Reactions
Surface Complexation Model Principles
Case Study
Phosphate-NZVI Modeling
Summary
Why P and Fe?
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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
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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
SO H + (M
2+
) aq  SO H (M
SO H + (M
2+
) aq  SO M  H
2 SO H + (M
2+
+
2+
0
inner-sphere complex
+
) aq  (SO ) 2 M  2 H
Pierre Glynn, USGS, March 2003
outer-sphere complex
) aq
+
bidentate inner-sphere complex
Surface Complexation Reactions
For all surface reactions:
 G total   G intrinsic   G coulom bic   G intrinsic   Z F 
0
K
app
0
 K
 G coulom bic
int
0
  ZF  
exp 

RT


0
Electrostatic or coulombic
correction factor
0
is variable and represents the electrostatic work needed to transport
species through the interfacial potential gradient.
Kint strictly represents the chemical bonding reaction.
Surface Complexation Model
Principles
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Sorption on oxides takes place at specific sites.
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Sorption reactions on oxides can be described quantitatively
via mass law equations.
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Surface charge results from the sorption reaction themselves.
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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?
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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
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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
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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
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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
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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.
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(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
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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
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PO4-3 Recovery using NZVI
 99%
removal of PO4-3
 80% recovery
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Idea: to use SCM to describe NZVI-phosphate
sorption reactions n aqueous solutions using
data from my research.
The Model – Input
Initial Species
Hfo_sO6%
Hfo_sOH2+
1%
Hfo_sPO4H1%
Hfo_sOH
19%
Hfo_sOFe+
73%
The Model- Output
30
20
10
0
SI
Goethite
-10
-20
-30
-40
-50
Fe(OH)3(a)
H2(g)
H2O(g)
Hematite
Fe2O3
O2(g)
Vivianite
Fe3(PO4)2:8H2O
Summary
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The concept of SCM was applied to Fe- PO4-3
reactions.
PHREEQC modeling results: ERROR!
Problem:
References
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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