Post on 14-Jan-2016
Nadine J. Kabengi
Measuring Surface Chemical Properties
Using Flow Adsorption Calorimetry: The Case of Amorphous Aluminum Hydroxides and Arsenic (V)
AcknowledgementsAcknowledgements(alphabetical order)(alphabetical order)
Initial Help Mrs Elizabeth Kennelly Dr. Rao Mylavarapu Mr. Joseph Nguyen
Mr. Bill Reve Dr. Jaimie Sanchez
Departmental Support Mrs. Heather Barley Mrs. Cheryl Combs Ms. Kelly Lewis Mrs. Pam Marlin Ms. An Nguyen Mrs. Laura Studstill Mrs. Joyce Taylor
Technical AssistanceDr. Chip AppelMr. Keith HollienMr. Thomas LuongoMr. Konstantinos MakrisMr. Bill Reve
Daily & Valiant FriendsDr. Chip AppelDr. Hector CastroMr. Bill ReveDr. Kanika Sharma
Committee MembersDr. Samira DaroubDr. Dean RhueDr. Nick ComerfordDr. Randy BrownDr. Willie HarrisDr. Mike Scott
All the other talented & wonderful personsI had the opportunity
to meet & interact with.
I have learned from each one of you !
Nadine J. Kabengi
Measuring Surface Chemical Properties
Using Flow Adsorption Calorimetry: The Case of Amorphous Aluminum Hydroxides and Arsenic (V)
Core Objective
was to demonstrate the application of Flow Adsorption Calorimetry as a powerful technique in probing chemical surfaces, thus obtaining information not readily accessible by other methods
developed flow calorimetry as an effective and rapid screening tool for surface studies.
Results
build a methodology template that can derive information
about the relation between surface chemical & structural properties and energetics, specificity and reversibility of surface processes.
succeeded in showing that flow adsorption calorimetry is a uniquely informative yet rapid experimental tool that can be applied to numerous application in surface chemistry studies.
in conjunction with existing technologies, Flow Adsorption Calorimetry can greatly improve our understanding of basic surfacial processes in
soil/clay systems,
This afternoon
An ILLUSTRATIVE EXAMPLE: the case of amorphous aluminum hydroxides (AHO)
and arsenic (V)
The case of AHO & Arsenic (V)
Why AHO ?
abundant in natural water and soils as high surface area minerals, mineral coatings, & colloids.
significant adsorptive properties, namely amorphous species.
Often times used as reference material for better understanding of basic processes.
The case of AHO & Arsenic (V)
Why Arsenate ?
focus of public attention & receive special attention of the scientific community
good representative of a classic inorganic oxyanion sorption (phosphate, chromate, molybdate…)
elucidate reactions mechanisms into unified model ?
Calorimetry Fundamentals
Instrumentation Several inexpensive flow calorimeters for measuring
heats of adsorption from solution onto solids were constructed in our lab.
Sensitivity and Precision High sensitivity: 10-5 ˚C Detection limit ≈ 1 mJ Low thermal drift and good signal-to-noise ratio
Interpreting a heat signal initial slope: rate of reaction peak width & shape: uniformity of surface sites energies areas under the curves: proportional to strength of
interaction
Calorimetry Fundamentals
-0.4
-0.3-0.2
-0.10
0.1
0.20.3
0.4
0 25 50 75 100 125
Time (mn)
V m
l
NO3 exotherm
Cl endotherm
20 s Heat pulse
AHO: Synthesis
precipitation of AlCl3 with NaOH to pH 6.5 - 7.
oven-dried at 60ºC, crushed and sieved through 150 m mesh
Four batches: 3 (our method) + 1 (Sims et al.)
AHO: Physical Properties
amorphous with no occluded salt.
Washed with DDI
untreated
AHO: Physical Properties
0
25
50
75
100
0 250 500 750 1000
Temp (deg C)
% w
eigh
t los
s
hydrated in nature
AHO: Physical Properties
porous in nature
AHO: Physical Properties
Batch 1 Batch 2 Batch 3 Batch 4
---------------------------------------m2 g-1----------------------------------
S.S.Aa 212 114 64 443
Table 1. Specific surface areas of the amorphous aluminum hydroxidesTable 1. Specific surface areas of the amorphous aluminum hydroxides
aa specific surface areas specific surface areas
possess high surface areas
AHO: Chemical Properties
Had 13 – 20 % Al content
High Anion Exchange Capacities : 94 to 131 cmol(+) kg-1 of solid or 198 to 264 cmol(+) kg-1 of Al(OH)3
1:6 mole ratio of (+) : Al
Working Rationale
changes in the heats and extent of ion exchange (Cl/NO3 and K/Na) BEFORE and AFTER arsenate treatment on a sample of AHO can be used as a probe of the surface and the mechanisms bywhich As(V) interacts with it.
Working Strategy
Conducted in such a way that pieces of evidence are collected through individuals experimentsand put together to offer a complete picture
Ion Exchange Properties, calorimetrically
Was rapid, reversible & reproducible over time & samples
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0 10 20 30 40 50Time (mn)
Vo
lts
Heat of exchange : 3.6 to 5.8 kJ mol-1 AEC 1.1 to 1.6 kJ mol-1 CEC
0.05
0.1
0.15
0.2
0.25
0 5 10 15 20 25 30
Time (mn)
Vo
lts
NO3 exotherm
Cl Cl endotherm
K exotherm
Na endotherm
Exhibited a ZPC around pH 9.5
AHO: ZPC determination, calorimetrically
-20
-10
0
10
20
30
40
50
60
4 5 6 7 8 9 10 11
pH
V m
lAnion
Cation
Calorimetric Determination of the Zero Point of Charge
AHO: Surface Charging, calorimetrically
2 pKa modelS—OH0 + H+ ↔ S—OH2
+ Ka1
S—O- + H+ ↔ S—OH0 Ka2
a “charge neutral” surface exists
1 pKa modelS—OH1/2- + H+ ↔ S—OH2
1/2+ KH
neutral surface when # of (+) = # of (-)“charge neutral” surface not possible
AHO: Surface Charging, calorimetrically
-20
-10
0
10
20
30
40
50
60
4 5 6 7 8 9 10 11
pH
V m
l
Anion
Cation
Was consistent with a 2pka model of surface charging based on the existence of the neutral species.
Ion Exchange “Other” Properties
The “Flip-Flop” effect K exotherm & Na endotherm pH 8.0: shift in sign K endotherm & Na endotherm return to original signs at pH 10.5
The two cases of surface behavior toward ion exchange weak field: surface charge beneath surface
energy of exchange hydrated radius strong field: surface charge near surface
energy of exchange ionic radius
Ion Exchange “Other” Properties
Suggestions
related to geometrical distribution of charge & charge
same charge density: spherical point charge 8 × stronger field than a distributed smear
Arsenate Sorption Properties
Was exothermic with majority of heats of adsorption between 40 to 60 kJ mole1- sorbed arsenate
a different peak shape than anion exchange indicating a kinetically different reactions
Was much slower reaction that ion exchange
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0 10 20 30 40 50
Time
Vo
lts K exotherm
Na endotherm
As slow burn
Arsenate Sorption Properties
B
0
0.1
0.2
0.3
0 15 30 45 60
Time (mn)
Vol
ts
C
-0.05
0.05
0.15
0.25
0 15 30 45 60
Time (mn)
Vol
ts
D
-0.1
0
0.1
0.2
0 15 30 45 60
Time (mn)
Vol
ts
A
0.1
0.2
0.3
0.4
0 15 30 45 60
Time (mn)
Vol
ts As exotherm
Cl endotherm
Reactive surface are regenerated: spatial rearrangement, diffusion along the surface to less accessible sites or into the interior.
Arsenate Sorption Properties
Molar Al:As ratios were always lower than Al:Clex ratio (6:1) indicating that the AHO maximum sorption
capacity was not satisfied.
Minimum Maximum
As Al:As As Al:As
g g-1 mole ratio g g-1 mole ratio
Batch 1 6,000 24.50 31,000 38.70
Batch 2 10,200 35.30 39,000 13.90
Batch 3 11,700 36.40 67,300 8.29
Batch 4 22,200 24.20 --a --
Table 2. Arsenate loadings and corresponding Al:As mole ratios
a not available
Heats of adsorption decreased with increasing As surface coverage (decreasing Al:As mole ratios)
Arsenate Sorption Properties
H As sorbed Al:As
Column name kJ mol -1 g g-1 mole ratio
Col 3 B1 63.5 6,920 44.98
Col 8 B1 37.4 15,536 22.0
Col 11 B1 18.0 21,053 17.93
Col 17 B2 48.9 10,178 35.51
Col 25 B2 15.9 11,030 37.54
Col 26 B2 6.8 39,142 13.86
Col 11 B3 33.0 11,667 36.35
Col 14 B3 6.2 39,656 14.99
Col 15 B3 4.7 67,290 8.29
Table 3. H values, amounts of sorbed arsenate and Al:As mole ratios.
Table 4. Effect of arsenate sorption on pH of solution
Arsenate Sorption Properties
AHO weight pH values
in mg Initial after 5 mn after 2 days
Batch 1 17.1 (1.27)a 5.93 (0.14) 7.05 (0.07) --b
Batch 2 6.60 (0.36) 5.37 (0.10) 5.98 (0.24) 4.80 (0.28)
Batch 3 4.27 (0.31) 5.48 (0.10) 6.23 (0.41) 4.81 (0.29)
Batch 4 1.77 (0.55) 5.03 (0.07) 4.35 (0.42) 4.30 (0.04)
Arsenate sorption resulted in OH- release followed by H+
aa number in parenthesis are standards deviations of the means number in parenthesis are standards deviations of the meansbb not measured at the time of the experiment not measured at the time of the experiment
Effects of Arsenate Sorption: on AEC
Loss in heats of exchange and AEC
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 15 30 45 60
Time (mn)
Vo
lts
before after
y = 1.10x
R2 = 0.54
0
0.25
0.5
0.75
1
1.25
0 0.25 0.5 0.75 1
Remaining fractional heat
Re
ma
inin
g f
rac
tio
na
l A
EC
Energetics of Cl/NO3 exchange (kJ/mol(+)) is not affected by sorbed arsenate
Effects of Arsenate Sorption: on AEC
Effects of Arsenate Sorption
1 mole of As sorbed eliminated about 1.61 mole of anion exchange
0
5
10
15
20
25
0 2 4 6 8 10 12
As mmoles
Lo
st
AE
C m
mo
les
(+)
2:1 line
1:1 line
Effects of Arsenate Sorption
it is easy to account for 1:1 mole ratio loss stoichiometry
—SOH2]1+ + H2AsO4- ↔ —S--H2AsO4]0 + OH2 monodentate
—(SOH2+)2 + H2AsO4
- ↔ —(S—OAsOH)2]1+ + 2H2O bidentate
to account for the 2:1 mole ratio loss stoichiometry, with OH- release & lack of negative charge conferred:
polydentate, namely tridendate ?!
Effects of Arsenate Sorption: on CEC
B2
0
0.1
0.2
0.3
0.4
0 15 30 45Time (mn)
Vol
tsK exotherm
Na endotherm
As does not confer any negative charge to the surface calorimeter detection limit is < 0.5 mol (+)
Effects of Arsenate Sorption: on CEC
EXCEPT: very high loadings.
As As sorbed Al:As CEC
Column g g-1 mol mole ratio cmolc Kg
1B4 22,200 7.80 37.70 0
12B1 6,000 1.20 69.9 0
14 B3 39,700 8.50 6.19 1.97
15 B3 67,300 13.90 4.69 3.98
Table 5. Comparisons between samples that showed an increase in CEC after As exposure and samples that did not.
Effects of Arsenate Sorption: on ZPC
IN A FLOW SYSTEM, the ZPC shifts by up to 1 pH unit
-40
-20
0
20
40
60
4 5 6 7 8 9 10 11
pH
V m
l AEC
CEC
IN A BATCH SYSTEM, the ZPC shifts by up to 4 units
Effects of Arsenate Sorption: on ZPC
-20
-15
-10
-5
0
5
10
15
20
4 5 6 7 8 9 10 11
pHV
ml
AEC
CEC
Effects of Arsenate Sorption: on ZPC
PZC shift As sorbed K/Na peak areas in V/ml after As Final CEC
Column in pH units g g-1 5.75 8.0 10.5 cmol(-) kg
flow 0.4 11,700 0 0.65 8.65 3.67
batch 3.9 25,800 1.80 3.06 14.3 12.90
Table 6. Comparisons in ZPC shifts and other data of B3 samples arsenated in flow and in batch.
sorbed more arsenate
measurable heat of CEC at pH 5.75 & bigger peaks at pHs 8.0 & 10.5
had almost 4 times more CEC.
Differences in arsenate coverage and its effect on surface charge
ZPC shifts: explained
Column Description
PZC
3B3 9B3
PZC after As
10B3 11B3
Al content in % 14.4 15.5 15.6 15
As sorbed in mmoles 0 n.A n.A 2.33
Cl/NO3 peak in V ml
initial 56.40 65.30 62.30 64.20
after As -- 32.20 42.70 38.20
pH 8.0 19.0 7.38 4.86 8.8
pH 10.5 0 0 0 0
K/Na peak in V ml
initial 0 0 0 0
after As -- 0 0 0
pH 8.0 0 0.93 0.23 0.65
pH 10.5 7.62 6.30 3.05 8.65
PZC 9.5 8.8 8.6 9
Final CEC in cmol (-) kg-1 2.47 6.69 8.43 3.67
By measuring ZPC on clean & arsenated samples (refer to previous table)
As sorption did not confer a negative charge
but it caused a measurable shift in ZPC
shift is caused by greater drop in AEC & greater increase in CEC as pH is raised
arsenated samples generated more CEC at pH 10.5 with fewer sites
contrast with generally accepted view that shift is caused by negative charge from As.
ZPC shifts: explained
ZPC shifts: explained
the K value is manifested through the magnitude of the heats of Cl/NO3 exchange.
a reduction in size of peak areas, upon increase in pH, is an indication of a decrease in the number of protonated surface sites
if pK=6 at pH = 6 50 % of SOH2
+ deprotonates to SOH0
vs pH = 4 100 % are protonated
Calorimetrically: as a loss of ½ of the AEC at pH 6
ZPC shifts: explained
Table 8. Reductions in Cl/NO3 peak areas with increase in solution pH for clean samples samples
Cl/NO3 peak areas in V ml Reduction
Columns pH 5.75 pH 7.25 pH 8.0 in %
Batch 3
3B3 56.40 19.0 66.30
16B3 48.40 15.80 66.70
5B3 59.60 22.60 62.20
7B3 51.90 16.2 68.80
a change in the fractional reduction in AEC can be interpreted as a change in pK.
the decrease in AEC peak areas as pH is raised was consistently uniform.
ZPC shifts: explained
Reductions in Cl/NO3 peak areas in %
Columns clean arsenated
Batch 3
3B3 66.30
9B3 77.0
10B3 88.60
11B3 77.0
Table 9. Reductions in Cl/NO3 peak areas with increase in solution pH from 5.75 to 8.0 for arsenated samples
Arsenated samples had higher reduction in AEC peak areas upon exposure to pH 8.0
SOH2+ become more acidic, losing a proton quicker
Change in pK could:
Explain a ZPC shift in absence of increase in surface negative charge
ZPC shifts: change in pK
Explain higher CEC at pH 10.5 with less reactive groups (&/or adsorbed arsenate deprotonates creating new negative sites. Need to partition between reactive SO- groups and adsorbed arsenate)
Account for a stoichiometry > 1:1 between AEC lost and As sorbed.
Effects of Arsenate Sorption
Possible mechanism for shifting the pKa:
electronegative As attracts electrons away from surface.
sites becomes more reactive towards arsenate
neutralize higher number of sites
Suggestions: on structure & morphology of AHO
AHO OPEN STRUCTURE
cotton like
formed of strands of AHO polymer, twisted and folded
no external surface per se, network of pores & conduit
reactive functional groups are dispersed throughout
loose and hydrated, permeable to hydrated ions
Suggestions: on AHO surface chemistry
FOR AHO:
necessary information (charge distribution, coordination
environment and neighboring sites) difficult to obtain
resolution of experimental data, rather than prepackaged model
must allow existence of neutral species (in a way or another)
Suggestions: on Arsenate sorption
Sorption of Arsenate on AHO can be interpreted in terms of physical and chemical processes
initial uptake phase: ligand exchange with aquo and hydroxo groups
Al—OH2]1+ + H2AsO4- ↔ Al—H2AsO4]0 + OH2
Al—OH]0 + H2AsO4- ↔ Al—H2AsO4]0 + OH-
reaction progresses: access to less accessible reactive sites not classical diffusion vs rapid anion exchange
regeneration of sites: spatial rearrangement, changes in physical structure. Entropy driven or very slow
at higher fractional saturation: change in mechanism, ol and oxo groups are attacked. CEC formed. AHO breaks up. New As/AHO solid. Energy consuming.
Suggestions: on Arsenate sorption
AlAl
AlAl
OHOH0 0 + H+ H22AsOAsO44--
Al—HAl—H22AsOAsO441/2-1/2-
Al—OHAl—OH1/2-1/2-
Wrapping up
By exposing the nature of the information accessible, I hope I have demonstrated the application of Flow
Adsorption Calorimetry as a powerful technique in probing and understanding chemical surfaces.
Thank YouThank You