Nadine J. Kabengi Measuring Surface Chemical Properties Using Flow Adsorption Calorimetry: The Case...

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


0

25

50

75

100

0 250 500 750 1000

Temp (deg C)

% w

eigh

t los

s

hydrated in nature


porous in nature


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


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.


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)


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


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


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


-20

-15

-10

-5

0

5

10

15

20

4 5 6 7 8 9 10 11

pHV

ml

AEC

CEC


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.



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


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.


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.


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

Nadine J. Kabengi Measuring Surface Chemical Properties Using Flow Adsorption Calorimetry: The Case...

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