Nadine J. Kabengi Measuring Surface Chemical Properties Using Flow Adsorption Calorimetry: The Case of Amorphous Aluminum Hydroxides and Arsenic ( V )

Acknowledgements (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 Assistance Dr. Chip Appel Mr. Keith Hollien Mr. Thomas Luongo Mr. Konstantinos Makris Mr. Bill Reve

Daily & Valiant Friends Dr. Chip Appel Dr. Hector Castro Mr. Bill Reve Dr. Kanika Sharma

Committee Members Dr. Samira Daroub Dr. Dean Rhue Dr. Nick Comerford Dr. Randy Brown Dr. Willie Harris Dr. Mike Scott

All the other talented & wonderful persons I 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: ˚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 NO 3 exotherm Cl endotherm 20 s Heat pulse

AHO: Synthesis precipitation of AlCl 3 with NaOH to pH 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 hydrated in nature

AHO: Physical Properties porous in nature

AHO: Physical Properties Batch 1Batch 2Batch 3Batch m 2 g S.S.A a Table 1. Specific surface areas of the amorphous aluminum hydroxides a 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/NO 3 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 by which As( V ) interacts with it.

Working Strategy Conducted in such a way that pieces of evidence are collected through individuals experiments and put together to offer a complete picture

Ion Exchange Properties, calorimetrically Was rapid, reversible & reproducible over time & samples Heat of exchange : 3.6 to 5.8 kJ mol -1 AEC 1.1 to 1.6 kJ mol -1 CEC NO 3 exotherm Cl Cl endotherm K exotherm Na endotherm

Exhibited a ZPC around pH 9.5 AHO: ZPC determination, calorimetrically Calorimetric Determination of the Zero Point of Charge

AHO: Surface Charging, calorimetrically 2 pKa model S—OH 0 + H + ↔ S—OH 2 + Ka 1 S—O - + H + ↔ S—OH 0 Ka 2 a “charge neutral” surface exists 1 pKa model S—OH 1/2- + H + ↔ S—OH 2 1/2+ K H neutral surface when # of (+) = # of (-) “charge neutral” surface not possible

AHO: Surface Charging, calorimetrically 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 mole 1- sorbed arsenate a different peak shape than anion exchange indicating a kinetically different reactions Was much slower reaction that ion exchange

Arsenate Sorption Properties 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:Cl ex ratio (6:1) indicating that the AHO maximum sorption capacity was not satisfied. MinimumMaximum AsAl:AsAsAl:As  g g -1 mole ratio  g g -1 mole ratio Batch 16, , Batch 210, , Batch 311, , Batch 422, 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H As sorbedAl:As Column namekJ mol -1  g g -1 mole ratio Col 3 B163.56, Col 8 B , Col 11 B , Col 17 B , Col 25 B , Col 26 B26.839, Col 11 B , Col 14 B36.239, Col 15 B34.767, 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 weightpH values in mgInitialafter 5 mnafter 2 days Batch (1.27) a 5.93 (0.14) 7.05 (0.07) -- b Batch (0.36) 5.37 (0.10) 5.98 (0.24) 4.80 (0.28) Batch (0.31) 5.48 (0.10) 6.23 (0.41) 4.81 (0.29) Batch (0.55) 5.03 (0.07) 4.35 (0.42) 4.30 (0.04) Arsenate sorption resulted in OH - release followed by H + a number in parenthesis are standards deviations of the means b not measured at the time of the experiment

Effects of Arsenate Sorption: on AEC Loss in heats of exchange and AEC before after

Energetics of Cl/NO 3 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 2:1 line 1:1 line

Effects of Arsenate Sorption it is easy to account for 1:1 mole ratio loss stoichiometry —SOH 2 ] 1+ + H 2 AsO 4 - ↔ —S--H 2 AsO 4 ] 0 + OH 2 monodentate —(SOH 2 + ) 2 + H 2 AsO 4 - ↔ —(S—OAsOH) 2 ] H 2 O 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 K 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. AsAs sorbedAl:AsCEC Column  g g -1  mol mole ratiocmol c Kg 1B422, B16, B339, B367, 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

IN A BATCH SYSTEM, the ZPC shifts by up to 4 units Effects of Arsenate Sorption: on ZPC

PZC shiftAs sorbed K/Na peak areas in V/ml after As Final CEC Columnin pH units  g g cmol (-) kg flow0.411, batch3.925, 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 10B311B3 Al content in % As sorbed in mmoles 0n.A 2.33 Cl/NO 3 peak in V ml initial after As pH pH K/Na peak in V ml initial0000 after As--000 pH pH PZC Final CEC in cmol (-) kg

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

the K value is manifested through the magnitude of the heats of Cl/NO 3 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 SOH 2 + deprotonates to SOH 0 vs pH = % are protonated Calorimetrically: as a loss of ½ of the AEC at pH 6

ZPC shifts: explained Table 8. Reductions in Cl/NO 3 peak areas with increase in solution pH for clean samples samples Cl/NO 3 peak areas in V mlReduction ColumnspH 5.75pH 7.25pH 8.0in % Batch 3 3B B B B 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/NO 3 peak areas in % Columnscleanarsenated Batch 3 3B B B B377.0 Table 9. Reductions in Cl/NO 3 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 SOH 2 + 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—OH 2 ] 1+ + H 2 AsO 4 - ↔ Al—H 2 AsO 4 ] 0 + OH 2 Al—OH] 0 + H 2 AsO 4 - ↔ Al—H 2 AsO 4 ] 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 sorptionAlAl OH 0 + H 2 AsO 4 -  Al—H 2 AsO 4 1/2- Al—OH 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 You