Speciation of Th(IV) in marine systems

Peter H. SantschiLaboratory for Oceanographic and Environmental

Research (LOER)

Depts. Of Oceanography and Marine Sciences

Texas A&M University

Galveston, TX

Outline

• Introduction: Chem. Ocng.• vs. Mar. Chem. • ThO2 solubility• Solution and surface

speciation: inorganic• Solution and surface

speciation: organic• Uniqueness of sticky

macromolecular ligand• Relationship to POC/234Th

ratio

Family of marine sticky spiderweb-like ligands with fractal and amphiphilic properties, persistenceLengths of 100s nm, contour lengthsof 100s - 1000s of nm and thickness of 1-2 nm

TEM picture of stained marine colloids in nanoplast (Santschi et al., 1998).

500 nm

Th(IV) as an oceanographic tracer( at [Th] ≤ 10-12 M)

• New Production by [POC/234Thp]*234Th-flux• Particle and Coagulation residence times• Colloidal Pumping• Particle sources• Deep water ventilation rates• Boundary scavenging

YET, WE TAKE TH(IV) SPECIATION FOR GRANTED

Myths about Th(IV)

• Reversibility of adsorption to solids at the molecular level.

• Silica and Carbonates as major adsorbents.• Th(IV) sorbs to almost anything, and thus, works

well as an oceanographic tracer. • Therefore, we do not need to know more about its

marine chemistry.

Particle-water partition coefficients, Kd:

Kd = fp/{(1-fp)Cp, with fp=fraction on particles, and Cp=particle

concentration; Kd {ml/g or L/kg}Sorbent Log Kd

- Th Log Kd - PaClean Unclean Clean Unclean

Chitin 1.7 3.5 - 3.7SiO2 2.5 4 2.7** 5CaCO3 2.7-3.7 5.6 - 3.7FeOOH 5.1 5.8 - 5.2MnO2 4.5 6.1 4.4** 6.2COM-bulk 5.0 - 6.8 - - -COM-100%PS 7.8 - - -APS(Carageenen) 7.8 8 - 7.5** Innoue et. al. (1978)

2)1) 2)

2) Simulating natural conditions; Guo et al., 2002.1) Abstracting natural conditions; Quigley et al., 2002, and references therein;

=> 1) ∆ clean/unclean; 2) Kd(Th(IV))~Kd(Pa(IV,V)); Kd(PS) is max.

Predicted fp for Th(IV) using lab-based Kd & field Cp

(compared to observed fp = 0.1 - 0.5)*

Mineral ororganic Phase

Log Kd(L/kg)

Reference* -Log Cp(kg/L)

Fp(predicted)

SiO2 3-5 Our data 8 <10-3

CaCO3 4-5 1,2 8 <10-3

Al2O3/Clays 5.6 3 9 <10-3

FeOOH 5.1-5.8 2,4 9 <10-3

MnO2 4.4 2,5 9 <10-4

EPS@100% PS 8 6 8 0.5*References:1. Edwards et al., 1987; Cochran, 1992. 2. Guo et al., 2002.3. Niven and Moore, 1993. 4. Quigley et al., 1996.5. Hunter et al., 1988. 6. Quigley et al., 2002.

*) fp = Kd*Cp/(1+Kd*Cp)

What do we need to know about Th(IV) speciation?

• Because of relatively low abundance of Fe-oxides and medium to low Kd values for more abundant SiO2 and CaCO3, we need to concentrate more on surface speciation/sorption to COM/POM than to inorganics.

• Likely organic phases need more focus, especially the acid polysaccharide-rich rigid exopolymers aggregated as “marine spiderwebs” acting as “sticky ligands”.

Fibrillar exopolymers (“TEP”) as marine snow and sticky ligands (“marine spiderwebs”) with fractal properties (voids,

scaling invariant)

TEM of spiderweb-like fibrils [nm-µm; Santschi et al., 1998, Wilkinson, unpublished]

Marine Snow [mm to cm; Alldredge and Gottschalk, 1989]

Dissolved matter

Colloidal matter

Small aggregates

Sedimenting aggregates

500nm

234Th deficiencies in the water column as a measure of particle scavenging intensity in surface and deep waters

(Santschi et al., 1999. CSR, 19, 609)

Aggregates ≥ 1.5 mm diameter

Solution and surface speciation: inorganic speciation

• Solution

• ThO2 solubility

• Iron oxide and silica surface

• Sorption reversibility

• Presence or absence of organic matter

Th(IV) complexation in pure water

• Langmuir and Herman, 1980. GCA 44, 1753

Th-hydroxo species dominant at pH=8, even at high phosphate or EDTA concentration=> ThO2 solubility ~ 10-13 M

ThO2 solubility ~ 10-8 M, regardless of crystallinity and size at pH=8 (Fanghaenel and

Neck, 2002. Pure Appl. Chem., 72, 1895)

-8

-15

Murphy et al., 1999. Coll. Surfaces A, 157, 47

=> Importance of hydroxy-carbonate complexes

Th(IV) sorbs more strongly on Iron oxides in presence of marine COM

U(VI)Th(IV)

Th(IV) forms inner-sphere complexes on hematite surface

No detectable desorption from hematite ( ) and COM ( ) colloids within 3 days after resuspension of tagged colloids into artificial seawater solutions (Quigley et al., 1996, 2001).

predicted

0

sorption

desorption

Disaggregation mascarading as “desorption” when clusters of 70 nm hematite particles are 0.4 µm

filtered, but [Th] = 0 when 0.03µm filtered (Quigley et al., 1996. Aquat. Geochem. 1, 277)

Silica: Östhols, 1995. GCA, 59, 1235

Th(IV) sorption to SiO2 in presence of Humic Acids (Moulin et al., 2004. In: Humic

Substances, Taylor and Francis, Inc., p.275)

Enhanced Partitioning Coefficients (Kc) to Polysaccharide Enriched Colloidal Organic Matter (COM) over

Unpurified COM (Quigley et al., 2002. L&O, 47, 367)

Increased Colloid-Water Partition Coefficient (Kc) of 234Th(IV) as a fct. of Polysaccharide Content (Quigley et al., 2002. L&O, 47, 367)

Enrichment through alcohol precipitation

(g-PS/g-OM)

Kc = Kc(o)*10(2.2fPS

)

=>Increase in Kc and ∆14C

Consequences for POC/[234Th] ratio

• The POC/[234Thp] ratio is a function of the particle-water partition coefficient,

Kd = Kd(o)*10(2.2fPS) (cm3 g-1), [234Thd] (in dpm/l) = dissolved 234Th concentration, [SPM] = the suspended particulate matter concentration ( in g/L), [OC] = organic carbon (µmol-OC/mg particles), fOC and fPS = fractions of OC (OC/SPM) and polysaccharides, CHO (PS/OC), respectively,

• POC/[234Thp] = ([POC]/SPM])/([234Thd]Kd) or

= [fOC]/([234Thd]Kd)

• => Log{[POC]/[234Thp]} = log(fOC) – log[234Thd] – logKd(o) – 2.2 fPS ≈ constant - 2.2 fPS

• [POC/234Th] 1/fPS (if fPS is small)

• Or [234Th/POC] fPS

Relationship between 234Th/POC ratio and POC-normalized APS and Carbohydrate Concentrations [Guo

et al., 2002, Mar. Chem. 78, 103-119; Santschi et al., GRL.30,C2, 1044]

2001 cruise to Gulf of Mexico:Filled circles: sinking particlesCollected at 65, 90, 120 m depth; Open circles: suspended particles (sum of 0.5, 1, 10, 53 µm fractions)

2000 cruise to Gulf of Mexico:Open circles: suspended particles

• CHO/OC~0.1

• URA,APS/OC~0.01

=> Need for lab experiments

“Sticky” macromolecular Th(IV)-binding ligand (Quigley et al., 1996, 2001, 2002;

Alvarado, 2004)

• Sticky coefficient of 0.9

• Low pKa of 2-3

• Low pHIEP of 2-3

• Molecular weight of ~ 10 kDa

• Functional groups: R-COO-, R-OPO3-, R-

OSO3-, alone or in concert

• Apparent irreversibility of sorption

2D PolyAcrylamide Gel Electrophoresis of Gulf of Mexico COM radiolabeled with 14C

and 234Th (Quigley et al., 2002, L&O 47, 367; Alvarado-Quiroz, 2004, PhD Dissertation,

TAMU, College Station, TX)

234Th labeled COM, with similar distribution as polysaccharide-enriched COM, and 14C-labeled sugar OH groups of COM (Quigley et al., 2002)

(Santschi et al., 2003, GRL 30(2), 1044). -> Th(IV)- binding molecule contains Phosphate and Sulfate(Alvarado, 2004)

0%

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1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

pH

Concentration

PhosphateSulphate

COM from GOM St. 4 – 72m: IC – PO4 & SO4

(Alvarado-Quiroz, 2004, PhD Dissertation, TAMU, College Station, TX)

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35%

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pH

Activity

234 Th IEF

234Th IEF from 2D SDS-PAGE

14C IEF from 2D SDS-PAGE

pHIEP ≈ 2-3 pHIEP ≈ 2-3

=> Family of ligand systems with varying MW and fct groups from COM & EPS harvested from marinephytoplankton and bacteria

Phosphate, Sulfate Th(IV)

Model Acid Polysaccharides withRCOO-, ROSO3

-, or R2OPO3- binding sites

- Carrageenan Teichoic Acid

Note: Carrageenans act as blood anticoagulants, while alginates act as blood coagulants, but both are used as emulsifiers and stabilizers in the food and pharaceutical industry

Surface water vs. Deep water: Middle Atlantic Bight

2600 m2 m

Fibrils in 1-200 nm COM documented by Atomic Force Microscopy (AFM, horizontal distance 10 µm) [Santschi et al., 1998. L&O 43, 896]

Fibrils in 1-200 nm COM documented by Atomic Force Microscopy (AFM, horizontal distance 10 µm) [Santschi et al., 1998. L&O 43, 896]

-> Forms and shapes of colloids: pearls on necklace most common colloidal form -> “spiderweb”; -> fibrils in surface and bottom waters, but not in mid-depth waters.-> modern radiocarbon ages of pure fibrils (e.g., ~100% CHO)

10 µm 10 µm

Fibrils on cell surface (Leppard, 1995, Sci. Total. Environ. 165:103)

TEM of bacterial exopolymers used for experiments (without cell lysis, bacterial and dust contamination); scale bar: 200nm

Origins of fibrillar EPSFunctions and roles of EPS•Floc formers (“marine snow”, “lake snow”), •Form matrix components of biofilms, •Play roles in colloid scavenging •Facilitate microbial adhesion to surfaces. •Bind extracellular enzymes in their active forms, as well as nutrients, •Scavenge trace metals from the water,•Templates for FeOOH, MnO2, CaCO3 and SiO2 growth •Can immobilize toxic substances, •Can alter the surface characteristics of suspended particles•Modify the solubility of associated molecules.

Calcium poly--L-guluronatel -eft handedhelix view dow n axis     

view along a xis , sh owing the hydroge n bond ing a nd ca lcium bind ing s ite s.

Where can Th(IV) go? - substitute for Ca2+ with similar ionic radius.Role of Metals: Stabilization of -helices through Ca2+ bridging

-> biosorbent for trace metals by, e.g., Ca substitution

=> Rigidity of polymer through Ca2+ stabilized alpha-helices

Characterization of Exopolymeric fibrils from Sagitulla st.

(Hung and Santschi, unpublished) • TEM

• Spectrophotometric Methods

• GC-MS

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Sagi. D. (>1kDa) Sagi. P. (>1kDa) E. HuxD.(>1kDa)

E. HuxP.(>1kDa)

TCHO/DOC (%)

RT: 12.17 - 65.49

15 20 25 30 35 40 45 50 55 60 65

Time (min)

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NL:

2.29E6

TIC MS

08140202

(#1,2,6,8,9 : unknown peaks, 3: glucose, 4: galactose, 5: galacturonic acid, 7: mannuronic acid).

Scale bare 200 nm

Lab: Problem of colloidal impurities in all reagents and tracers used in laboratory

experiments: Results ~ fct(purity of chemicals)

• Colloids are everywhere in laboratory reagents and tracers, at concentrations 10-8 to 10-10 M.

• Possible Sources: atmospheric dust, leaching from glassware.

• Possible compounds: Fe and Al hydroxides, silicates, bacterial EPS.

Importance of Experimental Conditions in Lab Experiments

• Clean-up of tracer and reagents• Clean-up of mineral phases, e.g., SiO2

• Neutralization method of acid: buffer vs. base (e.g., NaOH)

• Phase separation: Ultrafiltration vs. centrifugation for particle or colloid separation

Solution conditions: 0.1 M NaClO4, pH of 7-8, using Th(V) tracer (K. Roberts, TAMUG)

Problem Condition Log Kd

Neutralization of tracer acid, SiO2

NaOH

NaHCO3

4.35

3.74

Clean conditions (teflon, clean reagents, clean-room), SiO2

Reagent-grade reag.

TM-grade reagents

5.5

2.5)*

Acid/base clean-up of SiO2

Clean silica

Silica not precleaned

5.5

5.8*

Phase separation (Xanthan, alginate, carrageenan), clean

Centrifugation

1 kDa ultrafiltration

~ 4.5

6 - 7

Colloidal form of tracer ion

Only 1-8 % of tracer ion ≤ 1 kDa (pH 8)

-

*) In clean solutions there is greater wall loss=> Know about, or swamp colloidal impurities!?

Oceanographic Perspective

Guo et al., 1995. EPSL, 133, 117

Is 234Th representative for Th(IV)?

• particle-water partitioning same for short-lived 234Th as for long-lived 230Th in seawater

• particle concentration effect for Th-isotopes due to the presence of colloidal macromolecular ligands in seawater

=> Particle concentration effect across sizes indicates Th-binding ligands down to small (≤ 10 kDa) sizes

Guo et al., 1997. Coll. Surfaces A, 120, 255.

=> 234Th(IV) partitions between solution, colloid and particle phases broadly similar to organic carbon

“Th-complexing capacity”

• Hirose and Tanoue, 1998. Mar. Chem. 59, 235Valid for pH of 1

Hirose and Tanoue (2004. Sci.World J., 4, 67): Constant ratio of [232Thp] to [“Strong Organic Ligand”]

Profile shape similar to that of POC, PON, Proteins

Hirose and Tanoue, 2001. Mar. Env. Res., 59, 95.(L/C in SPM in surface Ocean: ~ 1.5, deep Ocean: ~ 4 mmol/mol-C)

=> Ligand site:carbon (L/C) of ~ 0.001, and proportional to surface area:volume ratios

{mmol/mol-C}

Fibrillar exopolymers (“TEP”) as marine snow and sticky ligands (“marine spiderwebs”) with fractal properties (voids,

scaling invariant)

TEM of spiderweb-like fibrils [nm-µm; Santschi et al., 1998, Wilkinson, unpublished]

Marine Snow [mm to cm; Alldredge and Gottschalk, 1989]

Dissolved matter

Colloidal matter

Small aggregates

Sedimenting aggregates

500nm

“Sticky coefficient” of 0.9 for polysaccharide-enriched colloidal macromolecular organic matter, and 0.8 for COM (Quigley et al., 2001. Mar. Chem., 76, 27)

• Amphiphilic Properties of EPS;• Surface Activity of Hydrocolloid by smaller amounts of covalently bound hydrophobic proteins (e.g.,

gum arabic; Dickinson, 2003, Food Hydrocolloids, 17, 25) or lipids

Th(IV) sorbed to natural particles and colloids with same end-state, even when at different initial conditions(tagged colloids, ortagged particles)

234Th-tagged OM:

LMW≤10kDa

HMW≥10kDa

Particles ≤0.4µm, ≥0.1 µm

P≥0.1µm

P≥0.1µm

P>0.1µm

Sorption kinetics: k1 a Cp≈ 0.3)observed in bot lab (Nyffeler et al., 1984; Honeyman and Santschi, 1989; Stordal et al., 1996; Wen et al., 1997; Quigley et al., 2001) and field (e.g., Honeyman and Santschi, 1989; Baskaran et al., 1992)

Results from Field Experiments:Sampling stations in the Gulf of Mexico

* S6* S4

Warm Core Rings (red), Cold Core Rings (blue)

Importance of Prymnesiophytes in 2001 and Cyanobacteria in 2000 [Santschi et al., 2003, GRL 30, 1044]

Different phytoplankton species appear, at times, to control acid polysaccharide (APS) e.g., uronic acid (URA), production and 234Th(IV) complexation

Abundance in ocean: CHO/POC ~ 0.1, APS/POC ~ 0.01, URA/POC ~ 0.01 (all: Santschi et al., 2003; Hung et al., 2003), L/POC ~ 0.001 (Hirose, 2004).

Relationship between bacterial production (BP) and a) total APS concentration (µg-C/L), and b) 234Th/POC ratios (May

2001, Gulf of Mexico) [Santschi et al., 2003, GRL 30(2), 1044]

=>Interplay betweensorptive, aggregatingand enzymaticProcesses;-> Microbial APS production and degradation coupled to coagulation of morerecalcitrant APS fragments provides a steady conveyor belt for 234Th to and from Particles, with the “ligand soup” being regulated by the microbial community in the water, as a self-regulating (autoporetic) system.

Summary and Conclusions• Experimental Lab Results with Th(IV) tracers at environmentally

relevant (low) concentration levels depend on experimental (ultra-clean vs. ambient impurities) conditions, with tracers likely present as pseudo-colloids at neutral pH, with environmental significance only when colloidal impurities are known or controlled.

• Family of Th(IV)-binding surface-active macromolecular ligands with varying functional groups and molecular weights, produced by phytoplankton and bacteria, partly degraded by bacterial enzymes but re-aggregating as smaller fibrillar units on their way to bottom, with aggregation pathway dominating for TEP and Th(IV), degradation pathway dominating for OC.

• EPS might act as “colloid trap”, like a marine spiderweb, sinking at speeds controlled by its fractal dimensions, and in proportion to the mineral ballast (SiO2, CaCO3, Al2O3).

Where do we go from here? => Constrain variability in POC/[234Th] ratios

• We need an improved relationship between the POC/[234Th] ratio and the ligand, CHO or OC content, whereby CHO/OC~0.1, APS(URA,LPS)/OC~0.01, L/OC~0.001.

• More insight into molecular mechanisms of Th(IV) “scavenging” needed; => coupling of complexation to colloids/particle aggregation into sinkable particles important.

• Importance of hydrophobic-lipophilic balance (HLB numbers) for parameterizing “stickiness”?

In summary, what is needed is:

• Better insight into the molecular mechanisms of the physical, chemical and biomolecular mechanisms of Th(IV) binding to a “sticky” macromolecular ligand family of compounds requires a paradigm shift.