Speciation of Th(IV) in marine systems

1
Speciation of Th(IV) in marine systems

• Peter H. Santschi
• Laboratory for Oceanographic and Environmental
  Research (LOER)
• Depts. Of Oceanography and Marine Sciences
• Texas AM University
• Galveston, TX

2
Outline
Family of marine sticky spiderweb- like ligands
with fractal and amphiphilic properties,
persistence Lengths of 100s nm, contour
lengths of 100s - 1000s of nm and thickness of
1-2 nm

• 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

500 nm
TEM picture of stained marine colloids in
nanoplast (Santschi et al., 1998).
3
Th(IV) as an oceanographic tracer( at Th
10-12 M)

• New Production by POC/234Thp234Th-flux
• Particle and Coagulation residence times
• Colloidal Pumping
• Particle sources
• Deep water ventilation rates
• Boundary scavenging
• YET, WE TAKE TH(IV) SPECIATION FOR GRANTED

4
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.

5
Particle-water partition coefficients, KdKd
fp/(1-fp)Cp, with fpfraction on particles, and
Cpparticle concentration Kd ml/g or L/kg
2)
2)
1)
1) Abstracting natural conditions Quigley et
al., 2002, and references therein
2) Simulating natural conditions Guo et al.,
2002.
gt 1) ? clean/unclean 2) Kd(Th(IV))Kd(Pa(IV,V))
Kd(PS) is max.
6
Predicted fp for Th(IV) using lab-based Kd
field Cp (compared to observed fp 0.1 - 0.5)
) fp KdCp/(1KdCp)
7
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.

8
Fibrillar exopolymers (TEP) as marine snow and
sticky ligands (marine spiderwebs) with fractal
properties (voids, scaling invariant)
Dissolved matter Colloidal matter Small
aggregates Sedimenting aggregates
500nm
TEM of spiderweb-like fibrils nm-µm Santschi
et al., 1998, Wilkinson, unpublished
Marine Snow mm to cm Alldredge and Gottschalk,
1989
9
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
10
Solution and surface speciation inorganic
speciation

• Solution
• ThO2 solubility
• Iron oxide and silica surface
• Sorption reversibility
• Presence or absence of organic matter

11
Th(IV) complexation in pure water

• Langmuir and Herman, 1980. GCA 44, 1753

• Th-hydroxo species dominant at pH8, even at high
  phosphate or EDTA concentration
• gt ThO2 solubility 10-13 M

12
ThO2 solubility 10-8 M, regardless of
crystallinity and size at pH8 (Fanghaenel and
Neck, 2002. Pure Appl. Chem., 72, 1895)
-8
-15
13
Murphy et al., 1999. Coll. Surfaces A, 157, 47
gt Importance of hydroxy-carbonate complexes
14
Th(IV) sorbs more strongly on Iron oxides in
presence of marine COM
U(VI)
Th(IV)
15
Th(IV) forms inner-sphere complexes on hematite
surface
Quigley et al., 1996. Aquat. Geochem. 1, 277
16
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
17
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)
18
Silica Östhols, 1995. GCA, 59, 1235
19
Th(IV) sorption to SiO2 in presence of Humic
Acids (Moulin et al., 2004. In Humic Substances,
Taylor and Francis, Inc., p.275)
20
Enhanced Partitioning Coefficients (Kc) to
Polysaccharide Enriched Colloidal Organic Matter
(COM) over Unpurified COM (Quigley et al., 2002.
LO, 47, 367)
21
Increased Colloid-Water Partition Coefficient
(Kc) of 234Th(IV) as a fct. of Polysaccharide
Content (Quigley et al., 2002. LO, 47, 367)
Kc Kc(o)10(2.2fPS)
Enrichment through alcohol precipitation
(g-PS/g-OM)
gtIncrease in Kc and ?14C
22
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)
• gt LogPOC/234Thp log(fOC) log234Thd
  logKd(o) 2.2 fPS constant - 2.2 fPS
• POC/234Th a 1/fPS (if fPS is small)
• Or 234Th/POC a fPS

23
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 particles Collected at 65, 90, 120 m
depth Open circles suspended particles (sum of
0.5, 1, 10, 53 µm fractions)
CHO/OC0.1 URA,APS/OC0.01
2000 cruise to Gulf of Mexico Open circles
suspended particles
gt Need for lab experiments
24
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

25
2D PolyAcrylamide Gel Electrophoresis of Gulf of
Mexico COM radiolabeled with 14C and 234Th
(Quigley et al., 2002, LO 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). -gt
Th(IV)- binding molecule contains Phosphate and
Sulfate (Alvarado, 2004)
26
COM from GOM St. 4 72m IC PO4 SO4
(Alvarado-Quiroz, 2004, PhD Dissertation, TAMU,
College Station, TX)
Phosphate, Sulfate Th(IV)
pHIEP 2-3
pHIEP 2-3
gt Family of ligand systems with varying MW and
fct groups from COM EPS harvested from
marinephytoplankton and bacteria
27
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
28
Surface water vs. Deep water Middle Atlantic
Bight
Fibrils in 1-200 nm COM documented by Atomic
Force Microscopy (AFM, horizontal distance 10 µm)
Santschi et al., 1998. LO 43, 896
10 µm
10 µm
2 m
2600 m
-gt Forms and shapes of colloids pearls on
necklace most common colloidal form -gt
spiderweb -gt fibrils in surface and bottom
waters, but not in mid-depth waters. -gt modern
radiocarbon ages of pure fibrils (e.g., 100 CHO)
29
Origins of fibrillar EPS

• Functions 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.

Fibrils on cell surface (Leppard, 1995, Sci.
Total. Environ. 165103)
TEM of bacterial exopolymers used for
experiments (without cell lysis, bacterial and
dust contamination) scale bar 200nm
30
Where can Th(IV) go? - substitute for Ca2 with
similar ionic radius. Role of Metals
Stabilization of a-helices through Ca2 bridging
gt Rigidity of polymer through Ca2 stabilized
alpha-helices
-gt biosorbent for trace metals by, e.g., Ca
substitution
31
Characterization of Exopolymeric fibrils from
Sagitulla st.(Hung and Santschi, unpublished)

• TEM
• Spectrophotometric Methods
• GC-MS

Scale bare 200 nm
32
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.

33
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

34
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) -
gt Know about, or swamp colloidal impurities!?
) In clean solutions there is greater wall loss
35
Oceanographic Perspective
36
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
Guo et al., 1995. EPSL, 133, 117
37
gt Particle concentration effect across sizes
indicates Th-binding ligands down to small ( 10
kDa) sizes
38
gt 234Th(IV) partitions between solution, colloid
and particle phases broadly similar to organic
carbon
Guo et al., 1997. Coll. Surfaces A, 120, 255.
39
Th-complexing capacity

• Hirose and Tanoue, 1998. Mar. Chem. 59, 235

Valid for pH of 1
Profile shape similar to that of POC, PON,
Proteins
Hirose and Tanoue (2004. Sci.World J., 4, 67)
Constant ratio of 232Thp to Strong Organic
Ligand
40
gt Ligand sitecarbon (L/C) of 0.001, and
proportional to surface areavolume ratios
mmol/mol-C
Hirose and Tanoue, 2001. Mar. Env. Res., 59,
95. (L/C in SPM in surface Ocean 1.5, deep
Ocean 4 mmol/mol-C)
41
Fibrillar exopolymers (TEP) as marine snow and
sticky ligands (marine spiderwebs) with fractal
properties (voids, scaling invariant)
Dissolved matter Colloidal matter Small
aggregates Sedimenting aggregates
500nm
TEM of spiderweb-like fibrils nm-µm Santschi
et al., 1998, Wilkinson, unpublished
Marine Snow mm to cm Alldredge and Gottschalk,
1989
42
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
43
234Th-tagged OM LMW10kDa HMW10kDa Particle
s 0.4µm, 0.1 µm
Quigley et al., 2001
Pgt0.1µm
Th(IV) sorbed to natural particles and colloids
with same end- state, even when at different
initial conditions (tagged colloids, or tagged
particles)
P0.1µm
P0.1µm
Sorption kinetics k1 a CpQ (Q 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)
44
Results from Field ExperimentsSampling stations
in the Gulf of Mexico
S4
S6
Warm Core Rings (red), Cold Core Rings (blue)
45
Importance of Prymnesiophytes in 2001 and
Cyanobacteria in 2000 Santschi et al., 2003, GRL
30, 1044
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).
Different phytoplankton species appear, at times,
to control acid polysaccharide (APS) e.g., uronic
acid (URA), production and 234Th(IV) complexation
46
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
gtInterplay between sorptive, aggregating and
enzymatic Processes -gt Microbial APS production
and degradation coupled to coagulation of
more recalcitrant 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.
47
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).

48
Where do we go from here? gt 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/OC0.1, APS(URA,LPS)/OC0.01,
  L/OC0.001.
• More insight into molecular mechanisms of Th(IV)
  scavenging needed gt coupling of complexation
  to colloids/particle aggregation into sinkable
  particles important.
• Importance of hydrophobic-lipophilic balance
  (HLB numbers) for parameterizing stickiness?

49
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.