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Speciation of Th(IV) in marine systems

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  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 A&M 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/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

  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, Kd:Kd = fp/{(1-fp)Cp,with fp=fraction on particles, and Cp=particle 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. => 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 = Kd*Cp/(1+Kd*Cp)

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

  12. ThO2 solubility ~ 10-8 M, regardless of crystallinity and size at pH=8 (Fanghaenel and Neck, 2002. Pure Appl. Chem., 72, 1895) -8 -15

  13. Murphy et al., 1999. Coll. Surfaces A, 157, 47 => 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. L&O, 47, 367)

  21. Increased Colloid-Water Partition Coefficient (Kc) of 234Th(IV) as a fct. of Polysaccharide Content (Quigley et al., 2002. L&O, 47, 367) Kc = Kc(o)*10(2.2fPS) Enrichment through alcohol precipitation (g-PS/g-OM) =>Increase 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), fOCand 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]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/OC~0.1 • URA,APS/OC~0.01 2000 cruise to Gulf of Mexico: Open circles: suspended particles => 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, 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)

  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 => 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. Fibrils in 1-200 nm COM documented by Atomic Force Microscopy (AFM, horizontal distance 10 µm) [Santschi et al., 1998. L&O 43, 896] Surface water vs. Deep water: Middle Atlantic Bight 10 µm 10 µm 2 m 2600 m -> 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)

  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. 165:103) 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 => Rigidity of polymer through Ca2+ stabilized alpha-helices -> biosorbent for trace metals by, e.g., Ca substitution

  31. (#1,2,6,8,9 : unknown peaks, 3: glucose, 4: galactose, 5: galacturonic acid, 7: mannuronic acid). 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) => 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. => Particle concentration effect across sizes indicates Th-binding ligands down to small (≤ 10 kDa) sizes

  38. => 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. => Ligand site:carbon (L/C) of ~ 0.001, and proportional to surface area:volume 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 hydrophobicproteins (e.g., gum arabic; Dickinson, 2003, Food Hydrocolloids, 17, 25) or lipids

  43. 234Th-tagged OM: LMW≤10kDa HMW≥10kDa Particles ≤0.4µm, ≥0.1 µm Quigley et al., 2001 P>0.1µm Th(IV) sorbed to natural particles and colloids with same end- state, even when at different initial conditions (tagged colloids, or tagged particles) P≥0.1µm P≥0.1µm 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 Experiments:Sampling 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] =>Interplay between sorptive, aggregating and enzymatic Processes; -> 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? => 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 ofcomplexation 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.