Table of Contents

• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

The uptake and binding of six particle-reactive and/or redox-sensitive radionuclides (210Pb, 234Th, 7Be, 59Fe, 237Np and 233Pa) with different organic functionalities of three size fractions.

Basically, the humic acids (HAs) from different soils were isolated and further purified according to an alkaline extraction method from the International Humic Substance Society (IHSS) (Xu et al., 2011). Briefly, the dried soil was pre-treated with a 1 M HCl solution to separate the supernatant (i.e., fulvic acid) from the soil, followed by the addition of 0.1 M KOH under N2 purging and 0.3 M K+, as KCl. Then, the supernatant was acidified to pH of 1.0 using 6 M HCl to precipitate HAs, which were pelleted by centrifugation and then suspended in a 0.1 M HCl/0.3 M HF solution overnight for five times to minimize ash content. After HF digestion, Milli-Q water was used to wash HA with the purpose of minimizing ions. HA characterization data were previously reported elsewhere (Fujitake et al., 2012) and on IHSS website (http://humic-substances.org/13c-nmr-estimates-of-carbon-distribution-in-ihss-samples/). 

The HA-groundwater suspension batch experiments were conducted basically similar to a previously reported procedure (Xu et al., 2014). In brief, 5-6 mg of the purified HAs were pre-equilibrated in artificial groundwater in the centrifuge tubes for 48 h at room temperature (20 ºC) to reach the dissolution equilibrium, since the HAs were extraceted under pH < 1 but the pH of artificial groundwater is 5.5. Then, ~50 Bq of each gamma emitting radionuclide, including 234Th, 237Np-233Pa, 210Pb, 7Be and 59Fe was added to the HA-groundwater slurry to a final volume of 4 mL. The radiolabeled HA-groundwater slurry was then mixed continuously for 7 days in the dark with an end-over-end mixer to ensure that quasi-equilibrium was attained15. After one-week period, particulate (>0.45 µm), colloidal (3 kDa to 0.45 µm), and truly dissolved (<3 kDa) phases were size fractionationated by using 0.45 µm centrifugal filter tubes followed by ultrafiltration with 3 kDa Microsep centrifugal filter tubes (Millipore). Each fraction was collected for the measurement of radionuclide activity and organic matter concentrations. All the size fractions, including the particulate, colloidal and truly dissolved phases were corrected to the same volume and geometry for the counting of 234Th, 233Pa, 237Np, 210Pb, 7Be and 59Fe activity concentrations by a Canberra ultrahigh purity germanium well gamma detector.

Concentrations of organic carbon and nitrogen in the colloidal and truly dissolved fractions were determined using a Shimadzu TOC-L analyzer. The organic carbon and nitrogen concentrations in the particulate phase of the HA-groundwater suspension were calculated as the difference between the total carbon/nitrogen contents of the added HAs and the sum of colloidal and truly dissolved phases. 

Microsoft Excel Ver. 15.15; KaleidaGraph Ver. 4.1.3

BCO-DMO Processing Notes:
* added conventional header with dataset name, PI name, version date
* modified parameter names to conform with BCO-DMO naming conventions
* reorganized data from three tables into one table
* added 'nd' for STD and AVG values which were not computed
* included additional site information as provided.

NSF Award Abstract:

Particle-associated natural radioisotopes are transported to the ocean floor mostly via silica and carbonate ballasted particles, allowing their use as tracers for particle transport. Th(IV), Pa (IV,V), Po(IV), Pb(II) and Be(II) radionuclides are important proxies in oceanographic investigations, used for tracing particle and colloid cycling, estimating export fluxes of particulate organic carbon, tracing air-sea exchange, paleoproductivity, and/or ocean circulation in paleoceanographic studies. Even though tracer approaches are considered routine, there are cases where data interpretation or validity has become controversial, largely due to uncertainties about inorganic proxies and organic carrier molecules. Recent studies showed that cleaned diatom frustules and pure silica particles, sorb natural radionuclides to a much lower extent (by 1-2 orders of magnitude) than whole diatom cells (with or without shells). Phytoplankton that build siliceous or calcareous shells, such as the diatoms and coccolithophores, are assembled via bio-mineralization processes using biopolymers as nanoscale templates. These templates could serve as possible carriers for radionuclides and stable metals.

In this project, a research team at the Texas A & M University at Galveston hypothesize that radionuclide sorption is controlled by selective biopolymers that are associated with biogenic opal (diatoms), CaCO3 (coccolithophores) and the attached exopolymeric substances (EPS), rather than to pure mineral phase. To pursue this idea, the major objectives of their research will include separation, identification and molecular-level characterization of the individual biopolymers (e.g., polysaccharides, uronic acids, proteins, hydroquinones, hydroxamate siderophores, etc.) that are responsible for binding different radionuclides (Th, Pa, Pb, Po and Be) attached to cells or in the matrix of biogenic opal or CaCO3 as well as attached EPS mixture, in laboratory grown diatom and coccolithophore cultures. Laboratory-scale radiolabeling experiments will be conducted, and different separation techniques and characterization techniques will be applied.

Intellectual Merit : It is expected that this study will help elucidate the molecular basis of the templated growth of diatoms and coccoliths, EPS and their role in scavenging natural radionuclides in the ocean, and help resolve debates on the oceanographic tracer applications of different natural radioisotopes (230,234Th, 231Pa, 210Po, 210Pb and 7,10Be). The proposed interdisciplinary research project will require instrumental approaches for molecular-level characterization of these radionuclides associated carrier molecules.

Broader Impacts: The results of this study will be relevant for understanding biologically mediated ocean scavenging of radionuclides by diatoms and coccoliths which is important for carbon cycling in the ocean, and will contribute to improved interpretation of data obtained by field studies especially through the GEOTRACES program. This new program will enhance training programs at TAMUG for postdocs, graduate and undergraduate students. Lastly, results will be integrated in college courses and out-reach activities at Texas A&M University, including NSF-REU, Sea Camp, Elder Hostel and exhibits at the local science fair and interaction with its after-school program engaging Grade 9-12 students from groups traditionally underrepresented.