Susanta Paikaray

As(III) retention kinetics, equilibrium and redox stability on biosynthesized schwertmannite and its fate and control on schwertmannite stability on acidic (pH 3.0) aqueous exposure.

High As(III) enrichment in schwertmannite precipitated acid mine impacted areas is a major concern considering its acute toxicity and mobility where the current knowledge of their interaction, redox conditions and schwertmannite metastability is inadequate. In this study we have investigated such aspects through batch isotherm, microscopic and spectroscopic techniques. Schwertmannite produced by biotic process with 14.7 m(2)g(-1) surface area demonstrated a rapid As(III) uptake followed by slow retention possibly into the internal absorbing sites through multilayer and heterogeneous sorption processes. Chemical, X-ray diffraction, infrared and microscopic examinations revealed that ionic exchange between schwertmannite SO(4)(2-) and As(III) and surface precipitation governed the total As(III) uptake where lower dissolved SO(4)(2-) and high sorbent mass enhanced As(III) retention. Redox instability of sorbed As(III) was evidenced from the near-edge spectroscopic analysis at extremely high Fe(III):As(III) ratio (5.5×10(5)) leading to surface oxidation to As(V), while As(III) was found as the predominant redox species at high As(III):Fe(III) ratios. Only 0.83% of sorbed As(III) was released which was subsequently re-adsorbed into schwertmannite during 4 months stabilization without any evidence of mineralogical transformation.

The effect of Fe(II) on schwertmannite transformation and its interference with As

Goldschmidt Conference Abstracts 2009 Spring residence times: Role in weathering rates F.A.L. P ACHECO 1 AND C.H. V AN DER W EIJDEN 2 Department of Geology and Centre for Chemistry, UTAD, Vila Real, Portugal (fpacheco@utad.pt) Utrecht University, The Netherlands (chvdw@geo.uu.nl) Estimation of plagioclase (Pl) weathering rates (W Pl = ([Pl]/t)×(Q/A Pl )) at the watershed scale of springs requires the prior evaluation of a number of parameters which include the mole fractions of Pl ([Pl]) and their fracture surface areas (A Pl ), the residence times of springs (t) and their annual discharge (Q). An atempt to relate the weathering of plagioclase to mixtures rich in halloysite and to quantify the W Pl for a number of very small spring watersheds from the Vila Pouca de Aguiar region (VPA, North of Portugal) is documented in [1]. In this paper we take a step further by focusing our attention on adjusting the previously used advective flow equation and introducing hydraulic turnovers for the assessment of t. Now, the advective flow equation (t = (n e /K)(F 2 /D h )) replaces the average watershed depth (D) by the average depth of the saturated aquifer (D h ), whereas hydraulic turnovers assign t = V h n e /Q. V h is the saturated volume of the aquifer characterized by an effective porosity n e and a hydraulic conductivity K, and F is the average lateral path from the recharge area to the spring site. The evaluation of n e , K, F and Q has been addressed by [1]. The D h of the VPA springs could be related to their isotopic composition ( 87 Sr/ 86 Sr) and to annual precipitation (P): D h = [( 87 Sr/ 86 Sr) spring – ( 87 Sr/ 86 Sr) rain ] / (5.62×10 –7 P – 4.66×10 –4 ). The corresponding V h ’s were determined from the total watershed volumes (V) as calculated by a terrain modeling software: V h = V×(D h /D). The plagioclase log rates (Figure 1) are: –12.4±1.8 (adjusted flow equation) and –13.5±1.1 (turnover times). Relative to the former results, there is a decrease in the average log rates, by 0.2 in the first case and 1.4 in se second case. Number of cases Methane oxidation rates by AMS M. P ACK 1 *, M. H EINTZ 2 , W.S. R EEBUR G H 1 , S.E. T RUMBORE 1 , D.L. V ALENTINE 2 AND X. X U 1 University of California Irvine, Irvine, CA 92697 (*correspondence: mpack@uci.edu) University of California Santa Barbara, Santa Barbara, CA 93106 (mbheintz@umail.ucsb.edu, valentine@geol.ucsb.edu) In the marine environment methane (CH 4 ) oxidation consumes up to 84% of the CH 4 produced and mitigates the release of CH 4 , a potent green house gas, to the atmosphere [1]. The microbialy mediated process is an important sink in the global CH 4 budget, yet it remains poorly quantified because only a small number of direct oxidation rate measurements are available. Traditional oxidation rate measurements use regulated levels of radiotracers ( 14 C- and 3 H-CH 4 ) in conjunction with scintillation counting and come with certain limitations: safety and contamination factors restrict the measurements to isotope vans, and radioisotope use may not be permitted in foreign venues and may complicate shipping. We have developed a rate measurement that utilizes non- regulated levels of 14 C-CH 4 tracer (<50nCi/g) [2] in conjunction with accelerator mass spectrometry (AMS). The high sensitivity of AMS allows for a 10 3 reduction in tracer activity which relaxes complications with tracer shipping, handling and waste disposal. Together with ease of performance, this method could provide a larger sample throughput and therefore a better quantification of the marine CH 4 oxidation sink. Further, it allows for easy quantification of the fraction of CH 4 taken up in microbial biomass as well as the fraction oxidized, thereby providing important information about the activity of methanotrophs in the ocean. Our rate measurements compared to 3 H-CH 4 rate measurements on water from the same Niskin bottles are generally consistent. The two measurements are similar when ambient rates are high, but diverge when rates are low. [1] Reeburgh (2007) Chem. Rev. 107, 486-513. [2] Vogel (2000) Nucl. Instrum. Methods Phys. Res. B 172, 885-891. Turnover times Flow equation Times A983 Log (W Pl ) Figure 1: Log rates of plagioclase. [1] Pacheco F.A.L., Van der Weijden C.H. (2008). Geochimica et Cosmochimica Acta, v. 72, no. 17, Page A715.