The impact of sequestration on the bioaccessibility of arsenic in long-term contaminated soils

Arsenic (As) contamination of soil poses a potential threat to human health, particularly for small children, through the incidental ingestion of soil from hand-to-mouth activity. In this study, we examined the relationship between As bioaccessibility using the simplified bioaccessibility extraction test (SBET) and the soil fractions that contribute to bioaccessible As in 12 long-term contaminated soils. Sequential fractionation of soils prior to As bioaccessibility assessment found that As was primarily associated with the specifically sorbed (3-26%), amorphous and poorly crystalline (12-82%), and the well crystalline (3-25%) oxyhydroxide Fe/Al phases with proportions varying depending on the mode of As input. Arsenic bioaccessibility in these soils ranged from less than 1% in the gossan soil to 48% in railway corridor soils. Soil fractions contributing to As bioaccessibility were found to be from the non-specifically (<1-11%), the specifically (<1-29%) sorbed and the amorphous and poorly-crystalline (30-93%) oxyhydroxide Fe/Al fractions. Significant correlations (p<0.05) were found between the As bioaccessible fraction and the amorphous and poorly-crystalline oxyhydroxide Fe/Al fractions indicating that this fraction is a key factor influencing As bioaccessibility in many anthropogenically contaminated soils.     

Prokaryotic life in a potash-polluted marsh with emphasis on N-metabolizing microorganisms

Prokaryotic life along the salt gradient of the potash marsh resulting from mining waste at Schreyahn, Northern Germany, was screened for the distribution of total prokaryote (assessed by the 16S rRNA gene) and of N2-fixing (nifH gene), denitrifying (nosZ) and nitrifying (amoA) microorganisms. Information on prokaryotes was retrieved from the different soil sites (a) by culturing in conventional media, (b) by isolating the DNA, amplifying the target genes by PCR followed by sequencing, (c) by employing the recently developed computer program (TReFID [R?sch, C., Bothe, H., 2005. Improved assessment of denitrifying, N2-fixing, and total-community bacteria by terminal restriction fragment length polymorphism analysis using multiple restriction enzymes. Applied and Environmental Microbiology 71, 2026-2035]) based on tRFLP data. New sequences were obtained as well as ones that were almost identical to those found at far distant locations. Whereas the distribution of plants strictly follows the salt gradient, this is apparently not the case with prokaryotes. Bacteria of hypersaline areas coexist with salt-non-tolerant species. The recently developed TReFID program is successfully applied to characterize a prokaryote community structure.     

Leaching of seven pesticides currently used in cotton crop in Mato Grosso State-Brazil

This study aimed to evaluate the leaching of pesticides and the applicability of the Attenuation Factor (AF) Model to predict their leaching. The leaching of carbofuran, carbendazim, diuron, metolachlor, alpha and beta endosulfan and chlorpyrifos was studied in an Oxisol using a field experiment lysimeter located in Dom Aquino-Mato Grosso. The samples of percolated water were collected by rain event and analyzed. Chemical and physical soil attributes were determined before pesticide application to the plots. The results showed that carbofuran was the pesticide that presented a higher leaching rate in the studied soil, so was the one representing the highest contamination potential. From the total carbofuran applied in the soil surface, around 6% leached below 50 cm. The other pesticides showed lower mobility in the studied soil. The calculated values to AF were 7.06E-12 (carbendazim), 5.08E-03 (carbofuran), 3.12E-17 (diuron), 6.66E-345 (alpha-endosulfan), 1.47E-162 (beta-endosulfan), 1.50E-06 (metolachlor), 3.51E-155 (chlorpyrifos). AF Model was useful to classify the pesticides' potential for contamination; however, that model underestimated pesticide leaching.     

Modeling in-situ uranium(VI) bioreduction by sulfate-reducing bacteria

We present a travel-time based reactive transport model to simulate an in-situ bioremediation experiment for demonstrating enhanced bioreduction of uranium(VI). The model considers aquatic equilibrium chemistry of uranium and other groundwater constituents, uranium sorption and precipitation, and the microbial reduction of nitrate, sulfate and U(VI). Kinetic sorption/desorption of U(VI) is characterized by mass transfer between stagnant micro-pores and mobile flow zones. The model describes the succession of terminal electron accepting processes and the growth and decay of sulfate-reducing bacteria, concurrent with the enzymatic reduction of aqueous U(VI) species. The effective U(VI) reduction rate and sorption site distributions are determined by fitting the model simulation to an in-situ experiment at Oak Ridge, TN. Results show that (1) the presence of nitrate inhibits U(VI) reduction at the site; (2) the fitted effective rate of in-situ U(VI) reduction is much smaller than the values reported for laboratory experiments; (3) U(VI) sorption/desorption, which affects U(VI) bioavailability at the site, is strongly controlled by kinetics; (4) both pH and bicarbonate concentration significantly influence the sorption/desorption of U(VI), which therefore cannot be characterized by empirical isotherms; and (5) calcium-uranyl-carbonate complexes significantly influence the model performance of U(VI) reduction.