Reduction of heavy metals in situ by plants may be a useful detoxification mechanism for phytoremediation. Using X-ray spectroscopy, we show that Eichhornia crassipes (water hyacinth), supplied with Cr(VI) in nutrient culture, accumulated nontoxic Cr(III) in root and shoot tissues. The reduction of Cr(VI) to Cr(III) appeared to occur in the fine lateral roots. The Cr(III) was subsequently translocated to leaf tissues. Extended X-ray absorption fine structure of Cr in leaf and petiole differed when compared to Cr in roots. In roots, Cr(III) was hydrated by water, but in petiole and more so in leaf, a portion of the Cr(III) may be bound to oxalate ligands. This suggests that E. crassipes detoxified Cr(VI) upon root uptake and transported a portion of the detoxified Cr to leaf tissues. Cr-rich crystalline structures were observed on the leaf surface. The chemical species of Cr in other plants, collected from wetlands that contained Cr(VI)-contaminated wastewater, was also found to be Cr(III). We propose that this plant-based reduction of Cr(VI) by E. crassipes has the potential to be used for the in situ detoxification of Cr(VI)-contaminated wastestreams.

(rabbitfoot grass, cattail and saltmarsh bulrush, respectively).Rates for the most dominant species, saltmarsh bulrush, varied during the year: the mean rates were 150, 70 and 25 p.1gSe m-z day" in February, June and October, respectively. We conclude that biological Se volatilization is a significant pathway of Se removal in wetlands.[Abstract figure chosen is IntroductionSelenium (Se) is a metalloid which exists in a variety of oxidation states including selenide (Se 2 "), elemental Se (Seo), selenite (Se 4 +), selenate (Se 6 +), and several organic (e.g., selenomethionine) and volatile Se compounds (e.g., dimethyl selenide, DMSe). The oxidized forms of Se, selenite and selenate, are highly soluble and therefore bioavailable and potentially toxic .. The reduced forms, selenide and elemental Se, are insoluble and much less bioavailable. The presence of Se in aqueous discharges from agriculture and industry is a cause for concern in many areas of the world. In the 1980's, selenate-rich agricultural drainage water was discharged into the Kesterson Reservoir marsh in California. This had very serious environmental consequences and resulted in the death and deformity of wildfowl due to Se poisoning (1). Another significant health concern is the rising Se levels in the San Francisco Bay. Much of the Se inflow comes from urban and industrial sources (2). Six oil refineries discharge into the Bay and Se discharge levels from several of the refineries substantially exceed those permitted under the guidelines established by both state and federal regulators.Constructed wetlands constitute a complex ecosystem, the biological and physical components of which interact to provide a mechanical and biogeochemical filter capable of removing many different types of contaminants from water. They have been used to c1eanupmunicipal wastewaters, stonnwater runoff, and many other types of polluted wastewaters in the USA (3), and in Europe (4). Constructed wetlands are orders of magnitude lower in cost than other treatment systems (5); however, the science of understanding of wetland detoxification mechanisms is in its infancy. The first indication that wetlands might be useful in the removal of Se from wastewaters came from a study of a 36-hectare constructed wetland located adjacent to the San Francisco Bay, California. Analysis of the wetland inlet and outlet waters showed that the constructed wetland was successful in removing at least 70% of the Se from the wastewater passing through it (6). Although effluent levels ofSe were well below the permit requirements, there remained the obvious concern that the Se retained in the wetland may eventually be harmful to wildlife. This concern led to a field study to determine Se partitioning among the sediments, water and plant material. The resulting inventory accounted for only 70% of the Se removed by the wetland (6); the fate of the remainder of the Se removed was unknown.One explanation for the «missing Se" at the constructed wetland is that the Se had been converted to volatile for...

An open‐flow chamber sampling system was developed for measurements of selenium (Se) volatilization from soil and vegetated areas under field conditions. This paper provides detailed information on the design, construction, and calibration of the chamber sampling system. Volatile Se was collected using a Plexiglas collection chamber, 0.71 m long, 0.71 m wide, and 0.76 in high; the chamber provided an internal volume of 0.38 m3 and enclosed a ground area of 0.5 m2. Volatile Se produced inside the collection chamber was trapped by activated carbon filters from the air leaving the chamber. The trapped Se was extracted from the charcoal filters in alkaline‐peroxide solution. The system has a maximum measurement capacity of 10 mg Se m−2 over a sampling period of 24 h at an air flow rate of 0.43 m3 h−1. Measurements of Se volatilization from the soil‐halophyte system in central California were carried out to demonstrate the efficiency of this sampling system under field conditions.

The retention of heavy metals in water treatment wetlands is well documented, but little understood. Fluxes to and from sediments for moderate concentrations of dissolved metals are particularly unknown. Treatment wetlands are dried out seasonally or occasionally for maintenance. The extent to which heavy metals may be released by drying/re-flooding is of particular concern because of the potential for toxic levels of metals to be mobilized. A 36 ha treatment wetland receiving treated oil refinery effluent in California was dried for 6 months, then re-flooded to an average depth of >10 cm. The concentrations of 11 metals, As and Se in inflow, outflow, and porewaters were measured weekly for 4 months. Mass flux rates showed that the wetland acted as a sink for As and Se, six metals (Co, Cr, Mg, Mn, Ni, and Sr) and S were overall sources and five showed zero net flux (Ba, Cu, Fe, Mo, and Zn). Porewater results indicate that oxidation of the sediments caused the source metals to be released. Removal for As > Cu, Fe, Mo, Zn > Co, Mn, Ni was consistent with the thermodynamically-predicted ‘sulfide ladder’, suggesting that available sulfide was insufficient to re-sequester the entire pool of mobile chalcophile elements. Our results suggest that less-soluble sulfide metals may be immobilized prior to more-soluble metals following drying/re-flooding in coastal systems with multiple metal contaminants. Ponding for up to several weeks, depending on the metals of concern, will facilitate metal re-immobilization within sediments before waters are released and minimize impacts downstream. Research on how to speed-up the conversion of soluble metals to their insoluble sulfides or other immobilized forms is urgently needed.