Detoxification of ferrocyanide in asoil-plant system

The detoxification of iron cyanide in a soil-plant system was investigated to assess the total cyanide extracted from contaminated soil and allocated in the leaf tissue of willow trees (Salix caprea). They were grown in soil containing up to 1000 mg/kg dry weight (dw) of cyanide (CN), added as ¹⁵N-labeled potassium ferrocyanide and prepared with a new method for synthesis of labeled iron cyanides. CN content and ¹⁵N enrichment were monitored weekly over the exposure in leaf tissue of different age. The ¹⁵N enrichment in the young and old leaf tissue reached up to 15.197‰ and 9063‰, respectively; it increased significantly over the exposure and with increasing exposure concentrations (p < 0.05). Although the CN accumulation in the old leaf tissue was higher, compared to the young leaf tissue (p < 0.05), the ¹⁵N enrichment in the two tissue types did not differ statistically. This indicates a non-uniform CN accumulation but a uniform ¹⁵N allocation throughout the leaf mass. Significant differences were detected between the measured CN content and the C¹⁵N content, calculated from the ¹⁵N enrichment (p < 0.05), revealing a significant CN fraction within the leaf tissue, which could not be detected as ionic CN. The application of labeled iron CN clearly shows that CN is detoxified during uptake by the willows. However, these results do not exclude other detoxification pathways, not related to the trees. Still, they are strongly indicative of the central role the trees played in CN removal and detoxification under the experimental conditions.

Toward Financially Viable Phytoextraction and Production of Plant-Based Palladium Catalysts

Although a promising technique, phytoextraction has yet to see significant commercialization. Major limitations include metal uptake rates and subsequent processing costs. However, it has been shown that liquid-culture-grown Arabidopsis can take up and store palladium as nanoparticles. The processed plant biomass has catalytic activity comparable to that of commercially available catalysts, creating a product of higher value than extracted bulk metal. We demonstrate that the minimum level of palladium in Arabidopsis dried tissues for catalytic activity comparable to commercially available 3% palladium-on-carbon catalysts was achieved from dried plant biomass containing between 12 and 18 g·kg^-1 Pd. To advance this technology, species suitable for in-the-field application: mustard, miscanthus, and 16 willow species and cultivars, were tested. These species were able to grow, and take up, palladium from both synthetic and mine-sourced tailings. Although levels of palladium accumulation in field-suitable species are below that required for commercially available 3% palladium-on-carbon catalysts, this study both sets the target, and is a step toward, the development of field-suitable species that concentrate catalytically active levels of palladium. Life cycle assessment on the phytomining approaches described here indicates that the use of plants to accumulate palladium for industrial applications has the potential to decrease the overall environmental impacts associated with extracting palladium using present-day mining processes.

Toxic effects, uptake, and translocation of Cd and Pb in perennial ryegrass

Cadmium (Cd) and lead (Pb) are regarded as the most toxic environmental pollutants because they are a threat not only to plants, but also to humans. Better understanding of the growth response, uptake and translocation of Cd and Pb in perennial ryegrass (Lolium perenne L.) would be helpful to evaluate the role of this grass species as a potential candidate used for phyto-remediation. Perennial ryegrass seedlings were subjected to three Pb levels (0, 0.5, and 3.2 mM) for 6 days and three Cd levels (0, 0.2, and 0.5 mM) for 7 days in a walk-in growth room. The Cd and Pb reduced the normalized transpiration and growth rate. Greater amount of Cd and Pb accumulated in roots relative to shoots. The enrichment coefficient (EC) and translocation factor (TF) were greater in plants treated with Cd than those with Pb. The dose of Cd had no effect on EC and TF. However, the plants subjected to higher dose of Pb had a greater TF and a lower EC relative to that with lower dose of Pb. Meanwhile, removal rates were decreased when the doses of Pb and Cd increased. The results indicated that perennial ryegrass preferred Cd assimilation to Pb, and can be considered as one of the plant species which accumulate high levels of Cd and Pb.

On the role of β-cyanoalanine synthase (CAS) in metabolism of free cyanide and ferri-cyanide by rice seedlings

A study was conducted to investigate the contribution of β-cyanoalanine synthase (CAS) to the botanical metabolism of free cyanide and iron cyanides. Seedlings of rice (Oryza sativa L. cv. XZX 45) were grown hydroponically and then amended with free cyanide (KCN) or ferri-cyanide [K(3)Fe(CN)(6)] into the growth media. Total cyanide, free cyanide, and Fe(3+)/Fe(2+) in aqueous solution were analyzed to identify the speciation of K(3)Fe(CN)(6). Activity of CAS in different parts of the rice seedlings was also assayed in vivo and results indicated that dissociation of K(3)Fe(CN)(6) to free cyanide in solution was negligible. Almost all of the applied KCN was removed by rice seedlings and the metabolic rates were concentration dependent. Phyto-transport of K(3)Fe(CN)(6) was apparent, but appreciable amounts of cyanide were recovered in plant tissues. The metabolic rates of K(3)Fe(CN)(6) were also positively correlated to the concentrations supplied. Rice seedlings exposed to KCN showed a considerable increase in the CAS activity and roots had higher CAS activity than shoots, indicating that CAS plays an important role in the botanical assimilation of KCN. However, no measurable change of CAS activity in different parts of rice seedlings exposed to K(3)Fe(CN)(6) was detected, suggesting that K(3)Fe(CN)(6) is likely metabolized by rice directly through an unknown pathway rather than the β-cyanoalanine pathway.

Effect of temperature on removal of iron cyanides from solution by maize plants

GOAL, SCOPE, AND BACKGROUND

Cyanide is commonly found in soils and groundwater complexed with iron as ferro- and ferri-cyanide. It is evident that plants are capable of tolerating, transporting, and assimilating iron cyanides. The objectives of this study were to investigate the influence of temperatures on the removal and bioaccumulation of two chemical forms of iron cyanides by maize seedlings.

MATERIALS AND METHODS

Maize (Zea mays L. var. ZN 304) seedlings were grown hydroponically and treated with ferro- or ferri-cyanide in solution for 5 days. Six different temperatures were tested ranging from 12 to 27 degrees C. Total cyanide in solution phase and plant tissues was analyzed spectrophotometrically. The temperature coefficient (Q (10)) was also determined for maize exposed to both iron cyanides.

RESULTS

The dissociation of both iron cyanides to free cyanide in solution was below the detection limit. Maize seedlings showed a significantly higher removal potential for ferro-cyanide than ferri-cyanide at all treatment temperatures. Analysis of mass balance revealed that the majority of these iron cyanides taken up from the hydroponic solutions was assimilated by maize, and roots were the major sink for cyanide accumulation. The Q (10) values were determined for ferro- and ferri-cyanide to be 2.31 and 2.75, respectively.

DISCUSSION

Due to the significant difference in the removal rate between the two species of iron cyanides by plant, the conversion of ferri- to ferro-cyanide in aqueous solution prior to uptake is unlikely. Compared to the treatments amended with ferro-cyanide, more cyanide was recovered in plant materials of maize when exposed to ferri-cyanide, implying that ferri-cyanide is less sensitive to degradation than ferro-cyanide. Although the velocity of botanical assimilation of ferro-cyanide was faster than that of ferri-cyanide at any of the treatment temperatures, the removal of ferri-cyanide by maize was more sensitive to changes in temperature than that of ferro-cyanide.

CONCLUSIONS

Removal of both iron cyanides by maize seedlings was observed to be positive in response to temperatures. Changes in temperatures have a substantial influence on not only the uptake and assimilation of ferro- and ferri-cyanide by maize but also cyanide accumulation in plant tissues.

RECOMMENDATIONS

As one of the crucial abiotic factors involved in phytoremediation, temperature shows a positive influence on the removal of iron cyanides by plants. Further investigation on the fate of ferro- and ferri-cyanide in plant tissues would have helped distinguish the differences in the botanical assimilation pathways between the two iron cyanides.

PERSPECTIVES

The ability of maize to remove iron cyanides has important implications on the vegetation management of environmental contamination.