Rufus L. Chaney

Phytoremediation of soil metals

The phytoremediation of metal-contaminated soils offers a low-cost method for soil remediation and some extracted metals may be recycled for value. Both the phytoextraction of metals and the phytovolatilization of Se or Hg by plants offer great promise for commercial development. Natural metal hyperaccumulator phenotype is much more important than high-yield ability when using plants to remove metals from contaminated soils. The hypertolerance of metals is the key plant characteristic required for hyperaccumulation; vacuolar compartmentalization appears to be the source of hypertolerance of natural hyperaccumulator plants. Alternatively, soil Pb and Cr6+ may be inactivated in the soil by plants and soil amendments (phytostabilization). Little molecular understanding of plant activities critical to phytoremediation has been achieved, but recent progress in characterizing Fe, Cd and Zn uptake by Arabidopsis and yeast mutants indicates strategies for developing transgenic improved phytoremediation cultivars for commercial use.

Zinc and Cadmium Uptake by Hyperaccumulator Thlaspi caerulescens and Metal Tolerant Silene vulgaris Grown on Sludge-Amended Soils

Two metal tolerant plants, Thlaspi caerulescens J. and C. Presl. (hyperaccumulator), and Silene vulgaris L. (indicator) were grown with Paris Island Cos Romaine lettuce (Lactuca sativa var. longifolia) on longterm sewage sludge plots. Metal uptake patterns by plants in relation to total soil metal and soil pH were examined. The 2-year study used four treatments and two pH levels. Zinc and Cd uptake were measured. Zinc and Cd for Silene and lettuce were as expected with increasing plant concentration in the more contaminated treatments and lower pH levels. Thlaspi followed the same pattern for Cd but not for Zn. Concentrations of Cd were not significantly different between Thlaspi and the other plants. Zinc concentrations in Thlaspi (2000 and 4000 mg kg -1 ) were 10-fold greater than in Silene. They showed no relation to available soil Zn. Although Thlaspi appears to hyperaccumulate Zn on mildly contaminated soils, Cd uptake follows predictable patterns.

Effects of vesicular-arbuscular mycorrhizal fungi on heavy metal uptake by soybeans

The uptake of heavy metals by soybeans may be affected by the colonization of roots with vesicular-arbuscular mycorrhizal (VAM) fungi. Soils with various heavy metal contents were collected from areas in close proximity to a Zn smelter in operation for nearly 100 yr. All soils were placed into pots and steam sterilized. One treatment was inoculated with mixed VAM fungi (600 spores per pot) and soil bacteria. The second treatment was inoculated with soil bacteria (minus VAM fungi) and the third treatment remained sterile. Soybeans (Glycine max L. Merr. “Essex”) were sown into each soil. After 6 weeks of growth, the plants were harvested and the dry weight and the content of N, P, Zn, Cd, Cu, Mn and Fe were determined in plant leaves. The amount of VAM fungal colonization, nitrogenase activity and number and weight of nodules were also determined on plant roots. Results indicate that inoculation with soil bacteria and VAM fungi increased plant dry weight and foliar P and N contents. Inoculation with VAM fungi reduced Zn, Cd and Mn concentrations in plant leaves grown in soil with high concentrations of these metals. VAM fungi enhanced foliar concentrations of these heavy metals when the plants were grown in soils with low heavy metal concentrations. The colonization of roots by VAM fungi was reduced at the highest soil metal concentrations. These results indicate that the effect of VAM fungi on heavy metal uptake is dependent upon the initial soil metal concentration.

Development of a technology for commercial phytoextraction of nickel: economic and technical considerations

In recent R&D work, we have made progress in developing a commercial technology using hyperaccumulator plant species to phytoextract nickel (Ni) from contaminated and/or Ni-rich soils. An on-going program is being carried out to develop a genetically improved phytoextraction plant that combines favorable agronomic and Ni accumulation characteristics. Genetically diverse Ni hyperaccumulator species and ecotypes of Alyssum were collected and then evaluated in both greenhouse and field using serpentine and Ni-refinery contaminated soils. Large genetic variation was found in those studies. Mean shoot Ni concentrations in field-grown plants ranged from 4200 to 20 400 mg kg−1. We have been studying several soil management practices that may affect the efficiency of Ni phytoextraction. Soil pH is an important factor affecting absorption of metals by plants. An unexpected result of both greenhouse and field experiments was that Ni uptake by two Alyssum species was reduced at lower soil pH and increased at higher soil pH. At higher pH, plant yield was improved also. In soil fertility management studies, we found that N application significantly increased plant biomass, but did not affect plant shoot Ni concentration. These findings indicate that soil management will be important for commercial phytoextraction. A number of field trials have been carried out to study planting methods, population density, weed control practices, harvest schedule and methods, pollination control, and seed processing. Such crop management studies have improved phytoextraction efficiency and provide a tool for farmers to conduct commercial production. We have done some work to develop efficient and cost-effective methods of Ni recovery. Recovery of energy by biomass burning or pyrolysis could help make phytoextraction more cost-effective. The progress made in our recent studies will enable us to apply this technology commercially in the near future.

Using municipal biosolids in combination with other residuals to restore metal-contaminated mining areas

High metal waste materials from historic mining at the Bunker Hill, Idaho (ID) Superfund site was amended with a range of materials including municipal biosolids, woody debris, wood ash, pulp and paper sludge, and compost. The existing soil or waste material has elevated metal concentrations with total Zn, Pb and Cd ranging from 6000 to 14 700, 2100 to 27 000 and 9 to 28 mg kg−1, respectively. Surface application of certain amendments including biosolids mixed with wood ash resulted in significant decreases in subsoil acidity as well as subsoil extractable metals. This mixture was sufficient to restore a plant cover to the contaminated areas. At the Bunker Hill site, a surface application of high N biosolids (44 or 66 tons ha−1) in combination with wood ash (220 tons ha−1) with or without log yard debris (20% by volume) or pulp and paper sludge (44 tons ha−1) was able to restore a vegetative cover to the metal contaminated materials for 2 years following amendment application. Plant biomass in 1999 was 0.01 mg ha−1 in the control versus a mean of 3.4 tons ha−1 in the residual amended plots. Metal concentrations of the vegetation indicated that plants were within normal concentrations for the 2 years that data were collected. Surface application of amendments was also able to reduce Ca(NO3)2 extractable Zn in the subsoil from about 50 mg kg−1in the control to less than 4 mg kg−1in two of the treatments. Use of conventional amendments including lime alone and microbial stimulants were not sufficient to support plant growth. These results indicate that surface application of biosolids in combination with other residuals is sufficient to restore a vegetative cover to high metal mine wastes.

Land Treatment of Wastewater

Publisher Summary This chapter describes land treatment of wastewater and discusses the selection and design of system for land treatment. Minimum impact on the environment and minimum total cost of operation are the two main design criteria for land treatment of liquid waste. The choice of system is largely controlled by soil and hydrogeologic conditions and by the availability of land. The chapter divides land treatment systems into three types: overland flow systems, low-rate application systems, and high-rate application systems. Overland flow systems are used where the soil is too impermeable or the suspended solids content of the wastewater is too high to allow significant infiltration rates, causing most of the wastewater to run off. With low-rate application systems, all wastewater apply infiltrates into the soil, but the dosages are rather small and of the same order as the water requirements of the crop or vegetation. With high-rate application systems, all wastewater again infiltrates into the soil, but the dosage is much greater than that necessary for crop growth.

Free metal activity and total metal concentrations as indices of micronutrient availability to barley [Hordeum vulgare (L.) ‘Klages’]

The form in which a micronutrient is found in the rhizosphere affects its availability to plants. We compared the availability to barley of the free hydrated cation form of Fe3+, Cu2+, Zn2+, and Mn2+ versus their total metal concentrations (free ion plus complexes) in chelator-buffered solutions. Free metal ion activities were estimated using the chemical equilibrium program GEOCHEM-PC with the corrected database. In experiment 1, barley was grown in nutrient solutions with different Fe3+ activities using chelators to control Fe levels. Chlorosis occurred at Fe3+ activities of 10−18 and 10−19M for barley grown in HEDTA and EDTA solutions, respectively. In experiment 2, barley was grown in nutrient solutions with the same calculated Fe3+ activity and the same chelator, but different total Fe concentrations. Leaf, root and shoot Fe concentrations were higher from CDTA buffered solutions which had the higher total Fe concentration indicating the importance of the total Fe concentration on Fe uptake. Results from treatments using EDTA or HEDTA, with one exception, were similar to the results from the CDTA treatment. This suggests differences in critical Fe3+ activities found in experiment 1 were due to differences in the total Fe concentration and not errors in chelate formation constants used to estimate the critical activities. Results for Cu, Zn, and Mn were similar to Fe; despite solutions with equal free Cu2+, Zn2+ and Mn2+ activities, plant concentrations of these metals were generally greater when grown in the solutions with the greater total amount of Cu, Zn, or Mn. When the free Zn2+ activity was kept constant while the total amount of Zn was increased from 4.4 to 49 μM, leaf Zn concentration increased from 77 to 146 μg g-1. In order to predict metal availability to barley and other species in chelator-buffered nutrient solutions, both free and total metal concentrations in solution must be considered. The critical Fe3+ activities required by barley in this study are much higher than those from tomato and soybean, 10-28M, which strongly supports the Strategy 2 model of Fe uptake for Poaceae. This is related to the importance of the Fe3+ (barley) and the Fe2+ (tomato and soybean) ions in Fe uptake. Fe-stressed barley is known to release phytosiderophores which compete for Fe3+ in the nutrient solution, while tomato and soybean reduce Fe3+ to Fe2+ at the epidermal cell membranes to allow uptake of Fe2+ from Fe3+ chelates in solution.

Bioavailability as an issue in risk assessment and management of food cadmium: A review

The bioavailability of cadmium (Cd) from food is an important determinant of the potential risk of this toxic element. This review summarizes the effects of marginal deficiencies of the essential nutrients zinc (Zn), iron (Fe), and calcium (Ca) on the enhancement of absorption and organ accumulation and retention of dietary Cd in laboratory animals. These marginal deficiencies enhanced Cd absorption as much as ten-fold from diets containing low Cd concentrations similar to that consumed by some human populations, indicating that people who are nutritionally marginal with respect to Zn, Fe, and Ca are at higher risk of Cd disease than those who are nutritionally adequate. Results from these studies also suggest that the bioavailability of Cd is different for different food sources. This has implications for the design of food safety rules for Cd in that if the dietary source plays such a significant role in the risk of Cd, then different foods would require different Cd limits. Lastly, the importance of food-level exposures of Cd and other potentially toxic elements in the study of risk assessment are emphasized. Most foods contain low concentrations of Cd that are poorly absorbed, and it is neither relevant nor practical to use toxic doses of Cd in experimental diets to study food Cd risks. A more comprehensive understanding of the biochemistry involved in the bioavailability of Cd from foods would help resolve food safety questions and provide the support for a badly needed advance in international policies regarding Cd in crops and foods.

Complexity of iron nutrition: Lessons for plant‐soil interaction research

Abstract Iron deficiency chlorosis remains an economically important plant nutrition problem after decades of research. However, basic research on Fe nutrition has provided much information important to a general understanding of plant nutrition, including: 1) chemical equilibria in nutrient solutions and xylem and phloem fluids; 2) regulation of the roots Fe uptake potential to meet the availability of Fe to the root; 3) localization of ferric reduction, Fe‐uptake, and proton excretion in young parts of roots (<3cm from tip) of main and lateral roots; 4) excretion of ligands (phytosiderophores) by graminae to facilitate cation diffusion and uptake; 5) excessive phosphate use as pH buffer in nutrient solutions confounds plant research compared to other buffers; and 6) uptake of Fe by plants under sterile conditions shows uptake is a fundamental plant capability; however, plants may also use Fe from some microbial siderophores. One of the most important iron nutrition research findings is regulatory contr...

Mitigation effects of silicon rich amendments on heavy metal accumulation in rice (Oryza sativa L.) planted on multi-metal contaminated acidic soil

The mechanisms of stabilization by silicon-rich amendments of cadmium, zinc, copper and lead in a multi-metal contaminated acidic soil and the mitigation of metal accumulation in rice were investigated in this study. The results from a pot experiment indicated that the application of fly ash (20 and 40gkg(-1)) and steel slag (3 and 6gkg(-1)) increased soil pH from 4.0 to 5.0-6.4, decreased the phytoavailability of heavy metals by at least 60%, and further suppressed metal uptake by rice. Diffusion gradient in thin-film measurement showed the heavy metal diffusion fluxes from soil to solution decreased by greater than 84% after remediation. X-ray diffraction analysis indicated the mobile metals were mainly deposited as their silicates, phosphates and hydroxides in amended treatments. Moreover, it was found metal translocation from stem to leaf was dramatically restrained by adding amendments, which might be due to the increase of silicon concentration and co-precipitation with heavy metals in stem. Finally, a field experiment showed the trace element concentrations in polished rice treated with amendments complied with the food safety standards of China. These results demonstrated fly ash and steel slag could be effective in mitigating heavy metal accumulation in rice grown on multi-metal contaminated acidic soils.