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 20400 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.     

Phytoextraction: an assessment of biogeochemical and economic viability

Phytoextraction describes the use of plants to remove metals and other contaminants from soils. This low-cost technology has potential for the in situ remediation of large areas of contaminated land. Despite more than 10 years of intensive research on the subject, very few commercial phytoextraction operations have been realised. Here, we investigate the viability of phytoextraction as an effective land-treatment technology. A Decision Support System (DSS) was developed to predict the effect of phytoextraction on soil metal concentration and distribution, as well as the economic feasibility of the process in comparison to either inaction or the best alternative technology. Changes in soil metal concentration are mechanistically predicted on the basis of plant water use, metal concentration in soil solution, soil density, plant root distribution and our so-called root-absorption factor. The root-absorption factor is a `lumped parameter' describing the xylem/soil solution metal concentration quotient. Phytoextraction is considered to be a viable option if it can satisfy environmental regulations and simultaneously be shown to be the most cost-effective technology, either alone, or in combination with other remediation technologies. To date, commercial phytoextraction has been constrained by the expectation that site remediation should be achieved in a time comparable to other clean-up technologies. However, if phytoextraction could be combined with a profit making operation such as forestry, then this time constraint, which has often been considered to be the Achilles heel of phytoextraction, may be less important.     

Field Evaluation of Willow Under Short Rotation Coppice for Phytomanagement of Metal-Polluted Agricultural Soils

Short rotation coppice (SRC) of willow and poplar might be a promising phytoremediation option since it uses fast growing, high biomass producing tree species with often a sufficient metal uptake. This study evaluates growth, metal uptake and extraction potentials of eight willow clones (Belders, Belgisch Rood, Christina, Inger, Jorr, Loden, Tora and Zwarte Driebast) on a metal-contaminated agricultural soil, with total cadmium (Cd) and zinc (Zn) concentrations of 6.5 ± 0.8 and 377 ± 69 mg kg^?1 soil, respectively. Although, during the first cycle, on average generally low productivity levels (3.7 ton DM (dry matter) ha^?1 y^?1) were obtained on this sandy soil, certain clones exhibited quite acceptable productivity levels (e.g. Zwarte Driebast 12.5 ton DM ha^?1 y^?1). Even at low biomass productivity levels, SRC of willow showed promising removal potentials of 72 g Cd and 2.0 kg Zn ha^?1 y^?1, which is much higher than e.g. energy maize or rapeseed grown on the same soil. Cd and Zn removal can be increased by 40% if leaves are harvested as well. Nevertheless, nowadays the wood price remains the most critical factor in order to implement SRC as an acceptable, economically feasible alternative crop on metal-contaminated agricultural soils.     

Arundo donax L., a Candidate for Phytomanaging Water and Soils Contaminated by Trace Elements and Producing Plant-Based Feedstock. A Review

Plants and associated microorganisms are used to remediate anthropogenic metal(loid) contamination of water, soils and sediments. This review focuses on the potential of Arundo donax L. (Giant reed) for alleviating risks due to soils, water, and sediments contaminated by trace elements (TE), with emphasis on its advantages and limits over macrophytes and perennial grasses used for bioenergy and plant-based feedstock. Arundo donax is relevant to phytomanage TE-contaminated matrices, notably in its native area, as it possesses characteristics of large biomass production even under nutrient and abiotic stresses, fast growth rate, TE tolerance and accumulation mainly in belowground plant parts. Cultivating A. donax on contaminated lands and in constructed wetlands can contribute to increase land availability and limit the food vs. plant-based feedstock controversy. To gain more tools for decision-taking and sustainable management, further researches on A. donax should focus on: interactions between roots, TE exposure, and rhizosphere and endophytic microorganisms; biomass response to (a)biotic factors; sustainable agricultural practices on marginal and contaminated land; integration into local, efficient, energy and biomass conversion chains with concern to biomass quality and production; Life-Cycle Assessment including contaminant behavior, as well as environmental, agricultural and socio-economic benefits and drawbacks.     

Soil moisture effects on uptake of metals by Thlaspi, Alyssum, and Berkheya

Most commonly used hyperaccumulator plants for phytoextraction of metals evolved on soils where moisture is limited throughout much of the year. As these plant species are commercialized for use, they are frequently moved from the point of evolution to locations where environmental conditions may be significantly different. Greatest among these potential differences is soil moisture. The objective of this study was therefore to determine whether these plants could grow in soils with much higher soil moisture and whether they would continue to hyperaccumulate metals as soils approach saturation. We examined extractable soil metal concentrations, plant growth, and metal accumulation for the Ni hyperaccumulators, Alyssum murale and Berkheya coddii and the Zn hyperaccumulators Thlaspi caerulescens cultivars AB300 and AB336. Non-hyperaccumulating control species for each were also examined. In general, extractable soil concentrations of Ni decreased with increasing soil moisture content. Few significant effects related to Zn extractability were observed for any of the soil moisture treatments. The biomass of all tested species was generally greater at higher soil moisture and inhibited at low soil moisture. Further, plants accumulated large amounts of metals from soil at higher soil moisture. Highest foliar concentrations of Zn or Ni were found at the two highest WHCs of 80 and 100%. These results show that hyperaccumulators grow well under conditions of high soil moisture content and that they continue to hyperaccumulate metals. Thus, growing Thlaspi, Alyssum, and Berkheya for commercial phytoextraction under nonnative conditions is appropriate and suggests that this technology may be applied to a wide and diverse range of soil types, climatic conditions, and irrigation regimes.     

The Phytoremediation Potential of Thallium-Contaminated Soils Using Iberis and Biscutella Species

Biscutella laevigata and Iberis intermedia were sampled from sites near St Laurent le Minier, Southern France, and B. laevigata was also sampled from Rocca San Silvestro, Tuscany, Italy. Soils associated with the rhizosphere of each plant were also sampled. Both Biscutella laevigata and Iberis intermedia accumulate inordinately high concentrations of thallium (1.94 and 0.4%, respectively) in their above-ground dry tissue. The levels of thallium accumulated by both species were strongly correlated with both the total and extractable concentrations of thallium in the soils. Concentrations of zinc, cadmium, and lead were below the threshold for hyperaccumulation. It is proposed that B. laevigata and/or I. intermedia could be used for phytoremediation or phytomining of thallium-contaminated soils. Such an operation would involve the repeated cropping of either species, until an acceptable level of thallium in the soils was reached. Additionally, the harvested plant material could be burnt and the resulting ash smelted to produce an economically viable ‘crop’ of thallium.     

Phytoremediation of lead (Pb) and Arsenic (As) by Melastoma malabathricum L. from Contaminated Soil in Separate Exposure

This study was conducted to investigate the uptake of lead (Pb) and arsenic (As) from contaminated soil using Melastoma malabathricum L. species. The cultivated plants were exposed to As and Pb in separate soils for an observation period of 70 days. From the results of the analysis, M. malabathricum accumulated relatively high range of As concentration in its roots, up to a maximum of 2800 mg/kg. The highest accumulation of As in stems and leaves was 570 mg/kg of plant. For Pb treatment, the highest concentration (13,800 mg/kg) was accumulated in the roots of plants. The maximum accumulation in stems was 880 mg/kg while maximum accumulation in leaves was 2,200 mg/kg. Only small amounts of Pb were translocated from roots to above ground plant parts (TF < 1). However, a wider range of TF values (0.01–23) for As treated plants proved that the translocation of As from root to above ground parts was greater. However, the high capacity of roots to take up Pb and As (BF > 1) is indicative this plants is a good bioaccumulator for these metals. Therefore, phytostabilisation is the mechanism at work in M. malabathricum's uptake of Pb, while phytoextraction is the dominant mechanism with As.     

Potential of Selected Canadian Plant Species for Phytoextraction of Trace Elements From Selenium-Rich Soil Contaminated by Industrial Activity

In this preliminary screening study, we tested the phytoextraction potential of nine Canadian native/well-adapted plant species on a soil highly polluted by trace elements (TE) from a copper refinery. Plant physiological parameters and soil cover index were monitored for a 12-week period. At the end of the trial, biomass yield, bioconcentration (BFC) and translocation (TF) factors for the main TE as well as phytoextraction potential were determined. Most plants were severely injured by the high pollution levels, showing symptoms of toxicity including chlorosis, mortality and very low biomass yield. However, Indian mustard showed the highest selenium extraction potential (65 mg m^?2), even under harsh growing conditions. Based on our results, tall fescue and ryegrass, which mainly stored As, Cu, Pb and Zn within roots, could be used effectively for phytostabilization.     

Enhancing Phytoremediation Potential of Pennisetum clandestinum Hochst in Cadmium-Contaminated Soil Using Smoke-Water and Smoke-Isolated Karrikinolide

The use of plant growth regulators (PGRs) and biostimulants to enhance phytoextraction is gaining popularity in phytoremediation technology. This study investigated the stimulatory effects of smoke-water (SW), a smoke-derived compound karrikinolide (KAR₁) and other known plant growth regulators (PGRs) [gibberellic acid (GA₃), kinetin (Kin) and indole-3-butyric acid (IBA)] to enhance the phytoextraction potential of Pennisetum clandestinum. Pennisetum clandestinum seedlings were grown for 10 weeks in vermiculite using Hoagland's nutrient solution and were treated with cadmium (Cd) (2, 5, and 10 mg L^?1) and SW, KAR₁ and PGRs. KAR₁ exhibited positive effects on shoot and root dry weight (140 and 137 mg respectively) at the highest concentration of Cd (10 mg L^?1) compared to all the other treatments. KAR₁ and SW treatments used in the present study significantly improved the phytoextraction potential of P. clandestinum (602 and 575 mg kg^?1 respectively) compared to the other tested PGRs. This is the first report on the use of SW and KAR₁ to enhance phytoremediation potential in P. clandestinum. Further studies are needed to elucidate the exact mechanisms of smoke constituents involved in phytoextraction potential of plant species.     

Metal hyperaccumulation and bioremediation

The phytoremediation is an environment friendly, green technology that is cost effective and energetically inexpensive. Metal hyperaccumulator plants are used to remove metal from terrestrial as well as aquatic ecosystems. The technique makes use of the intrinsic capacity of plants to accumulate metal and transport them to shoots, ability to form phytochelatins in roots and sequester the metal ions. Harbouring the genes that are considered as signatures for the tolerance and hyperaccumulation from identified hyperaccumulator plant species into the transgenic plants provide a platform to develop the technology with the help of genetic engineering. This would result in transgenics that may have large biomass and fast growth a quality essential for removal of metal from soil quickly and in large quantities. Despite so much of a potential, the progress in the field of developing transgenic phytoremediator plant species is rather slow. This can be attributed to the lack of our understanding of complex interactions in the soil and indigenous mechanisms in the plants that allow metal translocation, accumulation and removal from a site. The review focuses on the work carried out in the field of metal phytoremediation from contaminated soil. The paper concludes with an assessment of the current status of technology development and its future prospects with emphasis on a combinatorial approach.     

Metal Tolerance and Accumulation Ability of the Ni Hyperaccumulator Streptanthus polygaloides Gray (Brassicaceae)

High concentrations of metals occur in some plant species (termed hyperaccumulators), such as the Ni hyperaccumulator Streptanthus polygaloides. We determined the tolerance of S. polygaloides to, and its accumulation abilities for, six metals (Ni, Zn, Cu, Co, Mn, and Pb). Potting mix concentrations used for all metals ranged from 0 to 1200 μg/g dry weight. For Ni, a treatment of 1600 μg/g was included. For Mn, treatments of 1600, 2000, and 2500 μg/g also were used, and for Pb these concentrations plus 3500 μg/g were included. Germination, plant number per pot, and size at days 30 and 39, number of plants at the end of the experiment (day 49), flower production, and metal concentration in the aboveground biomass were documented. Lead and Ni showed no consistent effects on plant performance, but yielded increased tissue metal concentrations. Streptanthus polygaloides was more sensitive to Co, Cu, and Zn, as ≥ 400 mg/g significantly suppressed plant growth, survival, and flower production. Tissue metal concentrations also were increased to maxima of 1500 μg Co/g, 120 μg Cu/g, and 6000 μg Zn/g. Manganese affected S. polygaloides less markedly, as ≥ 800 mg/kg decreased growth, survival, and flower production. Maximum tissue Mn concentration was 2900 μg/g. We concluded that S. polygaloides would be an appropriate phytoextractor for soils contaminated with Ni or low levels of Co but would not be useful for Cu, Zn, Mn, and Pb.     

Healthy Response from Chromium Survived Pteridophytic Plant-Ampelopteris prolifera with the Interaction of Mycorrhizal Fungus-Glomus deserticola

Interaction between arbuscular mycorrhizal fungus Glomus deserticola and pteridophytic member Ampelopteris prolifera was found abundant on entire growth level based on elemental composition and gaseous exchange as a potential remediation system for phytoextraction of chromium. Inoculated A. prolifera (AM) and non-inoculated A. prolifera (Non-AM) were supplied with two Cr species: 12 mmol of trivalent cation (Cr⁺³) [Cr(III)] and 0.1 mmol of divalent dichromate anion (Cr₂O₇ ^?2) [Cr(VI)]. Both Cr species were found to be depressed in overall growth and inefficient stomatal conductance (g_s) and net photosynthesis (NP). Mycorrhizal association was found to be natural scavenger of Cr toxicity as indicated by greater growth in plants exposed to Cr species, and increased gas exchange of Cr(III) treated plants. Though, chromium reduction resulted lower level of nitrogen (N), phosphorus (P), and potassium (K) but interestingly elevated the level of aluminum (Al), iron (Fe), and zinc (Zn) uptake in many folds which is the significance of sustainable growth of plant.     

Phytoextraction of soil trace elements by willow during a phytoremediation trial in Southern Québec,Canada

The phytoextraction of the trace elements (TEs) As, Cd, Cu, Ni, Pb, and Zn by willow cultivars (Fish Creek, SV1 and SX67) was measured during a 3-year field trial in a mildly contaminated soil. Biomass ranged from 2.8 to 4.4 Mg/ha/year at 30,000 plants/ha. Shoots (62%) were the main component followed by leaves (23%) and roots (15%). Biomass was positively linked to soluble soil dissolved organic carbon, K, and Mg, while TEs, not Cd and Zn, had a negative effect. The TE concentration ranking was: Zn > Cu > Cd > Ni, Pb > As, and distribution patterns were: (i) minima in shoots (As, Ni), (ii) maxima in leaves (Cd, Zn), or (iii) maxima in roots (Cu, Pb). Correlations between soil and plant TE were significant for the six TEs in roots. The amounts extracted were at a maximum for Zn, whereas Fish Creek and SV1 extracted more TE than SX67. More than 60% (91–94% for Cd and Zn) of the total TE was in the aboveground parts. Uptake increased with time because of higher biomass. Fertilization, the selection of cultivars, and the use of complementary plants are required to improve productivity and Cd and Zn uptake.     

Phytoremediation of biosolids from an end-of-life municipal lagoon using cattail (Typha latifolia L.) and switchgrass (Panicum virgatum L.)

Land spreading of biosolids as a disposal option is expensive and can disperse pathogens and contaminants in the environment. This growth room study examined phytoremediation using switchgrass (Panicum virgatum L.) and cattail (Typha latifolia L.) as an alternative to land spreading of biosolids. Seedlings were transplanted into pots containing 3.9 kg of biosolids (dry wt.). Aboveground biomass (AGB) was harvested either once or twice during each 90-day growth period. Switchgrass AGB yield was greater with two harvests than with one harvest during the first 90-day growth period, whereas cattail yield was not affected by harvest frequency. In the second growth period, harvesting frequency did not affect the yield of either plant species. However, repeated harvesting significantly improved nitrogen (N) and phosphorus (P) uptake by both plants in the first period. Phytoextraction of P was significantly greater for switchgrass (3.9% of initial biosolids P content) than for cattail (2.8%), while plant species did not have a significant effect on N phytoextraction. The trace element accumulation in the AGB of both plant species was negligible. Phytoextraction rates attained in this study suggest that phytoremediation can effectively remove P from biosolids and offers a potentially viable alternative to the disposal of biosolids on agricultural land.     

Effects of Soil Trace Elements on Longevity Population in China

Based on background concentrations of elements in soils and the sixth population census data of China, this study discussed the distribution characteristics of soil elements and longevity population at province level in China. Percentages of the aging population are high in Southwest China and the eastern coastal region but low in western and northwestern regions. Provinces in South and Southwest China gain a high level of longevity, while the northern part of China has a low level of longevity. The background concentration of Se in soil has a significant positive correlation with longevity index, while Ba and Ni have a significant negative correlation with longevity indexes. By regression analysis, longevity index C/100,000 can be expressed as C/100,000?=?1.679?0.205 Ni?+?0.413 Co?+?0.006 Se (with R ²?=?0.402 and p?<?0.01), C/65+ can be expressed as C/65+=3.425?0.262 Ni?+?0.435 Co?+?0.006 Se (with R ²?=?0.369 and p?<?0.01).     

Wetland-based phytoremediation of biosolids from an end-of-life municipal lagoon: A microcosm study

This study examined the effectiveness of a wetland system for phytoremediation of biosolids from an end-of-life municipal lagoon. The microcosm experiment tested the effects of one vs. two harvests of cattail per growth cycle in biosolids without (PB) or with (PBS) the addition of soil on phytoremediation. Cattail (Typha latifolia) seedlings were transplanted into pots containing 4.5 kg (dry wt.) of biosolids, above which a 10-cm deep water column was maintained. Results showed that two harvests per growth cycle significantly increased N and P phytoextraction relative to a single harvest. Overall, the three cycles of cattail removed ～3.7% of N which was originally present in the biosolids and ～2% of the total P content. Phytoextraction rates are expected to be higher under field conditions where biomass yields are much higher than those obtained under growth room conditions in this study. These results indicate that wetland-based phytoremediation can effectively clean up nutrients from biosolids, and therefore presents a potential alternative to the spreading of biosolids on agricultural land, which may not be readily available in some communities. Phytoextraction rates of trace elements, however, were much lower (0.02–0.17%). Nonetheless, trace element concentrations were not high enough to be of significant concern.     

Negative Effects of Chelants on Soil Qualities of Five Soil Series

Due to the low phytoavailability of some heavy metals (HMs), a prolonged period is required when phytoextraction is used to remove these HMs from contaminated soils. The use of chelants and other chemical compounds are often used to increase the phytoavailability of the HMs for plant uptake. Negative effects of chemical agents on the soil and groundwater have rarely been reported during chemical-enhanced phytoextraction. This research applied chelants to various soil series with different characteristics to assess their impacts on soil quality. The experimental results showed that the application of 5 mmol kg^?1 of all chelants had a negative effect on the soil quality. This was especially true for electrical conductivity (EC) when diethylene trinitrilo pentaacetic acid (DTPA) was used as the chemical extracting agent.     

Magnesium and iron deficiencies alter Cd accumulation in Salix viminalis L.

Evidence exists that Cd and certain nutrient elements, such as Fe and Mg, could share similar mechanisms of plant uptake and accumulation. Here we report that Mg and Fe deficiency in mature plants of Salix viminalis, grown in hydroponic solutions containing 5 µg ml^?1 of Cd, caused a significant increase in Cd accumulation in roots, stems and leaves. Cd (µg g^?1 dry weight) was determined following three treatments: 1) Cd treatment in complete nutrient solution; 2) Cd treatment with Fe deficiency; and 3) Cd treatment with Mg deficiency, yielding, respectively: in young leaves (65.3, 76.1, and 92.2), mature leaves (51.5 to 76.3 and 87.1), upper stems (80.6, 116.8, and 130.6) lower stems (67.2, 119, and 102.3), roots (377.1, 744.8, and 442,5). Our results suggest that Cd utilizes the same uptake and transport pathways as Mg and Fe. Evidence exists that Mg and Fe uptake and translocation could be further facilitated by plants as an adaptive response to deficiency of these elements. Such physiological reaction could additionally stimulate Cd accumulation. Although Cd uptake was mostly confined in roots, high Cd content in aerial plant parts (51.5–130.6 µg g^?1) indicates that the analysed Salix viminalis genotype is suitable for phytoextraction.     

The Role of Trace Elements in Tinnitus

A growing understanding of antioxidant mechanisms and insulin-like actions of trace elements selenium and zinc has rekindled researchers’ interest towards their role in diabetes mellitus, nutritional management of which concentrates predominantly on macronutrient intake. However, selenium studies limiting largely to diabetes have yielded inconsistent results with sparse knowledge in the pre-diabetes population. This hospital-based cross-sectional study screened 300 people who came to the institutional hospital laboratory with fasting plasma glucose and glycosylated haemoglobin requisition over a period of 6 months. Thirty-five pre-diabetes subjects aged 25–45 years and 35 age-matched healthy controls were selected as per inclusion criteria and clinical history. Serum selenium was estimated by inductively coupled plasma-mass spectrometry, zinc and magnesium by colorimetric end-point methods and insulin by enzyme-linked immunosorbent assay, and insulin resistance was calculated using a homeostasis model assessment (HOMA) 2 calculator. Data analysis was done using SPSS ver. 16 employing an independent sample t test for intergroup comparison of means and Pearson’s correlation for correlation analysis. Serum mineral levels in the pre-diabetes group (selenium 63.01 ± 17.6 μg/L, zinc 55.78 ± 13.49 μg/dL, magnesium 1.37 ± 0.38 mg/dL) were significantly reduced (p < 0.05) in comparison to the healthy controls (selenium 90.98 ± 15.81 μg/L, zinc 94.53 ± 15.41 μg/dL, magnesium 2.12 ± 0.22 mg/dL). A significant negative correlation was seen with glycaemic indices and insulin resistance. This study conducted in pre-diabetes subjects highlights a considerable deficiency of serum selenium, zinc and magnesium observed at a much earlier pre-clinical phase. This coupled with the evidence of a strong inverse association with glycaemic indices and insulin resistance postulates the role of mineral alterations in the pathophysiology of hyperglycaemia and insulin resistance.     

Chelate-Enhanced Phytoremediation of Soils Polluted with Heavy Metals

In general, hyperaccumulators are low biomass, slow-growing plants. High biomass non-hyperaccumulator plants by themselves are not a valid alternative for phytoextraction as they also have many limitations, such as small root uptake and little root-to-shoot translocation. In this context, chemically-induced phytoextraction (based on the fact that the application of certain chemicals, mostly chelating agents, to the soil significantly enhances metal accumulation by plants) has been proposed as an alternative for the cleaning up of metal polluted soils. But chelate-induced phytoextraction increases the risk of adverse environmental effects due to metal mobilization during extended periods of time. In order to minimize the phytotoxicity and environmental problems associated with the use of chelating agents, nowadays, research is being carried out on the gradual application of small doses of the chelating agent during the growth period. However, EDTA utilization in the future will most likely be limited to ex situconditions where control of the leachates can be achieved. There are other mobilizing agents which are much less harmful to the environment such as citric acid, NTA, and particularly EDDS. Research should also be aimed towards more innovative agronomic practices. Environmentally safe methods of chelate-induced phytoextraction must be developed before steps towards further development and commercialization of this remediation technology are taken. Most importantly, more applied projects in this field are needed to clarify the real potential and risks of this technology.