Francesca Pedron

Strategies to use phytoextraction in very acidic soil contaminated by heavy metals

In microcosm experiments, the use of inorganic and organic amendments has been studied as potential agents to reduce heavy metal bioavailability in an acidic soil highly contaminated by Cu, Zn and Ni, that has to be remediated by phytoremediation. The concentrations of heavy metals in the original soil (O-Soil) produced phytotoxic effects with a strong reduction in biomass yield that hinder the utilization of this technology. To overcome phytotoxicity the use of three immobilizing agents was evaluated. The results obtained showed that all the strategies decreased the mobile fractions of heavy metals in soil and increased the metal removal efficiency. In the case of Brassica juncea the best results for Zn and Ni were obtained after zeolites addition (Z-Soil) with an increase of about 6 times with respect to the value found in the O-Soil. In the case of Cu, the more efficient treatment was Ca(OH)(2) addition (Ca-Soil). The B. juncea plants accumulated Cu amounts 8 times greater than in the O-Soil. For this metal, relevant results were obtained also with compost, that increased the amount of Cu in the plants of 7 times with respect to the O-Soil. Similar results were obtained with Helianthus annuus the highest Zn and Ni accumulation was detected in the Z-Soil and compost-treated soils (C-Soil), with an increase of nearly 11 times with respect to the accumulation in the O-Soil. In the case of Cu the highest increase of total uptake was found in the C-Soil: 28 times higher than in the O-Soil. Total accumulation in Poa annua plants showed the highest removal efficiency in the Z-Soil for all metals. The values obtained increased of 4, 11 and 12 times for Cu, Zn and Ni, respectively.

Nitrogen fertilizer improves boron phytoextraction by Brassica juncea grown in contaminated sediments and alleviates plant stress

In this study we evaluated the effect of different fertilizer treatments on Brassica plants grown on boron-contaminated sediments. Experiments were conducted in the laboratory and on the lysimeter scale. At laboratory scale (microcosm), five different fertilizers were tested for a 35-d period. On the lysimeter scale, nitrogen fertilization was tested at three different doses and plants were allowed to grow until the end of the vegetative phase (70 d). Results showed that nitrogen application had effectively increased plant biomass production, while B uptake was not affected. Total B phytoextracted increased three-fold when the highest nitrogen dose was applied. Phytotoxicity on Brassica was evaluated by biochemical parameters. In plants grown in unfertilized B-contaminated sediments, the activity of antioxidant enzymes superoxide dismutase (SOD), ascorbate peroxidase (APX) and pyrogallol peroxidase (PPX) increased, whereas catalase (CAT) decreased with respect to control plants. Addition of N progressively mitigated the alteration of enzymatic activity, thus suggesting that N can aid in alleviating B-induced oxidative stress. SOD activity was restored to control levels just at the lowest N treatment, whereas the CAT inhibition was partially restored only at the highest one. N application also lowered the B-induced increase in APX and PPX activities. Increased glutathione reductase activity indicated the need to restore the oxidative balance of glutathione. Data also suggest a role of glutathione and phytochelatins in B defense mechanisms. Results suggest that the nitrogen fertilizer was effective in improving B phytoextraction by increasing Brassica biomass and by alleviating B-induced oxidative stress.

Using a plant hormone and a thioligand to improve phytoremediation of Hg-contaminated soil from a petrochemical plant.

Mercury-contaminated soils from a petrochemical plant in southern Italy were investigated to assess the phytoextraction efficiency of crop plants treated with the phytohormone, cytokinine (CK foliar treatment), and with the thioligand, ammonium thiosulfate (TS, soil application). Plant biomass, evapotranspiration, Hg uptake and distribution in plant tissues following treatment were compared. Results indicate the effectiveness of CK in increasing plant biomass and the evapotranspiration rate while TS treatment promoted soil Hg solubility and availability. The simultaneous addition of CK and TS treatments increased Hg uptake and translocation in both tested plants with up to 248 and 232% in Brassica juncea (Indian mustard) and Helianthus annuus (sunflower) respectively. B. juncea was more effective in Hg uptake, whereas H. annuus gave better response regarding plant biomass production. The effectiveness of the treatments was confirmed by the calculation of Hg phytoextraction and evaluation of labile-Hg residue in the soil after plant growth. In one growing cycle the plants subject to simultaneous CK and TS treatment significantly reduced labile-Hg pools that were characterized by the soil sequential extraction, but did not significantly affect the pseudototal metal content in the soil. Results support the use of plant growth regulators in the assisted phytoextraction process for Hg-contaminated soils.

Remediation of a Mercury-Contaminated Industrial Soil Using Bioavailable Contaminant Stripping

Abstract The method to remove bioavailable amounts of heavy metals from a contaminated soil by using plants is defined as bioavailable contaminant stripping (BCS) and could safely be applied if the soils long-term ability to replenish the bioavailable pool is known. The aim of this study was to evaluate the ability of three common plant species selected, Brassica juncea, Poa annua, and Helianthus annus, to remove bioavailable amounts of mercury (Hg) from a contaminated industrial soil containing 15.1 mg kg−1 Hg. Trials were carried out under greenhouse conditions using pots (mesocosms). According to the precautionary principle, we modified the BCS remediation approach by adding a new step, in which mercury bioavailability was increased by the addition of a strong mobilizing agent, ammonium thiosulphate, (NH4)2S2O3, to obtain an estimate of the likely long-term bioavailable Hg pool. The modified BCS remediation approach was called enhanced bioavailable contaminant stripping (EBCS). After one growth cycle, nearly all the bioavailable mercury (95.7%) was removed and the metal remaining in the soil was considered inert since it was neither extractable by (NH4)2S2O3 nor taken up by plants during a second growth cycle. The results demonstrated that EBCS appeared promising since it removed the most dangerous metal forms while substantially shortening the cleanup time.

Mercury Mobilization in a Contaminated Industrial Soil for Phytoremediation

The aim of this work was to investigate the possibility of using plants for mercury (Hg) removal from a contaminated industrial soil, increasing the metals bioaccessibility by using mobilizing agents: ammonium thiosulphate [(NH4)2S2O3] and potassium iodide (KI). The selected plant species were Brassica juncea and Poa annua. The addition of the mobilizing agents promoted Hg uptake by plants, with respect to controls. Treatments promoted Hg translocation to aerial parts. In the case of Poa annua, greater Hg uptake was found in plants after the 100 mM KI treatment, reaching values that were nearly 400 mg kg−1 in the aerial part. In contrast, Brassica juncea plants accumulated in their aerial part the greatest Hg quantities, about 100 mg kg−1, after treatment with 0.27 M (NH4)2S2O3. The ratio between the concentration of Hg in the shoots and the initial concentration in the soil support the potential for successfully applying Hg phytoextraction on this soil.

Green remediation strategies to improve the quality of contaminated soils

Soil contamination has been identified as an important issue requiring action for soil protection in Europe. New approaches to remediation will be required if soils are to perform their essential functions. There is a need to find new strategies of remediation which, to date, have involved rather drastic technologies or landfill disposal. The US EPA is promoting strategies (green remediation) that restore contaminated sites to productive use with great attention paid to global environmental quality, including the preservation of soil functionality. As an example of this approach, a case study is reported in which phytoremediation as a ‘green remediation strategy’ has been selected to clean-up sites contaminated by polycyclic aromatic hydrocarbons (PAHs). Lupinus albus and Zea mays were tested and the efficiency of the remediation was determined comparing the decrease in PAH concentration in soil with and without plants growing (32 and 22%, respectively). This is a consequence of the presence of plants that stimulated the microbial biomass involved in PAH degradation. As an index of soil quality at the end of the phytoremediation test, soil stability structure was evaluated by means of wet aggregate stability (WAS). This parameter sharply increased from 35 to 60% after cultivating selected plants.

The Bioavailability Processes as a Key to Evaluate Phytoremediation Efficiency

Bioavailability is the key to understanding the risks from pollution and to defining remediation strategies, since organisms only respond to the fraction that is biologically available. Plants are able to uptake the substances only if present in available forms in the soil environment; this means that bioavailability is the key to evaluating the feasibility of phytoextraction as a remediation technology.

Phytoremediation Towards the Future: Focus on Bioavailable Contaminants

The hypothesis that one of the possible future trends of phytoextraction should be the removal of the bioavailable contaminants has recently received renewed and increasing interest. This fraction is the most hazardous to the environment and human health. The evaluation of contaminant bioavailability is essential for the appropriate application of the technology. Before selecting a phytoextraction process, it is necessary to consider the specific characteristics of the soil at the contaminated site in order to evaluate how the soil properties will influence the final result of remediation in the field. With this approach based on the concept of bioavailable contaminant stripping (BCS), an evaluation of the hazards of the residual fraction not removed by plants is left to a risk assessment procedure. This approach can be safely applied, if the soil ability to replenish the bioavailable pools in the long term is considered. This problem can be overcome by enhanced bioavailable contaminant stripping (EBCS) which evaluates this amount through the combined use of chemical extraction with a mobilizing agent, specific for each metal, and pot experiments in which successive growing cycles must confirm the absence of the bioavailable fractions. Phytoextraction can be selected as “green remediation” to minimize the mobile and bioavailable fractions of contaminants, while improving soil quality.

Protocols for Applying Phytotechnologies in Metal-Contaminated Soils

Contamination with heavy metals continues to pose a serious challenge for the remediation of polluted soil, as they are not degradable and must be physically removed. At present, most technologies used for removing heavy metals from the soil greatly affect the biogeochemical characteristics of the soil. In many cases, the soil can no longer be considered a useful and productive soil resource, and the treated soil has to be disposed of in landfills. Phytoremediation is the only solution that approaches the problem from an eco-sustainable point of view—it is environmentally friendly and relatively cheap. In this chapter, two phytotechnology approaches for remediating heavy metal-contaminated soil will be discussed, along with protocols for their implementation: phytoextraction and phytostabilization. Phytoremediation as a technique for rehabilitating heavy metal-polluted land therefore requires protocols and decision-support tools to assess the most appropriate approach, based on site-specific characteristics and requirements for soil status during and after remediation. Decisions have to be made on whether to use phytoextraction or phytostabilization, or even reject phytoremediation as a whole. Protocols and decision tools, from modeling and laboratory tests to full-blown feasibility studies, will be discussed.

Assisted phytoremediation of a multi-contaminated soil: Investigation on arsenic and lead combined mobilization and removal

The removal of contaminants from an earthy matrix by phytoremediation requires the selection of appropriate plant species and a suitable strategy to be effective. In order to set up an assisted phytoremediation intervention related to a disused industrial site affected by an arsenic and lead complex contamination, an extensive experimental investigation on micro and mesocosm scale has been conducted. Particular attention was given to the choice of plant species: using crop plants (Lupinus albus, Helianthus annuus and Brassica juncea) a series of parallel test campaigns have been realized to investigate different scenarios for the reclamation. With regard to the arsenic contamination, which is certainly the most worrying, the possibility of employing a hyper-accumulator species (Pteris vittata) has also been investigated, highlighting advantages and difficulties associated with such an approach. The application of various mobilizing agents in different concentrations was tested, in order to maximize the extraction efficiency of plants in respect of both contaminants, showing the necessity of a chemically assisted approach to promote their uptake and translocation in the shoots. Phosphate addition appears to produce the desired results, positively affecting As phyto-extraction for both hyper-accumulator and crop plants, while minimizing its toxic effects at the investigated concentrations. With regard to Pb, although tests with EDDS have been encouraging, EDTA should be preferred at present due to lower uncertainties about its effectiveness. The performed tests also improved the addition of mobilizing agents, allowing the simultaneous removal of the two metals despite their great diversity (which in general discourages such approach), with significant saving of time and an obvious improvement of the overall process.