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Poplar and willow demonstrate great potential for the phytoextraction of trace elements (TEs) from soils. In most cases, these species are managed as short-rotation coppice, producing high woody biomass yields, which could provide a valuable contribution toward reducing greenhouse gas emissions in the atmosphere. In the current study, we compared the TE extraction and CO2 sequestration rates in a four-year field trial in Southern Italy of two arboreal species (willow and poplar). The results show that, once established in the study area, willow extracted more Cd and Cu and less Pb than poplar. The two species demonstrated the same average Ni and Zn extraction rates. Greater biomass yields in poplar suggest that this species was able to fix greater amounts of CO2 (28.7 Mg ha−1 yr−1) than willow (24.9 Mg ha−1 yr−1). We argue that the choice of the species to be used in phytoextraction should first be made considering the TE-specific affinity and phytoextraction rates. For TEs whose extraction rates were the same (i.e., Ni and Zn), poplar is to be preferred because of its ability to fix greater amounts of CO2 than willow. [ABSTRACT FROM AUTHOR]

We developed a methodology for the cost optimization of electrokinetic treatment porous media contaminated by toxic metals. A two-dimensional reactive-transport model was implemented to simulate the transport of chemical species by diffusion, electromigration and electroosmosis, coupled with a geochemical model which calculates precipitation and dissolution of species, adsorption and desorption reactions, and aqueous speciation. The model was applied to the case study of an electrokinetic remediation prototype plant built in Livorno (Italy), treating 150 m 3 of dredged sediments contaminated by toxic metals. The plant consisted of an ex-situ treatment basin equipped with electrodic wells arranged on a rectangular grid, connected to an electrolyte management system for catholyte and anolyte pH control. We validated the model by comparing the simulated electric field with the measured electric potential and the simulated pH profiles with the pH values of field samples. Good agreement was achieved between the modelled and measured data. On the basis of the validated model, we performed a parametric study to evaluate the influence of electrode distance and sediment buffering capacity on treatment costs and calculated the overall cost as a function of these two parameters. The results and costs were evaluated in terms of Pb removal, which was taken as the representative toxic metal. The results revealed the existence of distinct minima, representing the best set of parameters which optimized the overall treatment costs. We believe that the methodology and results obtained can be employed as a valuable tool to support the evaluation and design of electrokinetic remediation systems. [ABSTRACT FROM AUTHOR]

We implemented a numerical model to simulate transport of multiple species and geochemical reactions occurring during electrokinetic remediation of metal-contaminated porous media. The main phenomena described by the model were: (1) species transport by diffusion, electromigration and electroosmosis, (2) pH-dependent buffering of H + , (3) adsorption of metals onto particle surfaces, (4) aqueous speciation, (5) formation and dissolution of solid precipitates. The model was applied to simulate the electrokinetic extraction of heavy metals (Pb, Zn and Ni) from marine harbour sediments, characterized by a heterogeneous solid matrix, high buffering capacity and aged pollution. A good agreement was found between simulations of pH, electroosmotic flow and experimental results. The predicted residual metal concentrations in the sediment were also close to experimental profiles for all of the investigated metals. Some removal overestimation was observed in the regions close to the anode, possibly due to the significant metal content bound to residual fraction. [ABSTRACT FROM AUTHOR]

The suitability of electrokinetic remediation for removing heavy metals from dredged marine sediments with high acid buffering capacity was investigated. Laboratory-scale electrokinetic remediation experiments were carried out by applying two different voltage gradients to the sediment (0.5 and 0.8 V/cm) while circulating water or two different chelating agents at the electrode compartments. Tap water, 0.1 M citric acid and 0.1 M ethylenediaminetetraacetic acid (EDTA) solutions were used respectively. The investigated metals were Zn, Pb, V, Ni and Cu. In the unenhanced experiment, the acid front could not propagate due to the high acid buffering capacity of the sediments; the production of OH ions at the cathode resulted in a high-pH environment causing the precipitation of CaCO and metal hydroxides. The use of citric acid prevented the formation of precipitates, but solubilisation and mobilisation of metal species were not sufficiently achieved. Metal removal was relevant when EDTA was used as the conditioning agent, and the electric potential was raised up to 0.8 V/cm. EDTA led to the formation of negatively charged complexes with metals which migrated towards the anode compartment by electromigration. This result shows that metal removal from sediments with high acid buffering capacity may be achieved by enhancing the electrokinetic process by EDTA addition when the acidification of the medium is not economically and/or environmentally sustainable. [ABSTRACT FROM AUTHOR]

Electrokinetic remediation is an emerging technology for extracting heavy metals from contaminated soils and sediments. This method uses a direct or alternating electric field to induce the transport of contaminants toward the electrodes. The electric field also produces pH variations, sorption/desorption and precipitation/dissolution of species in the porous medium during remediation. Since heavy metal mobility is pH-dependent, the accurate control of pH inside the material is required in order to enhance the removal efficiency. The common approach for monitoring the remediation process both in laboratory and in the field is the chemical analysis of samples collected from discrete locations. The purpose of this study is the evaluation of Spectral Induced Polarization as an alternative method for monitoring geochemical changes in the contaminated mass during remediation. The advantage of this technique applied to field-scale is to offer higher resolution mapping of the remediation site and lower cost compared to the conventional sampling procedure. We carried out laboratory-scale electrokinetic remediation experiments on fine-grained marine sediments contaminated by heavy metal and we made Spectral Induced Polarization measurements before and after each treatment. Measurements were done in the frequency range 10 − 3 –10 3 Hz. By the deconvolution of the spectra using the Debye Decomposition method we obtained the mean relaxation time and total chargeability. The main finding of this work is that a linear relationship exists between the local total chargeability and pH, with good agreement. The observed behaviour of chargeability is interpreted as a direct consequence of the alteration of the zeta potential of the sediment particles due to pH changes. Such relationship has a significant value for the interpretation of induced polarization data, allowing the use of this technique for monitoring electrokinetic remediation at field-scale. [ABSTRACT FROM AUTHOR]