162 research outputs found
Model-based optimization of field-scale electrokinetic treatment of dredged sediments
No full textWe 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 m3 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
Determination of trace elements in seawater samples by on-line column extraction/graphite furnace atomic absorption spectrometry
No full textAn innovative procedure for the on-line coupling of ion chromatography with graphite furnace atomic absorption spectrometry is described, which is particularly effective for the determination of trace metals in seawater samples. The Capillary Injection Device (CID) is used as an interface which allows the eluent to be transferred from the chromatographic column into the graphite tube at a flow rate of as high as 2 ml/min. The analytical procedure is based on the metal complex formation with 8-hydroxyquinoline in the sample solution, followed by the preconcentration of the complexes in a chromatographic column packed with XAD-2 resin. The complexes were then eluted from the column with methanol, and quantitatively injected into the furnace. The procedure was validated by determining cadmium and lead in certified reference seawater samples at a level of 30–40 pg/g, with a typical reproducibility of 10% and an accuracy of better than 5%. Finally, it was tested on a real sample of seawater. Due to the high reproducibility, a pg/g concentration level can be measured
Preliminary tests to select operating conditions for the accurate determination of stability constant by cation exchange chromatography: the Cd2+-Cl- and Cd2+-NO3- systems
No full textIon chromatography (IC) has been demonstrated to be a powerful tool for equilibrium constant determination, related to various cation-ligand systems. Nevertheless, no systematic research has been carried out to develop preliminary checks in order to verify whether the variation in ligand concentration in the eluent at constant ionic strength affects the exchange mechanism for the system of interest in the selected chromatographic column. In this paper, tests are proposed which allow one to determine beforehand the experimental conditions to be used in cation-exchange chromatography, whereby parallel mechanisms of elution (mainly in the reversed-phase mode) are avoided. In this way IC becomes an independent rather than an auxiliary means to obtain accurate βi values. Cd(II)Cl- and Cd(II)NO3 - systems were considered and are discussed
Decontaminazione elettrocinetica di sedimenti marini in vasca di colmata: risultati preliminari di un'applicazione pilota
No full textLa bonifica elettrocinetica viene proposta come tecnica di rimozione di vari contaminanti, in particolare metalli pesanti, da matrici come terreni o sedimenti caratterizzate da bassa permeabilità idraulica. La sua implementazione più comune applica un campo elettrico alla matrice da trattare mediante coppie di elettrodi collocati in pozzetti a pareti porose in cui viene fatta circolare acqua opportunamente condizionata. Il campo elettrico provoca l’elettrolisi di tali soluzioni, generando un fronte acido, che avanza dagli anodi ai catodi favorendo il desorbimento e la mobilizzazione dei contaminanti, e un fronte basico che avanzando in senso opposto ostacola tale processo e viene dunque contrastato dosando acidi al catolita. Le reazioni di elettrolisi, gli agenti condizionanti e i fenomeni di trasporto dovuti al campo elettrico arricchiscono gli elettroliti dei contaminanti rimossi e di altri macroelementi, rendendo necessaria una loro gestione per: 1. evitare rischi di precipitazione sugli elettrodi, sulle pareti dei pozzetti elettrodici o nella matrice da trattare; 2. impedire la possibile inversione dei processi di trasporto e il conseguente rischio di ricontaminazione della matrice trattata; 3. contenere l'incremento di conduttività elettrica degli elettroliti e della soluzione interstiziale che riduce il rendimento di rimozione e causa inutile dispendio di energia elettrica. La letteratura scientifica, essenzialmente focalizzata sul trattamento del mezzo poroso, raramente considera l’implementazione a scala reale e le problematiche di gestione degli elettroliti. Pertanto, il presente contributo è mirato ad analizzare le opzioni di gestione e di trattamento degli elettroliti evidenziandone tecniche, criticità ed aspetti economici
Multispecies reactive transport modelling of electrokinetic remediation of harbour sediments
No full textWe 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
Numerical modelling of electrokinetic extraction of heavy metals from harbour sediments
No full textWe implemented a numerical model able to simulate transport of multiple species and geochemical reactions occurring during the remediation of metal-contaminated sediments characterized by a heterogeneous solid matrix, high buffering capacity and aged pollution. The main phenomena described by the model were: (1) chemical species transport through the porous matrix by electromigration and electroosmosis, (2) pH-dependent buffering of H+, (3) adsorption of contaminants onto sediment particle surfaces, (4) aqueous speciation and (5) formation/dissolution of solid precipitates. A constitutive relationship between zeta-potential and pH was used to compute the local electroosmotic permeability. The electroosmotic flow was computed by the volume averaging the electroosmotic permeability. The results of three electrokinetic tests, carried out with different treatment durations (32, 63 and 120 days) were used to validate the model. A good agreement was found between the experimental data and model predictions. In particular, pH and electroosmotic flow were predicted with good accuracy. The predicted metals profiles were also close to experiments profiles for all of the investigated metals (Pb, Zn and Ni) but an overestimation of the removal was observed in the regions close to the anode, possibly due to the significant metal content bound to the residual fraction, quantified with sequential extraction technique. These results encourage the use of the discussed modelling approach as an engineering tool for the design, implementation and removal efficiency prediction of electrokinetic technology at the field scale
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