1,721,082 research outputs found
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
Full text linkWe 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
Capillary injection device (CID) for in situ pre-concentration of trace elements in Graphite Furnace-Atomic Absorption Spectrometry (GF-AAS)
No full textA device for in situ pre-concentration of trace elements in graphite furnace-atomic absorption spectrometry (GF-AAS), which is based on the direct introduction of the sample solution into the furnace by a fused silica capillary, is described. Experimental conditions have been optimized for the determination of several elements at the sub pg/g level with a CV always better than 15%. A deposition yield of approximately 95% was estimated by depositing a solution containing NaCl 0.002 M. A very simple and effective device for handling a sample under contamination-free conditions is also described which allowed a CV less than 10% to be obtained for replicate determinations of Cd in blank samples at 0.4 pg/g. The overall analytical system was validated by recovery studies carried out on standard solutions and spiked snow samples. Measurements were performed at 1, 5 and 50 pg/g, and the error was always lower than 20, 10 and 5%, respectively. The evaluation of the experimental detection limits which ranged in the interval 0.1-4 pg/g is finally discussed
Model-based optimization of field-scale electrokinetic remediation of marine sediments
Full text linkSeveral technologies are available for the remediation of contaminated marine sediments, most of which derived from soil remediation techniques, such as sediment washing, thermal treatment, chemical oxidation, bioslurry processes and biopiles. The applicability of such technologies is strongly affected by specific characteristics of sediments which can adversely influence both operation and removal efficiency. Unfortunately, none of available technologies is effective when main pollution is represented by heavy metals and when sediment matrix is characterized by low hydraulic permeability. These conditions pose severe limitations to remediation efficiency, as most decontamination techniques available for treating high-permeability soils are not effective for fine-grained matrices. In this context, electrokinetic remediation is widely recognized as an efficient technique for removing a broad range of contaminants from low-permeability soils and sediments .
Despite this technique has demonstrated being very promising for the remediation of marine sediments, the selection of the best operating conditions remains elusive, due to the variety of mechanism involved. Complicating factors include the high complexity of sediment matrix and the strong non-linearity of the processes occurring during electric field application. Complexity is one of the main limiting factors to the spread of electrokinetic remediation technology for "real-world" applications. Complexity directly affects the reliable prediction of achievable results and induces technical challenges when the technique is implemented at the field scale [4]. A thorough understanding of the mechanisms involved in electrokinetic remediation through experimental studies is always needed, for an appropriate design of full-scale treatment schemes. In some cases, the prescribed laboratory experiments can get excessively time consuming, and modelling could become a necessary tool to assess the main remediation parameters and predict achievable results. Despite several mathematical models have been developed to predict electrokinetic extraction of contaminants, their application to plant design and optimization is still unavailable.
This work presents a model-based approach for the optimization of field-scale electrokinetic processes, including the possibility of estimating optimum design parameters and minimizing the costs. We developed a numerical model to simulate the electrokinetic remediation of real contaminated sediments characterized by a heterogeneous solid matrix and aged heavy metal pollution. The numerical model couples a transport model able to simulate electromigration and electroosmosis with a geochemical model, which calculates aqueous speciation, precipitation-dissolution, adsorption and surface complexation reactions. Laboratory-scale experiments were carried out to calibrate and validate the model. The model was able to reproduce experimental data with adequate accuracy. We identified the acid buffering capacity as the most significant factor for the extraction of heavy metals from the sediments as their speciation and mobility were strictly dependent on sediment capacity to prevent the pH shift to the acidic range.
The developed model was then “scaled up”, to reproduce electrokinetic processes occurring at field scale, considering a typical field setup consisting of vertical electrodes arranged on a rectangular grid. A parametric study was performed to evaluate the influence of electrode distance and sediment properties on treatment costs. The simulations allowed us to calculate time-dependent contaminant removal rate and to define cost curves for each set of parameters. The resulting curves of total cost were obtaining by summing up all individual costs. A clear point of minimum could be identified in each case, corresponding to the optimum set of parameters which minimized the costs.
In conclusion, we strongly believe that the methodology and results obtained can be employed as a valuable tool to support evaluation and design of electrokinetic remediation systems
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