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ABSTRACT: Adsorption is an effective process to remove mercury from polluted waters. In spite of the great number of experiments on this subject, the assessment of the optimal working conditions for industrial processes is suffering the lack of reliable models to describe the main adsorption mechanisms. This paper presents a critical analysis of mercury adsorption on an activated carbon, based on the use of chemical speciation analysis to find out correlations between mercury adsorption and concentration of dissolved species. To support this analysis, a comprehensive experimental study on mercury adsorption at different mercury concentrations, temperatures and pH was carried out in model aqueous solutions. This study pointed out that mercury capture occurs mainly through adsorption of cationic species, the adsorption of anions being significant only for basic pH. Furthermore, it was shown that HgOH(+) and Hg(2+) are captured to a higher extent than HgCl(+), but their adsorption is more sensitive to solution pH. Tests on the effect of temperature in a range from 10 to 55 °C showed a peculiar non-monotonic trend for mercury solution containing chlorides. The chemical speciation and the assumption of adsorption exothermicity allow describing this experimental finding without considering the occurrence of different adsorption mechanisms at different temperature.
Journal of hazardous materials 07/2011; 192(3):1842-50. · 4.14 Impact Factor
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ABSTRACT: A procedure to optimize the design of a Permeable Adsorptive Barrier (PAB) for the remediation of a contaminated aquifer is presented in this paper. A computer code, including different routines that describe the groundwater contaminant transport and the pollutant capture by adsorption in unsteady conditions over the barrier solid surface, has been developed. The complete characterization of the chemical-physical interactions between adsorbing solids and the contaminated water, required by the computer code, has been obtained by experimental measurements. A case study in which the procedure developed has been applied to a tetrachloroethylene (PCE)-contaminated aquifer near a solid waste landfill, in the district of Napoli (Italy), is also presented and the main dimensions of the barrier (length and width) have been evaluated. Model results show that PAB is effective for the remediation of a PCE-contaminated aquifer, since the concentration of PCE flowing out of the barrier is everywhere always lower than the concentration limit provided for in the Italian regulations on groundwater quality.
Journal of Environmental Management 01/2011; 92(1):23-30. · 3.24 Impact Factor
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ABSTRACT: The design of adsorber units is mainly dependent on the equilibrium adsorption capacity of the sorbent in the working conditions. At the moment, these data are available in a limited number of experimental conditions and, for the case of activated carbon, there are no predictive models to assess the adsorption capacity as a function of the process parameters. This makes the adsorber design a complex and approximated task. In this work, a model for the description of metallic ions adsorption onto activated carbon is presented. The model starts from an evaluation of ion speciation and it considers the approach of the multi-component Langmuir model to correlate the metal uptake to the ion concentration in solution. The model has been used to analyse available experimental data on the adsorption of As(V), Cd(II), Cr(III) and Cr(VI) ions on activated carbon. A good matching between experimental results and model predictions has been obtained for all the investigated conditions.
Journal of hazardous materials 04/2009; 169(1-3):360-9. · 4.14 Impact Factor
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ABSTRACT: In this work the adsorption of pentavalent arsenic on a granular activated carbon (GAC) has been experimentally studied. The effects of arsenic concentration, pH, temperature and salinity on equilibrium adsorption capacity have been investigated. Experimental results show that the adsorption capacity is the highest at neutral pH conditions, low salinity levels and high temperatures. A model for the description of the arsenic adsorption mechanism is reported. This is based on the multicomponent Langmuir adsorption theory applied to the ionic species in solution. The model points out that the adsorption capacity is proportional to the concentration of arsenic anions in solution and decreases by increasing the concentration of competitive ions such as hydroxides and chlorides, allowing a correct interpretation of the pH and salinity effects on the adsorption capacity. Finally, one of the main goals of the proposed model is to preserve the exothermicity of the adsorption phenomena despite the observed trend of experimental results: the increase of adsorption capacity with temperature appears to be related to a higher arsenic dissociation.
Water Research 05/2008; 42(8-9):2007-16. · 4.86 Impact Factor
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ABSTRACT: This work studies the reliability of an activated carbon permeable reactive barrier in removing cadmium from a contaminated shallow aquifer. Laboratory tests have been performed to characterize the equilibrium and kinetic adsorption properties of the activated carbon in cadmium-containing aqueous solutions. A 2D numerical model has been used to describe pollutant transport within a groundwater and the pollutant adsorption on the permeable adsorbing barrier (PRB). In particular, it has been considered the case of a permeable adsorbing barrier (PAB) used to protect a river from a Cd(II) contaminated groundwater. Numerical results show that the PAB can achieve a long-term efficiency by preventing river pollution for several months.
Journal of Hazardous Materials 04/2008; 160(2-3):428-34. · 4.17 Impact Factor
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ABSTRACT: Adsorption isotherms of chromium ions in aqueous solution have been experimentally measured on a granular activated carbon (GAC) and on a char of South African coal (CSAC). Experimental results show that the adsorption capacity for the GAC strongly depends on solution pH and salinity, with maximum values around 7mg/g at neutral pH and low salinity levels. On the contrary, the CSAC shows a smaller adsorption capacity, near 0.3mg/g, which slightly decreases by increasing pH and salinity levels. Chromium adsorption mainly depends on the availability of chromium ions in solution and on the occurrence of redox reactions between the surface groups and the Cr(VI) which lead to the formation of Cr(III). The reduction of Cr(VI) and the following sorption of Cr(III) cations appears as the leading mechanism for chromium uptake on the CSAC. A similar behaviour can be observed for the GAC at pH below 3. On the contrary, at pH>7, the multicomponent competitive adsorption of Cr(VI), OH(-) and Cl(-) has to be considered.
Journal of Hazardous Materials 08/2007; 145(3):381-90. · 4.17 Impact Factor
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ABSTRACT: In this paper the technical feasibility of various adsorbents for mercury removal from contaminated waters has been studied. Adsorption isotherms of mercury ions in aqueous solution have been experimentally measured on a granular activated carbon (Aquacarb 207EA), a char, a pozzolana and a yellow tuff. The experimental evidences show that the mercury capture capacity of yellow tuff and char is of few tenths of milligrams per gram of sorbent while for the pozzolana and the activated carbon this value is of the order of 1mg/g of sorbent. Moreover, for a mercury concentration as high as 3000 microg/l the pozzolana shows the highest adsorption capacity. This result seems to be quite interesting, especially in consideration of the extremely low cost of this natural sorbent.
Journal of Hazardous Materials 06/2006; 132(2-3):220-5. · 4.17 Impact Factor
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ABSTRACT: Among different flue gas desulfurization processes for control of sulfur dioxide emissions from combustion of fossil fuels, wet limestone scrubbing is the most widely used. Forced oxidation in the scrubber loop substantially improves the dewatering properties of the sludge, leading to the formation of gypsum (CaSO4·2H2O). In view of this, the present paper reports the experimental study of calcium bisulfite oxidation in the presence of catalysts (ferrous and manganese ions) both separately and simultaneously added in the reaction vessel. A laboratory-scale apparatus was used; the experiments were performed at a fixed oxygen partial pressure (21.3 kPa) and at a temperature of 45 °C. In particular, the effect of the simultaneous addition of both catalysts has been studied. The analysis of the experimental results, carried out by using the theory of mass transfer with chemical reaction, indicated that the slow reaction regime has been explored and the transition from the kinetic to the diffusional subregime identified. Experimental results, as compared with those obtained in the presence of the single catalytic species (Mn2+ alone and Fe2+ alone) allowed one to observe the synergistic effect that the two catalysts added simultaneously have on the oxidation reaction.
07/2004;
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Environmental Science and Technology 06/1994; 28(6):1031-6. · 5.23 Impact Factor
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Combustion Science and Technology 01/1993; 93(1):277-289. · 0.86 Impact Factor
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ABSTRACT: In the present work attention was focused on a wet flue gas desulfurization process using limestone suspensions, which is the most common method used to reduce SO2 emissions from power plant exhaust gases. The SO2 absorption rate was measured varying both the SO2 concentration in the gas phase and the limestone concentration in the suspension. The experiments were performed by bubbling mixtures of sulfur dioxide and nitrogen in the continuous limestone aqueous suspension.The absorption phenomenon was studied by making use of the film theory to describe the liquid-side mass transfer. It was assumed that the liquid-phase diffusional resistance is concentrated in a layer the thickness of which depends on fluid dynamics, but which is independent of the nature of the reactions taking place. The equations considered by the model describe conditions of thermodynamic equilibrium as well as material and electrical balances. Furthermore, they take into account the effect of the gradient of the electric potential of diffusion on the diffusive transport of ions and molecules in the film surrounding the gas-liquid interface.The SO2 absorption rate and the limestone dissolution rate experimentally determined were used to integrate the model equations, yielding the value of the film thickness, and allowing the determination of the concentration profiles of the different species in the liquid film and of the enhancement factor for chemical absorption. Consistency between model and experimental results, on the basis of the hypothesis of the model, was found.
Chemical Engineering Science.
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ABSTRACT: In this work, the removal of arsenate from synthetic wastewater by adsorption onto a coal combustion fly ash (CCA) has been experimentally studied. To this aim, a multiple analysis that included the evaluation of metal releases in aqueous solution, the assessment of arsenate adsorption capacity and a possible beneficiation treatment has been performed. The release of different metallic ions (mainly Ca, Al, K, and Si) was observed, including arsenic itself whose total content on CCA resulted to be around 0.05 mg/g. Arsenate adsorption isotherms on raw CCA at constant temperature (20 °C) have been determined in two synthetic arsenic aqueous solutions, a distilled water and a mineral water simulating a groundwater. In both the experimental conditions, CCA shows almost the same arsenate adsorption capacities. In order to increase CCA adsorption capacity and to simultaneously remove the arsenic originally present on CCA, a HCl treatment has been performed; the treated sample showed a higher adsorption capacity, likely related to a surface oxidation. Finally, the effect of equilibrium pH on arsenate adsorption on both raw and treated CCA samples has been investigated. A speciation analysis of arsenate ions revealed that for each investigated pH, the treated CCA sample showed the highest adsorption capacity.
Desalination.
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ABSTRACT: Adsorption isotherms of chromium ions in aqueous solution have been experimentally measured on a granular activated carbon (GAC) and on a char of South African coal (CSAC). Experimental results show that the adsorption capacity for the GAC strongly depends on solution pH and salinity, with maximum values around 7 mg/g at neutral pH and low salinity levels. On the contrary, the CSAC shows a smaller adsorption capacity, near 0.3 mg/g, which slightly decreases by increasing pH and salinity levels. Chromium adsorption mainly depends on the availability of chromium ions in solution and on the occurrence of redox reactions between the surface groups and the Cr(VI) which lead to the formation of Cr(III). The reduction of Cr(VI) and the following sorption of Cr(III) cations appears as the leading mechanism for chromium uptake on the CSAC. A similar behaviour can be observed for the GAC at pH below 3. On the contrary, at pH > 7, the multicomponent competitive adsorption of Cr(VI), OH- and Cl- has to be considered.
Journal of Hazardous Materials.
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ABSTRACT: In this work, an experimental study aimed at the assessment of the factors affecting the adsorption of trichloroethylene (TCE) from water solutions onto activated carbons is presented. The influence of sorbent properties, such as B.E.T. surface area, micropore volume, chemical composition and acid/basic surface functional groups on TCE adsorption capacity is experimentally assessed by testing a set of 12 sorbents. Moreover, the effect of the presence of other species in solution, such as sodium acetate and tetrachloroethylene (PCE), is studied through parametric TCE adsorption isotherms realization. The experimental results show that the TCE adsorption capacity is promoted by a high B.E.T. surface area, micropore volume and C content and it is significantly affected by the presence of a non-ionic compound of similar structure (PCE), however it does not depend on the presence of an organic salt (sodium acetate). These results confirm that neither TCE–carbon ionic interaction nor sorbent ionization phenomena are involved in the TCE adsorption, since its mechanism is based on dispersion forces (London–Van Der Walls interaction). A thorough analysis of the experimental data set suggests that, in consideration of the TCE adsorption mechanism, the maximization of basal plane extent (as the B.E.T. surface area) and its effective fraction (as the C content) is a valid criterion to select or synthesize a new suitable sorbent for TCE adsorption from waters.
Applied Surface Science.
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ABSTRACT: Adsorption processes have long been used in water and wastewater treatment and a deterministic approach has been often adopted for the analysis of fundamental mechanisms.In this research, a new approach based on statistical treatment of modelling data is adopted for the analysis of trichloroethylene (TCE) adsorption on granular activated carbon (GAC) from synthetic groundwater. The work starts from an experimental analysis aimed at the assessment of the influence of thermodynamic parameters (concentration, pH, salinity, temperature and the presence of an organic non-ionic compound, i.e. ethyl acetate) on TCE adsorption capacity. A wide range of process parameters have been considered to simulate highly polluted groundwater, as when leachate infiltrations occur at solid waste landfill.Experimental results show that adsorption capacity decreases by increasing the temperature and the ethyl acetate concentration, while neither pH nor salinity have any influence.A thorough understanding of adsorption mechanism and the individuation of the model with highest physical and mathematical significance is carried out by a thermodynamic analysis and a statistical methodology based on Analysis of Variance (ANOVA). The latter indicates that in dilute solutions the Freundlich model is the best data fitting model for TCE adsorption, among those which are commonly adopted.
Chemical Engineering Journal.
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ABSTRACT: Wet limestone scrubbing is one of the most widely used flue gas desulfurization processes for control of sulfur dioxide emissions from combustion of fossil fuels. A key step in the process is forced oxidation in the scrubber loop, which substantially improves the dewatering properties of the sludge, leading to the formation of gypsum (CaSO4·2H2O). This paper deals with the experimental study of calcium bisulfite oxidation in the presence of catalysts (ferrous and cuprous ions) added to the reaction vessel. A laboratory-scale apparatus was used; the experiments were performed at a fixed oxygen partial pressure (21.3 kPa) and at a temperature of 45 °C. In particular, the effect of the simultaneous addition of both catalysts has been studied. The analysis of the experimental results, carried out by using the theory of mass transfer with a chemical reaction, indicates that the slow reaction regime has been explored, and the transition from the kinetic to the diffusional sub-regime has been reached. Experimental results have been compared with those obtained in the presence of the single catalytic species (Cu+ alone and Fe2+ alone), showing that copper is less effective than iron as a catalyst; no synergy between the two catalysts added simultaneously has been observed.
Chemical Engineering Journal.
