D. Musmarra

Study of mercury absorption and desorption on sulfur impregnated carbon

Abstract In this work the attention was focused on the adsorption and desorption of elemental mercuric on HGR activated carbon produced by Calgon-Carbon Corp. The study was performed in an apparatus at laboratory scale in which Hg 0 vapors in a nitrogen gas stream, at a given temperature and mercury concentration, flowed through a fixed bed of adsorbent material. The experiments showed that the adsorption phenomena are faster than the desorption phenomena. SEM micrographs of the fresh carbon and of the carbon after adsorption or desorption had occurred have shown that mercury is adsorbed on the surface on particular sites where high sulfur concentration exists.

Equilibrium and dynamic study on hexavalent chromium adsorption onto activated carbon

In this work, the results of equilibrium and dynamic adsorption tests of hexavalent chromium, Cr (VI), on activated carbon are presented. Adsorption isotherms were determined at different levels of pH and temperature. Dynamic tests were carried out in terms of breakthrough curves of lab-scale fixed bed column at different pH, inlet concentration and flow rate. Both the adsorption isotherms and the breakthrough curves showed non-linear and unconventional trends. The experimental results revealed that chromium speciation played a key role in the adsorption process, also for the occurrence of Cr(VI)-to-Cr(III) reduction reactions. Equilibrium tests were interpreted in light of a multi-component Langmuir model supported by ion speciation analysis. For the interpretation of the adsorption dynamic tests, a mass transfer model was proposed. Dynamic tests at pH 11 were well described considering the external mass transfer as the rate controlling step. Differently, for dynamic tests at pH 6 the same model provided a satisfying description of the experimental breakthrough curves only until a sorbent coverage around 1.6mgg(-1). Above this level, a marked reduction of the breakthrough curve slope was observed in response to a transition to an inter-particle adsorption mechanism.

Experimental and modelling analysis of As(V) ions adsorption on granular activated carbon

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.

Mercury adsorption on granular activated carbon in aqueous solutions containing nitrates and chlorides

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.

Groundwater protection from cadmium contamination by permeable reactive barriers

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.

A descriptive model for metallic ions adsorption from aqueous solutions onto activated carbons

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.

Model of oxygen absorption into calcium sulfite solutions

Abstract Wet limestone scrubbing is the most common flue gas desulfurization process, and in this process sulfite oxidation plays a major role determining the dewatering properties of the sludge produced and leading to the production of gypsum of high quality. A literature analysis showed that the results obtained for sulfite oxidation under homogeneous conditions are relatively easily interpreted, while the results relative to heterogeneous reaction conditions (i.e. conditions in which gaseous oxygen and a sulfurous solution are brought in contact) are much more difficult to interpret, owing to the interaction between mass transfer and chemical reaction. In this work a model is proposed to describe the interaction between oxygen absorption and the oxidation reaction under heterogeneous conditions, which takes into account the peculiar characteristics of absorption with zero order reaction. A kinetic equation of order zero in dissolved O2, 3/2 in HSO3− and 3/2 in the catalyst concentration was used to match experimental results presented in the literature and model results. Integration of the model equations led to evaluation of the oxygen absorption rate as a function of the catalyst concentration together with the concentration profiles in the liquid film for the different species.

Dynamic waves in fluidized beds

Abstract Published results on the velocity of propagation and the change of amplitude of dynamic waves in fluidized beds are reviewed and re-evaluated. A distinction is drawn between pressure waves and solids concentration waves. Over the range of frequencies of interest in fluidized beds, predictions of the velocity of compression waves by pseudo-homogeneous models and by models which treat the two phases separately differ little. Therefore the experimental results are compared against the simpler pseudo-homogeneous models. The behaviour of dynamic waves is inconsistent with models which ignore particle-particle contacts. To distinguish unambiguously between compression and dynamic waves, experiments at elevated pressure are needed. The results are not consistent with pseudo-homogeneous description of compression waves.

Adsorption of Mercuric Chloride Vapours from Incinerator Flue Gases on Calcium Hydroxide Particles

ABSTRACT The adsorption of mercuric chloride vapours on a fixed bed of calcium hydroxide fine particles has been experimentally studied. The study has been conducted at laboratory scale using simulated flue gases, constituted of mercuric chloride vapours in nitrogen, which have been in contact with a Ca(OH)2 fixed bed. The effect of the bed temperature, the inlet HgCl2 concentration, and the relative gas-solid velocity on the HgCl2 removal efficiency has been studied. The experimental results indicated that high removal efficiencies of up to 95% can be obtained. The bed temperature has been found to be the most relevant parameter; particularly as the experimental results show that the lower the bed temperature, the higher the removal efficiency. Moreover, the removal efficiency increases when the HgCl2 concentration in the inlet gas increases and when the relative gas-solid velocity decreases. A model based on a simplified expression of the Freundlich adsorption isotherm has been proposed, which is able t...

Remediation of an aquifer polluted with dissolved tetrachloroethylene by an array of wells filled with activated carbon.

In this work, an array of deep passive wells filled with activated carbon, namely a Discontinuous Permeable Adsorptive Barrier (PAB-D), has been proposed for the remediation of an aquifer contaminated by tetrachloroethylene (PCE). The dynamics of the aquifer in the particular PAB-D configuration chosen, including the contaminant transport in the aquifer and the adsorption onto the barrier material, has been accurately performed by means of a computer code which allows describing all the phenomena occurring in the aquifer, simultaneously. A PAB-D design procedure is presented and the main dimensions of the barrier (number and position of passive wells) have been evaluated. Numerical simulations have been carried out over a long time span to follow the contaminant plume and to assess the effectiveness of the remediation method proposed. The model results show that this PAB-D design allows for a complete remediation of the aquifer under a natural hydraulic gradient, the PCE concentrations flowing out of the barrier being always lower than the corresponding Italian regulation limit. Finally, the results have been compared with those obtained for the design of a more traditional continuous barrier (PAB-C) for the same remediation process.