Aspects of Hexavalent Chromium Pollution of Thebes Plain Aquifer, Boeotia, Greece

Koilakos, D.

doi:10.3390/w9080611

Cited by 17 publications

(2 citation statements)

References 40 publications

(65 reference statements)

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“…The Cr isotope data for groundwater from the C. Evia and Assopos-Thiva Basins [38,46,70] imply oxidative mobilization of Cr(VI) from the ultramafic host rocks, and successive backreduction of the Cr(VI), as is exemplified by the recorded range of the δ 53 Cr values between 0.8 and 1.99‰. In addition, a series of leaching experiments carried out for ultramafic rock (natural outcrops) from C. Evia, have shown δ 53 Cr values ranging between 0.56 and 0.96‰ for the rock leachates (Table 2).…”

Section: The Cr Isotopes (δ 53 Cr Values) As An Evidence Of the Cr Rementioning

confidence: 99%

The Cr(VI) Stability in Contaminated Coastal Groundwater: Salinity as a Driving Force

Eliopoulos

Economou-Eliopoulos

2021

Minerals

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Chromium concentrations in seawater are less than 0.5 μg/L, but the Cr(VI) in contaminated coastal groundwater affected by Cr-bearing rocks/ores and/or human activities, coupled with the intrusion of seawater may reach values of hundreds of μg/L. A potential explanation for the stability of the harmful Cr(VI) in contaminated coastal aquifers is still unexplored. The present study is an overview of new and literature data on the composition of coastal groundwater and seawater, aiming to provide potential relationships between Cr(VI) with major components in seawater and explain the elevated Cr(VI) concentrations. It is known that the oxidation of Cr(III) to Cr(VI) and the subsequent back-reduction of Cr(VI) processes, during the transport of the mobilized Cr(VI) in various aquifers, facilitate the natural attenuation process of Cr(VI). Moreover, the presented positive trend between B and Cr(VI) and negative trend between δ53Cr values and B concentration may suggest that seawater components significantly inhibit the Cr(VI) reduction into Cr(III), and provide insights on the role of the borate, [B(OH)4]− ions, a potential buffer, on the stability of Cr(VI) in coastal groundwater. Therefore, efforts are needed toward the prevention and/or minimization of the contamination by Cr(VI) of in coastal aquifers, which are influenced by the intrusion of seawater and are threatened by changes in sea level, due to climate change. The knowledge of the contamination sources, hotspots and monitoring of water salinization processes (geochemical mapping) for every coastal country may contribute to the optimization of agricultural management strategies.

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Section: The Cr Isotopes (δ 53 Cr Values) As An Evidence Of the Cr Rementioning

confidence: 99%

The Cr(VI) Stability in Contaminated Coastal Groundwater: Salinity as a Driving Force

Eliopoulos

Economou-Eliopoulos

2021

Minerals

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“…Todos los residuos líquidos con Cr(VI) deben ser tratados antes de verterse a un cuerpo de agua o al drenaje municipal, obligación que no siempre es bien atendida por el sector privado, lo que ha provocado la contaminación de ríos [9], y acuíferos [10]. Como resultado de estas acciones, las autoridades han generado normas que establecen el límite máximo permisible de estos compuestos en efluentes residuales descargados y en agua potable.…”

Section: Panorama Del Cromo En La Industria Y El Ambienteunclassified

Intensificación del proceso electroquímico para la remoción de cromo en residuos líquidos

Varela¹

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Chromium is a metal commonly used in the electroplating industry, which generates rinsing water containing hexavalent chromium concentrations above those allowed by environmental regulations. Several treatment alternatives have been proposed to attack this problem, among which electrochemical methods stand out. Among the latter, an attractive process is the one that uses iron electrodes, which are the source for the in situ electrochemical generation of the reducing agent, which chemically reduces the chromium in the liquid from its hexavalent to its trivalent state. The effluent is passed to a mixing tank to promote the precipitation of the metals into metal hydroxides by increasing the pH of the medium in order to separate them from the treated water. In this work, different strategies are presented to intensify this treatment process in order to improve the hydrodynamic performance of the electrochemical reactor and the precipitation stage. To assess the processes performance, in both systems, the electrochemical reactor and in the precipitation reactor, experimental tests were carried out in stirred tank reactors and jars test, which were complemented with studies using computational fluid dynamics to integrate the information on the performance of the processes under the different scenarios tested. Chapter two details the study carried out using computational fluid dynamics tools on the rotating rings electrochemical reactor, which has shown high removal efficiencies of hexavalent chromium in pollulet water. This part of the project shows the most appropriate modeling approach to simulate the hydrodynamics inside the reactor. Comparison of the results predicted by three variants of the κ-ε turbulence model (stadard, RNG and realizable) coupled to the multi-reference frame model to simulate the electrode rotation is performed. The effect of position of the rotational reference frame boundaries relative to the stationary reference frame boundaries is also evaluated. It is shown that the prediction with the κ-ε realizable model in conjunction with the position at 0° generates the results with the closest approach to the experimental mixing time measurements obtaining a 6% error. Chapter three shows the results of the incorporation of a novelty, which consists of a reactor equipped with the static electrode of electro-baffles agitated by two inclined vane impellers, known as PBT. The results of this modification are compared with those obtained with the performance of the electrochemical reactor equipped with the dynamic rotating ring electrode. The comparison is made theoretically to evaluate their differences in terms of hydrodynamic performance, and experimentally to know their efficiency against the reduction of hexavalent chromium. For the comparison, the reactors were operated at the same stirring speed and Reynolds number. The results of the hydrodynamic analysis show that the arrangement of static electro-baffles together with the pair of impellers improves the mixing time by 36%, increasing the hydraulic efficiency by 85% when the reactor is operated at the same Reynolds number. It is evident that the circulation capacity of the reactor directly affects the reduction rate of hexavalent chromium, since the treatment times have a similar trend to the axial circulation times. It is also shown that there is a reduction in energy consumption of at least 21% when the reactor is equipped with the electro-baffles and the two-impeller agitation system. In chapter four, the operating conditions of the electrochemical reactor are evaluated, such as: geometric configuration of the electrode, stirring speed and current intensity. To evaluate the change in the electrode configuration, the static ring electrode and electro-baffles are used. When operating the electrode with static rings, the need to incorporate conventional baffles is also evaluated. The reactor agitation systems are composed of two PBT impellers, of which the separation between them is also evaluated. In performing the simulations, the interaction of the liquid-gas interface was considered. The results reveal that due to the position of the impellers it is necessary to take into account the liquid-air interaction in the model to obtain a more realistic prediction of the flow pattern in the electro-baffles configuration. As a result of the studies, it was found that the configuration with the lowest energy consumption was the electro-baffles with a separation between impellers equal to their diameter, since its geometrical and hydrodynamic characteristics allow it to be more efficient. The effect of agitation speed and current intensity was explored in this system. The agitation speed increases the reduction rate of hexavalent chromium, finding its limit at 300 rpm. At this agitation speed, the effect of the current intensity was explored, from which a linear dependence of the reactor energy consumption on this variable was found in the range evaluated. In chapter five, the precipitation stage is studied. The studies are carried out in an agitated jar system with two types of impellers, one radial and the other axial. The evaluation of the effect of the pH at which the effluent is adjusted to perform precipitation at values of 4, 6, 7 and 9 is carried out. The results show that after precipitating the treated water at pH = 9.0, a clarified product with neutral pH is obtained and all species are precipitated. The hydrodynamic environment of the jars is also evaluated experimentally and numerically, determining that the radial impeller dissipates more turbulent energy than the axial impeller. Therefore, the axial impeller provides a favorable hydrodynamic environment for the development of flocs, which results in higher sedimentation velocities than those achieved when the jar is operated with the radial impeller. In addition, the axial impeller consumes only 50% of the energy consumed by the radial impeller.

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Cr (VI) adsorption from aqueous solutions on grafted chitosan

et al. 2020

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Chitosan was grafted on the surface of a cotton gauze (20, 50, and 100 mg chitosan g−1 cotton) to improve its stability in aqueous solutions. The adsorption of hexavalent chromium ions from water on the grafted chitosan was evaluated to determine, by means of linear and nonlinear models, the kinetic and isotherm adsorption of the process. The kinetics of pseudo second‐order, pseudo first‐order, and adsorption isotherms type II were obtained, that is, a monolayer adsorption on nonporous adsorbents with physical adsorption was present. The most probable energy of adsorption corresponded to a physisorption with hydrogen bond interactions between chromium ions and ammonium groups. Moreover, three different cross‐sectional areas of hexavalent chromium ions were calculated and used to estimate the specific surface area employed by active sites to adsorb metal ions in terms of chitosan or cotton mass. Finally, the percentage of the area occupied by chromium ions on the surface was estimated by dividing the resulting specific surface area in terms of cotton mass by the specific surface area of cotton reported in literature. As a result, it was determined that the occupied area is between 6% (for 20 mg chitosan g−1 cotton)‐24% (for 100 mg chitosan g−1 cotton) from the total area of cotton.

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Aspects of Hexavalent Chromium Pollution of Thebes Plain Aquifer, Boeotia, Greece

Cited by 17 publications

References 40 publications

The Cr(VI) Stability in Contaminated Coastal Groundwater: Salinity as a Driving Force

The Cr(VI) Stability in Contaminated Coastal Groundwater: Salinity as a Driving Force

Intensificación del proceso electroquímico para la remoción de cromo en residuos líquidos

Cr (VI) adsorption from aqueous solutions on grafted chitosan

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