A. Xenidis | National Technical University of Athens (original) (raw)

Papers by A. Xenidis

Chromium (Cr) is a heavy metal that naturally occurs in the Earth’s crust, and can be found in gr... more Chromium (Cr) is a heavy metal that naturally occurs in the Earth’s crust, and can be found in groundwater in either the trivalent, Cr(III), or in the hexavalent, Cr(VI), form. While Cr(III) is considered an essential trace element for the human body, Cr(VI) is potentially dangerous for human health although the type and intensity of the effect are still being investigated. The widespread use of chromium in industrial applications along with careless waste disposal practices has in many cases resulted in serious soil and groundwater contamination. Until recently, this was considered the only Cr(VI) source in groundwater, although new research has shown that relatively high levels of Cr(VI) can also be attributed to natural processes. This mainly occurs in areas where the levels of naturally occurring Cr(III) are high and natural oxidation processes convert Cr(III) to Cr(VI). Such phenomena take place in several areas worldwide, including Greece. Naturally occurring high contents of ...

Chemosphere, 2021

Cr(VI) is widely used in industry and often large quantities have been accidently or due to lack ... more Cr(VI) is widely used in industry and often large quantities have been accidently or due to lack of precautions released at the environment. The presence of high concentrations of Cr(VI) in waste streams causes concern due to its toxicity and solubility. However the Cr(III) is a useful micronutrient in human diet. The reduction of Cr(VI) to Cr(III) is a promising technology for a clean environment. Polyphenols are commonly found in plants and they are considered as strong natural antioxidants. In this study the reduction of hexavalent to trivalent chromium using gallic acid (GA), a polyhydroxyphenolic compound, was evaluated. The effect of pH, temperature, and Cr(VI) concentration on the kinetics of Cr(VI) reduction by GA were examined by conducting batch tests. It was found that reduction of Cr(VI) by GA is a rapid reaction under mild acidic conditions. Reduction was found to follow a 2nd order kinetics with respect to Cr(VI) concentration in the pH range 3-5. The results of the present study indicated that the treatment of contaminated waters with initial concentration of Cr(VI) 100 μΜ by gallic acid, below the environmental limit of 0.96 μM, required 20 min at pH 3 and 40 min at pH 4. At higher pH a different reduction mechanism seems to prevail and the required time for Cr(VI) reduction was approximately 10 h at pH 5 and 42 h at pH 6. The increase of temperature25 °C to 35 °C, at pH 6, was slightly accelerated the reaction.

In this study the results of a pilot-scale injection of green iron nanoparticles (GT-nZVI) and th... more In this study the results of a pilot-scale injection of green iron nanoparticles (GT-nZVI) and the formation of a reaction zone (RZ) for Cr(VI) removal are presented. A pilot scale tank was constructed with dimensions 1.5m (height) × 2.5m (width) × 3.75m (length) which was filled with 24 tons of soil. A network of 48 sampling points was installed inside the tank to monitor water quality. Nano ZVI was synthesized on-site by batches of 300 L. A total amount of 2.8 m 3 GT-nZVI suspension was injected via 3 injection wells in order to create a RZ zone, perpendicular to the groundwater flow. After the injection of GT-nZVI suspension, the supply of contaminated water containing 5 mg/L Cr(VI) was initiated. Pore water samples taken upstream and downstream of the RZ indicated that the zone operated effectively, since all the downstream samples were not found to contain any Cr(VI) and Cr(tot) had very low concentrations varying between <6 and 15 μg/L. Monitoring data for a period of 12 mo...

Applied Geochemistry, 2020

Abstract Nano zero valent iron (nZVI) is a promising agent for in situ remediation of contaminate... more Abstract Nano zero valent iron (nZVI) is a promising agent for in situ remediation of contaminated groundwater and soil. However, the limited mobility of iron nanoparticles in calcareous soils may inhibit the efficiency of this technology. Aim of this study is the evaluation of the mobility of nanoiron synthesized with green tea polyphenols in porous media containing limestone by conducting columns tests. The effect of grain size of limestone particles added in columns on nanoiron suspension transport was evaluated. It was shown that the mobility of iron nanoparticles was strongly affected by the residual acidity and the aquatic iron (Fe(III)) of the nZVI suspension in contact with limestone which caused the neutralization of acidity and the precipitation of iron oxyhydroxides on limestone surface. It was shown that the interference of Fe(III) precipitation on the nZVI suspension mobility becomes more important as the size of calcite grains decreases. Namely, the total Fe content in the solids at the end of column tests, was 4.4 mg/g in the column containing the coarser CaCO3 grains (d = 3.4 mm) and increased up to 44.8 mg/g in the column with the finer grains (d = 0.22 mm), due to higher available specific surface area. The corresponding amount of retained nanoiron, nFe (0), was 0.75 and 4.6 mg/g in the columns with the coarser and finer CaCO3 grains respectively.

Journal of Environmental Chemical Engineering, 2016

In the present research work the possibility of spilite valorization, a mining waste produced dur... more In the present research work the possibility of spilite valorization, a mining waste produced during the exploitation of nickeliferous laterites from LARCO Mining and Metallurgical Company, as an alternative material for the construction of compacted landfill liners was examined. For that purpose, different types of mixtures were tested with bentonite content up to 3 wt%. Particle size distribution, chemical analysis, mineralogical analysis by X-ray diffraction (XRD), thermogravimetric analysis (TG-DTG) and microstructure examination by scanning (SEM) and transmission (TEM) electron microscopy were used for the spilitic sand characterization. The evaluation of the produced spilitic sand-bentonite mixtures was carried out by determining the liquid limit (WL), the plasticity index (PI), the organic matter content, the Proctor compaction and the hydraulic conductivity. According to the results, the hydraulic conductivity coefficient of the mixture with 3 wt% bentonite addition was less than 10 À9 m/s, satisfying the requirement for landfill liner materials.

Lindsay/Light, 2011

ABSTRACT The reductive smelting of Greek bauxite residues was investigated for the production of ... more ABSTRACT The reductive smelting of Greek bauxite residues was investigated for the production of an iron product that meets some crucial industrial requirements as a blast furnace feed. Fine-grained Greek bauxite residue - either as is or in the form of pellets - and solid fuel reducing agents -lignite and coke-, were used as raw materials. The effect of parameters such as the smelting temperature, the amount of the reducing agent in the mixture, the retention time and the addition of fluxes on the quality of the metallic product, as well as the basicity and the desulfurization capability of the slag, was investigated. The results obtained regarding the chemical properties of the metallic product were very promising, providing input for further research on the optimization of the proposed pyrometallurgical method for the production of an attractive ferrous raw material for the iron ore industry.

Bulletin of environmental contamination and toxicology, 2015

Alternative plant extracts were examined as raw materials for the synthesis of nZVI from ferric s... more Alternative plant extracts were examined as raw materials for the synthesis of nZVI from ferric solutions. Four plants were selected for evaluation, i.e. Camellia sinensis (green tea, GT), Syzygium aromaticum (clove, CL), Mentha spicata (spearmint, SM) and Punica granatum (pomegranate, PG). Based on the results obtained, it was concluded that the reduction of Fe(III) with the herb extracts is not complete. Using the GT extract, approximately 28 mM of the initial 66 mM of Fe (42.4 %) are reduced to the elemental state Fe(0). The highest reduction of Fe(III), about 53 %, was achieved with PG and the lowest, only 15.6 %, with the SM extract. Additional batch experiments have been carried out to evaluate the effectiveness of nZVI, synthesized with GT, CL, SM and PG, for the removal of hexavalent chromium from a 0.96 mM solution. The highest reduction of Cr(VI) (96 %) was obtained using the nZVI suspension produced with PG juice. The other three nZVI suspensions, i.e. CL-nZVI, GT-nZVI, a...

Heavy Metals in the Environment: Origin, Interaction and Remediation, 2005

Publisher Summary A variety of methods for the remediation of both solid and liquid media such as... more Publisher Summary A variety of methods for the remediation of both solid and liquid media such as water, groundwater, industrial wastewaters, soils, sediments, and sludges is known. This chapter provides a general overview over different technologies by dividing them into three major categories: first, physical methods such as soil washing, encapsulation, or electrokinesis; second chemical methods such as solidification, precipitation, or ion exchange; and third, biological methods that use plants to remove heavy metals. Innovative approaches such as passive treatment technologies for soil and groundwater contaminations have been developed. The chapter introduces the various remediation technologies. Physical remediation techniques include soil washing, vitrification, encapsulation of contaminated areas by impermeable vertical and horizontal layers, electrokinesis, and permeable barrier systems. Encapsulation of contaminated areas is commonly used for remediation by containment or pollution prevention. Most of these techniques have been adapted to the use in the field of environmental engineering from the water-tight encapsulation of construction pits.

ChemInform, 2009

ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance t... more ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Journal of Hazardous Materials, 2015

h i g h l i g h t s • Chrysotile and chromite minerals confirmed the ultramafic origin of Vergina... more h i g h l i g h t s • Chrysotile and chromite minerals confirmed the ultramafic origin of Vergina soil. • Cr(VI) concentration in Vergina soil decreased continuously with increasing depth. • Mn(III)-Mn(IV) oxides responsible for Cr oxidation are present in the tested soils. • Cr(VI) and Crtot concentrations in groundwater decrease linearly with depth. • Increasing soil pH is considered responsible for decreased Cr oxidation with depth.

The effectiveness of lignite fly ash for stabilization of Pb, Zn and Cd in tailings and contamina... more The effectiveness of lignite fly ash for stabilization of Pb, Zn and Cd in tailings and contaminated soils was examined. Fly ash was mixed with the contaminated soil or oxidic tailings samples at various doses and pot experiments were performed. The effectiveness of stabilization was mainly evaluated by the standard US EPA TCLP toxicity test. The 5-stage sequential extraction procedure was also applied to determine the form of contaminants in the fly ash amended soil or tailings samples. Complementary EDTA extraction tests were also carried out. The fly ash treatment resulted in the reduction of TCLP solubility of Pb, Zn and Cd to below the respective regulatory limits at 5 and 10% w/w fly ash addition rates in tailings and soil, respectively. Speciation of lead, in the treated soil and tailings samples indicated that there was a significant transition of the heavy metals form from the exchangeable and carbonate fractions to reducible and residual, suggesting that the potential mechanisms of heavy metals retention are, apart from increase of pH, sorption on the oxides and hydroxides surfaces and binding with the hydrated fly ash compounds.

Soil and Sediment Contamination: An International Journal, 2001

... Contaminated Soil By Means of Natural Zeolite Aggeliki Moirou, Anthimos Xenidis,* and Ioannis... more ... Contaminated Soil By Means of Natural Zeolite Aggeliki Moirou, Anthimos Xenidis,* and Ioannis Paspaliaris ... The tailing's average thickness is 0.3 to 1.2 m. The zeolitic tuff was provided by Silver and Baryte SA that mines the deposit of the village Pentalofos, Evros. ...

Applied Mathematical Modelling, 2016

A 2D, steady-state computational fluid dynamics (CFD) analysis of an industrial electric arc furn... more A 2D, steady-state computational fluid dynamics (CFD) analysis of an industrial electric arc furnace (EAF) is presented. The analysis accounts for the electrode shape and immersion depth, as well as for the dependence of Joule heating on the properties of the slag. The equations for the electric potential, momentum and heat transfer were solved across four distinct regions (i.e. air, slag, ferronickel and firebricks) and the final profile of the slag/metal interface was calculated as a function of the operating parameters of the furnace. The results indicate that the amount of Joule heat produced by the Söderberg electrodes increased with increasing applied voltage and electrical conductivity of the slag; the Joule heat peaked for a value of slag electrical conductivity equal to 3 S/m. A highly conductive slag along with a greater electrode immersion depth was found to facilitate the melting process. The maximum slag velocity values computed were of the order of 0.8 m/s in the vicinity of the electrode tips.

Bulletin of Environmental Contamination and Toxicology

The objective of the present study was to evaluate the performance of a nanocomposite material co... more The objective of the present study was to evaluate the performance of a nanocomposite material consisting of nano zero valent iron and a cation exchange resin, for the reduction of chromate, by conducting column tests. A cationic resin, Amberlyst 15, was selected as porous host material. The synthesis of the nanocomposite material (R-nFe) was carried out using Green Tea extract to obtain the reduction of adsorbed Fe(III) to the elemental state Fe(0). Three column tests were implemented with different dimensions, corresponding to variable contact times between the aqueous solution and the resin beads loaded with Fe(0), namely 168, 744 and 1260 s respectively for columns I, II and III. The results indicated that the removal of Cr(VI) follows a first order kinetic law with a chemical constant equal to 0.0526 min−1 (8.8 × 10–4 s−1).

Royal Society Open Science

A transient mathematical model was developed for the description of fluid flow, heat transfer and... more A transient mathematical model was developed for the description of fluid flow, heat transfer and electromagnetic phenomena involved in the production of ferronickel in electric arc furnaces. The key operating variables considered were the thermal and electrical conductivity of the slag and the shape, immersion depth and applied electric potential of the electrodes. It was established that the principal stimuli of the velocities in the slag bath were the electric potential and immersion depth of the electrodes and the thermal and electrical conductivities of the slag. Additionally, it was determined that, under the set of operating conditions examined, the maximum slag temperature ranged between 1756 and 1825 K, which is in accordance with industrial measurements. Moreover, it was affirmed that contributions to slag stirring due to Lorentz forces and momentum forces due to the release of carbon monoxide bubbles from the electrode surface were negligible.

Journal of Cleaner Production, 2015

Science of The Total Environment, 2016

Jiang/3rd International, 2012

Desalination and Water Treatment, 2014

ABSTRACT Nano Zero Valent Iron (nZVI) is considered as a highly efficient reductant with wide app... more ABSTRACT Nano Zero Valent Iron (nZVI) is considered as a highly efficient reductant with wide applicability in the remediation of contaminated sites, due to the small particle size, large specific surface area and high reactivity. In this study, nZVI was synthesized by mixing Camellia Sinensis extract (green tea, GT) with a ferric iron solution. Polyphenols in the GT extract act as: (a) strong reductants, reducing thus Fe(III) to its elemental state, and (b) capping agents that stabilize the Fe(0) nanoparticles and inhibit their potential rapid oxidation. The remediation potential of ZVI nanoparticles, produced with the Green Tea method (GT-nZVI), for hexavalent chromium (Cr(VI)) was evaluated by column tests. The tests were carried out using polyethylene columns, with 2.63 cm internal diameter and ~10 cm length. The columns were filled with 65 g of natural soil and pure silica sand at a ratio of 50% w/w. The reductive capacity of GT-nZVI was examined for high (5mg/L) concentration of hexavalent chromium in aqueous solution. The soil was initially treated in the first column by introducing 11 pore volumes (PV) of GT-nZVI suspension through the packed column while in the second column 24 pore volumes were introduced. It was found that about 210 mg and 420 mg of ZVI nanoparticles were retained by the soil during this stage in column I and in column II, respectively. After this step the solution of hexavalent chromium was introduced in the columns at a flowrate of 1.2 mL/min, in order to determine the breakthrough curve of Cr(VI) in the immobilized GT-nZVI on the soil. The total amount of reduced Cr(VI) was equal to 17 mg in column I and 33 mg in column II. Based on the available experimental data, it was estimated that a GT-nZVI treated zone in the subsurface, with a length (L) of 0.75 m, could provide a sufficiently long contact time in order to reduce Cr(VI) levels from 5 mg/L to less than 5 μg/L, under the condition that groundwater Darcy velocity is equal or less than 0.1 cm/min.