•High concentrations (up to 120μg/L) of Cr(VI) were locally observed in the groundwaters of Sarigkiol/Kozani area.•Unconfined aquifers are more vulnerable to Cr(VI) pollution from ophiolitic rocks, ...as well as from dispersed fly ash.•Fly ash produced from a power plant burning lignite, was found to contribute to groundwater pollution with Cr(VI).•Positively fractionated δ53Cr values in groundwater and fly ash leachate point to partially anthropogenic origin of Cr(VI).•The natural (geochemical) contribution, regarding Cr (VI) pollution, was found up to 60μg/L.
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Hexavalent chromium constitutes a serious deterioration factor for the groundwater quality of several regions around the world. High concentrations of this contaminant have been also reported in the groundwater of the Sarigkiol hydrological basin (near Kozani city, NW Greece). Specific interest was paid to this particular study area due to the co-existence here of two important factors both expected to contribute to Cr(VI) presence and groundwater pollution; namely the area's exposed ophiolitic rocks and its substantial fly ash deposits originating from the local lignite burning power plant. Accordingly, detailed geochemical, mineralogical, hydro-chemical, geophysical and hydrogeological studies were performed on the rocks, soils, sediments and water resources of this basin. Cr(VI) concentrations varied in the different aquifers, with the highest concentration (up to 120μgL−1) recorded in the groundwater of the unconfined porous aquifer situated near the temporary fly ash disposal site. Recharge of the porous aquifer is related mainly to precipitation infiltration and occasional surface run-off. Nevertheless, a hydraulic connection between the porous and neighboring karst aquifers could not be delineated. Therefore, the presence of Cr(VI) in the groundwater of this area is thought to originate from both the ophiolitic rock weathering products in the soils, and the local leaching of Cr(VI) from the diffused fly ash located in the area surrounding the lignite power plant. This conclusion was corroborated by factor analysis, and the strongly positively fractionated Cr isotopes (δ53Cr up to 0.83‰) recorded in groundwater, an ash leachate, and the bulk fly ash. An anthropogenic source of Cr(VI) that possibly influences groundwater quality is especially apparent in the eastern part of the Sarigkiol basin.
This study aims to specify the source minerals of geogenic chromium in soils and sediments and groundwater and to determine the favorable hydrogeological environment for high concentrations of Cr(VI) ...in groundwaters. For this reason, chromium origin and the relevant minerals were identified, the groundwater velocity was calculated and the concentrations of Cr(VI) in different aquifer types were determined. Geochemical and mineralogical analyses showed that chromium concentrations in soils and sediments range from 115 to 959mg/kg and that serpentine prevails among the phyllosilicates. The high correlation between chromium and serpentine, amphibole and pyroxene minerals verifies the geogenic origin of chromium in soils and sediments and, therefore, in groundwater. Manganese also originates from serpentine, amphibole and pyroxene, and is strongly correlated with chromium, indicating that the oxidation of Cr(III) to Cr(VI) is performed by manganese–iron oxides located on the surface of Cr–Mn-rich minerals. Backscattered SEM images of the soils revealed the unweathered form of chromite grains and the presence of Fe–Mn-rich oxide on the outer surface of serpentine grains. Chemical analyses revealed that the highest Cr(VI) concentrations were found in shallow porous aquifers with low water velocities and their values vary from 5 to 70μg/L. Cr(VI) concentrations in ophiolitic complex aquifers ranged between 3 and 17μg/L, while in surface water, karst and deeper porous aquifers, Cr(VI) concentrations were lower than the detection limit of 1.4μg/L.
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•Fe–Mn oxides are located on the surface of mafic minerals and coexist with Cr.•Cr(VI) is released directly from serpentine, chlorite, amphibole and pyroxene.•Higher Cr(VI) concentrations are detected in shallow porous aquifer (<150m).•Low groundwater velocity favors Cr(VI) generation in sedimentary aquifer.
Catalytic ozonation was applied for the removal of small concentrations (4 μM) of micropollutants benzotriazole, carbamazepine, p-CBA from aqueous solutions at pH 7. These compounds present different ...physicochemical characteristics and different kinetic rate constants, when reacting with ozone or hydroxyl radicals in the ranges of <0.15–3 × 105, 5.2 × 109, and 8.8 × 109 M−1s−1, respectively. Calcite was used as heterogeneous catalyst and its catalytic activity evaluated, by applying (and optimized) different experimental conditions (i.e., pH, temperature, ozone concentration), concerning the removal efficiency of p-CBA. Study of micropollutants' removal showed all examined organic compounds can be sufficiently removed by application of catalytic ozonation either by use of calcite, or by presence of Co(II) or Fe(II) (applied as homogeneous catalysts), while the optimum catalyst between them was found to be calcite. Carbamazepine with kO3 = 3 × 105 M−1s−1 can be easily removed, even by application of single ozonation, while benzotriazole and p-CBA resulted in 50% and 68.2% higher removal after application of catalytic ozonation within 3 min of oxidation reaction, due to acceleration of hydroxyl radicals' production by presence of calcite in the ozonation system. The contribution of hydroxyl radicals in removal of all three micropollutants was evaluated by extraction of Rct and f•OH parameters. HIGHLIGHTS Benzotriazole, carbamazepine and p-CBA were successfully removed by catalytic ozonation.; Among examined catalysts calcite was defined as the best one.; The removal of micropollutants is based on the mechanism of radicals’ production.; Benzotriazole removal was in best agreement with the 1st order kinetic model.; The kinetic model of p-CBA depended on the applicable catalyst.;
•Cr(VI) was determined in 600 tap waters of small-medium sized communities of Greece.•Cr(VI) concentration in water samples accounts for 96% of total chromium in average.•Typical Cr(VI) containing ...waters have a pH>7.5 and Ca concentration <50mg/L.•Contact with ultramafic rocks activates Cr(VI) formation by a catalytic mechanism.•Highest Cr(VI) values observed in shallow porous aquifers downstream of ophiolites.
This study provides a survey on potential Cr(VI) exposure attributed to drinking water in Greece. For this reason, a wide sampling and chemical analysis of tap waters from around 600 sites, supplied by groundwater resources, was conducted focusing on areas in which the geological substrate is predominated by ultramafic minerals. Results indicate that although violations of the current chromium regulation limit in tap water are very rare, 25% of cases showed Cr(VI) concentrations above 10μg/L, whereas Cr(VI) was detectable in 70% of the samples (>2μg/L). Mineralogy and conditions of groundwater reservoirs were correlated to suggest a possible Cr(VI) leaching mechanism. Higher Cr(VI) values are observed in aquifers in alluvial and neogene sediments of serpentine and amphibolite, originating from the erosion of ophiolithic and metamorphic rocks. In contrast, Cr(VI) concentration in samples from ophiolithic and metamorphic rocks was always below 10μg/L due to both low contact time and surface area, as verified by low conductivity and salt concentration values. These findings indicate that under specific conditions, pollution of water by Cr(VI) is favorable by a slow MnO2-catalyzed oxidation of soluble Cr(III) to Cr(VI) in which manganese products Mn(III)/Mn(II) are probably re-oxidized by oxygen.
Ball-milling of magnetite powder is studied as a potential method to produce large quantities of nanostructured particles for As(III) and As(V) adsorption. Surfactant-assisted milling resulted in ...30nm isolated nanoparticles with high specific surface area but limited adsorption capacity. On the opposite, when no surfactant is added, large aggregates (300nm) surrounded by a thick α-Fe₂O₃ shell were formed. Hematite-coated particles appear to be very effective in the removal of As(III) and As(V) from aqueous solutions being able to adsorb around 1.0 and 2.1μg/mg, respectively, while keeping the residual arsenic below the regulation limits for drinking water. In addition, the existence of a magnetic core facilitates their application in a magnetic separation system.
The qualitative and quantitative characterization of several mining by-product samples, were collected from the magnesite mine of “Grecian Magnesite SA” company (Gerakini, Chalkidiki, North Greece), ...was aiming to evaluate the possibility of upgrading their refractory properties by applying thermal treatment. The concentration range of main components for the selected best qualified samples was 38.7–43 wt% for MgO, 37.5–44.1 wt% for SiO2 and 6.5–7.3 wt% for FeO. The mineralogical characterization revealed the presence of olivine, pyroxenes and serpentine, with the concentration of the latter positively correlated to LOI. Microprobe analyses clarified the presence of olivine (Mg1.79Fe0.19Ni0.01)SiO4, consisting mainly of 90 wt% of forsterite (Mg2SiO4) and 10 wt% of fayalite (Fe2SiO4), as well as of pyroxene group minerals (Mg0.87Fe0.08Ca0.01Cr0.01)(Si0.98Al0.04)O3, consisting mainly of 91 wt% enstatite (MgSiO3) and 9 wt% of ferrosilite (FeSiO3), respectively. The thermal treatment of the qualified samples demonstrated that at the temperature of 650–680 °C serpentine is almost totally decomposed and at the temperature of 850 °C it has been totally recrystallized to olivine and pyroxenes. At higher temperature treatment (1300 °C), it seems that there is a recrystallization process that favors the deformation of olivine and the further formation of pyroxenes, due to the excess of Si available from the initial decomposition of serpentine, while the presence of magnesite resulted to the restriction of olivine deformation through the partial capture of available Si. For increasing the olivine percentage and, subsequently, the improvement of refractory properties of this material, at temperature > 1300°C, the ideal theoretical addition dose of wt% MgO for optimizing the formation of olivine was calculated, ranging from 7.4 to 17.5 wt%. The latter calculations are reported for the first time in the literature regarding this kind of materials.
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•Liquidus temperature of mining wastes was calculated between 1257 and 1332 °C.•High correlation of serpentine content with LOI•850 °C is the optimum thermal treatment for second-generation of olivine.•Thermal treatment at 1300 °C favors the formation of pyroxenes•MgO dose for refractory material production was estimated between 7.4 and 17.5 wt%.
•An industrial solid waste is used to stabilize hazardous tannery waste.•Aluminum anodizing sludge is used as an alternative stabilization additive.•Aluminum anodizing sludge managed to stabilize ...both chromium and organic compounds.•Phytotoxicity tests were used to examine the potential toxic effect to plants.•The simplicity and the low cost of the stabilization process.
A global demand for efficient re-utilization of produced solid wastes, which is based on the principles of re-use and recycling, results to a circular economy, where one industry's waste becomes another's raw material and it can be used in a more efficient and sustainable way. In this study, the influence of a by-product addition, such as aluminum anodizing sludge, on tannery waste (air-dried sludge) stabilization was examined. The chemical characterization of tannery waste leachate, using the EN 12457-2 standard leaching test, reveals that tannery waste cannot be accepted even in landfills for hazardous wastes, according to the EU Decision 2003/33/EC. The stabilization of tannery waste was studied applying different ratios of tannery waste and aluminum anodizing sludge, i.e. 50:50, 60:40, 70:30 and 80:20 ratios respectively. Subsequently, the stabilization rate of the qualified as optimum homogenized mixture of 50:50 ratio was also tested during time (7, 15 and 30days). Moreover, this stabilized product was subjected to phytotoxicity tests using the Lepidium sativum, Sinapis alba and Sorghum saccharatum seeds. The experimental results showed that aluminum anodizing sludge managed to stabilize effectively chromium and organic content of tannery waste, which are the most problematic parameters influencing its subsequent disposal. As a result, tannery waste stabilized with the addition of aluminum anodizing sludge at 50:50 ratio can be accepted in non-hazardous waste landfills, as chromium and dissolved organic carbon concentrations in the respective leachate are below the relevant regulation limits, while the stabilized waste shows decreased phytotoxicity.
This study demonstrates an optimization procedure for the development of an Hg-specified adsorbent able to comply with the regulation limit for drinking water of 1μg/L. On this purpose, the synthesis ...of Mn(IV)-feroxyhyte was modified to achieve high negative charge density by combining alkaline and extreme oxidizing conditions. In particular, precipitation of FeSO4 at pH9 and excess of KMnO4 follows a very fast nucleation step providing a product with very small nanocrystal size (1–2nm), high specific surface area (300m2/g) and maximum negative charge density (1.8mmol H+/g). The adsorbent was validated for Hg removal in batch experiments and column tests using natural-like water indicating an adsorption capacity as high as 2.5μg/mg at equilibrium concentration 1μg/L under reliable conditions of application. Importantly, the adsorption is an exothermic spontaneous process, resulting in the formation of inner sphere complexes by sharing both A-type and B-type oxygen atoms with the metal surface octahedral as revealed by the X-ray absorption fine structure results.
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•Negatively charged Mn(IV)-feroxyhyte was optimized for Hg removal from drinking water.•Adsorption capacity at the regulation limit approaches 2.5μg/mg.•Intense oxidizing conditions during Mn(IV)-feroxyhyte synthesis promotes Hg2+ capture affinity.•Hg adsorption takes place through an exothermic chemisorption.•Hg2+ is adsorbed as a bidentate inner sphere complex.
The potential of magnetite nanoparticles to be applied in drinking water treatment for the removal of hexavalent chromium is discussed. In this study, a method for their preparation which combines ...the use of low-cost iron sources (FeSO4 and Fe2(SO4)3) and a continuous flow mode, was developed. The produced magnetite nanoparticles with a size of around 20nm, appeared relatively stable to passivation providing a removal capacity of 1.8μg Cr(VI)/mg for a residual concentration of 50μg/L when tested in natural water at pH7. Such efficiency is explained by the reducing ability of magnetite which turns Cr(VI) to an insoluble Cr(OH)3 form. The successful operation of a small-scale system consisting of a contact reactor and a magnetic separator demonstrates a way for the practical introduction and recovery of magnetite nanoparticles in water treatment technology.
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•Iron sulfates were used for the kilogram scale production of Fe3O4 nanoparticles.•Studied particles showed a Cr(VI) removal capacity of 2μg/mg in natural water.•Cr(VI) uptake is mostly based on its reduction and precipitation as Cr(OH)3.•A continuous flow reactor–magnetic separator operated with nanoparticles
•A tannery sludge was incinerated at oxic and anoxic conditions without any pretreatment.•Chromium speciation was investigated by monitoring Cr-bearing crystal phases.•Complete oxidation of Cr(III) ...to Cr(VI) ensues at 500°C, at oxic conditions.•At higher temperatures, partial reduction of Cr(VI) to Cr(III) ensues.•Main Cr-bearing crystal phases were CaCrO4 and MgCr2O4.
A tannery sludge, produced from physico-chemical treatment of tannery wastewaters, was incinerated without any pre-treatment process under oxic and anoxic conditions, by controlling the abundance of oxygen. Incineration in oxic conditions was performed at the temperature range from 300°C to 1200°C for duration of 2h, while in anoxic conditions at the temperature range from 400°C to 600°C and varying durations. Incineration under oxic conditions at 500°C resulted in almost total oxidation of Cr(III) to Cr(VI), with CaCrO4 to be the crystalline phase containing Cr(VI). At higher temperatures a part of Cr(VI) was reduced, mainly due to the formation of MgCr2O4. At 1200°C approximately 30% of Cr(VI) was reduced to Cr(III). Incineration under anoxic conditions substantially reduced the extent of oxidation of Cr(III) to Cr(VI). Increase of temperature and duration of incineration lead to increase of Cr(VI) content, while no chromium containing crystalline phase was detected.
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