Occurrence of Cr(VI) in drinking water of Greece and relation to the geological background

Implementing magnetically-active Sn-based nanocomposites in hexavalent chromium removal from drinking water

A novel nanocomposite consisting of Fe3O4-loaded tin oxyhydroxy-chloride is demonstrated as an efficient adsorbent for the removal of hexavalent chromium in compliance to the new drinking water regulation. This study introduces a continuous-flow production of the nanocomposite through the separate synthesis of (i) 40 nm Fe3O4 nanoparticles and (ii) multilayered spherical arrangements of a tin hydroxy-chloride identified as abhurite, before the application of a wet-blending process. The homogeneous distribution of Fe3O4 nanoparticles on the abhurite's morphology, features nanocomposite with magnetic response whereas the 10 % loaded nanocomposite preserves a Cr(VI) uptake capacity of 7.2 mg/g for residual concentrations below 25 μg/L. Kinetic and thermodynamic examination of the uptake evolution indicates a relative rapid Cr(VI) capture dominated by interparticle diffusion and a spontaneous endothermic process mediated by reduction to Cr(III). The efficiency of the optimized nanocomposite was validated in a pilot unit operating in a sequence of a stirring reactor and a rotary magnetic separator showing an alternative and competitive application path than typical fixed-bed filtration, which is supported by the absence of any acute cellular toxicity according to human kidney cell viability tests.

Groundwater Cr(VI) contamination and remediation: A review from 1999 to 2022

Hexavalent chromium (Cr(VI)) contamination of groundwater has traditionally been an environmental issue of great concern due to its bioaccumulative and highly toxic nature. This paper presents a review and bibliometric analysis of the literature on the interest area "Cr(VI) in groundwater" published in the Web of Science Core Collection from 1999 to 2022. First, information on 203 actual Cr(VI)-contaminated groundwater sites around the world was summarized, and the basic characteristics of the sources and concentrations of contamination were derived. 68.95% of the sites were due to human causes and 56.43% of these sites had Cr(VI) concentrations in the range of 0-10 mg/L. At groundwater sites with high Cr(VI) contamination due to natural causes, 75.00% of the sites had Cr(VI) concentrations less than 0.2 mg/L. A total of 936 papers on "Cr(VI) in groundwater" were retrieved for bibliometric analysis: interest in research on Cr(VI) in groundwater has grown rapidly in recent years; 59.4% of the papers were published in the field of environmental sciences. A systematic review of the progress of studies on the Cr(VI) removal/remediation based on reduction, adsorption and biological processes is presented. Out of 666 papers on Cr(VI) removal/remediation, 512, 274, and 75 papers dealt with the topics of reduction, adsorption, and bioremediation, respectively. In addition, several studies have demonstrated the potential applicability of natural attenuation in the remediation of Cr(VI)-contaminated groundwater. This paper will help researchers to understand and investigate methodological strategies to remove Cr(VI) from groundwater in a more targeted and effective manner.

Leachability of hexavalent chromium from fly ash-marl mixtures in Sarigiol basin, Western Macedonia, Greece: environmental hazard and potential human health risk

Hexavalent chromium (Cr (VI)) is an environmental contaminant brining high concerns due to its higher toxicity and mobility in comparison with trivalent chromium Cr(III). Cr (VI) has been linked with several adverse health effects, including respiratory diseases, lung cancer, and skin irritation. The primary sources of it in the environment are industrial activities.Most of the time, fly ash made of lignite can release Cr(VI) when it comes into contact with water in an aquatic environment. The objective of this study is the investigation of Cr (VI) concentration in leachates of fly ash and marl mixtures and the determination of its solubility under different pH conditions. Samples of fly ash were collected from the Power Plant of Agios Dimitrios. Additionally, samples of marl were collected from the mine of South Field, and both samples were mixed and prepared in in different proportions (% w.t.). The leaching experiments were carried out according to the EN-12457/1-4 (2003) standard under different pH conditions and chemical analysis of the leachates were performed by spectrophotometry with diphenylcarbazide (DPC). The environmental footprint of Cr (VI) in the study area was significant, especially in mixtures containing higher concentrations of fly ash. A critical pH range between 6 to 12 is observed. At acidic pH values, a high release of Cr (VI) was observed, while at the mentioned critical values (pH 10-12), a gradual decrease in its leachability was noticed. The high concentrations of Cr (VI) in the industrial area studied require immediate actions in terms of managing and limiting the potential hazardous impacts on the environment and by extension on the public health by developing appropriate prevention strategies.

Tin Oxide Nanoparticles via Solar Vapor Deposition for Hexavalent Chromium Remediation

Tin oxide nanoparticles optimized to capture low concentrations of hexavalent chromium from water were developed through a facile, scalable, and low-cost one-step solar vapor deposition methodology. Considering the preservation of high electron donation capacity as the key to support the reduction of mobile Cr(VI) into insoluble forms, the growth of SnO nanoparticles was favored by the co-evaporation of SnO2 with Fe powders at various mass ratios. Characterization techniques indicated that the percentage and the stability of SnO is proportional to the Fe content in the target with a requirement of at least 50% wt to inhibit the formation of a passive SnO2 surface layer. The produced particles were evaluated regarding their efficiency to capture Cr(VI) under conditions similar to water treatment for drinking purposes (pH 7). It was revealed that passivation-free SnO nanoparticles deliver significant improvement in the adsorption capacity corresponding to the residual concentration of 25 μg/L, reaching a value of 1.74 mg/g for the sample prepared with 50% wt Fe in the target. The increase of water acidity was found responsible for the activation of more reduction sites on the particle surface, as reflected through the elevation of efficiency by more than 20% at pH 6.

Characterization of trace elements in thermal and mineral waters of Greece

Natural thermal and mineral waters are widely distributed along the Hellenic region and are related to the geodynamic regime of the country. The diverse lithological and tectonic settings they are found in reflect the great variability in their chemical and isotopic composition. The current study presents 276 (published and unpublished) trace element water data and discusses the sources and processes affecting the water by taking into consideration the framework of their geographic distribution. The dataset is divided in groups using temperature- and pH-related criteria. Results yield a wide range of concentrations, often related to the solubility properties of the individual elements and the factors impacting them (i.e. temperature, acidity, redox conditions and salinity). Many elements (e.g. alkalis, Ti, Sr, As and Tl) present a good correlation with temperature, which is in cases impacted by water rock interactions, while others (e.g. Be, Al, Cu, Se, Cd) exhibit either no relation or an inverse correlation with T possibly because they become oversaturated at higher temperatures in solid phases. A moderately constant inverse correlation is noticed for the vast majority of trace elements and pH, whereas no relationship between trace element concentrations and Eh was found. Seawater contamination and water-rock interaction seem to be the main natural processes that influence both salinity and elemental content. All in all, Greek thermomineral waters exceed occasionally the accepted limits representing in such cases serious harm to the environment and probably indirectly (through the water cycle) to human health.

Biochar facilitated bacterial reduction of Cr(VI) by Shewanella Putrefaciens CN32: Pathways and surface characteristics

Biochar can facilitate the microbial reduction of various pollutants in soil and groundwater environments, but its impact on Cr(VI) reduction by dissimilatory metal reducing bacteria (DMRB) remains to be systematically investigated. In this study, we prepared biochars at 500 °C and 700 °C from wheat straw and grass, and investigated the impact of these biochars on Cr(VI) reduction by a model DMRB, Shewanella Putrefaciens CN32 (CN32). Pristine biochars abiotically reduced Cr(VI), which decreased the concentration and toxicity of chromium to CN32 cells, and brought about higher overall Cr(VI) removal extent after CN32 were added sequentially; on the other hand, no enhancement effect were observed when biochars and CN32 were added simultaneously. Further tests between biologically reduced biochars and Cr(VI) revealed that the reaction rates between bioreduced biochars and Cr(VI) are relatively sluggish compared to that of direct Cr(VI) reduction by CN32, which prohibited biochars from directly accelerating the Cr(VI) reduction by CN32 in simultaneous-addition scenario. The relative importance of biochars' surface functional groups and surface areas on their reactivities towards Cr(VI) reduction were also investigated. This study deepened our understanding towards the role of biochar played during bacterial Cr(VI) reduction and could potentially contribute to optimizing the biochar-based Cr(VI) bioremediation strategies.

A novel hybrid ferrous sulfide impregnated anion exchanger for trace removal of hexavalent chromium from contaminated water

To effectively remove trace concentration of potential cancer-causing Cr(VI) from impaired drinking water, a novel hybrid material was synthesized through an in-situ synthesis process by dispersing ferrous sulfide nanoparticles within an anion exchanger. Characterization studies revealed that the hybrid material, named hybrid ferrous sulfide impregnated anion exchanger (HISIIX), contained uniformly distributed ferrous sulfide nanoparticles of size 10-40 nm within the anion exchanger host. Apart from FeS₂ nanoparticles, it also included nanoparticles of FeO and FeOOH. The incorporation of ferrous sulfide nanoparticles within the anion exchanger contributed to the significant differences in the Cr(VI) uptake capacity of HISIIX. Validation studies using fixed-bed column proved that HISIIX had significantly high Cr(VI) uptake capacity and was able to run for 4200 bed volumes (BVs) before a breakthrough of 50 μg L^-1 when subjected to a synthetic aqueous solution containing 200 μg L^-1 Cr(VI). Cr(VI) uptake capacity of the parent anion exchanger and HISIIX were determined to be 1.39 mg g^-1 and 3.44 mg g^-1, respectively, when the columns were allowed to run until exhaustion. Ferrous sulfide nanoparticles acted as a reducing agent transforming Cr(VI) anions into Cr(III) precipitates. It also produced sites for further removal of Cr(VI) anions through ligand sorption upon oxidation. The anion exchanger substrate attracted anions selectively via the Donnan membrane principle, resulting in a synergy of three different processes - ion exchange, redox reaction, and ligand sorption that gave the HISIIX a high capacity for the selective Cr(VI) removal from contaminated water.

In-situ growing of metal-organic frameworks on iron mesh as a recyclable remediation material for removing hexavalent chromium from groundwater

The recovery and reuse of adsorbents is crucial for the effectiveness and sustainability of mitigation methods for groundwater pollution. Considering the difficulty in recovering powder materials and the low mechanical strength of membrane materials, we developed a sheet material with good recyclability and certain mechanical strength. In this study, an in situ synthesized MIL-100(Fe) film sample was produced by hydrothermal reaction using a commercially available iron mesh as the substrate. The MIL-100(Fe) samples were characterized by SEM, XRD, XPS, and FT-IR. The experiments showed that the material presented excellent removal ability toward Cr(VI) and good recovery performance. In the fourth cycle test, the Cr(VI) removal rate reached more than 95%. The material characterization and adsorption kinetics indicated that the removal mechanism was oxidation-reduction reaction and electrostatic adsorption. The removal experiments at different pH values and with different co-existing ions demonstrated that the material can maintain good removal capacity at pH values between 2 and 8, and common ions in groundwater can promote the removal of Cr(VI) under neutral conditions. The recycling test demonstrated that the sample can be reused. After the sample was recovered and calcined in an inert environment, a network sample containing zero-valent iron was obtained, and it removed Cr(VI) from water at a low pH in 20 min. This study provides a new alternative for the practical removal of Cr(VI) from groundwater.

Mechanisms of chromium isotope fractionation and the applications in the environment

Chromium (Cr) is a toxic heavy metal that gives rise to environmental pollution and human risk. Chromium stable isotopes have a wide range of applications in both environmental field and earth science field. In this contribution, we focus on the application of the Cr isotope in both tracing pollution sources and monitoring Cr(Ⅵ) pollution. Meanwhile, we also provide a description of the main influencing factors controlling Cr isotope fractionation, chromium isotope analytical methods, and terrestrial Cr release. Chromium isotope tracing of contaminant sources is a new application method, it has a tremendous advantage in searching for the source of Cr pollution, which has not been covered in previous reviews. At the end of the article, the current status of Cr isotope applications in the paleo-environment is explained. Although there are still some uncertainties in practical applications, chromium isotope system shows great promise in the environmental aspects.

Identification of sources and transformations of nitrate in Cr(VI)-impacted alluvial aquifers by a hydrogeochemical and δ15N-NO3- and δ18O-NO3 - isotopes approach

A coupled methodology of nitrogen isotopes, hydrogeochemical characterization, multivariate statistical analysis, and SIAR Bayesian modeling has been employed to identify the sources of NO₃^- and N transformation processes in three alluvial aquifers (Schinos, Thiva, and Central Evia) located in central Greece where geogenic Cr(VI) co-occurs with agricultural activity and rural development. Hexavalent chromium concentrations exceed 50 μg/L in many sampling stations of the studied groundwater bodies, while nitrate contamination is evident in all three study areas with concentrations well over 50 mg/L. The mean δ¹⁵N-NO₃^- and δ¹⁸Ο-NO₃^- values are 6.67 ± 1.77‰ and 2.68 ± 1.77‰ in C. Evia, 8.72 ± 4.74‰ and 3.96 ± 4.57‰ in Schinos and 4.44 ± 1.71‰ and 2.91 ± 1.02‰ in Thiva, respectively. Domestic sewage and N-bearing fertilizers are contributing in various degrees to the observed nitrification which is the dominant transformation process of N in the studied aquifers. Multivariate statistics indicated that the main processes identified in the study areas are salinization, silicate dissolution, and groundwater contamination due to fertilizer use. It is suggested that ultramafic rock-related alluvial aquifers must be closely monitored in terms of nutrient inputs as an effective measure for controlling Cr(VI) release in groundwater.

Chromium speciation, mobility, and Cr(VI) retention-release processes in ultramafic rocks and Fe-Ni lateritic deposits of Greece

Water contamination by geogenic hexavalent chromium is an emerging issue in areas developed on ultramafic rocks and their weathering products. In this study, samples of serpentinites, soil, and laterites were collected and analyzed for the levels of Cr species, distribution into phases of different mobility, mineralogy, Cr oxidation capacity, and leaching of Cr(VI). Total chromium (2176-21,929 mg kg^-1) was mainly found in Cr spinels (~ 50% wt as Cr₂O₃) and Fe (hydr)oxides (2.5% wt). Total Cr(VI) contents (0.49-11.5 mg kg^-1) increased from the serpentinites to the soil and lateritic deposit, i.e., during the advanced stages of weathering, which were accompanied by increased Cr mobility. Batch experiments of 500-h duration showed that Cr(VI) released in water at rates of 0.25-1.20 nM h^-1. Rates were higher in water of pH 5.7 rather than pH 8.5, because more Cr(VI) was formed during the experimental period. Asbolane-type Mn oxides and Mn-bearing crystalline Fe oxides were responsible for Cr(III) oxidation. Most of the generated Cr(VI) (52-79% of total Cr(VI)) remained solid-bound by adsorption and/or precipitation processes. Because all samples had a self-capacity to oxidize Cr(III), it seems that retention processes will ultimately define the extent of geogenic Cr(VI) contamination of aquatic systems.

Natural source of Cr(VI) in soil: The anoxic oxidation of Cr(III) by Mn oxides

Cr(VI) from oxidation of geogenic Cr(III) minerals is gradually becoming the primary source of Cr(VI) in soils and groundwater instead of direct emissions. Thermodynamically, natural oxidants of Cr(III) are limited to O₂ and Mn oxides. The oxidation of Cr(III) occurs commonly in oxic soils but the difference in the oxidative dissolution of Cr(III) by Mn oxides in different redox soils (especially under anoxic conditions) is not fully understood and field evidence is lacking. Here, the relationship between Cr(VI) and Mn oxides in basalt-origin soil profiles under three different redox conditions (anoxic, suboxic and oxic) was studied. The oxidative dissolution of chromite was validated by synthesising δ-MnO₂ that was close to biogenic Mn oxides under anoxic and oxic conditions. In anoxic soils, high levels of Cr(VI) were detected in the same horizons as those where Cr(III)-minerals co-existed with Mn(III/IV) oxides, suggesting an exclusive pathway for Cr(VI) generation through oxidation by Mn oxides where there was a deficiency of other oxidants, such as O₂. In oxic soils, the highly abundant Fe oxides combined with Cr(III) to form Cr(III)-Fe(III) oxyhydroxides and Cr(VI) was generated mainly via slow oxidation by O₂. The chromite oxidation experiment results also indicated that a high abundance of Mn oxides could promote chromite oxidative dissolution to generate Cr(VI), even under anoxic conditions. Additionally, the form of Cr and the reactivity and abundance of Mn oxides and reducing agents controlled the net content of Cr(VI) in the soil. This study showed that, even under reducing conditions, Cr(III) is readily oxidised by Mn oxides to generate Cr(VI) in reductant-deficient and Mn-rich soils, which may lead to the continuous introduction of Cr(VI) into groundwater and agricultural soils.

Effects of Chromium Toxicity on Physiological Performance and Nutrient Uptake in Two Grapevine Cultivars (Vitis vinifera L.) Growing on Own Roots or Grafted onto Different Rootstocks

Chromium toxicity is considered within the most severe and dangerous nutritional disorders, and it can often be observed in crops grown in industrial areas. The present study aims to determine the effects of Cr(VI) toxicity on the growth, nutrition, and physiological performance of grapevines. In a pot hydroponic experiment, own-rooted Merlot and Cabernet Franc grapevine cultivars or cultivars grafted onto 1103P and 101-14 Mgt rootstocks were exposed to 120 μM Cr(VI). Leaf interveinal chlorosis appeared after forty-five days of treatment. Overall leaf chlorosis and brown root coloration after sixty days was reported. A significant effect on the majority of the measured parameters due to the Cr(VI) treatment was observed. Chromium stress increased the total Cr concentrations in all parts of the vines, i.e., leaves, shoots, roots, and trunks. When comparing between the studied plant sections, the roots presented the highest Cr concentrations, ranging from 396 to 868 mg kg−1 d. w., and then, in descending order, the Cr concentrations ranged from 41 to 102 mg kg−1 d. w. in the trunks, from 2.0 to 3.3 mg kg−1 d. w. in the leaves, and from 1.9 to 3.0 mg kg−1 d. w. in the shoots. Between the assessed rootstocks, 1103P was identified to be a better excluder of Cr concentration in the roots and other aerial parts of the vines. Additionally, chromium toxicity negatively affected the concentrations and compartmentalization of the most important nutrients. Leaf chlorophyll (Chl) concentration decreased down to approximately 53% after sixty days of Cr stress. Chromium toxicity significantly reduced the stem water potential (SWP), net CO2 assimilation rate (A), stomatal conductance (gs), and PSII maximum quantum yield in all the cases of grafted or own-rooted vines. At this stage, chromium stress increased the leaf total phenolic content from 46.14% in Merlot vines to 75.91% in Cabernet Franc vines.

Genesis of high hexavalent chromium groundwater in deep aquifers from loess plateau of Northern Shaanxi, China

Hexavalent chromium (Cr(VI)) groundwater usually exists in shallow aquifers related to ultramafic and serpentine formations, but knowledge of the genesis of dissolved Cr(VI) in deep sandstone aquifers is limited. Both groundwater and aquifer sediments were taken from the Jingbian County in the Loess Plateau of Northwestern Shaanxi to investigate distribution and genesis mechanism of high Cr(VI) groundwater in deep sandstone aquifers. Results showed that the Cr concentrations (median 142 μg/L) in groundwater from deep aquifers (>100 m) were relatively high, while the Cr concentrations in shallow groundwater were low (median 33.8 μg/L). Dissolved Cr mainly existed in the species of Cr(VI) (average, 90%). Deep groundwater with higher Cr(VI) concentrations generally had higher pH, Eh, and DO than shallow groundwater, indicating that the high Cr(VI) groundwater existed in relatively oxic environment. Cretaceous sandstones in deep aquifers had anomalously high contents of total Cr (average 115 mg/kg), where Cr was mainly present in silicates-bound form, and secondly in strongly adsorbed form. There were positive correlations between Mn and Cr in the unweathered silicate-bound form and adsorbed form, which were conducive to Cr(III) oxidation into Cr(VI) in an alkaline-oxic environment. The different ionic ratios (i.e. (Ca²⁺ + Mg²⁺)/(HCO₃^- + SO₄^2-)) also supported silicate weathering as the dominant rock-water interactions in the deep groundwater, which enhanced the release of the unweathered silicate-bound Cr. Relatively high pH and ionic strength mobilized the adsorbed Cr(VI) into groundwater. This investigation emphasizes the geological origin of high Cr(VI) groundwater in deep sandstone aquifers containing Mn oxides, which deserves more concerns for the purpose of drinking water supply.

Chromium(VI) Removal from Water by Lanthanum Hybrid Modified Activated Carbon Produced from Coconut Shells

Cr(VI) is considered to be the most hazardous and toxic oxidation state of chromium and hence the development of effective removal technologies, able to provide water with Cr(VI) below the drinking water limits (US EPA 100 μg/L, European Commission 50 μg/L, which will be reduced to 25 by 2036) is a very important issue in water treatment. This study aimed at examining the performance of activated carbon produced from coconut shells, modified by lanthanum chloride, for Cr(VI) removal from waters. The structure of the formed material (COC-AC-La) was characterized by the application of BET, FTIR and SEM techniques. The effect of the adsorbent's dosage, pH value, contact time, initial Cr(VI) concentration and water matrix was examined with respect to Cr(VI) removal. The results indicated that the maximum Cr(VI) removal was observed at pH 5; 4 h contact time and 0.2 g/L of adsorbent's dosage was adequate to reduce Cr(VI) from 100 μg/L to below 25 μg/L. Freundlich isotherm and pseudo-second order kinetic models fitted the experimental data sufficiently. The maximum adsorption capacity achieved was 6.3 μg/g at pH 5. At this pH value, the removal percentage of Cr(VI) reached 95% for an initial Cr(VI) concertation of 30 μg/L. At pH 7 the corresponding efficiency was roughly 60%, resulting in residual Cr(VI) concentrations below the anticipated drinking water limit of 25 μg/L of total chromium, when the initial Cr(VI) concentration was 50 μg/L. Consecutive adsorption and regeneration studies were conducted using 0.01 M of NaOH as an eluent to evaluate the reusability of the adsorbents, Results showed 20% decrease of adsorption capacity after 5 regeneration cycles of operation.

Reductive Transformation of Hexavalent Chromium in Ice Decreases Chromium Toxicity in Aquatic Animals

In this study, the toxicity of hexavalent chromium [Cr(VI)] reduced by citric acid in ice was measured using representative aquatic model invertebrates (i.e., rotifer, water flea, amphipod, and polychaete) and a vertebrate (zebrafish) by analyzing short- and/or long-term endpoints that are frequently applied to each animal. Cr(VI) reduction in the presence of citric acid was markedly enhanced in the ice phase compared to that in an aqueous solution through the freeze concentration effect. The highly concentrated Cr(VI) and citric acid in ice grain boundaries were also confirmed using in situ cryogenic confocal Raman spectroscopy. Overall, exposure to Cr(VI) resulted in higher acute and/or chronic effects on aquatic animals, such as drastic mortality, growth inhibition, and decrease in offspring number, whereas the animals were increasingly tolerant to Cr(VI) that was reduced in the ice phase. Sublethal concentrations of Cr(VI) significantly decreased the antioxidant capacity in the aquatic animals. However, when the same concentrations of Cr(VI) were reduced in ice, these treatments showed no modulation or increase in the antioxidant defense system. Taken together, our results suggest that Cr(VI) reduction into Cr(III) was successfully achieved in ice and that this methodology can decrease the actual toxicity of Cr(VI) in aquatic animals.

Chromium biogeochemical behaviour in soil-plant systems and remediation strategies: A critical review

Chromium (Cr) is a toxic heavy metal that is heavily discharged into the soil environment due to its widespread use and mining. High Cr levels may pose toxic hazards to plants, animals and humans, and thus have attracted global attention. Recently, much progress has been made in elucidating the mechanisms of Cr uptake, transport and accumulation in soil-plant systems, aiming to reduce the toxicity and ecological risk of Cr in soil; however, these topics have not been critically reviewed and summarised to date. Accordingly, based on available data-especially from the last five years (2017-2021)-this review traces a plausible link among Cr sources, levels, chemical forms, and phytoavailability in soil; Cr accumulation and translocation in plants; and Cr phytotoxicity and detoxification in plants. Additionally, given the toxicity and hazard posed by Cr(VI) in soils and the application of reductant materials to reduce Cr(VI) to Cr(III) for the remediation of Cr(VI)-contaminated soils, the reduction and immobilisation mechanisms by organic and inorganic reductants are summarised. Finally, some priority research challenges concerning the biogeochemical behaviour of Cr in soil-plant systems are highlighted, as well as the environmental impacts resulting from the application of reductive materials and potential research prospects.

Use of LDH- chromate adsorption co-product as an air purification photocatalyst

This work deals with the use of layered double hydroxides for a double environmental remediation. The residue obtained in the use of these materials as a chromate sorbent in water, was subsequently studied as a photocatalyst for the removal of NO_x gases. With this aim, MgAl-CO₃ layered double hydroxides were synthesized by the coprecipitation method with a divalent/trivalent metal ratio of 3. After its calcination at 500 °C, the mixed oxide was obtained and MgAl-CrO₄ were synthesized by the reconstruction method. A complete chemical, morphological and photochemical study of the samples was carried out with techniques such as XRD, FT-IR, TGA, XRF, PL, DRIFTS and UV-Vis spectroscopy. Results showed that LDH materials presented no significant changes in their structure after their use as a sorbent. Photocatalytic tests of the samples showed a very good NO removal efficiency, as well as a high selectivity (low NO₂ emissions) through complete oxidation of these oxides to nitrate. The incorporation of chromate into the LDH structure improved the absorption of light in the visible region of the spectra, producing an improvement of 20% in the NO elimination compared with the LDH without chromate.

Hydrogeochemical Processes and Natural Background Levels of Chromium in an Ultramafic Environment. The Case Study of Vermio Mountain, Western Macedonia, Greece

The hydrogeochemical processes and natural background levels (NBLs) of chromium in the ultramafic environment of Vermio Mountain, Western Macedonia, Greece, were studied. Seventy groundwater samples were collected from 15 natural springs between 2014–2020, and an extensive set of physical and chemical parameters were determined. The ultramafic-dominated environment of western Vermio Mt. favors elevated groundwater concentrations of dissolved magnesium (Mg2+), silicon (Si), nickel (Ni), and Cr in natural spring waters. Chromium was the principal environmental parameter that exhibited a wide range of concentrations, from 0.5 to 131.5 μg/L, systematically exceeding the permissible limit of 50 μg/L for drinking water. Statistical evaluation of hydrogeological, hydrochemical, and hydrological data highlighted the water-ultramafic rock process as the predominant contributor of Cr in groundwater. The NBL assessment for Cr and Cr(VI) was successfully applied to the typical ultramafic-dominated spring “Potistis” that satisfied all the methodology criteria. The NBLs of Cr and Cr(VI) were defined at 130 μg/L and 100 μg/L, respectively, revealing that a natural ultramafic-dominated environment exhibits the geochemical potential to contribute very high concentrations of geogenic Cr to groundwater. The holistic methodology, proposed herein, could be implemented in any catchment scale to assess geogenic and anthropogenic Cr-sources that degrade groundwater quality.

Lead, cadmium, mercury, and chromium in urine and blood of children and adolescents in Germany - Human biomonitoring results of the German Environmental Survey 2014-2017 (GerES V)

Metals reach humans through food and drinking water intake and inhalation of airborne particles and can have detrimental health effects in particular for children. The metals presented here (lead, cadmium, chromium, and mercury) could lead to toxic effects such as neurotoxicity, mutagenicity, and have been classified as (possible) carcinogens. Using population representative data from the German Environmental Survey 2014-2017 (GerES V) from 3- to 17-year-old children on lead and cadmium in blood (n = 720) and on cadmium, chromium, and mercury in urine (n = 2250) we describe current internal exposure levels, and socio-demographic and substance-specific exposure determinants. Average internal exposure (geometric means) in blood was 9.47 μg/L for lead and below 0.06 μg/L (limit of quantification) for cadmium, and in urine 0.072 μg/L for cadmium, 0.067 μg/L for mercury, and 0.393 μg/L for chromium, respectively. Younger children have higher concentrations of lead and chromium compared to 14-17-year-old adolescents, and boys have slightly higher mercury concentrations than girls. With respect to substance specific determinants, higher lead concentrations emerged in participants with domestic fuel and in non-smoking children with smokers in the household, higher levels of cadmium were associated with smoking and vegetarian diet and higher levels of mercury with the consumption of seafood and amalgam teeth fillings. No specific exposure determinants emerged for chromium. The health based guidance value HBM-I was not exceeded for mercury and for cadmium in urine it was exceeded by 0.6% of the study population. None of the exceedances was related to substantial tobacco smoke exposure. Comparisons to previous GerES cycles (GerES II, 1990-1992; GerES IV, 2003-2006) indicate continuously lower levels.

Chromium (VI) removal kinetics by magnetite-coated sand: Small-scale flow-through column experiments

Magnetite nanoparticles are promising materials for treating toxic Cr(VI), but safe handling is challenging due to their small size. We prepared flow-through columns containing 10% or 100% (v/v) magnetite-coated sand. Cr(VI) removal efficiency was determined for different Cr(VI) concentrations (0.1 or 1.0 mM), neutral or alkaline pH, and oxic/anoxic conditions. We formulated a reactive-transport model that accurately predicted total Cr removal, accounting for reversible and irreversible (chemi)sorption reactions. Our results show that the material removes and irreversibly sequesters Cr(VI). For the concentration range used 10% and 100% (v/v) -packed columns removed > 99% and 72% of influent Cr(VI), respectively. Two distinct parameter sets were necessary to fit the identical model formulation to the 10 or 100% (v/v) columns (e.g., maximum sorption capacities (q_max) of 1.37 µmol Cr/g sand and 2.48 µmol Cr/g, respectively), which we attributed to abrasion-driven magnetite micro-particle detachment during packing yielding an increase in reactive surface area. Furthermore, experiments under oxic conditions showed that, even when handled in the presence of O₂, the magnetite-coated sand maintained a high removal capacity (47%). Our coupled experimental and modelling analyses indicates that magnetite-coated sand is a promising and suitable medium for treating Cr(VI)-contaminated water in fixed-bed reactors or permeable reactive barriers.

A DPSIR Approach to Selected Cr(VI) Impacted Groundwater Bodies of Central Greece

The holistic approach of Driver-Pressure-State-Impact-Response (DPSIR) methodology was applied to selected Cr(VI) impacted groundwater bodies of Central Greece. The main driving forces in the study areas are agricultural activities, urban and industrial development as well as tourism. The main pressures induced by the anthropogenic activities are fertilizer use, uncontrolled urban sewage disposal and industrial effluents discharges. Groundwater stress is caused by the qualitative degradation due to Cr(VI), NO₃^-, Cl^- and SO₄^2- contamination. Hexavalent chromium occurrence is attributed to both geogenic and anthropogenic sources. The maximum Cr(VI) concentration (11.7 mg/L) was measured in Oinofyta area. Important impacts are the deterioration of groundwater body chemical status as well as the decline of groundwater use efficiency. Based on the applied DPSIR, a management framework is proposed in order to address the complex environmental issue of Cr(VI) in the study areas.

Ultramafic geoecosystems as a natural source of Ni, Cr, and Co to the environment: A review

Ultramafic soils are in equal parts fascinating and dangerous. Developed on rocks derived predominately from the Earth's mantle and metamorphosed at the ocean floors, ultramafic soils form in the places where tectonic forces brought these rocks from mantle depths to the surface. As it is common in nature, both ultramafic rocks and soils are site-specific, and vary in character and composition; however, they have one thing in common, they are enriched in certain elements and three metals in particular form an "ultramafic" triad: Ni, Cr, and Co. These three metals are far from being human-friendly and strict legislative limits are established for maximum allowable concentrations of these metals in soils, but mostly in the case when the metals are of anthropogenic origin. However, ultramafic soils are a natural phenomenon where increased metal content is not the result of pollution, but rather referred as a peculiar geochemical background, therefore there is no reason for their remediation. At the same time, it is not that easy to actually find an ultramafic soil that does not overstep the limits (for the sake of this paper we use median world Regulatory Guidance Values - RGVs). Often, mobile Ni and Co concentrations are above the guidelines when doing tests to estimate the bioavailable fraction (EDTA and DTPA), and high concentrations of Ni are also commonly present in excluder plants (also edible ones). Also waters in ultramafic areas often exceed Ni and Cr(VI) limits. It is therefore expected that the ultramafic metals are present in the food chain and they might constitute a potential health risk. Thus, there is a need for additional research focused on assessment of the potential health consequences of chronic high exposure on naturally occurring Ni, Cr, and Co.

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

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.

Graphene-modified graphite paper cathode for the efficient bioelectrochemical removal of chromium

Metal-free electrocatalysts have been widely used as cathodes for the reduction of hexavalent chromium [Cr(VI)] in microbial fuel cells (MFCs). The electrocatalytic activity of such system needs to be increased due to the low anodic potential provided by bacteria. In this study, graphite paper (GP) was treated by liquid nitrogen to form three-dimensional graphite foam (3DGF), improving the Cr(VI) reduction by 17% and the total Cr removal by 81% at 30 h in MFCs. X-ray absorption spectroscopy confirmed the Cr(VI) reduction product as Cr(OH)₃. Through the spectroscopy characterizations, electrochemical measurements, and density functional theory calculations, the porous structures, edges, and O-doped defects on the 3DGF surface resulted in a higher electroconducting rate and a lower mass transfer rate, which provide more active sites for the Cr(VI) reduction. Additionally, the scrolled graphene-like carbon nanosheets and porous structures on the 3DGF surface might limit the OH^- diffusion and result in a high local pH, which accelerated the Cr(OH)₃ formation. The results of this study are expected to provide a simple method to manipulate the carbon materials and insights into mechanisms of Cr(VI) reduction in MFCs by the 3DGF with in situ exfoliated edges and O-functionalized graphene.

Assessment of the intake of selected minerals in population of premenopausal women based on specific socio-demographic indicators

Eating behavior interventions are a modifiable risk factor for chronic diseases. The aim of this study was to monitor the intake of selected minerals – calcium, phosphorus, zinc, copper, selenium, and chromium in the diet of premenopausal women (n = 30) and to highlight the possible adverse effects of disbalanced intake of these elements. At the same time, we investigated whether socio-demographic factors and choice of food store have an impact on the intake of these substances. We assessed the intake of selected minerals using three-day nutritional protocols and subsequently processed the data. The participants were women between 40 and 50 years old, from districts of Partizánske and Prievidza, for whom menopause has not yet begun. Women provided information about their place of residence (urban or rural area), type of home they live in (family house or apartment), and the type of food store where they grocery shop. The results indicate an impact of the place of residence: higher intake of zinc (p = 0.012) and selenium (p = 0.020) were observed in the participants from the urban area. The impact of the type of home was proven on the intake of chromium (p = 0.049), copper (p = 0.048), and carbohydrates (p = 0.021) with higher intake in the apartment-dwelling group. The impact of food store choice has not been confirmed. Based on the observed values, we conclude that the observed population might be at a higher risk of skeletal disorders and osteoporosis due to deficient calcium intake and the unfavorable ratio of Ca:P; increased Zn and Se intake levels may stimulate the development of cardiovascular risk factors and may also elevate the risk for type 2 diabetes mellitus.

Simultaneous Removal of Arsenate and Chromate from Ground- and Surface- Waters by Iron-Based Redox Assisted Coagulation

Arsenic (As) and chromate (Cr(VI)) contamination of ground and surface waters is a major problem worldwide. Given that a new drinking water limit is anticipated for Cr(VI) and that the limit of arsenic in drinking water is quite low (10 μg/L), there is an urgent need for evaluating technologies that could be efficient for removal of both contaminants simultaneously. In this work, the use of Fe(II) redox assisted coagulation was investigated to simultaneously remove the contaminants of interest. The basic principle of this technology is that Fe(II) could react with Cr(VI) and form Fe(III)-hydroxides and insoluble Cr(III) species, while the freshly formed Fe(III) hydroxides are very efficient adsorbents for As(V). The effect of pH, the water matrix composition, Fe(II) dose, initial contaminant concentrations, NOM presence and phosphate concentration were the examined parameters. The results revealed that with a dose of 2 mg/L Fe(II), residual As(V) and Cr(VI) concentrations were both below 10 μg/L, from initial concentrations of 50 μg/L. Though, this is effective only at circumneutral pH values. This is however not a big obstacle, since most natural waters, especially groundwaters, have near neutral pH values. At these pH values, residual iron concentration was far below 200 μg/L. The presence of phosphate anions inhibited As(V) removal but had no effect on Cr(VI) removal. Increasing Fe(II) concentrations eliminated the effect of phosphate and provided simultaneous phosphate removal. Therefore, Fe(II) coagulation can be applied, with secured results, for simultaneous As(V), Cr(VI) and phosphate removal from waters.

Cr(VI) Femoval from Ground Waters by Ferrous Iron Redox-Assisted Coagulation in a Continuous Treatment Unit Comprising a Plug Flow Pipe Reactor and Downflow Sand Filtration

Chromium(VI) (Cr(VI)) is the main chromium species found in groundwater and is considered as a highly toxic and carcinogenic element to humans. In the present study, removal of Cr(VI) by coagulation with ferrous iron is studied in a continuous flow treatment unit comprising pipe flocculation reactors followed by a sand filter. The studied parameters, regarding their effect on the removal of hexavalent chromium, were the ferrous iron dose, the effect of linear velocity, and the effect of the starting Cr(VI) concentration. The experiments have shown that the Cr(VI) removal achieved was very efficient and residual Cr(VI) and total Cr concentration in the treated water was lower than 10 μg/L, provided that the required dose of ferrous iron is provided. In particular, the study demonstrated that the removal of hexavalent chromium, from initial concentration of 50 μg/L and 100 μg/L, was more than 90% with ferrous doses of 1 mg/L and 2 mg/L respectively, applying linear velocity of 8 m/h, at an initial pH value of 7.3. Iron concentration in treated water was very low, far below 200 μg/L, which is the limit for iron in drinking water. This unit comprises a simple treatment option, for applications at the household level, with minimum maintenance requirements capable of removing Cr(VI) to concentrations below 10 μg/L, which might be the future limit for chromium in drinking water.

An Optimized Cr(VI)-Removal System Using Sn-based Reducing Adsorbents

Despite significant risks to human health due to elevated Cr(VI) concentrations in drinking water, a selective adsorbent capable of purifying water before consumption is still not commercially available. This work introduces an integrated household water filtration setup, for point-of-use applications, loaded with a tin-based Cr(VI)-oriented adsorbent that was tested under various contact times, pH values and Cr(VI) concentrations. The adsorbent comprises a chloride-substituted stannous oxy-hydroxide with a structure resembling that of the mineral abhurite. It demonstrated high reducing capacity that triggered the formation of insoluble Cr(III) hydroxides and the complete removal of Cr(VI) in considerably high volumes of polluted water. Test operation of the filtration system verified its ability to produce Cr(VI)-free water in compliance with the impending drinking water regulation, even for extreme initial concentrations (1000 μg/L). Apart from its high efficiency, the potential of the studied material is enhanced by its minimal-cost synthesis method carried out in a continuous-flow reactor by tin chloride precipitation under acidic conditions.

Cr(VI) reduction by Fe(II) sorbed to silica surfaces

The reaction kinetics of groundwater contaminants are integral to evaluating the fate and transport of toxic metals in the environment. For redox sensitive contaminants, such as chromium, the kinetics of different reaction pathways can vary by orders of magnitude. Species-specific rate constants for iron-chromium oxidation-reduction reactions are unknown for many systems, especially in the presence of sorbing surfaces. We investigate the role of quartz and amorphous silica (SiO_2(am)) surfaces in mediating abiotic reduction of Cr(VI)_aq by aqueous and sorbed Fe(II) using batch sorption and redox experiments. Sorption edges indicate outer-sphere (Fe(II)_ads,OS) and inner-sphere (Fe(II)_ads,IS) complexes are present on both silica surfaces, and their abundance depends on pH, ionic strength, and surface disorder. The rate constants for Cr(VI)_aq reduction by Fe(II) species increase in the following order: k_aq ≪ k_{ads,OS,quartz} < k_{ads,OS,SiO2(am)} < k_{ads,IS,quartz} < k_{ads,IS,SiO2(am)}, suggesting that increasing proximity of Fe(II) to the negatively charged silica surface enhances the rate of reduction of Cr(VI)_aq. However, we observe that experiments with larger amounts of sorbed Fe(II) reduce less total Cr(VI)_aq over time, which we attribute to a portion of the sorbed Fe(II) being sequestered into the Cr(III)-Fe(III)-oxyhydroxide precipitates forming on the silica surface. Therefore, the balance between increases in the rate and decreases in the total amount of Cr(VI)_aq reduction by various sorbed Fe(II) species must be considered when devising remediation strategies.

Occurrence and cycling of trace elements in ultramafic soils and their impacts on human health: A critical review

The transformation of trace metals (TMs) in natural environmental systems has created significant concerns in recent decades. Ultramafic environments lead to potential risks to the agricultural products and, subsequently, to human health. This unique review presents geochemistry of ultramafic soils, TM fractionation (i.e. sequential and single extraction techniques), TM uptake and accumulation mechanisms of ultramafic flora, and ultramafic-associated health risks to human and agricultural crops. Ultramafic soils contain high levels of TMs (i.e. Cr, Ni, Mn, and Co) and have a low Ca:Mg ratio together with deficiencies in essential macronutrients required for the growth of crops. Even though a higher portion of TMs bind with the residual fraction of ultramafic soils, environmental changes (i.e. natural or anthropogenic) may increase the levels of TMs in the bioavailable or extractable fractions of ultramafic soils. Extremophile plants that have evolved to thrive in ultramafic soils present clear examples of evolutionary adaptations to TM resistance. The release of TMs into water sources and accumulation in food crops in and around ultramafic localities increases health risks for humans. Therefore, more focused investigations need to be implemented to understand the mechanisms related to the mobility and bioavailability of TMs in different ultramafic environments. Research gaps and directions for future studies are also discussed in this review. Lastly, we consider the importance of characterizing terrestrial ultramafic soil and its effect on crop plants in the context of multi-decadal plans by NASA and other space agencies to establish human colonies on Mars.

Expounding the origin of chromium in groundwater of the Sarigkiol basin, Western Macedonia, Greece: a cohesive statistical approach and hydrochemical study

The aim of this paper is to provide a methodology including statistical tools and spatial techniques, in order to identify the various potential sources of chromium (Cr_tot) in the Sarigkiol basin, Western Macedonia, Greece, where elevated concentrations of Cr_tot in groundwater have been recorded since 1996. Integrated hydrochemical approach and statistical analyses including Pearson's correlation coefficient, multivariate statistical analyses (factor analysis and hierarchical cluster analysis), and spatial techniques (Moran's I spatial autocorrelation index and bivariate local indicator spatial association cluster map) were applied to evaluate the chemical analyses of 73 water samples, from irrigation wells, natural springs, and surface water. Both natural and anthropogenic sources of Cr_tot were recorded; the first (ultramafic-dominated environment) is strongly depicted on the natural spring water, in which Cr_tot concentrations as high as ~ 130 μg/L were recorded, whereas the second (agricultural activities) acts synergistically in the irrigation wells of the Sarigkiol basin, in which strong correlations of Cr_tot, P, and NO₃^- were defined. The paper highlights its findings by outlining the potential sources of elevated concentrations of Cr⁶⁺ in the Sarigkiol basin, stressing the need for a closer attention on the role of agricultural activities as an important, though commonly neglected, anthropogenic source of Cr_tot in groundwater.

The synergistic role of agricultural activities in groundwater quality in ultramafic environments: the case of the Psachna basin, central Euboea, Greece

In the present study, we approach the geochemical processes affecting the hydrochemistry and resulting in elevated concentrations of hexavalent chromium (Cr⁶⁺) in groundwater of the Psachna basin, central Euboea, Greece. Sixty-five groundwater samples and 16 topsoil (5-20 cm) samples were studied in order to examine groundwater and soil quality in relation to geogenic processes and anthropogenic activities. Specifically, the origin of Cr and Cr⁶⁺ in groundwater was investigated by co-evaluating (a) hydrochemical cross plots of major ions; (b) spatial distribution maps of Cl^-, Mg²⁺, NO₃^-, and Cr⁶⁺; (c) multivariate statistical analyses such as factor analysis (FA) and hierarchical cluster analysis (HCA) of groundwater geochemistry; (d) chemical analyses of soil samples; and (e) chemical analyses of fertilizers. The major factors that control the hydrochemistry of the study area are reverse ion exchange, dissolution of silicate minerals, and intense agricultural activities. According to FA, three factors explain 73.2% of the total variance of data, whereas according to HCA, the groundwater samples were classified into three groups indicating both geogenic (water-rock interaction) and anthropogenic (agricultural activities) impact. The high concentration of NO₃^-, up to 540 mg L^-1; the strong positive correlation between NO₃^- and Cr as well as between NO₃^- and other parameters such as SO₄^2- and Mg²⁺ in groundwater samples; and the very high content of P, up to 2444 mg kg^-1, in soil samples of the Psachna basin, imply the synergistic, although commonly neglected, role of the use of fertilizers in groundwater quality.

Release and fate of Cr(VI) in the ophiolitic aquifers of Italy: the role of Fe(III) as a potential oxidant of Cr(III) supported by reaction path modelling

Reaction path modelling of serpentinite dissolution in meteoric water was performed, varying Fe₂O₃/(FeO + Fe₂O₃) weight ratios of serpentine and reproducing the analytical concentrations of relevant solutes, including Cr(VI), in the Mg-HCO₃ groundwaters hosted in the ophiolitic aquifers of Italy. The occurrence of geogenic Cr(VI) in these groundwaters appears to be potentially controlled by the oxidation of trivalent Cr to the hexavalent redox state, driven by the reduction of trivalent Fe to the divalent redox state. In fact, trivalent Fe is the only oxidant present in suitable amounts in serpentinite rock, and even serpentine contains a high content of trivalent Fe as proven by recent studies. In contrast, the generally accepted hypothesis that geogenic Cr(VI) in waters interacting with serpentinites is driven by the reduction of trivalent and tetravalent Mn is questionable, since serpentinite rock has a low Mn content and it is necessary to invoke adsorption of trivalent Cr ions onto the surface of Mn oxides, which are oxidised as a surface complex to hexavalent Cr. Moreover, Mn oxides are present in the aqueous solution as suspended particles or in rocks as coatings or nodules.

Increased prevalence of Sjogren's syndrome in where soils contain high levels of chromium

Previously, we showed that farm soil levels of chromium were strongly correlated with people's serum chromium levels and an increase (3.6 fold) in both the incidence and prevalence of sicca syndrome in areas where farm soil chromium was high. Because Sjogren's syndrome (SS) is the major disease causing a dry mouth and dry eyes, we aimed in the study to investigate whether these areas with high soil chromium have a high SS prevalence. We used a database from the authority in charge of catastrophic illness certificates. Heavy metal concentrations in farm soils were retrieved from nationwide surveys. We used spatial regression models to study the relationships between the SS prevalence and soil metal concentrations. There were 11,220 people, 1165 men and 10,055 women who received a SS certificate from 2000 to 2011. The SS prevalence was 31 per 105 people, 5.59 for men and 55.01 for women. The highest SS prevalence in Taiwan (53 per 105) was located in an area where farm soils contain the highest amounts of chromium. In contrast, other types of heavy metal did not show such a strong association. In conclusion, the SS prevalence is significantly increased in areas where soils contain high levels of chromium. Chromium is likely a risk for SS.

Reductive precipitation and removal of Cr(VI) from groundwaters by pipe flocculation-microfiltration

Chromium (Cr(VI)) is a very toxic and carcinogenic element, which is widely present in groundwaters, mainly due to geogenic conditions. The limit of Cr(VI) in drinking water is expected to be reduced to 10 μg/L in both the USA and the European Union. Recent literature findings indicated that the most efficient process in reducing Cr(VI) levels to below 10 μg/L proved to be Cr(VI) reduction by Fe(II), by applying a molar ratio Fe(II)/Cr(VI) of around 9. In the present work, we investigated the reduction of Cr(VI) by Fe(II) in pipe flocculation reactors followed by filtration of insoluble products by microfiltration. The proposed technology involves re-circulation of a part of the sludge in the pipe reactors, in order to improve kinetics and efficiency of the process. The obtained results showed that with a Fe(II) dose of around 1 mg/L, Cr(VI) was reduced to below 10 μg/L, by even an initial concentration as high as 300 μg/L of Cr(VI), corresponding to a molar ratio Fe(II)/Cr(VI) of around 3, thus reducing the overall quantity of reductive reagents and of the produced sludge. This ratio was also confirmed by the XPS analysis, which also showed that Cr(VI) was reduced to Cr(III) and then precipitated either as Cr(OH)₃ or associated with the produced iron oxides.

Environmentally available hexavalent chromium in soils and sediments impacted by dispersed fly ash in Sarigkiol basin (Northern Greece)

Hexavalent chromium is one of the most toxic and carcinogenic species known and can be released into the environment from several sources. In Sarigkiol basin (N Greece) the presence of Cr(VI) in soil, sediments and groundwater may originate from both natural (ophiolitic rocks and their weathering products) and anthropogenic (dispersed fly ash produced from lignite power plants) sources. In this study, the distribution of contents and origin of environmentally available Cr(VI) in soils, sediments, regoliths and fly ash of Sarigkiol basin is presented. Detailed geochemical and mineralogical studies were performed on soil samples (up to 1 m) and regoliths, while leaching tests were also applied to fresh and old fly ash samples. Leachable chromium from soil and sediment samples generally increased with depth and the highest concentrations were observed near to the power plant of Agios Dimitrios. The speciation of chromium in leachates revealed that Cr(VI) concentrations accounted for more than 96% of total Cr. Leaching tests of regoliths established that the natural contribution of Cr(VI) is up to 14 μg kg^-1. Therefore, the measurement of higher concentrations (up to 80 μg kg^-1) of environmentally available Cr(VI) in soils and sediments can be attributed to the impact/presence of dispersed fly ash in the soils and sediments of the same area. This was also supported by the low correlation recorded between environmentally available chromium and Cr-bearing minerals (mainly serpentine and talc). The influenced zone is located in the eastern part of the basin near the local power plant and surrounds an open conveyor belt that transfers fly ash to an open temporary storage pit. This zone overlies an unconfined porous aquifer thus explaining the elevated concentrations of Cr(VI) in groundwater (up to 120 μg L^-1) previously reported in this area.

The use of Sn(II) oxy-hydroxides for the effective removal of Cr(VI) from water: Optimization of synthesis parameters

The development of a novel adsorbent based on Sn(II) oxy-hydroxide nanoparticles and the optimization of main synthesis parameters was examined for the efficient removal of hexavalent chromium at low residual concentration levels. The aqueous hydrolysis of Sn(II) salts in a continuous-flow process was evaluated as an effective method to synthesize an appropriate material able to operate both as an electron donor for Cr(VI) reduction, and provide a suitable crystal structure that favors strong complexation with the formed Cr(III) species. Experimental results revealed that the main hydrolysis parameters, such as pH value and tin origin/source, can be used to determine the chemical formula of the produced materials and thereby, eventually improve their uptake capacity for Cr(VI). Among the tested sorbent materials, the synthetic nanostructured hydroromarchite, Sn₆O₄(OH)₄, prepared by the hydrolysis of SnCl₂ in a highly acidic environment (pH2), was deemed the best sorbent material and it was further investigated for its Cr(VI) uptake performance under reliable conditions (column experiments) for drinking water treatment. Specifically, Rapid Small-Scale (laboratory) Column Tests indicated that aggregates of the Sn₆O₄(OH)₄ nanomaterial can achieve a maximum uptake capacity of around 19mg/g, keeping the levels of outflow Cr(VI) below 10μg/L during the treatment of natural-like water at pH7. The high efficiency is mainly attributed to the stabilization of Sn(II) content in nanoparticles, as well as the improved surface charge density, reaching 1.0mmol[OH^-]/g, whereas the obtained thermodynamic data indicate a combined reduction-sorption process. The latter aspect was further verified by XPS, showing that even in the highly-loaded sorbent materials with adsorbed chromium, its trivalent form is the predominant one. These specific characteristics suggest that the product is a more favorable candidate for wider applications in water treatment units, regarding Cr(VI) removal, compared to other examined sorbent materials.

Evaluation of activated carbon fiber supported nanoscale zero-valent iron for chromium (VI) removal from groundwater in a permeable reactive column

An activated carbon fiber supported nanoscale zero-valent iron (ACF-nZVI) composite for Cr(VI) removal from groundwater was synthesized according to the liquid phase reduction method. The techniques of N₂ adsorption/desorption, FESEM, EDX, XRD and XPS were used to characterize the ACF-nZVI composite and the interaction between the ACF-nZVI composite and Cr(VI) ions. Batch experiments were conducted to evaluate the effects of several factors, including the amount of nZVI on activated carbon fiber (ACF), pH value, initial Cr(VI) concentration, and co-existing ions on Cr(VI) removal. The results indicate that presence of ACF can inhibit the aggregation of nanoscale zero-valent iron (nZVI) particles and increase its reactivity, and the Cr(VI) removal efficiency increases with increasing amounts of nZVI on ACF and a decrease in the initial Cr(VI) concentration. In acidic conditions, almost 100% of Cr(VI) in solution can be removed after 60 min of reaction, and the removal efficiency decreases with increasing initial pH values. The Cr(VI) removal is also dependent on the co-existing ions. Reusability experiments on ACF-nZVI demonstrate that the ACF-nZVI composite can keep a high reactivity after five successive reduction cycles. The removal mechanisms are proposed as a two-step interaction including the physical adsorption of Cr(VI) on the surface or inner layers of the ACF-nZVI composite and the subsequent reduction of Cr(VI) to Cr(III) by nZVI.

Origin of hexavalent chromium in groundwater: The example of Sarigkiol Basin, Northern Greece

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.

Heavy Metal Contamination of Ground Water from an Unlined Landfill in Bulawayo, Zimbabwe

BACKGROUND

Developing countries such as Zimbabwe deal with challenges in solid waste management such as insufficient waste recycling, hazardous wastes that are not separated for safe disposal, and landfills that are not properly engineered to prevent groundwater pollution. For these reasons, landfills in developing countries pose serious environmental and public health hazards.

OBJECTIVES

The objective of this study was to assess heavy metal release and groundwater pollution from an unlined landfill in Bulawayo, Zimbabwe. The purpose of the study was to explore environmental and public health risks posed by improperly managed landfills in developing countries.

METHODS

We assessed levels of metal release from Richmond landfill in the city of Bulawayo, Zimbabwe by measuring lead, cadmium, chromium and copper levels in landfill soil, leachate and plants. We also monitored metal levels in groundwater from boreholes located in a residential area in the vicinity and downgradient of the landfill within a range of 800-2135 m. Soil was characterized at the landfill to assess potential sources of heavy metals.

RESULTS

All metals that were assessed were present in landfill soil and in leachate. There was high metal accumulation in weeds that were growing at the landfill, indicating mobility and bioavailability of the metals. Groundwater from nearby boreholes had high levels of lead (Pb) and cadmium (Cd) which were negatively correlated to distance from the landfill (p<0.01), indicating contamination from the landfill. The Pb and Cd levels exceeded World Health Organization standards for drinking water quality, posing health hazards to the communities who rely on the water. Solid waste at the landfill consisted of soft plastics (33%), hard plastics (18.6%), metals (3%), paper (8%), electronic waste (0.8%), organics (15.3%) and various other types (21.3%).

DISCUSSION

A combination of factors may be attributed to groundwater contamination. These include the co-disposal of metallic and electronic wastes at the landfill, lack of membrane lining at the landfill, inadequate leachate management and the porous geo-physical characteristics of the sub-surface at the landfill site.

CONCLUSIONS

Our study highlights adverse environmental and public health consequences of co-disposal of metals and electronic wastes at improperly engineered municipal landfills. This is a 'wake-up' call for policy makers in developing countries to improve solid waste management.

Characterization of regional cold-hydrothermal inflows enriched in arsenic and associated trace-elements in the southern part of the Duero Basin (Spain), by multivariate statistical analysis

Naturally occurring arsenic in groundwater exceeding the limit for potability has been reported along the southern edge of the Cenozoic Duero Basin (CDB) near its contact with the Spanish Central System (SCS). In this area, spatial variability of arsenic is high, peaking at 241μg/L. Forty-seven percent of samples collected contained arsenic above the maximum allowable concentration for drinking water (10μg/L). Correlations of As with other hydrochemical variables were investigated using multivariate statistical analysis (Hierarchical Cluster Analysis, HCA and Principal Component Analysis, PCA). It was found that As, V, Cr and pH are closely related and that there were also close correlations with temperature and Na⁺. The highest concentrations of arsenic and other associated Potentially Toxic Geogenic Trace Elements (PTGTE) are linked to alkaline NaHCO₃ waters (pH≈9), moderate oxic conditions and temperatures of around 18°C-19°C. The most plausible hypothesis to explain the high arsenic concentrations is the contribution of deeper regional flows with a significant hydrothermal component (cold-hydrothermal waters), flowing through faults in the basement rock. Water mixing and water-rock interactions occur both in the fissured aquifer media (igneous and metasedimentary bedrock) and in the sedimentary environment of the CDB, where agricultural pollution phenomena are also active. A combination of multivariate statistical tools and hydrochemical analysis enabled the distribution pattern of dissolved As and other PTGTE in groundwaters in the study area to be interpreted, and their most likely origin to be established. This methodology could be applied to other sedimentary areas with similar characteristics and problems.

Sorption mechanisms of chromate with coprecipitated ferrihydrite in aqueous solution

Hexavalent chromium (Cr(VI)) attracted researchers' interest for its toxicity, natural availability and removal difficulty. Nevertheless, its sorption mechanism is not clearly understood yet. In this work, we elucidated the sorption mechanism of the co-precipitation of chromates with ferrihydrite through quantitative analysis. The influence of Cr/Fe molar ratio on sorption was investigated by zeta potential measurements, X-ray diffraction (XRD) and X-ray adsorption fine-structure analysis (XAFS). Coprecipitation at pH 5 showed almost twice the sorption density of adsorption at pH 5. In co-precipitation, a shift of the XRD peak due to inner-sphere sorption of chromate was observed at Cr/Fe molar ratio 0.5. For adsorption, the same peak shift was confirmed at Cr/Fe molar ratio of 1. Zeta potential at pH 5 suggested that the sorption mechanism changed at Cr/Fe molar ratio 0.25 for coprecipitation and at Cr/Fe molar ratio of 1 for adsorption. Fitting of Cr and Fe K-edge extended X-ray adsorption fine-structure suggested that ferrihydrite immobilized Cr(VI) via outer sphere surface complexation for lower Cr/Fe ratios and via inner-sphere surface complexation for higher molar ratios. At higher molar ratios, bidentate binuclear CrFe bonds were well established, thus resulting in the expansion of the ferrihydrite structure.

Enhanced Cr(VI) removal from groundwater by Fe0-H2O system with bio-amended iron corrosion

A one-pot bio-iron system was established to investigate synergetic abiotic and biotic effects between iron and microorganisms on Cr(VI) removal. More diverse iron corrosion and reactive solids, such as green rusts, lepidocrocite and magnetite were found in the bio-iron system than in the Fe⁰-H₂O system, leading to 4.3 times higher Cr(VI) removal efficiency in the bio-iron system than in the Fe⁰-H₂O system. The cycling experiment also showed that the Cr(VI) removal capacity of Fe⁰ in the bio-iron system was 12.4 times higher than that in the Fe⁰-H₂O system. A 62days of life-span could be achieved in the bio-iron system, while the Fe⁰-H₂O system lost its efficacy after 30days. Enhanced effects of extra Fe²⁺ on Cr(VI) removal was observed, largely contributed to the adsorbed Fe²⁺ on iron surface, which could function as an extra reductant for Cr(VI) and promote the electron transfer on the solid phase. The results also showed that the reduction of Cr(VI) by microorganisms was insignificant, indicating the adsorption/co-precipitation of Cr by iron oxides on iron surface was responsible for the overall Cr(VI) removal. Our study demonstrated that the bio-amended iron corrosion could improve the performance of Fe⁰ for Cr(VI) removal from groundwater.

Occurrence and fractionation of Cr along the Loushan River affected by a chromium slag heap in East China

This study investigated the chromium (Cr) occurrence and distribution along the Loushan River adjacent to a chromium slag heap. The speciation and chemical fractionation of Cr in different environmental media were determined. The potential ecological risks for the surrounding environment were assessed on the basis of both potential ecological risk index (RI) and risk assessment code (RAC). The results show that the surface soil experienced severe Cr contamination with Cr(T) and Cr(VI) values of 3220 ± 6266 and 64 ± 94 mg/kg, respectively, even though the chromium slag heap had already been removed. The chromium slag enhanced the Cr concentration level in the surface soil, water, and sediment samples more than the background level to different extents, which indicates that Cr released from the chromium slag actually affects the surrounding environment. The spatial distribution variety of Cr implies that their transport might have been affected by soil leaking, atmospheric transport, and fluvial hydraulics. The chemical fractionation results demonstrate that the residual fraction was the dominant form, accounting for 54.6 and 66.1% Cr(T) in surface soil and sediment samples, respectively. The content of bioavailable exchangeable Cr fraction correlated with the organic matter (OM), cation exchange capacity (CEC), and pH value. The ecological risk assessment suggests no considerable ecological risk toward the biota despite a relatively high Cr(T) level. Nevertheless, attention should be paid to the potential long-term risks owing to the slow release of oxidizable and residual fractions.

Hexavalent Chromium Generation within Naturally Structured Soils and Sediments

Chromium(VI) produced from the oxidation of indigenous Cr(III) minerals is increasingly being recognized as a threat to groundwater quality. A critical determinant of Cr(VI) generation within soils and sediments is the necessary interaction of two low-solubility phases-Cr(III) silicates or (hydr)oxides and Mn(III/IV) oxides-that lead to its production. Here we investigate the potential for Cr(III) oxidation by Mn oxides within fixed solid matrices common to soils and sediments. Artificial aggregates were constructed from Cr(OH)₃- and Cr_0.25Fe_0.75(OH)₃-coated quartz grains and either mixed with synthetic birnessite or inoculated with the Mn(II)-oxidizing bacterium Leptothrix cholodnii. In aggregates simulating low organic carbon environments, we observe Cr(VI) concentrations within advecting solutes at levels more than twenty-times the California drinking water standard. Chromium(VI) production is highly dependent on Cr-mineral solubility; increasing Fe-substitution (x = 0 to x = 0.75) decreases the solubility of the solid and concomitantly decreases total Cr(VI) generation by 37%. In environments with high organic carbon, reducing conditions within aggregate cores (microbially) generate sufficient Fe(II) to suppress Cr(VI) efflux. Our results illustrate Cr(VI) generation from reaction with Mn oxides within structured media simulating soils and sediments and provide insight into how fluctuating hydrologic and redox conditions impact coupled processes controlling Cr and Mn cycling.