Mobility Of Cr Research Articles

Behavior of trace metals during composting of mixed sewage sludge and tropical green waste: a combined EDTA kinetic and BCR sequential extraction study in New Caledonia.

The aim of the study was to assess the impact of composting on the release dynamics and partitioning of geogenic nickel (Ni), chromium (Cr)and anthropogenic copper (Cu) and zinc (Zn) in a mixture of sewage sludge and green waste in New Caledonia. In contrast to Cu and Zn, total concentrations of Ni and Cr were very high, tenfold the French regulation, due to their sourcing from Ni and Cr enrichedultramafic soils. The novel method used to assess the behavior of trace metals during composting involved combining EDTA kinetic extraction and BCR sequential extraction. BCR extraction revealed marked mobility of Cu and Zn: more than 30% of the total concentration of these trace metals was found in the mobile fractions (F1 + F2) whereas Ni and Cr were mainly found in the residual fraction (F4). Composting increased the proportion of the stable fractions (F3 + F4) of all four trace metals studied. Interestingly, only EDTA kinetic extraction was able to identify the increase in Cr mobility during composting, Cr mobility being driven by the more labile pool (Q1). However, the total mobilizable pool (Q1 + Q2) of Cr remained very low, < 1% of total Cr content. Among the four trace metals studied, only Ni showed significant mobility, the (Q1 + Q2) pool represented almost half the value given in the regulatory guidelines. This suggests possible environmental and ecological risks associated with spreading our type of compost that require further investigation. Beyond New Caledonia, our results also raise the question of the risks in other Ni-rich soils worldwide.

Mobility, bioaccumulation in plants, and risk assessment of metals in soils

Heavy metal contamination of soils is one of the main factors contributing to soil quality decline and loss of biodiversity, which is also associated with plant contamination, as metals accumulate in the surface layer of soils and then enter the trophic chain. The aims of the study were to assess the mobility and bioavailability of metals in soils to plants, and to estimate the ecological and health risks associated with heavy metal content in soils. 320 topsoil and 206 plant samples were collected. Fractional analysis showed that for most of the samples, there was no or low risk associated with the mobility of Cr, Pb, Cu, Ni, Zn, and low and medium for Cd. High and very high metal release risk was only shown for Cd (28 % of samples), and Zn and Pb (2 % of samples). The bioaccumulation factor found moderate levels of accumulation for Cd, Zn, Cu, Ni. High accumulation of Cd and Zn was found in 38 % and 15 % of plant samples. Alivibrio fischeri proved to be a more sensitive indicator of soil ecotoxicity compared to Sinapis alba. In the 81 % of the soil samples found a low probability of adverse effects on ecological receptors associated with exposure to soilborne metals. In the case of human health risk, no harmful health effects were observed due to accidental ingestion of metal-containing soils in the study area. In assessing metal risks, the choice of indicators is crucial. Moreover, the properties of soils have a significant impact on the mobility of metals and their bioaccumulation by plants. This means that the more varied the choice of indicators, the more comprehensive, reliable and close to reality the risk assessment of heavy metals in soils will be.

The mobility of Cr(VI) on the ferrihydrite-Cr(VI) co-precipitates: The effect of co-existing tartaric acid and Cu(II)

Ferrihydrite is a crucial scavenger for hexavalent chromium (Cr(VI)) via coprecipitation processes. Foreign impurities widely exist in soils and sediments, which influence mineral recrystallization and transformation processes. Under the circumstances, the speciation and migration behavior of Cr(VI) will change accordingly. In this research, we explored the impact of tartaric acid and Cu(II) on ferrihydrite transformation, as well as the sequestration of Cr(VI) on iron minerals. Our results manifested that tartaric acid affected the valence of Cr on iron oxides. The adsorbed Cr(VI) was reduced to Cr(III) by tartaric acid on the surface of iron oxides and not released into the aqueous phase, which greatly reduced its toxicity. It was conducive to the conversion of ferrihydrite into hematite when tartaric acid concentration was 1.0 mM, because tartaric acid could facilitate the hematite nucleation to induce the generation of hematite from ferrihydrite. Moreover, Cu(II) promoted the formation of hematite and played a synergistic role in Cr(VI) fixation. Therefore, tartaric acid and Cu(II) are important factors decreasing the toxicity of Cr(VI) when considering Cr(VI) retention on iron minerals in complicated environments. This work has a vital implication for understanding the impact of the co-existent metals and organic substances on Cr(VI) retention in complex environments.

Concurrent reductive decontamination of chromium (VI) and uranium (VI) in groundwater by Fe(0)-based autotrophic bioprocess

The co-presence of chromium (VI) [Cr(VI)] and uranium (VI) [U(VI)] is widely found in groundwater, imposing severe risks on human health. Although zerovalent iron [Fe(0)] supports superb performance for bioreduction of Cr(VI) and U(VI) individually, the biogeochemical process involving their concurrent removal with Fe(0) as electron donor remains unexplored. In the 6-d batch study, 86.1% ± 0.7% of Cr(VI) was preferentially eliminated, while 78.4% ± 0.5% of U(VI) removal was achieved simultaneously. Efficient removal of Cr(VI) (100%) and U(VI) (51.2% ∼ 100%) was also obtained in a continuous 160-d column experiment. As a result, Cr(VI) and U(VI) were reduced to less mobile Cr(III) and insoluble U(IV), respectively. 16 S rRNA sequencing was performed to investigate the dynamics of microbial community. Delftia, Acinetobacter, Pseudomonas and Desulfomicrobium were the major contributors mediating the bioreduction process. The initial Cr(VI) and hydraulic retention time (HRT) incurred pronounced effects on community diversity, which in turn altered the reactor’s performance. The enrichment of Cr(VI) resistance (chrA), U(VI) reduction (dsrA) and Fe(II) oxidation (mtrA) genes were observed by reverse transcription qPCR. Cytochrome c, glutathione and NADH as well as VFAs and gas metabolites also involved in the bioprocess. This study demonstrated a promising approach for removing the combined contaminants of Cr(VI) and U(VI) in groundwater.

Release and mobility of hexavalent chromium in contaminated soil with chemical factory waste: Experiments, Cr isotope analysis and reactive transport modeling

Our study focused on the leaching processes in soil contaminated with hexavalent chromium (Cr(VI)), traced to industrial waste from a disused site and resulting in groundwater contamination. Mineral and geochemical characterization of the soil by means of XRD, SEM-EDS, total digestion and sequential extractions revealed that the main Cr content was from solid waste located in the upper meter of the soil profile. Flow-through and column experiments were carried out to investigate the processes responsible for Cr(VI) release. Cr(VI) mobility along the soil profile was also assessed. Moreover, Cr isotope signatures were used to evaluate a potential Cr(VI) reduction process, which preferably could immobilize toxic Cr(VI) complexes. One-dimensional (1D) numerical simulations reproduced the Cr(VI) release from the flow-through experiment containing the Cr(VI) rich-solid waste and also the Cr(VI) mobility along the column experiment. These results enabled us to interpret quantitatively the processes resulting in Cr(VI) contamination and mobility along the soil profile. Cr(VI) was released from dissolving Cr(VI)-rich phases (e.g., sodium chromate, Cr(VI)-hydrocalumite and Cr(VI)-ettringite) of the solid waste layer. Cr(VI) reduction and Cr(VI) adsorption did not take place along the column. Such accurate characterization of these processes is necessary for the mitigation of Cr(VI) mobility in contaminated soils.

The mobility and fate of Cr during aging of ferrihydrite and ferrihydrite organominerals

The fixation of environmentally hazardous Cr(VI) in soils is largely attributed to its retention by iron (Fe) (oxyhydr)oxides in the soil environment, which are prevailingly associated with organic carbon (OC). The effect of OC with different binding strengths on Cr(VI) adsorption and the mobility and fate of Cr(VI) during aging of Fe (oxyhydr)oxide however, are unknown. Here we investigate how OC binding strength influences Cr(VI) adsorption to ferrihydrite (Fh) organominerals, the aging of Fh organominerals and the subsequent retention or release of Cr(VI) as aging proceeds. We conduct Cr(VI) adsorption experiments onto Fh organominerals, apply surface complexation modelling to study the relative binding strengths between OC and Cr(VI), and then conduct aging experiments to track the redistribution of Cr(VI). We show based on the relative binding strength between OC and Cr(VI), that weakly-bound OC at different C loadings has almost no discernable influence on Cr(VI) adsorption, while strongly-bound OC significantly suppresses Cr(VI) adsorption via surface site blocking and/or electrostatic repulsion. The mobility and fate of Cr(VI) with Fh organominerals during the aging process is strongly influenced by the presence of OC. Weakly-bound carboxyl-poor OC not only near doubles the rate of Cr(VI) retained (i.e., Cr(VI) redistribution from a weakly-bound to a strongly-bound fraction) in neoformed Fe minerals at pH 5.0, but also increases the final proportion of Cr(VI)strongly-bound, which we attribute to the loose and porous structure of Fh organominerals and easier replacement of weakly-bound OC by Cr(VI). More strongly-bound carboxyl-rich OC and higher pH however, reduces the influence of OC, such that the rate of Cr(VI) retained is similar to or lower than that for the pure Fh system, which we attribute to the fact that the strongly-bound OC is less easily replaced by Cr(VI) and hence Cr(VI) is less able to access surface sites or surface pores and thus to be retained inside neoformed minerals. Overall we suggest that OC with higher carboxyl-richness will further suppress the rate of Cr(VI) retained and overall proportion of Cr(VI)strongly-bound, and thus that the mobility and fate of Cr(VI) in soils is strongly controlled by the carboxyl-richness of OC in Fe organominerals.

Geochemical Speciation and Principal Component Analysis of Toxic Metals in River Ogun Coastal Sediments, Southwestern Nigeria

This study investigated the influence and interactions of geochemical fractions of toxic metals in sediments of River Ogun in Nigeria. Fe, Mn, Zn, Pb, Cr, and Co were significantly present across the area, while Cu, Ni, and Cd were negligible. All the metals, except Zn, were found highly above their permissible limits in sediments. The bulk metal concentrations followed the trend: Fe &gt;&gt; Mn &gt; Zn &gt; Pb &gt; Co &gt;&gt; Cu ? Cr ?Ni &gt;&gt; Cd. At p = 0.05, the concentrations (ppm) of Ni, Zn, Co, Mn, Fe, Pb, Cr, Cd, and Cu in the F1 were 1.72–5.84, 13.2–15.0, 1.16–3.41, 56.1–77.2, 198–212, 9.32–14.3, 1.96–3.66, 0.55–1.61, and 0.56–1.98, respectively, while the values were 12.1–13.6, 113–270, 67.8–152, 206–275, 514–637, 131–179, 18.5–27.0, 1.31–1.75, and 6.11–8.11 for the F5, respectively. Overall, Cd and Fe showed the least and highest distributions in the various geochemical forms, respectively. Moreover, the abundance and mobility of Mn, Ni, Fe, Cr, Cd, and Zn correlated significantly in exchangeable and carbonate-bond fractions. Hence, environmental monitoring of the sediments and Pisces should be carried out while remediation of the location is recommended.

The sensitive mobility of Cr in ashes studied by SiO2–Al2O3–Fe2O3–CaO system

Since incineration is a feasible method for stabilization/solidification of chromium (Cr)-enriched wastes, the species, distribution, and mobility of Cr in ashes deserve more studies, especially as the function of ash composition. Synthetic Cr-bearing ashes (SAs) were synthesized by SiO2–Al2O3–Fe2O3–CaO systems to investigate Cr mobility under 1100 °C. A study from simplicity to complexity. The Cr in SiO2–CaO is of high mobility with CrO42− formation, in contrary to the moderate mobility in SiO2–Al2O3 and poor mobility in SiO2–Fe2O3. However, species and mobility of Cr are affected by the values of CaO/SiO2, Al2O3/SiO2, and Fe2O3/SiO2 ratios. When other oxides are added to the two-phase systems above, the fate of Cr is affected more considerably. With the SiO2 content of 70%, adding a slight amount of CaO (<10%) strengthens the stabilization/solidification of Cr, due to the favorable solid integration under Ca2+ fusion. However, the Cr mobility is higher with increasing the CaO content further. The minimum content of CaO is ∼20% to sufficiently decrease the proportion of residual Cr (QCr-S5) in SiO2–Al2O3–CaO, much lower than in SiO2–Fe2O3–CaO, which confirms the easier release of Cr immobilized in Si–Al matrixes. Considering the opposite effects of Fe2O3 and CaO on Cr mobility, increasing Fe2O3/CaO ratios >3/2 can limit the effect of CaO, leading to the efficient stabilization/solidification of Cr waste. Additionally, the QCr-S5 is 83% with the Fe2O3 content of 15% in SiO2–Al2O3–Fe2O3, higher than in SiO2–Al2O3 and SiO2–Fe2O3. This suggests the intense stabilization/solidification of Cr, probably due to the formation of amorphous Fe-rich glass. Based on these above, an equation is developed to describe the relationship between ash compositions and QCr-S5 (QCr-S5 = −39.37X1 + 24.96X2 + 5.34X3 – 2.51X4 + 54.29).

Chromium toxicity, speciation, and remediation strategies in soil-plant interface: A critical review.

In recent decades, environmental pollution with chromium (Cr) has gained significant attention. Although chromium (Cr) can exist in a variety of different oxidation states and is a polyvalent element, only trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)] are found frequently in the natural environment. In the current review, we summarize the biogeochemical procedures that regulate Cr(VI) mobilization, accumulation, bioavailability, toxicity in soils, and probable risks to ecosystem are also highlighted. Plants growing in Cr(VI)-contaminated soils show reduced growth and development with lower agricultural production and quality. Furthermore, Cr(VI) exposure causes oxidative stress due to the production of free radicals which modifies plant morpho-physiological and biochemical processes at tissue and cellular levels. However, plants may develop extensive cellular and physiological defensive mechanisms in response to Cr(VI) toxicity to ensure their survival. To cope with Cr(VI) toxicity, plants either avoid absorbing Cr(VI) from the soil or turn on the detoxifying mechanism, which involves producing antioxidants (both enzymatic and non-enzymatic) for scavenging of reactive oxygen species (ROS). Moreover, this review also highlights recent knowledge of remediation approaches i.e., bioremediation/phytoremediation, or remediation by using microbes exogenous use of organic amendments (biochar, manure, and compost), and nano-remediation supplements, which significantly remediate Cr(VI)-contaminated soil/water and lessen possible health and environmental challenges. Future research needs and knowledge gaps are also covered. The review's observations should aid in the development of creative and useful methods for limiting Cr(VI) bioavailability, toxicity and sustainably managing Cr(VI)-polluted soils/water, by clear understanding of mechanistic basis of Cr(VI) toxicity, signaling pathways, and tolerance mechanisms; hence reducing its hazards to the environment.

Thermodynamic assessment of the Co–Cr–Fe system and atomic mobility study of its fcc phase

In the present work, the Co–Cr–Fe system was thermodynamically assessed by using the CALPHAD approach coupled with first-principles calculations and experimental data from the literature. Subsequently, the fcc Co–Cr–Fe ternary diffusion couples annealed at 1273 and 1473K were scanned by using electron probe microanalysis (EPMA) to obtain the composition profiles, from which the interdiffusion coefficients were extracted by the Whittle-Green method. Based on these interdiffusion coefficients and present thermodynamic parameters, the atomic mobilities of Co, Cr, and Fe in the fcc Co–Cr–Fe alloys were assessed by means of DICTRA software. The calculated interdiffusion coefficients, composition profiles and diffusion paths of the fcc Co–Cr–Fe alloys were consistent with the experimental data, which verifies the accuracy of the assessed atomic mobilities.

Remediation and its biological responses to Cd(II)-Cr(VI)-Pb(II) multi-contaminated soil by supported nano zero-valent iron composites

Multi-metal contaminated soil has received extensive attention. The biochar and bentonite-supported nano zero-valent iron (nZVI) (BC-BE-nZVI) composite was synthesized in this study by the liquid-phase reduction method. Subsequently, the BC-BE-nZVI composite was applied to immobilize cadmium (Cd), chromium (Cr), and lead (Pb) in simulated contaminated soil. The simultaneous immobilization efficiencies of Cd, Cr(VI), Crtotal, and Pb were achieved at 70.95 %, 100 %, 86.21 %, and 100 %, respectively. In addition, mobility and bioavailabilities of Cd, Cr, and Pb were significantly decreased and the risk of iron toxicity was reduced. Stabilized metal species in the contaminated soil (e.g., Cd(OH)2, Cd-Fe-(OH)2, CrxFe1-xOOH, CrxFe1-x(OH)3, PbO, PbCrO4, and Pb(OH)2) were formed after the BC-BE-nZVI treatment. Thus, the immobilization mechanisms of Cd, Cr, and Pb, including adsorption, reduction, co-precipitation, and complexation co-exist with the metals. More importantly, bacterial richness, bacterial diversity, soil enzyme activities (dehydrogenase, urease, and fluorescein diacetate hydrolase), and microbial activity were enhanced by applying the BC-BE-nZVI composite, thus increasing the soil metabolic function. Over all, this work applied a promising procedure for remediating multi- metal contaminated soil by using the BC-BE-nZVI composite.

Recent advances in biochar amendments for immobilization of heavy metals in an agricultural ecosystem: A systematic review

Over the last several decades, extensive and inefficient use of contemporary technologies has resulted in substantial environmental pollution, predominantly caused by potentially hazardous elements (PTEs), like heavy metals that severely harm living species. To combat the presence of heavy metals (HMs) in the agrarian system, biochar becomes an attractive approach for stabilizing and limiting availability of HMs in soils due to its high surface area, porosity, pH, aromatic structure as well as several functional groups, which mostly rely on the feedstock and pyrolysis temperature. Additionally, agricultural waste-derived biochar is an effective management option to ensure carbon neutrality and circular economy while also addressing social and environmental concerns. Given these diverse parameters, the present systematic evaluation seeks to (i) ascertain the effectiveness of heavy metal immobilization by agro waste-derived biochar; (ii) examine the presence of biochar on soil physico-chemical, and thermal properties, along with microbial diversity; (iii) explore the underlying mechanisms responsible for the reduction in heavy metal concentration; and (iv) possibility of biochar implications to advance circular economy approach. The collection of more than 200 papers catalogues the immobilization efficiency of biochar in agricultural soil and its impacts on soil from multi-angle perspectives. The data gathered suggests that pristine biochar effectively reduced cationic heavy metals (Pb, Cd, Cu, Ni) and Cr mobilization and uptake by plants, whereas modified biochar effectively reduced As in soil and plant systems. However, the exact mechanism underlying is a complex biochar-soil interaction. In addition to successfully immobilizing heavy metals in the soil, the application of biochar improved soil fertility and increased agricultural productivity. However, the lack of knowledge on unfavorable impacts on the agricultural systems, along with discrepancies between the use of biochar and experimental conditions, impeded a thorough understanding on a deeper level.

Dissolved organic matter regulates aggregation and deposition of chromium (hydr)oxide colloids: molecular-scale investigation using ESI-FT-ICR-MS

Soil dissolved organic matter (SDOM) forms the corona and affects the colloidal mobility of Cr (hydr)oxides.

Alleviation of Cr(VI) Toxicity and Improve Phytostabilization Potential of Vigna radiata Using a Novel Cr(VI) Reducing Multi-Stress-Tolerant Plant Growth Promoting Rhizobacterial Strain Bacillus flexus M2

Chromium (Cr) is a toxic heavy metal discharged into the environment through various anthropogenic sources, which affects soil properties and fertility. Hence, an effective soil restoration strategy is the need of the hour. In this study, a potent Cr(VI)-reducing strain M2 was isolated from the rhizosphere of Zea mays L. grown in leather industrial effluent contaminated sites and identified as Bacillus flexus through 16S rDNA sequencing. Strain M2 exhibited strong tolerance to multi-stresses such as temperature (up to 45 °C), pH (up to 9.0), Sodium chloride (NaCl) (up to 7%) and PEG 6000 (up to 50%) and showed strong Cr(VI) reduction with the presence of multi-stresses. The interaction of Cr(VI) with strain M2 was elucidated through various instrumentation analyses. Fourier Transform Infra-red (FTIR) Spectroscopy analysis confirmed that Cr(VI) exposures induce significant changes in the cell-surface functional groups. Raman spectrum and Transmission Electron Microscopy–Energy Dispersive X-ray spectroscopy (TEM-EDX) analysis confirmed the bio-reduction of Cr(VI) to Cr(III) and their intracellular localization as Cr(III). Further, strain M2 produced a significant quantity of Indole acetic acid (IAA), ammonia, and exopolysaccharide (EPS) and showed positive results for various plant-growth-promoting activities with the presence of Cr(VI). In greenhouse experiments, the strain M2 inoculation progressively increased the plant growth parameters and stabilized the antioxidant system of Vigna radiata under Cr stress. However, Cr(VI) exposure decreased the growth parameters and increased the level of proline content, Hydrogen peroxide (H2O2) accumulation, and antioxidant enzymes expression in V. radiata. Interestingly, strain M2 inoculation significantly reduced the accumulation of Cr in root and shoot of V. radiata when compared to the uninoculated Cr(VI) treatment. Hence, this study confirms that rhizobacterial inoculation markedly reduced the negative impact of Cr toxicity and improved V. radiata growth even in harsh environments by stabilizing the mobility of Cr in the rhizosphere.

Long-distance mobilization of chromium(III) in soil associated with submicron Cr2O3

Trivalent chromium is generally assumed to form insoluble species, resulting in low mobility of Cr(III) in soils. Here, we report continuous distributions (0–19 m) of a high concentration of Cr(III) in the alkaline soils of a historically industrial site for producing Na2Cr2O7, CrO3, and Cr2O3, which challenges this abovementioned conventional wisdom. The thermodynamic equilibrium model showed the low possibility of Cr(III) originating from Cr(VI) reduction under the redox conditions of this study. The AF4-MALLS-ICP-MS and μ-XRF-XANES were used to identify the particle size distribution of Cr(III)-containing colloids and Cr(III) species in mobile colloids. In any soil layer, Cr(III) accounts for 71.1–94.3% of the total Cr in submicron soil colloids and is composed of submicron intrinsic Cr2O3 (55.2%−63.8%), Cr(OH)3 (0–33.0%), and Cr(III) adsorbed by ferrihydrite (0–19.0%) and clay montmorillonite (11.1%−21.1%) colloid. On the contrary, Cr(VI) was mainly distributed in bulk soil (> 2 µm) except for the topsoil, accounting for 62.6–90.0% of total Cr(VI). Organic matter content and soil texture are the most critical factors driving the mobilization of submicron colloids in soils by principal component analysis. Humic acid (HA) formed HA-corona on Cr2O3 surface and enhanced colloidal dispersion, thereby accelerating the long-distance mobilization of submicron Cr2O3 colloids in alkaline soil layers, whereas the heteroaggregation of clay colloid with Cr2O3 was only favorable for short-distance mobilization. Our findings help to re-recognize the potential migration risks of insoluble heavy metals in soils.

Chromium removal from contaminated soil using a novel FeOx/granular activated carbon-based three-dimensional electrokinetic system

A novel three-dimensional electrokinetic remediation (3D EKR) system, with FeOx/granular activated carbon (GAC) composite constituting a third electrode was investigated for the removal of Cr from a real contaminated soil. Such third electrode particles were distributed evenly throughout the contaminated soil and their additional presence enabled the generation of a higher and more stable current density and reduced acid-base polarization of the soil, when compared against a conventional 2D EKR (cathode-anode) system. This resulted in a higher Cr removal, which was primarily driven by enhanced oxidation of Cr(III) to more mobile Cr(VI) aqueous complexes, which electromigrated towards the anode and into the anolyte. Analysis of FeOx/GAC and soil samples before and after EKR using XRD, FTIR, SEM-EDS, and XPS confirmed that: (i) both oxidation of Cr(III) and reduction of Cr(VI) occurred simultaneously on particle electrode surfaces; and (ii) aqueous Cr transport governed by electromigration was enhanced by Electro-Fenton reactions and hydrolysis on FeOx/GAC. A further benefit of 3D EKR was the promotion of residual Cr(III), in proximity to the cathode, to barely soluble crystalline silicate phases, therefore further decreasing the overall toxicity of the soil. This study therefore provides a highly promising first insight into the use of 3D EKR for the removal of Cr from contaminated land. Future work will seek to examine this emerging new technology at large scale and for different contaminated land scenarios, therefore further developing it towards a potential commercial application.

Wildfires on Cr-rich Ferralsols can cause freshwater Cr(VI) pollution: A pilot study in New Caledonia

In the context of global warming, environmental forcing of wildfires on continental ecosystems is on a growing trend. Among other environmental impacts, wildfires can enhance trace metals concentration in freshwater systems. In this regard, chromium is of particular environmental and health concern because the temperature-induced oxidation of the less mobile and toxic Cr(III) form to the most mobile and toxic Cr(VI) one might represent a risk of freshwater pollution. The aim of the present study was to make a first assessment of this risk in New Caledonia by quantifying the influence of soil heating on Cr(VI) mobility in Cr-rich Ferralsols that cover about 1/3 of this French overseas territory. The results obtained indicated an enhanced Cr(VI) mobility upon soil heating from 200 °C, with a marked turnaround from 400 °C. Based on these results, a solid/solution mass balance approach allowed to confirm the risk of freshwater Cr(VI) pollution as a function of suspended particulate matter (SPM) concentration issued from burned Cr-rich Ferralsols. In addition, the results obtained revealed an apparent control of the Fe-oxides/silicates ratio on Cr(VI) mobility upon soil heating. Since this ratio decreases from the top to the bottom of ultramafic catchments, this latter result suggests that the risk of freshwater Cr(VI) pollution could depend on the actual location of the wildfires at the top or slope/base of these catchments. Since freshwaters represent the major water resource for the inhabitants of New Caledonia, these results point to the need for a larger evaluation of the risk of wildfires-induced freshwater Cr(VI) pollution at other catchments composed of Cr-bearing soils like Cambisols and Vertisols. At the larger scale, they also point to the need to foster studies aimed at better evaluating the risk of wildfires-induced freshwater Cr(VI) pollution at Cr-bearing pedological settings worldwide.

Effect of natural organic matter on Cr(VI) reduction by reduced nontronite

Fe(II)-bearing clay minerals are abundant in soils and are critical to control the fate of the highly toxic hexavalent chromium (Cr(VI)). Even though the reduction of Cr(VI) by Fe(II)-bearing clay minerals has been extensively studied, the effect of the commonly co-existing natural organic matter (NOM) on this process is poorly understood. This study aimed to understand the kinetics of Cr(VI) reduction by a typical Fe(II)-bearing clay mineral (i.e., reduced nontronite (rNAu-2)) in the presence of Suwannee River NOM and to unveil the possible mechanisms. Cr(VI) reduction experiments were conducted with different NOM concentrations in a range of 0 to 100 mg C L−1 under circumneutral pH. The time-course aqueous-phase measurements and solid-phase characterizations were performed with a combination of wet chemical methods, transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses. The results demonstrated that NOM inhibited Cr(VI) reduction overall, mainly due to the decrease of Cr(VI) sorption on rNAu-2 by mineral-bound NOM. Besides the predominant solid Cr(III) species, soluble and mobile Cr(III) complexes were formed during reduction. Interestingly, the reduction rate (k) decreased with the increase of NOM concentration from 0 to 20 mg C L−1, but kept increasing when the concentration of NOM increased from 30 to 100 mg C L−1. The contrary pattern is due to the increasing promotion effects that offset the inhibitory effect caused by competitive sorption between Cr(VI) and NOM. The promotion effects may include weakening the mineral passivation by forming stable aqueous NOM-Cr(III) complexes, accelerating the electron transfer rate by dissolving rNAu-2, and donating electrons through NOM-Fe(II/III) complexes and/or NOM-Cr(V) complexes. This study extends our knowledge of Cr(VI) reduction by clay minerals in organic matter-rich soils and sediments and has important implication for understanding the transport and long-term behavior of Cr in the environment.

SPECIATION, MOBILITY AND POTENTIAL TOXICITY OF METALS (CR, CO, CU AND MN) IN SOIL SAMPLES FROM DUMPSITES IN KANO METROPOLIS

Soil pollution by heavy metals is a global environmental problem. The need to keep the environment clean is increasingly becoming a global concern. Heavy metals are one group of pollutants that are widespread in our environment. They tend to persist indefinitely once they enter the soil. Their mobility and toxicity depend on their forms and oxidation states in the soil. This research presents the speciation, mobility and potential toxicity of Cr, Co, Cu and Mn in soils from some dumpsites in Kano, using a six-step sequential extraction method and AAS analysis. The results revealed the pH of the soils to range from 6.20-7.80. Percentage composition of clay and organic matter ranged from 6.00-16.58 % and 0.48- 4.30 % respectively. The results showed that the metals are bioavailable at different forms evident from their distribution pattern in the different fractions and therefore the soils should not be used for cultivation to avoid the metals getting into the food chain. The mobility factor values ranging between 14.38 % and 61.48 % for dry season; 15.63 % and 74.74 % for rainy season showed the metals to be very mobile especially Co, Cu, and Mn. This could lead to potential pollution of the agricultural produce and ground water in this area and hence have toxic effects on human lives. This result can serve as a guide to researchers on the appropriate method to reclaim the soils in this area based on the pattern of distribution of the metals in different geochemical phases.

Assessment of natural and anthropogenic contamination sources in a Mediterranean aquifer by combining hydrochemical and stable isotope techniques

The Atalanti basin is an intensively cultivated area in central Greece, facing groundwater quality deterioration threats due to natural and anthropogenic-related contamination sources. A combination of statistical and hydrogeochemical techniques, and stable isotope compositions (δ2H-H2O and δ18Ο-Η2Ο, δ15Ν-ΝΟ3− and δ18Ο-ΝΟ3−, δ34S-SO42− and δ18O-SO42−) were applied to elucidate the origin of salinity and nitrate contamination, and shed light on the potential associations between geogenic Cr(VI) and NO3− sources and transformations. Nitrate and Cr(VI) concentrations reached up to 337 mg L−1 and 76.1 μg L−1, respectively, exceeding WHO threshold values in places. The cluster of samples with the high salinity was mostly influenced by irrigation return flow and marine aerosols, and less by seawater intrusion, as evidenced by the ionic ratios (e.g., Na+/Cl−) and the stable isotopes of oxygen and hydrogen in water, and sulphur and oxygen in sulphates. The δ15Ν-ΝΟ3− and δ18O-NO3− values ranged from +2.0 ‰ to +14.5 ‰ and + 0.3 ‰ to +11.0 ‰, respectively. We found that the dominant sources of NO3− in groundwater were fertilizers in the central part of the area and sewage waste in the northern part around the residential area of Livanates. The occurrence of denitrification was evident in the northern part of the basin, where the DO levels were lowest (≤ 2.2 mg L−1), whereas nitrification of NH4+-fertilizers prevailed in the central part. Elevated Cr(VI) values (≥ 20 μg/l) were associated with the lowest deviation of the measured from the theoretical nitrification δ18Ο-NO3− values, whereas the lowest Cr(VI) values were observed in the denitrified water samples. Our isotope findings revealed the strong influence of redox conditions on the biogeochemical transformations of N species and the mobilization of Cr(VI) that will help improve the understanding of the fate of these contaminants from the unsaturated zone to the groundwater in areas of agricultural and urban land use.