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

Ecological impacts and potential hazards of nickel on soil microbes, plants, and human health

Nickel (Ni) contamination poses a serious environmental concern, particularly in developing countries: where, anthropogenic activities significantly contributes to Ni accumulations in soils and waters. The contamination of agricultural soils with Ni, increases risks of its entry to terrestrial ecosystems and food production systems posing a threat to both food security and safety. We examined the existing published articles regarding the origin, source, accumulation, and transport of Ni in soil environments. Particularly, we reviewed the bioavailability and toxic effects of Ni to soil invertebrates and microbes, as well as its impact on soil-plant interactions including seed germination, nutrient uptake, photosynthesis, oxidative stress, antioxidant enzyme activity, and biomass production. Moreover, it underscores the potential health hazards associated with consuming crops cultivated in Ni-contaminated soils and elucidates the pathways through which Ni enters the food chain. The published literature suggests that chronic Ni exposure may have long-term implications for the food supply chain and the health of the public. Therefore, an aggressive effort is required for interdisciplinary collaboration for assessing and mitigating the ecological and health risks associated with Ni contamination. It also argues that these measures are necessary in light of the increasing level of Ni pollution in soil ecosystems and the potential impacts on public health and the environment.

Nickel stocks and fluxes in a tropical agromining 'metal crop' farming system in Sabah (Malaysia)

Nickel hyperaccumulator plants play a major role in nickel recycling in ultramafic ecosystems, and under agromining the nickel dynamics in the farming system will be affected by removal of nickel-rich biomass. We investigated the biogeochemical cycling of nickel as well as key nutrients in an agromining operation that uses the metal crop Phyllanthus rufuschaneyi in the first tropical metal farm located in Borneo (Sabah, Malaysia). For two years, this study monitored nine 25-m2 plots and collected information on weather, biomass exportation, water, and litter fluxes to the soil. Without harvesting, nickel inputs and outputs had only minor contributions (<1 %) to the total nickel budget in this system. The nickel cycle was mainly driven by internal fluxes, particularly plant uptake, litterfall and throughfall. After two years of cropping, the nickel litter flux corresponded to 50 % of the total nickel stock in the aerial biomass (3.1 g m-2 year-1). Nickel was slowly released from the litter; after 15 months of degradation, 60 % of the initial biomass and the initial nickel quantities were still present in the organic layer. Calcium, phosphorus and potassium budgets in the system were negative without fertilisation. Unlike what is observed for nickel, sustained agromining would thus lead to a strong depletion of calcium stocks if mineral weathering cannot replenish it.

Deciphering endemic rhizosphere microbiome community's structure towards the host-derived heavy metals tolerance and plant growth promotion functions in serpentine geo-ecosystem

Environmental microbes in rhizosphere soil and surrounding plants have the potential to alter ecosystem functions. We investigated the microbial communities inhabiting the rhizosphere soils of both serpentine and non-serpentine rhizosphere zones to evaluate their heavy metal tolerance and ability to promote plant growth, utilizing 16S rRNA metabarcoding. The Biolog-EcoPlate technique was employed to determine how abiotic stress factors affect carbon utilization capacity by rhizospheric microbial communities in the serpentine geo-ecosystem. The phyla Proteobacteria, Acidobacteria, Bacteroidetes, and Nitrospirae colonized in the roots of Miscanthus sp., Biden sp., and Oryza sp. showed noticeable differences in different rhizosphere zones. The PICRUSt2-based analysis identified chromium/iron resistance genes (ceuE, chrA) and arsenic resistance genes (arsR, acr3, arsC) abundant in all the studied rhizosphere soils. Notably, nickel resistance genes (nikA, nikD, nikE, and nikR) from Arthrobacter, Microbacterium, and Streptomyces strongly correlate with functions related to solubilization of nickel and an increase in siderophore and IAA production. The abundance of Arthrobacter, Clostridium, Geobacter, Dechloromonas, Pseudomonas, and Flavobacterium was positively correlated with chromium and nickel but negatively correlated with the calcium/magnesium ratio. Our results contribute to a better understanding of the functions of plant-tolerant PGPR interaction in the heavy metal-contaminated rhizosphere and eco-physiological responses from long-term biological weathering.

Enhanced silicate remediation in cadmium-contaminated alkaline soil: Amorphous structure improves adsorption performance

Silicates have been used as soil heavy metal passivators, but low remediation efficiency limited their development. In order to solve this problem, in this paper, an economical and environmentally friendly amorphous iron silicate was prepared by a simple co-precipitation method. It could be proved from the passivation experiments that the remediation efficiency of amorphous iron silicate (AIS) on Cd-contaminated soil was better than that of natural silicates (montmorillonite and diatomite), which reflected the superiority of amorphous materials. Plant experiments showed that AIS could effectively inhibit the absorption and accumulation of Cd²⁺ in the edible parts of garlic. In addition, it may effectively reduce the potential ecological risk assessment of soil, and its immobilization mechanism of Cd²⁺ includes electrostatic adsorption, co-precipitation, ion exchange, and complexation of surface functional groups. This study demonstrates the advantages of amorphous iron silicate as a new functional material in the remediation of Cd-contaminated soil and provides a reference for the development and application of environment-friendly passivators.

Unravelling the ultramafic rock-driven serpentine soil formation leading to the geo-accumulation of heavy metals: An impact on the resident microbiome, biogeochemical cycling and acclimatized eco-physiological profiles

In the present study, we have underpinned the serpentine rock, serpentinized ultramafic soil and rhizosphere's microbial communities, signifying their heavy metals-exposed taxa signatures and functional repertoires in comparison to non-serpentine soils. The results revealed that the serpentine rock embedded soil highlighted the geo-accumulation of higher amount of Cr and Ni impacting soil microbial diversity negatively by metal stress-driven selection. Biolog Ecoplate CLPP defined a restricted spectrum of C-utilization in the higher heavy metal-containing serpentine samples compared to non-serpentine. The linear discriminant analysis (LDA) score identified a higher abundance of Desulfobacterota, Opitutales, and Bacteroidales in low Cr and Ni-stressed non-serpentine-exposed samples. Whereas the abundance of Propionibacteriales and Actinobacteriota were significantly enriched in the serpentine niche. Further, the C, N, S, Fe, and methane biogeochemical cycles linked functional members were identified, and showing higher functional diversity in low Cr and Ni concentration-containing rhizosphere JS-soils. The Pearson correlation coefficient (r) value confirmed the abundance of functional members linked to specific biogeochemical cycle, positively correlated with relevant pathway enrichment. Ultimately, this study highlighted the heavy metal stress within a serpentine setting that could limit the resident microbial community's metabolic diversity and further select the bacteria that could thrive in the serpentine-associated heavy metal-stressed soils. These acclimatized microbes could pave the way for the future applications in the soil conservation and management.

Influence of local geological data and geographical parameters to assess regional health impact in LCA. Tomsk oblast', Russian Federation application case

The research paper is aimed to modify the human health impact assessment of Cr in soils. The current article presents the input of several critical parameters for the human health Impact Score (IS_hum) assessment in soils. The modification of the IS_hum is derived using geological data - results of neutron activation analysis of soils are used in the IS_hum calculation; research area is divided using the watersheds and population size and density. Watersheds reflect the local environmental conditions of the territory unlike the administrative units (geographical areas of the studied region) due to their geological independence. The calculations of the characterization factor value underestimate the influence of the population size and density on the final result. Default characterization factor values cannot be considered during the assessment of the potential human health impact for the big sparsely inhabited areas. In case of very low population density, the result will be overrated and underestimated in the opposite case. The current approach demonstrates that the geographical separation in the USEtox model should be specified. The same approach can be utilized for other geo zones due to the accessibility of this information (area size, population size, and density, geological, and landscape features).

Metabolic and genetic derangement: a review of mechanisms involved in arsenic and lead toxicity and genotoxicity

Urbanisation and industrialisation are on the rise all over the world. Environmental contaminants such as potentially toxic elements (PTEs) are directly linked with both phenomena. Two PTEs that raise greatest concern are arsenic (As) and lead (Pb) as soil and drinking water contaminants, whether they are naturally occurring or the consequence of human activities. Both elements are potential carcinogens. This paper reviews the mechanisms by which As and Pb impair metabolic processes and cause genetic damage in humans. Despite efforts to ban or limit their use, due to high persistence both continue to pose a risk to human health, which justifies the need for further toxicological research.

Release and mobilization of Ni, Co, and Cr under dynamic redox changes in a geogenic contaminated soil: Assessing the potential risk in serpentine paddy environments

The release dynamics and mobilization of geogenic Ni, Co, and Cr in serpentine paddy soils under fluctuating redox conditions have not yet been well studied. Here we investigated the release dynamics of Cr, Co, and Ni and controlling factors (e.g., Fe, Mn, Mg, Cl^-, PO₄^3-, SO₄^2-, and dissolved organic carbon (DOC)) in a geogenic-contaminated serpentine soil under wide range of redox potential (E_H) changes. The effects of re-oxidation process have been also investigated. The soil was incubated for 28 days and E_H was controlled from oxidation (+200 mV) to reduction (-200 mV) and re-oxidation (+240 mV) using a microcosm setup in duplicates. The slurry pH increased, along with decreasing E_H. The average concentration of dissolved Co (17.1-23.6 μg L^-1) decreased under low E_H/high pH and vice versa. The average concentration of dissolved Cr decreased sharply from 624 μg L^-1 to 54.4 μg L^-1 with decreasing E_H from +200 mV to 0 mV and the associated increase of pH from 7.8 to 8.5; then, it was constant around 24.5 μg L^-1. Concentration of dissolved Ni was lower (73.5-84.6 μg L^-1) under high E_H at the first week of incubation; then, increased to 108.5 μg L^-1 under low E_H (-200 mV); thereafter, increased more at the end up to 124.5 μg L^-1 at high E_H (+240 mV), because of the pH decrease. A factor analysis identified that Cr and Co formed one group with Mn and Mg, while Ni was clustered together with Cl^-, DOC, and SO₄^2-. This indicates that the redox-induced release dynamic of Cr and Co was mainly governed by MnMg compounds, while the release of Ni was mainly affected by the aliphatic compounds of DOC and the redox chemistry of chlorides and sulfur in this soil. The re-oxidation increased the mobilization of Ni and Co and did not affect the release of Cr. These findings suggest that the redox-induced mobilization of geogenic Co, Ni, and Cr from soil to water in serpentine rice soils should be considered due to the high solubility and thus the associated bioavailability and potential environmental and human health risks, when such metal-enriched soils will be used for agricultural flood-dry cycle systems.

Distance impacts toxic metals pollution in mining affected river sediments

The study of metals mobility derived from mining activities in an ultramafic lithology is limited. This study investigates the effects of distance on potentially toxic metals such as Co, Cu, Fe, Mn, Ni, Pb, and Zn pollution, and the geochemical processes of fluvial system downstream of an ex-copper mine (Mamut River). The toxicity level of the river was evaluated using various sediment quality guidelines, ecotoxicological risks (ecological risk and risk index) and pollution indices. The geochemical behavior and stability of these toxic metals in the solid-phase samples were also examined. The results show that elevated concentrations of Ni, Cu, and Fe in the sediments can be linked to the adsorption and precipitation of metals from the aqueous-phase samples. We found that the metal scavenging rate as a function of distance is more evident in tropical environments than it was previously thought (10 km downstream). Such an inference could be explained by the greater amount of rainfall (pH 5.5-6.5) received in the tropics and higher weathering products that could react and form stable complexes. Geochemical analysis of the river sediment indicates that Ni, Cu, and Fe in the river sediment have increased 44-, 81-, and 90-fold compared to the background values, respectively. A significant decrease in the concentration of the potentially toxic metals was found at 5.5 km downstream. The scavenging rate of Fe is the highest (1485.82 μg g^-1 km^-1) followed by Cu (141.48 μg g^-1 km^-1), Ni (10.23 μg g^-1 km^-1), Pb (8.12 μg g^-1 km^-1) and Zn (5.01 μg g^-1 km^-1) in the tropical river system. In contrast, the concentration of Co and Mn in the river sediments doubled as the river flows approximately 5 km downstream due to the higher mineral solubility and weaker metal partition coefficient. This study also discusses the possibility of asbestos (mainly as chrysotile in the X-ray diffraction) as a potential hidden risk present within the ultramafic setting. This case study can be extrapolated to explain the dispersion of inorganic pollutants in an ultramafic environment in a global context.

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.

Source apportionment and health risk assessment of potentially toxic elements in soil from mining areas in northwestern China

Soil contaminated with toxic elements from mining activities is a public health concern. In order to obtain a comprehensive understanding of the status and potential risks of inorganic toxic elements in soil resulting from mining activities, Cu, Pb, Cr, Zn, Ni, As, and Cd were selected to evaluate a total of 42 soil samples collected from Gannan mining areas in northwestern China. The concentrations of As and Cd were much higher than their respective background values, while the concentrations of the other elements fluctuated around their background values. Results of combined multivariate statistical analyses and the distribution patterns of the individual pollutants imply that the toxic elements were originated from different sources even for one element in different sampling locations. The pollution index values indicated that As and Cd have a moderate to high pollution levels. The geo-accumulation indexes (I_geo) indicated that Cu, Pb, Cr, Zn, and Ni are likely of geologic origin, while As and Cd have been significantly affected by anthropogenic activities. Potential ecological risk indexes further showed that soils from mining areas within the study area pose a high potential ecological risk, and As and Cd were major risk contributors. Based on the calculated Hazard Index, the ingestion of soil particles appeared to be the main exposure route resulting in a higher risk, followed by dermal contact. The potential health risks of children and adults for As were greater than the safe level. The carcinogenic risk associated with As for local residents was also higher than the accepted levels, indicating a serious health risk to local residents. These results suggest that proper management strategies and various remediation practices should be implemented in the Gannan mining area in northwestern China.

Biotic and Abiotic Biostimulation for the Reduction of Hexavalent Chromium in Contaminated Aquifers

Hexavalent chromium is a carcinogenic heavy metal that needs to be removed effectively from polluted aquifers in order to protect public health and the environment. This work aims to evaluate the reduction of Cr(VI) to Cr(III) in a contaminated aquifer through the stimulation of indigenous microbial communities with the addition of reductive agents. Soil-column experiments were conducted in the absence of oxygen and at hexavalent chromium (Cr(VI)) groundwater concentrations in the 1000–2000 μg/L range. Two carbon sources (molasses and EVO) and one iron electron donor (FeSO4·7H2O) were used as ways to stimulate the metabolism and proliferation of Cr(VI) reducing bacteria in-situ. The obtained results indicate that microbial anaerobic respiration and electron transfer can be fundamental to alleviate polluted groundwater from hazardous Cr(VI). The addition of organic electron donors increased significantly Cr(VI) reduction rates in comparison to natural soil attenuation rates. Furthermore, a combination of organic carbon and iron electron donors led to a longer life span of the remediation process and thus increased total Cr(VI) removal. This is the first study to investigate biotic and abiotic Cr(VI) removal by conducting experiments with natural soil and by applying biostimulation to modify the natural existing microbial communities.

Risk assessment of heavy metals in soils and edible parts of vegetables grown on sites contaminated by an abandoned steel plant in Havana

Food production in areas contaminated by industrial wastes poses a serious risk to farmers and consumers. Here, we evaluate Cd, Cr, Ni, and Pb concentrations in the soils and the edible parts of lettuce, chives, tomatoes, pepper, and cassava plants grown by small farmers in areas contaminated by slag from an abandoned steel plant in Havana, Cuba. The total, environmentally available, and bioavailable concentrations of metals in the soils and the metals bioconcentration factor in the plants were determined. The risks to human health from food and soil ingestion were estimated. The total and environmentally available concentrations of Cd, Cr, and Pb were above values considered safe by international standards, with likely adverse effect on human health. Cadmium was the most bioavailable metal, reflected in the highest accumulation in the crops' edible parts. Even with negligible DTPA-available Cr concentrations in soils, the Cr concentrations in edible parts of the crops exceeded regulatory levels, suggesting that rhizosphere mechanisms may increase Cr availability. The consumption of vegetables represented 70% of the daily intake dose for Cr, Cd, and Ni, while accidental ingestion of contaminated soil is the predominant human exposure route for Pb. Our results demonstrated the health risks associated with cultivating and consuming vegetables grown on metal contaminated soils in Havana and can assist public policies capable of guaranteeing the sustainability of urban agriculture and food security.

Coupled Mn(II) and Cr(III) Oxidation Mediated by Ascomycete Fungi

Manganese (Mn) oxides are considered as the primary oxidant of trivalent chromium [Cr(III)] in the environment. Microbial activities are responsible for the majority of Mn oxide formation in nature, thus likely influencing Cr(III) oxidation. Previous studies have been limited to Cr(III) oxidation by bacterial Mn oxides. Herein, we report coupled Mn(II) and Cr(III) oxidation in the presence of three Mn(II)-oxidizing Ascomycete fungi. In contrast to the previously reported inhibitory effect of Cr(III) on bacterial Mn(II) oxidation, varying effects of Cr(III) on fungal Mn(II) oxidation were observed, which may be linked to their Mn(II)-oxidation mechanisms. Under the concentrations of Mn(II) and Cr(III) applied in this study, Cr(III) promoted Mn(II) oxidation if it was mediated by hyphae-associated processes, but inhibited Mn(II) oxidation if it was achieved via extracellular enzymes/metabolites. The Cr(III) oxidation rate and extent were affected by Cr(III) speciation, Cr(VI) removal capacity (i.e., adsorption/reduction) of fungi, and organic content. The morphology and spatial relationship of Mn oxides with fungi varied, depending on fungal species and Cr(III) presence. Our findings highlight the importance of Mn(II)-oxidizing fungi in biogeochemical cycles of Mn and Cr and have significant implications for the origin of geogenic Cr(VI) and stability of reduced chromium in contaminated environments.

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.

No escape: The influence of substrate sodium on plant growth and tissue sodium responses

As an essential micronutrient for many organisms, sodium plays an important role in ecological and evolutionary dynamics. Although plants mediate trophic fluxes of sodium, from substrates to higher trophic levels, relatively little comparative research has been published about plant growth and sodium accumulation in response to variation in substrate sodium. Accordingly, we carried out a systematic review of plants' responses to variation in substrate sodium concentrations.We compared biomass and tissue-sodium accumulation among 107 cultivars or populations (67 species in 20 plant families), broadly expanding beyond the agricultural and model taxa for which several generalizations previously had been made. We hypothesized a priori response models for each population's growth and sodium accumulation as a function of increasing substrate NaCl and used Bayesian Information Criterion to choose the best model. Additionally, using a phylogenetic signal analysis, we tested for phylogenetic patterning of responses across taxa.The influence of substrate sodium on growth differed across taxa, with most populations experiencing detrimental effects at high concentrations. Irrespective of growth responses, tissue sodium concentrations for most taxa increased as sodium concentration in the substrate increased. We found no strong associations between the type of growth response and the type of sodium accumulation response across taxa. Although experiments often fail to test plants across a sufficiently broad range of substrate salinities, non-crop species tended toward higher sodium tolerance than domesticated species. Moreover, some phylogenetic conservatism was apparent, in that evolutionary history helped predict the distribution of total-plant growth responses across the phylogeny, but not sodium accumulation responses.Our study reveals that saltier plants in saltier soils proves to be a broadly general pattern for sodium across plant taxa. Regardless of growth responses, sodium accumulation mostly followed an increasing trend as substrate sodium levels increased.

Origin of Critical Metals in Fe–Ni Laterites from the Balkan Peninsula: Opportunities and Environmental Risk

As the global energy sector is expected to experience a gradual shift towards renewable energy sources, access to special metals in known resources is of growing concern within the EU and at a worldwide scale. This is a review on the Fe–Ni ± Co-laterite deposits in the Balkan Peninsula, which are characterized by multistage weathering/redeposition and intense tectonic activities. The ICP-MS analyses of those laterites indicated that they are major natural sources of Ni and Co, with ore grading from 0.21 to 3.5 wt% Ni and 0.03 to 0.31 wt% Co, as well as a significant Sc content (average 55 mg/kg). The SEM-EDS analyses revealed that fine Fe-, Ni-, Co-, and Mn-(hydr)oxides are dominant host minerals and that the enrichment in these elements is probably controlled by the post-formation evolution of initial ore redeposition. The paucity of rare earth element (REE) within the typical Fe–Ni laterite ore and the preferential occurrence of Co (up to 0.31 wt%), REE content (up to 6000 mg/kg ΣREE), and REE-minerals along with Ni, Co, and Mn (asbolane and silicates) towards the lowermost part of the Lokris (C. Greece) laterite ore suggest that their deposition is controlled by epigenetic processes. The platinum-group element (PGE) content in those Fe–Ni laterites, reaching up to 88 μg/kg Pt and 26 μg/kg Pd (up to 186 μg/kg Pd in one sample), which is higher than those in the majority of chromite deposits associated with ophiolites, may indicate important weathering and PGE supergene accumulation. Therefore, the mineralogical and geochemical features of Fe–Ni laterites from the Balkan Peninsula provide evidence for potential sources of certain critical metals and insights to suitable processing and metallurgical methods. In addition, the contamination of soil by heavy metals and irrigation groundwater by toxic Cr(VI), coupled with relatively high Cr(VI) concentrations in water leachates for laterite samples, altered ultramafic rocks and soils neighboring the mining areas and point to a potential human health risk and call for integrated water–soil–plant investigations in the basins surrounding laterite mines.

Influences of species and watersheds inputs on trace metal accumulation in mangrove roots

Mangrove forest is a key ecosystem between land and sea, and provides many services such as trapping sediments and contaminants. These contaminants include trace metals (TM) that can accumulate in mangroves soil and biota. This paper innovates by the comparative study of the effects of the watershed inputs on TM distribution in mangrove soil, on roots bioconcentration factors of two species (Avicennia marina and Rhizophora stylosa), and on Fe plaque formation and immobilization of these TM. Two mangrove forests in New Caledonia were chosen as study sites. One mangrove is located downstream ultramafic rocks and a Ni mine (ultrabasic site), whereas the second mangrove ends a volcano-sedimentary watershed (non-ultrabasic site). TM concentrations (Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Zn) were measured in soil, porewaters, and roots of both species via ICP-OES or Hg analyzer. Analyzed TM were significantly more concentrated in soils at the ultrabasic site with Fe, Cr, and Ni the most abundant. Iron, Mn, and Ni were the most concentrated in the roots with mean values of 9,651, 192, and 133 mg kg^-1 respectively. However, the bioconcentration factors (BCF) of Fe (0.16) and Ni (0.11) were low due to a lack of ions in the dissolved phase and potential uptake regulation. The uptake of TM by mangrove trees was influenced by concentrations in soil, but more importantly by their potential bioavailability and the physiological characteristics of each species. TM concentrations and BCF were lower for R. stylosa probably due to less permeable root system. A. marina limits TM absorption through Fe plaque formation on its pneumatophores with a capacity to retain TM up to 94% for Mn. Mean Fe plaque formation is potentially correlated to Fe concentration in soil. Eventually, framboids of pyrite were observed within root tissues in the epidermis of A. marina's pneumatophores.

Profiling metal contamination from ultramafic sediments to biota along the Albanian shoreline of Lake Ohrid (Albania/Macedonia)

Ultramafic sediments exhibit high levels of geogenically-derived and potentially toxic metals, with Ni, Cr and Co often exceeding benchmark values. As yet, a holistic understanding of the bioavailability, mobility, potential ecotoxicity and trophic transfer of trace elements in both benthic and pelagic food chains in aquatic ultramafic environments (UME) is lacking. We investigated potential environmental health issues due to metal contamination by jointly implementing chemical, ecological and toxicological tools, along the Albanian shoreline of Lake Ohrid. It is an aquatic system of worldwide importance, representative of temperate UME with a legacy of Ni and Cr contamination from mining activities. Levels of Ni, Cr, Cd, Cu, Co, Fe, Mn and Zn were determined in waters, sediments and native biota. The potential environmental mobility of sediment-bound elements was further assessed using CaCl₂, EDTA and acetic acid extractions. Whole-sediment ecotoxicity tests were also carried out using ostracods and chironomids, according to standardized procedures. Despite Ni and Cr concentrations above the sediment quality guidelines for probable effect levels, we did not observe ecotoxic effects in laboratory tests. However, these elements were bioavailable to native organisms under field conditions, especially to benthic primary producers in direct contact with sediments (up to 139 mg Cr kg^-1 and 785 mg Ni kg^-1). Although biomagnification was not observed, further investigations of metal translocation, metabolism and elemental trophic transfer along benthic food webs appears to be a general research priority in the management of temperate UME. The present study shows that proper management of temperate UME requires not only the integration of data from different lines of evidence, but also laboratory vs. field approaches to understand the subtler, long-term effects of increased elemental body burdens in native organisms.

Contamination of the Soil–Groundwater–Crop System: Environmental Risk and Opportunities

The increasing development of industries, resulting in a large volume of mining, smelting, and combustion wastes, and intense agricultural activities, due to demand for food and energy, have caused environmental hazards for food quality and ecosystems. This is a review on the contamination of the soil–groundwater–crop system and a potential reduction of the contamination by a gradual shift towards green economy within the European Union and on a worldwide scale. Available mineralogical and geochemical features from contaminated Neogene basins have shown a diversity in the contamination sources for soil and groundwater, and highlighted the need to define the contamination sources, hot spots, degree/extent of contamination, and provide ways to restrict the transfer of heavy metals/metalloids into the food chain, without the reduction of the agricultural and industrial production. Among harmful elements for human health and ecosystems, the contamination of groundwater (thousands of μg/L Cr(VI)) by industrial activities in many European countries is of particular attention. Although Cr(VI) can be reduced to Cr(III) and be completely attenuated in nature under appropriate pH and Eh conditions, the contamination by Cr(VI) of coastal groundwater affected by the intrusion of seawater often remains at the hundreds μg/L level. A positive trend between B and Cr(VI) may provide insights on the role of the borate [B(OH)4]− ions, a potential buffer, on the stability of Cr(VI) in coastal groundwater. Efforts are needed towards reducing toxic metal(loids) from the industrial wastewaters prior to their discharge into receptors, as well as the transformation of hazardous mining/industrial wastes to new products and applications to the optimization of agricultural management strategies.

Uptake of hexavalent chromium by Lactuca sativa and Triticum aestivum plants and mediated effects on their performance, linked with associated public health risks

Plants exposed to toxic Cr(VI) concentrations show reduced biomass production and yield. The present study examined (1) the impacts of increasing Cr(VI) exposure (0.05, 0.5, 1, 5 and 10 mg L-1 K2Cr2O7 in irrigation water) on the growth and development, yield, and stress physiology of two important crop species, such as wheat (Triticum aestivum L.) and lettuce (Lactuca sativa L.), and (2) the associated human health risks due to the consumption of these commodities. Plants in all Cr(VI) treatments preserved similar growth rates and phenotypes with control, untreated plants, with no remarkable modulations in biomass yield (fresh/dry weight, plant height, straw/grain weight). Grains harvested from plants exposed to all Cr(VI) treatments had increased size and 1000 grain weight, partially suggesting hormetic effect. Elevated K2Cr2O7 concentrations (5 and 10 mg L-1) resulted in increased photosynthetic pigments' levels in lettuce leaves. Lipid peroxidation and H2O2 content also revealed the absence of oxidative stress in lettuce plants. Non-carcinogenic (target hazard quotient) and carcinogenic risks from the consumption of these commodities grown under the conditions described in this study were found de minimis. However, more studies are needed in order to obtain more solid information on the safety of Cr(VI)-contaminated water irrigation.

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.

Potential of using a new aluminosilicate amendment for the remediation of paddy soil co-contaminated with Cd and Pb

Cadmium (Cd) and lead (Pb) are toxic heavy metals that impact human health and biodiversity. Removal of Cd/Pb from contaminated soils is a means for maintaining environmental sustainability and biodiversity. In this study, we applied a newly modified material fly ash (NA), zeolite (ZE), and fly ash (FA) to the paddy soils and evaluated the effects of Cd/Pb accumulation in rice via a one-year field experiment. The results showed that the application of NA and ZE enhanced the soil pH and nutrients to a large extent and reduced the availability of Cd/Pb in soil. The Cd and Pb concentrations in rice grains decreased by 32.8% and 62.9%, respectively, with the NA treatments. Similarly, the application of ZE reduced the Cd and Pb concentrations in rice grains by a factor of 27.9% and 63.5%, respectively, which indicates that the amendments can promote the transfer of Cd and Pb from acid-exchangeable fraction to oxidizable and residual fractions. The Cd/Pb showed a significant positive correlation to other metal ions and a negative correlation to the nutrients. Generally, the application of NA and ZE was effective in reducing Cd/Pb accumulation and improving rice yield. Moreover, the NA was more cost-effective than ZE. Hence, this study proves that NA may be a better amendment for remediation of Cd/Pb contaminated soils.

Factors Controlling the Chromium Isotope Compositions in Podiform Chromitites

The application of Cr isotope compositions to the investigation of magmatic and post-magmatic effects on chromitites is unexplored. This study presents and compiles the first Cr stable isotope data (δ53Cr values) with major and trace element, contents from the Balkan Peninsula, aiming to provide an overview of the compositional variations of δ53Cr values in ophiolite-hosted chromitites and to delineate geochemical constraints controlling the composition of chromitites. The studied chromitites exhibit δ53Cr values ranging from −0.184‰ to +0.159‰, falling in the range of so-called “igneous Earth” or “Earth’s mantle inventory” with values −0.12 ± 0.11‰ to 0.079 ± 0.129‰ (2sd). A characteristic feature is the slightly positively fractionated δ53Cr values of all chromitite samples from Othrys (+0.043 ± 0.03‰), and the occurrence of a wide range of δ53Cr values spanning from positively, slightly negatively to the most negatively fractionated signatures (Pindos, δ53Cr = −0.147 to +0.009‰; Skyros, δ53Cr = −0.078 to +0.159‰). The observed negative trend between δ53Cr values and Cr/(Cr + Al) ratios may reflect a decrease in the δ53Cr values of chromitites with increasing partial melting degree. Alternatively, it may point to processes related to magmatic differentiation, as can be seen in our data from Mikrokleisoura (Vourinos).