Water-soluble fraction of mercury, arsenic and other potentially toxic elements in highly contaminated sediments and soils

Arsenic pollution concerning surface water and sediment of Jie River: A pilot area where gold smelting enterprises are concentrated

A comprehensive monitoring and risk assessment of arsenic (As) pollution concerning surface water and sediment is performed in the Jie River basin, where gold smelting enterprises are concentrated. The study area is divide into six regions, labeled as A, B, C, D, E, and F, from sewage outlets to downstream. Results shows that with far away from the sewage outlets, the total As concentrations in water and sediment gradually decrease from regions A to F. However, in region F, the concentration of bioavailable As significantly increases in the sediment due to the higher pH, leading to the transformation of As(V) into more mobile As(III). In sediment, Paracladius sp. exhibits strong resistance to As pollution in sediment, which can potentially elevate the risk of disease transmission. In water bodies, diatoms and euglena are the main phytoplankton in the Jie River while toxic cyanobacteria exhibits lower resistance to As pollution. Overall, measures should be taken to ecologically remediate the sediment in downstream while implementing appropriate isolation methods to prevent the spread of highly contaminated sediments from regions near sewage outlets.

Ecotoxicological impacts of metals in single and co-exposure on mussels: Comparison of observable and predicted results

Used in high-tech and everyday products, mercury (Hg), cobalt (Co), and nickel (Ni) are known to be persistent and potentially toxic elements that pose a serious threat to the most vulnerable ecosystems. Despite being on the Priority Hazardous Substances List, existing studies have only assessed the individual toxicity of Co, Ni and Hg in aquatic organisms, with a focus on the latter, ignoring potential synergistic effects that may occur in real-world contamination scenarios. The present study evaluated the responses of the mussel Mytilus galloprovincialis, recognized as a good bioindicator of pollution, after exposure to Hg (25 μg/L), Co (200 μg/L) and Ni (200 μg/L) individually, and to the mixture of the three metals at the same concentration. The exposure lasted 28 days at 17 ± 1 °C, after which metal accumulation and a set of biomarkers related to organisms' metabolic capacity and oxidative status were measured. The results showed that the mussels could accumulate metals in both single- and co-exposure conditions (bioconcentration factors between 115 and 808) and that exposure to metals induced the activation of antioxidant enzymes. Although Hg concentration in organisms in the mixture decreased significantly compared to single exposure (9.4 ± 0.8 vs 21 ± 0.7 mg/kg), the negative effects increased in the mixture of the three elements, resulting in depletion of energy reserves, activation of antioxidants and detoxification enzymes, and cellular damage, with a hormesis response pattern. This study underscores the importance of risk assessment studies that include the effects of the combination of pollutants and demonstrates the limitations of applying models to predict metal mixture toxicity, especially when a hormesis response is given by the organisms.

Selenium/sulfur-modified montmorillonite materials mitigate mercury pollution in farmland

Selenium (Se) amendment could reduce mercury (Hg) bioaccumulation in crops, but sometimes it could cause excessive Se accumulation in crops and potential Se exposure risks for humans. In this study, we designed and synthesized selenium and sulfur-modified montmorillonite materials (Se/S-Mont) to effectively reduce mercury levels and avoid excessive Se enrichment in plants. The results of pot experiments (1 g Se/S-Mont/100 g soil) and field microplot trials (0.3 g Se/S-Mont/100 g soil, 8 t/ha) showed that Se/S-Mont amendments significantly reduced the Hg concentrations in water spinach and hybrid Pennisetum by 28-68% and 57%-92% (P < 0.05), respectively, while they did not lead to excessive Se bioaccumulation in the plants. Se/S-Mont was more efficient in mitigating soil Hg pollution than adding raw materials (e.g., NaSeO₃) and their combinations, and they significantly reduced the available Se fraction in the soil and the Se levels in the plants (P < 0.05). The potential mechanisms revealed by X-ray absorption near-edge spectra (XANES) and pot experiments were the adsorption and slow release of Hg, S, and Se by Se/S-Mont, the high affinity between Hg and Se, competition between Se and S, and the formation of stable complexes containing Se-S-Hg. The Se/S-Mont immobilizer was easy to prepare and required the application of small amounts, and the remediation effect was relatively stable and exhibited few negative effects; therefore, the approach showed high environmental and economic potentials.

Metal(oid)s accumulation (Hg and As) and their biochemical effects in Halimione portulacoides (Ria de Aveiro, Portugal)

For decades, mercury (Hg) and arsenic (As) were discharged into the Aveiro Lagoon. This study was designed to assess the natural attenuation process evolution by: (1) evaluating Hg and As concentration in sediments and in Halimione portulacoides, and assess the long-term evolution of a historically contaminated salt-marsh system; (2) evaluating the stress levels imposed by Hg and As, and the mechanisms triggered to squash oxidative damage. Concentrations of Hg and As in sediments varied considerably between sampling locations. H. portulacoides did not bioconcentrate Hg and As, restricted translocation to aerial biomass and immobilized contaminants in cell walls. The ions that reached the cytosol induced oxidative stress, restrained by antioxidant mechanisms, especially SOD and CAT activity. Results show that after 25 years of natural attenuation, contamination decreased in the sediment, but remains above non-contaminated systems and continues to induce toxicity in the saltmarsh halophyte community, evidencing the persistent effect of Hg and As contamination.

Bismuth-based photocatalyst for photocatalytic oxidation of flue gas mercury removal: A review

Photocatalytic oxidation method is a promising technology for solving flue gas mercury (Hg) pollution from industrial plants. Semiconductor photocatalysts have been widely applied in energy conversion and environmental remediation. However, key issues such as low light absorption capacity, wide energy band gap, and poor physicochemical stability severely limit the application of photocatalysts in practical industrial plants. In recent years, bismuth-based (Bi-based) photocatalysts, including bismuth oxide halide BiOX (X = Cl, Br or I), bismuth salt oxymetal BiVO₄, and BiOIO₃ etc., have increasingly aroused scientists' attention due to their peculiar crystalline geometric structures, tunable electronic structure and high photocatalytic performance. In present review, we firstly review the photocatalytic reaction mechanism and main photocatalytic oxidation mechanism of mercury. Secondly, the synthetic methods of Bi-based photocatalysts are summarized. Then, according to the mechanism of mercury removal, the experimental modifying approaches including heterojunction making, external atoms doping, defect creating, and crystal face regulating to promote the photocatalytic oxidation of mercury removal are summarized, as well as the determination of the band gap and electronic density of states (DOS) of Bi-based photocatalysts to elucidate the photocatalytic oxidation mechanism via density functional theory (DFT) calculation. Furthermore, constructing electronic transmission channels is an efficient way to improve the photocatalytic activity. Finally, challenges and perspectives of Bi-based photocatalyst for photocatalytic oxidation of mercury removal are presented. In addition, the excellent performance photocatalysts and efficient pollution removal equipment for mercury removal in industrial plants are still required in-depth study.

Mercury methylation in cyanide influenced river sediments: A comparative study in Southwestern Ghana

Studies on the influence of CN on Hg methylation rates in aquatic systems draining gold mining (artisanal and small-scale) communities in Africa are rare. The study assessed the influence of CN on Hg methylation in aquatic sediments of two major river systems draining artisanal and small-scale gold mining (ASGM) communities of the Prestea-Huni Valley district, Southwestern Ghana. The miners extract gold (Au) through exclusive amalgam [Hg-Au] formation or cyanidation of Au-rich Hg-contaminated tailings, or a combination of both techniques. Hg water solubility and probable mercuric compounds in sediments of Hg-contaminated CN-loaded (River Aprepre) and Hg-contaminated non-CN (River Ankobra) aquatic systems within the district were investigated. THg was determined by CV-AAS after HF/HNO₃/HCl digestion. MeHg in sediments were extracted with H₂SO₄/KBr/CuSO₄-CH₂Cl₂; followed by aqueous-phase propylation, preconcentration-on-Tenax, and GC-CV-AFS. River Aprepre showed 4.58-14.83 ngMeHg/g as Hg (1.4-3.7% THg as MeHg), with 241-415 ngTHg/g, and 0.05-0.21 mgCN/kg. For River Ankobra, MeHg ranged 0.24-1.21 ngMeHg/g (0.08-0.35% THg as MeHg) with 162-490 ngTHg/g dw and CN < 0.001 mg/kg. There was positive correlation (r² = 0.5974; p < 0.01) between MeHg and CN in River Aprepre. The water-soluble fraction of Hg in sediment from both rivers was < 1% of THg. Hg in sediments from River Aprepre were generally more soluble than that from River Ankobra, indicating that Hg in sediments from River Aprepre were potentially more bioavailable for methylation. Accordingly, the presence of CN in Hg-dominated river sediments potentially influences and enhances the solubility and mobility of Hg, resulting in increased Hg methylation rates.

Simultaneous immobilization of Pb, Cd and As in soil by hybrid iron-, sulfate- and phosphate-based bio-nanocomposite: Effectiveness, long-term stability and bioavailablity/bioaccessibility evaluation

Here, the bio-nanocomposite (n-HFP + n-HFS)@An was developed to simultaneously immobilize Pb, Cd and As in the severely contaminated soil. The immobilization rates of diethylenetriaminepentaacetic acid (DTPA)/decarbonate-extracted bioavailable Pb, Cd and As were 59.87%, 31.28% and 62.30%, and the immobilization rates of their water-soluble forms were 63.12%, 60.02% and 89.39%, respectively. Moreover, the ten-year acid rain simulated leaching assay showed that the maximum cumulative release contents of Pb, Cd and As in the treated soil samples were decreased by 2.94, 2.46 and 40.60 times, comparing to the un-treated ones. Additionally, the results of SBRC (Solubility Bioaccessibility Research Consortium) revealed that the bioaccessible rates of the three metals in intestinal phase were lower than in gastric phase, and both of them decreased with increasing the immobilization time. The gastric bioaccessibility of Pb, Cd and As had a higher correlation with the contents of water-soluble forms, while the intestinal bioaccessibility was more strongly positively associated with the bioavailable forms.

Evaluating the relationships between specific drainage area characteristics and soil metal concentrations in long-established bioswales receiving suburban stormwater runoff

Bioswales are used to attenuate stormwater pollution, but their long-term sustainability regarding sequestered metals is relatively unknown, and a clear rationale for prioritizing soil management is lacking. Impervious areas draining into four 14-year-old suburban bioswales were delineated, for which surface soils (top 10 cm; 72 samples) were sampled; soils from 4 adjacent reference sites were also sampled. Total and water soluble metals (Cd, Cu, Pb, Zn) were quantified, and the relationships between metal concentrations and drainage area characteristics evaluated. Annual metal loads were estimated using regional runoff data to simulate current and future metal concentrations; risks to soil biota were assessed by comparing metal concentrations to ecological screening levels. The drainage areas' percent imperviousness (37-71%) and ratios of impervious drainage area to bioswale area (2.0-5.7) varied, owing to differing proportions of rooftops, paved surfaces, lawns, and natural soils. Total Cu and Zn ranged from 10.0 to 43.2 mg/kg dry soil, and 15.6 to 129.5 mg/kg dry soil, respectively. Across all bioswales, total Zn was positively correlated to percent impervious area (r = 0.32, p = 0.0073), the ratio of connected impervious drainage area to infiltration area (r = 0.32, p = 0.0073), and percent drainage area as paved surfaces (r = 0.46, p = 5.6 E-05), but negatively correlated to percent drainage area as lawns (r = -0.48; p = 2.4 E-05). Water soluble metal concentrations were orders of magnitude lower than total metals. Given annual metal loads (0.2-0.4 mg Cu/kg dry soil; 1.5-3.1 mg Zn/kg dry soil), replacing bioswale soils to constrain metal concentrations would be unnecessary for decades. Taken together, this study proposes a transferable approach of estimating, then verifying via sampling and analysis, bioswale soil metal concentrations, such that soil management decisions can be benchmarked to ecological screening levels.

Assessing Mercury Mobility in Sediment of the Union Canal, Scotland, UK by Sequential Extraction and Thermal Desorption

The mobility of mercury (Hg) was assessed in sediment from the Union Canal, Scotland, UK. Samples collected from the vicinity of a former munitions factory that manufactured mercury fulminate detonators were subjected to sequential extraction followed by cold vapor atomic absorption spectrometry (CVAAS) and direct analysis using thermal desorption (TD). The sequential extraction indicated that > 75% of mercury (up to 429 mg kg^-1) was in mobile forms, with < 12% semimobile and < 23% nonmobile species. In the TD method, > 67% of the total Hg content was desorbed in the temperature range 100-250 °C consistent with species weakly attached to the mineral matrix [tentatively identified as an iron (oxy)hydroxide-associated species]. This predominance of mobile mercury species may arise from a lack of association between Hg and either organic matter or sulfur in the sediments. Further investigation of Hg mobilization, transport, and assimilation/biomagnification is required both to determine whether there is a need for remediation of the sediment and to improve understanding of the biogeochemical cycling of Hg in shallow, oxic, freshwater systems.

Phosphoric acid purification sludge: Potential in heavy metals and rare earth elements

The present study was carried out to show the potential in heavy metals (HM) and the rare earth elements (REE) which presents the residues of phosphoric-acid(PA) purification. Three different cadmiferous solid residues (according to the nature of the purification process of the PA: BG, BC and BS) were collected from an industrial site located in the south of Tunisia. The mineralogical study showed the predominance of anhydrite, accompanied by quartz, malladrite; calcium sulfate hemihydrate and fluorapophyllite. The microanalysis showed (i) the association of cadmium and zinc, (ii) as well as the presence of associated REEs. The chemical analysis showed that (i) the calcium sulfate concentrations are majority in samples BS, BG and BC (44, 34 and 44%, respectively), (ii) significant concentrations of phosphoric acid (28, 18 and 21% P₂O₅, respectively), (iii) the HM: Cd, Zn, Cr, Ni, V, Cu, Pb, Co, Mo, Mn and U have proportion in the order of 0.1%. The concentrations of Cd, Zn and Cr are respectively in the order of: 230, 149 and 189 mg/kg for BS, 346, 243 and 153 mg/kg for BG and 183, 129 and 440 mg/kg for BC and (iv) the REEs: La, Ce, Nd, Eu, Y et Yb present considerable mass percentages able to reach 0.2%. A series of extraction tests was led on the cadmiferous sludges to evaluate the rates of HM (Cd, Zn) and REE dissolution, using two solvents (deionized water (DW) and aqueous sodium based alkaline metal solution). The results showed that the dissolution rates of Cd and Zn are respectively in the order of (12-29% and 41-45% for DW; 67-86% and 83-93% for Na₂SO₄ solution). The extractability of HM and REE is strongly influenced by pH, solvent nature and mineral load in the cadmiferous sludges. The water-soluble metals represent a significant mobile fraction, making the toxic elements more sensitive to mobilization processes, such as leaching and erosion. Whereas, the metals extractable by the Na₂SO₄ solution represent a very important exchangeable and "co-crystallization" fraction, which reflects the bioavailability of these metals.

Investigations of water-extractability of As in excavated urban soils using sequential leaching tests: Effect of testing parameters

Excavated soils with low-level As contamination obtained from construction projects during city development have been of great concern in Japan. Water-extractable As represents the most easily mobilized and ecotoxicologically relevant fraction in the soil environment. In the present study, the water-extractability of As in excavated alkaline urban soils was assessed using sequential leaching tests (SLTs) with a focus on the effects of test parameters. In addition, the potentially water-leachable As over an extremely long period was assessed using the pollution potential leaching index (PPLI), from which one can estimate the number of extractions required to reduce the As in the cumulative leachates to below the Japanese environmental standard (10 μg L^-1). Total As concentrations varied from 6.75 to 79.4 mg kg^-1, and As was continuously detectable among replicate SLT experiments. The water-extractable As obtained in the first step of the SLT accounted for 0.41%-7.60% of total As (average: 2.36%), while the cumulative released As in the SLTs corresponded to 1.30%-21.6% of the total (average: 10.6%). The variability of the water-soluble fractions was sensitive to the test conditions. The shaking time at each SLT step had the largest effect on the As water-extractability; followed by sample storage, shaking speed and shaking interruption. A longer shaking time in the standard leaching test of excavated soils is suggested for regulatory purposes in Japan. The use of the PPLI concept for quick estimation of the potential As leachability from excavated soils was supported by the good reproducibility of PPLI results obtained from SLTs under different test parameters.

Trace metal(loid) mobility in waste deposits and soils around Chadak mining area, Uzbekistan

The assessment of potential trace metal(loid) contamination in tailing dumps and soils was characterized in the Chadak mining area (Uzbekistan). Concentrations of trace elements (V, Cr, Co, Ni, Cu, Zn, As, Cd, Sb, Pb) were determined by X-ray fluorescence analysis and compared with background and intervention values (IV). The concentrations of As, Zn, Sb, and Pb were higher in the abandoned than in the active tailing dump, ranging from 42-1689mg/kg for As, 73-332mg/kg for Zn, 14-1507mg/kg for Sb, and 27-386mg/kg for Pb. Selective extractions were applied in order to assess the mobility and availability of trace metal(loid)s in samples. Oxyanion-forming elements such as As and Sb were immobilized by Fe oxides, although to some extent also extractable with acetic acid and soluble-in-water forms were detected, indicating potential bioavailability that can impose a potential toxicity risk for the environment. Selective extractions data also showed that Zn and Pb were relatively immobile, although in higher contamination sites significant amounts of these elements were also extractable with acetic acid. In tailing materials Zn and Pb mobility were negatively correlated by the cation-exchange capacity (CEC) and clay content, indicating the importance of these factors in the reduction of the potential toxicity for these elements. Total concentration of As, Sb, and Pb were also negatively correlated with soil pH, indicating that the oxidation process of sulphide tailings and thus the generation of acidic conditions may lead to release of contaminants over time. However, due to the calcium carbonate content, the acid neutralization capacity of the tailings is not yet exhausted and contaminant concentrations in soil-pore water are still relatively low. The results of our investigation suggest that environmental risk associated with these wastes in semi-arid climate is therefore not a short-term problem but rather requires constant monitoring and additional ecotoxicological studies.

Linking toxicity profiles to pollutants in sludge and sediments

Obtaining a complex picture of how pollutants synergistically influence toxicity of a system requires statistical correlation of chemical and ecotoxicological data. In this study, we determined concentrations of eight potentially toxic metals (PTMs) and four groups of organic pollutants in 15 sewage sludge and 12 river sediment samples, then linked measured contaminant concentrations to the toxicity of each matrix through constrained correspondence analysis (CCA). In sludge samples, Hg, As, hexachlorohexane (HCH), polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecane (HBCD) influenced the toxicity profiles, with the first four having significant effects and HBCD being marginally significant. In sediment samples, Hg, As, PBDEs, hexachlorobenzene (HCB), dichlorodiphenyltrichloroethane (DDT), HBCD, HCH and polycyclic aromatic hydrocarbons (PAHs) were found to explain toxicity profiles with Hg, As, PBDEs, HCB, DDT, HBCD, and HCH having significant effects and PAHs being marginally significant. Interestingly, HCH was present in small amounts yet proved to have a significant impact on toxicity. To the contrary, PAHs were often present in high amounts, yet proved to be only marginally significant for sediment toxicity. These results indicate that statistical correlation of chemical and ecotoxicological data can provide more detailed understanding of the role played by specific pollutants in shaping toxicity of sludge and sediments.

Bioaccumulation and molecular effects of sediment-bound metals in zebrafish embryos

Predicting the bioavailability and effects of metals in sediments is of major concern in context with sediment risk assessment. This study aimed to investigate the bioavailability and molecular effects of metals spiked into riverine sediments to zebrafish (Danio rerio) embryos. Embryos were exposed to a natural and an artificial sediment spiked with cadmium (Cd), copper (Cu), nickel (Ni) and zinc (Zn) individually or as a mixture at concentrations ranging from 150 to 3000 mg/kg dry weight (dw) over 48 h, and uptake of metals was determined. Furthermore, transcript abundances of the metallothioneins MT1 and MT2, the metal-responsive element-binding transcription factor (MTF) and the genes sod1, hsp70 and hsp90α1 were measured as indicators of metal-induced or general cellular stress. D. rerio embryos accumulated metals from sediments at concentrations up to 100 times greater than those spiked to the sediment with the greatest bioaccumulation factor (BAF) for Cu from artificial sediment (275.4 ± 41.9 (SD)). Embryos accumulated greater concentrations of all metals from artificial than from natural sediment, and accumulation was greater when embryos were exposed to individual metals than when they were exposed to the mixture. Exposure of embryos to Zn or the mixture exhibited up to 30-fold greater transcript abundances of MT1, MT2 and hsp70 compared to controls which is related to significant uptake of Zn from the sediment. Further changes in transcript abundances could not be related to a significant uptake of metals from sediments. These studies reveal that metals from spiked sediments are bioavailable to D. rerio embryos directly exposed to sediments and that the induction of specific genes can be used as biomarkers for the exposure of early life stages of zebrafish to metal-contaminated sediments.

Soil-pore water distribution of silver and gold engineered nanoparticles in undisturbed soils under unsaturated conditions

Release of engineered nanoparticles (ENPs) to soil is well documented but little is known on the subsequent soil-pore water distribution of ENPs once present in soil. In this study, the availability and mobility of silver (Ag) and gold (Au) ENPs added to agricultural soils were assessed in two separate pot experiments. Pore water samples collected from pots from day 1 to 45 using porous (<0.17 μm) membrane samplers suggest that both Ag and Au are retained almost completely within 24 h with less than 13% of the total added amount present in pore water on day 1. UV-Vis and TEM results showed that AuENPs in pore water were present as both homoaggregates and heteroaggregates until day 3 after which the concentration in pore water was too low to detect the presence of aggregates. A close relation between the concentration of Au and Fe in pore water suggests that the short term solubility of Au is partly controlled by natural soil colloids. Results suggest that under normal aerated soil conditions the actual availability of Ag and AuENPs is low which is relevant in view of risk assessment even though the impact of environmental conditions and soil properties on the reactivity of ENPs (and/or large ENPs aggregates) retained in the solid matrix need to be addressed further.

Occurrence, distribution, and ecological risk assessment of potentially toxic elements in surface sediments of Lake Awassa and Lake Ziway, Ethiopia

Microwave-assisted acid digestion and modified aqua regia (HNO3:HCl:HF:H3BO3) leaching techniques were used for the determination of 15 potentially toxic elements (V, Cr, Fe, Mn, Co, Ni, Cu, Zn, As, Se, Ag, Cd, Sn, Hg and Pb) in sediment samples from Lake Awassa and Lake Ziway, Ethiopia. The digests were subsequently analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-optical emission spectrometry (ICP-OES). Mercury was directly determined in the solid samples using an elemental mercury analyzer. The precision and accuracy of the digestion procedures were verified using certified reference materials. The experimental results were in good agreement with the certified values (P < 0.05) and the recoveries were quantitative (>90%). The average relative standard deviations were below 10%. There is significant correlation between the two lakes at the 0.01 level (2-tailed). Using the sediment quality guidelines, both lakes are heavily polluted with Zn and some of the sites are heavily polluted with Cu, Ni and Pb. Based on effect range low (ERL) - effect range medium (ERM), in both lakes for Ag were greater than the ERM, indicating that the areas could be toxic to aquatic organisms, while for Cr, Cu, As and Hg the values were less than ERL.

Study of Selected Metals Distribution, Source Apportionment, and Risk Assessment in Suburban Soil, Pakistan

Composite soil samples collected from suburban areas were analyzed for Cd, Co, Cr, Cu, Fe, Mn, Pb, Sr, and Zn by atomic absorption spectrophotometry. Based on pseudototal metal analysis, Fe, Mn, Sr, and Zn were the prevailing metals while Cd, Co, Cr, and Pb were the least participants. However, based on bioavailability, Cd, Co, Pb, and Sr were easily leachable and might pose adverse effects to soil biota. In ecological risk assessment, contamination factor demonstrated moderate contamination by Co, Sr, and Zn and high contamination by Cd, Cu, and Pb; geoaccumulation index indicated heavy to extreme contamination by Cd and heavy contamination by Pb; enrichment factor revealed significant enrichment by Co, Cr, Cu, Mn, Sr, and Zn and extreme enrichment by Cd and Pb. Substantial human inputs for Cd, Co, Cr, Cu, Mn, Pb, Sr, and Zn were also revealed by principal component analysis in the examined soil. Overall the study area was found to be contaminated at considerable/high degree.

Interactive effects of mercury and arsenic on their uptake, speciation and toxicity in rice seedling

Rice can take up and translocate more As and Hg than other cereal crops. A hydroponic experiment was conducted to investigate their interactive effects on their uptake and toxicity in rice seedling after exposing to As(III) (0.1, 0.5 or 2.5 mg L−1) and Hg (0.05, 0.25 or 1.25 mg L−1) for 14 d. Rice was much more effective in taking up Hg than As and sequestered both in the roots. As and Hg reached 339 and 433 mg kg−1 in the roots, and 48.5 and 16.1 mg kg−1 in the shoots at As2.5 + Hg1.25. Though Hg inhibited As uptake and translocation, it enhanced As(III) toxicity to rice seedling. However, As inhibited Hg uptake at Hg0.05, but the opposite was observed at Hg0.25 and Hg1.25. Arsenite (54–100%) and inorganic Hg (100%) were the predominant form in the plant based on speciation analysis via HPLC–ICP–MS. Malondialdehyde in the roots and shoots increased with increasing As and Hg concentrations, with the highest being 54 μmol g−1 at As0.5 + Hg1.25 in the roots. Root cell structural damage and organelles number reduction with increasing As and Hg concentration were observed based on TEM. As and Hg transformation and toxicity can help to understand the metabolic mechanisms of As and Hg in rice plant when co-present.

Applications of organic and inorganic amendments induce changes in the mobility of mercury and macro- and micronutrients of soils

Both soil organic matter and sulfur (S) can reduce or even suppress mercury (Hg) mobility and bioavailability in soil. A batch incubation experiment was conducted with a Chernozem and a Luvisol artificially contaminated by 440 mg·kg⁻¹ Hg showing wide differences in their physicochemical properties and available nutrients. The individual treatments were (i) digestate from the anaerobic fermentation of biowaste; (ii) fly ash from wood chip combustion; and (iii) ammonium sulfate, and every treatment was added with the same amount of S. The mobile Hg portion in Chernozem was highly reduced by adding digestate, even after 1 day of incubation, compared to control. Meanwhile, the outcome of these treatments was a decrease of mobile Hg forms as a function of incubation time whereas the contents of magnesium (Mg), potassium (K), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), and phosphorus (P) were stimulated by the addition of digestate in both soils. The available calcium (Ca) contents were not affected by the digestate addition. The experiment proved digestate application as the efficient measure for fast reduction of mobile Hg at extremely contaminated soils. Moreover, the decrease of the mobile mercury portion was followed by improvement of the nutrient status of the soils.

Non-carcinogenic and carcinogenic health risk assessment of selected metals in soil around a natural water reservoir, Pakistan

Contaminants in surface soil can directly pose significant human health risks through oral ingestion, dermal contact and particle inhalation, especially for children. Both non-cancer and cancer risks associated with selected metal levels (Cd, Cr, Cu, Fe, Mn, Pb and Zn) were evaluated in surface soil around Mangla Lake during summer and winter. The results based on average pseudo-total metal concentrations followed the decreasing order: Fe (4038mg/kg)>Mn (394mg/kg)>Zn (40mg/kg)>Pb (17mg/kg)>Cr (21mg/kg)>Cu (15mg/kg)>Cd (1.3mg/kg) during summer and Fe (3673mg/kg)>Mn (407mg/kg)>Zn (30mg/kg)>Cr (26mg/kg)>Pb (26mg/kg)>Cu (14mg/kg)>Cd (1.8mg/kg) during winter. Present metal levels in the soil were also compared with other studies and guideline values which showed significant increase in the metal concentrations in this study. The metal levels also showed considerable spatial variations around the lake. The correlation study and multivariate principal component analysis revealed significant anthropogenic contributions of the metals in soils. Aqua-regia extractable (pseudo-total) contents were used as the reference to establish the percentage of bioavailability. After considering the bioavailability (Cd>Pb>Cr>Zn>Cu>Fe>Mn during both seasons), the non-cancer and cancer risks posed by Cd, Pb and Cr was relatively higher than rest of the metals, though the overall hazard index (HI) and cancer risk levels were within the safe limits (1.0 and 1.0E-06, respectively). The overall cancer risk to the adults based on pseudo-total metal concentrations exceeded the target value (1.0E-06), mainly contributed by Cr. A method considering bioavailability is suggested to produce a more realistic estimation for human health risks of trace metals contamination in soil.

Mobility and ecological risk assessment of trace metals in polluted estuarine sediments using a sequential extraction scheme

The three-stage sequential extraction procedure for the fractionation of Cd, Cr, Cu, Ni, Pb, and Zn, proposed by the Commission of the European Communities Bureau of Reference, was applied to sediment five samples collected from the Sal estuary, Sergipe State, northeast Brazil, in September 2009. The method showed satisfactory recoveries, detection limits, and standard deviations for determinations of trace metals in the sediments. Cd and Pb were the metals most prevalent in the bioavailable fractions (carbonates, Fe and Mn oxides, organic matter, and sulfides), while Ni, Zn, Cu, and Cr showed higher percentages in the inert fraction. The order of mobility of the metals was Cd (66%) > Pb (65%) > Zn (59%) > Ni (57%) = Cr (57%) > Cu (56%). Possible toxicity related to these metals was examined using the risk assessment code, and by comparing the chemical data with sediment quality guideline ERL-ERM values. Results obtained using the two methods were in agreement, and showed low to medium risk for all metals, indicating that adverse effects on aquatic biota should rarely occur.

Statistical apportionment and risk assessment of selected metals in sediments from Rawal Lake (Pakistan)

The present study was carried out in order to evaluate the statistical apportionment and risk assessment of selected metals (Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Pb, Sr, and Zn) in freshly deposited sediments in Rawal Lake, Pakistan. Composite sediment samples were collected, oven-dried, grounded, homogenized, and processed to assess the water-soluble and acid extractable concentrations of the metals in the water extract and acid extract of the sediments using flame atomic absorption spectrophotometer. Statistical methods were used to identify the possible sources of the metals. Sediment quality guidelines and potential acute toxicity were used to evaluate the ecotoxicological sense of selected metals. Non-carcinogenic health risk assessment was also carried out to determine the potential adverse health risks to the inhabitants. Relatively higher concentration was noted for Ca, Fe, Mg, Na, K, Mn, and Sr in the sediment samples. Principal component analysis and cluster analysis revealed anthropogenic contributions of Cd, Pb, Cr, Mn, Fe, and Li in the sediments. Enrichment factors of the metals in sediments showed severe to moderate enrichment of Cd, Pb, Ca, Fe, Li, Mn, and Sr. Geoaccumulation indices and contamination factors evidenced significant contamination by Cd and Pb, although, on the whole, low degree of contamination was noted. The levels of some metals exceeded the sediment quality guidelines, which revealed frequently adverse biological effects to the dwelling biota in the aquatic ecosystem. The sediments were found to be significantly contaminated by Cd, Pb, Cr, Mn, Fe, and Li.

Role of EDTA in arsenic mobilization and its uptake by maize grown on an As-polluted soil

EDTA amendments are widely used for micronutrient fertilization in arid soils, besides their effectiveness in the remediation process of heavy metal from contaminated soils. However, the persistence of EDTA in arsenic contaminated soil may have further negative effects on the grown plants. To investigate the influences of EDTA on soil As, a pot experiment was conducted using a sandy clay loam As-polluted soil treated with gradual rates of EDTA (0, 1.0, 2.5 and 5 mmol kg(-1)) and planted with maize for two months. The key findings reveal that EDTA applications increased AB-DTPA extractable and water soluble As significantly. Such increases seemed to be the main reasons behind the increase in As uptake by maize plants as the addition of EDTA at the rates of 1.0, 2.5 and 5.0 mmol kg(-1) increased significantly As uptake by shoots 1.5, 2.4 and 3.0 folds, respectively compared to the untreated soil. On the other hand, As uptake by roots did not increase significantly except with the highest application rates of 2.5 and 5.0 mmol kg(-1). The results also show that arsenic translocation factor (TF) values were too low to attain successful phytoextraction. In conclusion, the bioavailable fraction of As is important to investigate the phytoextraction and phytotoxicity of As.

Assessment of background levels of trace metals in water and soil from a remote region of Himalaya

Selected trace metals were estimated by atomic absorption spectrometry in the water and soil samples collected from the remote region of Himalaya. The soil samples were analysed for soluble and acid extractable fraction of trace metals. In water samples, Ca, Na, Mg and K emerged as dominant contributors, whereas, Ca, Na, K, Mg, Fe and Pb were estimated at comparatively higher levels in the water extract of the soil. In case of acid extract of the soil samples, Ca, K, Fe, Mg, Mn and Na were found at elevated concentrations. Based on mean levels of the metals, following decreasing concentration order was observed in water samples: Ca > Na > Mg > K > Pb > Co > Cu > Zn > Mn > Cr > Fe > Cd > Li, however, in the acid extract of the soil, following order was noted: Ca > K > Fe > Mg > Mn > Na > Pb > Zn > Cr > Li > Cu > Co > Cd. The correlation study revealed appreciably diverse mutual relationships of trace metals in the water and soil samples. The multivariate cluster analyses exhibited divergent apportionment of trace metals in water and soil samples. Among the trace metals, Cd, Pb, Li, Zn, Cr, Cu, Mn and Co exhibited extreme to significant anthropogenic enrichment in the soil samples, while the rest of the metals were mostly contributed by the natural processes.

The water-soluble fraction of potentially toxic elements in contaminated soils: relationships between ecotoxicity, solubility and geochemical reactivity

To better understand the impacts posed by soil contamination to aquatic ecosystems it is crucial to characterise the links between ecotoxicity, chemical availability and geochemical reactivity of potentially toxic elements (PTE's) in soils. We evaluated the adverse effects of water extracts obtained from soils contaminated by chemical industry and mining, using a test battery including organisms from different trophic levels (bacteria, algae and daphnids). These tests provided a quick assessment of the ecotoxicity of soils with respect to possible adverse effects on aquatic organisms although the ecotoxicological responses could be related to the solubility of PTE's only to a limited extent. The analysis of results of bioassays together with the chemical characterisation of water extracts provided additional relevant insight into the role of conductivity, pH, Al, Fe, and Mn of soil extracts on toxicity to organisms. Furthermore, an important conclusion of this study was that the toxicity of extracts to the aquatic organisms could also be related to the soil properties (pH, Org C and Fe(ox)) and to the reactivity of PTE's in soils which in fact control the soluble fraction of the contaminants. The combined assessment of ecotoxicity in water fractions, solubility and geochemical reactivity of PTE's in soils provided a more comprehensive understanding of the bioavailability of inorganic contaminants than ecotoxicological or chemical studies alone and can therefore be most useful for environmental risks assessment of contaminated soils.

Distribution, correlation and risk assessment of selected metals in urban soils from Islamabad, Pakistan

Urban soil samples were analyzed for Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Pb, Sr and Zn by atomic absorption spectrophotometric method. Multivariate statistical approach was used to study the apportionment of selected metals in the soil samples during summer and winter. The degree of contamination along with the geoaccumulation index, enrichment factor and contamination factor was also evaluated. In water-extract of the soil samples, relatively higher levels were noted for Na, Ca, K, Fe, Mg, and Pb with average concentrations of 56.38, 33.82, 12.53, 7.127, 5.994, and 1.045mg/kg during summer, while the mean metal levels during winter were 76.45, 38.05, 3.928, 0.627, 8.726, and 0.878mg/kg, respectively. In case of acid-extract of the soils, Ca, Fe, Mg, Na, K, Mn and Sr were found at 27,531, 12,784, 2769, 999.9, 737.9, 393.5, and 115.1mg/kg, during summer and 23,386, 3958, 3206, 254.6, 1511, 453.6, and 53.30mg/kg, during winter, respectively. Most of the metals showed random distribution with diverse correlations in both seasons. Principal component analysis and cluster analysis revealed significant anthropogenic intrusions of Cd, Pb, Co, Cr, Cu, Li, Zn and Na in the soils. Geoaccumulation indices and contamination factors indicated moderate to heavy contamination for Pb and Cd in the soils, while enrichment factor exhibited significant enrichment (EF>5) of Cd, Pb, Ca, Co, Li, Mn and Zn by anthropogenic activities. Overall, on the average basis, considerable degree of contamination (C(deg)>16) was observed in both seasons, although it was higher in winter. Present metal levels were also compared with those reported from other areas around the world.

Mycelial growth and solid-state fermentation of lignocellulosic waste by white-rot fungus Phanerochaete chrysosporium under lead stress

Lignocellulosic biomass is an abundant renewable resource difficult to degrade. Its bioconversion plays important roles in carbon cycles in nature, which may be influenced by heavy metals in environment. Mycelial growth and the degradation of lignocellulosic waste by lignin-degrading fungus Phanerochaete chrysosporium under lead stress were studied. It was shown that P. chrysosporium could grow in liquid media with 400 mg L⁻¹ Pb(II), and mycelial dry weight was reduced by 54% compared to the control. Yellow mycelia in irregular short-strip shape formed in Pb-containing media, whereas the control showed ivory-white regular mycelial pellets. Two possible responses to Pb stress were: dense hyphae, and secretion from mycelia to resist Pb. During solid-state fermentation of straw, fungal colonization capability under Pb stress was positively correlated with the removal efficiency of soluble-exchangeable Pb when its content was higher than 8.2 mg kg⁻¹ dry mass. Carboxymethyl cellulase activity and cellulose degradation were inhibited at different Pb concentrations, whereas low Pb concentrations increased xylanase and ligninolytic enzyme activities and the hemicellulose and lignin degradation. Cluster analyses indicated that Pb had similar effects on the different microbial indexes related to lignin and hemicellulose degradation. The present findings will advance the understandings of lignocellulose degradation by fungi under Pb pollution, which could provide useful references for developing metal-polluted waste biotreatment technology.