Diffusive gradients in thin films technique coupled to X-ray fluorescence spectrometry for the determination of bioavailable arsenic concentrations in soil

Many of the procedures for assessing the bioavailability of contaminant including arsenic (As) in soil are time-consuming, thus there is need to develop more effective methods. In this regard, a direct analysis of the binding resin in the diffusive gradient in thin film (DGT) by using wavelength dispersive X-ray fluorescence spectrometry (WDXRF) was tested in determining bioavailable As concentrations in soil. The binding resin obtained from the DGT was dried at room temperature in a desiccator with silica gel for 2 h, and directly analyzed by the WDXRF. The mass of As loaded in the DGT binding resin was plotted against the X-ray intensity obtained from the WDXRF analysis to a draw calibration curve, which showed good linearity (R-2 = 0.997) with a limit of quantification of 0.2 mu g. A correction factor (CF) for compensating the spectral interference between As-K alpha and Pb-L alpha was determined by considering the slope between the X-ray intensity measured at a Bragg angle of 48.781 degrees for As-K alpha and the Pb mass on the DGT binding resin. The use of the derived CF value (0.113) is reasonable to obtain As concentrations with a high accuracy. The relation between phytotoxicity of As to barley Hordeum vulgare and bioavailable As concentrations in soils, which were determined by means of the combined use of DGT and WDXRF, was observed. The study supports that the DGT-WDXRF can be a promising tool to predict soil phytotoxicity for As-contaminated soil risk management.N

Identification of strontium substitution mechanism in hematite via calcium solution

Nonradioactive strontium (Sr) are produced as a result of radioactive decay of heavier elements such as uranium and thorium. Nonradioactive Sr shares physicochemical similarities with Ca and can replace it during bone formation, which may cause bone cancer in humans. Hence, concerning the potential hazards associated with strontium, it is imperative to eliminate it. The present study aimed to investigate the removal mechanisms of hematite-adsorbed strontium by calcium solution. Strontium was adsorbed to hematite at pH 8 and 10 and washed with calcium solution. X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), scanning electron microscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (after Ca washing) were performed on the samples before and after washing. Analyses and fitting by XANES and EXAFS confirmed the formation of an inner-sphere complex of strontium at pH 10. The XRD spectra showed that SrCO3 and SrFe2O4 formed at pH 8 and 10, respectively. After washing with the calcium solution, strontium was directly substituted to form CaCO3 and CaFe2O4. The X-ray photoelectron spectroscopy results provided a systematic analysis of the proportions of hematite and strontium, confirming the substitution of strontium with calcium. This substitution could be attributed to the physicochemical similarities between calcium and strontium. This study confirms the substitution of Sr with Ca, highlighting the physicochemical similarity of the Sr and Ca that facilitates substitution reactions.N

Determination of human health risk incorporating experimentally derived site-specific bioaccessibility of arsenic at an old abandoned smelter site

This study was conducted to investigate the contribution of a site-specific bioavailability of arsenic (As) to human health risk at an old abandoned smelter site in Korea. The site was contaminated with As for over 60 years with the same source (As2O3 in flue gas), but concentration and in vitro bioaccessibility (IVBA) of As differed by operable units (OU), which consequently resulted in difference in estimated risk. Soil samples collected from six OUs showed that aqua regia-extractable As concentrations ranged from 9.8 to 52.8 mg/kg (average 34.1 mg/kg) at OUs 1-5, which had been used as rice paddy field and farmland, and a forest region OU 6 showed much higher As concentrations (14.4-169.8 mg/kg, average 85.9 mg/kg). IVBA of As, determined from the ratio of Solubility/Bioavailability Research Consortium (SBRC)-extractable As to aqua regia-extractable As had a wide range of values (90th percentile values of 28.2-65.8%). Carcinogenic risk calculated with total soil As concentration was the highest (1.4 x 10(-4)) at OU 6 and the risk at the other OUs ranged from 3.8 x 10(-5) to 5.7 x 10(-5). In contrast, when site-specific relative bioavailability (i.e., IVBA values) was incorporated, the estimated risk was reduced by 29.5-62.0% and the decrease was the highest at OUs 1 and 5 with the lowest IVBA of 28.2%. The results demonstrate that the chemical forms of As may be different although the source of contamination is similar, and site-specific bioavailability affected by the chemical forms is an important factor in determining human health risk. (C) 2014 Elsevier Inc. All rights reserved.N

Differential in vitro bioaccessibility of residual As in a field-aged former smelter site and its implication for potential risk

Chemical forms of arsenic (As) present in a former smelter site were determined. A five-step sequential extraction showed that about 94.8 to 99.2% of total As concentration was found to be present as residual form, and interestingly some of the residual As seemed to be still bioaccessible, when determined with an in vitro bioaccessibility test. However, the extents of bioaccessible As greatly varied among the three soils tested. Soil B showed the highest bioaccessibility being 17.18 mg-As/kg (11.9%) followed by 12.71 (2.02%) and 14.03 mg-As/kg (0.64%) in soils C and A, respectively. When the residual As was treated with hydrofluoric acid (i.e., HF) 65.3 to 80.9 mg-As/kg was extracted and only 4.17 to 725% of the HF-extractable As was found to be bioaccessible. In contrast, when the residual As was treated with hydroperchloric acid (i.e., HClO4) only 5.64 to 8.01 mg-As/kg was recovered but 64.5 to 92.5% of the HClO4-extractable As was bioaccessible. The results suggest the presence of differential bioaccessibility of residual As, which apparently depends on the solid phase that As is associated with (i.e., organic matter or clay minerals). Of the As present as residual fraction, the As mainly bound to silicate mineral showed extremely low bioaccessibility and the As associated with refractory organic matter was highly bioaccessible. (C) 2013 Elsevier B.V. All rights reserved.N

Increased ecological risk due to the hyperaccumulation of As in Pteris cretica during the phytoremediation of an As-contaminated site

Ecological risk due to the hyperaccumulation of As in Pteris cretica during phytoremediation was evaluated at an abandoned As-contaminated site. Five receptor groups representing terrestrial invertebrates, avian insectivores, small mammals, herbivores, and omnivores were selected as potentially affected ecological receptors. Soil and food ingestion were considered as major exposure pathways. Phytoremediation was performed with P. cretica only and with both P. cretica and siderophores to enhance plant uptake of As. Ecological hazard index (EHI) values for the small mammal greatly exceeded 1.0 even after three weeks of growth regardless of siderophore application, probably due to its limited home range. For the mammalian herbivore, which mainly consumes plant foliage, the EHI values were greater than 5.73 after seven weeks without siderophore application, but the value increased sharply to 293 at seven weeks when siderophores were applied. This increased risk could be attributed to the facilitated translocation of As from roots to stems and leaves in P. cretica. Our results suggest that, when a phytoremediation strategy is considered for metals remediation, its ecological consequences should be taken into account to prevent the spread of hyperaccumulated heavy metals throughout the food chain of ecological receptors. Uncertainties involved in the ecological risk assessment process were also discussed. (C) 2014 Elsevier Ltd. All rights reserved.N

Enhanced uptake and translocation of arsenic in Cretan brake fern (Pteris cretica L.) through siderophorearsenic complex formation with an aid of rhizospheric bacterial activity

Siderophores, produced by Pseudomonas aeruginosa, released slightly more Fe (53.6 mu mol) than that chelated by ethylenediaminetetraacetic acid (EDTA; i.e. 43.7 mu mol) in batch experiment using As-adsorbed ferrihydrite. More importantly, about 1.79 mu mol of As was found to be associated with siderophores in the aqueous phase due to siderophore-As complex formation when siderophores were used to release As from ferrihydrite. In contrast, As was not detected in the aqueous phase when EDTA was used, probably due to the readsorption of released As to ferrihydrite. A series of pot experiment was conducted to investigate the effect of siderophores as a microbial iron-chelator on As uptake by Cretan brake fern (Pteris cretica L.) during phtoextraction. Results revealed that P. cretica, a known As hyperaccumulator, grown in the siderophore-amended soil showed about 3.7 times higher As uptake (5.62 mg-As g(-1) -plant) than the plant grown in the EDTA-treated soil (1.51 mg-As g(-1) -plant). In addition, As taken up by roots of P. cretica in the presence of siderophores seemed to be favorably translocated to shoots (i.e. stems and leaves). About 79% of the accumulated As was detected in the shoots in the presence of siderophores after ten weeks. Fluorescence microscopic analysis confirmed that As in the roots was delivered to the leaves of P. cretica as a siderophore-As complex. (C) 2014 Elsevier B.V. All rights reserved.N

Identification of pH-dependent removal mechanisms of lead and arsenic by basic oxygen furnace slag: Relative contribution of precipitation and adsorption

Removal mechanisms, precipitation and adsorption, of lead (Pb) and arsenic (As) by basic oxygen furnace (BOF) slag were studied at pH 7epH 13 range. Specifically, the relative contribution of precipitation and adsorption on Pb and As removal was investigated. Pb was mainly removed by precipitation at the pH values tested, as evidenced by the presence of Pb-hydroxide precipitate confirmed by X-ray diffractometry. In contrast, As seemed to be removed mainly by adsorption at the pH range tested. But, precipitation of amorphous calcium (Ca) arsenate was observed at above pH 11. An experiment with Ca reduced BOF slag provides a line of evidence implying the involvement of Ca2+ in the adsorption of As, probably by bridging between slag surface and As oxyanions. X-ray photoelectron spectroscopy and thermogravimetric analysis also elucidated that calcium hydroxide and calcium carbonate were coated on the slag surface at pH 13, which probably blocked As adsorption. When tested following the toxicity characteristic leaching procedure, the precipitation seemed more stable than the adsorption. The removal mechanism and efficiency demonstrated in this study would contribute to the reuse of BOF slags for Pb and As removal. (C) 2020 Elsevier Ltd. All rights reserved.N

Survival of introduced phosphate-solubilizing bacteria (PSB) and their impact on microbial community structure during the phytoextraction of Cd-contaminated soil

This study was conducted to investigate whether or not phosphate-solubilizing bacteria (PSB) as a kind of plant growth promoting rhizobacteria enhance the uptake of Cd by plants. In addition, the effect of PSB augmentation during phytoextraction on the microbial community of indigenous soil bacteria was also studied. In the initial Cd-contaminated soil, the major phyla were Proteobacteria (35%), Actinobacteria (38%) and Firmicutes (8%). While Proteobacteria were dominant at the second and sixth week (41 and 54%, respectively) in inoculated soil, Firmicutes (mainly belonging to the Bacilli class-61%), dramatically increased in the eight-week soil. For the uninoculated soil, the proportion of alpha-Proteobacterin increased after eight weeks (32%). Interestingly, Actinobacteria class, which was originally present in the soil (37%), seemed to disappear during phytoremediation, irrespective of whether PSB was inoculated or not. Cluster analysis and Principal Component Analysis revealed that the microbial community of eight-week inoculated soil was completely separated from the other soil samples, due to the dramatic increase of Bacillus aryabhattai. These findings revealed that it took at least eight weeks for the inoculated Bacillus sp. to functionally adapt to the introduced soil, against competition with indigenous microorganisms in soil. An ecological understanding of interaction among augmented bacteria, plant and indigenous soil bacteria is needed, for proper management of phytoextraction. (C) 2013 Elsevier B.V. All rights reserved.N

Contribution of precipitation and adsorption on stabilization of Pb in mine waste by basic oxygen furnace slag and the stability of Pb under reductive condition

A basic oxygen furnace (BOF) slag was used to stabilize lead (Pb) in a mine waste. Stabilization efficiencies differed depending on the slag contents (i.e., 3, 5, and 10 wt.%) and the water contents (i.e., 0.05-5.0 L/kg), varying from 52.2 to 98.0%, and both the slag contents and the water contents positively affected the stabilization efficiency. X-ray photoelectron spectroscopy suggested an evidence that precipitation and adsorption mechanisms were involved. When the contribution of each mechanism was determined, the increase in the BOF slag content mainly increased adsorption mechanism probably because of the increase in the adsorption sties. The increase in the water content, on the other hand, facilitated precipitation mechanism by lowering the ionic strength. Stabilized Pb could be mobilized at redox potential of 20-85 mV due to the reductive dissolution of Fe and Mn oxides. Sequential extraction results demonstrated that the adsorbed Pb became mobilized, and the fraction of exchangeable Pb increased. (C) 2020 Elsevier Ltd. All rights reserved.N

Mechanism for alkaline leachate reduction through calcium carbonate precipitation on basic oxygen furnace slag by different carbonate sources: Application of NaHCO3 and CO2 gas

Carbonate treatment was tested as a means to mitigate the generation of alkaline leachate from basic oxygen furnace (BOF) slag. BOF slag was treated with 0.1, 0.5, and 1.0 M concentrations of NaHCO3 solution for 48 h at a liquid/solid ratio of 5 L/kg. At 1.0 M NaHCO3, the pH of the leachate decreased from 12.0 to 11.3 because less free CaO was dissolved from the treated slag. Approximately 1.59 mg-Ca2+/g-slag of free CaO was dissolved from the untreated BOF slag while only 0.06 mg-Ca2+/g-slag was liberated from the treated slag. When the data from X-ray photoelectron spectroscopy and thermogravimetric analysis were taken together, formation of CaCO3 precipitates on the surface of the treated BOF slag was evident. Surface precipitation of CaCO3 was more pronounced when CO2 gas was used as an alternative carbonate source. Carbon dioxide treatment further decreased the leachate pH to 8.3, probably because it liberated more Ca2+ from BOF slag during the treatment than 1.0 M NaHCO3 solution due to the pH difference (pH 6.6 and 9.6, respectively), in turn generating more CaCO3 precipitates. Scanning electron microscopy analysis revealed that more CaCO3 was precipitated on the CO2 gas-treated slag surface than on the NaHCO3-treated slag. This study identifies the leachate pH reduction-mechanism and the effect of carbonate source which are expected to contribute to the environmentally safe management of BOF slags. (C) 2019 Elsevier Ltd. All rights reserved.N