Seulki Jeong

Application of phosphate-solubilizing bacteria for enhancing bioavailability and phytoextraction of cadmium (Cd) from polluted soil

In this study, phosphate-solubilizing bacteria (PSB), Bacillus megaterium, were used to enhance Cd bioavailability and phytoextractability of Cd from contaminated soils. This strain showed a potential for directly solubilizing phosphorous from soils more than 10 folds greater than the control without inoculation. The results of pot experiments revealed that inoculation with B. megaterium significantly increased the extent of Cd accumulation in Brassica juncea and Abutilon theophrasti by two folds relative to the uninoculated control. The maximum Cd concentrations due to inoculation were 1.6 and 1.8 mg Cd g(-1) plant for B. juncea and A. theophrasti after 10 wk, respectively. The total biomass of A. theophrasti was not significantly promoted by the inoculation treatment, yet the total biomass of B. juncea increased from 0.087 to 0.448 g. It is also worth to mention that B. juncea predominantly accumulates Cd in its stems (39%) whereas A. theophrasti accumulates it in its leaves (68%) after 10 wk. The change of the Cd speciation indicated that inoculation of B. megaterium as PSB increased the bioavailabilty of Cd and consequently enhanced its uptake by plants. The present study may provide a new insight for improving phytoremediation using PSB in the Cd-contaminated soils.

Prediction of Cd and Pb toxicity to Vibrio fischeri using biotic ligand-based models in soil.

Biotic ligand-based models to predict site-specific toxicity of Cd and Pb contaminated soil were developed by using a Vibrio fischeri toxicity test. Firstly, competition effect by cations (i.e., Ca, Mg, K) commonly found in soil solution was incorporated into the models. For this purpose, biotic ligand-based model parameters including conditional binding constants of cations and metal ions to binding sites (i.e., biotic ligands) and the fractions of binding sites occupied by the metal ions were determined. Data from aqueous phase toxicity test showed that the difference between model-predicted EC(50) values of Cd and Pb and experimentally determined EC(50) values ranged within a factor of two, suggesting that the developed model parameters were reliable. Secondly, the use of soil solution to predict soil toxicity of Cd and Pb was experimentally verified with freshly spiked and field-aged soils. The results showed linear relationships in both soils, meaning that toxicity of soil solution can be representative of toxicity of soil. Finally, applicability of the developed models in Cd- or Pb-spiked soils was investigated by comparing predicted toxic effects (i.e., % bioluminescence inhibition at given cations and metal activities in soil solution) and experimentally obtained toxic effects determined by Microtox(®) solid phase toxicity test. Our data demonstrate that toxicity of Cd- or Pb-contaminated soil can be predicted by using the developed biotic ligand-based model with the chemical analysis data of soil solution as input data.

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 52.8mg/kg (average 34.1mg/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 (14.4-169.8mg/kg, average 85.9mg/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 (1.4×10(-4)) at OU 6 and the risk at the other OUs ranged from 3.8×10(-5) to 5.7×10(-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.

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 μmol) than that chelated by ethylenediaminetetraacetic acid (EDTA; i.e. 43.7 μmol) in batch experiment using As-adsorbed ferrihydrite. More importantly, about 1.79 μmol 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-Asg(-1)-plant) than the plant grown in the EDTA-treated soil (1.51 mg-Asg(-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.

Study on Heavy Metal Contamination Characteristics and Plant Bioavailability for Soils in the Janghang Smelter Area

Potential risk of heavy metals to various receptors including humans depends on the bioavailability of the heavy metals in soil. In this study, the heavy metal extraction methods using 0.1N HCl and aqua regia were compared with the Tessier`s sequential extraction method to assess whether these two methods can be used to determine the plant-available heavy metal concentrations. The contamination characteristics of copper (Cu), cadmium (Cd), lead (Pb), and arsenic (As) found in soils collected from 75 sites around the closed Janghang smelter were analyzed by extracting heavy metals using 0.1 N HCl, aqua regia, and the Tessier`s sequential extraction method. The portion of metals bioavailable to plants is considered as the sum of the fraction 1 (exchangeable) and the fraction 2 (carbonates binding) of the Tessier`s 5-step sequential extraction method, which were determined to be 3.1 3.82, 0.6 0.15, 20.6 18.78, and 7.0 6.48 mg/kg for Cu, Cd, Pb, and As, respectively, in this study. When the extraction using aqua regia and the Tessier`s extraction method were compared, the extracted Cu and Pb concentrations did not show significant differences, whereas the extracted Cd and As concentrations showed significant differences. These results indicate that the portion of Cd and As in the fraction 5 of the Tessier`s sequential extraction can not be extracted using aqua regia. Using aqua regia, which is the official test method, higher concentrations of Cu, Cd, Pb and As were extracted than the sum of the fraction 1 and 2. The results show that only 9, 40, 39 and 10% of Cu, Cd, Pb and As using aqua regia can be uptaken by plants (i.e., plant-available). Using 0.1N HCl, the portion of Cd equivalent to about 66% the fraction 1 could be extracted, while, with Pb, the portion of the fraction 1 and about 90% of the fraction 2 could be extracted. With As, the portion equivalent to the fraction 1, 2 and 79% of the fraction 3 was extracted, while with Cu, the portion equivalent to the fraction 1, 2, 3 and 20% of the fraction 4 was extracted using 0.1N HCl.

Ecological Risk Characterization in a Military Heavy Metals– and Explosives-Contaminated Site

ABSTRACT Potential ecological risks of two heavy metals (Cu, Pb) and three explosives (TNT, RDX, HMX) were determined for a military gunnery range. Since a portion of the site will be submerged after the construction of a flood control reservoir, risk assessment of the site was conducted over two time points: at the current state and after the construction of the flood control reservoir. Terrestrial plants, terrestrial invertebrates, birds, mammals, and aquatic invertebrates were selected as affected ecological receptors at the study site. Potential noncarcinogenic risk was found only in one terrestrial site for Cu and RDX at both time points: For terrestrial plants, ecological hazard quotient (EHQ) by Cu was 1.22. For terrestrial invertebrates, EHQs for Cu and RDX were 1.38 and 6.47, respectively. EHQs in the aquatic environment increased to some extents after the reservoir construction, but the values were less than 1.0. A comparison between EHQs before and after the construction for the same site shows that the reservoir construction would not significantly increase the overall ecological risk, suggesting that the reservoir construction influenced the potential ecological risks at the study site, but the changes were acceptable. Uncertainties involved in the assessment process were identified and discussed.

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 29.3 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.

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 7.25% 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.

Importance of chemical binding type between As and iron-oxide on bioaccessibility in soil: Test with synthesized two line ferrihydrite

Bioaccessible concentrations of As associated with Fe oxide as different chemical binding types were determined in soils using the in vitro Physiologically Based Extraction Test (PBET). When compared to the five-step sequential extraction data, most of the As extracted by in vitro PBET originated from the amorphous Fe oxide-bound fraction, and more importantly, the bioaccessibility of As ranged from 0 to 58.8% in 24 soil samples. Two batches of ferrihydrite were synthesized separately. For one batch, As was adsorbed onto the ferrihydrite after synthesis; for the other one, As was added while synthesizing ferrihydrite to co-precipitate. The bioaccessible concentration of As determined by in vitro PBET of the former was 415mg of As/kg of ferrihydrite and that of the latter was 67mg of As/kg of ferrihydrite. X-ray photoelectron spectra (XPS) analysis indicated that As-O-Fe bonds were evident in As-associated ferrihydrite sample and especially, As was found within the Fe oxide lattice in the co-precipitated sample. Our data suggest that binding type between As and Fe oxide should be considered when determining the bioaccessibility of As in soil, which, in turn, greatly influences the realistic risk of As present in soil.

Photodegradation of tetracycline and sulfathiazole individually and in mixtures

Antibiotics in environment can be of concern as they can enter the food chain posing risks to ecosystems and human health. Photodegradation has been considered as a promising way of naturally degrading antibiotics in environment. Antibiotics are usually present in mixtures in environment; however, previous studies focused on individual compounds. Therefore, this study investigated the effect of UV irradiation on the degradation of tetracycline (TC) and sulfathiazole (STH) in individual solutions and mixtures. Under dark conditions, the initial masses of TC and STH were reduced by about 35% and 26%, respectively, over a 35 d-reaction period. With UV irradiation TC and STH were completely removed within 14 d and 35 d, respectively, regardless of the initial concentrations. Both the TC and STH removals were faster (i.e., 2-4 times) when they were in mixtures. This may be partly attributed to the byproducts such as sulfate that can promote indirect photolysis and partly to the enhanced hydrolysis due to changes in the solution pH. Overall, this study suggests that when photodegradation is used to remove antibiotics in water, the removal kinetics of antibiotics individually and in mixtures can be considered to develop more efficient treatment technologies.