INTERNATIONAL ARTICLES

Iron (Fe) oxide precipitation is a promising method for stabilizing arsenic (As) in contaminated soils; however, the addition of salts during the process can negatively affect soil functions. This study investigated the effects of in situ Fe oxide precipitation on As stabilization and the impact of salt stress on soil functions and microbial communities. Fe oxide precipitation reduced the concentration of bioaccessible As by 84% in the stabilized soil, resulting in the formation of ferrihydrite and lepidocrocite, as confirmed by XANES. Nevertheless, an increase in salt stress reduced barley development, microbial enzyme activities, and microbial diversity compared to those in the original soil. Despite this, the stabilized soil exhibited natural resilience and potential for enhanced microbial adaptations, with increased retention of salt-tolerant bacteria. Washing the stabilized soil with water restored EC_1:5 to the level of the original soil, resulting in increased barley growth rates and enzyme activities after 5-d and 20-week incubation periods, suggesting soil function recovery. 16 S rRNA sequencing revealed the retention of salt-tolerant bacteria in the stabilized soil, while salt-removed soil exhibited an increase in Proteobacteria, which could facilitate ecological functions. Overall, Fe oxide precipitation effectively stabilized soil As and exhibited potential for restoring the natural resilience and ecological functions of soils through microbial adaptations and salt removal.

Food waste compost (FWC) is a sustainable recycling approach employed in soil media, offering extensive advantages to urban areas by promoting resource circulation and effectively managing water pollution. To improve value, Bacillus subtilis (B. subtilis)-induced FWC-based biomedia (BIBM_FWCs) was produced via a secondary treatment involving selective meso-thermophilic stages. During the production of BIBM_FWCs, physicochemical properties were found to have favorable characteristics for the efficient removal of metal ions. The produced organic-carbonate complex structure demonstrated the synergistic effect involving simultaneous sorption/precipitation mechanisms for the removal of Pb(II) and Cr(III). Also, the dose of B. subtilis has an impact on the pseudo-second-order (PSO) and intra-particle diffusion (IPD) reaction, leading to distinct removal capacities for Pb(II) and Cr(III) [24.26–24.74 mg g⁻¹ in Pb(II) and 12.7–23.93 mg g⁻¹ in Cr(III)]. Furthermore, B. subtilis, an inducing mediator for microbial metabolites, exhibits the potential to facilitate the removal of Pb(II) and Cr(III) through biological modification of raw materials, which are transformed, facilitating the presence of hydroxyl groups, immobilizing metal ions, and enabling ion exchange via biogenic carbonate formation processes. Finally, the developed BIBM_FWCs could be used as a nature-based solution (NBS) material without in-situ pH control.

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.

The overuse of chelating soil washing agents for removal of heavy metal can release soil nutrients and negatively affect organisms. Therefore, developing novel washing agents that can overcome these shortcomings is necessary. In this study, we tested potassium as a main solute of novel washing agent for cesium-contaminated field soil, owing to the physicochemical similarities between potassium and cesium. Response surface methodology was combined with a four-factor, three-level Box–Behnken design to determine the superlative washing conditions of the potassium-based solution for the removal of cesium from the soil. The parameters that were considered were the following: potassium concentration, liquid-to-soil ratio, washing time, and pH. Twenty-seven sets of experiments were conducted using the Box–Behnken design, and a second-order polynomial regression equation model was obtained from the results. Analysis of variance proved the significance and goodness of fit of the derived model. Three-dimensional response surface plots displayed the results of each parameter and their reciprocal interactions. The washing conditions that achieved the highest cesium removal efficiency (81.3%) in field soil contaminated at 1.47 mg/kg were determined to be the following: a potassium concentration of 1 M, a liquid-to-soil ratio of 20, washing time of 2 h, and a pH of 2.

Acid or alkali spills destroy the physicochemical properties of soils and cause irreversible damage to their ecological functions. This study examined changes in physicochemical properties (i.e., organic matter, clay content, and cation exchange capacity (CEC)) as well as pH buffering capacity (indicator of soil ecological function) of 20 field soils in response to the spills. Also, we identified the characteristics of soils vulnerable to the spills. Although the spills did not substantially change the clay content, organic matter decreased by approximately 50%, consequently resulting in a 41% decrease in pH buffering capacity. When we classified soils into three groups based on soil properties and pH buffering capacity, the extent of change in soil properties by spill differed by group. As the organic matter content increased or clay content decreased, the soil tended to be more vulnerable to spills in terms of the degree to which the soil function was changed. Considering that the protonation-deprotonation characteristics of clay sized fraction were not remarkably changed by the spills, this result was mainly attributed to the dissolution of organic matter. Together with the successful prediction of CEC and pH buffering capacity by multiple linear regression models using organic matter and clay content, our findings enable the easy classification of soils based on their vulnerability and site-specific management of areas with a high probability of spills.

The upstream of Nakdong River is contaminated by heavy metals such as Cd, Cu, Zn, As, and Pb. Although the origin of the contamination is unequivocal, it is suspected that the heavy metals have been leached from several mine tailings and a refinery. Here, receptor models, absolute principal component score (APCS) and positive matrix factorization (PMF), were used to identify the contamination sources. Source markers representing each source (factor) were investigated using correlation analysis for five major contaminants (Cd, Zn, As, Pb, and Cu) and identified as following: Cd and Zn for the refinery (factor 1), As for mine tailings (factor 2). The categorization of sources into two factors was statistically validated via the cumulative proportion and APCS−based KMO test score with the values >90 % and > 0.7 (p < 0.001), respectively. High R² values of linear regressions between the predicted data from receptor models and observed data indicate the reliability of the model prediction; moreover, the predicted initial concentrations of contaminants were validated using a sediment sample collected from near the refinery (chi-test: p > 0.200). Concentration distribution and source contribution using GIS revealed the heavy metal contaminated zones affected by the precipitation.

The mobility of arsenic (As) in soil is highly affected by the change in the form of iron oxides present in the soil, which has a strong correlation with the change in redox potential. In this study, the altered mobility of As under repetitive redox conditions and the effect of organic substrates (i.e., glucose) on such change during four anoxic-oxic cycles were studied. During the 1st anoxic period, 37.1% of soil As was released into the soil solution, but the As in the soil solution decreased to 25.2% after the 1st oxic period. Moreover, the As in the soil solution further decreased during the 2nd to 4th oxic periods, indicating further re-adsorption of aqueous As. The analysis of As speciation revealed that inorganic arsenate (As(V)) increased under the redox-oscillating conditions, probably due to the depletion of electron donors. When glucose was re-spiked at the beginning of the 4th cycle, aqueous As increased to 47.3% again in the anoxic period and decreased to 27.6% in the subsequent oxic period, indicating inhibition of As re-adsorption. During the same period, the amount of highly sorptive As(V) in the solution decreased sharply to less than 3.3%. The X-ray absorption near edge structure analysis with linear combination fitting confirmed that the transformation of Fe oxides to poorly crystalline structures such as ferrihydrite occurred during repetitive cycles. These results imply that the mobility of As can be increased in As-contaminated redox transition zones by the introduction of rainfall with labile organics or by the fluctuation of organic-rich groundwater.

The biotic ligand model (BLM) was applied to derive ecotoxicologically acceptable Cu concentrations at 12 monitoring stations in the Han River Basin, South Korea, considering temporal variations in water characteristics. During the monitoring period, pH, dissolved organic carbon (DOC), and water temperature varied instantaneously, resulting in spatiotemporal variations in the half-maximal effective concentrations (EC₅₀[Cu]_T) of Daphnia magna. The effect of dissolved Ca²⁺ concentration was evaluated to determinate EC₅₀[Cu]_T using the Visual MINTEQ 3.1 speciation model. Dissolved Ca²⁺ concentration was directly proportional to EC₅₀[Cu]_T values, indicating that a higher Ca²⁺ in the solution will result in the lesser toxic effects on D. magna due to the competition between Ca²⁺ and Cu²⁺ ions. The Ca²⁺ concentration was set at 0.4 mM while deriving EC₅₀[Cu]_T, which is the geometric mean concentration in the Han River Basin. The lower confidence limit (LCL) of EC₅₀[Cu]_T was 28.7–67.8 μg/L in the monitoring stations. Among the water characteristics, DOC was more strongly positively correlated with EC₅₀[Cu]_T than that with pH and temperature. DOC concentration was significantly related to Cu²⁺ activity, pH was less explicitly related to EC₅₀[Cu]_T than to DOC, and water temperature had the weakest correlation coefficient. Compared to the 5% hazardous concentration (HC5) derived from the toxicity data for 171 aquatic species and Cu criteria in different countries, the computed LCL concentrations had similar orders of magnitude. With more information on actual Ca²⁺ concentrations at monitoring sites, a more accurate Cu concentration that reflects spatiotemporal variations of water characteristics can be obtained.

Lactate-based carbon chain-elongating open culture systems have gained increasing research attention as they help to upgrade carbon-rich wastewater to the valuable platform chemical n-caproate. However, microbial competitions occurring in this system have markedly lowered n-caproate specificity. This study investigated possible microbial competitions occurring during long-term operating periods of anaerobic membrane bioreactors (AnMBRs) inoculated with shaped chain-elongating microbiomes to develop l-lactate-to-n-caproate conversion systems with high specificity. We demonstrated both semicontinuously (Sc-AnMBR) and continuously (C-AnMBR) fed AnMBRs and identified three possible competitions between (i) potential chain-elongating bacterial species, (ii) chain-elongating and propionate-fermenting bacterial species, and (iii) chemolithotrophic acetogens and hydrogenotrophic methanogens. In both AnMBRs, Caproiciproducens galactitolivorans achieved an ecological niche among chain-elongating species, occupying more than 79% of the relative abundance. During the Sc-AnMBR operating period, the n-caproate specificity was only 32.4% due to the intervention of the propionate-fermenting species Propionibacterium freudenreichii in l-lactate consumption. In contrast, during the C-AnMBR operating period, C. galactitolivorans prevailed, while P. freudenreichii was thoroughly excluded; thereby, the n-caproate specificity reached 81.3%. Even though propionate fermentation is thermodynamically favored in lactate-fed anaerobic bioreactor systems, this study showed that chain elongation reaction selectivity can be highly improved by controlling the kinetic parameters and suggested a bioreactor operation strategy to ensure high n-caproate specificity.

n-Caproate production by reactor microbiomes is receiving more attention due to its high economic and environmental benefits. However, the ecological complexity within the bioreactor obstructs the development of functional microbiome shaping technology, which is expected to further unlock the potential of microbiomes. This study adopted the “Design-Build-Test-Learn approach” to develop a fast and reliable microbiome shaping technology. In the first step, 10 mM n-butyrate was a better electron acceptor than 10 mM acetate when 50 mM L-lactate was used as an electron donor, but the n-caproate selectivity was only 33.9%. This was ascribed to the high H₂ and CO₂ partial pressure causing a high homoacetogenic activity of the microbiome, hindering n-caproate production. The second step was conducted by continuously removing headspace CO₂, and as a result n-caproate selectivity increased to 68.6%. This process was repeated five times to apply selection pressures, and a stable n-caproate specificity (i.e., 57.3–62.6%) was achieved using 50 mM L-lactate and 10 mM n-butyrate. Finally, a n-caproate-producing microbiome with a high productivity (10.0 gCOD/L/day, 0.19 g/L/h) was shaped in a substantially shorter start-up period (84 h) than other studies had achieved. An 16S rRNA sequencing analysis of the shaped microbiome identified the abundance of C. carboxidivorans, which is a chain-elongating bacterial species.

Solutions of monovalent and divalent ions, including calcium, magnesium, ammonium, and potassium, were tested in the removal of Sr and Cs from soil near a nuclear power plant. The Ca²⁺ and K⁺ solutions exhibited removal efficiencies of 68.2% and 81.3% for Sr and Cs, respectively. This high performance was probably due to the physicochemical similarities between ‘Ca and Sr’ and ‘K and Cs’. Alternatively, the Mg²⁺ and NH₄⁺ solutions performed much worse, despite having the same valences as Ca²⁺ and K⁺, respectively. Ca²⁺ and K⁺ solutions could also simultaneously remove cationic toxic metals present with the nuclides, albeit much less efficiently (30–40%). For anionic metalloid As and anionic toxic metal Cr, the efficiency was even lower (< 20%). The five-step sequential extraction experiment confirmed that all chemical forms of Sr and Cs, except the residual form, were extensively removed by the Ca²⁺ and K⁺ solutions^, respectively. For comparison, widely used washing agents exhibited removal efficiencies of 25–30%. Notably, Fe²⁺ and Mn²⁺ ions were hardly detected in the Ca²⁺ solution, while their concentrations were much higher in the common washing agents, suggesting the involvement of an ion-exchange mechanism in Sr and Cs removal, rather than a Fe/Mn oxide dissolution mechanism.

Owing to its physicochemical similarity to strontium (Sr), calcium (Ca) was tested as a key component of a soil washing solution for Sr-contaminated soil collected near a nuclear power plant. A four-factor, three-level Box–Behnken experimental design combined with response surface modeling was employed to determine the optimal Sr washing condition for Ca-based solution. The Ca concentration (0.1–1 M), liquid-to-soil ratio (5–20), washing time (0.5–2 h), and pH (2.0–7.0) were tested as the independent variables. From the Box–Behnken design, 27 sets of experimental conditions were selected, and a second-order polynomial regression equation was derived. The significance of the independent parameters and interactions was tested by analysis of variance. Ca concentration was found to be the most influential factor. To determine whether the four variables were independent, three-dimensional (3D) response surface plots were established. The optimal washing condition was determined to be as follows: 1 M Ca, L/S ratio of 20, 1 h washing, and pH = 2. Under this condition, the highest Sr removal efficiency (68.2%) was achieved on a soil contaminated with 90.1 mg/kg of Sr. Results from five-step sequential extraction before and after washing showed that 84.0% and 82.9% of exchangeable and carbonate-bound Sr were released, respectively. In addition, more tightly bound Sr, such as Fe/Mn oxides-bound and organic matter-bound Sr, were also removed (86.2% and 64.5% removal, respectively).

A site-specific, ecologically acceptable concentration of Cu in soil pore water was determined with four trophic levels of soil-residing organisms. Specifically, soil pore water was periodically collected from a site contaminated with heavy metals using in-situ samplers. Dissolved Cu concentration, Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl⁻, SO₄²⁻, NO₃⁻, dissolved organic carbon, pH, and temperature were analyzed to derive a half-maximal effective concentration of Cu (EC₅₀[Cu]_T) using a biotic ligand model (BLM). The BLM parameters, such as binding constants (logK_XBL) and the fraction of biotic ligand sites occupied by Cu ions (f), were adapted from previous studies. The EC₅₀{Cu²⁺} values were used to construct a species sensitivity distribution (SSD) curve from which the hazardous concentration, protecting 95% of the soil-residing organisms (HC5), was determined. Using ten BLM-based acceptable concentrations of Cu obtained by combining BLM and SSD, time series analysis was conducted with the fixed monitoring benchmark method to obtain maximum Cu concentration as an endpoint exhibiting no-adverse-effect which was found to be 0.084 mg/L of Cu in soil pore water at the test site. This study provides a systematic tool for determining an ecologically acceptable concentration of Cu in the soil by incorporating soil pore water chemistry and time series analysis.

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.

Application methods (i.e., pouring and mixing method) of Microbially Induced Calcite Precipitation (MICP) and its effect on wind erosion were investigated on four soil types (i.e., medium sand, fine sand, loamy fine sand and loam). With mixing method, calcite precipitated evenly throughout the upper part (0 – 5 cm) of all the soils tested, but with pouring method, only medium sand showed even calcite distribution. The reason can be ascribed to the limited permeability of MICP-inducing solution (i.e., calcium, urea and Sporosarcina pasteurii) through loamy fine sand and loam due to their low hydraulic conductivity (i.e., < 10^-5 cm/s). Moreover, bacterial penetrability was also reduced by calcium (i.e., 70 to 20%) in fine sand. Hence, pouring method for medium sand and mixing method for the others were applied with various MICP-inducing solution concentrations (i.e., 0.1 to 1 M of urea and calcium). When exposed to wind of 15 m/s after MICP application, 0.25 M solution in medium and fine sand, and 0.1 M solution in loamy fine sand and loam showed little or no soil loss. The results suggest that a proper application method be chosen considering soil properties that affect even calcite distribution to mitigate soil erosion.

Removal mechanisms, precipitation and adsorption, of lead (Pb) and arsenic (As) by basic oxygen furnace (BOF) slag were studied at pH 7–pH 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.

The bioaccessibility of heavy metals in soil is closely related to their potential risk. Therefore, developing techniques for reducing it needs considerable attention. In this study, we aimed to co-precipitate soil As(V) through an in situ formation of Fe oxides, thereby reducing its bioaccessibility. Soil As(V) was co-precipitated by introducing 2% Fe-nitrate (w/w) and 30% water (v/w) into soil at pH ~7. Two different neutralizing agents (NaOH and CaO) were used to induce the precipitation of Fe oxides, and their effects on the speciation of As were investigated. In all the stabilized soils, the exchangeable As fraction decreased, and the fraction of As bound to amorphous Fe oxides increased by a factor of more than 1.4. In contrast, a marked decrease in bioaccessibility of As was achieved using NaOH (40% to 7%). X-ray absorption spectroscopy analysis demonstrated that highly bioaccessible forms of calcium iron arsenate (yukonite and arseniosiderite) could be generated in CaO-stabilized soil. Our study found that neutralizing agents may play an important role in stabilizing As(V) and lowering its bioaccessibility through determining the type of formed Fe oxides in soil.

Urea hydrolysis is an initiating step of microbially induced calcium carbonate precipitation (MICP) which can be used as a stabilization technology in heavy metals contaminated soil. In this study, inhibition of urea hydrolysis was investigated in Cu-contaminated soil. At soil Cu concentration from 0 to 1000 mg/kg, the amount of urea hydrolyzed (i.e., initial urea 450 mM) ranged from 449.3 ± 1.4 to 10.5 ± 0.8 mM. Correspondingly, decrease in calcium carbonate precipitation was commensurate with the inhibition of urea hydrolysis. Interestingly, 2.75 times more urea were hydrolyzed in 350 days-aged soil than in freshly spiked soil even at the same soil Cu concentration of 250 mg/kg, suggesting the inhibitory effect of Cu in soil solution. Indeed, the concentrations of Cu in soil solution were 4.9 ± 0.1 and 21.0 ± 0.3 mg/L, respectively. Since MICP application involved an increase in Ca²⁺ concentration in soil, its effect was also determined. In the freshly spiked soil with 250 mg-Cu/kg, the Cu concentration in the soil solution increased from 7.6 ± 0.1 to 21.0 ± 0.3 mg/L as the calcium concentration increased from 0 to 450 mM. Accordingly, urea hydrolysis was significantly reduced from 217.5 ± 59.0 to 11.9 ± 0.2 mM. The effect of pH was also determined, showing that 32.8 ± 3.4 and 205.9 ± 32.5 mM of urea was hydrolyzed at soil pH of 4.5 and 7.8, respectively. The reason was attributed to the great difference in free Cu concentration in soil solution (i.e., 3.3 and 0.3 mg/L at pH 4.5 and 7.8, respectively). The relationship between amounts of urea hydrolyzed and free Cu concentrations was established and half-maximal inhibition concentration (IC₅₀) of free Cu concentration in soil solution was predicted to be 0.39 mg/L.

Purpose

Soil loss by rainfall is a serious problem in civil and environmental engineering. In this study, microbially induced calcite precipitation (MICP) was applied to reduce rainfall-induced soil loss. Furthermore, the effects of particle size and organic matter content were investigated.

Materials and methods

A mixture of Sporosarcina pasteurii, 450 mM urea, and 450 mM calcium ions was introduced to sand and sandy loam with 2.6% organic matter content to induce MICP. Artificial rainfall and penetrometer tests were conducted to analyze the soil loss and surface strength, respectively, of the MICP-applied soils.

Results

As MICP was applied, the concentration of CaCO₃ precipitates increased linearly (9.8 mg CaCO₃/g-soil/application), but the strength of the soil reached a plateau of 23.9 ± 1.2 N/mm after five repeated applications (54.5 ± 3.6 mg CaCO₃/g-soil). Only after two repeated MICP applications, up to 84% of reduction in loss rate was accomplished in sand under the worst conditions (rainfall intensity of 75 mm/h, slope of 15°), while only 58% of reduction was obtained after five repeated applications in sandy loam with 2.6% organic matter. For the same amount of CaCO₃ precipitates, the strength was higher in sand with larger particle size. Lower organic matter content led to the higher strength. SEM revealed that larger CaCO₃ precipitates were obtained in sand with lower organic matter content.

Conclusion

Our results indicate that the effect of MICP on the prevention of rainfall-induced soil loss is promoted when the particle size is larger and organic matter content is lower.

Biotic ligand model (BLM) was extended to predict the toxicity of inorganic arsenate (iAs(V)) to the luminescent bacteria, Aliivibrio fischeri. As the pH increased from 5 to 9, the HAsO₄^2- form predominated more than the H₂AsO₄^– form did, and the EC₅₀[As]_T (50% effective iAs(V) concentration) decreased drastically from 3554 ± 393 to 39 ± 6 μM; thus, the HAsO₄^2- form was more toxic to A. fischeri than H₂AsO₄^–. As the HPO₄^2- activity increased from 0 to 0.44 mM, the EC₅₀{HAsO₄^2-} values (50% effective HAsO₄^2- activity) increased from 31 ± 6 to 859 ± 128 μM, indicating that the toxicity of iAs(V) decreased, owing to the competition caused by the structural similarity between iAs(V) and phosphate ions. However, activities of Ca²⁺, Mg²⁺, K⁺, SO₄^2-, NO₃^–, and HCO₃^– did not significantly affect the EC₅₀{HAsO₄^2-} values. The BLM was reconstructed to take into account the effects of pH and phosphate, and the conditional binding constants for H₂PO₄^–, HPO₄^2-, H₂AsO₄^–, and HAsO₄^2- to the active binding sites of A. fischeri were obtained; 3.424 for logK_XH2PO4, 4.588 for logK_XHPO4, 3.067 for logK_XH2AsO4, and 4.802 for logK_XHAsO4. The fraction of active binding sites occupied by iAs(V) to induce 50% toxicity (f_mix^50%) was found to be 0.616.

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 (R² = 0.997) with a limit of quantification of 0.2 μg. A correction factor (CF) for compensating the spectral interference between As-Kα and Pb-Lα was determined by considering the slope between the X-ray intensity measured at a Bragg angle of 48.781° for As-Kα 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.

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.

Pyrosequencing analyses to determine soil bacterial communities were conducted with forty-two soil samplescollected from rice paddy and forest/farmland soils (Group A and B, respectively) at a long-term As-con-taminated site. Soil physicochemical properties, such as the concentrations of As, Fe, Al, and Mn, pH, organicmattercontent,andclaycontent,werefoundtobesignificantlydifferentwithlanduse,andmoreimportantly,strongly affected thebacterial community structure of the soil samples. When fitting the soil properties onto anonmetric multidimensional scale plot of soil bacterial communities, clay content was found to be the mostimportant factor in clustering the bacterial communities (R2=0.4831,p-value=0.001). PhylumChloroflexi(-1.03 of bioplot score) andPlanctomycetes(1.31 of bioplot score) showed a significant relationship with claycontentinsoilsamples.Interestingly,thebacterialphylotypeslinkedtoclaycontentwereonlyfoundinthesoilsamplesofgroupBwithlowclaycontent,andhadastrongrelationshiptoAscontaminationintheredundancyanalysis and the correlation analysis.Our results suggest that clay content seems to be negatively related to Ascontamination in soils, which, in turn, strongly influences the structure of bacterial communities in As-con-taminated soil.

Meeting the regulations based on the short-term leaching tests may not necessarily assure the environmental and human health safety of reusing mine wastes. This study investigated heavy metal leachability of four metal mine waste samples (e.g., Z, Y, H, and M) and human health risk of reusing them as construction materials. The heavy metal leachability did not depend on the total heavy metal contents. For example, the Z sample contained greater amounts of As and Fe than Zn, but the leachates contained only Zn at a detectable level. This can be attributed to the crystalline structure and heavy metal fractions of the mine wastes. The leaching test results suggested that the four mine waste samples are potentially reusable. But the Z and M samples reused in industrial areas imposed carcinogenic risks. This was largely attributed to As that is exposed via dermal contact. The Y and H samples reused in residential areas imposed carcinogenic risk. The major exposure route was the ingestion of crops grown on the mine wastes and Cr was the major concern. The two-stage assessment involving leaching tests and risk assessment can be used to promote safe reuse of mine wastes.

This study presents a promising approach that enhances the sludge fermentation by using basic oxygen furnace (BOF) slag as analkaline source for the first time.BOF slag added to thereactors could maintain astable alkaline condition due to continuous release of Ca(OH)2 from slag. The reactor pH could be adjusted to a target value by the choice of the BOF slag dose. Concentrations of soluble chemical oxygen demand (sCOD) and short-chain carboxylates (SCCs) were substantially increased in the presence of BOF slag. At a BOF slag mass to sludge volume ratio of 1/10g slag/L sludge, the reactor pH was maintained at 10 and the concentration of SCCs produced was the highest (i.e., 3510mg COD L−1 from 14,000mg VS L−1 of sludge mixture), followed by B/S ratios of 1/20, 1.50, 1/5, and 1/2.5g slag L−1 sludge with reactor pH of 9.4, 8.9, 10.5, and 11, respectively. Our data suggestthatthe pHvalue thatbest facilitates the degradation ofsludge into SCCs and inhibit theconversion of SCCs into biogas is around 10. Interestingly, compositions of the accumulated SCCs varied greatly depending on the BOF slag dose. BOF slag showed phosphorus removal ability due to enhanced precipitation of Ca-PO43–P complexes, which significantly lowered PO43− concentration of the reactor effluent.

The effectiveness of in situ stabilization in the long-term As-contaminated soil was assessed. In situ stabilization of As was conducted through a Fe-based sorbent amendment. Chemical extractability of As was first determined by solubility/bioavailability research consortium extraction method and any change in human health risk through oral ingestion was characterized. Also, nonspecifically bound As in soil was determined by five-step sequential extraction. The results indicate that such extractable fractions of As decreased, and consequently risk through oral ingestion decreased probably due to hematite transformed from both the goethite in the original soil and the Fe-based sorbent, which was identified through the X-ray absorption spectroscopy. In ecotoxicity test with Hordeum vulgare, root and shoot elongation and germination rate decreased which was contrary to the chemical extraction data. Such increase in As toxicity is because of increased exchangeable Ca²⁺concentration causing As accumulation in the membrane surface of H. vulgare. Also, adsorption of phosphorus onto the Fe-based sorbent decreased available phosphorus concentration causing phosphorus deficiency for growth. Our results demonstrate that the effectiveness of in situ stabilization should be evaluated by means of both chemical extractability and biological response, as chemical analysis alone may not be sufficient to assess the ecotoxicity.