Use of clay and organic matter contents to predict soil pH vulnerability in response to acid or alkali spills.

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.

Effect of organic substrate and Fe oxides transformation on the mobility of arsenic by biotic reductive dissolution under repetitive redox conditions.

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.

Response surface modeling with Box-Behnken design for strontium removal from soil by calcium-based solution.

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).

Effect of monovalent and divalent ion solutions as washing agents on the removal of Sr and Cs from soil near a nuclear power plant.

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.

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.

Reduction of bioaccessibility of As in soil through in situ formation of amorphous Fe oxides and its long-term stability.

The bioaccessibility of As in soil, rather than its total concentration, is closely related to its potential risk. In this study, the in situ formation of amorphous Fe oxides was applied to As-contaminated soil to induce As-Fe coprecipitates that can withstand the gastric digestion condition of human beings. To promote the formation of Fe oxides, 2% ferric nitrate (w/w) and 30% water (v/w) were introduced, and the pH was adjusted to ~7. The chemical extractability of As in soil was determined using the solubility/bioavailability research consortium method and five-step sequential extraction. In situ formation of Fe oxides resulted in a remarkable increase in the As associated with amorphous Fe oxides, decreasing most of the exchangeable As (i.e., the sum of SO42- and PO43- extractable As), and thereby reducing the bioaccessibility of As. The types of association between As and Fe oxides were investigated using X-ray absorption spectroscopy analysis. Linear combination fit (LCF) analysis demonstrated that As bound to amorphous Fe oxides could exist as coprecipitates with ferrihydrite and schwertmannite after stabilization. The bioaccessibility of the coprecipitated As in soil further decreased as amorphous Fe oxides transformed to crystalline form with time, which was supported by the LCF results showing an increase of goethite in aged soil.

Effect of neutralizing agents on the type of As co-precipitates formed by in situ Fe oxides synthesis and its impact on the bioaccessibility of As in soil.

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.

Inhibition of urea hydrolysis by free Cu concentration of soil solution in microbially induced calcium carbonate precipitation.

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.

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.

Stabilization mechanism of arsenic in mine waste using basic oxygen furnace slag: The role of water contents on stabilization efficiency.

Arsenic stabilization mechanism in a mine waste was investigated using a basic oxygen furnace (BOF) slag. A lab-scale batch test was carried out to stabilize As in the mine waste samples for 1 h, where various amounts of the BOF slag and distilled water were introduced. Different stabilization efficiencies were observed depending on the stabilizing conditions (i.e., BOF slag content and water to mine waste (L/S) ratio). The stabilization efficiencies ranged 75-92% and 92-95% for 5% (w-slag/w-mine waste) and 10% BOF slag treated mine waste samples, respectively. Interestingly, a notable effect of the L/S ratio on the stabilization efficiency was observed (78% at 0.05 L/kg, and 23% at 1.0 L/kg) at the 3% BOF slag treatment. The point of zero charge and the stabilizing pH indicated that the BOF slag surface was negatively charged. Based on the comparison of fresh and Ca-reduced BOF slags, As stabilization mechanism was determined to be adsorption through cation bridges by Ca2+. The Surface analysis using X-ray photoelectron spectroscopy (XPS) and the stabilization experiment conducted at lower pH provided evidence that the hindrance of As adsorption resulted from Ca(OH)2 precipitation on the BOF slag surface when excess water (1.0 L/kg) was added. Such effect of water content seemed to be overcome by providing an excessive amount of the BOF slag. When an ample amount of Ca2+ is provided and pH is maintained around 11, not only As adsorption but also calcium arsenate precipitation occur, and both contributed to the stabilization mechanisms of As.

Applicability of a submersible microbial fuel cell for Cr(VI) detection in water.

Two-chamber microbial fuel cells (MFCs) were used to study the applicability of MFCs for hexavalent chromium (Cr(VI)) detection in water. The microbial acetate oxidation in the anode and the Cr(VI) reduction in the cathode together generated voltages, which were used to indicate the change in Cr(VI) concentrations of the cathode under varying conditions of pH, ionic strength, co-existing Fe(II) concentration, and organic matter concentration. The MFC-based Cr(VI) detector showed a significant change in voltage with increasing Cr(VI) concentration at pH 1 and 2, but not at higher pH conditions. The detector also successfully measured the changes in Cr(VI) concentration at a range of ionic strength (i.e., 10-300 mM), and in the presence of different concentrations of fulvic acid (0-50 mg/L), which was used as a surrogate of organic matters, without interference. The Cr(VI) detection was not interfered by the presence of Fe(II) at the Cr(VI)/Fe(II) ratio of 1:1 and 1:15, but it was interfered at higher ratios (i.e., 1:164 and 1:848). The detector could measure the Cr(VI) concentration from 0.1 to 15 mg/L at pH 2. Overall, the MFC-based Cr(VI) detector may be applied to meet the growing need of real-time Cr(VI) monitoring in water.