Removal of arsenic in tailings by soil flushing and the remediation process monitoring

In order to investigate the optimum conditions for the application of soil flushing of arsenic, a batch test was carried out using EDTA at various concentration and pH levels. Based on the optimum condition derived from the batch test, a column test was conducted to examine the feasibility of the soil-flushing technology under field-equivalent conditions. In this column test, a low flushing solution flow rate showed a significantly higher As removal efficiency (71.6%) than a high flow rate (56.3%). TCLP (toxicity characteristic leaching procedure) and a seed germination test were carried out to monitor the toxicity both during and after the treatment. The finally treated tailings were shown to be significantly remediated, having a reduced toxicity by both the TCLP and seed germination tests.     

Removal of arsenic in tailings by soil flushing and the remediation process monitoring

In order to investigate the optimum conditions for the application of soil flushing of arsenic, a batch test was carried out using EDTA at various concentration and pH levels. Based on the optimum condition derived from the batch test, a column test was conducted to examine the feasibility of the soil-flushing technology under field-equivalent conditions. In this column test, a low flushing solution flow rate showed a significantly higher As removal efficiency (71.6%) than a high flow rate (56.3%). TCLP (toxicity characteristic leaching procedure) and a seed germination test were carried out to monitor the toxicity both during and after the treatment. The finally treated tailings were shown to be significantly remediated, having a reduced toxicity by both the TCLP and seed germination tests.     

Microbial removal of uranium in uranium-bearing black shale

The effects of several conditional factors on efficiency of U bioleaching using an iron-oxidizer, Acidithiobacillus ferrooxidans, from U-bearing black shale (349 mg kg-1 of U) were investigated. When batch-type reactors containing black shale were initially inoculated with the cells, lower pH, higher redox potential and higher amount of aqueous Fe3+ than those of non-inoculated reactor were observed until 200 h. Such development of condition, which was facilitated by microbial activity, can enhance the rate and extent of U leaching from the solid substrate. However, under the condition of enough nutrients and energy source (Fe2+) supplied, indigenous Fe-oxidizers in the non-inoculated black shale were activated over time. They exerted almost same influence on the leaching efficiency with the inoculated samples after 250 h. Low initial Fe2+ supply (5 g l-1) and no addition of inorganic nutrients resulted in nearly identical extent of U leaching with that of 9 g l-1 of initial Fe2+ and nutrients supply. The results indicate that, in a practical process of bioleaching, the expenses for Fe2+ and nutrients addition can be reduced.     

The effects of different carbon sources on microbial mediation of arsenic in arsenic-contaminated sediment

Changes in speciation and mobility of As by indigenous bacteria in As-contaminated sediments (339 mg/kg) from an abandoned Au–Ag mine area in Korea were investigated after biostimulation with a variety of carbon sources, including acetate, lactate and glucose in batch experiments. Sequential extraction analysis designed to determine the form of As occurrence revealed that 40 and 47% of As were present in the sediment as Fe-associated and residual fractions, respectively. After 22-day incubation with acetate and lactate, the presence of indigenous bacteria increased the amount of total dissolved As from both Fe-associated and residual fractions in the sediment. More than 99% of dissolved As existed as As(V) in biotic slurries in contrast to sterile controls (less than 50% of total dissolved As), which indicated that indigenous bacteria transformed some dissolved As(III) to As(V). In real environments, depending on the pH, microbially-produced aqueous As(V) may be either immobilized through adsorption or reduced to As(III) after migration to the anoxic subsurface.     

The effects of <Emphasis Type="Italic">Acidithiobacillus ferrooxidans</Emphasis> on the leaching of cobalt and strontium adsorbed onto soil particles

Bioleaching from soil artificially contaminated with analogues of radionuclides, Co and Sr, was carried out using a Fe-oxidizing bacterium, Acidithiobacillus ferrooxidans. Due to bacterial metabolism, the pH and dissolved Fe³⁺ concentration in a biotic slurry decreased and increased respectively, over time, but the concentrations of Co and Sr extracted from the soil showed no significant enhancement compared with those under abiotic control. In both cases, Co and Sr were leached from the soil during the initial period of the experiment, due to the initially low solution pH of 2.0, and the dissolved concentrations remained almost constant for the duration of the experiment (300 h). Since oxidation of Fe²⁺ by A. ferrooxidans led to the production of Fe precipitates and colloidal suspensions, the Co and Sr extracted into solution were most likely re-adsorbed onto the Fe solids. Also, A. ferrooxidans, without an external supply of Fe²⁺, extracted almost equal or greater amounts of Co and Sr from the soil than when Fe²⁺ was supplied. Under the same leaching conditions, the extent of Sr removal was much lower than that of Co. On the contrary to the high efficiency of microbial metal leaching in biohydrometallurgy for low-graded sulfide ores, which has been widely documented, conventional bioleaching techniques with A. ferrooxidans supplied with enough Fe²⁺ showed low efficiency for the removal of radionuclides loosely bound onto soil particle surfaces.     

The role of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans in arsenic bioleaching from soil

Bioleaching of As from the soil in an abandoned Ag–Au mine was carried out using Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. A. ferrooxidans is an iron oxidizer and A. thiooxidans is a sulfur oxidizer. These two microbes are acidophilic and chemoautotrophic microbes. Soil samples were collected from the Myoungbong and Songcheon mines. The main contaminant of the soil was As, with an average concentration of 4,624 mg/kg at Myoungbong and 5,590 mg/kg at Songcheon. A. ferrooxidans and A. thiooxidans generated lower pH conditions during their metabolism process. The bioleaching of As from soil has a higher removal efficiency than chemical leaching. A. ferrooxidans could remove 70 % of the As from the Myoungbong and Songcheon soils; however, A. thiooxidans extracted only 40 % of the As from the Myoungbong soil. This study shows that bioleaching is an effective process for As removal from soil.     

Enhancement of arsenic mobility by indigenous bacteria from mine tailings as response to organic supply

Arsenic leaching by indigenous bacteria in abandoned Au-Ag mine tailings which contained approximately 3200 mg/kg of As was investigated after supply of various organic substrates. Sequential extraction analysis designed to determine the mode of As occurrence in the tailings revealed that most As (90%) was closely associated with the Fe fraction. When glucose was supplied as a C source, indigenous bacteria significantly enhanced the extent of As release from the tailings into solution under both aerobic and anaerobic conditions. Anaerobic indigenous bacteria leached more amount of As from the tailings than aerobes. Highly positive correlation between the extracted amounts of As and Fe implied that microbial dissolution of Fe(III)-oxides, whether it was ligand- and proton-promoted dissolution or reductive dissolution, might be dominantly responsible for the As release. Bacterial strains which were resistant to up to 100 mM As(V) was aerobically isolated from the tailings. One of the isolates appeared to reduce some aqueous As(V) to likely As(III) in a batch type experiment, which indicated that indigenous bacteria can mediate the electrochemical speciation and thus the mobility of As in the tailings. The results suggest that indigenous bacteria in As-contaminated tailings can increase As mobility from the solid media when microbially available organic substrates are supplied, and thus enhance the risk of As dispersion to nearby soil, sediment and groundwater.     

Baseline Study on Essential and Trace Elements in Polished Rice from South Korea

In 2000, 63 (polished) white rice samples were collected in eight administrative areas all over South Korea and analyzed for 16 elements by inductively coupled plasma atomic emission spectrometry (ICP-AES). Potassium had the highest content, next to Mg, Ca, Si, Zn, Na, Al and Fe. Most of the samples contained worldwide average concentrations of essential and trace elements in rice grains reported by various researches. For inter-area differences in those elements in the rice, the statistical analysis showed no significant differences (p > 0.05) among the eight administrative areas, suggesting that inter-area differences were not substantial in most cases. Thus, the present data can be used as national background levels of elements in rice produced in South Korea. Using the published data on daily consumption of rice in South Korea, it was possible to estimate the daily intake of As, Cd, Cu, Pb and Zn via rice. The results showed that a regular consumption of rice produced in Korea plays an important role in accumulation of essential and trace elements in Korean, especially for farm-households consuming relatively large amounts of rice.     

Competitive adsorption characteristics of Co2+, Ni2+, and Cr3+ by IRN-77 cation exchange resin in synthesized wastewater

Adsorption properties of Co2+, Ni2+, and Cr3+ on an Amberlite IRN-77 cation exchange resin were investigated in batch systems. Levels of adsorption rapidly approached an equilibrium state within 1 h. The adsorption characteristics of each metal onto the resin were accurately represented by Langmuir isotherms. Co2+ and Ni2+, which have an equivalent electrovalence, displayed similar levels of adsorption onto the resin when they coexisted in the solution. However, when Cr3+ was added to the solution it competitively replaced Co2+ and Ni2+ ions that had been previously adsorbed onto the resin, resulting in the desorption of these metals into the solution. The result was likely due to a higher adsorption affinity of Cr3+ relative to Co2+ and Ni2+. This implies that interactively competitive adsorption of multi-cations onto the resin should be thoroughly considered when contemplating the efficient operation of an ion exchange process in the treatment of industrial wastewater.     

Microbiological reduction of hexavalent chromium by indigenous chromium-resistant bacteria in sand column experiments

Indigenous bacteria that are resistant to high concentrations of Cr(VI) were isolated from a Cr-contaminated sediment. Sand column experiments were conducted using the isolated bacteria to investigate microbial effects on Cr(VI) reduction in open systems that simulated subsurface conditions. The indigenous Cr-resistant bacteria appeared to reduce Cr(VI) in the column experiments. When 12 mg/L of Cr(VI) was injected into the columns, the removal efficiencies of Cr(VI) by the isolated bacteria were 39.1%, 62.5%, and 63.6% at 24, 48, and 72 h retention times of Cr(VI) solution, respectively. These results imply that the linear velocity of groundwater or pore water should be less than 0.63 cm/h for effective removal of Cr(VI) in subsurface conditions. In comparison, the noninoculated control column did not show a significant variation in dissolved Cr(VI) concentration. The results indicated that reduction of Cr(VI) was occurring in the column due to the activity of the indigenous bacteria.