Selenium chemistry in amorphous iron (hydr)oxide

Soil Science and Plant Nutrition

ISSN: 0038-0768 (Print) 1747-0765 (Online) Journal homepage: http://www.tandfonline.com/loi/tssp20

Selenium chemistry in amorphous iron (hydr)oxide-applied soil as influenced by redox potential (Eh) and pH Md. M. Rashid , Yumei Kang & Katsutoshi Sakurai To cite this article: Md. M. Rashid , Yumei Kang & Katsutoshi Sakurai (2002) Selenium chemistry in amorphous iron (hydr)oxide-applied soil as influenced by redox potential (Eh) and pH, Soil Science and Plant Nutrition, 48:2, 261-269, DOI: 10.1080/00380768.2002.10409199 To link to this article: http://dx.doi.org/10.1080/00380768.2002.10409199

Published online: 22 Nov 2011.

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Date: 10 May 2016, At: 21:52

261

Soil Sci. Plant Nutl:. 48 (2), 261-269,2002

Selenium Chemistry in Amorphous Iron (hydr )oxide-Applied Soil as Influenced by Redox Potential (Eh) and pH Md. M. Rashid, Yumei Kang*, and Katsutoshi Sakurai* The United Graduate School of Agricultural Science. Ehillle University. Matsllyallla, 790-8566 Japan; alId *Faculty of Agriculture. Kochi University, Nankoku, 783-8502 Japan

Downloaded by [Kochi University] at 21:52 10 May 2016

Received September 12, 2001; accepted in revised form November 28,2001

A laboratory study was carried out that focused on the stability of immobilized Se in amorphous iron (hydr)oxide-applied soils in relation to the redox potential (Eh) and pH. Applied soils were incubated at 28"C (± 0.1) under submerged conditions. Without the addition of a microbial nutrient source, no significant changes in the incubated soil Eh and, hence, in the amount of soluble Se were observed. On the other hand, the Eh changed within a few days of incubation with the addition of 1% glucose to the samples. Under oxidative conditions (Eh > 200 mV), the amount of soluble Se in the incubated samples ranged from 2 to 22 jJ.g L -1. Under mildly oxidative conditions (200 to 0 mV), the Eh decreased and pH increased simultaneously, resulting in a subsequent increase in the amount of soluble Se. After 28 d of incubation, the Eh drastically decreased to a strongly reductive state in both soil and supernatant phase, and the amount of soluble Se increased to a maximum value followed by a decrease to a low level. Reductive dissolution of applied amorphous iron (hydr)oxide occurred during this transition period that led to the subsequent desorption of immobilized Se into the solution phase. The amount of desorbed Se further reduced to insoluble Se forms in a strongly reductive condition. Se species in total soluble Se in soils with mixed [Se(IV) + Se(VI)] contamination were determined under different Eh-pH conditions. The amount of soluble Se(VI) inversely followed the changes in the Eh values in an oxidative to mildly reductive condition and that closely followed the changes in the Eh values in a reductive to strongly reductive condition in this experiment. The amount of soluble Se(VI) increased followed by a decrease in the amount of total soluble Se; however, Se(VI) in soluble Se was the major species throughout the incubation period. The increase in the amount of soluble Se(IV) and organic Se species was inversely related to the decrease in the Eh values throughout the incubation period. These results suggest that immobilized Se in amorphous iron (hydr )oxide-applied soil is stable either under oxidative or strongly reductive environment.

Key Words: amorphous iron (hydr)oxide, pH, redox potential (Eh), Se-contaminated soil, Se immobilization.

As a widespread and potentially toxic trace element in various natural and anthropogenic disturbed environments, Se has received a great deal of attention from the scientific community (Rosenfeld and Beath 1964; Sharma and Singh 1983; Frankenberger and Benson 1994). The main sources of elevated levels of Se are the drainage discharge from seleniferous soil leaching or inigated agriculture (U.S. Environmental Protection Agency 1975). Industrial wastes containing Se, municipal sewage sludge, and laboratory wastes or water are also source of Se contamination to the sunounding environment.

To mitigate the Se pollution to a nontoxic leveL several physical, chemical, and biochemical technologies have been proposed. Phytoremediation (Banuelos et al. 1997) and bioremediation (Thompson-Eagle and Frankenberger Jr. 1992) are two of them. We have proposed a method in a previous report whereby amorphous iron (hydr)oxide is an effective remedial agent for immobilizing Se in polluted soil and water (Kang et al. 2002). Se in soil and water is frequently affected by the factors influencing the environment, including air-drying, submergence, pH, Eh, etc. In another previous report, we confirmed the stability of immobilized Se in amorphous


262

M.M. RASHID, Y. KANG. and K. SAKURAI

iron (hydr)oxide-applied soils in relation to air-drying and pH (Rashid et al. 2002). We found that immobilized Se in amorphous iron (hydr)oxide-applied soils was stable under moderately acidic conditions upon air-drying. Under submerged conditions, soils frequently experience a reductive state at low redox values. Un submerged soils also experience various degrees of redox fluctuations (Cresser et al. 1993; Skladany and Baker 1994). Anaerobic conditions in soils can rapidly develop in the absence of O2 (Ponnamperuma 1972). Even in well-aerated soils, there are still numerous micro sites are anaerobic (Cresser et al. 1993). The influence of the redox potential on Se chemistry is evident. However, few studies have investigated the effects of the changes in soil or sediment redox status on the environmental behavior of Se (Geering et al. 1968; Masscheleyn et al. 1990, 1991; Jayaweera and Biggar 1996; Tokunaga et al. 1996). The chemical speciation of Se is particularly important in the soil-water environment as the release of Se from soil to water system depends on the speciation of Se. Changes in the chemical speciation of Se in soil are controlled principally by the redox potential and pH of the system (Geering et al. 1968; Brookins 1988; Jayaweera and Biggar 1996). Besides the investigations of the effects of air-drying and pH on the stability of immobilized Se in amorphous iron (hydr)oxide-applied soils, the effect of the redox potential should be considered as the soils frequently experience water-logging conditions. Improved understanding of the effect of the redox potential and pH on Se in applied soils may enable to improve the management of amorphous iron (hydr)oxide-app1ied soils. In this study, amorphous iron (hydr)oxide-applied soil contaminated with Se was incubated under submerged conditions with and without the addition of a microbial nutrient source. The objective of this study was to confirm the stability of immobilized Se in amorphous iron (hydr)oxide-applied soil as a function of the redox potential and pH.

MATERIALS AND METHODS Properties of soil used. A soil of silty clay loam in texture was used in this study. The soil was collected from Sanpoh Mountain in Kochi Prefecture, Japan. To obtain a soil not disturbed by anthropogenic factors and to avoid the effect of organic matter content, the soil samples were taken from the C horizon. The soil samples were air-dried, sieved (2.0 mm mesh), analyzed for physico-chemical properties, and used for Se immobilization experiment. Physico-chemical analyses included the determination of pHw (5.00), content of total C (0.58 g kg-I), content oftotal N (0.03 g kg-I), content

of ammonium oxalate-oxalic acid-extractable Fe (Fea ) fraction (4.90 g kg-I) & dithionite-citrate-bicarbonate (DCB)-extractable Fe (Fed) fraction (1l.90 g kg-I), and content of total Se (0.03 mg kg-I). Amorphous iron (hydr )oxide. Amorphous iron (hydr)oxide used in this study was synthesized according to the method of Okazaki et al. (1989). Briefly, 1 M NaOH was added to a 0.1 M Fe(N0 3)3' 9Hp solution until the solution pH reached a value of 7. Then the precipitate was allowed to stand for 3 h at room temperature (20'C). It was dialyzed with deionized water until excess salt was removed. Selected characteristics of the synthetic amorphous iron (hydr)oxide are shown in Table 1. The amount of NH4 -oxalate-oxalic acid-extractable Fe (Fea> fraction of the synthetic amorphous iron (hydr)oxide was determined according to the method of Schwertmann (1964). The amount of soluble Fe was determined by atomic absorption spectrophotometry (Shimadzu, AA-610S, Kyoto, Japan). The surface area of the synthetic amorphous iron (hydr)oxide was determined by the ethylene glycol mono-ethyl ether (EGME) method proposed by Eltantawy and Arnold (1973). Zero point of charge (ZPC) of the synthetic amorphous iron (hydr)oxide was determined according to the method proposed by Sakurai et al. (1988). X-ray diffraction analysis of the synthetic amorphous iron (hydr)oxid~ was performed on powder mounts using Ni-filtered CuKa. radiation generated at 30 kV and 20 rnA (Shimadzu, XD-Dlw). Preparation of Se-contaminated soil and application of amorphous iron (hydr )oxide. Samples of soils with eight levels of contamination were prepared by the addition of SeelY) and Se(VI), either alone or in combination, ranging from 10 to 150 mg kg- I (Table 2). Ten to 150 mg L -I stock solutions of Se(IV) and Se(VI) were prepared from N~Se03 and Na 2 Se04 by the addition of deionized water. Finally, Se content of the contaminated soils thus prepared ranged from 10.9 to 155.6 mg kg-I. One or 2% of amorphous iron (hydr)oxide was applied to 250 g of each contaminated soil samples. One hundred mL of deionized water was added to it and mixed homogeneously. The paste in the beakers was air-dried by keeping it barefaced in a dry room for 12 d followed by gentle grinding, and storage for the subsequent incubation experiment.

Table 1. Selected characteristics of a synthetically produced amorphous iron (hydr)oxide (freeze-drying basis). Characteristics Oxalate-extractable Fe (Fe o) Specific surface area Zero point of charge X-ray diffraction analysis

Results 460 g kg- I 273.6 m 2 g-I pH 7.64 (ZPC) Poorly crystalline

Selenium in Amorphous Fe (hydr)oxide-Applied Soil

Table 2.

Se content in contaminated soils and % amorphous Fe (hydr)oxide applied. Am-Fe . d Added concen- Measured concen. C ontammate. '1 tratlon (mg Se tratlOn (mg Se (hydr)oxide SOl s 1 . 1 • kg- sOlI) kg- SOlI) applied (%) Se(IV) 20 2l.8 50 50.4 100 103.9 Se(VI)


Se(IY) + Se(VI)

10 20 50

10.9 2l.6 51.4

2

20 + 10 50 + 20 100 + 50

3l.8 72.8 155.6

2 2

Measurement of Eh and pH of the incubated samples. A 6 g portion of amorphous iron (hydr)oxide-applied soil was taken in an airtight-capped glass bottle. Two sets of experiments were carried out: in one set I % glucose was added as a microbial nutrient source and in the other set glucose was not added. Thirty milliliters of deionized water was added to each sample bottle and the bottles were fully capped to prevent air penetration. The sample bottles were incubated at 28"C (:±: 0.1) for different periods of time (2, 6, 8, 14,21,28, 37, 46, and 60 d). Under controlled N2 atmosphere, a 15 mL portion of each incubated sample was set aside for the analysis of soluble Se and Fe. Soon after the Eh and pH of the remaining portion of the incubated sample was measured in the same controlled N2 atmosphere. The kept portions were filtrated through a 0.45 J-l-m filter and used for the determination of the amount of total soluble Se, Se species, and total soluble Fe. Determination of total soluble Se and Fe. Total soluble Se content was determined by the method proposed by Yamada et al. (1987) and Yamada and Hattori (1990). Two milliliters of kept portion (diluted if necessary) was mixed with 8 mL of deionized water and decomposed by the addition of 4 mL of 3% potassium peroxodisulfate (K2Sps) dissolved in 1 M NaOH for 30 min at 120"C. After cooling, 3 mL (0.5 g mL -I) of potassium bromide (KEr) and 5 mL of 6 M HCI were added, then the samples were heated for 20 min at 100"C. Eight milliliters of the resultant solution was taken into the extracting glass tube. One milliliter of a 0.1 M EDTA-sodium fluoride solution and 2 mL of 20% acetic acid were added to the solution, and pH was adjusted to l.0 with HCI or ammonium hydroxide. A 2.5 mL of 0.2% 2,3-diaminonaphthalene (DAN) in 0.1 M HCI was added to the solution and then warmed for 30 min at 50"C. After being cooled to room temperature, the solution was extracted with 3 mL of cyclohexane by shaking for 5 min. A 200 J-l-L aliquot of the cyclohexane extract was analyzed for the determination of total solu-

263

ble Se. The kept portion was also used for the determination of total soluble Fe. The amount of total soluble Fe was measured by atomic absorption spectrophotometry. Determination of soluble Se species. To determine the amount of inorganic Se [SeelY) + Se(Vl)], 2 mL of kept portion (diluted if necessary) was mixed with 8 mL of deionized water and 250 J-l-L of 1,000 mg L -1 CuS04 solution. Then the sample solution was extracted according to the same procedure as that adopted for the determination of the amount of total soluble Se but by omitting the decomposition of organic Se by potassium peroxodisulfate treatment. To determine the amount of SeelY), 250 J-l-L of 1,000 mg L -1 CuS0 4 solution was added to 8 mL of kept portion and then extraction was carried out according to the same procedure as that adopted for the determination of the amount of total soluble Se and inorganic Se but by omitting the decomposition of organic Se by potassium peroxodisulfate treatment, and reduction of Se(Vl) by the addition of potassium bromide and hydrochloric acid. The amount of total soluble Se, inorganic Se, and SeelY) in the respective extracts were determined by high performance liquid chromatography (Shimadzu, RF-lOAXL) with a fluorescence detector using a mixture of cyclohexane and ethyl acetate (92 : 8, v / v %) as the mobile phase (flow rate I mL min-I). The amount of Se(Vl) was calculated by subtracting the amount of SeelY) from that of inorganic Se. The amount of organic Se was calculated by subtracting the amount of inorganic Se from that of total soluble Se.

RESULTS AND DISCUSSION Changes in Eh and pH during incubation The changes in the Eh and pH values in amorphous iron (hydr)oxide-applied soils with Se contamination during different incubation periods are shown in Fig. 1. The values of both Eh and pH of the applied soils changed during the incubation under submerged conditions. Without the addition of a microbial nutrient source (e.g., glucose) to the incubated samples, no significant changes in the values of Eh, pH, and consequently in the amount of soluble Se were observed even with the prolongation of the incubation period, probably due to the insufficient supply of nutrient source for microbial activity. On the contrary, changes in the values of both Eh and pH were found within a few days of incubation after the addition of 1% glucose to the incubated samples. Addition of organic matter (e.g., glucose) to the submerged soil enhanced the decrease in the Eh value of the soil (Bloomfield 1950). Ponnamperuma (1972) reported that the extent to which soil Eh de-

M.M. RASHID, Y. KANG, and K. SAKURAI


264

creased, however, was a function of soil organic C. This author reported that readily available C sources might playa significant role in the chemical and microbiological behavior of Se, resulting in a limited movement of soluble Se in the anaerobic environment. At the onset of this experiment, the Eh and pH values of amorphous iron (hydr)oxide-applied soil suspensions were around 350 mV and 4.11, respectively. We observed that the changes in Eh status during the incubation of the applied soils seemed to occur in three steps. In the first step, a slow decrease in the Eh (Eh: 31491 mY) and a gradual increase in pH (4.08-5.23) occurred simultaneously during the 2 to 21 d period of incubation. The Eh values decreased faster in the soil phase than that in the supernatant phase during the 21 to 28 d of the transition period. In the second step, the Eh values drastically decreased (Eh: = - 300 mY) in both soil and supernatant phase and pH reached a maximum value (around 6) after 28 d of incubation. In the third step, a slight increase in the Eh values (- 38 to - 72 mY) and a slight decrease in the pH values (5.29-5.72)

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Stability of amorphous iron (hydr )oxide in applied soil during incubation The amount of soluble Fe in amorphous iron (hydr)oxide-applied soils during various incubation periods is shown in Fig. 2. Under oxidative conditions, soluble Fe could not be detected. The Eh values decreased in a

50 mg Se(IV) kg- 1

400

..c: 1LI

in Se(IV) and Se(VI) soils were also observed after 37 d of incubation, presumably due to the insufficient supply of nutrient source for microbial activity. The data showed an inverse relationship between Eh and pH, which has been reported by other researchers in reduced systems, such as submerged soils (Ponnamperuma 1972; Jayaweera and Biggar 1996; Li et al. 1997). Jayaweera and Biggar (1996) reported in an oxic-anoxic-oxic transition Se experiment in a Kesterson Reservoir soil that Eh decreased to a low value of - 151 mV and pH increased to 8.2 during the anoxic phase. The Eh decreased in soils to values as low as - 250 to - 300 m V subject to anoxic conditions (Patrick and Reddy 1978; Cresser et al. 1993; Liss and Baker 1994).

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Fig. 1. Relationship between Eh (mY) and pH inamorphous iron (hydrloxide-applied soils under different incubation periods (d). Experimental conditions: solid (g) to solution (mLl ratio = 1 : 5, the doses of amorphous iron (hydr)oxide 1 or 2%, glucose 1%.


Selenium in Amorphous Fe (hydrloxide-Applied Soil

mildly oxidative environment (Eh: 150-0 mV) while the amount of soluble Fe gradually increased. Under reductive conditions (0 to - 300 mV), Fe solubility was high (10-18%, and 19% of the amount of total Fe added in SeelY) and Se(VI) soils, respectively, was dissolved). The decrease in the value of Eh was inversely related to the increase in the amount of soluble Fe under mildly oxidative to reductive conditions. Many species of microorganisms are capable of reducing Fe oxide (Ottow and Glathe 1971). When 02 is lacking at an Eh value of approximately 100 mY, Fe H can take up electrons produced during the metabolic oxidation of organic compounds and subsequently the amount of Fe2 + in the solution phase increases (Schwertmann and Taylor 1989). In this study, reductive dissolution of applied amorphous iron (hydr)oxide led to the subsequent mobilization of adsorbed Se into the solution phase in a mildly oxidative to reductive environment. The divalent FeU reoxidizes to Fe3+ hydroxide whenever 02 is again introduced, or moves to the zones with a higher Eh value

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.>g L -1 for all the 8 separate contamination levels of the applied soils. At 8 d after the incubation in Se(IV) soils, the amount of total soluble Se ranged between 2 and 8 jJ.>g L -1 in an oxidative environment (Eh: 316-244 mY; pH 4.01-4.16). Between 14 and 21 d of incubation, the amount of total soluble Se increased

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(higher 02 concentration) and reoxidizes there, or precipitates as solid Fe H phase or green rust (Schwertmann and Taylor 1989; Li and Horikawa 1998).

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