Kinetics of Radiocesium Released from Contaminated Soil by Fertilizer Solutions.
Po-Neng Chiang1, Ming-Kuang Wang1, P.M. Huang2, and Jeng-Jong Wang3. (1) National Taiwan Univ, No.1, Sec. 4, Roosevelt Road, Taipei, Taiwan 106, Taipei, Taiwan, (2) Dept of Soil Science, Univ of Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N 5A8, Canada, (3) Institute of Nuclear Energy Research, No. 1000, Wunhua Rd., Jiaan Village, Longtan Township, Taoyuan County 32546, Taoyuan, Taiwan
137Cs is one of the major radionuclides of contaminated environments. 137Cs contaminates soil either through accidental spillage or leakage, deposition of airborne material during nuclear testing and incinerator processing, the operation of nuclear facilities, and plume development from evaporation ponds, liquid-storage tanks, and burial grounds (Nabyvanets, et al., 2001, Zachara, et al., 2002). In early 1990, the Institute of Nuclear Energy Research (INER, Taiwan) accidentally discharged radionuclide wastewater into an irrigation ditch and contaminated the agricultural ecosystem (Nabyvanets, et al., 2001). If these contaminated soils are not treated, they pose an immediate danger to human health and constitute a sustained environmental hazard. Traditionally, the management of soils contaminated with radiocesium has relied on natural attenuation and soil disposal, or the use of countermeasures such as heavy fertilization, deep-ploughing or mulching to minimize radiocaesium uptake by plants (Chiang et al,. Zhu and Shaw, 2000). MonoAmmonium Phosphate (MAP) is one of the most used fertilizers (Tisdale, et al., 1993). The nature of the reaction of phosphate fertilizers with soil constituents may vary with the distance from the fertilizer granules because of the change in phosphate concentration and pH. High concentrations of phosphate combined with a low pH cause substantial alterations of soil constituents and the subsequent Cs release. The objectives of this study were to investigate the kinetics and mechanisms of the phosphate-induced Cs release from soil under acidic conditions, and quantify the desorption kinetics using mathematical models. Ten grams of the contaminated soil samples were placed in a 100 mL polypropylene cup and then 100 mL 1 M NH4H2PO4, NH4Cl, KCl, and NaCl, or 0.5 M (NH4)2SO4 solutions were added. These chemicals were presumed as common applications of nitrogen (N), phosphorous (P) and potassium (K) fertilizers in agriculture. The soil suspensions were collected and reacted at various reaction times, between 0.5 and 48 h (0.083, 0.25, 0.5, 0.75, 1, 2, 3, 4, 8 12, 24, and 48 h). Batch renewing method was employed in this experiment. Ten grams of the contaminated soil samples were placed in a 100 mL polypropylene cup and then 100 mL 1 M NH4H2PO4, NH4Cl, KCl, and NaCl, or 0.5 M (NH4)2SO4 solutions were added. After 2 h equilibrated, the solution was extracted and centrifuged and substituted by 100 mL of the solution containing fertilizers, which was then shaken for 2 h and centrifuged again. The overall renewing experiment consisted of four cycles. The 137Cs radioactivity released from contaminated soils extracted by 1 M NH4H2PO4, 0.5 M (NH4)2SO4, 1 M NH4Cl, 1 M KCl, and 1 M NaCl solutions (pH 4.0) for 48 h were 1201, 1220, 1101, 971, and 158 Bq kg-1, respectively. It is suggested that NH4Cl, KCl, and NaCl solutions had the same anion but different cations, the Na+ had higher hydration energy than that of NH4+ and K+. Therefore, NH4+ and K+ could more easily exchange Cs+ from the soil surface to solution than what Na+ could. After renewing the fertilizer solutions four times, the 137Cs radioactivity of soil suspension extracted by the NH4H2PO4, (NH4)2SO4 and NH4Cl solutions were 2235, 1835, and 1703 Bq kg-1 soil. It is suggested that the NH4+ and Cs+ concentration in the solution reached a dynamic equilibrium, thus 137Cs+ would not be continually released without continuous renewal of the solutions. After renewing, 137Cs can continually release into the solutions. When comparing the anion effect of NH4H2PO4, (NH4)2SO4 and NH4Cl, it could be found that more 137Cs release from contaminated soil after the 3rd and 4th renewal of the (NH4)2SO4 and NH4Cl solutions. The rate of 137Cs released to (NH4)2SO4 and NH4Cl solutions remained the “same” but the rate of 137Cs released from soil to the NH4H2PO4 extracting solution still increased. The combined effect of phosphate and proton was the major mechanism of 137Cs release from contaminated soils in NH4H2PO4 solution. Application of NH4H2PO4 fertilizer to soil is recommended to promote 137Cs release from soil, and thus also increase opportunity for plant uptake, migration to groundwater and entry into the food chain. References: (1) Chiang, PN, Wang MK, Wang JJ,Chiu CY. Low molecular weight organic acids exudation of rape (Brassica campestris) roots in cesium-contaminated soils. Soil Sci. 2005.(In press). (2) Nabyvanets YB, Gesell TF, Jen MH, Chang WP. Distribution of 137Cs in soil along Ta-han river valley in Tau-yuan county in Taiwan. J. Environ. Radioact. 2001;54:391-400. Tisdale SL, Nelson WL, Havlin JL. Soil Fertility and Fertilizers. Macmillan Publishing Company, New York, NY, 1993. (3) Zachara JM, Smith JT, Liu CG, McKinley JP, Serne RJ, Gassman PL. Sorption of Cs to micaceous subsurface sediments from the Hanford site, USA. Geochim. Cosmochim. Acta 2002;66:193-211. (4) Zhu YG, Shaw G. Soil contamination with radionuclides and potential remediation. Chemosphere 2000;41:121-8.