Interactions of Arsenic and Organic Substances

• Main Authors: Haw-Tarn Lin, 林浩潭
• Other Authors: Min-Chao Wang
• Format: Others
• Language: zh-TW
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/53958357376805661595

博士 === 國立中興大學 === 土壤環境科學系 === 87 === The interactions of environmental toxicants with organic substances deserve increasing attention. These interactions affect the speciation and dynamics of toxicants, and subsequently affect the toxicity, mobility, and fate of toxicants in the environment. Furthermore, they indirectly affect the quality of crops and human health, when the toxicants are absorbed and accumulated by plants. The objectives of this study were to investigate the binding strength of arsenic and organic substances, as well as the effects of organic substances on the mobility and reduction of arsenate. In addition, the influence of organic fertilizer application on arsenic mobility in soil and arsenic content of rice (Oryza Sativa L.) plant in paddy field condition were also studied with lysimeter experiment. Arsenate containing solution was prepared to react with the crude compost extract (CCE) which was extracted from swine compost with deionized (DI) water, and to incubate for 48 hours at 25℃. The formation of As-organic complex was investigated by dialysis and ion-exchange methods. The results show that arsenic bonds to organic substances in the presence of cations, which act as a cation bridge in As-humic complex. The effect of organic substances on the adsorption of arsenate by soil was conducted by using CCE as a complex anion source in batch experiment. Two higher arsenic content of calcareous slate alluvial soils of an Inceptisol (Fine-silty, mixed, hyperthermics, Typic Epiaquept, Chiwulan series, with arsenic content of 23.7 mg kg-1), and of an Entisol (Sandy-skeletal, mixed, hyperthermic, Lithic Udorthents, Shuipientou series, with arsenic content of 12.9 mg kg-1) were used. The adsorption of arsenate by both soils decreases in the presence of CCE when the amount of arsenate added in the reaction systems are lower than the arsenic adsorption maximum of both soils. However, the adsorption increases in the presence of CCE. When the amount of arsenate added in the reaction system are higher than the arsenic adsorption maximum of both soils. These may be attributed to competitive adsorption for binding sites on the soils between arsenic oxyanion and organic anions. The data of arsenate reacted with 1% of CCE reveal that As(V) is capable of reducing to As(III). The reduction of As(V) at 25℃, but not at 4℃ and 60℃ verifies the biotic process. The same soil samples as used in batch experiment were used in lysimeter experiment. Rice plants were grown in the soils amended with various fertilizers in lysimeter column. The leachates of lysimeters were collected for arsenic determination every two weeks during the growing period of rice plants. The As contents of soil samples including surface soils (0-20 cm in depth), bottom soils (20-40 cm in depth), as well as rice plants were also determined after the rice plants were harvested. The results show that prolonged flooding of the soils in the lysimeter columns leads to a markedly decrease in soil redox potential for all treatments. This may result in the precipitation of the free arsenics in soil solution as solid arsenics or arsenate sulfides. Quite low arsenic concentrations (< 20 μg L-1) were detected in all leachates. In addition, there is no significant difference in arsenic concentrations in the leachates among the treatments. The sequence of the arsenic concentrations in rice plants was: root > leaf > stem > brown rice. By comparing the arsenic contents in rice roots of each treatment, it was found that both treatments with no fertilization and with organic fertilizer fertilization were higher than those with chemical fertilizer fertilization and with half chemical-half organic fertilizer fertilization.As a result of phosphate deficiency in soils with no fertilization and with organic fertilizer fertilization. Arsenic may become the substitute for the phosphate and be absorbed by rice roots. Moreover, the organic anions may compete with arsenic oxyanions for the adsorption sites on the soils, and subsequently increase the release of arsenic from the soils. These thus increase the arsenic absorption by rice plants from soils with no fertilization and with organic fertilizer fertilization. In conclusion, organic substances may decrease the adsorption of arsenic by soils, and/or cause the reduction of As(V) to more toxic and mobile form of As(III), leading to both groundwater and crop contamination by arsenic. Therefore, the consideration of the application of organic fertilizer to high arsenic content soils deserve close attention.