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Interactions of Arsenic and Organic Substances
|關鍵字:||Arsenic;砷;Organic substances;adsorption;Cation bridge;Soil;Rice;有機物;吸附作用;陽離子鍵橋;土壤;水稻||出版社:||土壤環境科學系||摘要:||
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.
環境毒物與有機物之交互作用會影響毒物之物種變化與動態，進而影響其在環境中之生物毒性、移動性與宿命，間接影響作物品質與人體健康。本研究之目的在於探討砷與有機物之結合以及有機物對砷被土壤吸附與有機物對砷酸還原之影響，再者，亦探討有機肥施用對砷含量偏高土壤中砷移動與作物中砷含量之影響。經由吸附試驗、透析法與離子交換法等探討砷酸與堆肥之水抽出液中有機物之結合，發現砷酸可與堆肥水抽出液中腐植質結合，此砷酸與有機物之結合是藉由陽離子鍵橋之鍵結。以淇武蘭系(Fine-silty, mixed, hyperthermics, Typic Epiaquept) 及水汴頭系 (Sandy-skeletal, mixed, hyperthermic, Lithic Udorthents)二種砷含量偏高之石灰性黏板岩沖積土(砷含量分別為23.7 mg kg-1及 12.9 mg kg-1 )，利用批次平衡試驗探討有機物對砷酸被土壤吸附之影響，結果顯示土壤溶液中有機陰離子與砷酸相互競爭吸附位置，當砷酸添加量低於土壤砷之最大吸附量時，堆肥抽出液之加入，可降低土壤對砷酸之吸附；當砷酸添加量高於土壤砷之最大吸附量時，砷酸之加入，可降低土壤對有機物之吸附。不同濃度堆肥水抽出液與砷酸反應之結果，發現砷酸與有機物共存之系統中，砷酸可還原為亞砷酸，依據反應在室溫 (25℃)下可發生，在 4℃ 及 50℃ 無法發生之現象，可推知此反應為生物性還原反應。利用盆栽試驗施用不同肥料種植水稻，探討前述二種砷含量偏高土壤中砷之移動性及生物累積性，發現水田狀態下，不論施用有機及或無機肥料，土壤皆呈強還原性，游離砷還原為元素態砷或形成硫化砷沉殿。稻株中砷含量由高而低依序為：根>葉>莖>糙米。稻根中砷含量以不施肥者最高，其次為施用有機肥者，而以施用化肥及半有機半化肥者為最低，由於不施肥者缺乏磷肥供應，施用有機肥者磷肥供應速率太慢或供應量不足，作物吸收砷以替代磷，再者，土壤中有機陰離子與砷酸陰離子產生競爭吸附，促進砷之釋出，致使作物吸砷量增高。由於有機物可降低砷在土壤中之吸附或使砷酸還原為亞砷酸，增加砷在土壤中之移動性及生物毒性，提高作物吸砷量及地下水砷含量，因此，吾人須慎重考量關於砷含量偏高土壤施用有機肥之問題。
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