| 摘要: | 過往的文獻已證實藉由二價鐵與硫酸鹽(sulfate)的適度添加，可讓汞(Hg)與鎘(Cd)穩定於底泥跟土壤介質中，達到兩金屬生物有效性顯著降低之功效。然而，實驗室前期盆栽試驗的結果，卻發現僅有零價鐵可發揮降低水田根系土壤中鎘之生物有效性的功能，硫酸鹽的效果卻遠不如預期。由於後續菌種組成的調查顯示試驗土壤中的硫酸鹽還原菌佔比極低，故本研究從生態氧化序列(ecological redox sequence)的角度切入，試圖調查硫
酸鹽無法發揮功效的原因，是否與前期試驗時所用的肥料含有比硫酸鹽更容易被現地微生物利用的最終電子受體有關。有鑑於此，本研究利用原有的污染土壤及定期加入與所測得的根系分泌物等量的有機碳(葡萄糖與醋酸鹽)，並分成有無添加肥料的組別，進行為期兩個月的土壤縮模培養試驗，以探討(兩種)肥料、硫酸鹽及零價鐵三者於模擬稻田覆水時期根圈土壤達厭氧狀態時，彼此的兢爭關係，並以孔隙水中重金屬濃度的減少與否，做為該金屬在試驗期間於土壤介質中生物有效性降低的證據。研究結果發現未含肥料組別的土壤及孔隙水樣品，隨時間皆能順利觀察到硫化物與二價鐵存在的跡象，且鎘於孔隙水中的濃度得以有效降低。而在含有肥料的組別中，零價體與硫酸鹽的還原速度相較於未含肥料組則來得緩慢、甚至毫無反應，並在孔隙水中檢測到硝酸鹽，其濃度隨時間逐漸降低，過程中也一度發現亞硝酸鹽的蹤影，藉此可確定系統中正進行硝酸鹽還原(或脫硝)反應，也由此可推測該土壤中的硝酸鹽會與零價鐵及硫酸鹽競爭，並優先成為系統中微生物呼吸作用的電子受體，最後使得硫酸鹽所受的影響最大，因無法被有效還原，以至於無法顯著降低鎘在系統中的生物有效性。這些結果表明故當利用硫酸鹽進行受鎘或其他重金屬污染的稻田土整治時，必須優先檢測所用肥料是否含有一定濃度的硝酸鹽，因其最終效用會與此含有硝酸鹽的氮肥形成衝突，兩者在厭氧根圈中競爭的後果將造成硫酸鹽帶來的整治效果不彰；但假使環境中缺少充足硝酸鹽作為電子受體，硫酸鹽的添加仍具有良好的效果，可以有效降低稻田土孔隙水中的鎘濃度，且其價格低廉，是值得考慮的土壤改良劑。;Previous literatures have confirmed that by moderate addition of ferrous iron and sulfate,
mercury and cadmium can be stabilized in the sediment and soil medium, and the bioavailability
of the two metals can be significantly reduced. However, the results of the previous pot
experiments in the laboratory found that only zero-valent iron can play a role in reducing the
bioavailability of cadmium in the root soil of paddy fields, while the effect of sulfate is far less
than expected. Since the subsequent investigation of bacterial species composition showed that
the proportion of sulfate-reducing bacteria in the test soil was extremely low, this study started
from the perspective of ecological redox sequence, and tried to investigate whether the reason
why sulfate could not exert its effect was the same as that in the previous period. Fertilizers
used in the experiments contained final electron acceptors that were more readily available to
local microorganisms than sulfate. In view of this, this study used the original contaminated
soil and regularly added the same amount of organic carbon (glucose and acetate) as the
measured root exudates, and divided them into groups with or without added fertilizers for a
period of two months. Soil miniature model culture experiment to explore the relationship
between (two) fertilizers, sulfate and zero-valent iron when the rhizosphere soil reaches
anaerobic state in the simulated paddy water-covered period, and the reduction of heavy metal
concentration in pore water Whether or not, as evidence of reduced bioavailability of the metal
in soil media during the test period.The results of the study found that the presence of sulfide
and ferrous iron could be successfully observed over time in soil and pore water samples
without fertilizers, and the concentration of cadmium in pore water was effectively reduced. In
the group containing fertilizer, the reduction rate of zerovalent body and sulfate was slower than
that of the group without fertilizer, and there was no response. Nitrate was detected in pore
water, and its concentration gradually decreased with time. During the process, traces of nitrite
were also found, which can confirm that the nitrate reduction (or denitrification) reaction is
going on in the system, and it can be speculated that the nitrate in the soil will compete with
zero-valent iron and sulfate, and It preferentially becomes the electron acceptor for microbial respiration in the system, and finally makes the sulfate most affected, because it cannot be effectively reduced, so that the bioavailability of cadmium in the system cannot be significantly reduced.These results indicate that when using sulfate to remediate paddy soil contaminated with cadmium or other heavy metals, it is necessary to first detect whether the fertilizer used contains a certain concentration of nitrate, because its final effect will conflict with this nitrogen fertilizer containing nitrate, and the two The result of the competition between the two in the
anaerobic rhizosphere will cause the ineffective remediation effect brought by sulfate; but if there is a lack of sufficient nitrate as an electron acceptor in the environment, the addition of sulfate still has a good effect, which can effectively reduce the amount of soil in paddy fields.Cadmium concentration in pore water, and its low price, is a soil conditioner worth considering. |
