以離子交換樹脂埋入法評估重金屬鉛於底泥之生物有效性及毒性效應

Abstract

底泥重金屬汙染是重要的環境議題，重金屬在水體環境中容易被有機質、黏土顆粒、鐵錳氧化物等環境基質吸附鍵結，或形成碳酸鹽、硫化物等低溶解度的化合物，而沉積在底泥當中。然而水體的擾動或是生物活動可能使底泥重金屬重新釋放到水體，因而提高水生生物的暴露風險。為了瞭解底泥重金屬對生物體的危害潛勢，本研究旨在利用離子交換樹脂埋入法，搭配青鱂魚胚胎─魚苗試驗(Medaka embryo-larval assay, MELA)，以評估此方法應用於預測重金屬鉛於底泥之生物有效性及毒性效應的可行性。本實驗選用不同來源及物化特性之人造或環境底泥，分別配製成汙染程度不一的含鉛底泥樣品，再利用全底泥暴露(Whole sediment exposure)方法，將青鱂魚胚胎(Medaka embryos, Oryzias latipes)置於含鉛底泥中連續暴露7天，觀察胚胎及魚苗的死亡率、生物累積量及半致死效應。此外，暴露期間之底泥表層水及孔隙水也用以量測鉛離子濃度及生物毒性效應，並比較利用不同生物暴露法－包括全底泥、覆蓋水(Overlying water)及孔隙水(Pore water)暴露法－評估鉛汙染底泥之胚胎毒性效應是否有差異。結果顯示以全底泥暴露法測得之胚胎死亡率和底泥總鉛濃度呈現較顯著之劑量反應關係:青鱂魚胚胎暴露於含鉛人造底泥(50-800 mg/kg)，第7天胚胎死亡率(Mortality)為5%-53%，總畸形率(Deformation rate in hatchlings)為10%-100%。實驗證明鉛離子對胚胎的急毒性較低(LD50≒13.8 mg/L)，但有很高的致畸性(顯著致畸濃度>0.65 mg/L)，其中以脊柱發育異常(Lordosis /kyphosis)發生比例最高。此外，孔隙水暴露法所得毒性結果低於全底泥暴露法與覆蓋水暴露法的結果，顯示全底泥暴露法與覆蓋水暴露法應較能反映現地環境底泥重金屬汙染對底棲生物的實際毒性效應。研究進一步利用對數邏輯模式(Log-logistic model)，將離子交換樹脂(Chelex 100)埋入法抽出底泥(人造及夢湖底泥)中之游離性鉛含量，與全底泥暴露法與覆蓋水暴露法所得之胚胎存活率(Survival rate of embryos)、非畸形率(Non-malformation rate in hatchalings)與生物累積量等指標繪製劑量反應關係，結果皆呈現高度相關性(R > 0.90)，說明水體中游離態鉛確實為引起胚胎毒性的主要原因，並且透過生物累積量與兩種方法抽出鉛量所計算之相關性，說明離子交換樹脂埋入法更能準確預測底泥鉛之生物有效性及危害潛勢(R = 0.98)。本研究利用青鱂魚胚胎，透過全底泥暴露法的毒性試驗成果，證實離子交換樹脂埋入法抽出之底泥鉛含量能夠有效預測底泥重金屬之生物有效性及生物毒性潛勢，可進一步應用於評估現地底泥重金屬污染之危害風險。

Sediment pollution by heavy metals is an important environment issue for a long time. Heavy metals are easily adsorbed to the organic matter, clay particles and/or iron or manganese oxides in water and then deposited in sediments as a pollutant source. They are likely to be released to the water column via disturbance by water flow or benthic organisms, thus enhancing the exposure risk of aquatic organisms to heavy metals. In order to evaluate the toxic and bioavailable potential of lead (Pb)-contaminated sediments for aquatic organisms, this study aims to establish the medaka (Oryzias latipes) embryo-larval assay (MELA) and selective ion exchange resin (Chelex 100) embedding in artificial or environmental sediments. First, we demonstrated that lead spiked artificial sediments (50-800 mg-pb/kg sediment dry weight) evoked dose dependent toxic effects in mortality and developmental toxicity (e.g., malformation) of medaka embryos or larvae. The lead exposure from the contaminated sediment induced high developmental toxicity in medaka embryos, especially for lordosis/kyphosis. As well, the dose-response relationship is more significant with the whole sediment exposure, as compared to those results with aqueous exposures of pore water (extracting with vacuum suction) or overlaying water collected from the lead contaminated sediments. Also, the concentration of soluble lead ion in pore water was lower than that measured in overlaying water. These results indicated that aqueous exposure of pore water from lead contaminated sediments with vacuum suction may not well express lead bioavailability in sediment to medaka embryos because it had less lead ion and caused lower embryonic mortality. On the other hand, we embedded the selective ion exchange resin (Ca-saturated Chelex 100, 1 g per batch, 2 days) in lead spiked sediments (including artificial sediments and environmental sediment from Moon-lake) to extract lead with bioavailable potency. Finally, the log-logistic model was used to compute the correlation of Chelex 100-extracable lead or soluble lead in overlaying water with the observed toxicity from the medaka biomarker assays. We demonstrated that Chelex 100-extracable lead had significantly negative correlation with the survival rate and non-malformation of medaka embryos/hatchlings in both sediments. Overall, this study demonstrate that ion exchange resin Chelex 100 embedded method is a promising approach to determine bioavailable portion of heavy metals in sediments in situ.