台灣關渡平原土壤重金屬(砷與鉛)劃分、移除與吸附之研究

Abstract

本研究選擇兩個砷與鉛汙染剖面(pedon 1 及 2)以及兩個未汙染剖面(pedon 3 及 4)，來分析土壤物理化學性質及砷與鉛之吸附研究。關渡地區土壤為弱酸性、高鹽基之灰色黏壤土，土壤之高鹽基受到海風與地下水位之影響甚大。礦物組成以蛭石、伊萊石與高嶺石為主。本區為典型或暗色的漂白潮濕淋溶土。 以X-ray diffraction (XRD)礦物鑑定、掃瞄式電子顯微鏡(Scanning electron microscopy, SEM) 、X光能譜散佈分析儀 (Energy dispersive spectrometer , EDS)與同步輻射分析技術 (X光吸收光譜技術, X-ray absorption spectroscopy, XAS)等物理學的方式，分析關渡地區砷與鉛污染土壤；輔以還原實驗瞭解重金屬釋放之情形；並以淋洗與萃取劑之研究來瞭解是否有更效率的移除方式；最後以低汙染土壤對於砷與鉛進行吸附之研究，瞭解當地土壤對於砷與鉛之吸附行為。 土壤樣本經粉末XRD礦物分析鑑定，發現汙染土壤剖面各層次皆有砷鉛鐵礬(beudantite)的存在；以掃瞄式電子顯微鏡分析，發現似砷鉛鐵礬之八面體構造，輔以X光能譜散佈分析儀，發現該八面體構造中同時含有砷與鉛元素之存在；X光近緣結構 (X-ray absorption near edge structure, XANES)圖譜顯示砷原子在污染土壤樣本中主要為+5價，而鉛原子高於+4價鉛離子(PbO2)，所有證據說明土壤中有砷鉛鐵礬之存在。 在還原實驗中，砷在還原條件下釋放速率相當緩慢，XANES分析土壤中的砷主要還是以五價砷為主，而鐵的價數主要是以三價為主，但隨還原時間有晶性改變之現象。液相層析儀分析土壤釋放砷之型態主要是以三價砷為主。 以淋洗實驗來看，鉛並未被洗出； pH 2之磷酸對於污染土壤中的砷有最強的洗出效果。由萃取實驗來看，對汙染土壤的砷而言，以0.1 M磷酸之移除效果最強，其次是pH 12的萃取劑；對汙染土壤的鉛而言，所有萃取劑之移除效果皆不顯著。所有萃取劑的移除效果，皆隨著剖面深度增加而下降，間接證明鉛是以礦物型態向下移動。 所有受測土壤樣品中以pedon 4之20-40 cm對砷酸氫鈉有最大吸附量，為2.44 mg g-1；而pedon 3之40-60 cm對硝酸鉛有最大吸附量，為10.56 mg g-1。溫度變化對於砷酸氫鈉的吸附過程有顯著影響，但對硝酸鉛的吸附過程影響不明顯。實驗結果顯示砷酸氫鈉與硝酸鉛之脫附過程為緩慢的反應。

This study collected two pedons of arsenic (As) and lead (Pb) contaminated soils (i.e. Pedon 1 and 2) and uncontaminated soils (pedon 3 and 4) for the soil physical and chemical analysis, and As and Pb adsorption and desorption experiments. Guandu areas contaminated soils is weak acid and high base, it can be classified as clay loam, mixed, thermic, Typic or Umbric Albaqualfs. Soil minerals are vermiculite, illite and kaolinite. X-ray diffraction mineral identification (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray absorption spectroscopy (XAS). The reduction experiment can help us to understand the release situation of arsenic and lead in contaminated soils. The leaching and extractant tests may find out is there a more efficient way to remove arsenic and lead. On the other hand, using uncontaminated soil for As and Pb adsorb experiments. All experiments will help us to understand the role of heavy metals in this region. The powder XRD of contaminated pedon has found beudantite existed at different soil horizons. Scanning electron microscope (SEM) analysis found octahedral structure as beudantite. Energy dispersive spectrometer (EDS) also proof that arsenic and lead are on the octahedral shape. In soil samples, XAS shows the valence of arsenic atoms is +5 and lead atoms is near +4. All evidence show that existence of beudantite contained in the soil. Under reducing conditions, the release rate of arsenic is very slow. XANES analysis soil samples shows that the dominant specie of arsenic is As(V), and the valence of iron in soil sample is +3. Liquid chromatography analysis of released arsenic as the types of As(III) is a majored species. In leaching experiments, pH 2 phosphate has the best removal efficiency to replace As in contaminated soils, but Pb was not leached out from soil samples. In contaminated soils, 0.1 M phosphate shows the best removal efficiency of arsenic, followed by pH 12 extractant. On the other hand, the effects of removal lead by all extractant are insignificant. Removal effects of extractants are decreased with increasing soil depth. It cans beshows indirect evidence of mineral forms of lead is moving down in soil profiles. For the soil adsorption experiment, the soil sample of pedon 4 at 20-40 cm has the strongest adsorbing capacity to Na2HAsO4, the adsorbance is 2.44 mg g-1. On the other hand, the soil sample of pedon 3 at 40-60 cm has show the strongest adsorbing capacity to Pb(NO3)2, the adsorbance is 10.56 mg g-1. Temperature changes significantly impact the effects of arsenic adsorption, but it is noe shown any significant for lead adsorption. Desorption experiments show that As and Pb desorption is a slow reaction.