以活性碳/黏土薄層覆蓋法整治含汞底泥之縮模研究

Abstract

近年來，現地薄層覆蓋法技術之突破為整治汞汙染底泥帶來新的可能性，也提供除疏濬法以外新的整治策略。本研究分為兩部分，利用數種活性碳/黏土基之薄層覆蓋層在微型系統中評估整治汞底泥之可能性。在本研究第一部分中，自製之含硫活性碳(SAC)在等溫吸附實驗中發現對於二價汞及甲基汞之吸附親和性(KD值分別為9.42×104及7.66×105)比起其原始活性碳(AC)有顯著提升(KD值分別為3.69×104及2.25×105)。但在底泥競爭吸附實驗中，AC對比SAC對於汞底泥(14.2‒235.8 mg-Hg/kg)中具有較佳之汞溶出抑制能力，並在3%添加量下具有最佳之汞溶出抑制效率(99.88%)。其原理可能為SAC在吸附實驗平衡後形成穩定的HgS (s)奈米顆粒，使底泥競爭吸附實驗之AC抑制汞溶出之能力優於SAC。另外，研究之各式覆蓋層(SAC+皂白土、SAC+低汞底泥、AC+皂白土)在上流式微型系統中對於總汞及甲基汞均具有良好溶出抑制能力。在本研究第二部分為探討底泥擾動事件對於覆蓋穩定之影響，開發一具有自製震盪系統之橫向流微型系統。三種不同黏土材料之活性碳/黏土基覆蓋層施加於實場汞底泥中(76.0±2.59 mg-Hg/kg)，發現AC(3%)+皂白土(3%)以及AC(3%)+高嶺土(3%)在模擬橫向流及表層底泥擾動之條件下，對於實場底泥中總汞及甲基汞皆能達到約75‒95%及64‒98%之溶出抑制效果達75天之久(實驗全時程)。而AC(3%)+蒙脫土(3%)的薄層覆蓋層由於蒙脫土在水中沉降性及穩定性較差，使總汞及甲基汞溶出抑制效率不佳。本研究發現穩定性高之薄層覆蓋層有較佳之汞溶出抑制能力，並於間歇性底泥擾動下有較佳之抵抗能力；而穩定性較差之薄層覆蓋層可能造成甲基汞大量溶出。

The breakthrough of in-situ thin layer capping technology in recent years has shed light on the remediation of Hg-contaminated sediment and provides a promising alternative besides traditional dredging. In this thesis, the plausibility of several activated carbon (AC)/clay-based thin layer caps were demonstrated in two microcosm studies. In the first study, a lab-synthesized sulfurized activated carbon (SAC) performed greater sorption affinity to both aqueous Hg2+ (KD=9.42×104) and MeHg (KD=7.66×105) compared to those for raw activated carbon (KD=3.69×104 and 2.25×105, respectively) in isotherm adsorption tests. However, AC appeared to have greater sequestration ability than SAC in Hg-spiked sediment (14.2‒235.8 mg-Hg/kg), with the optimistic dosage of 3wt% AC causing reduction of THg with 99.88%. It may suggests that possibly formed nano-HgS particles could be released thus elevates the porewater Hg when SAC existed. Also, a 83-d trail of up-flow microcosms was demonstrated with various caps (SAC + bentonite, SAC + clean sediment, and AC + bentonite) and all observed significant inhibition of both THg and MeHg. In the second study, a horizontal-flow microcosm with lab-made vibration system was designed to evaluate the capping efficiency during turbation events. AC/clay-based caps with clay combinations were applied to actual Hg-contaminated estuary sediment (76.0±2.59 mg-Hg/kg). The caps with AC + bentonite and AC + kaolin were efficient in reducing both total mercury (THg) and methylmercury (MeHg) concentrations in overlying water by 75−95% and 64−98%, respectively in the later stage of 75-d operation. In contrast, the AC (3%) + montmorillonite (3%) cap did not show a significant reduction on THg and MeHg in overlying water, probably due to the unstable, suspension property of montmorillonite. The stable caps showed higher resistance to Hg breakthrough under occasional turbation events; however, a labile cap appeared to have dramatic Hg breakthrough when turbation occurred. It is therefore essential to note that with unstable caps, turbation events may result in unwanted secondary resuspension of contaminants.