二價硫、氯離子與溶解性有機物對活性碳吸附水中二價汞之影響

Abstract

近年來，汞汙染底泥對於水體環境造成嚴重的危害與影響，而使用薄層覆蓋法被許多研究發現能有效抑制汞從底泥中溶出，並被認為是一個較適當且具彈性的整治方法。活性碳被許多相關研究報導具有有效吸附水中的二價汞之能力，是相當有潛力的覆蓋材料。然而環境因子像是二價硫、氯離子以及溶解性有機物往往能影響汞在水體環境中的宿命，而環境因子對於活性碳吸附汞行為之影響卻鮮少被探討。因此本研究之目標為釐清汞在存在二價硫、氯離子以及溶解性有機物情況下被活性碳吸附之機制與影響。 本研究主要分為兩個部分，其一為活性碳吸附汞之表現，另一部分為汞分布於各相之探討。研究發現，二價硫存在下活性碳具有最高的吸附反應速率，次之為氯離子存在，DOM存在條件下則最低。在動力模式模擬部分，添加氯離子以及DOM下，活性碳吸附汞之行為以直接吸附Hg-Cl和Hg-DOM複合物為主，並以擴散為速率限制步驟。添加二價硫則以偏向擬二階動力模式去除水中溶解態之汞。在汞去除率方面，添加氯離子範圍在0‒400 mM下，去除率皆在落在92.1‒95.2%。添加二價硫(2‒20 μM)的情況下，汞去除率則落在65‒75%，其中液相顆粒汞的形成貢獻23.6‒28.7%的去除率，而約有30%的汞是以凡德瓦力吸附在活性碳上。添加DOM (0.25‒20 mg-C L-1)的情形下，隨著DOM濃度上升，溶解態的汞也隨之提升了30.6%，並且同時可看到活性碳相上的汞減少46.8%。綜觀而言，氯離子的存在提升了活性碳的吸附量，然而二價硫與DOM皆對汞之吸附造成負面影響，這是未來將活性碳應用在薄層覆蓋法上必須要克服的重要議題。

Mercury-contaminant sediments have posed a serious threat to aquatic ecosystems. Using thin layer capping to reduce mercury (Hg) released from contaminated sediment is a feasible and durable remediation approach. Activated carbon (AC) has been reported as an effective adsorbent to capture Hg. However, there are various environmental factors such as sulfide, chloride and dissolved organic matter (DOM) that could significantly affect the Hg fate in the aquatic system. The main objective of this research is thus to clarify the Hg adsorption mechanism by AC with the presence of sulfide, DOM, and chloride. The lab-scale batch experiments can be divided into two parts: (1) AC adsorption performance and (2) Hg distribution test by operational definition method. The rate of Hg adsorption on AC was various in the presence of sulfide, chloride, and DOM (from fast to slow). Hg adsorption might be directly bonded on AC with Hg-Cl and Hg-DOM complexes and mainly controlled by intraparticle diffusion. In contrast, “Hg+S” results was shown better fitted to pseudo-second order model. The Hg removal efficiency was (92.1‒95.2%) in the presence of chloride (0‒400 mM). The Hg removal efficiency was around 65‒75% in the “Hg+S” condition. Among removal of Hg, 23.6‒28.7% of Hg was formed into aqueous-phase particles and around 30% of Hg was adsorbed on AC by Van der Wall's force under sulfide concentration (2‒20 μM). Increasing DOM concentration resulted in more dissolved phase of Hg in the “Hg+DOM” condition. The proportion of “dissolved Hg” increased 30.6% by increasing DOM concentration from 0.25 to 20 mg-C L-1. Simultaneously, the proportion of “Hg in AC” decreased by 46.8%. Overall, the presence of chloride increases the Hg adsorption on AC. However, the presence sulfide and DOM causes a negative effect that should be concerned in future application.