利用硫脲還原氧化石墨烯進行廢水中選擇性吸附金之研究

Abstract

隨著科技的迅速發展，電子電器設備不斷的推陳出新，對於電子電器設備的需求量同時快速上升，導致電子電器設備的廢棄量也有逐年增加的趨勢。根據聯合國大學研究指出，2016年的電子電器設備之廢棄量達44.2百萬噸，然而，印刷電路板為電子電器設備中的重要元件，電刷電路板富含高價值的金，另外，印刷電路板的製造過程中會產生含金廢水，因此，如果能從廢棄電子電器設備及含金廢水中回收金，不僅能使資源再利用，同時也落實城市採礦的前瞻思維。 本研究係利用Hummers 法合成氧化石墨烯並改質成硫脲還原氧化石墨烯進行廢水中選擇性吸附金之實驗，首先進行氧化石墨烯吸附廢水中銅、鉛及鋅，結果顯示，由於氧化石墨烯氧含量為40%，因此對於銅、鉛及鋅有良好的吸附效果，然而對於金並無選擇性吸附效果，因此透過改質成硫脲還原氧化石墨烯，增加對金之選擇性吸附效率。經由元素分析結果顯示，經改質後之硫脲還原氧化石墨烯硫及氮含量分別為22.8 wt.%及2.0 wt.%，氧含量從40 wt.%下降至10 wt.%，再進一步探討劑量、pH及時間對吸附金之影響，結果顯示硫脲還原氧化石墨烯在酸性溶液中對於金有較佳之吸附效率，然而需要96小時才可達吸附平衡；另外在同時含有銅、鉛、鋅及金之廢水中進行選擇性吸附實驗，在0.1 mg L-1至10 mg L-1 金含量溶液，硫脲還原氧化石墨烯之金選擇性吸附效率為95-99%，對於鉛僅有1-2%的吸附效率，對銅及鋅都不具吸附效果，因此硫脲還原氧化石墨烯具有相當高的金選擇性吸附效果。另外利用硫代硫酸銨進行金脫附實驗，結果顯示經由0.2 M硫代硫酸銨處理，金脫附效率達93.7%；為了使金能夠脫附完全，本研究嘗試利用二階段脫附增加脫附效率，結果顯示脫附效率只增加1.4%；本研究再接續探討材料之再生能力，經由五次之吸附脫附實驗，結果顯示，吸附效率從99.0%下降至77.6%，根據元素分析結果顯示，硫含量也下降至5.5 wt.%。此結果主要歸因於金吸附於硫脲還原氧化石墨烯表面時被硫官能基還原成元素金，硫官能基則會被氧化成硫化物，導致硫脲還原氧化石墨烯表面上硫含量的損失。

With the rapid technology development, massive improvement and innovation on the performance of electric and electronic equipment (EEE) has been achieved, causing an increasing demand for EEE. Hence, the amount of waste electric and electronic equipment (WEEE) has markedly increased in recent years. According to the Global E-waste Monitor 2017 of the United Nations University report, 44.2 Mt of WEEE has been generated in 2016. Notably, the printed circuit boards (PCBs), which contain a valuable amount of Au are also key components of EEE. The massive production of EEE makes the content of Au existing in the PCBs of WEEE much larger than that in ores. Additionally, the wastewater generated from the process of PCBs manufacturing also contains metals and precious metals. As a result, the recovery of Au from WEEE and wastewater could be an important Au source for urban mining. In this study, selective adsorption experiment of Au was carried out by using graphene oxide (GO) and thiourea reduced graphene oxide (TU-rGO). Firstly, GO was synthesized through Hummers’ method, and modified by thiourea to thiourea reduced graphene oxide. The adsorption experiments of Cu, Pb, and Zn were carried out in wastewater. The experimental results showed that GO had excellent adsorption of Cu, Pb, and Zn because of its abundant oxygen content (over 40%). However, GO cannot selectively adsorb Au. Therefore, TU-rGO was subsequently prepared to selectively adsorb Au. The element analysis showed that the content of sulfur and nitrogen was 22.8 wt.% and 2.0 wt.% for TU-rGO, respectively. The content of oxygen decreased from 40 wt.% to 10 wt.% as compared to GO. Then, the effect of TU-rGO dosage, pH, and contact time was discussed. The results showed that TU-rGO had greater adsorption efficiency for Au in the acidic environment as compared to basic environment. Then, selective adsorption experiment of TU-rGO for Au was carried out with Cu, Pb, Zn and Au coexisting in the wastewater. TU-rGO still demonstrated a high efficiency of 95-99% Au selective adsorption with the initial Au concentration of 0.1-10 mg L-1. However, 96 h are needed to reach the Au adsorption equilibrium on TU-rGO. In order to separate Au from TU-rGO, desorption experiment was further carried out. The experimental result indicated that desorption efficiency of Au reached 93.7% when 0.2 M ammonium thiosulfate was added. To complete the desorption of Au, two-step desorption was carried out to examine the desorption efficiency of Au. The experimental result showed that desorption efficiency of Au only slightly increased about 1.4%. The regeneration of TU-rGO was subsequently discussed. The adsorption efficiency of TU-rGO decreased from 99.0 to 77.6% after five cycles of adsorption and desorption, with the content of sulfur and nitrogen decreasing to 5.5 wt.%, respectively. These results suggest that the Au adsorbed on the TU-rGO was consequently reduced to elemental Au. In addition, the sulfur on the surface of TU-rGO would be oxidized, resulting in forming sulfide.