開發硫脲交聯自組裝還原氧化石墨烯球從回收電子廢棄物的廢水中選擇性吸附/還原黃金

Selective Adsorption/Reduction of Au in E-waste Recycling Wastewater by Self-Assembly of Thiourea-Crosslinked Reduced Graphene Oxide Framework Ball

陳廷鈞(Ting-Chun Chen)

指導教授 ： 席行正

共同指導教授 ： 林正芳(Cheng-Fang Lin)

國立臺灣大學/工學院/環境工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202100854

摘要

現今科技快速發展帶來生活品質的提升，電器和電子設備生產及消費的增加導致些許原材料供應與需求的比例失衡；特別是金具有獨特的性質，例如耐腐蝕性和高導電性，因此在印刷電路板中是不可替代的材料。黃金是回收廢棄電子電器設備（WEEE）中最有價值的金屬之一，在2019年全球共產生的電子廢棄物總量為5360萬噸， 所以能從電子廢棄物中回收貴金屬已引起全世界的廣泛關注。 然而，在濕法冶金的實際應用中很少有選擇性吸附“低濃度”的金離子。本論文研究目的是設計一種吸附劑從溶液中選擇性捕捉低濃度金離子，且能應用在連續式固定床反應器中。

本論文開發硫脲交聯自組裝還原氧化石墨烯骨架球（TU-rGOF球）的合成方法來選擇性吸附/還原金離子。首先，必須通過（3-氨基丙基）三甲氧基矽烷對作為石墨烯載體的沸石球進行官能基化處理，這能使初始的氧化石墨烯（GO）沉積在球型沸石球載體上。 隨後，將官能基化的沸石球浸潤在硫脲（TU）與氧化石墨烯的去離子水溶液中，並且在不攪拌的情況下加熱溶液。更具體來說，加熱過程分為兩個階段：（1）將混合溶液的溫度保持在60℃持續半小時，以便通過硫脲交聯的方式使氧化石墨烯層層自組裝；（2）接著將溫度升至80℃持續3.5小時，將氧化石墨烯還原成還原氧化石墨烯（rGO）。此外，硫脲嫁接在石墨烯表面上並與石墨烯產生反應讓表面具有選擇性吸附金和還原金的能力，所得的TU-rGOF球在低濃度金溶液中展現出極佳的選擇性和吸附性質。由批次吸附實驗結果，一顆TU-rGOF球在共存銅、鉛、鋅和鎳二價金屬離子中吸附1 mg L-1金離子溶液，材料展現出對於金高度選擇性吸附作用，在pH 2的溶液中經過24小時吸附效率高達90％。最重要的是，TU-rGOF球在來自臺灣貴金屬回收公司的複雜廢水中同樣具有吸附/還原金的能力。根據等溫吸附實驗，TU-rGOF球吸附金更擬合Langmuir等溫吸附線，其理論的最大吸附量為97.09 mg g-1，該理論描述黃金在TU-rGOF球的表面上形成單層覆蓋，也符合在SEM顯微照片的觀察。在脫附實驗中，使用重量濃度5%的硫脲溶液可以將金顆粒從TU-rGOF球上在攝氏50度下24小時脫附，其脫附效率為90%。但是，再重複吸脫附3次時吸附效率降到57%。

透由表面分析技術:元素分析儀、掃描式電子顯微鏡-X射線能譜儀、X光光電子能譜儀、X光繞射儀、衰减全反射法-傅立葉轉換紅外線光譜、拉曼光譜和界達電位可以為TU-rGOF球的結構和表徵提供有效的證據。總結來說，我們提出的選擇性吸附機制， TU-rGOF球可能通過以下方式發生金吸附/還原：（1）由硫脲引起靜電相互作用、配位相互作用，（2）還原氧化石墨烯的sp2混成碳原子和含氧的官能基團供給電子給金離子還原成元素金。這項研究提供了一種新型且極具潛力的材料TU-rGOF球用於濕法冶金技術從電子廢物廢水中回收黃金，並且有望實現電子廢棄物資源可持續利用化和循環經濟的概念。

關鍵字

還原氧化石墨烯 ； 硫脲 ； 選擇性吸附還原 ； 金 ； 電子廢棄物

並列摘要

With technological advancements to improve the quality of life, the increase in the production and consumption of electrical and electronic equipment (EEE) has led to maladjustment in some raw materials supply-demand ratio; especially, gold (Au) is an irreplaceable material in PCBs because of unique properties such as corrosion resistance and high electrical conductivity. Au is one of the most valuable metals in waste electric and electronic equipment (WEEE), which in 2019 amounted to 53.6 million metric tons. Therefore, the recovery of precious metals from WEEE has considerable attention around the world. However, there has been relatively little conducted on selective adsorption "low concentration" of Au ion in the hydrometallurgical process for the practical application. This study aims to design an adsorbent that selectively captures a low concentration of Au from the solution in the continuous fixed-bed reactor.
This research reports synthesis of self-assembly of thiourea-crosslinked reduced graphene oxide framework ball (TU-rGOF ball) for selective Au adsorption/reduction. First, zeolite ball as graphene support must be functionalized via (3-aminopropyl)trimethoxysilane, which makes initial graphene oxide (GO) deposit on the spherical carrier. Subsequently, functionalization of zeolite ball is immersed in the mixed solution of thiourea (TU), graphene oxide, and deionized water. The solution is heated with no stirring. More specifically, the heating process is divided into two stages: (1) the temperature of the mixture is maintained 60℃ in 0.5 h for self-assembly of graphene oxide by thiourea crosslinking (2) the temperature is then increased to 80℃ for 3.5 h to reduce graphene oxide into reduced graphene oxide (rGO).
Moreover, the TU molecule grafted and interacting with graphene endows the surface with significantly enhanced selectivity and reducing power. The resultant TU-rGOF ball features excellent selective and adsorptive capabilities in low Au concentration. The batch adsorption experiments demonstrate that high selectivity and excellent adsorption efficiency of up to 90% with one TU-rGOF ball for 1 mg L-1 of Au in coexisting base metal ions, Cu(II), Pb(II), Zn(II), and Ni(II) solution, at pH 2 for 24 h. Most importantly, the TU-rGOF ball has the ability of Au adsorption/reduction in the real and complicated wastewater originated from a precious metal recycling company in Taiwan. According to the isotherm adsorption experiments, Au adsorption by TU-rGOF ball appears to fit better with the Langmuir isotherm than the Freundlich isotherm theoretical maximum adsorption capacity is 97.09 mg g-1. Hence, this result demonstrates that Au forms a monolayer coverage on the surface of the TU-rGOF ball in conformity with SEM observation. For desorption experiments, a 5 wt% TU solution can desorb Au particle from TU-rGOF ball, which desorption efficiency is 90% in 24 h at 50℃. However, the adsorption efficiency is down to 57% in repeated 3 cycles.
Surface analysis technology by EA, SEM-EDS, XPS, XRD, ATR-FTIR, Raman spectroscopy, and Zeta potential can exhibit valid evidence for the structure and characterizations of TU-rGOF ball. In summary, we propose a selective adsorption mechanism that Au adsorption/reduction by TU-rGOF ball possibly occurs through (1) the electrostatic interaction, coordination interaction due to thiourea, and (2) Au reduction by sp2 carbon and O-containing functional groups of graphene for electron donation to Au. This study provides a novel and potential material, TU-rGOF ball, in the hydrometallurgical recovery of Au from E-waste wastewater, and it is expected to achieve sustainable use of resources of WEEE and the concept of a circular economy.

並列關鍵字

Reduced graphene oxide ； Thiourea ； Selective adsorption/reduction ； Gold ； Electronic waste

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