以硫脲改質球狀活性碳進行印刷電路板廢水中金之選擇性吸附

Abstract

城市採礦，代表著從廢棄電子電機設備(WEEE)產品等回收有價物質，是一種循環經濟的概念。根據文獻顯示，2016年的廢棄電子電機設備已達到44.7百萬噸且預估在2021年將成長至52.2百萬噸。將資源循環永續利用，在全球越來越受到重視，而對缺乏天然礦產的台灣來說，更是重要的議題。在廢棄電子電機設備中，以印刷電路板(PCB)為例，含有金、銀、鉑等貴重金屬，雖然這些貴重金屬的佔比相對少，但其價值卻相當高；而金在貴金屬中，依據其佔比換算價值，可獲得最高的回收價值。 本研究使用硫脲改質活性球狀碳以吸附方法回收金，首先將碳化過的酚醛樹酯放在直立高溫爐中並通入二氧化碳，經過4小時、900oC的活化後，形成球狀活性碳(ACS)，再將硫脲溶入水與酒精混合溶液，倒入裝好球狀碳的燒杯中，經過8小時、60oC的加熱攪拌後，過濾烘乾而得到硫脲改質活性球狀碳(TUACS)。經過初步選擇性吸附實驗測試中可發現，在含有金、銅、鉛、鋅、鎳的水溶液中，硫脲改質活性球狀碳對金具有選擇性吸附的特性，因此，進一步對於此材料進行物性化性分析，根據元素分析(EA)結果可得知，硫含量從0上升至10%；而BET比表面積從2445.3 m2/g下降至1948 m2/g，顯示硫官能基成功吸附在球狀活性碳上造成表面積與孔體積下降。經過XPS分析，可得知硫官能基可能為C-S鍵與亞硫酸鹽離子sulfite ion (SO32-)，可協助金離子還原為元素金的鍵結官能基。另外，探討時間、劑量、pH值對硫脲改質活性球狀碳吸附金的影響，發現達到飽和吸附平衡時間為96小時、且在pH=2、劑量為0.01 g L-1，其吸附金的回收效率可達70%左右，而劑量增加至0.06 g L-1時，其回收效率可上升至99%左右。在等溫吸附實驗中，可得知其較符合Langmuir等溫吸附曲線。在脫附實驗中，利用0.3 M的硫代硫酸銨(ammonium thiosulfate)進行脫附實驗，脫附時間為24小時，得到的脫附效率可達到94%，證實使用硫代硫酸銨脫附元素金為可行方法。最後進行重複吸附與脫附試驗，了解此材料之重複利用率，經過五次的重複吸脫附，其金回收效率仍可達到90%，表示此材料很適合重複使用。

Urban mining represents the recycling of valuable materials from waste electrical and electronic equipment (WEEE) products, and it is a concept of circular economy. According to the literature, the waste electrical and electronic equipment in 2016 has reached 44.7 million tons and is expected to grow to 52.2 million tons in 2021. The sustainable use of resources is increasingly important all over the world. For Taiwan, which lacks of natural minerals, sustainable use of resources in waste is even more vital. Among the WEEE, printed circuit boards (PCBs) contain precious metals such as gold, silver, and platinum. Although these precious metals account for a relatively small proportion, their values are very high. Gold is obviously one of the metals with the highest recycling value and worth to be recycled. In this study, thiourea modified activated carbon sphere (TUACS) was used to recover gold by adsorption. First, carbonized phenol-formaldehyde (PF) resin was carbonized using carbon dioxide in an upright high-temperature furnace at 500oC for 1 h. After 4 hours of activation at 900oC, activated carbon sphere (ACS) is formed. ACS was then immensed in to a mixed solution of DI water and ethanol (EtOH) with thiourea dissolved in. The solution was heated and stirred at 60oC for 8 h, then filtered and dried to obtain TUACS. The selective adsorption experiment tests showed that TUACS has excellent ability to selectively adsorb gold in aqueous solution containing gold, copper, lead, zinc, and nickel ions. Elemental analysis results showed that the sulfur content increased from 0 to 10% and the BET specific surface area determined by N2 adsorption decreased from 2445.3 to 1948 m2 g-1, indicating that sulfur functional groups were successfully impregnated in TUACS, causing a decrease in surface area and pore volume. XPS analysis showed that the sulfur functional groups were mainly as C-S bonds or sulfite ion (SO32-), which may be the key functional groups that bond and reduce gold ions to elemental gold. The time to reach an adsorption equilibrium of gold was 96 h at pH= 2 with the dosage of 0.01 g L-1, and the gold recovery can reach about 70%. When the dosage increased to 0.06 g L-1, the gold recovery efficiency of TUACS can rise to about 99%. The isotherm adsorption experiments suggest that the gold adsorption by TUACS is better fitted with the Langmuir isotherm than the Freundlich isotherm.by using 0.3 M ammonium thiosulfate ((NH4)2S2O3) to desorb gold, the desorption efficiency could reach 94% in 24 h. These results confirm that the element gold present in TUACS can be successfully desorbed with ammonium thiosulfate. Finally, repeated adsorption and desorption tests indicate that after five repeated adsorption and desorption, the gold recovery efficiency of TUACS can still reach 90%, confirming that the TUACS fabricated in this study is a novel gold-selective adsorber with high adsorption efficiency and reuse ability.