許多研究指出，活性碳對於六價鉻有優異的去除能力，亦提出吸附劑對於六價鉻會發生還原機制，然而這些文獻中卻未區隔吸附劑的吸附能力與將六價鉻還原為三價鉻的能力，因此本研究欲探討活性碳對於鉻酸鹽的還原以及吸附機制。 由pH=1~10的實驗得知，總鉻去除率在pH=3的環境下去除效果最佳，約可達64%，而隨著pH值與pH3差距越大，總鉻去除效果越差，去除率的反應以擬二階動力模式最符合反應速率的變化，計算可得活性碳對於六價鉻濃度100ppm在pH=3的平衡吸附量為6.4mgCrtot/gGAC ，而pH值越低、活性碳將六價鉻還原為三價鉻的速率就越快，以一階動力模式最符合反應速率的變化，若使用2克的活性碳欲將200ml六價鉻濃度100ppm去除率達99%，在pH=1的條件下，需52.5分鐘，在pH=6條件下則需1439.1分鐘。由於pH>3以後，活性碳表面帶正電荷之官能基減少，因此吸附容量下降，然而當pH值越低、活性碳將六價鉻還原為三價鉻的效果越好，造成低pH值時六價鉻大量還原為無法被吸附的三價鉻，因而形成總鉻吸附率減少。 另外、溫度的上升可加速還原反應速率，在pH=1時，去除時間由25℃的52.45分鐘縮短為35℃的41.90分鐘，不過溫度的上升不利於活性碳對六價鉻的吸附，在相同pH之下，35℃的飽合吸附量<30℃的飽合吸附量<25℃的飽合吸附量。 吸附飽和後的活性碳若以強酸為再生液使活性碳帶有正電荷的官能基，反而不利於六價鉻的脫附，必須依賴強酸將吸附於活性碳上的六價鉻還原為三價鉻後才能脫附，反之、氫氧化鈉使活性碳表面帶有負電荷，於是可輕易將六價鉻脫附而出。 Hexavalent chromium ions, Cr(VI), have been shown to be adsorbed or reduced on the surface of GAC. The tendency of Cr(VI) reduction by various adsorbents has been mentioned by several researchers, but the mechanisms have not been investigated in great details. Removal of Crtot and Cr(VI) was studied at pH ranging from 1 to 10. The results show that remvoal efficiency of Crtot increases with increasing pH, reaches maximum (64%) at pH3, and decreases with pH afterward. The kinetics of Crtot removal can be fitted by the pseudo-second-order model. On the other hand, the Cr(VI) remvoal increases with decreasing pH, and kinetics of Cr(VI) removal can be described by pseudo-second-order model. The amount of positively charged functional groups on activated carbons decreases with increaseing pH, resulting in reducing sorption capacity of GAC for Cr(VI). As pH of less than 3, Cr(VI) was reduced to Cr(III) readily, and as the result the amount of Cr(VI) removed by adsorption process is much less than reduction. Increasing temperature from 25℃ to 35℃ accelerates reduction rate but retard adsorption rate of GAC for Cr(VI). Chromium-loaded GAC was regenerated with HCl or NaOH. The former increases positively charged functional groups on GAC surface and Cr(VI) is not desorbed readily. On the other hand, the latter makes negatively charged functional groups on GAC surface and Cr(VI) is desorbed easily.
