21182
Since commercial raw MWCNT contain impurities that may affect their bioactivity , HCl refluxing was exploited to purify raw " as - received " MWCNT by removing the amorphous carbon layer on the MWCNT surface and reducing the metal impurities ( e . g . Ni ) .
 HCl, carbon, Ni
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 HCl, carbon, Ni