Why is 就 used in ...我实际上在立法会内曾就胶袋问题提出质询...? 兩個月前，我其實在立法會內曾就膠袋問題提出質詢，因為我們有一次 到沙灘和路邊的郊野公園去，看見那情況感到很恐怖。 I looked it on MDBG but didn't get anything of important meaning.
ABC calls this a 介词 (converb) and defines it like this:


  with regard to; concerning


Here's the examples they use:


  就A 来说 
  
  Jiù A lái shuō
  
  with regard to A


And:


  就A 而言 
  
  Jiù A ér yán
  
  with regard to A


就膠袋問題提出質詢 = as for the inquires raised in regard to the problems with the pastic bags