[Minifig]

Hi All,
Just a quick question that has arisen during my chemistry investigation this year.
I've been testing the adsorption of citric acid onto activated carbon as part of my coursework and every time I do there is always slight fizzing and a smell of sulfur.
I was skeptical it was a reaction with sulphur impurities in the carbon, but I tried reacting the citric acid with sulphur and sure enough, no reaction.
I can only think it would be displacement of SO₂ from the carbon surface, but I'm somewhat skeptical that so much would be produced.
Any ideas?
For reference I'm using 50mL of 0.2 mol dm^-3 citric acid and about 2g of activated carbon in each test.
Thanks in advance,
Mini