text: A thiol is a molecule with a covalently bonded -SH group on it (like an alcohol has a covalently bonded -OH).  A thiol is quite nucleophilic (more than oxygen in alcohol); in other words, its an electron-donor (i.e. Lewis base).  They react with oxidizing agents to form disulfides or in extreme cases sulfonic acids.  \n\nI hope this helps you get started on your problem; you're going to have to spend a little time digging on your own to understand their role in phytoremediation (particularly in reference to whatever homework you're doing, your reading or teacher should've given you a clue but your question is too vague right now for me to give much help without answering it outright).