Use Google Scholar with the search terms lime ash phytoremediation you will get approximately 2,700 citations. 

The mode of action of heavy metal immobilization is through a change of soil pH. Under acid soil conditions the heavy metals are mobilized from the acidity under the neutral and alkaline state these are immobilized. 

Even with high content many of these heavy metals are sparingly soluble at pH 7.3 to 8.0.

If you have high content and low soil pH your first step is to lime to a neutral to slightly alkaline state and reassay the contents which are found in your plants of interest. 

Both lime and ash are alkaline in nature and will lead to increasing soil pH. If you had Mo heavy metal issue liming mobilizes the metal and you might have to acidify soil to immobiilize the issues.

In relation to these issues increasing soil organic matter can be helpful. A caveat is that many municipal biosolids can be high in heavy metals because of industrial mixture in the municipal waste streams. This is generally not a problem in municipalities without industrial input into their sewage system. 

Good luck on your project if you found the information helpful give me a recommendation. 

Much appreciated. 

Dear Paul Thank You.

Hi,

I think Paul has nicely summarized several of the key points.

These articles may be of interest to you.

Hope it helps!

Good luck with your research!

The only thing I would add is that you should check the lime and especiallythe ash for heavy metal contaminants