Figure 13. Citric acid is a naturally occurring chelating agent. The chelation process is water activated. EDTA, NTA, citrate, oxalate, malate, succinate, tartrate, phthalate, salicylate and acetate etc. have been used for "chelate-induced hyperaccumulation". Synthetic soil amendments such as ammonium thiocyanate and natural zeolites have yielded promising results in inducing hyperaccumulation of metals.