Role of Organic Acids in Desorption of Mercury from Contaminated Soils in Eastern Shandong Province, China

Batch experiments were conducted to study the effects of low-molecular-weight organic acids (LMWOAs) on desorption of Hg
from two kinds of soils in the eastern Shandong Province, China. Of all LMWOAs, oxalic and citric acids were chosen as the
representatives in this experiment because they are most common in soils. Desorption of Hg increases with the increase of extracting
time, until the highest desorption amount at 6–8 h, and then declines. It indicates that timeliness is very important when organic acids
are used for phytoremediation in contaminated soils. When the concentrations of organic acids increase from 1.0 mmol/L to 10.0 mmol/L,
Hg desorption increases significantly regardless of oxalic or citric acid. Compared with oxalic acid, citric acid is more effective to enhance
the desorption of Hg, especially from fluvo-aquic soil. With the increase of initial pH value from 3 to 8, Hg desorption decreases directly in
the presence of oxalic acid. However, Hg desorption first decreases, then increases, and eventually decreases again in the presence of
citric acid due to the variation of competitive ability between chelating of citric acid and adsorption of soil in different pH values. Citric
acid showed greater ability to release Hg from soils than oxalic acid. According to the results of the present study, it is found that the
bioavailabilities of heavy metal can be improved with selecting suitable types and concentrations of organic acid amendment and
reasonable soil condition.