Enhanced transport of heavy metal ions by low-molecular-weight organic acids in saturated porous media: Link complex stability constants to heavy metal mobility.

Environment-ubiquitous low - molecular-weight organic acids (LMWOAs) can interact with heavy metal ions and thus affect their mobility in subsurface aquifers. Herein, the effects of LMWOAs (including acetic acid, tartaric acid, malonic acid, oxalic acid, and citric acid) on the mobility of heavy metal ions (including Cd2+, Zn2+, Ni2+, Mn2+, and Co2+) in porous media were investigated to reveal the role of the stability constants of metal-LMWOA complexes in the mobility of heavy metal ions in porous media. The results showed that the mobility of different metal ions followed the order of Cd2+ < Zn2+ < Ni2+ < Mn2+ < Co2+ despite of LMWOAs-free or LMWOAs-addition. For each heavy metal, all the organic acids enhanced its transport by forming stable non-adsorbing metal-LMWOA complexes and the enhanced ability followed the order of citric acid > oxalic acid > malonic acid > tartaric acid > acetic acid. An interesting finding was that there was a significantly positive correlation between the enhanced abilities of LMWOAs to metal mobility and the complex stability constants (log K) (R 2 = 0.801–0.961, p < 0.05), indicating that the complex stability of metal-LMWOA was the dominant factor responsible for the enhanced transport of heavy metal ions. Meanwhile, the linear slope indicated the intensity of enhancement of LMWOAs on heavy metal mobility was heavy metal type-dependent. This study proposed that the complex stability of metal-LMWOA could be an indicator to quantify and predict the impact of LMWOAs on the mobility of heavy metals. [Display omitted] • LMWOAs facilitate metal mobility due to the formation of non-adsorbing complexes. • For a given metal ion, the extent of enhanced effects depends on LMWOA species. • The complex stability is the dominant factor responsible for the enhanced effects. • The intensity of enhanced effects of LMWOAs on metal mobility depends on metal type. [ABSTRACT FROM AUTHOR]