Low-molecular-weight organic acids (LMWOAs) are ubiquitous in the aquatic environment and consequently may affect the heavy metal transport in aquifer systems. However, the low-molecular-weight organic acids (LMWOAs) based studies on transport of Cd²⁺ have been paid limited interest.

In this study, three LMWOAs such as acetic acid, tartaric acid, and citric acid were employed. Under acidic conditions (pH 5.0), the results indicated that LMWOAs inhibited the transport of Cd²⁺ even at the low concentrations of organic acids (i.e., 0.05 and 0.1 mM). The inhibition effects might be attributed to the complexation role of the sand surface-bound organic acids and also electrostatic interaction. Meanwhile, the inhibition effects of LMWOAs on Cd²⁺ transport in the following order of citric acid > tartaric acid > acetic acid, which was also in agreement with the decreasing complex stability constants between Cd²⁺ and LMWOAs. However, under neutral conditions (pH 7.0), LMWOAs generally enhanced the transport of Cd²⁺. The transport-enhancement of LMWOAs was ascribed to the formation of stable aqueous non-adsorbing Cd–organic acid complexes.

These findings demonstrate that LMWOAs may inhibit or facilitate Cd²⁺ transport under different environmental conditions. Thus, environmental assessment concerning the transport of heavy metals should consider the roles of organic acids.

Dr Zhichong Qi, teacher of College of Chemistry and Chemical Engineering, Henan University, is the corresponding author of this paper. This project was supported by the National Natural Science Foundation of China (Grants 21707081), the Opening Foundation of Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria (2018-06), the program for Science & Technology Innovation Team in Universities of Henan Province (19IRTSTHN029), and Henan Joint International Research Laboratory of Environmental Pollution Control Materials.

Article links: https://www.sciencedirect.com/science/article/pii/S004313541930956X