Although
recent research points to Mo as a candidate for anthropogenically
induced contamination of natural waters, there has been little research
on Mo contamination in river water. The Erdenet mine in Mongolia is
the one of the largest Mo mines in the world. Our systematic temporal
and spatial monitoring of Mo concentrations in river waters around
the Erdenet mine from March to November 2018 showed that in the Khangal
River, which is downstream of the Erdenet mining area, Mo concentrations
exceeded 70 μg/L (the World Health Organization health-based
guideline) during summer. The sources of the Mo in river water are
groundwater used for cooling in the Erdenet power plant and seepage
from mine tailing ponds. Mo concentrations had a positive correlation
with the pH of the river water. Laboratory adsorption experiments
confirmed that the pH dependency of Mo concentrations originates from
the adsorption/desorption reactions of iron oxide in the river sediments.
Mo concentrations in river water were still higher 10 km downstream
of the contamination sources. Our observations provided field evidence
of high solubility and mobility of Mo in river water that were caused
by the lower distribution coefficients of Mo adsorbed on iron oxides
in water at pH >8.
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