Tracing the behavior of metallic oxyanions in an acid mine drainage polluted estuary (Tinto River estuary, SW Spain)

Estuaries are dynamic systems that play a crucial role in the transfer of trace elements from continent to ocean. The Ría de Huelva estuary is formed by the confluence of the Tinto and Odiel rivers, which are severely contaminated by acid mine drainage (AMD), transporting high pollutant loads of sulfates and metal(loid)s to the estuary and ultimately to the Atlantic Ocean. Despite extensive research on this estuary, the behavior of oxyanions has not yet been adequately addressed. This work assesses the variations of the concentration of oxyanion-forming elements such as As, Sb, V, and Mo within the fluvial domain of the Tinto River estuary. Two sampling campaigns were conducted under low river flow, one with a low tidal coefficient (26-30) and the other with a high tidal coefficient (99-102). Dissolved and particulate (>0.45 µm) As, Sb, V, and Mo concentrations were measured in samples of surface water collected by an autosampler Teledyne ISCO with high temporal resolution (hourly) for 24 hours. Additionally, physicochemical parameters were measured for each sample. During both sampling periods, several complete tidal cycles were recorded. The pH and EC values ranged from 4.0 to 6.3 and from 43.2 to 51.9 mS cm^-1, respectively, with low tidal coefficient. With high tidal coefficient, the pH and EC values ranged from 4.4 to 7.6 and from 43.7 to 53.0 mS cm^-1, respectively. Highest values were recorded during high tide conditions due to greater influence of seawater. Particulate oxyanions proportion displayed a general increase along with increasing pH, being retained by Fe-oxyhydroxisulfate precipitates like schwertmannite as demonstrated in previous research [1]. Data obtained suggest that during high tide with low tidal coefficient, oxyanions remain mainly in the particulate phase (55-100%). Nevertheless, at high tidal coefficient, a significant decrease in the particulate phase proportion is observed when pH is above 6.43 with total concentrations found in the dissolved phase (reaching 100%). This increase in the dissolved phase is probably associated with desorption processes related to the zero-point charge of schwertmannite in circumneutral water environments. During the seawater neutralization of the Tinto River’s acidic waters, when pH values are above 6, the schwertmannite surface becomes negatively charged, and since oxyanions are negatively charged, their desorption is expected [1]. Special attention should be paid to As, Sb, V, and Mo, as they return to solution when precipitates reach pH values above 6.43, contributing to the total amount discharged by the Tinto River, which later reaches the ocean.

Acknowledgments

This work is part of the I + D + i TRAMPA project (PID2020-119196RB-C21), funded by MCIN/AEI/10.13039/501100011033.

[1] Pérez-López, R., Millán-Becerro, R., Basallote, M. D., Carrero, S., Parviainen, A., Freydier, R., Macías, F., Cánovas, C. R. (2023). Effects of estuarine water mixing on the mobility of trace elements in acid mine drainage leachates. Marine Pollution Bulletin, 187, 114491.