Towards understanding formation of polytungstates in tungsten-bearing hot springs and its environmental implications

Polytungstates have toxicity, mobility, and bioavailability quite different from those of monomeric tungstates, the most common tungsten (W) species. To date, relevant studies of polytungstates have been conducted under laboratory conditions, and there is a lack of studies on such W species in natural waters. In the present study, polytungstates at environmentally relevant concentrations were detected in a number of investigated hot springs. All the hot springs with high polytungstates concentrations (> 10 μg/L) have vent temperatures above 70 °C and pH values above 8, and polytungstates are immeasurable in all the acidic hot springs. Further inspection showed that the detected polytungstates were not isopolytungstates but likely heteropolytungstates forming via incorporation of silicate or borate enriched in the hot springs into tungstates at elevated temperatures. These detectable polytungstates, however, might also be polymeric thiotungstates in view of their close correlation with aqueous sulfide and thiotungstates and their specific retention time during the chromatographic separation. More precise identification of the polytungstates depends on improvement of the used analytical technique to achieve a further chromatographic separation of different kinds of polytungstates. Upon the discharge of the hot springs, the aqueous W was transported to the hot spring sediments due to its adsorption onto the Fe-bearing minerals there. Although the pH values of the hot springs were the critical parameter for the W adsorption, the substantial formation of polytungstates in some neutral to alkaline hot springs could inhibit the adsorption to some degree, thereby contributing somewhat to the enrichment of W in such springs. This is the first study reporting polytungstates and their environmental behavior in natural hot springs. It is possible that polytungstates are widely present in high-temperature, neutral to alkaline hot springs hydrochemically similar to those involved in this study.