Metal scavenging by calcium carbonate at the Eyjafjallajökull volcano: A carbon capture and storage analogue

- We investigated the ongoing travertine precipitation in the Hvanná River, Iceland.
- The river water was heavily charged with cations and dissolved CO2.
- The travertine was identified; it was dominated by calcite with evidence to suggest initial formation of ikaite.
- Heavy metals present initially were taken up by the travertine.
- Partition coefficients derived from the travertine correlated extremely well with data reported in the literature.

The reaction of CO2 and water with basaltic rock can release trace heavy metals, which pose a serious threat to the quality of surface waters. The pH of the carbonated water increases during dissolution of the host rock or dilution by pore fluids. This leads to precipitation of carbonate and other secondary minerals that often scavenge the released heavy metals. However, very little is known about uptake capacity of the precipitates in natural systems or how much divergence there could be, compared with behavior in laboratory experiments. The spring 2010 eruption of the Eyjafjallajökull volcano, Iceland, provides a unique opportunity to study the mobility of heavy metals that are released during CO2 injection into shallow basaltic aquifers and the ensuing precipitation of carbonate minerals.Following the Eyjafjallajökull eruption, rapid and constant travertine formation was discovered in the Icelandic river, Hvanná, in the vicinity of the volcano. The river water emerged from under the lava flow and was heavily charged with cations and dissolved CO2. The concentration of the major dissolved constituents was: dissolved inorganic carbon (DIC), 33.08 mM; calcium, 6.17 mM; magnesium, 4.27 mM; sodium, 2.78 mM and sulfur, 1.92 mM. Carbon dioxide degassing of the river water increased pH from 6.6 to 8.5 and travertine precipitated for hundreds of meters downstream, rendering the stream bed white with calcite. Rapid crystallization rate produced dendritic structures or sometimes very porous material. Boxwork textures were observed within the porous calcite that probably originated from transformation of a metastable phase such as ikaite (CaCO3·6 H2O). A gradual decrease of conductivity from 1.8 mS/cm at the river water outlet to 1.1 mS/cm downstream and a clear drop in dissolved metal concentration strongly correlated with the precipitated calcite. Considering the complexity of the natural system, the estimated partition coefficients for Ba, Cd, Co, Cu, Mg, Mn, Na, Ni, Sr and Zn are in good agreement with the values derived from laboratory experiments under rather ideal conditions. Other elements were also scavenged from the river water, including Al, Fe, K, P, S, Si, Ti, V and the rare earth elements (REE). Our thermodynamic modeling suggests that, in addition to calcite and ikaite, silica, clay minerals, ferrihydrite, gibbsite and amorphous Ca, Mg carbonate minerals were supersaturated as the spring water degassed its CO2. Our results provide a valuable base for assessing the environmental impact of volcanic eruptions in basaltic terrain and carbon capture and storage (CCS) in basaltic rock.