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

The reaction of CO2 andwaterwith basaltic rock can release trace heavymetals,which pose a serious threat to the

quality of surfacewaters. The pH of the carbonatedwater 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.