Rapid release of metal salts and nutrients from the 2011 Grímsvötn, Iceland volcanic ash

Abstract

On May 21st, 2011, one of Iceland’s most active volcano, Grímsvötn, started its strongest eruption in a century. Grímsvötn produced hundreds of megatonnes of fine basaltic ash, which spread over Iceland, the North Atlantic and Europe. Such fine grained ash can impact human activity and health both with fertilization and with toxicity potential for aquatic environments. The purpose of this study was: (1) to investigate the basic physical and chemical properties of the ash from the 2011 Grímsvötn eruption, (2) to identify surface salts deposited on the ash during the eruption, (3) to identify which elements are released during ash–water interactions and their release rates, (4) to characterize the secondary phases formed during water exposure, and (5) to assess impact of the ash on humans and the environment.During the eruption, we collected a unique set of pristine ash samples over a range of 50–115km from the source and examined them with X-ray powder diffraction (XRPD), X-ray fluorescence (XRF), surface area analysis (BET), laser absorption, electron microprobe (EMPA), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The ash could be classified as glassy tholeiitic basalt with <10mass% crystalline plagioclase (Al1.6Ca0.6Na0.4Si2.4O8) and pyroxene. The particles were small (<125μm) and elongated with sharp edges. About 50% of the particle mass was fine ash (<63μm), which could travel long distances, and ∼8% was very fine ash (<10μm), which is harmful if inhaled. The specific surface area of the pristine ash ranged from 0.4 to 0.7m2/g. Material taken up on particle surfaces contributed the equivalent of <0.5nm thick layer, consisting of salts such as CaSO4, Na2SO4, NaCl, CaF2, CaCl2, MgSO4 and MgCl2.We exposed the pristine ash to ultrapure deionised (MilliQ) water in a single pass, plug, flow through reactor and the effluent was analyzed for 73 elements using inductively coupled plasma sector field mass spectroscopy (ICP-SFMS) and ion chromatography (IC). High release rates of mainly S, Na, Ca, Mg, F and Cl were observed during the first 10min but after 12h, the most abundant element released was Si. The effluent was alkaline. Secondary phases of mainly Al and Fe precipitated on the ash surfaces and these were suspected of scavenging As, Ba, Cr, Co, Cu, Ga, Mn, Mo, Ni, P, Te, V and Zn. The maximum total of surface salts containing F, Cl and S, carried by the erupted ash, was 11, 13 and 117kilotonne. The flux of nutrient and toxic elements from the Grímsvötn ash was low compared with that from other eruptions and would not have been expected to cause poisoning of mammals or aquatic life. The elements most likely to affect sensitive biota were F, Se, Cu and Zn.