Abstract: Volcanic ash (VA) from the 2021 Tajogaite eruption (La Palma, Spain) is a high-volume aluminosilicate residue with potential for low-carbon construction materials, yet its environmental behaviour remains insufficiently characterised. This study evaluates VA-based alkali-activated binders (AABs) through standardized leaching tests UNE-EN 12457-4 (UNE, 2003), ICP-MS trace-element quantification, and two marine bioassays: the Vibrio fischeri luminescence inhibition test (ISO 11348-3:2007) and the Paracentrotus lividus embryo-larval development test. Activation with 6 M and 8 M NaOH substantially modified the geochemical release profile of the precursor, increasing pH, conductivity, and the mobilisation of oxyanion-forming elements (As, Mo, V). Mo exceeded EU inert-landfill limits, while Sb and Se approached regulatory thresholds. Seawater extractants buffered alkalinity but did not suppress oxyanion release, indicating that high ionic strength maintains MoO42-2 and VO4 3- solubility. The consistent release hierarchy (V > Mo > As) highlights the role of alkali-driven speciation. Bioassays showed marked taxon-specific sensitivity. V. fischeri classified all eluates as non-toxic (LID menor o igual que 8; TU < 0.4), suggesting limited acute effects on bacterial metabolism. Conversely, P. lividus exhibited significant sublethal toxicity, especially for the 8 M binder (EC50 = 56.90%), with developmental inhibition correlating with elevated oxyanion concentrations. These results demonstrate that VA-based AABs are technically viable but exhibit activation-dependent environmental behaviour that is not detected by bacterial assays alone. Results support integrating sensitive marine invertebrate bioassays into regulatory and weight-of-evidence frameworks and provide guidance for the sustainable management of volcanic residues and the deployment of alkali-activated materials in coastal and marine-influenced environments.

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