Leaching, Acute and Chronic Toxicity of Nano-ZnO Incorporated into Photocatalytic Concrete for Pavement Applications Toward Daphnia magna

Urban air pollution has driven the use of photocatalytic concrete for pavement applications, in which nano-ZnO has emerged as an alternative to nano-TiO2. However, its higher solubility raises concerns regarding the release of Zn2+ into the aquatic environment. In this context, this study evaluated whether the incorporation of nano-ZnO, at a proportion of 9% relative to the mass of Portland cement, increases the toxicity of pavement concrete leachates toward D. magna, compared to conventional concrete. The nano-ZnO powder was physicochemically characterized (XRD, EDXRF, TEM, FTIR, TG/DTG, and colloidal behavior), and the leachates from the conventional and 9% nano-ZnO concretes, obtained by an accelerated TCLP-based leaching of crushed concrete, were characterized by ICP-MS and FTIR. Acute (48 h) and chronic (21 days) toxicity were evaluated for the nano-ZnO powder, for the reference leachate (conventional concrete; data from our previous study), and for the leachate from the nano-modified concrete. ICP-MS indicated that at most ~0.07–0.08% of the total incorporated zinc was leached, and no Zn–O band was detected by FTIR in the freeze-dried leachates, which is consistent with, although it does not prove, the retention of ZnO in the cementitious matrix. In the acute assay, the EC50, 48 h of the nano-modified leachate (10.5% v/v of the concentrated extract) did not differ significantly from that of the reference leachate (10.4% v/v; EC50 ratio 1.01, 95% CI 0.87–1.17) and was not attributable to dissolved zinc alone. In a 21-day assay based on OECD TG 211, the nano-modified leachate caused no adverse effect on longevity, size, reproduction, or age at first brood up to the highest concentration tested, whereas the reference leachate caused a slight reduction in size. Overall, nano-ZnO incorporation did not produce additional toxicity toward D. magna under the tested conditions.