Ecotoxicity screening of novel phosphorus adsorbents used for lake restoration

Short-term standardized laboratory tests were carried out for evaluating acute and chronic toxicological effects of novel phosphorus (P) adsorbents on Raphidocelis subcapitata (algal growth rate inhibition) and on Daphnia magna (immobilization, with direct and indirect exposure to adsorbents, and uptake-depuration tests). Four P adsorbents were tested: two magnetic (HQ and Fe₃O₄) and two non magnetic (CFH-12^® and Phoslock^®). For the case of the algal growth inhibition test, the EC₅₀ was 1.5 and 0.42 g L^-1 for HQ and CFH-12^®, respectively, and no inhibition patterns were observed neither for Fe₃O₄ nor for Phoslock^®. When organisms were exposed to a direct contact, in the D. magna immobilization test, no statistically significant differences were found in the EC₅₀ values among the four studied adsorbents. The huge difference between direct and indirect contact experiments suggests that toxicity is mainly physically mediated. The uptake-depuration test evidenced a much faster uptake and depuration rates for Phoslock^®, which was precisely the adsorbent with the highest particle size. In a realistic worst-case scenario using data from Honda lake (Almería, Spain), where lake restoration is carried out by a adding a single large dose to bind surplus P in the lake, the predicted environmental concentrations for all adsorbents were lower than EC₅₀ for all adsorbents and they were found to exceed a provisional limit value for ecotoxicity after a short-term exposure. All in all, since neither accumulation nor longer term effects of P adsorbents in the pelagic phase is expected, this risk may however, on a case-to-case basis, be acceptable.