Nanocomposite Metal Oxide/Hydroxide Adsorbents for Advanced Wastewater Treatment and Toxicological Risk Assessment for the Aquatic Environment ^†

Phosphorus (P) is a key nutrient for agriculture [1], but is also an environmental pollutant that causes eutrophication and is commonly removed from wastewater [2]. Engineered nanostructured materials, predominantly metal oxides/hydroxides, are frequently reported as excellent adsorbents for phosphate [3], able to selectively remove P from wastewater to ultra-low concentrations [4], and facilitate P-recovery through reversible sorption [5]; however, their environmental safety is rarely addressed. This study assesses the ecotoxicological hazard of 10 highly efficient metal oxide/hydroxide nanocomposite P-adsorbents using toxicity tests involving two different food-web level test organisms: the naturally bioluminescent marine bacterium Vibrio fischeri, and the crustacean Daphnia magna. Nanocomposites were synthesized based on published procedures [6,7] via co-precipitation of 2-, 3- and 4-valent metal precursors (Zn²⁺, Ca²⁺, Mg²⁺, Fe³⁺, Zr⁴⁺) at different molar ratios, and characterized with laser diffraction, ICP-OES, XRD and SEM. Among these, the pilot-scale tested ZnFeZr-6:1:1-oxyhydroxide [8] was modified by reducing the zinc fraction to minimize leaching of toxic Zn²⁺ ions. The composites’ stability was investigated in deionized water and 2% NaCl (V. fischeri test medium), addressing agglomeration, settling and solubilization (the release of metal ions and/or potentially hazardous nanoparticles). All composites, their filtered supernatants and precursor metal salts were evaluated for their toxic potency (half-effective concentration, EC₅₀ and minimum bactericidal concentration, MBC) using three different tests: a Vibrio fischeri 30 min bioluminescence inhibition assay (ISO-21338:2010) [9], a V. fischeri 24 h viability assay (‘Spot test’) and a Daphnia magna 48 h acute immobilization test (OECD-202) [10]. Only the Zn-containing composites showed inhibitory effects on both organisms. Those with the highest zinc fraction (ZnFeZr-18:5:1; ZnFeZr-10:1:1) were classified “harmful” to V. fischeri (10 < EC₅₀ ≤ 100 mg/L), and toxic to D. magna (1 < EC₅₀ ≤ 10 mg/L); therefore, they are environmentally unsafe for engineering applications. The ZnFeZr-6:1:1 (V. fischeri EC₅₀ = 118 mg/L; D. magna EC₅₀ = 7.7 mg/L) proved assumingly safe for both aquatic organisms once deposited on magnetic particles ZnFeZr-6:1:1@MPs (EC₅₀ >> 100 mg/L, MBC > 1000 mg/L). All other composites without Zn were non-toxic, both to V. fischeri, and to the more sensitive D. magna.