Synthesis and ecotoxicity screening of reusable, magnetically harvestable metal oxide/hydroxide nanocomposites for safe and sustainable removal and recovery of phosphorus from wastewater

This study assesses the ecotoxicological hazard and suggests measures to improve the ecosafety of 10 highly-efficient metal oxide/hydroxide nanocomposite adsorbents for phosphorus removal/recovery from wastewater using the marine bacterium Vibrio fischeri - an ecotoxicological test organism commonly used for the screening of chemicals which is especially sensitive to zinc. Nanocomposites were synthesized through simple co-precipitation of two-, three- and four-valent metal precursors (Ca2+, Mg2+, Zn2+, 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 was modified by reducing the zinc-fraction to minimize the leaching of harmful Zn2+. The presence of Zn, however, makes the materials technologically powerful by improving the adsorbent selectivity towards phosphate and the material reusability. Composites stability (d50 < 10 μm) was investigated in deionized water and 2% NaCl (V. fischeri test medium) addressing agglomeration, settling and solubilization (releasing metal ions and/or potentially hazardous nanoparticles). All composites, their filtered supernatants and precursor metal salt solutions were evaluated for their toxic potency (half-effective concentration, EC50 and minimum bactericidal concentration, MBC) using 30-min kinetic bioluminescence inhibition test (ISO-21338:2010) and 24-h viability assay with bacteria V. fischeri. Only the Zn-containing composites showed inhibitory effects to Vibrio fischeri. Those with highest zinc-fraction (ZnFeZr-18:5:1; ZnFeZr-10:1:1) were classified "harmful" (EC50 < 100 mg/L), thus environmentally unsafe for engineering applications. The ZnFeZr-6:1:1 (EC50 = 118 mg/L, MBC = 250 mg/L) proved assumingly safe once deposited on magnetic particles ZnFeZr-6:1:1@MPs (EC50 = 1123 mg/L, MBC > 1000 mg/L). All other composites without Zn were non-toxic to Vibrio fischeri.