Synthesis and ecotoxicity screening of reusable, magnetically harvestable metal oxide/hydroxide nanocomposites for safe and sustainable removal and recovery of phosphorus from wastewater
Asya Drenkova-Tuhtan, Mariliis Sihtmäe, Kevin Uke, Heiki Vija, Maximilian Oppmann, Johannes Prieschl, Karl Mandel, Anne Kahru
Abstract
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.