Emerging investigator series: environmental safety assessment of 11 novel metal oxide/hydroxide nanocomposite adsorbents for advanced magnetic removal and recovery of phosphorus from wastewater

Abstract

This study evaluated the ecotoxicity of 11 metal oxide/hydroxide nanocomposite adsorbents for advanced magnetic removal/recovery of phosphorus from wastewater using four test organisms representing different aquatic trophic levels: bacteria Vibrio fischeri, crustaceans Daphnia magna, algae Raphidocelis subcapitata and midge Chironomus riparius larvae. The nanocomposites (d₅₀ < 10 μm) were synthesized as co-precipitates of 2-, 3- and/or 4-valent metal precursors (Zn²⁺, Ca²⁺, Mg²⁺, Fe³⁺, Zr⁴⁺) at varying molar ratios. The shedding of precursor metals in toxic concentrations was observed only for the Zn-containing adsorbents. The acute toxicity of the Zn-containing composites ranged from “harmful” to bacteria (10 < 30 min EC₅₀ ≤ 100 mg L⁻¹), “toxic” to crustaceans (1 < 48 h EC₅₀ ≤ 10 mg L⁻¹) and “very toxic” to midge larvae and algae (24 h LC₅₀ ≤ 1 mg L⁻¹). As a rule, their toxicity correlated with the concentration of shed Zn-ions. All nanocomposites, regardless of their composition, proved very toxic to algae, i.e. remarkably inhibited algal growth (72 h EC₅₀ ≤ 1 mg L⁻¹). The latter effect could be explained by (i) shed Zn-ions in case of Zn-containing materials as algae are very sensitive to heavy metals, (ii) composites-induced phosphorus removal from the algal growth medium and (iii) entrapment of algal cells into particle agglomerates. Importantly, the most-promising benchmark material ZnFeZr-6 : 1 : 1 (V. fischeri EC₅₀ = 118 mg L⁻¹; D. magna EC₅₀ = 7.7 mg L⁻¹; C. riparius LC₅₀ = 0.59 mg L⁻¹) proved safe for bacteria and crustaceans once deposited on magnetic particles ZnFeZr-6 : 1 : 1@MPs yielding EC₅₀ > 100 mg L⁻¹. Summing up, although Zn enhances the adsorbent selectivity and reusability, all Zn-containing P-adsorbents are questionable in terms of ecosafety and thus not recommended for engineering applications in open systems.