Asya Drenkova-Tuhtan

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.

Metal–phenolic network nanocomposites are demonstrated as effective agents for mitigating the toxicity elicited by copper-based nanoagrochemicals commonly used in environmental applications.

Phosphorus (P) is a key nutrient for agriculture

Magnetic microparticles functionalized with tailored ZnFeZr oxyhydroxide adsorbent were used for the reversible sorption of orthophosphate and recalcitrant organo-phosphonates from wastewater. The loaded particles were harvested magnetically from water, regenerated in an alkaline solution and reused numerous times. The applicability of the technology to treat brackish water reverse osmosis concentrates was tested under controlled synthetic conditions by investigating the influence of typical metals (Ca2+, Pb2+, Cu2+) on the removal of common phosphonates (HEDP, NTMP, EDTMP), and vice versa. When present at equimolar concentrations, metal cations enhanced the adsorption of phosphonates and were co-adsorbed at pH 4.0–4.5 (with removals of 83–93% for Pb2+ and 53–73% for Cu2+), likely through ternary complex formation. In the absence of metals, at pH > pHPZC ∼ 7 (the material point of zero charge), a drop in adsorption efficiency was observed for orthophosphate and all phosphonates. Thus, at pH 7, an increased adsorbent dose (>0.1 g/L) was necessary to remove 1 mg/L NTMP-P in 30 min. The reusability and effluent polishing potential of the ZnFeZr particles was demonstrated in a pilot test with municipal wastewater throughout 55 adsorption/desorption cycles without any drop in performance. Consistent removal of the non-reactive phosphorus species to ultra-low concentrations (<0.05 mg/L Ptot) and complete orthophosphate elimination (<0.005 mg/L PO4-P) was maintained under optimal conditions.

Systematic investigations of the acidic dissolution of P, Al, Fe and Ca from Al-containing tertiary sludge were carried out in this work. The results were then compared with the dissolution behaviour of Al-containing anaerobically digested sludge to evaluate the P–recovery potential in the form of struvite from tertiary sludge versus anaerobically digested sludge. Additional investigations of synthetically produced Al–sludge served as a comparison for the dissolution behaviour of P and Al without the influence of other contaminants (metals, biomass). In addition, the acid consumption was analyzed as a function of the target pH during the dissolution. The dissolution efficiency of PO₄-P in tertiary and anaerobically digested sludge after acid treatment at pH 2 was ∼90%. The dissolution efficiency of Al and Ca in tertiary sludge was also ∼90% at pH 2, while the release efficiency of Al and Ca in anaerobically digested sludge was lower, ∼70% at pH 2. In tertiary sludge, about 75% of Fe was found dissolved at pH 2, whereas in anaerobically digested sludge this value was higher, ∼90%. Based on the experimental data, it can be concluded that significant dissolution of phosphorus from Al–containing tertiary sludge can take place at pH < 3. The highest sulphuric acid consumption for P–dissolution was observed in the case of tertiary sludge at pH 2.