Organic Contaminants Removal And Algal Toxicity Of Iron-based Nanomaterials

Due to the high removal capacity for organic and inorganic compounds,nanoscale zero-valent iron(nZVI)is an excellent candidate for environmental remediation and wastewater treatment.However,bare nZVI would undergo serious aggregation and oxidation in the environment,and is not easy to be recovered after usage,which greatly hinders its wide application.Thus,suitable modifications need to be introduced to improve the application potential of nZVI.Meanwhile,the application of nZVI and its modified materials would inevitably lead to their release to the environment,and pose a threat to the living organisms.In this study,entrapped nZVI and Fe/C composites were synthesized based on sodium alginate(SA),and investigated their performance in organic contaminants removal.For nZVI and its modified materials,the involved nZVI might cause adeverse effects on living organisms.To fully evaluate the potential ecological effects of nZVI and its modified materials,representative nZVI and its environtmental aging products,Fe2O3 and Fe3O4 nanoparticles(NPs)were introduced for toxicity assessment.A green algae(Chlorella prenoiosa)was chosen as the model organism.The factors influencing the toxicity of iron-based nanomaterials(NMs)and the involved toxic mechanisms were also discussed.The results could contribute to the development of environmental friendly iron-based NMs with high reactivity.The main conclusions were as follows:(1)Bare and entrapped nanoscale zero-valent iron(nZVI)were introduced for the removal of carbon tetrachloride(CT)and 1,1,2-trichloroethane(TCA)in model high-salinity fracturing wastewater(FWVW).With increasing ionic strength from Day-1 to Day-90 FWW,bare nZVI presented significantly lower removal efficiency for CT(from 53.5%to 38.7%)and 1,1,2-TCA(from 71.1%to 21.7%)and underwent more serious Fe dissolution.The immobilization of nZVI in SA with/without polyvinyl alcohol presented superior performance in Day-90 FWW for 1,1,2-TCA removal(62.6-72.3%),which also mitigated Fe dissolution.Both adsorption(by polymer matrix)and reduction(by immobilized nZVI)were involved in the 1,1,2-TCA removal for entrapped nZVI.After 1-month immersion in FWW,a marked drop in the reactivity of entrapped nZVI for 1,1,2-TCA removal was observed although the oxidation of the immobilized nZVI was mitigated by the polymer matrix.(2)Fe/C composites were successfully synthesized via high temperature pyrolysis with SA as carbon source and Fe(NO3)3 as iron source.Ferric ion was only transformed into Fe3O4/y-Fe2O3 with low pyrolysis temperature.The formation of Fe0/Fe3C and graphitic carbon was observed with elevated temperature(≥700 ℃).The performance of Fe/C composites in atrazine(ATZ)removal was not significantly influenced under the condition of pH 3-9,and the 24 h ATZ removal efficiency of Fe/C-600,Fe/C-700 and Fe/C,800 was 19.4±1.6%,76.8±2.2%and 93.5±0.9%,respectively.The adsorption from carbon phase was primarily responsible for ATZ removal by Fe/C composites.The contribution from degradation for ATZ removal by Fe/C-700(26.1±4.0%)and Fe/C-800(28.7±0.9%)was observed under acidic condition(pH 3),while only adsorption was present under neutral(pH 6)and alkalic condition(pH 9).(3)nZVI with 4 different particle sizes,Fe2O3 NPs with 2 different crystal phases and 1 type of Fe3O4 NPs were chosen as representatives to study the toxicity of iron-based NMs to Chlorella pyrenoidosa.Results show that the algal toxicity of nZVI increased significantly with decreasing particle size;with similar particle sizes(20-30 nm),the algal toxicity decreased with oxidation of the NMs with an order of nZVI>Fe3O4 NPs>Fe3O4 NPs,and a-Fe2O3 NPs presented higher toxicity than y-Fe2O3 NPs.The NM-induced oxidative stress was the main toxic mechanism,and the NM-cell heteroagglomeration and physical interaction also contributed to the algal toxicity,whereas the effect from NM dissolution could be ignored.The aging in 3 surface water samples increased surface oxidation of the iron-based NMs especially nZVI,which meanwhile decreased the toxicity to algae.