Toxicity of ZnFe-SO4 layered double hydroxide in Tetradesmus obliquus and evaluation of some physiological responses of the microalgae for stress management

Layered double hydroxides (LDHs), regarding their physical and structural properties, have different and wide applications industry and their increasing use may raise ecological and human health concerns. However, the potential toxicity mechanisms of LDHs in different organisms are still unclear. In the present work, after synthesizing of ZnFe-SO4 LDH and studying of its characterization by XRD, FT-IR, SEM, EDX-mapping, TEM and Raman, its toxicity in Tetradesmus obliquus was evaluated. According to experimental results, the growth of the algae and content of photosynthetic pigments were significantly decreased after treatment with 100 mg/L of ZnFe-SO4 LDH. The high dose exposure to the LDH also inhibited the activity of SOD and POD enzymes, possibly due to the LDH- catalyzed reactive oxygen species production. In addition, lipid peroxidation and the content of phenolic compounds, as no-enzymatic antioxidants were increased by enhancement of the LDH concentration. The rise of phenol, flavonoids and MDA contents could be regarded as some manifestations and responses to the toxic effects of the contaminant in the algae cells. The results provided a better understanding of the undesirable effects and toxicity of LDHs in aquatic organisms.

Ultrasonic-assisted photocatalytic degradation of various organic contaminants using ZnO supported on a natural polymer of sporopollenin

Water resource pollution by organic contaminants is an environmental issue of increasing concern. Here, sporopollenin/zinc oxide (SP/ZnO) was used as an environmentally friendly and durable catalyst for sonophotocatalytic treatment of three organic compounds: direct blue 25 (DB 25), levofloxacin (LEV), and dimethylphtalate (DMPh). The resulting catalyst had a 2.65 eV bandgap value and 9.81 m²/g surface area. The crystalline structure and functional groups of SP/ZnO were confirmed by X-ray diffraction (XRD) and Fourier transforms infrared spectroscopy (FTIR) analyses. After 120 min of the sonophotocatalysis, the degradation efficiencies of DB 25, LEV, and DMPh by SP/ZnO were 86.41, 75.88, and 62.54%, respectively, which were higher than that of the other investigated processes. The role of reactive oxygen species were investigated using various scavengers, enhancers, photoluminescence, and o-phenylenediamine. Owing to its stability, the catalyst exhibited good reusability after four consecutive cycles. In addition, the high integrity of the catalyst was confirmed by scanning electron microscopy (SEM), XRD, and FTIR analyses. After four consecutive examinations, the leaching of zinc in the aqueous phase was < 3 mg/L. Moreover, gas chromatography-mass spectrometry (GC-MS) analyses indicated that the contaminants were initially converted into cyclic compounds and then into aliphatic compounds, including carboxylic acids and animated products. Thus, this study synthesized an environmentally friendly and reusable SP/ZnO composite for the degradation of various organic pollutants using a sonophotocatalytic process.