Removal of selected emerging PPCP compounds using greater duckweed (Spirodela polyrhiza) based lab-scale free water constructed wetland

Greater duckweed (Spirodela polyrhiza) based lab-scale free water constructed wetland (CW) was employed for removing four emerging pharmaceuticals and personal care products (PPCPs) (i.e. DEET, paracetamol, caffeine and triclosan). Orthogonal design was used to test the effect of light intensity, aeration, E.coli abundance and plant biomass on the target compounds. Synthetic wastewater contaminated with the target compounds at concentration of 25 μg/L was prepared, and both batch and continuous flow experiments were conducted. Up to 100% removals were achieved for paracetamol (PAR), caffeine (CAF) and tricolsan (TCS) while the highest removal for DEET was 32.2% in batch tests. Based on orthogonal Duncan analysis, high light intensity (240 μmolmm^-2s^-1), full aeration, high plant biomass (1.00 kg/m²) and high E.coli abundance (1.0 × 10⁶ CFU/100 mL) favoured elimination of the PPCPs. Batch verification test achieved removals of 17.1%, 98.8%, 96.4% and 95.4% for DEET, PAR, CAF and TCS respectively. Continuous flow tests with CW only and CW followed by stabilization tank (CW-ST) were carried out. Final removals of the PPCP contaminants were 32.6%, 97.7%, 98.0% and 100% for DEET, PAR, CAF and TCS, respectively, by CW system alone, while 43.3%, 97.5%, 98.2% and 100%, respectively, were achieved by CW-ST system. By adding the ST tank, PPCP concentrations decreased significantly faster (p < 0.05) compared with continuous flow CW alone. In addition, after removing aerators during continuous flow CW experiments, the treatment systems presented good stability for the PPCP removals. CW-ST showed better chemical oxygen demand (COD) and total organic carbon (TOC) removals (89.3%, 91.2%, respectively) than CW only (79.4%, 85.2%, respectively). However, poor DEET removal (<50%) and high E.coli abundance (up to 1.7 log increase) in the final treated water indicated further treatment processes may be required. Statistical analysis showed significant correlations (p < 0.05) between PPCPs and water quality parameters (e.g. COD, nitrate, phosphate), and between the four PPCP compounds for the continuous flow CW and CW-ST systems. Positive results encourage further test of Greater duckweed at pilot scale CW using real wastewater.

Response of Spirodela polyrhiza to cerium: subcellular distribution, growth and biochemical changes

Rare earth elements are new and emerging contaminants in freshwater systems. Greater duckweed (Spirodela polyrhiza L.) is a common aquatic plant widely used in phytotoxicity tests for xenobiotic substances. In this study, the cerium (Ce) accumulation potential, the distribution of Ce in bio-molecules, and ensuing biochemical responses were investigated in greater duckweed fronds when they were exposed to Ce (0, 10, 20, 40, and 60μM). There was a concentration dependent increase in Ce accumulation, which reached a maximum of 67mgg^-1 of dry weight (DW) at 60μM Ce after 14 d. The Ce concentrations in bio-macromolecules followed the order: cellulose and pectin > proteins > polysaccharides > lipids. In response to Ce exposure, significant chlorosis; declines in growth, photosynthetic pigment and protein contents; and cell death were noted at the highest Ce concentration. Photosystem II inhibition, degradation of the reaction center protein D1, and damage to chloroplast ultrastructure were observed in Ce treated S. polyrhiza fronds, as revealed by chlorophyll a fluorescence transients, immunoblotting, and transmission electron microscopy (TEM). O₂^.- accumulation and malondialdehyde (MDA) content in the treated fronds increased in a concentration dependent manner, which indicated that oxidative stress and unsaturated fatty acids (C18:3) were specifically affected by Ce exposure. These results suggest Ce exerts its toxic effects on photosynthesis, with a primary effect on PS II, through oxidative stress.