Ecotoxicity: Pharmaceutical Micropollutants

The presence of pharmaceuticals as micropollutants in wastewater is an environmental and health concern. Indeed, effluents from urban wastewater treatment plants are the likely source of downstream surface and marine water contamination. One such pharmaceutical, antidepressant venlafaxine (VFX)—which, along with its metabolite d,l-O-desmethylvenlafaxine (DVFX), belongs to the serotonin-norepinephrine reuptake class—widely persists in wastewater and surface water even after treatment of those water sources.

García-Galán et al. (2016) recently evaluated the impact of UV/H₂O₂ oxidation (an advanced oxidation process, or AOP) applied after conventional wastewater treatment on overall ecotoxicity of the final effluent, given the remaining transformation products after VFX and DVFX removal.¹ To do this, the research team applied ultra-high-pressure liquid chromatography–mass spectrometry (UHPLC-MS) with an LTQ Orbitrap mass spectrometer (Thermo Scientific) to trace VFX and DVFX degradation and detect persistent compounds.

The researchers report successful removal of both VFX and DVFX by AOP. For VFX, this depletion was rapid, with 99.9% removal after five minutes. Achieving the same level of removal (99.9%) for DVFX required 30 minutes of treatment.

They detected 11 transformation products for VFX, including three isobaric compounds. Most of these demonstrated highest intensities after 2.5 minutes of treatment (at 86% depletion of VFX). The maximum value of the transformation products corresponded with 3.7% of the initial VFX concentration and occurred after 15 minutes of oxidation.

The team also detected six transformation products for DVFX at higher values than those detected for VFX. For these transformation products, they report a total sum of 18% and a maximum value of 8.4% after 2.5 minutes of oxidation. These results indicate a higher transformation for DVFX when compared with VFX.

In ecotoxicity assays measuring bioluminescence inhibition against marine bacteria Vibrio fischeri, García-Galán et al. observed slightly higher values after 30 minutes of treatment versus 15 minutes. For VFX, they report a toxicity increase (from 2.3–2.5 TUs to 8–8.7 TUs) through the first five minutes of oxidation as the transformation products increased in abundance, followed by a toxicity decrease nearly concurrent with depletion of a transformation product (TP 216), which disappeared after 15 minutes. A second toxicity increase occurred between the 10 and 15 minute time points in agreement with observed maximum levels for three transformation products (TP 104, 118, 160) at 15 minutes. For DVFX, they observed a drastic toxicity increase after 2.5 minutes of reaction (maximum toxicity value 12.1 TUs), followed by toxicity decreases corresponding to transformation product depletion, plateauing at values similar to the initial values after 45 minutes of treatment.

Overall, the research team indicates that UV/H₂O₂ oxidation as applied in this study is an efficient tool for VFX and DVFX removal. They report observation of a high number of transformation products during oxidation and note a positive correlation between transformation product abundance and ecotoxicity increase. They call for further studies following the fate of these oxidation byproducts as they enter the environment and also applying the treatment studied here in real wastewater matrices and environmental conditions.

Reference

1. García-Galán, M.J., et al. (2016) “UV/H2O2degradation of the antidepressants venlafaxine and O-desmethylvenlafaxine: Elucidation of their transformation pathway and environmental fate,” Journal of Hazardous Materials, 311 (pp. 70–80).