Physico-chemical Characteristics and Environmental Fate of a Hydrolytically Degradable Cationic Flocculant
physiologically based extraction test , degradable flocculant , degradation product , kaolin , thermogravimetric analysis , nuclear magnetic resonance spectroscopy
Polyacrylamide (PAM) flocculants are used frequently for wastewater treatment processes (WWTP). However, PAM is hydrophilic and generates loosely packed sediments, which makes further removal of water difficult. Therefore, a novel hydrolytically degradable flocculant, poly(lactic acid) choline iodide ester methacrylate [poly(PLA4ChMA)], was developed. This is a promising alternative because further dewatering occurs as this flocculant undergoes partial hydrolytic degradation. However, the characterization of poly(PLA4ChMA) and its environmental fate must be investigated. The objectives of this research were to characterize the degradation of poly(PLA4ChMA), and its interactions with mature fine tailings (MFT) and municipal wastewater, especially with respect to human exposure. In an initial study, the degradation products of poly(PLA4ChMA) were identified and quantified by proton nuclear magnetic resonance (1H-NMR) spectroscopy. Poly(PLA4ChMA) releases products with a molar ratio of lactyl lactate and lactic acid to choline iodide of ~2. The partially degraded water-insoluble flocculant contains on average two lactate units attached to each methacrylate unit of the backbone. Poly(PLA4ChMA) was then subjected to in-vitro gastrointestinal (GI) digestions with simulated MFT (kaolin), a model contaminant (naphthalene), and simulated municipal wastewater (SMW). Modified physiologically based extraction tests (PBETs) were developed to estimate the bioaccessibility of poly(PLA4ChMA) and naphthalene; i.e., the fraction of a contaminant solubilized in GI fluids and available for systemic absorption. Extracts were analyzed by 1H-NMR, gravimetry, and ultraviolet-visible spectrophotometry. Both poly(PLA4ChMA) and naphthalene have elevated bioaccessibility (85.8 100%) in gastric and intestinal PBET solutions in the absence of kaolin and SMW components. In the presence of kaolin, the bioaccessibility of poly(PLA4ChMA) is ~0%, which indicates that this flocculant sorbs onto kaolin throughout the PBET. Similarly, kaolin lowers the bioaccessibility of naphthalene in the gastric and intestinal phases to 78.4% and 32.7%, respectively. However, a negligible amount of poly(PLA4ChMA) sorbs onto SMW components during PBET. Thus, the poly(PLA4ChMA) exhibits nil bioaccessibility during PBET in the presence of SMW components. Conversely, the bioaccessibility of naphthalene dropped to 13.3% in the gastric solution but remained constant in the intestinal solution (~35%) after PBET. These results may be applied in appropriate wastewater treatments and to estimate potential risks associated with exposure to the flocculant.