Experimental investigation and comparative environmental impact analysis of conventional and naturally occurring kinetic hydrate inhibitors in offshore environments using toxicity and bioconcentration tools

Abstract

In this present study, an in vivo experiment was carried out to determine the tendencies of a naturally occurring high-phenolic compound red-onion skin extract (HPC-ROSE) and conventional N-Vinylcaprolactam (N-VCAP) kinetic hydrate inhibitors (KHIs) to bio-accumulate in offshore environment through the analysis of their uptake and depuration kinetics by a Fish-Chemical Model using Cyprinus carpio marine invertebrate. This test was performed in the laboratory in a flow through experimental set up according to the guidelines by [49] (Fish-chemical exposure test). The uptake duration was 10 days with a depuration period of twenty days. The inhibitor toxicity test was evaluated by determining the 50 % lethal concentration (LC50) as highlighted by OSPAR Commission protocols. The solubility of the inhibitors was determined by a column elution technique. Similarly, a proton-enabled nuclear magnetic resonance (1HNMR) technique was used to measure the partitioning characteristics of the inhibitors in an immiscible mixture of octanol and water (Kow) by 1HNMR spectroscopy using a benchtop low-field NMR spectrometer. OSPAR Commission and ASTM standard protocols were used for the laboratory investigation of the static sediment toxicity tests. A reference chemical substance of known BCF and solubility (Ethyl Acetate, EtOAc) was used as control to check the experimental progression. The stock solution was prepared by solid phase desorption mechanisms. The HPC-ROSE does not constitute environmentally significant risk to aquatic life owing to its lower BCF values between 215 L/kg - 251.5L/Kg, Log-Kow of 1.2 and higher lethal concentration of 25140 mg/L. Unlike the NVCAP which is very toxic with lower lethal concentration of 1280 mg/L and higher BCF values in the range of 442.5L/Kg to 485 L/kg with Log-Kow of 1.5. Furthermore, Nuclear magnetic resonance (1HNMR) is a simple and reliable method of estimating partition coefficient characteristics (Kow) because the obtained Log-Kow values showed good agreements with that of shake flask and high-performance liquid chromatography techniques. Also, the chemical concentration in water has an inverse relationship with uptake rate constant (K1) and steady-state bioconcentration factor (BCFss). Finally, the theoretically estimated BCFs were higher than the steady-state (BCFss) values that were obtained from laboratory experiments for the different inhibitor samples and at all concentrations. This is due to the route by which Log-Kow was estimated since most of the empirical models are usually one-factor model consisting of partition coefficient and BCF.