Journal of Petroleum & Environmental Biotechnology

Abstract

Tracking Interfacial Adsorption/Desorption Phenomena in Polypropylene/Biofuel Media using Trace Cr3+/Cr6+ and As3+/As5+-A Study by Liquid Chromatography-plasma Mass Spectrometry

Stephen S*, Pillay AE, Shah T and Siores E

The effect of adsorption/desorption of biofuels on polymers cannot be directly measured unless the fuel is infused with a marker or tracer to track the impact of the phenomenon. Certain trace chemical species in biodiesel can serve as natural markers and our study investigated the use of such natural markers in neem biodiesel to indirectly evaluate the kinetics of adsorption and desorption. Chemical species Cr3+/Cr6+ and As3+/As5+ naturally present in biodiesel were employed as tracers to measure the rates of adsorption and desorption in polypropylene material following immersion in biofuel media for selected intervals. Ultra-sensitive hyphenated HPLC linked to ICP-MS was deployed to monitor these effects. Elution followed by mass spectrometry for simultaneous detection was conducted for convenient resolution of all four species. A C-8 reinforced silica column formed the stationary phase (150 mm in length; internal diameter: 4.6 mm; particle size: 5.0 μm). The mobile phase was a special mixture prepared by following a specific protocol. The elution rate was ~1.2 mL/min; and associated retention times (min) were: As3+: 1.61; As5+: 4.06; Cr3+: 3.71; and Cr6+: 5.80. The system is equipped with a dynamic reaction cell (DRC) to facilitate mass detection. The oxygen flow in the DRC was controlled to give a rate of ~ 0.4 mL/min. Following mass detection m/z values of 52 characterized the Cr3+/Cr6+ species; while the arsenic components were detected as adduct ions, AsO+, m/z 91, in association with oxygen. Certified reference materials were used to standardise the instrument. The results showed that the phenomenon is influenced by ionic charge and polarity of the medium. Water contamination of the biodiesel tends to augment the adhesion rate. The rates of adsorption/desorption based on the species of interest follow first order kinetics. Pronounced adsorption rate constants were observed in water infected biodiesel, between 0.3–0.5 wk-1 for Cr6+ and As5+. The rate constants in “water free” samples dropped to about 0.1 wk-1 for all species. The impact of our study is discussed in terms of the attrition that biofuels can have on polymeric materials.