Impact of non-aqueous phase liquid on Cr(VI) removal by nano zerovalent iron particles: effects of contact time, pollution load, and pH

Nanoscale zerovalent iron (nZVI) was synthesized and employed for treating chromium (VI) containing aqueous solution through adsorption using a series of microcosm experiments. The experiments were then performed to study the effects of a light non-aqueous phase liquid (LNAPL), toluene, on Cr(VI) removal by nZVI. Results showed that a maximum removal efficiency of almost 90% was achieved within 300 minutes of the treatment process, while it was suppressed by ~25% in toluene’s co-presence. The efficiency was found higher (>80%) at pH 3-7 and reduced to 30.3% at pH 9. No significant change in the removal was observed due to change in pH under toluene’s co-presence (57-65% removal for all pH ranges). The removal efficiency also reduced from 62.7% to 37.1% and 82.1 to 28.3% with increase in Cr(VI) concentration (25 to 200 mg/L), with and without co-existing toluene, respectively. Surface morphology and structure of nZVI pre- and post-treatment with Cr(VI) and toluene was analyzed through scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The results confirmed the presence of Cr(VI) and toluene on the nZVI surface. The adsorption of Cr(VI) by nZVI was best described by pseudo second order (PSO) kinetics (R2 = 0.97 in both the cases) and Langmuir isotherm equilibrium model (R2 = 0.95, 0.81). The maximum adsorption capacity (Qmax) for Cr(VI) without and with toluene was 64.52 mg/g and 58.47 mg/g of the nZVI particles, respectively. Findings from this study provide crucial information about co-contamination and its noticeable effects on remediation of the contaminants of concern.