Biochars for the removal of naphthenic acids from water: A prospective approach towards remediation of petroleum refinery wastewater

Abstract

We report methodology for developing three different surface modified biochars produced from rice straw, bagasse, and eucalyptus wood for the adsorptive removal of benzoic acid as a model pollutant for remediation of naphthenic acid-rich petroleum refinery wastewater. Solid-phase extraction or miniaturized column format packed with (a) untreated biochars; (b) acid-treated (HCl and HNO3) biochars; and (c) non-covalent PDDA (cationic polyelectrolyte) wrapped biochars were employed for the adsorption studies. The benzoic acid loading capacities were in the range of 0.46 and 7.97 mg g−1, with an order of loading as: HCl-treated > HNO3-treated > PDDA-treated > untreated biochar. The acid-treated biochars revealed an increase in the oxygen content (wt %) owing to –COOH groups, corroborated by FTIR studies and point of zero charges (pHzpc) = 2.40. The best loading result, attributed to dipole-based or π-π stacking, was obtained for HCl-treated rice straw biochars. Its BET surface area increased from 18.45 m2 g−1 (untreated) to 49.01 m2 g−1 HCl-treated, with pore volume (0.067 cm3 g−1) and pore diameter (55.27 Å), corroborated by observation of pores of different sizes and patterns by scanning electron microscopy. The optimum pH for maximum adsorption of benzoic acid was 2, and 3 mL methanol was used to regenerate the biochar column with up to 90% removal of the adsorbate. Validation of experimental parameters was performed through naphthenic acid spiked water, and the results were also compared with adsorption over activated carbon. Our results are optimistic towards developing biochars as a low-cost and eco-friendly option to manage agricultural waste for removing pollutants from water.