Chemical Pretreatment of Rice Straw Biochar: Effect on Biochar Properties and Hexavalent Chromium Adsorption

The primary objective of this study was to evaluate how chemical pretreatment may affect the ability of biochar to adsorb hexavalent chromate [Cr(VI)]. Rice straw biochars produced at three different pyrolytic temperatures (300 °C, 500 °C and 700 °C) were treated with 0.5 mol L−1 hydrochloric acid (A), 0.5 mol L−1 sodium hydroxide (B); deionized water (C); or ethanol (E) and the changes in surface properties and chemical composition were then determined. The results showed that the elemental composition, functional groups and point zero charge (PZC) of the biochars were all affected by the pyrolytic temperatures and chemical pretreatment methods. Chemical pretreatment dramatically altered the adsorption of Cr(VI), as shown by the trend in Kd (the partition coefficient): 300B > 300A > 300E ≈ 300W, 500A > 500B ≈ 500E ≈ 500W and 700A > 700B > 700E ≈ 700W (number before the letter indicates the pyrolytic temperature). NaOH treatment exerted the greatest effect on the biochar produced at 300 °C, resulting in 300B having the highest PZC and rich in aromatic CO‒ and phenolic ‒OH groups, and hence the strongest capacity for Cr(VI) adsorption under slightly acidic condition. The ash content of the biochar was greatly reduced by HCl treatment, giving rise to reduced PZC and the protonation of functional groups. Thus, all the acid-pretreated biochars (300A, 500A and 700A) yielded the best Cr sorption capacity, with over 90% of the total Cr removed by the acid-treated biochar at pH 2. Both ethanol- and water treatment had little effect on the ability of the biochar to adsorb Cr. The results of Cr K-Edge X-ray absorption Near-Edge Spectroscopy (XANES) revealed Cr(III) as the dominant species adsorbed to the biochars, suggesting that the adsorption of Cr(VI) occurred partly through reduction of Cr(VI) to Cr(III) as well as via anionic adsorption. The finding of this study demonstrated that pretreatment could markedly alter the Cr sorption ability of rice straw-derived biochar, a phenomenon that could have practical application in the remediation of Cr-polluted water.