Effect of pH on hexavalent chromium removal driven by henna (Lawsonia inermis) fermentation

pH is significant to fermentation, species distribution of chromium, and removal of toxic Cr(VI). At pH 5, 7, 9, and 11, the coupled process of Cr(VI) removal driven by henna fermentation was investigated. In despite of the low adsorption capacity of Cr(VI) on henna biomass (4.10-2.73 mg/g), the fermentation process benefited Cr(VI) reductive removal by providing bio-available electron donors (volatile fatty acids, VFAs) and mediator (lawsone). During the 1st batch (days 1-4), the generation of sufficient electron donors ensured a nearly complete Cr (VI) removal (>99 %), and the removal rate was slightly higher at pH 9. During the 2nd (days 5-13) and 3rd (days 14-23) batches, the removal rate declined due to the lack of electron donors. At pH 11, although more VFAs and lawsone were produced, the removal of total chromium and the immobilization of Cr(III) were lowest. This was ascribed to the formation of more soluble organic-Cr(III) complex rather than Cr(OH) 3 precipitation, thereby increasing the solubility of Cr(III). Furthermore, X-ray photoelectron spectroscopic analysis showed that it was more suitable for Cr(III) immobilization on henna biomass at pH 9 (64.4 %) than at pH 11 (54 %). Therefore, mild pH 9 is optimum for Cr(VI) removal in henna fermentation-driven system, in which the specific removal rate of 23.7 mg Cr(VI) / (g d) was achieved.