High pulp density bioleaching of coal fly ash by Lactobacillus pentosus with waste citrus peel as the carbon source

Coal fly ash (CFA) disposal poses environmental concerns, yet offers a source for valuable metal recovery. This study introduces a sustainable heterotrophic bioleaching approach utilizing the citrus peel autoclave liquor as a novel and economical carbon source. Hydrothermal pretreatment of citrus peel yielded a liquor rich in glucose (14.9 g/L), fructose (18.4 g/L), and organic acids like citric, gluconic, and malic acid. This liquor effectively supported the growth of two previously verified strains with outstanding bioleaching performance ( Thermotoga sedimenticola and Lactobacillus pentosus ). L. pentosus demonstrated superior metal extraction from CFA, achieving efficiencies of 66.2 % for Zn, 40.1 % for Sr, and 11.8 % for V at 3 wt% pulp density. The innate buffering capacity of the citrus peel autoclave liquor was essential for maintaining a near-neutral pH at high pulp densities (20 wt%), counteracting the high alkalinity of CFA and sustaining L. pentosus ' bioleaching activity. Conversely, under similar high pulp density, pure sugar media failed to support the growth of L. pentosus or effectively leach metals due to the extreme alkalinity. This research successfully validates citrus peel autoclave liquor as an effective and economical substrate for valuable metal recovery from CFA, offering a promising "waste-to-value" strategy and an alternative to conventional methods for CFA bioleaching.