Fungal Generation of Organic Acids for Removal of Lead from Contaminated Soil

Abstract

The use of solid-state fermentation (SFF) of low cost substrates by fungal species to generate organic acid solutions for washing of lead from a contaminated soil was evaluated in this study. SFF filtrates were generated by fermentation of four substrates (corn cobs, apple pomace, rice and hay) with three fungal species (Aspergillus niger NRRL 2001 (A. niger 1), Aspergillus niger ATCC 64065 (A. niger 2), Aspergillus foetidus NRRL 337) at three fermentation times. The concentration and speciation of organic acids of the filtrates was found to be a function of the substrate type, the fungal species and the fermentation time. Fermentation of rice resulted in the highest concentrations of citric acid while fermentation of corn cobs, apple pomace and hay tended to generate oxalic acid with an increasing fraction of this acid as the fermentation progressed. Batch extraction tests that employed the SSF filtrates revealed that soluble lead concentrations as high as 35 mg/l could be achieved. Filtrates containing elevated concentrations of citric acid resulted in the greatest lead extraction while oxalic acid inhibited solubilization. Due to the buffering of pH that was provided by the soil in the batch tests this factor did not appear to influence lead extraction. Lead extraction was observed over an extended period of time in a column test. Lead extraction was strongly influenced by the pH of the soil column and less strongly influenced by the organic acid content of the SSF filtrate. The speciation of organic acids was substantially modified from primarily citric acid in the SSF filtrate to gluconic acid in the soil column discharge.

Firing Range Soils Yield a Diverse Array of Fungal Isolates Capable of Organic Acid Production and Pb Mineral Solubilization

“…Most of our environmental isolates originating from highly Pb-contaminated soils produced large quantities of mixed acids (Ն10 mM). This high level of organic acid production, as well as variable compositions of the acid mixture, has been documented in the model organisms A. niger and Penicillium bilaiae due to varying substrate C and N sources (3,36,39).…”

“…The well-characterized fungal model species A. niger has been repeatedly tested, with mixed results, for organic acid secretion, bioleaching, and remediation potential for heavy metals from various substrates, including municipal waste and fly ash (5), lowgrade mining ores (6,36), and a number of soils (3,6,39,44). A. niger is reported to produce organic acids in large quantities under specific laboratory culture conditions, with as much as 90% of the total mass of acids produced consisting of oxalic and citric acids (39).…”

“…A. niger is reported to produce organic acids in large quantities under specific laboratory culture conditions, with as much as 90% of the total mass of acids produced consisting of oxalic and citric acids (39). These two acids have been found to be the most powerful LMWOAs in terms of Pb release from minerals and Pb-contaminated soils (9).…”

“…These two acids have been found to be the most powerful LMWOAs in terms of Pb release from minerals and Pb-contaminated soils (9). A. niger culture exudates have been applied to a variety of substances to affect metal leaching, and in the case of Pb, this has resulted in concentrations as high as 35 mg liter Ϫ1 of soluble Pb in leachates (39) and up to 52% of the total mass of Pb removed from the solid phase (5).…”

“…Fungal production of low-molecular-weight organic acids (LMWOAs) has been almost exclusively studied in model organisms, such as Aspergillus spp. (2,3,16,27,36,39,56). Despite the fact that it is a process that may contribute significantly to Pb mobilization in soils, no thorough studies of these processes in indigenous soil fungi have been reported.…”