Assessment of the Pb 2+ biosorption potential of the fungus Penicillium citrinum under geothermal conditions

One solution for reducing the scaling risk of lead (Pb)-containing phases is to remove the aqueous Pb ²⁺ ions from the brine by a sorption process before oversaturation of Pb ²⁺ phases at unwanted locations within the geothermal fluid loop. This study investigated the known capacity of fungal biomass to bind Pb ²⁺ ions (biosorb) and thus remove Pb ²⁺ from the brine. So far, biosorption studies have neither been done at high temperatures or salinity, nor under high pressure, three conditions that have to be considered within geothermal power plants. Thus, the overall goal of this study was to assess the Pb ²⁺ biosorption potential of dead biomass of the fungus Penicillium citrinum strain HEK1 under conditions mimicking those of natural highly saline geothermal fluids. This specific strain was isolated from a geothermal power plant in which Pb ²⁺ scaling occurs. The dead biomass of P. citrinum was added to synthetic solutions containing 260 g/L NaCl, 1g/L Pb, and (in half of the treatments) 60 mg/L acetic acid. These synthetic solutions, including the dead biomass, were then incubated at high pressure (8 bar) at different temperatures (25°C, 60°C, 98°C) and for different time intervals (1 h, 2 h, 3 h). Results showed that the biomass was stable in such conditions, at all temperatures tested, but small amounts of organic compounds, with a wide variety of low molecular weight (< 350 Da to 10,000 Da) have been released into the fluids from the biomass. In general, increased temperature resulted in an increase of dissolved organic carbon (DOC) concentration. Moreover, the potential for biosorption of P . citrinum HEK1 biomass was not affected by changes in temperature, time of exposure or by the presence of organic acids within the fluids. However, the biosorption potential was overall low (1.4% of total Pb ²⁺ ). It nevertheless increased to about 5 to 10% of Pb ²⁺ removal in experiments with non-saline conditions. Therefore, the high salinity of the fluids was the factor limiting the biosorption to the highest extent, highlighting that working with highly saline geothermal fluids might be limiting for biosorption processes to happen efficiently.