Elsevier

Minerals Engineering

Volume 81, 1 October 2015, Pages 173-178
Minerals Engineering

Potential use of native fungal strains for assisted uranium retention

https://doi.org/10.1016/j.mineng.2015.04.003Get rights and content

Highlights

  • Up to 100% removal of uranium was achieved, of which 58–90% within ten minutes.

  • An increased production of organic compounds in the presence of uranium was observed.

  • Ligand candidates of high molecular weight might be used in remediation strategies.

  • Malt extract accounted for 70%, metabolites for 20% and biomass for 10% removal.

Abstract

Uranium-stabilizing ligands can be useful complexing agents for uranium in aqueous solution. The discovery of novel ligand candidates for selective uranium capture in artificial and natural waters could provide scope for their use in water remediation and metal recovery from low- and high grade ores. In this study we used seven fungal strains, isolated from shale waste, to monitor the uranium retention capacity from an aqueous solution. After four weeks of incubation, suspensions containing the fungal strains were filtered, and up to 100% of the total uranium inventory was removed from a 10 mg L−1 solution. Approximately 70% of the total uranium removal is attributed to complexation and/or adsorption by particles in the malt extract and some 10% is adsorbed by the fungal biomass. The additional 20% uranium removed could be related to the excretion of fungal metabolites. From 58% to 90% of the uranium is removed within ten minutes. The formation of colloidal/particulate uranium is proposed to be controlled by organic ligands in the culture medium and organic ligands excreted by the fungi where phosphorus moieties seem to be important. Membrane fouling by the hydrocarbons is also suggested to contribute to a loss of uranium from the aqueous phase.

Introduction

Black shale was mined in Kvarntorp, Sweden, some 200 km WSW of Stockholm, between 1942 and 1966 for oil production by pyrolysis. This shale contains 15–18% kerogen and 6–9% sulphur and is rich in vanadium, molybdenum, uranium (up to 0.030%) and rare earth metals (Andersson et al., 1985, Swanson, 1960). Besides the petroleum production, a small amount of uranium was also extracted. After mine closure most of the open pits were water filled. In the area there is a single waste pile containing some 40 million m3 as a mix of processed and unprocessed shale with unknown proportions. Weathering of the pyrite rich waste has greatly affected surface and groundwater nearby with elevated concentrations of both major and trace elements (Allard et al., 2014). Many efforts are made regarding remediation of heavily contaminated sites but low- or moderately contaminated mine tailings and waters are often neglected since they often do not possess an immediate health danger, even though a long term exposure of low levels of toxic metals might be just as severe.

Microorganisms play an important role in the biosphere regarding metal mobilization as well as immobilization (Gadd, 2007) and have been extensively studied in order to find alternatives to technical/chemical processes. Many fungal species are able to tolerate high levels of toxic metals, and microorganisms such as bacteria, fungi and algae are being used in large scale removal of metals from industrial and domestic effluents (Zapotoczny et al., 2007). Microbial processes encompass both solubilization as well as immobilization of metals (Gadd, 2000), involving fungal biosorption, bioaccumulation and precipitation (Fomina et al., 2007). Since free UO22+ is bioavailable it may be accumulated by fungi, both intra- and extracellularly, through fixation by phosphate- and/or carboxylate ligands (Kelly et al., 2002) but can also be complexed by amino ligands present in chitin or chitosan (Guibal et al., 1995). Uranyl complexation with phosphate-, carboxylate- and amino ligands is highly dependent on solution pH (Kelly et al., 2002, Guibal et al., 1995), competing ligands in solution as well as surface bound ligands. With increasing pH, hydroxyl-, carboxyl- and amino groups (e.g. mannans, chitin/chitosan, beta-glucans and amino acids) become deprotonated thus increasing cell wall charge (Naeem et al., 2006). As shown by Kelly et al. (2002) this is important in the estimation of the number of uranyl ions bound to phosphoryl and carboxyl ligands. The metal binding capacity of different microbial cell wall components has been extensively studied and shown to depend on many factors such as external pH, temperature, culture medium composition, residence time, metal concentration, element speciation, cell wall component abundance and affinity, and microbial species (Ashkenazy et al., 1997, Baik et al., 2002, Dursun et al., 2003; Guibal et al., 1995, Mishra et al., 2010).

The uranyl ion, UO22+ which is thermodynamically stable under oxidizing conditions at low to intermediate pH (Rai et al., 2003), can form complexes with a number of microbial exudates e.g. low molecular weight organic acids (LMWOAs) such as citric and oxalic acid (Gadd, 2007, Renshaw et al., 2003) but also polysaccharides (Huang and Yang, 2004, Sharma et al., 2009). Uranyl may also form complexes with many other organic compounds (Jamet et al., 1993, Lenhart et al., 2000). Oxalate, together with other LMWOAs, has been suggested to play major roles in metal detoxification (Jones, 1998). Metal stress often induces an increased production of fungal exudates as shown by Green and Clausen (2003) who reported that CaO treatment of a copper-tolerant brown-rot fungi generated a 2–17 times higher production of oxalic acid than in an untreated system. Besides oxalate and citrate, numerous other LMWOAs have been reported, such as gluconic, malic, malonic, formic, fumaric and tartaric acid (Galkin et al., 1998, Mäkelä et al., 2002, Takao, 1965). Moreover, hydroxycarboxylic acids like gluconic and citric acid as well as cellulose decomposition products like isosaccharinic acid and also humic substances (fulvic acids), would be strong complexing agents at pH above pKa, usually 9–10 and above due to hydroxyl groups present (Nordén, 1994, Pavasars, 1999, Dario, 2004). However, in many systems LMWOAs only represent a minority of fungal exudates (Ogar et al., 2014k, Aliferis and Jabadi, 2010). A study of metabolite composition from Rhizoctonia solani by Aliferis and Jabaji (2010) identified 109 different organic compounds. The exudate mainly consisted of 17.4% phenolics, 12.79% carboxylic acids, 6.08% carbohydrates, 3.78% fatty acids and 3.47% amino acids.

Fungal biomass as well as fungal metabolites may be of great use as clean-up tools for metals through adsorption and complexation. Studies have focused on the use of living or dead biomass as biosorbents. In this study we screen for site specific uranium tolerant fungal strains to investigate the production of uranium stabilizing ligands under uranium stress. The specific aim is to find novel ligand candidates for selective uranium capture in artificial and natural waters.

Section snippets

Fungal cultures

Pieces of unprocessed but weathered alum shale from Kvarntorp, Sweden, approx. size 0.5 × 3 mm, were spread on malt extract agar plates (Sigma, Germany) with pH 3.5, adjusted with autoclaved lactic acid solution according to a protocol for pH adjustment of malt extract (www.scharlab.com). Malt extract contains a mixture of carbohydrates, enzymes, amino acids and also minerals and trace elements (DiaMalt and Sons, 2014, www.diamalt.co.uk/nutritional.htm). There are two kinds of commercial malt

Size exclusion chromatography

In this study size exclusion chromatography (SEC) was used for the separation of organic compounds in the aqueous phase of fungal cultures and controls. The column used for this purpose can separate organic compounds of sizes between 0.10 and 100 kDa, where sizes below 1 kDa are denoted as intermediate- and low molecular weight organic compounds. The total separation time was 30 min during which the larger compounds had a shorter retention time than the smaller compounds. Fractions were collected

Conclusions

All fungal strains were able to tolerate uranium at the given concentration of 10 mg L−1 in this study. An increase in production of organic compounds in the low-, intermediate- and high molecular weight range was observed in all cultures except for in culture of strain BS D, where only a minor increase in the low molecular weight range was observed. Analysis of uranium concentration in the dissolved solution phase show a continuous decrease of dissolved uranium species of up to nearly 100%,

Acknowledgements

Financial support to S. Karlsson from the Academy of Economy, Science and Technology at Örebro University is acknowledged. This work was also supported by Foundation of Polish Science, international PhD projects Program co-financed by the EU European Regional Development fund (MPD/2009-3/5).

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