By Dr. Elise Morrison, Assistant Professor, Department of Environmental Engineering Sciences, University of Florida
Phosphorus is an element that essential for life. It is also critical for domestic food security, since crop production depends on fertilizers derived from scarce phosphate reserves. The United States is the world’s third largest phosphate producer behind China and Morocco, and phosphorus supports lucrative mining, fertilizer, and agricultural industries. Yet the United States’ phosphate supply is vulnerable to supply chain and global market disruptions, including political/economic events and natural disasters. Due to this, it is important to follow sustainable phosphorus practices, increase our reuse of phosphorus waste, and move towards a more circular phosphorus economy.
Fertilizer production generates phosphogypsum (CaSO4∙nH2O) waste. Phosphogypsum is formed during wet acid treatment, when phosphate rock (fluorapatite) is treated with sulfuric acid (H2SO4) to produce phosphoric acid (H3PO4) for fertilizers. Only ~15% of phosphogypsum produced annually is recycled because its reuse is often hindered by contaminants such as heavy metals and radionuclides, although the type and amount of contaminants can vary depending on the source of phosphate rock. Instead of reusing phosphogypsum, it is often stored in large stockpiles known as gypstacks, and it is estimated that there are over 6 billion tons of phosphogypsum stored in stacks globally. These gypstacks can be costly to operate and can have negative economic and environmental impacts.
A considerable body of research has been conducted on the extraction and remediation of phosphogypsum and other waste materials, with topics generally focusing on their reuse as agricultural amendments, the remediation of contaminants such as fluoride and uranium, and the recovery of critical minerals. Critical minerals are 50 minerals considered essential to US national or economic security with a vulnerable supply chain and can include aluminum, magnesium, manganese, titanium, and rare earth elements.
Phosphogypsum’s rare earth element content ranges from 0.01 to 0.60 wt%, and it is estimated that 1 million tons of rare earth elements are stored in global phosphogypsum stacks (2024 values). Beyond rare earth elements, other valuable materials (e.g. phosphorus and sulfur) can be recovered from phosphogypsum. But the question remains, what is the best way to extract valuable materials from phosphogypsum waste? Here, microbes may be able to help.
Microorganisms naturally exhibit a wide range of metabolisms, some of which are capable of secreting acids that liberate critical minerals from phosphogypsum. The innate capacity of microorganisms to extract valuable materials can be harnessed through biomining.
Biomining is the extraction and recovery of metals from ores and wastes using biological organisms. Biomining can include bioleaching (which solubilizes the target element), biosorption (which binds the target element in biomass), and bio-oxidation (a biological pretreatment prior to a secondary mining process) 1. Bioleaching is used to solubilize the target element and frequently uses biolixiviants, which are biologically produced chemicals used to extract ions from an ore, often derived from spent media.2 Some bacteria, most notably, Gluconobacter oxydans, have been usedto produce biolixiviants and leach valuable materials, such as critical minerals and rare earth elements, from waste products including phosphogypsum and spent fluorescent lightbulbs. These processes use naturally occurring microorganisms and are often safer for the environment, require less energy and can be more cost-effective than traditional mining methods.
However, scaling up these processes can be challenging, and it is unclear if microbes may be affected by the heavy metals and radioactive elements in phosphogypsum. Various biomining techniques have been successfully applied to target other elements at scale. These approaches include irrigation-based techniques (e.g. to leach copper, nickel, and pre-treat for gold), stirred tank bioreactors (e.g. for the bio-oxidation of refractory gold), and in situ mining (e.g. for uranium or copper). Irrigation-based techniques involve stacking waste into piles (2-10 m high), which are irrigated with dilute sulfuric acid to enhance the growth of existing mineral-oxidizing bacteria, and then collecting the leach solutions for the extraction of elements, such as copper. Stirred tank bioreactors (> 1300 m3) are often continuous-flow, aerated systems that are used to bio-oxidize refractory gold and cobalt, while other bioreactor designs have coupled aerobic and anaerobic bioreactors to acquire sulfur from phosphogypsum waste. Lastly, in situ mining involves fracturing ore bodies, flooding mines, then pumping the solution to the surface to recover the element of interest. Interestingly, in the middle ages, a similar approach was used when underground mines were intentionally flooded for mineral recovery, although the groundwork for modern in situ biomining was laid in 1940s with the discovery that Acidithiobacillus (previously Thiobacillus) ferrooxidans could convert metal sulfides to water-soluble metal sulfate.
While biomining techniques have advanced since the 1940s, more work is needed to explore the full metabolic capacity of microorganisms to address the challenge of managing phosphogypsum waste. One exciting innovation at the intersection of mining and agricultural sustainability is to use agricultural byproducts to fuel the microbes used for biomining. This approach is most relevant for processes that use microbial heterotrophs and has been successfully applied to Gluconobacter oxydans, which has been grown on potato waste instead of refined glucose, with equivalent biolixiviant performance.3,4 The approach has multiple benefits, including reducing operating costs, using agricultural waste, and recovering valuable materials from wastes. So in the future, perhaps crops fertilized with mined phosphates could fuel the microbes mining phosphogypsum waste, supporting a more circular phosphorus economy.
References for Further Reading
1. Johnson, D. B. Biomining—biotechnologies for extracting and recovering metals from ores and waste materials. Curr. Opin. Biotechnol. 30, 24–31 (2014).
2. Brown, R. M., Mirkouei, A., Reed, D. & Thompson, V. Current nature-based biological practices for rare earth elements extraction and recovery: Bioleaching and biosorption. Renew. Sustain. Energy Rev. 173, 113099 (2023).
3. Hatch, C. Potato wastewater could feed bacteria used to recycle high-tech devices. Idaho National Laboratory https://inl.gov/integrated-energy/potato-wastewater-could-feed-bacteria-used-to-recycle-high-tech-devices/ (2021).
4. Jin, H. et al. Sustainable Bioleaching of Rare Earth Elements from Industrial Waste Materials Using Agricultural Wastes. ACS Sustain. Chem. Eng. 7, 15311–15319 (2019).
Opinions expressed here do not necessarily reflect those of the Sustainable Phosphorus Alliance or its members.