The global consumption of metals and metalloids is increasing rapidly owing to an ever-growing demand from technology and renewable energy industries. Most of these metal(loid)s are defined as critical elements, since their supply is expected to be critically limiting in the near future. These include selenium, cobalt, and nickel for which the 2050 projected consumption is expected to be 5 to 10-folds higher than today. At the same time, ore grades are globally declining, and consequently the extraction of these elements will lead to serious environmental impacts connected to the production of large quantities of contaminated mineral wastes. In this scenario, MinE-UP aims at providing a tangible contribution to the recovery of these critical elements from alternative unconventional sources, such as sewage sludge (SS) and mine wastes. Sewage sludge is the final residue of the urban wastewater treatment process and derives from the particulate matter suspended in the sewage itself plus the microbial biomass produced during the water treatment process. SS contains appreciable amounts of N (2.5–3.4%), P (1.0–1.3%), K (0.2–0.4%), metal(loid)s and organic matter (20–25%). In Europe around 10 million tons of SS are produced each year and used in agriculture (EU directive 86/278/EEC), incinerated or landfilled. Incineration and landfilling entail the highest disposal costs, up to 350 €/ton of dry matter, while land disposal costs are about half that figure. In EU, more than 1/3 of SS still goes to landfill or incineration because of their high pollutant load which makes it unsuitable for safe agricultural re-use. However, SS contaminants, even though being potentially toxic, are often valuable metal(loid)s that may be considered an opportunity rather than problem. These include Se, Co, and Ni, which can reach concentrations of 100, 300 and 1000 mg/kg dry weight (DW) respectively in SS.
Considering the global increasing demand for industry critical metal(loid)s, the use of SS as alternative resource of these raw materials can thus constitute an appealing alternative to make ‘treasure out of trash’ and thereby promote the circular economy. Unlike mine wastes, SS are fertile substrates in which plants can thrive, and thus phytomining can constitute a valid environmental alternative to conventional extraction methods. Phytomining is an in-situ remediation technique that uses hyperaccumulator plants to purposely bio-concentrate target metal(loid)s in plant shoots, removing them from the substrate while allowing their recovery. Hyperaccumulators can concentrate these elements more than 1000-fold over normal plants thereby creating a metal(loid)-enriched biomass, called bio-ores. This technique has been extensively validated for the removal of metal(loid)s from contaminated soils, however its potential application to SS has not been properly investigated. Sewage sludge phytomining however requires space and suitable locations this activity can take place are mined areas which cover more than 31,000 km2 in the EU. In these areas SS has already been successfully tested as an amendment since its addition proved to be beneficial in speeding up the land reclamation process, enhancing soil fertility, water retention and revegetation. Is important to consider that the background concentration of soil metal(loid)s in these lands is already so high that additional contribution by SS will be negligible, and hyperaccumulator plants grown on this soils will also take up metals present in mine soil along with those added by the SS, thus achieving even higher concentrations in the harvestable biomass.
The overall aim of MinE-UP is to provide a crucial proof-of-concept for the recovery of critical elements (Se, Co, Ni) from an alternative resource using a circular and climate-neutral method. More specifically, the project will pursue the following 4 objectives:
1) Develop an efficient recovery process for the exploitation of metal(loid) enriched SS as an alternative resource of Se, Co, and Ni using targeted phytomining, leading to a reduction of the overall amount of disposed SS. 2) Characterize the new plant-based products (Se, Co, Ni bio-ores) and evaluate their applications and markets. 3) Test an innovative and sustainable use of mined areas for SS phytomining and evaluate the socio-environmental impact of the process. 4) Raise awareness within stakeholders, policy makers and the public on the new opportunity of coupling SS phytomining and mined-land reclamation.
