A circular economic approach to the phytoextraction of Zn from basic oxygen steelmaking filtercake using Lemna minor and CO₂

Two billion tonnes of alkaline metallurgical waste is generated per year as a product of industry, mining, and metal processing. Filtercake is one such residue formed as a bi-product of steelmaking. Metal rich bi-products can be both an environmental concern and potential resource. High concentrations of heavy metals, if accessed, could be utilised and reprocessed reducing both pollution and the demand for raw metal ores. Phytoextraction is one such method of recovering metals from contaminated mediums. Research interest in Lemna sp. has grown due to their phytoremediation potential. Facilitated by rapid growth and accumulation of nutrients and metals, Lemna minor has been described as one of the most effective macrophytes for remediating contaminated water. The present study outlines a system using L. minor to extract Zn from filtercake when submerged in static water. To facilitate phytoremediation, CO₂ carbonation can be employed to solubilise elements and utilise this greenhouse gas, another a bi-product of steel industry. The addition of CO₂ to vessels of water containing filtercake lowered the pH from as high as 8.8 to 5.6 and significantly increased Zn in solution compared to vessels receiving no CO₂. Results suggest the potential of L. minor to accumulating 68.7 kg Zn per year from 20.5 Mt. filtercake ha⁻¹. This system facilitates a circular economy with re-use of multiple existing bi-products. In addition, the potential employment of biomass in biofuel production and use of remediated filtercake in carbon sequestration adds further environmental and socio-economic impact. The extent to which the approach was consistent with circular economy was discussed and its wider integration considered.