## 4 Discussion
### Research Highlights and Future Implications  
The study results suggest that _Paulownia_ can be an optimal candidate for transformation of the contaminated DS into legislation-compliant peat substitutes. This potential is supported by the plant's ability to accumulate 41 out of 67 elements, including 8 noble metals and 16 rare earth elements, from flotation tailings, which can be classified as \"mandade sediments\" [103]. Recent studies have highlighted the growing interest in cultivating _Paulownia_ trees in European countries, positioning them as a strong alternative to native tree species [104; 105]. It is anticipated that the profitability of growing _Paulownia_ will become apparent soon, given its multi-purpose biomass utilised as an industrial raw material, i.e., pulp, pencils, crayons, energy production, paper, nanocellulose, biochar, biopolymers, wood plastics, and composites [85; 86; 104]. In light of Europe's goal to become the first \"climate-neutral\" continent by 2050 [106], the region is encouraged to cultivate fast-growing lignocellulosic species, such as _Populus_, _Eucalyptus, Salix_, _Robinia_, and _Paulownia_, as short rotation coppice species capable of thriving on marginal and abandoned lands [105]. A techno-economic analysis of _Paulownia_ dual-use (timber and woodchip) production in Southern Italy showed an annual gross margin of EUR 358 ha\({}^{-1}\), compared to EUR 237 ha\({}^{-1}\) for wine grapes [85], further confirming the profitability of _Paulownia_ cultivation. In addition to its economic viability, _Paulownia_ has been proposed as a sustainable tool for CO\({}_{2}\) mitigation. [PERSON] et al. [107] reported that planting 1.5 million _Paulownia_ trees on 2400 ha could capture 1.04 Mt of atmospheric CO\({}_{2}\) over 10 years while producing 568,301 t of biomass. Considering an investment cost of USD 1128 billion and the marketing value of the wood produced over 10 years (USD 1137 billion), the economic analysis suggests that _Paulownia_ trees could serve as a highly profitable, large-scale CO\({}_{2}\) capture tool with the potential to contribute to global climate goals by 2050 [106; 107]. Therefore, _Paulownia_ should be considered a promising phytoagent for the ex situ remediation of contaminated sediments. The urgent need to restore DS was further highlighted by [PERSON] et al. [27] during the evaluation of the economic and environmental life cycle assessment (LCA) of phytoremediation of chloride-contaminated DS using _Arundo donax_. The biomass was valorised through the Combined Heat and Power value chain. The study found that the costs and environmental impact of the phytoremediation of 1 m\({}^{3}\) of DS, whether conducted alone or integrated with biomass valorisation, were significantly lower than those of sediment landfilling. While the LCA model tested scenarios with no additives, mycorrhiz, humic acid, and combinations of these additives, the incorporation of biochar could further reduce costs and shorten the revenue time, making the approach even more cost-effective than conventional sediment management methods [27; 81]. Biochar has proven to be a valuable addition to the phytoremediation system [28; 29; 98; 108; 109]. Applying biochar derived from _Miscanthus \(\times\) giganteus_ (600 \({}^{\circ}\)C) at a 5% application rate improved the dry weight of stems and the leaves-to-stem ratio in _Paulownia_ plants grown in flotation tailings during the first year of vegetation [103]. In line with the circular economy concept, i.e., closing the economic loop of DS phytoremediation, post-phytoremediation _Paulownia_ biomass can be converted into biochar and reintegrated into the phytoremediation process. _Paulownia_-derived biochar, produced at 700-800 \({}^{\circ}\)C, has shown promising results when applied to contaminated soil at 2, 4, and 6% application rates. At the highest dose, it reduced the acid-soluble fraction of Cu by 35.0% and decreased the Fe/Mn oxide fraction of Pb by 9.85 and 8.37% at 2 and 6% application rates, respectively [110].