**1. Introduction**

The demand for metals is increasing in our society. The importance of a secure metal supply has been acknowledged by the European Commission, which has recommended the utilization of secondary raw materials as a way to achieve this [1]. When waste is incinerated using the most common technique, Waste-to-Energy (WtE) mass burn combustion, about 20% of the mass becomes what is known as bottom ash. In Sweden alone, almost 1 million tons of this type of bottom ash is produced annually; in the EU, Norway, and Switzerland, the corresponding amount is approximately 18 Mt/year [2,3]. These ashes contain significant amounts of metals, and the most common treatment method for bottom ash is mechanical separation, where pieces of metal are recovered and combined with natural weathering, i.e., carbonation. Ash having had this treatment is referred to as MIBA, the Mineral fraction of Incinerator Bottom Ash, to distinguish it from untreated bottom ash [3]. This treatment does not only recycle solid metals but also stabilizes the material [4,5] and opens it up for utilization. In some countries, e.g., Denmark and the Netherlands, MIBA is used for conventional road construction, whereas in others, including Sweden, Norway, and Switzerland, it is landfilled or used within landfills [3]. The variance in managemen<sup>t</sup> approaches is a result of the different guidelines and legislation used in different countries [3]. Current guidelines and legislation are strongly correlated to the presence and potential leaching of metals, as even after mechanical metal recovery, a significant amount of metal remains in the MIBA, either bound in chemical compounds or as small pieces (typically < 1 mm); see, e.g., [6,7]. If more metals were to be removed, the

**Citation:** Karlfeldt Fedje, K.; Edvardsson, V.; Dalek, D. Initial Study on Phytoextraction for Recovery of Metals from Sorted and Aged Waste-to-Energy Bottom Ash. *Soil Syst.* **2021**, *5*, 53. https://doi.org/ 10.3390/soilsystems5030053

Academic Editors: Matteo Spagnuolo, Paola Adamo and Giovanni Garau

Received: 17 August 2021 Accepted: 26 August 2021 Published: 31 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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potential fields of application for MIBA would increase significantly, and most importantly, these resources could be utilized instead of virgin metals needing to be mined.

Recovery of the metals remaining in MIBA cannot be carried out using physical separation, but thermal or hydrometallurgical processes could potentially be used. The content of, e.g., Cu in MIBA is comparable to workable ores, but as the MIBA matrix is different from a rock, the amounts are still too low to be of interest to the energy-intensive metal refining industry [8]. Additionally, the presence of unwanted elements such as Cl and As may harm the recovery process [8]. Instead, MIBA must be treated to generate concentrates with higher contents of interesting metals and lower amounts of unwanted elements. Hydrometallurgical processes, i.e., leaching combined with, e.g., chemical precipitation, could be a way to ge<sup>t</sup> such material. However, the main challenge is the huge amounts of leaching media needed for MIBA treatment. Pure water is not efficient enough to leach metals, and assuming a liquid-to-solid ratio of 4, almost 4 million m<sup>3</sup> leaching agents would be required for Sweden alone. Even if part of the leachate was re-circulated, the large amounts of liquid would still be problematic. In this perspective, phytoextraction is an interesting alternative for metal recovery from MIBA, as it offers a way to extract and concentrate the metals of interest. Phytoextraction uses specific plants, so-called hyperaccumulators, that cannot only survive in contaminated areas but can also extract the contaminants into their tissues. Once harvested, these plants can be incinerated, and metals can be recovered from the resulting ash. Successful laboratory-scaled Ni recovery experiments which combine phytoextraction with separate incineration of the Ni-enriched plants have been reported [9]. However, separate incineration is usually not an alternative if used on a full scale. Instead, incineration of the enriched plants in conventional full-scale WtE plants is favorable, and the metals enriched in plants would likely be found in the fly ash [10]. This is positive, as the recovery of metals from WtE fly ash has been in focus for many years. Zn especially is of high interest, and several successful initiatives for Zn recovery from ash are under development or present as full-scale processes [11–13]. This makes the production of a Zn-rich fuel generated from the growth of hyperaccumulators in MIBA especially interesting, as this would transfer the Zn from the challenging MIBA matrix into a material from which Zn can be recovered efficiently. Besides, in the future, methods for the recovery of other valuable metals in fly ash are likely to be developed as well.

Phytoextraction is used for treating contaminated soils all over the world; however, a drawback for the Nordic countries is that many of the known Zn hyperaccumulators, e.g., water hyacinth (*Eichhornia crassipes*), do not grow naturally in their climate [14]. Further, several of the potential plants have limited biomass, including alpine pennycress (*Thlaspi caerulescens*) [15]. However, sunflowers (*Helianthus annuus*), rapeseed (*Brassica napus*), corn (*Zea mays*), and haricot verts (*Phaseolus vulgaris*), as well as larger plants, such as salix trees, have also been shown to accumulate Zn [15–22]. Generally, experiments carried out so far have used contaminated soils with few contaminants, and there is a lack of studies of scientifically controlled cultivation on ash containing several potential pollutants. Most earlier studies have used coal ash [23,24]; however, there is one published study where manually sorted and washed bottom ash from WtE incineration was used [25]. The results of this study showed that Ni and Zn were extracted by Alyssum (*serpyllifolium*) and a succulent (*S. plumbizincicola*), respectively, although the biomass production was low. To the best of the authors' knowledge, no study on phytoextraction from conventional MIBA has been scientifically considered.

This project aimed to study phytoextraction, mainly of Zn, from conventional MIBA during the period of one cultivation season in the Nordic climate. The specific goals were to evaluate the survival of the plants in MIBA and to evaluate the potential to generate Zn-enriched fuel, which is to be used for metal recovery. Additionally, the simultaneous decrease in the metal content in the remaining MIBA was measured.

#### **2. Material and method**
