Recovery of europium from E-waste using redox active tetrathiotungstate ligands

Rare-earth elements (REEs) are critical to our modern economy, yet their mining from natural ores bears a profound environmental impact. Traditional separation techniques are chemical and energy-intensive because their chemical similarities make REEs very challenging to purify, requiring multiple extraction steps to achieve high purity products. This emphasizes the need for sustainable and straightforward separation methods. Here we introduce a strategy for the direct separation of europium (Eu) from complex mixtures under ambient conditions, leveraging on the redox non innocence of purely inorganic tungsten tetrathiolate (WS42−) ligands. The recovery of Eu is achieved upon reduction of Eu(III) to a Eu(II) coordination polymer, driven by an induced internal electron transfer from the tetrathiotungstate ligand. Applying this strategy to unconventional feedstock such as spent energy-saving lamps allows selective europium recovery with separation factors over 1000 and recovery efficiency as high as 99% without pre-treatment of the waste.

techniques. 6.In the synthesis of [NEt4]2[EuII(WS4)2] (1), I found that 1 was obtained by slow diffusion, and by analyzing the structure of the precipitate that appeared during the synthesis process, it was consistent with 1, and I would like to ask if it is possible to obtain the crystal of 1 by reducing the concentrafion of the reacfion and delaying the reacfion fime in the process of synthesizing 1. 7. I found that the condifions for the synthesis of 1 and 2 are basically the same, but 2 needs to be synthesized at -35 °C and protected from light, and the Eu in 2 is trivalent, what is the mechanism?8.In the cyclic process of europium recovery and ligand recycling, ammonium oxalate is added to obtain a red-brown precipitate, can its structure be confirmed?

Response to Reviewers
A point-by-point answer to the reviewers' comments can be found below.For clarity, the latter appear in bold/italics, and our response is in normal text; modifications in the manuscript are highlighted in yellow.

Reviewer 1 comments:
1. "The proposed closed-loop recycle and reuse of phosphor in Fig. 3  We are somehow surprised by the comment that the proposed loop is oversimplified but apologize for the confusion that this figure may have provided.We would like to clarify that this is not a hypothetical recycling loop, but the extraction process we carried out in the lab along the strategy defined in this paper, illustrated by 14 pictures taken along the way, as snapshot of the whole process (which is reported in the supporting video).As such, the scheme was not intended to describe existing procedures for the extraction of Ln ions from spent flurorescent lamps, but the specific strategy introduced here.In this work, we do not intend to integrate our strategy in existing extraction processes, using different acids than the one used in our study, but to develop an integrated extraction strategy, starting from the waste material, to the pure europium oxide.In the present case, our strategy uses triflic acid, and the selection of this acid allows the extraction of Europium with separation factors over 1000 in a single extraction step from the lamp waste, without any interference from the other metal present in the system and mentioned by the reviewer.This is significantly more energy efficient that classical Eu extraction strategies, showing much lower extraction factors.In particular, classical leaching strategies used prior to that work generally involve multiple leaching steps with inorganic acids of different concentrations, therefore generating consequential amounts of acidic waste.What we report is the use of a single leaching step which allows the rare earth extraction to be almost quantitative, therefore limiting the amount of waste generated.We apologize if this was not clear enough in our initial version of the text, and we have now reformulated figure 3 caption to better illustrate that fact.It now reads: "Fig.3. Illustrated circular process for europium recovery from a spent compact fluorescent lamp according to the process described in the present work.The 14 photos illustrate the main steps of the process, namely the separation of europium in a single step from the triflic acid extract of a lamp powder using tetrathiotungstate, the extraction of Eu(II) from the ligand using oxalate in water and its subsequent calcination to yield europium oxide.This process is further illustrated in the supplementary video."

"The method is effective in separating Y from Eu. Could authors elaborate on the advantages of the method in comparison with conventional ones, such as liquid-liquid extraction, membrane, and ion exchange?"
We thank the reviewer for their comment.Indeed, comparing the effectiveness of our separation strategy with respect to those described in the literature is critical.As highlighted in the original version of the text, the method reported in this work possesses, to the best of our knowledge the highest separation factor SEu/Y reported in the literature.As exemplified in table S14, the separation factor of our strategy using directly lamp phosphor powder sources is over 17 times higher than conventional liquid-liquid extraction, ion-exchange and photochemical reduction methods.Due to their typical lower extraction factors, we had not initially reported other extraction strategies.We have now added the highest reported factors for each methods in Table S14 to better address the comment of the reviewer.We hope this now better illustrate the unique performance of our system.topic of the present work.The whole series will be disclosed in a separate publication, focusing of the physical properties of these complexes.

"The valence state of W in the crystal structures need to be confirmed by relevant characterization techniques."
We thank the reviewer for this suggestion and apologize we had not sufficiently explicated that point in our original manuscript.We have now provided the XAS (W L3-edge), XPS and bond valence sum analysis of the complexes, all concurring to an oxidation state of +6 for the W centers in both Eu(II) and Eu(III) complexes.We have now added these in the ESI in sections 7, 8 and 11.

"In the synthesis of [NEt4]2[EuII(WS4)2] (1), I found that 1 was obtained by slow diffusion, and by analyzing the structure of the precipitate that appeared during the synthesis process, it was consistent with 1, and I would like to ask if it is possible to obtain the crystal of 1 by reducing the concentration of the reaction and delaying the reaction time in the process of synthesizing 1."
We thank the reviewer for this suggestion.It is indeed possible to obtain single crystals of 1 from three different strategies: using a dilute solution of the reactants, by slow diffusion, or by letting a dilute solution of 2 stand at room temperature for 24h.We have now added these 3 routes to single crystals of 1 in the ESI.

"I found that the conditions for the synthesis of 1 and 2 are basically the same, but 2 needs to be synthesized at -35 °C and protected from light, and the Eu in 2 is trivalent, what is the mechanism?"
We thank the reviewer for their question regarding the synthesis conditions of complexes 1 and 2. The necessity to synthesize 2 at -35 °C and shield it from light is crucial for maintaining the Eu in its trivalent state.This condition is essential to our mechanism, where the tetrathiotungstate ligand engages in thermal or photochemical processes to reduce Eu(III) to Eu(II), leading to the precipitation of complex 1.This point is in fact central to our study, designed to showcase the ligand's role in the redox behavior of europium.While we discussed this point extensively in the original version of the text, we acknowledge from the reviewer comment for the need for greater clarity in our manuscript on this point and appreciate the opportunity to further highlight the importance of synthesis conditions in directing the outcome of the reaction.We have now rephrased the following paragraph in the main text : "Interestingly, when the same reaction was conducted at −35 °C in the absence of light, the solution retained its dark red color, and no precipitate was observed, underscoring the critical role of synthesis conditions in stabilizing Eu in its trivalent state.Layering this dark red solution with Et2O yielded dark red crystals of the trivalent complex [NEt4]3[Eu III (MeCN)2(WS4)3] (2) (Fig. 2a).In-situ Raman spectroscopy monitoring of complex 1 synthesis revealed the emergence of a new lower symmetry W-S vibration at 491 cm −1 immediately after mixing the two reactants, indicative of the fast coordination of the tetrathiotungstate ligand, while the appearance of a vibration above 500 cm −1 after 10 minutes suggests the formation of a disulfide complex (Fig. S6), 33 highlighting the ligand's involvement in a redox process.This observation, coupled with the mild reduction potential of Eu(III) to Eu(II) exhibited in the cyclic voltammogram of 2, (E1/2 = − 0.55 V vs. Fc/Fc + , Fig. S16), suggests the role of the tetratungstate ligand in promoting an internal electron transfer (IIET) mechanism, initiated by an external stimuli such as light or heat, which likely triggers the reduction of europium and the formation of 1, as depicted in Fig. 2b."

"In the cyclic process of europium recovery and ligand recycling, ammonium oxalate is added to obtain a red-brown precipitate, can its structure be confirmed?"
The identity of the precipitate observed after addition of ammonium oxalate could be confirmed by both FTIR and XPS spectroscopy, and perfectly matches literature data.We have now added these data to may be oversimplified.For example, main challenges facing Y and Eu recovery from phosphor of spent fluorescent lamps are the acid extraction part and subsequent separation and purification processes.For example, instead of triflic acid, mostly inorganic acids, such as sulfuric acid or hydrochloric acid are used instead triflic acid as illustrated in the work.In that case, other metals including zinc, lead, and iron will be present.How will they interfere with the redox reactions?Besides, the acid extraction unit is energyintensive, time consuming, and with a large carbon footprint.Relatively speaking, the separation of Y and Eu may not be the top issue to be tackled."

Table S14 .
Separation factors from Y/Eu phosphor powders with different extractants.
*average value from duplicate experiments -see tableS13