This is the astonishing title of a paper published on plos.org last month. In this work, conducted in Japan, the feasibility of using salmon milt for REE recovery and separation was examined, along with the identification of the binding site of REEs in salmon milt. Results showed that (i) salmon milt has a sufficiently high affinity to adsorb REEs and (ii) the adsorption capacity of the milt is 1.04 mEq/g, which is comparable with that of commercial cation exchange resin. Heavier REEs have higher affinity for milt.
A comparison of stability constants and adsorption patterns of REEs discussed in the literature suggests that the phosphate is responsible for the adsorption of REE in milt. The results were supported by dysprosium (Dy) and lutetium (Lu) LIII-edge extended x-ray absorption fine structure (EXAFS) spectroscopy. The REE-P shell was identified for the second neighbouring atom, which shows the importance of the phosphate site as REE binding sites.
The comparison of REE adsorption pattern and EXAFS results between the milt system and other adsorbent systems (cellulose phosphate, Ln-resin, bacteria, and DNA-filter hybrid) revealed that the coordination number of phosphate is correlated with the slope of the REE pattern. The separation column loaded with milt was tested to separate REE for the practical use of salmon milt for the recovery and separation of REE. However, water did not flow through the column possibly because of the hydrophobicity of the milt. Thus, sequential adsorption–desorption approach using a batch-type method was applied for the separation of REE. As an example of the practical applications of REE separation, Nd and Fe(III) were successfully separated from a synthetic solution of Nd magnet waste by a batch-type method using salmon milt.
This study shows that freeze-dried salmon milt can be used as adsorbent for REEs and possibly other cations. The EXAFS results show that the phosphate site in the milt is responsible for REE adsorption onto the milt. The adsorption capacity of REE or cation was 1.0 mEq/g, which is comparable with that of commercial cation exchange resin. Although milt cannot be used for the column method because water does not flow through the milt-loaded column, milt powder can still be used for batch-type recovery and separation. For example, assuming its application to Nd magnet, separation of Fe and REE (Nd or Dy) is possible through adsorption on milt at pH 4 and subsequent REE desorption from the milt at pH 2. Considering that milt, an industrial waste from fishery industries, is a low-cost material, its ion-exchange capacity has a substantial potential for use in different fields such as the recovery of cations (e.g., REE) and wastewater treatment.
The diagram shows total recovery of Nd, Dy, and Fe(III) by repetitive leaching into HCl solution at pH 2 from the milt powder after the initial adsorption of these ions on the milt at pH 4 from their solution assuming the decomposed solution of Nd magnet.
