Biosorption of rare earths and critical metals

Biosorption is passive. Metal ions bind to functional groups on a biomass surface — living or dead, and in most designs dead — by ion exchange and complexation. No metabolism is involved, the process is fast, and it is reversible with acid. That last property is what makes it usable, and the first is what limits it.

Fixed sites, fixed capacity

The binding groups are the ordinary chemistry of a cell wall: carboxylates from uronic acids in alginate and bacterial exopolysaccharide, phosphoryl groups from teichoic acids and phospholipids, amines from chitosan in fungal walls, and hydroxyls. Their number per gram of biomass is finite, so the loading follows a Langmuir-type isotherm to a plateau. A gram of sorbent binds a set number of millimoles and then stops. Reported capacities for lanthanides on fungal, algal and bacterial biomass typically fall in the range of tens of milligrams per gram.

This is the difference that governs process design. An enzyme turns over: a small amount of catalyst treats an unlimited amount of substrate. A sorbent does not. Throughput is bought only by cycling — load, elute with dilute acid, re-condition, load again — and every cycle mechanically and chemically degrades the biomass, so operating life measured in cycles is a first-order economic variable rather than a detail.

The pH window is narrow at both ends

Carboxyl groups have pKa values around 3 to 5. Below that they are protonated, they do not exchange, and capacity collapses; a rare-earth leachate from coal ash, phosphogypsum or a tailings stream is usually strongly acidic, which puts the feed on the wrong side of that line and forces a neutralisation step. The window does not extend far upward either, because trivalent rare-earth ions begin to hydrolyse and precipitate as hydroxides above roughly pH 6. Practical operation therefore sits in a band around pH 4 to 6, and holding it against an acid feed is part of the plant, not a laboratory nicety.

Competing ions close the window further. Real leachates carry iron and aluminium at concentrations one to three orders of magnitude above the rare earths, and both are trivalent and bind to the same carboxyl and phosphoryl sites. A sorbent with high absolute capacity for neodymium in a clean solution can be almost fully occupied by iron in the stream it was built for. Pre-treatment to remove iron and aluminium is usually where the process cost actually lands.

Selectivity is the unsolved part

Separating the rare earths from everything else is tractable. Separating them from each other is not, because adjacent lanthanides differ by around one per cent in ionic radius and behave nearly identically toward a generic carboxylate surface. Protein-based approaches attack exactly this: lanmodulin, a lanthanide-binding protein identified in methylotrophic bacteria, binds lanthanides with very high affinity and discriminates strongly against calcium and other common competitors, and engineered display of such motifs is the current research direction. Separation factors between neighbouring lanthanides remain modest, and no biosorbent yet replaces the multi-stage solvent extraction that does that job industrially. Claims that biosorption produces separated rare-earth products should be read as claims about a concentrate.