Can a material capture lithium from salty water and release it again when the temperature changes? In our study in Green Energy & Environment, we explored this idea using a temperature-responsive polymer combined with alginate, a biopolymer derived from brown algae.
The study connects two questions in separation science: how to distinguish a target ion from competing ions, and how to recover that target after capture. Our approach used an aluminum-crosslinked polymer network to influence ion adsorption and a thermal response to help release the captured ions.
Why lithium recovery is a separation challenge
Lithium is an important component of lithium-ion batteries, but recovering it from a complex salt solution requires more than simply making an adsorbent that attracts positively charged ions. Sodium, magnesium, potassium, and other ions also interact with the material. A useful recovery process must manage that competition while allowing the adsorbent to be reused.
This study investigated Li-spiked seawater containing 60 mg/L of lithium, alongside much higher concentrations of sodium and magnesium. These were controlled experimental conditions with added lithium, so the results should not be interpreted as a demonstration of recovery from untreated seawater at its natural lithium concentration.
A material with two complementary functions
We combined poly(N-isopropylacrylamide), or PNIPAAm, with aluminum-crosslinked alginate to form an interpenetrating polymer network, abbreviated PNP/Alg(Al). Alginate provides negatively charged groups, while the aluminum crosslinks create a local environment that also repels cations.
The proposed mechanism depends on the balance of these interactions. Strong repulsion can hinder the binding of competing ions with high adsorption affinity, while hydrated lithium ions can enter the network through physical adsorption. The detailed ion-selection mechanism remains an open question because the polymer structure and ion interactions are complex.
PNIPAAm adds a second function: the network responds to temperature. Heating changes its interaction with water and can promote the release of hydrated ions. This offers a route to desorption without using an acidic solution for that step.
What the experiments showed
In situ transmission electron microscopy and infrared spectroscopy tracked structural and molecular changes during heating. These measurements showed that the composite has a more complex thermal response than a simple, isolated polymer network.
In adsorption experiments, the material reached a lithium adsorption capacity of approximately 0.031 mmol/g after one hour. Extending contact to 24 hours did not increase the measured capacity. The paper reported a lithium recovery ratio of approximately 7.3% under the tested conditions.
For repeated adsorption and desorption, the loaded material was transferred into deionized water at 60°C to release adsorbed ions. Performance was maintained over the three cycles examined, supporting the feasibility of thermal regeneration within this limited test.
Why this matters
The contribution is a material-design strategy that addresses both capture and release. Selectivity depends on the charged environment inside the network, while regeneration is linked to a temperature-responsive change in the material.
Avoiding acid treatment during desorption could be useful when developing alternative recovery processes. However, this study did not establish the energy consumption, environmental footprint, or economics of a complete industrial system. Those questions require process-level evaluation.
What still needs improvement
Lithium adsorption capacity was modest, and other ions were also captured. The experiments therefore do not demonstrate production of a high-purity lithium product. Testing at natural seawater lithium concentrations, improving selectivity and capacity, and measuring durability over many more cycles would be important next steps.
The central result is a proof of concept: a responsive polymer network can combine ion separation with thermally assisted release. Its value lies in opening a design route that can be refined, rather than establishing a finished extraction technology.
Connecting water treatment and resource recovery
This work complements our research on solar desalination and electricity generation. The applications differ, but both investigate how designed materials can control water or ion transport to obtain useful outputs from saline water. That shared theme extends beyond desalination to water and resource recovery.
Reference
Park, S. H., & Lee, S. J. (2022). Thermoresponsive Al3+-crosslinked poly(N-isopropylacrylamide)/alginate composite for green recovery of lithium from Li-spiked seawater. Green Energy & Environment, 7, 334–344. https://doi.org/10.1016/j.gee.2020.10.006. First available online October 14, 2020.