Utilisation of adsorption as a resource recovery technique for lithium in geothermal water

Geothermal water is rich in lithium (Li) and it can be recovered by adsorption and other methods. Product recovery by adsorption is important for the achievement of a circular economy and the reduction of geogenic Li in the biosphere. This paper aims to review the performance of different adsorbents for the removal of Li from geothermal water. The reported adsorption capacity was between 6 and 69 mg/g for most materials and the optimal pH was about 12. Ion-exchange was the major mechanism of Li uptake onto nano-adsorbents of inorganic origin. Other important mechanisms observed were intercalation with associated hydrogen bonds, electrostatic attraction, intermolecular and intramolecular complexation. The Langmuir isotherm and pseudo-second-order model were best-fit to describe the Li uptake in most cases. Adsorption is observed to have some technical advantages over other processes. It is relatively cheaper, does not have chemical resistance observed in membranes, does not have the limited selectivity and difficulty of integration into industrial processes, and does not possess the high electrical requirement of electrochemical techniques. HCl is the most effective eluent for Li desorption and most adsorbents can be reused over 5 times with good retention of adsorption capacity. Future studies can look into column adsorption of Li, molecular modelling of Li interaction with sorbents, used adsorbent disposal and cost analysis.