Chitosan-tripolyphosphate-tannic acid cryogels as a biocompatible adsorbent for the removal of Cu²⁺ ions

A chitosan (CH) cryogel is a supermacropore material that can be used as an adsorbent. However, its utilization as a metal adsorbent is limited due to its low stability in acidic conditions. A CH cryogel has been modified using tripolyphosphate (TPP) and tannic acid (TA) as biocompatible cross-linkers. TPP increases the stability of the cryogels in acidic solutions, and TA increases their mechanical properties. CH-TPP, CH-TPP2.5-TA, CH-TPP5-TA and CH-TPP7.5-TA cryogels were applied for the adsorption of Cu²⁺ ions. The CH-TPP-TA cryogels reached optimum conditions at pH 5.0 with a removal efficiency of 91-96%. The adsorption of Cu²⁺ ions by CH-TPP-TA cryogels fits the nonlinear Freundlich isotherm, or Cu²⁺ is adsorbed on the heterogeneous cryogels' surfaces. CH-TPP5-TA has an adsorption capacity up to 109 mg g^-1 at an initial Cu²⁺ concentration of 500 mg l^-1. The adsorption kinetics of Cu²⁺ ions follows pseudo-second order. It is interpreted that the adsorption mechanism is generally through chemical interactions. Copper elements are proven to be adsorbed by the cryogel, which is detected by energy-dispersive X-ray analysis. The photographic results show that Cu²⁺ ions can be adsorbed into the inner layer of the cryogel. Finally, the CH-TPP-TA cryogel is promising to be used as a biocompatible adsorbent for copper.