Abstract
Tannin gel particles that have extremely high adsorption capacity for hexavalent chromium Cr(VI) have been developed by controlling gelation of Mimosa tannin extracted from the bark of plants. The gelation process is composed of two stages; partial gelation of Mimosa tannin by reaction with formaldehyde (cross-linking agent) and granulation by dispersing the partially gelated solution into a liquid mixture of decalin and a polyether nonionic surfactant with vigorous stirring. The structure of the tannin gel particles is controlled by a combination of operating conditions such as reaction time and temperature at both stages. The water content of the tannin gel particles that reflects their structure can be widely changed from 40 to 79% (wet basis), even for a constant composition of 23 g Mimosa tannin and 6 ml formaldehyde (37 wt%). It is found that it is very important to increase the water content without losing mechanical strength and supply a large amount of protons during adsorption for achievement of high adsorption capacity. The maximum adsorption capacity, 540 mg-Cr(VI)/(g-dry tannin gel) is obtained at a water content of 77.6% with an acidic solution of pH ≈ 2.0. Based on this knowledge, we propose a new system that generates little waste in removing and recovering hexavalent chromium.
