Removal and recovery system of hexavalent chromium from waste water by tannin gel particles

Tannin gel particles that have extremely high adsorption capacity for hexav alent 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 formalde hyde (cross-linking agent) and granulation by dispersing the partially gela ted solution into a liquid mixture of decalin and a polyether nonionic surf actant 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 particl es 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 mi form aldehyde (37 wt%), It is found that it is very important to increase the wa ter 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 t his knowledge, we propose a new system that generates little waste in remov ing and recovering hexavalent chromium.