Extraction of Nickel from Spent Catalyst Using Biodegradable Chelating Agent EDDS

Literature suggests that ethylene-diamine-tetraacetic acid (EDTA) has been proved as a successful chelating agent for the extraction of metals from soils and spent catalysts. EDTA, however, is quite persistent in the environment due to its low biodegradability, thus its use becomes a matter of environmental concern. Therefore, to minimize the potential environmental risks, a new chelating agent [S,S]-ethylene-diamine-disuccinic-acid ([S,S]-EDDS) can be considered as an environmentally benign substitute for EDTA due to its easy biodegradation capability. The present study focuses on the effectiveness of biodegradable chelating agent [S,S]-EDDS for extraction of nickel from the spent catalyst of fertilizer industry. Experiments were carried out in batch mode under reflux conditions and process design parameters were optimized to maximize the extraction efficiency. Ni extraction of 84% was attained at optimum reaction condition in one cycle run. Dechelation of Ni-EDDS complex was performed at pH 5 where more than 96% EDDS was recovered. Results of the present study were compared with the previously studied chelating agent EDTA at optimum reaction conditions reported in literature. It was observed that [S,S]-EDDS requires a narrower pH range as compared to EDTA for chelation-dechelation process. Thus milder reaction conditions were employed for metal extraction using EDDS which is favorable to select the material of construction of equipment, in addition to the added advantage of biodegradability. Kinetic study was also performed for the noncatalyzed extraction process using shrinking core model and the process was found to be diffusion controlled under experimental conditions.