Optimization and estimated production cost of a citric acid-modified soybean hull ion exchanger

Soybean hulls modified with citric acid have enhanced capabilities for adsorbing metals from wastewater and have the potential to compete in the marketplace as ion-exchange resins. The objective of this study was to determine the optimal conditions for citric acid modification of soybean hulls and to estimate the cost of the modification process. Adsorption capacities for copper ion (Cu2+) were determined on unwashed, water-washed and 0.1 N NaOH-washed soybean hulls. All three samples had the same adsorption capacities. Therefore, unwashed hulls were selected as the feedstock for citric acid modification. The citric acid-treated hulls required a drying step before modification could proceed. However, the drying and modification steps could be combined into a single process step when the wet hulls were heat-treated at 120 degreesC for 1.5 h. A process flow diagram was developed based on these results. From the process description and equipment needed to manufacture the modified product, the capital and operating costs were determined. The product cost was estimated, based on a daily input of 10 000 kg of soybean hulls and a daily output of modified product of 7900 kg, with a yield of 79%. An estimated production cost for the modified product was $1.17/kg versus typical retail costs of $4-40/kg for commercial, petroleum-based cation-exchange resins when purchased in bulk quantities. Published by Elsevier Science B.V.