Acid recycling to optimize citric acid-modified soybean hull production

Modification of soybean hulls with citric acid (CA) improves the metal ion binding capacity of the hulls. A process for this modification has been developed. However, the process must be optimized for it to be cost effective. In this regard, the objectives of this study were (1) to develop a wash procedure to remove non-reacted or residual CA after soybean hull modification in order to maximize the amount of non-reacted acid removed, but minimize the subsequent effect on the product’s ability to adsorb copper ion (Cu ), and (2) to determine whether recycling non-reacted acid in the modification process, along with unused acid, can produce a product with similar copper ion adsorption compared with the use of unused acid only. To meet these objectives, an effective water washing procedure was developed to remove the residual CA associated with the modified soybean hulls. This washing procedure included removal of nonreacted acid, evaporation of excess water to the original volume of acid solution used, and addition of unused CA to reconstitute the solution to its original molarity. Reconstituted CA solutions prepared in this manner were used through three cycles of soybean hull modification. The effectiveness of the reconstituted CA to modify soybean hulls after each cycle was measured by the modified hulls ability to bind copper ions in solution. After three reaction cycles, there was a 7% reduction in copper ion adsorption. In general, two water wash cycles were necessary to remove sufficient nonreacted CA so that metal ion binding of the hulls was not affected, and the use of residual CA to modify soybean hulls can be successfully accomplished with only a small reduction in copper ion binding. Published by Elsevier B.V.