Submitted to: Environmental Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 10, 1999
Publication Date: N/A
Interpretive Summary: The use of low density, porous agricultural by-products such as soybean hulls as adsorbent material for metals has been problematic due to the small number of metal binding sites on the surface of these materials. In this study, the number of surface binding sites for metals has been greatly increased on soybean hulls by pre-treatment with a solution of base (sodium hydroxide) followed by a treatment with citric acid, a common food additive. The modified hulls were compared to commercial metal binders (cation exchange resins) for their ability to remove toxic metals, commonly found in industrial wastewater, from solution. Modified hulls were as good as and, in some cases, better than the commercial products at metals removal. Modified hulls could provide industries that currently use cation exchange resins with an effective, inexpensive alternative.
Technical Abstract: Soybean hulls, extracted with 0.1 N NaOH (BE) and modified in the presence of 0.6 M citric acid (CA), were compared to similarily treated peanut shells and the hulls of almonds, cottonseed and macadamia nut for their ability to adsorb copper ion as a typical metal ion. BE, CA-modified soybean hulls had the highest metal ion uptake but similarly treated almond hulls had the highest total negative charge. BE, CA-modified soybean hulls also were compared to BE hulls modified in the presence of 0.6 M concentrations of four different dicarboxylic acids (maleic, malic, succinic, tartaric) for their copper ion adsorption potential. Hulls modified with CA had the highest uptake of copper ion by virtue of their largest total negative charge. Adsorption capacities and affinity constants for the metal ions cadmium, copper, nickel, lead and zinc were determined for BE, CA-modified hulls at pH 4.8. Adsorption capacities for all ions were greater than 1.0 mmole/g hull. These hulls were also compared to two commercial cation exchange and two commercial chelating resins for adsorption of the five metals at three metal ion concentrations either in a solution containing all five ions or as individual metals. When all five metal ions were present at the highest concentration (7 mM), modified soybean hulls removed more metal ions than any of the commercial products. For individual metal ions, all adsorbents were generally quite effective at metals removal at the three concentration levels. Our comparative results demonstrate that modified soybean hulls have excellent metals removal properties and can be considered as a product with possible commercial potential for metal ion remediation.