Metal Ion Adsorption by Soybean Hulls Modified with Citric Acid: A Comparative Study

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 (Cu2+) as a typical metal ion. BE, CA-modified soybean hulls had the highest metal ion adsorption 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 adsorption of copper ion by virtue of their largest total negative charge. Adsorption capacities and affinity constants for the metal ions cadmium (Cd2+), copper (Cu2+), nickel (Ni2+), lead (Pb2+) and zinc (Zn2+) were determined for BE, CA-modified hulls at pH 4.8. Adsorption capacities for all ions were greater than 1.0 mmol g−1 hull. These hulls were 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 generally were 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.