Metal ion adsorption by soybean hulls modified with citric acid: A comparative study

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 shell s 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 hu lls had the highest total negative charge. BE, CA-modified soybean hulls al so were compared to BE hulls modified in the presence of 0.6 M concentratio ns 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 ca dmium (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 t o two commercial cation exchange and two commercial chelating resins for ad sorption of the live metals at three metal ion concentrations either in a s olution containing all five ions or as individual metals. When all five met al ions were present at the highest concentration (7 mM), modified soybean hulls removed more metal ions than any of the commercial products. For indi vidual metal ions, all adsorbents generally were quite effective at metals removal at the three concentration levels. Our comparative results demonstr ate that modified soybean hulls have excellent metals removal properties an d can be considered as a product with possible commercial potential for met al ion remediation.