Citric acid modified agricultural by-products as copper ion adsorbents

Agricultural by-products, base-extracted and reacted with citric acid (CA), were compared to demonstrate their ability to adsorb copper ions (Cu2+) from solution. Soybean hulls exhibited the highest copper ion uptake (1.44 mmol/g) of the 12 biomaterials tested. The by-products with the highest bulk densities (> 0.6 g/cm(3)), namely pecan, black walnut, and English walnut shells, showed the lowest copper ion uptake after citric acid modification. Those materials with a bulk density less than 0.6 cm(3)/g and a low lignin content had the best potential of becoming ion exchange resins using citric acid modification. There appeared to be a linear inverse relationship between total negative charge and lignin content. Chemical modification of these materials results in a low cost value-added product which can benefit industry. (C) 2000 Elsevier Science Ltd. All rights reserved.