Adsorption of copper ions from aqueous solution by citric acid modified soybean straw

The objectives of the present study were to convert soybean straw to a metal ion adsorbent and further to investigate the potential of using the adsorbent for the removal of Cu2+ from aqueous solution. The soybean straw was water or base washed and citric acid (CA) modified to enhance its nature adsorption capacity. The morphological and chemical characteristics of the adsorbent were evaluated by spectroscopy and N2-adsorption techniques. The porous structure, as well as high amounts of introduced free carboxyl groups of CA modified soybean straw makes the adsorbent be good to retain Cu2+. The adsorption capacities increased when the solution pH increased from 2 to 6 and reached the maximum value at pH 6 (0.64 mmol g−1 for the base washed, CA modified soybean straw (CA-BWSS)). The Cu2+ uptake increased and percentage adsorption of the Cu2+ decreased with the increase in initial Cu2+ concentration from 1 mM to 20 mM. Both the Langmuir and Freundlich adsorption isotherms were tested, and the Freundlich model fited much better than the Langmuir model. It was found that CA-BWSS have the highest adsorption capacity of the four kinds of pretreated soybean straw.