Characterization of adsorption gels prepared from plant biomaterials

The adsorption of Pb(II), Cu(II), Cd(II), and Zn(II) by new biomaterial gels synthesized in cross-linked reactions from agricultural wastes such as lemon, yuzu (Citrus junos), and coffee was investigated. Experimental data of adsorption equilibrium from the heavy metal solutions correlate well with Freundlich isotherm equations. The adsorption mechanism involved acid ion-exchange reactions between the heavy metal and the carboxylic acid of pectic acid in the lemon and yuzu gels, improving the amounts of Pb(II), Cu(II), Cd(II), and Zn(II) adsorbed onto the lemon and yuzu gels by approximately 5- and 6-fold, respectively. The maximum adsorption capacities of the yuzu and lemon gels for Pb(II) and Cu(II) were high. Among the biomaterial gels investigated, the lemon gel possessed the highest complexation ability with these heavy metals. In addition, the adsorption of nitrate ion (NO3−) was especially strong by the Pb(II)-adsorbed lemon gel, as shown by IR absorption spectrophotometry. These results indicate that lemon and yuzu gels can function, as new “green chemistry” adsorbents, which can be generated from recycled wastes, involve zero emissions, and provide effective recovery of precious resources.