Adsorption mechanism of cadmium on juniper bark and wood

In this study the capacity of sorbents prepared from juniper wood (JW) and bark (JB) to adsorb cadmium (Cd) from aqueous solutions at different pH values was compared. Adsorption behavior was characterized through adsorption kinetics, adsorption isotherms, and adsorption edge experiments. Results from kinetics and isotherm experiments showed that JB (76.3–91.6 μmol Cd g−1 substrate) had 3–4 times higher adsorption capacity for Cd than JW (24.8–28.3 μmol Cd g−1). In addition to higher capacity, JB exhibited a higher strength of adsorption (45.3 versus 9.1 L mmol−1) and faster uptake kinetics (0.0119 versus 0.0083 g μmol−1 min−1) compared to JW. For both these adsorbents, increasing Cd adsorption with increasing solution pH in the range of 2–6 suggests that surface carboxyl groups (RCOOH) might be involved in interaction with Cd. Diffuse reflectance infrared Fourier transform (DRIFT) spectra showed that the surface concentration of carboxyl groups was higher on JB compared to JW. The ratio of Ca released to Cd adsorbed was 1.04 and 0.78 for JB and JW, respectively, indicating that Ca–Cd ion-exchange was the primary mechanism involved. The higher Ca content in JB (15 times more) and the surface RCOOH concentration (2.5 times more) can be attributed to the observed differences in Cd adsorption behavior between the two lignocellulosic adsorbents.