Equilibrium and kinetic modelling of Cd(II) biosorption by algae Gelidium and agar extraction algal waste

In this study an industrial algal waste from agar extraction has been used as an inexpensive and effective biosorbent for cadmium (II) removal from aqueous solutions. This biosorbent was compared with the algae Gelidium   itself, which is the raw material for agar extraction. Equilibrium data follow both Langmuir and Redlich–Peterson models. The parameters of Langmuir equilibrium model are qmax=18.0mgg-1, b=0.19mgl-1 and qmax=9.7mgg-1, b=0.16mgl-1, respectively for Gelidium and the algal waste. Kinetic experiments were conducted at initial Cd(II) concentrations in the range 6–91 mg l−1. Data were fitted to pseudo-first- and second-order Lagergren models. For an initial Cd(II) concentration of 91 mg l−1 the parameters of the pseudo-first-order Lagergren model are k1,ads=0.17k1,ads=0.17 and 0.87 min−1; qeq=16.3qeq=16.3 and 8.7 mg g−1, respectively, for Gelidium and algal waste. Kinetic constants vary with the initial metal concentration. The adsorptive behaviour of biosorbent particles was modelled using a batch reactor mass transfer kinetic model. The model successfully predicts Cd(II) concentration profiles and provides significant insights on the biosorbents performance. The homogeneous diffusivity, Dh, is in the range 0.5–2.2×10−8 and 2.1–10.4×10−8 cm2 s−1, respectively, for Gelidium and algal waste.