Adsorption behavior of cadmium ion onto synthetic ferrihydrite: effects of pH and natural seawater ligands

Batch experiments were conducted to study the adsorption behavior of cadmium ion onto the synthetic ferrihydrite. The adsorbent was characterized using X-ray diffraction and N-2 adsorption-desorption measurements. The effects of pH value and the major seawater anions (nitrate, carbonate, chloride and sulphate) at their natural concentrations on the adsorption of cadmium ion were investigated. The results showed that the adsorption of cadmium ion was highly pH-dependent and suppressed in the presence of chloride, sulphate and nitrate, while carbonate was found to enhance the adsorption of cadmium ion over the examined range of 4.0-7.5. The maximum percentage of cadmium ion adsorbed can reach 89.96 % at pH 7.5. It is proposed that the competition for binding sites played a key role in the reduction of cadmium ion adsorption by nitrate, chloride and sulphate. X-ray diffraction spectroscopic data for cadmium adsorbed on ferrihydrite showed that CdCO3(s) was formed on ferrihydrite; the formation of CdCO3 precipitation or surface precipitation in carbonate system may be the reason for the increase of cadmium adsorption.