CHARACTERIZATION OF CHROMATE ADSORPTION ON GAMMA ALUMINA

The sorption behavior of chromate (CrO₄^2-) was studied on an aluminum oxide model solid in order to better understand the environmental conditions that may lead to its removal from natural waters. This work is part of a broader study to assess the molecular-scale mechanisms of the sorption of oxyanion complexes on common minerals.  The hexavalent form of chromium is the focus of this study because it is toxic and a likely carcinogen. We use gamma alumina (γ-Al₂O₃) as a model sorbent to represent potential behavior of aluminum components in minerals.  

Chromate adsorption was characterized by a combination of electrophoretic measurements and batch experiments performed under varying conditions of pH and CrO₄^2- concentrations. The electrophoretic mobility experiments were conducted over the pH range 3-10, and showed that CrO₄^2- adds negative charge to the surface of alumina, but does not change its point of zero charge (pH 9.2). Results from batch uptake experiments performed over the same pH range provide additional insight into the uptake behavior of CrO₄^2- on alumina, and allow us, in combination with the electrophoretic mobility results, to propose that CrO₄^2- interacts with the Al₂O₃ surface primarily via electrostatic interactions, possibly involving hydrogen bonding. This implies that alumina minerals may not be an effective sink for CrO₄^2- removal from aqueous solutions.