Influence of iron/aluminium ratio on the retention of lead and copper by amorphous iron–aluminium oxides

Abstract

Results are presented from a study on the influence of pH on the retention of Pb and Cu by amorphous iron, aluminium and mixed iron–aluminium oxides, plus kaolinite clay. The composition (Fe/Al molar ratio) of the oxides significantly affects their surface properties. Increasing the Fe/Al ratio generally enhances the surface area although the maximum surface area is found at Fe/Al=1. The cation exchange capacity (CEC) was dependent upon pH and Fe/Al ratio, decreasing in order Fe>3FeAl>Al>Fe3Al>FeAl≫kaolinite at pH 6.3–7. The oxide points of zero charge could be divided into two groups: Fe/Al≤1 (Al, Fe3Al, FeAl; pH ZPC=6.2–6.4) and Fe/Al>1 (3FeAl, Fe; pH ZPC=5.3–5.8). Batch equilibrium suspension tests were conducted with Pb and Cu (single metal solutions) to assess contaminant retention capability of each oxide and kaolinite. Pb retention by mixed oxides at a particular pH increases with Fe/Al ratio in the order Fe>3FeAl>FeAl>Fe3Al>Al. The same order is observed at low to medium Cu concentrations (0.5, 2.5, 5 mmol/l) but at higher metal loading (25 mmol/l), only Fe oxide has a clearly greater retention capacity. Oxides with Fe/Al<1 (Al, Fe3Al) retain more Cu than Pb in single contaminant tests at equal metal concentration. Bulk precipitation tests (no oxides or kaolinite) combined with surface charge measurements suggest that adsorption is the dominant retention mechanism, particularly at less than pH 6. Amorphous oxides demonstrate considerably greater heavy metal accumulation capacity, specific surface area, and surface charge than kaolinite.