Abstracts Engineering

A study of the retention of heavy metals by amorphous iron-aluminium oxides and kaolinite

by Hugh Potter

Amorphous iron-aluminium oxides are ubiquitous in the environment and control the availability of heavy metals (Pb, Cu). Oxide physicochemical properties are affected by the proportion of Fe and Al, the Fe/Al ratio. Five amorphous oxides containing Fe and/or Al were synthesised: "Fe" (only Fe); "3FeAl" (3 moles Fe per mole Al, Fe/Al = 3); "FeAl" (Fe/Al = 1); "Fe3Al" (Fe/Al = 0.3); "Al" (only Al). Kaolinite clay was also investigated along with kaolinite to which Fe-Al oxides had been added. The effect of Fe/Al ratio on these physicochemical properties was studied: (1) Specific surface area – EGME retention; (2) pH-dependent cation and anion exchange capacity – Ca(NO3)2 saturation; (3) Particle size density – sieving, pipette methods; (4) Surface charge density – potentiometric titrations. The Fe/Al ratio influenced the specific surface area: Fe and 3FeAl had significantly greater surface areas than Fe3Al and Al; FeAl exhibited the maximum surface area. The cation exchange capacity increased rapidly above pH 5, with Fe > 3FeAl > Al > Fe3Al > FeAl at pH 6 to 7. Surface charge density of low Fe/Al oxides was more sensitive to pH changes implying ≡AlOH surface hydroxyl groups are more easily protonated and de-protonated than ≡FeOH species. An open (porous) structure of loosely linked small particles was proposed with SO42- anions forming bridges between Al3+ and Fe3+ ions during oxide precipitation while inhibiting extensive crystal growth. Pb and Cu retention was measured using 24 hour batch suspension tests with varying pH and contaminant concentrations (0.5 to 25 mmol/L). Higher Fe/Al ratio oxides retained more Cu and Pb at all pH values. Cu retention (0.5 to 5 mmol/L) and Pb retention (all concentrations) followed Fe > 3FeA > FeAl > Fe3Al > Al but at 25 mmol Cu/L, only Fe accumulated more Cu. However, low Fe/Al ratio oxides preferentially sorbed Cu over Pb from multi-contaminant solutions. Adsorption rather than bulk precipitation was the dominant accumulation mechanism. Geochemical modelling suggested surface precipitation occurred at higher contaminant concentrations. Adding oxides to kaolinite significantly increased the clay's specific surface area, surface charge density and contaminant retention capacity; higher Fe/Al ratio oxide-amended clays retained more Pb and Cu. Amorphous Fe-Al oxides substantially enhance heavy metal accumulation by soils with higher Fe/Al ratio oxides being particularly effective.