Adsorption of heavy metals on mixed Fe-Al oxides in the absence or presence of organic ligands

A study was carried out on the adsorption of Co2+, Cu2+, Pb2+, and Zn2+ ions on mixed Fe-Al oxides in the absence or presence of increasing concentrations of oxalate or tartrate. Mixed Fe-Al oxides were prepared by precipitating at pH 5.5 mixtures of Fe and Al ions at initial Fe/Al molar ratios (R) of 0, 1, 2, 4, 10 and infinity ( R0, R1, R2, R4, R10 and Rinfinity). The oxides aged 7 days at 20degreesC or 30 days at 50degreesC showed different chemical composition and physico-chemical and mineralogical properties. All the mixed Fe-Al oxides showed presence of poorly crystalline materials ( ferrihydrite) even after prolonged aging. The heavy metals were selectively adsorbed on the oxides. For all the precipitates aged 7 days at 20degreesC, the selectivity sequence was Pb2+ > Cu2+ > Zn2+ > Co2+, but the pH at which 50% of each cation was adsorbed (pH(50)) was different from sample to sample. It was found that usually the greater the amounts of Fe in Fe-Al gels the lower the pH50 for each metal, but the adsorption of a heavy metal was not linearly related to Fe content. The pH50 usually did not change significantly when the oxides were aged 30 days at 50degreesC. Competitive adsorption of Cu and Zn on ferrihydrite (Rinfinity) showed that Cu strongly prevented Zn adsorption even at an initial Zn/Cu molar ratio of 8, whereas Cu sorption was not inhibited. In the presence of oxalate ( OX) or tartrate (TR) ( organic ligand/Pb molar ratio (r(L)) from 0 to 7) the quantities of Pb adsorbed on the Fe-Al oxides usually increased with increasing r(L). The adsorption increase of Pb was particularly high on the oxides richer in Fe (R4-Rinfinity), but a significant increase was also observed on R0-R2 samples. The adsorption of Pb on the oxides has been influenced not only by the presence and concentration of organic ligands but also by the sequence of addition of Pb and tartrate on the sorbents. It has been ascertained that on each oxide the greater amounts of Pb were adsorbed when tartrate was added before Pb and usually according to the following sequence: Tr before Pb > Pb before Tr > Pb + Tr > Pb.