2021 Volume 25 Issue 3-4 Pages 39-46
The adsorption behavior of sulfate (SO42−) on pseudoboehmite with or without surface complexation of citrate, oxalate, and acetate was investigated to elucidate the effect of low-molecular-weight organic acids on their sulfate adsorption. The adsorption experiments were conducted using reaction systems containing 0.01 g of pseudoboehmite with or without organic complexation and 10.0 mL of a mixed solution consisting of 10, 100, or 1000 µmol/L sulfate and background electrolyte of 1.0 mmol/L NaCl at pH conditions ranging from 3 to 10. An attenuated total reflectance-Fourier transform infrared analysis (ATR-FTIR) was conducted to confirm the surface organic complexes, and geochemical calculations were performed to examine the reaction process in solution. The results indicated that sulfate adsorption on the pseudoboehmite with no organic complex (PBM-NO) increased progressively as solution pH decreased from 9 to 6, whereas the adsorbed amount decreased successively at pH <5 because of the formation of free Al-sulfate complex ions mainly as AlSO4+ in this acidic region. The pseudoboehmite complexed with citrate (PBM-Cit) and oxalate (PBM-Oxa) exhibited lower sulfate adsorption than the PBM-NO due to the site blocking by the organic acids. However, the sulfate adsorption increased successively until the solution pH decreased to about 3 with no decrease in sulfate adsorption at pH <5 as observed in the PBM-NO. The ATR-FTIR and geochemical calculations indicated that the no decrease in sulfate adsorption was attributed to inhibition of the Al-sulfate complex formation due to the preferential complexation of dissolved free Al3+ with citrate or oxalate molecules released from the surface of the PBM-Cit and PBM-Oxa. The pseudoboehmite complexed with acetate exhibited a sulfate adsorption behavior similar to that of the PBM-NO due to the low complexation ability of acetate with the pseudoboehmite surface.
