Effect of salicylic acid upon trace-metal sorption (CdII, ZnII, CoII, and MnII) onto alumina, silica, and kaolinite as a function of pH

The sorption of four trace metals (Cd-II, Zn-II, Co-II, and Mn-II) onto alumina, silica, and kaolinite, in the presence or absence of salicylic acid (as well as the salicylic acid sorption), was investigated in batch experiments in the pH range from 4 to 9. The sorption was interpreted in terms of surface complexation using the diffuse layer model (DLM). Equilibrium parameters were optimized using the FITEQL program. The salicylic acid was only significantly sorbed onto the alumina and the sorption was modeled using the anionic monodentate surface complex. In the absence of salicylic acid, the sorption of the trace metals presented different pH edge behaviors, depending on the substrate. Using the cationic monodentate surface complex, the model fitted the experimental data well. In the presence of salicylic acid, at a given pH and depending on the substrate, the sorption of metals was (i) increased, suggesting the occurrence of ternary complexes; (ii) reduced (sometimes totally inhibited), due to the complexation with dissolved salicylic acid; or (iii) very weakly changed, in terms of net effect compared to free-organic-ligand systems. Modeling of the trace-metal sorption in the presence of salicylic acid was performed using ternary surface complexes. In the acidic pH range, this allowed the experimental data to be simulated, but in the alkaline pH range, the model failed to simulate the decrease in sorption. Probable causes of the discrepancies between the experimental data and modeling results are discussed.