Proton binding to humic acids and sorption of Pb(II) and humic acid to the corundum surface

An experimental investigation of proton binding to a humic acid, and of the co-adsorption of humic acid and Pb(II) to the corundum surface was conducted in 0.01 M NaNO3. We attempt to model the acid–base properties of the humic acid using both discrete and continuous distributions of proton affinity constants, also testing both nonelectrostatic and electrostatic approaches. The best-fitting models indicate that an average of three dominant proton-active functional groups are present on the humic acid with proton affinity constant logβ1,1,0 (int) equal to 2.6, 5.2 and 7.6. These values may be applied as intrinsic constants in a Constant Capacitance Model (CCM) or as means of distribution functions in a Langmuir–Freundlich isotherm where the humate is treated as a mixture of three monoprotic acids. The adsorption of humic acid onto corundum is best described using a Surface Complexation Model (SCM) with the Extended Constant Capacitance Model. The humate surface complexes, which best describe the experimental data, are the outer-sphere (>AlOH2+)0.1Hf1∑L(0.1–3.0+f1) and (>AlOH2+)0.1(H(f1+f2)∑L)(0.1–3.0+f1+f2) complexes, where f1 and f2 are the mole fractions of the sites with logβ1,1,0 (int)=7.6 and logβ1,1,0 (int)=5.2, respectively. Experimental data also show that the presence of calcium increases the adsorption of humate. The adsorption of Pb in the presence of humate was interpreted to proceed by the formation of the surface ternary complex >AlOH2+–L−z–Pb+2 at low to circumneutral pH. At high pH, aqueous Pb–humate complexation competes with Pb surface complexes and significantly reduces Pb adsorption.