RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

In this study, we provide a single model capable of describing the behavior of mercury(II) at two different solid/water interfaces. Mercury(II) sorption on amorphous ferric oxide (HFO) and α-SiO₂ can be simulated using the surface complexation approach. In both cases, experiments are best reproduced when ternary surface complexes between the surface (≡S-OH⁰), Hg²⁺, OH^- or Cl^- are included in the model. Sorption or ternary surface complexes predominates in the case of α-SiO₂. In addition, in the case of HFO, additional precipitation reactions of a nonideal Hg(OH)₂-Fe(OH)₃ solid solution are needed at high sorbate to sorbent ratios. The modeling leads to a new set of surface complexation constants for the interaction between Hg²⁺ and these oxide surfaces which are described by the following equilibria (298.15 K, I = 0M):

≡S–OH⁰ + Hg²⁺ ↔ ≡S–OHg⁺ + H⁺

with log K^int_SOHg+ = 6.9 ± 0.2 for HFO;

≡S–OH⁰ + Hg²⁺ + H₂O ↔ ≡S–OHgOH⁰ + 2 H⁺

with log K^int_SOHgOH = -3.2 ± 0.1 for α-SiO₂, log K^int_SOHgOH = -0.9 ± 0.2 for HFO; and

≡S–OH⁰ + Hg²⁺ + Cl⁻ ↔ ≡S–OHgCl⁰ + H⁺

with log K^int_SOHgCl = 7.0 ± 0.1 for α-SiO₂, log K^int_SOHgCl = 9.8 ± 0.4 for HFO.