Soil Humic Acid and Arsenite Binding by Isothermal Titration Calorimetry and Dynamic Light Scattering: Thermodynamics and Aggregation

The arsenite-humic acid binding process was investigated using Isothermal Titration Calorimetry (ITC), Dynamic Light Scattering and Laser Doppler Electrophoresis techniques. The ITC data were successfully (R2 = 0.996–0.936) interpreted by applying the MNIS model, enabling thermodynamic parameters to be determined. The MNIS model was adjusted to the arsenite-HA binding process assuming that hydrogen bonding is the dominant type of interaction in the system. Negative enthalpy change values indicated the arsenite-HAs binding as an exothermic process. Negative ΔG values (-(26.83–27.00) kJ mol−1) pointed out to spontaneous binding reaction, leading to the formation of the arsenite-HA complexes. The binding constant values ((7.57–5.02) 105 M−1) clearly demonstrate pronounced binding affinity. As ΔS values are obviously positive but close to zero, and ΔH>ΔS, the reaction can be considered enthalpy driven. Reaction heats and ΔH values (-(18.96–15.64) kJ mol−1) confirmed hydrogen bonds as the most ascendant interaction type in the arsenite-HA complex. Negative zeta potential values (−45 to −20 mV) had shown that arsenite-HA aggregates remained negatively charged in the whole molar charge ratio range. The HAs' aggregate size change is evident but not particularly pronounced (Zav = 50–180 nm). It can be speculated that aggregation during the titration process is not expressive due to repulsive forces between negatively charged arsenite-HA particles. Thermodynamic and reaction parameters clearly indicated that arsenite-HA complexes are formed at common soil pH values, confirming the possible influence of humic acids on increased As mobility and its reduced bioavailability.