Arsenite–soil humic acid binding by isothermal titration calorimetry: Thermodynamics and MNIS model

Arsenite–humic acid-binding process was investigated using the Isothermal Titration Calorimetry(ITC). 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 hydrogen bonding as the dominant type of interaction in the system.Negative enthalpy change values indicated the arsenite–HAs binding as an exothermic process.Negative ΔG values (-(27.85-26.83) kJ mol-1) pointed out to spontaneous binding reaction, leading tothe formation of the arsenite–HA complexes. High binding constants values ((7.57-5.02)105 M-1)clearly demonstrated pronounced binding affinity. As ΔS values (0.041-0.027) kJ mol-1 K-1) wereapparently positive, but close to zero, and ΔH>ΔS, the reaction could be considered enthalpy driven.Reaction heats and H values (-(18.96-15.64) kJ mol-1) confirmed hydrogen bonds as the mostascendant interaction type in the arsenite–HA complex. Thermodynamic and reaction parametersclearly indicated that arsenite–HA complexes are formed at common soil pH values, confirming thepossible influence of humic acids on increased As mobility, and its reduced bioavailability.