The effect of temperature on tight metal binding by peat and soil derived solid humic acids

The brown biomaterials called humic acids (HA) in pears and soils retain water and bind metal cations and other solutes. Studies of the interactions of purified solid peat and soil-derived HA from different countries with metal cations in water probe HA microstructures and help to characterize the metal binding sites. Labile cations such as Ca-aq(2+), Co-aq(2+), Cu-aq(2+), Fe-aq(3+), Mg-aq(2+) and Mn-aq(2+) tightly bind to solid HAs in sequential steps. The isotherms A vs. c are well fitted with the Langmuir model and plots of 1/A vs. 1/c are linear for each step. Here, A is mmol bound metal g(-1) HA and c is the equilibrium cation concentration (M). This paper compares the stoichiometric site capacities v(i) and equilibrium constants K-i for tight binding of Ca-aq(2+), Co-aq(2+), Cu-aq(2+), Fe-aq(3+), Mg-aq(2+) and Mn-aq(2+) at different solid HA sites. Measurements Measurements at different temperatures give linearly correlated metal binding enthalpy and entropy changes, indicating that conformational changes and cation/HA hydration/dehydration are important factors in metal binding and release by solid HAs.