Discussion on the thermodynamic calculation by distribution constant in Journal of Contaminant Hydrology 243 (2021) 103906

Discussion on the thermodynamic calculation and adsorption spontaneity re Ofudje et al. (2023)

Accurate calculations and precise results are very important for the dissemination of scientific knowledge, whereas the errors of calculation will diminish the academic value of the paper. This discussion focuses on the calculation of thermodynamics and the determination of the spontaneity of adsorption processes in the paper of Ofudje et al. (2023). Ofudje et al. found that the apatite synthesized by chemical method (CHAp) has excellent adsorption properties for cadmium ions, which is an important contribution to the remediation of cadmium pollution. However, the calculation results of standard Gibbs free energy change (ΔGo), standard enthalpy change (ΔHo) and standard entropy change (ΔSo) of the adsorption of Cd2+ onto CHAp surface need to be corrected due to an incorrect calculation. Firstly, the partition coefficient (KD) with a dimension cannot be used for thermodynamic calculation. Secondly, the adsorbent mass (m) described by Ofudje et al. in different Sections is inconsistent, leading to incorrect results of Ko and ΔGo. When the appropriate value of the adsorbent mass is selected and the partition coefficient is converted to the standard adsorption equilibrium constant Ko, the calculated ΔGo is less than zero, which means that the adsorption is spontaneous. This discussion provides the correct calculation method of standard adsorption equilibrium constants and thermodynamic parameters, which can improve the reader's judgment and understanding of adsorption spontaneity.