Synthesis of [H₂₂·Zr₅·WO₄·10 P₂O₇]_n·26n H₂O by response surface methodology to adsorb Ca(II) in manganiferous wastewater

The presence of calcium challenges the manganese recovery from manganiferous wastewater. In this paper, a kind of mesoporous material named [H₂₂·Zr₅·WO₄·10 P₂O₇]n·6n H₂O is investigated as an ion exchanger to remove calcium ion from manganese slag percolate. The synthesis of zirconium tungstopyrophosphate (ZWPP) was optimized by response surface methodology , and its adsorption capacity and equilibrium were tested. The adsorption data have been confirmed by the use of various techniques such as Fourier transform infrared spectroscopy, scanning electron microscopy and Brunauer, Emmett and Teller. An empirical formula of ZWPP was obtained by X-ray diffraction and thermal analysis. The adsorption process conformed to pseudo-second-order kinetics and the Langmuir isotherm, which described the equilibrium powerfully. Furthermore, different thermodynamic parameters were evaluated. And it was found that Gibbs free energy change is negative, indicating the adsorption process was spontaneous, whereas the enthalpy change and entropy change are positive indicating endothermy and increased randomness nature of the adsorption process. As a result, ZWPP could be a possible ion exchanger material in the area of removing Ca²⁺ from processing water or wastewater.