Simultaneous Removal of Pb, Ni, Mn, and Cr from Phosphogypsum with Na2EDTA: A Box-Behnken Design Modelling and Optimisation

Phosphogypsum (PG) is considered a by-product and is classified as solid waste generated during the production of phosphoric acid by the reaction of sulphuric acid with phosphate rock via the wet method. The presence of heavy metals in PG causes a range of environmental issues and poses limitations on its potential applications. In this study, the Box-Behnken design (BBD) technique was employed to simultaneously reduce concentrations of lead, nickel, manganese, and chromium in PG using Na2EDTA. This approach aimed to optimise factors such as Na2EDTA concentration, solid-to-liquid ratio (S/L), and contact time, while also developing a mathematical model. The optimal process points were identified through analysis of variance (ANOVA) and the response surface plots. Furthermore, the physicochemical properties of PG before and after purification were analysed using XRF, XRD, and FTIR methods. Based on the results obtained through response surface design methodology, the coefficients of correlation (R2) of the experimental data and the second-order regression model for Pb, Ni, Mn, and Cr were found to be 89.75, 95.37, 98.18, and 94.53%, respectively. Generally, high R-squared values indicate that the experimental data are consistent with the data predicted by the model under optimum conditions: c(Na2EDTA)=0.055M, solid/liquid ratio=1:20gml−1, contact time=157min. Under these conditions, the removal efficiencies for Pb, Ni, Mn, and Cr were 46.65, 42.31, 67.02, and 77.9%, respectively.