Simultaneous Removal of Pb, Ni, Mn and Cr Heavy Metals with NA2EDTA: Modelling and Optimisation Using Box-Behnken Design

Phosphogypsum (PG) is considered a by-product, characterized as solid waste that occurs for manufacturing phosphoric acid by reacting of the sulfuric acid with the phosphate rock according to the wet method. Heavy metals in PG cause a series of drawbacks and negative impacts for global environment and create various restrictions in PG applications. In this article, the Box-Behnken Design (BBD) technique was chosen for the simultaneous decrease of lead, nickel, manganese and chromium heavy metals from PG with Na ₂ EDTA which lead to optimizing factors such as Na ₂ EDTA concentration, liquid to solid (L/S) ratio and contact time and creating a mathematical model. The optimal process points were identified using the analysis of variance (ANOVA) and the surface response plots. Moreover, the physicochemical properties, before and after purification of PG were studied by XRF, XRD and FTIR methods. Regarding the results found by applying the response surface design methodology, the coefficients of correlation (R ² ) of the experimental data and the second-order regression model for Pb, Ni, Mn and Cr were determined as 89.75%, 95.37%, 98.18% and 94.53%, respectively. Generally, high R-squared values ​​state that the experimental data are compatible that the data predicted by the model Under optimum conditions, Na ₂ EDTA concentration: 0.055 M, liquid/solid ratio: 20:1 ml/g, contact time: 157 minutes and the removal of lead, nickel, manganese and chromium were 46.65%, 42.31%, 67.02% and 77.9% respectively.