Figure 2: Contour plots showing the interaction effect between pair of variables (a) KH2PO4 and (NH4)2SO4, (b) KH2PO4 and MnSO4, (c) KH2PO4 and phenol, (d) (NH4)2SO4 and MnSO4, (e) (NH4)2SO4 and phenol, (f) MnSO4 and phenol on phenol degradation (%) by keeping the other two variables constant at middle level.