Fig. 3. (a) Photographs of a solution of PS beads (90 μm) and PS beads after adding Fe³⁺ to the surface phenolic beads. Optical microscopic images of (b) the untreated PS beads and (c) the PS beads after adding Fe³⁺, at 200× magnification. (d) Optical microscopic images of PS beads after adding Fe³⁺ on the surface treatment of various concentrations of tannic acid (TA), at 40× magnification.