Application of activated phosphate by-products for removing methylene blue and cadmium

Abstract

The use of phosphate salts as a source of phosphate ions instead of phosphoric acid for activating phosphate by-products (especially those activated with sodium dihydrogen phosphate (NaH₂PO₄)) remains largely unexplored in adsorption applications. In this study, three different materials were formulated following the geopolymerization processes using phosphate waste as a precursor with varying NaH₂PO₄ contents. X-ray diffraction (XRD) analysis identified hydroxyapatite as the dominant crystalline phase formed after activation, with the content increasing with higher phosphate salt concentrations. Meanwhile, transmission electron microscope (TEM) analysis revealed that the amorphous gel type changed with increasing NaH₂PO₄ content. Initial methylene blue (MB) adsorption tests showed high efficiencies, with the 30 % NaH₂PO₄ activated clay by-product achieving 98 % adsorption within 15 min and reaching equilibrium within 1 h. Adsorption followed the Temkin isotherm, with a complementary contribution from the Freundlich model. It also followed pseudo-second-order kinetics, with 20 mg of adsorbent achieving a maximum removal efficiency of 99 %. Neither temperature nor pH had a significant effect on MB adsorption. Cadmium (Cd) adsorption was also evaluated in an acidic medium (5 % P₂O₅), showing a lower efficiency of 29.9 % due to the high acidity. However, when the pH of the solution was increased to 6, the adsorption efficiency improved to 77.3 %. These findings imply that activated clay by-products are highly effective for dye removal, such as MB, and can achieve considerable heavy metal removal under optimized conditions.

Keywords: Adsorption; Hydroxyapatite; Mining wastes; Phosphate salts; Wastewater.