Enhanced fixed-bed column adsorption using a ternary MgO-impregnated eggshell–kaolin composite: Toward an efficient and sustainable fluoride removal technology

Abstract

In this study, a ternary MgO-impregnated eggshell–kaolin (EKM) composite was synthesized and evaluated for fluoride removal from water under fixed-bed column adsorption. The composite, prepared by co-precipitation and wet impregnation, was characterized using XRD, FTIR, SEM–EDX, and Brunauer–Emmet–Teller analyses. Results confirmed successful integration of the three components, yielding a mesoporous structure with a surface area of 158.5 m 2 g −1 . The composite exhibited good defluoridation performance under varying operational conditions. Higher bed depth and lower flow rate enhanced adsorption capacity, achieving up to 6.35 mg g −1 at 10 mg L −1 influent fluoride concentration. Breakthrough data were well described by the Thomas, Clark, and Yoon–Nelson models ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msup> <mml:mi>R</mml:mi> <mml:mn>2</mml:mn> </mml:msup> <mml:mspace width="0.25em"/> <mml:mo>≥</mml:mo> <mml:mn>0.94</mml:mn> </mml:math> ) at lower influent concentration, moderate flow rate, and greater bed depth, while the bed depth service time model confirmed a linear increase in service time with bed depth. The adsorbent maintained good regeneration ability across four cycles. Co-existing anions showed interference with the uptake of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">F</mml:mi> </mml:mrow> <mml:mo>−</mml:mo> </mml:msup> </mml:math> , with inhibitory effects following the order: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">PO</mml:mi> </mml:mrow> <mml:mn>4</mml:mn> <mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mo>&gt;</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">SO</mml:mi> </mml:mrow> <mml:mn>4</mml:mn> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>−</mml:mo> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mo>&gt;</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">NO</mml:mi> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> <mml:mo>&gt;</mml:mo> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">Cl</mml:mi> </mml:mrow> <mml:mo>−</mml:mo> </mml:msup> </mml:math> . The study estimated that 11.11 g of EKM composite yields 2 L of safe water for less than USD1. These results highlight the EKM composite as an efficient and sustainable adsorbent for practical defluoridation applications.