Simultaneous recovery and retention of fluoride resources using esterified cellulose filter papers loaded with environmentally friendly calcium and magnesium extracted from seawater
Journal
Separation and Purification Technology
Journal Volume
325
Date Issued
2023
Author(s)
Abstract
Resource reclamation with low carbon emissions is highly desirable from the perspective of the circular economy. We propose a green solution in this study, which involves mining calcium and magnesium from seawater as an effective reagent for the recovery of valuable fluoride. This was achieved by esterifying the cellulose filter papers with citric acid to associate carboxyl groups onto their surfaces. Consequently, the high affinity between the carboxyl group and alkaline earth elements enabled efficient and selective loading of Ca2+ (0.193 ± 0.001 mmol-Ca2+/g) and Mg2+ (0.127 ± 0.001 mmol-Mg2+/g) onto the filter papers, reaching up to 96% and 60% of the available active sites, respectively. When applied in natural seawater, the esterified filter papers successfully captured both Ca2+ (0.110 ± 0.007 mmol-Ca2+/g) and Mg2+ (0.078 ± 0.005 mmol-Mg2+/g) simultaneously. The presence of IR singlet symmetric (νs COO, ≈ 1450 cm−1) stretching vibrations indicated that both Ca2+ and Mg2+ ions were complexed with carboxyl groups in a bridging unidentate mode. The adsorption capacity of Ca2+ and Mg2+ was likely influenced by the varying thickness of their hydration shells within the highly overlapped electric double layers presented in the micropores of the esterified filter papers. The papers loaded with Ca2+ and Mg2+ were subsequently utilized for fluoride recovery in simulated HF wastewater. The formation of CaF2/MgF2 colloids effectively recovered the stoichiometric amount of fluoride (CaF2.46 and MgF1.72), which were retained on the surface of the filter papers. Additional flocculation for colloid separation is thus avoided, resulting in a significant increase in the content of reclaimed fluoride. Increasing the density of associated carboxyl groups is crucial for effectively complexing a greater amount of seawater Ca2+/Mg2+ and, consequently, enhancing the capacity of fluoride reclamation by esterified filter papers. © 2023
Subjects
Cellulose filter paper
Circular economy
Esterification
Fluoride reclamation
Green seawater mining
SDGs
Type
journal article