Ca treated Palygorskite and Halloysite clay minerals for Ferrous Iron (Fe+2) removal from water systems

Abstract Palygorskite fibers and halloysite nanotubes (HNT) were used for the Fe (II) uptake from aqueous solutions under various experimental conditions. Palygorskite and halloysite samples were characterized using XRD, FTIR, SEM, BET and CEC and then were saturated by exchanging Ca + 2 cations. The Ca-Palygorskite (Ca-Pal) and Ca-Halloysite (Ca-Hall) were characterized as well, and applied in batch kinetic experiments series. It was shown that both Ca-treated clay minerals were more efficient adsorbents for the lowest ferrous concentrations removal (5 mg/L), especially when the highest solid: liquid ratio (20 g/L) have been applied, reaching 99.8% and 91.2% removal with Ca-Pal and Ca-Hall respectively, within 10 min at the optimal room temperature (20 ± 1 °C). The pH value affected the adsorption’s efficiency, as Ca-Pal was more efficient adsorbent at acidic values (4–6), while Ca-Hall efficiency is positively correlated with pH increase (7 ). Moreover, the competitive ions found to prohibit Ca-Hall capacity for Fe (II), following the order Mn K Cu Zn Ba Mg Na, but Ca-Pal is slightly affected by K Na=Mn Zn Cu Mg Ba, highlighting its preference to a Fe cations. According to the thermodynamic analysis the Fe (II) uptake is a physical and spontaneous process, while both Ca-Pal and Ca-Hall fit in Langmuir isotherm instead of Freundlich, indicating monolayer sorption. The exact mechanism of Ca-Pal efficient adsorption was verified and further explained with molecular simulation optimization by Material Studio 4.3.