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Contamination of arsenic in the form of arsenite (As 3+ ) and arsenate (As 5+ ) in potable water causes serious illness in the human body even at a very low concentration (10 µg / L). As 3+ is comparatively 60 times more toxic than As 5+ species. Hence, this work is focused on the preparation of adsorbents for efficient removal of As 3+ with higher adsorption affinity at trace level (≤50 µg/L). Here, iron-doped hydroxyapatite (Fe-HAp) synthesized by hydrothermal technique is employed as an efficient adsorbent. A very less quantity of Fe 2+ ion (0.5 wt %) is incorporated in the tetrahedral and octahedral sites of the HAp lattice along the c-axis which lead to drastic reduction in particle size (400%) and enhancing the specific surface area (105%), colloidal stability, and adsorption affinity. The maximum adsorption capacity of As 3+ is 139±2 µg/g and 183±2 µg/g for HAp and Fe-HAp, respectively. The adsorption rate of Fe-HAp is very rapid which is 538% higher compared to HAp and also the As 3+ adsorption affinity or sensitivity (0.71 µg/L) significantly improved 83-99% when compared to the adsorbents reported in the previous literature so far. The monolayer adsorption of As 3+ is purely strong chemisorption as confirmed by the Langmuir and Dubinin-Radushkevich (DKR) isotherm. The structure and morphology of HAp and Fe-HAp remain unchanged after the adsorption of As 3+ ions and also no secondary toxic products were observed. Hence, the above results reveal Fe-HAp as an efficient and low-cost adsorbent for removal of highly toxic As 3+ ions at the trace level.

Congo red (CR) is a stable anionic diazo dye that causes allergic reactions with carcinogenic properties. The rapid removal of CR using cation-incorporated nanohydroxyapatite (pristine HAp: X (X = Fe, Ni, Zn, Co, and Ag)) was investigated. The pristine and cation ion-doped HAp adsorbents were coprecipitated and subjected to hydrothermal and ultrasound treatments and subsequent microwave drying. The dopant ions significantly engineered the crystallite size, crystallinity, particle size (decreased 38–77%), shape (a rod to sphere modification by the incorporation of Ag+, Ni2+, and Co2+ ions), and colloidal stability (CS) of the adsorbent. These modifications aided in the rapid removal of the CR dye (98%) within one minute, and the CR adsorption rate was found to be significantly higher (93–99%) compared to previously reported rates. Furthermore, the kinetic, Langmuir, Freundlich, and DKR isotherms and thermodynamic results confirmed that the CR adsorption on the HAp was due to the strong chemical adsorption process. The order of the maximum CR adsorption capacity was Fe-HAp > HAp > Ag-HAp > Co-HAp > Zn-HAp. Whereas the CR regeneration efficiency was Fe-HAp (92%) > Ag-HAp (42%) > Ni-HAp (30%), with the other adsorbents exhibiting a poor recycling efficiency (1–16%). These results reveal Fe-HAp as a potential adsorbent for removing CR without the formation of byproducts.

The presence of F − ions in water is either a helpful or harmful outcome, contingent upon the concentration level of the pollutant. In this work, hydroxyapatite/chitosan (HAp/CS) nanocomposites were prepared incorporating varying weight percentages of chitosan with HAp, using co-precipitation followed by lyophilization without employing any cross-linking agents for adsorption of F − ions. Increase in the concentration of chitosan with HAp matrix signi cantly enhanced the colloidal stability, F − ions adsorption capacity and reduced the particle size (length 38% and width 86%) compared to HAp. The maximum F − adsorption capacity of HAp/CS1, HAp/CS2, and HAp/CS4 nanocomposites are 56 mg/g, 80 mg/g and 100 mg/g at pH 7, which was considerably higher (10 times) than HAp when compared to other existing reports. The adsorption isotherm and kinetics of HAp and HAp/CS nanocomposite adsorbent con rm the monolayer and strong chemisorption. After adsorption of F − ions, the surface morphology of adsorbent changed from rod to sphere along with an increase in particle size, as con rmed by HR-TEM. Moreover, FTIR and XPS analysis revealed the surface interaction between adsorbent and F − ions through ion exchange and electrostatic interaction, which lead to formation of uorapatite. HAp/CS4 showed 2.5 times higher sustainable regeneration e ciency (90% in 30 mins) up to 7th cycle than HAp. Furthermore, the solution pH (5-7) of treated water was maintained at neutral after uoride adsorption, making the water potable. Hence, these results demonstrate that chitosan plays a major role in removal of F − ion and this composite is ideal for F − adsorption application. Highlights 1. First report on the synthesis and Fadsorption by the cross-linker-free mesoporous lyophilized HAp/chitosan nanocomposites 2. The HAp/CS4 exhibited signi cantly higher Fadsorption capacity (60%-98%) than the existing reports 3. Higher and sustained regeneration e ciency (90% up to7 th cycle) 4. Enhanced colloidal stability and along with signi cant reduction in the particle size (38% length and 86% width) with an increased chitosan incorporation.

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