Abstract
Contamination of arsenic in the form of arsenite (As3+) and arsenate (As5+) in potable water causes serious illness in the human body even at a very low concentration (10 µg/L). As3+ is comparatively 60 times more toxic than As5+ species. This work is focused on the preparation of adsorbents for efficient removal of As3+ with higher adsorption affinity at trace level (≤ 50 µg/L). The iron-doped hydroxyapatite (Fe-HAp) synthesized by hydrothermal technique is employed as an efficient adsorbent. A significantly less quantity of Fe2+ ion (0.5 wt%) is incorporated in the tetrahedral and octahedral sites of the HAp lattice along the c-axis, which leads to a drastic reduction in particle size (400% in length and 225% in width) and enhances the specific surface area (105%), colloidal stability, and adsorption affinity. The maximum adsorption capacity of As3+ 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 than HAp. Also, the As3+ adsorption affinity or sensitivity (0.71 µg/L) significantly improved 83–99% when compared to the adsorbents reported in the previous literature. The adsorption capacity of As3+ ion was unaffected by other competing ions and obtained 97% recycling efficiency up to 7 cycles using Fe-HAp. The monolayer adsorption of As3+ 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 As3+ ions and also no secondary toxic products were observed. Hence, the above results reveal Fe-HAp as an efficient and low-cost adsorbent for the removal of highly toxic As3+ ions at the trace level.Graphic Abstract
Highlights
Arsenic (As) is a familiar toxic metal ion that can be found in groundwater as a contaminant
A very less quantity of Fe2+ 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 adsorption rate of Fe-HAp is very rapid which is 538% higher compared to HAp and the As3+ adsorption affinity or sensitivity (0.71 μg/L) significantly improved 83-99% when compared to the adsorbents reported in the previous literature so far
Summary
Arsenic (As) is a familiar toxic metal ion that can be found in groundwater as a contaminant. The World Health Organization (WHO) recommended that the limit of arsenic in drinking water is ≤10 μg/L. More than 60 million peoples are said to be affected by arsenic contamination, which is exceeding WHO limits [1,2,3,4,5]. As3+ is found in neutral species such as As(OH), H3AsO3, As(OH)4−, AsO2OH2−, and AsO33− , whereas As5+ is found in anionic forms such as AsO43-or H2AsO− or HAsO42− [12,13]. The prevalence of As3+ contaminated groundwater is approximately 67-99% higher than As5+ ions and the toxicity rate of As3+is 60 times greater than the As5+ [14]. The anionic compound has improved attractive force between the adsorbent and adsorbate ions [7]
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