Novel design of Fe⁰-and AC-doped hydroxyapatite-alginate composites for efficient arsenic clean-up: Mechanistic...

Despite extensive research, the development of novel, highly effective technologies to remove water contaminants remains a critical need. In the present study, novel and efficient composite beads of Hydroxyapatite-Sodium Alginate (HA/SA), Activated Carbon/HA/SA (AC/HA/SA), Zero-valent Iron-doped/HA/SA (Fe⁰/HA/SA), and Fe⁰/AC/HA/SA were synthesized for As removal from water. The as-synthesized composite beads were characterized comprehensively using advanced analytical techniques (FTIR, XRD, EDX, SEM, and TGA). Batch-column-scale experiments revealed superior As adsorption performance by Fe⁰/AC/HA/SA (92.7%) at pH 2.5 compared to HA/SA (68%), AC/HA/SA (73.7%), and Fe⁰/HA/SA (84.7%). The as-synthesized beads effectively removed 91.2% As at low concentrations (50 μg L^-1) under acidic conditions (pH 2.5). Complete As removal (100%) was achieved within 15 min using a 2 g adsorbent dose in column experiments. The adsorption data were best described by the Freundlich isotherm (R² > 0.99) and pseudo-second-order kinetics (R² > 0.99). Multivariate analysis further confirmed that Fe⁰/AC/HA/SA outperformed the other composites. The Fe⁰/AC/HA/SA composite showed the highest reusability compared with other composite beads. The coexisting ions revealed a minimum decline in As removal (2%) with Fe⁰/AC/HA/SA. The study highlights the exceptional adsorption capacity and reusability potential of Fe⁰/AC/HA/SA beads for practical As removal, even under complex adsorption mechanisms.