Pb-zeolite alginate granules for arsenate removal from water: static and dynamic studies

Arsenic is a toxic and widespread contaminant of waters and soils, originating from both natural geochemical processes and anthropogenic activities e.g. mining, metallurgy, and agriculture. Its removal remains as a global environmental challenge. Arsenic immobilization is commonly achieved through sorption, ion exchange, or precipitation, often involving iron-, aluminum-, or calcium-based phases. However, many of these methods suffer from limited capacity, sensitivity to water chemistry, or poor long-term stability.

In this study we present an approach for arsenate AsO₄^3- removal from water based on reactive sorption on granulated Pb-zeolite. Arsenate is precipitated in the form of low-solubility solid phase - mimetite (Pb₅(AsO₄)₃Cl) - by reaction with Pb²⁺ desorbed from granulated Pb-zeolite (clinoptilolite). Pb-modified granulated zeolite was prepared by (1) sorption of Pb²⁺ from solution, (2) intensive washing to remove excess Pb and (3) granulation. For granulation, suspension of zeolite in sodium alginate solution was dropwise added to calcium chloride solution, resulting in formation of beads 2 - 4 mm in size. Dried beads provide with mechanically stable, porous and easy to handle sorbent.

Arsenate removal was investigated under static and dynamic flow-through conditions. In static batch experiments using [As] = 0.1 to 5 mg/L, arsenate removal exceeded 90% within minutes and approached 99% with increasing contact time, indicating that under well-mixed conditions the process is not limited by external mass transfer.

Column experiments were performed at varying flow rates, arsenate concentrations, sorbent masses, and column diameters in order to evaluate the effect on removal efficiency. Increased flow rate resulted in decreased arsenate removal due to shortened residence time, whereas higher sorbent mass enhanced removal by increasing the reactive contact area and the effective diffusion path length. Arsenic concentrations in the effluent increase gradually with time and the breakthrough curves follow a logarithmic rather than sinusoidal trend. This indicates sustained arsenate removal over extended periods (up to 6 days of continuous operation at the conditions of the experiment).

SEM/EDS analyses of reacted granules revealed the formation of a porous crust composed of mimetite needles on the granule surface. Mimetite precipitation does not passivate the reactive interface. The reaction front was located on the surface of granules indicating that precipitation kinetics was faster than lead desorption. Also, the porous structure of the alginate granules allowed for diffusion of Pb²⁺ from the interior outward.  Comparison of arsenic removal and lead release rates indicates a gradual, approximately linear depletion of lead with time. Precipitation of Pb₅(AsO₄)₃Cl on the surface of granules created a Pb concentration gradient which was a driving force for further Pb desorption from zeolite in the interior of the granules: lead was consumed progressively from the outer regions of the granule inward. This mechanism likely governs the extended tailing observed in the breakthrough curves.

Proposed application of Pb-zeolite alginate granules enables efficient arsenate removal through induced mimetite precipitation under both static and flow-through conditions. The combination of high removal efficiency, long-term reactivity, and physical immobilization of lead highlights the potential of this approach for water treatment applications.

This research was funded by National Science Centre project No 2024/53/N/ST10/01763.