Lead-modified zeolite for arsenate removal via mimetite (Pb5(AsO4)3Cl) precipitation

The toxicity of arsenic and its detrimental impact on human health have driven extensive research into effective removal methods (Smedley and Kinniburgh, 2002). Arsenate AsO₄^3- can be efficiently sequestered from aqueous solutions by inducing the precipitation of mimetite (Pb₅(AsO₄)₃Cl), a stable and sparingly soluble phase with an apatite-like structure (Magalhães & Silva, 2003; Bajda et al., 2007; Bajda, 2010). This study introduces a novel application of lead-modified zeolite for arsenate removal, leveraging mimetite precipitation for efficient and environmentally sustainable water treatment. Pb bound to the zeolite surface exhibits optimal binding strength: strong enough to prevent the release of lead into water, yet weak enough to facilitate its reaction with arsenate and chloride ions present in solution.

This reaction led to the precipitation of mimetite directly on the surface of zeolite aggregates, ensuring efficient arsenic removal:

 

5Pb²⁺_{desorbed from zeolite} + 3AsO₄^3-_{contamination} + Cl^-_{supplied extra} = Pb₅(AsO₄)₃Cl _{precipitated on zeolite}

 

The source of lead was a lead-modified natural clinoptilolite (ZEOCEM, Bystré, Slovak Republic), originally in its Ca form but transformed to Na-clinoptilolite after a 24-hour reaction in 2 M NaCl solution. This sodium form of clinoptilolite was then prepared by sorption of Pb²⁺ from solution, followed by intensive washing to remove excess Pb and desorption of loosely bound ions. 800 mg of Na-clinoptilolite was reacted with 40 mL of solution containing 4000 mg Pb/L at pH 4 for 24 hours, followed by washing with redistilled water and centrifugation (seven times) until Pb was below the detection limit of the Atomic Absorption Spectroscopy (AAS) method. The Pb-modified zeolite thus obtained, containing about 70 g of bound Pb per kg of zeolite, was subsequently reacted with arsenate solutions (500 mg of Pb-zeolite in 40 mL of solution containing 50 mg As(V)/L) in the presence of chloride ions (20 mg Cl/L) at pH 2 and 7 for up to 7 days.

At pH 7, arsenate levels decreased below the detection limit within 24 hours, while at pH 2, 95% of arsenate was removed. X–Ray Powder Diffraction (XRPD) and Scanning Electron Microscopy with Energy Dispersive Spectrometry (SEM-EDS) analyses confirmed the formation of mimetite, which precipitated on the zeolite aggregates, forming incrustations of distinct needle-like crystals up to 2 μm in length. Induced precipitation of mimetite produces a highly stable crystalline product, suggesting its potential for effective arsenate sequestration. This approach, with further testing in complex systems, holds significant promise for scalable and cost-effective arsenate remediation in contaminated waters.

References:

Bajda, T., Szmit, E., & Manecki, M. "Removal of As (V) from solutions by precipitation of mimetite Pb₅(AsO₄)₃Cl." Environmental Engineering, (2007): 119-124.

Bajda, T. “Solubility of mimetite Pb₅(AsO₄)₃Cl at 5–55 C.” Environmental Chemistry 7(3) (2010): 268-278

Magalhães, M. C. F., and Silva, M. C. M. „Stability of lead (II) arsenates.” Monatshefte fuer Chemie/Chemical Monthly, 134, (2003):735-743.

Smedley, P. L., and Kinniburgh, D.G. "A review of the source, behaviour and distribution of arsenic in natural waters." Applied geochemistry 17.5 (2002): 517-568.