Arsenate immobilisation and mimetite formation on Pb-modified zeolite under variable aqueous chemistry

Arsenic contamination in water remains a global challenge, and understanding the mineralogical controls on arsenate sequestration is essential for predicting its behaviour in natural and engineered aqueous systems. This study investigates arsenate immobilisation on Pb-modified clinoptilolite through the precipitation of mimetite (Pb₅(AsO₄)₃Cl), a sparingly soluble apatite-group mineral. The Pb-modified zeolite (Na-exchanged clinoptilolite containing ~ 70 g Pb/kg), was reacted with As(V) solutions (0.1-50 mg/L) containing 20 mg/L Cl^- across pH 2-8 for up to 168 h. Arsenate concentrations decreased rapidly, with measurable uptake within 1 min and up to 99% immobilisation achieved within 1 h at pH 6-8. Lower immobilisation at pH 2-4 reflects reduced mimetite stability and pH-dependent arsenate speciation. Mimetite precipitated directly at the solid-solution interface through the reaction of surface-released Pb²⁺ with arsenate and chloride, indicating an interface-controlled process driven by local supersaturation. The mechanism was largely unaffected by competing cations (K⁺, Mg²⁺, Ca²⁺, Al³⁺, Mn³⁺, Cu²⁺, Ni²⁺, Zn²⁺, Cd²⁺) and anions (F^-, SO₄^2-, PO₄^3-). Arsenate immobilisation in natural waters from Bad Wildbad (Germany), Złoty Stok (Poland), and the Upper Mur Valley (Austria) varied substantially with solution composition, ranging from 51 to 99%. These results demonstrate that arsenate sequestration in this system is governed by coupled surface-mediated Pb release and secondary mineral precipitation, providing new insight into the controls on arsenic mobility in Pb-bearing environments.