Optimization of tetravalent manganese feroxyhyte's negative charge density: A high-performing mercury adsorbent from drinking water

This study demonstrates an optimization procedure for the development of an Hg-specified adsorbent able to comply with the regulation limit for drinking water of 1 μg/L. On this purpose, the synthesis of Mn(IV)-feroxyhyte was modified to achieve high negative charge density by combining alkaline and extreme oxidizing conditions. In particular, precipitation of FeSO₄ at pH 9 and excess of KMnO₄ follows a very fast nucleation step providing a product with very small nanocrystal size (1–2 nm), high specific surface area (300 m²/g) and maximum negative charge density (1.8 mmol H⁺/g). The adsorbent was validated for Hg removal in batch experiments and column tests using natural-like water indicating an adsorption capacity as high as 2.5 μg/mg at equilibrium concentration 1 μg/L under reliable conditions of application. Importantly, the adsorption is an exothermic spontaneous process, resulting in the formation of inner sphere complexes by sharing both A-type and B-type oxygen atoms with the metal surface octahedral as revealed by the X-ray absorption fine structure results. - Highlights: • Negatively charged Mn(IV)-feroxyhyte was optimized for Hg removal from drinking water. • Adsorption capacity at the regulation limit approaches 2.5 μg/mg. • Intense oxidizing conditions during Mn(IV)-feroxyhyte synthesis promotes Hg²⁺ capture affinity. • Hg adsorption takes place through an exothermic chemisorption. • Hg²⁺ is adsorbed as a bidentate inner sphere complex.