Sorption of mercury(II) by manganese(IV) oxide

Abstract

Manganese(IV) oxide avidly adsorbed mercury(II) species, with freshly prepared material (HMO; capacity, 280 mmol kg⁻¹ abstracting most of the Hg present in 1 to 12 mg litre⁻¹ Hg(II) solutions. α cryptomelane also adsorbed in accordance with a Langmuir isotherm (capacity, 250 mmol kg⁻¹) but the binding constant (4400) was only 1% of the HMO value. A λ MnO₂ sample had an adsorption capacity of 15 mmol kg⁻¹. Uptake was not influenced by pH over the range 5 to 11, but decreased in the 5 to 3 region due to protonation of sites. The amount adsorbed was reduced in the presence of chloride ions, with the extent, and pH of minimum uptake, being a function of Cl⁻ level. As HgCl₂ and Hg(OH)₂ are the dominant solution species, and as protons were not released in the adsorption process, it is proposed that the adsorption mechanism involves migration of the Hg(II) species into the internal structure of the Mn(IV) oxide, where they either undergo ligand exchange with surface hydroxyl groups or are transformed into a solid hydroxy phase. The presence of sulphate ions further lowered Hg uptake. Extraction studies showed that adsorbed Hg could be totally released by EDTA, DTPA or 0·5m HCl. Decreasing quantities were displacedby 1m HNO₃, Na₄P₂O₇, Na₃cit. Equilibration with 0·5m NaCl extracted about half, while 1m NH₄NO₃ or CH₃COONH₄ recovered < 5%. It was concluded that the formation of manganese(IV) oxides in waterways would aid the maintenance of low solution-Hg levels.