Kinetics and mechanisms of pH-dependent Mn(II) reactions in plant-growth medium

The general objectives of this study were (i) to determine the kinetics and mechanisms of Mn(II) removal from solution by a representative plant-growth substrate; (ii) to assess the role of biotic components on Mn(II) cycling in this system; and (iii) to assess the roles of pH and temperature in determining Mn(II) partitioning between the solution and the solid phases. The Mn(II) solubility in typical growth medium (perlite) was predominantly controlled by pH-dependent reactions, i.e., adsorption and oxidation. The role of precipitation, including formation of new solid phases of Mn-P or Mn-carbonates, as possible contributing factors to Mn(II) removal from perlite suspensions was assessed and ruled out. Under acidic pH (below pH 6), Mn(II) was elevated above concentrations expected in an inert medium; it was attributed to releases from indigenous sources, on one hand, and both low adsorptive capacity and Mn-bacteria activity, on the other hand. The Mn(II) concentration declined sharply above pH 6, and at pH 7.7 dropped almost to zero, 48 h after application. On a time scale of seconds to a few hours after application to perlite media the Mn(II) solubility was controlled by adsorption reactions, mainly onto the organic constituents accumulated from root excretions or decomposition and rhizosphere biota. With advancing time, biotic oxidation became important and it became the predominant mechanism of Mn(II) removal.