pH-dependent kinetics of tuff dissolution

Despite the widespread use of commercial tuffs as growth substrates, their dissolution kinetics and the time variation of their solution compositions have not hitherto been investigated. To fill this gap in our knowledge, rates of ion release by two tuffs were measured as a function of pH in batch and in steady-solution-flow suspensions. Reaction kinetics was characterized by rapid H+/OH- consumption, and Si, Al, Ca, Mg, and P release during the first day. This was followed by a period in which H+ consumption and ion release were linear functions of time for at least 720 h. On the basis of reaction stoichiometry and equilibrium calculations, rapid short-term H+ consumption of the studied tuffs was related to cation exchange and rapid dissolution of fine particles of volcanic glass and hydroxyapatite (HA). The rate laws for long-term acid consumption were fractional-order with respect to H+ concentration in the pH ranges 3.4-6.5 and 3.6-9.6, black and yellow tuffs, respectively. On the basis of proton-consumption rate data and ionic solution composition, long-term acid consumption was attributed to the dissolution of halloysite-like allophane in yellow tuff, volcanic glass in black tuff, and HA in both tuffs. Aluminum dissolution from black tuff under batch conditions at pH below 4 may create a toxic environment for plant roots. (C) 1999 Elsevier Science B.V. All rights reserved.