Amorphous silica precipitation (60 to 120°C):: Comparison of laboratory and field rates

Amorphous silica precipitation behavior was investigated in simple laboratory experiments and more complex field experiments in the Wairakei, New Zealand, geothermal area. Both the laboratory and field precipitation rates an dependent on reaction affinity for (AB1) SiO2(Am.Si.) +/- 2H(2)O double left right arrow H4SiO4 In simple laboratory solutions supersaturated with respect to amorphous silica by a factor less than 1.3 and in the absence of chemical impurities, precipitation rates have a first-order dependence on f(hG(f)) (AB2) Rate(ppt) ([Si] m(-2) s(-1)) = k(ppt) exp (-E-a/RT) (1 - exp Delta Gr/RT) where k(ppt) = 10(-1. 9) [Si] m(-2) s(-1) and E-a = 61 +/- 1 kJ mol(-1). In more supersaturated and chemically complex field solutions, amorphous silica precipitation rates have a nonlinear dependence on f(Delta G(r)) and may be described by (AB3) Rate(ppt) ([Si] m(-2) s(-1)) = 10-(10.00+/-0.06) (exp Delta G(r)/RT)(4.4+/-0.3) or (AB4) Rate(ppt) ([Si] m(-2) s(-1)) = 10(-9.29+/-0.03) (Delta G(r)/RT)(1.7+/-0.1) The changes in reaction order, form of f(Delta G(r),), and chemical impurities suggest that the dominant amorphous silica precipitation mechanism changes from elementary reaction control in the simple laboratory experiments to surface defect/surface nucleation control in the complex field experiments. Copyright (C) 1998 Elsevier Science Ltd.