The effect acid rain has on soil is dependent on the mineral content. This effect varies based on the mineral systems which can buffer the pH of the soil once exposed to the acid rain. Calcareous minerals (reaction 1), silicate clays (reaction 2), and aluminosilicate clays (reaction 3) are the three mineral systems that provide soil the capacity to withstand acid attack before a major pH drop occurs. One way to differentiate between these systems could be to observe the reaction orders of the underlying mechanisms.
CaCO3(s) 2 H+(aq) ⇌ Ca2+(aq) + H2O(l) + CO2(g) (1)
M-soil(s) + n H+(aq) ⇌ Hn-soil(s) + M+n(aq) (2)
Al-soil(s) + 3H+(aq) ⇌ H3-soil(s) + Al3+(aq) (3)
One hypothesis that this project sought to test was that each of these reactions would have a different kinetic mechanism and/or different rate constants.
If the mechanisms are different and they can be detected, then it might be possible to extract the mineral % composition in an environmental sample which is likely to be a mixture. Therefore, the purpose of this project was to determine the reaction orders of the kinetic mechanisms of these mineral systems. Specifically fayalite (Fe2SiO4) and forsterite (Mg2SiO4) were used as representatives of reaction (2). Previous experiments showed that the addition of strong acid to these samples results in an initial, rapid decrease in the pH followed by a partial recovery of in the pH as a result of the buffering mechanism. These recovery curves (see Figure 2) decrease in amplitude until the buffering system is exhausted. The recovery curve provides the opportunity to observe the reaction kinetics.
Overway, Ken and Rees, Hillary, "Soil Buffering Mechanisms" (2012). Chemistry Faculty Scholarship. 12.