58 Based on observations described above, there was clear evidence that acetic-
citric acid dissolved more amorphous than crystalline materials from milled silicate rocks, promoting a high dissolution rate during the early stage of the dissolution
process, and at the same time some initially dissolved structural polyvalent cations, mainly Al, were re-precipitated producing secondary amorphus materials. This
dissolution mechanism differs from mechanisms suggested by other workers Huang and Keller 1970; Welch and Ullman 1996; Blake and Walter 1999; Oelkers and
Scott 2001; Oelkers and Gislason 2001 as mentioned earlier that dissolution mechanisms are ion exchange reactions in which soluble surface cations are
exchanged equivalently by H
+
from the acidic solvent and complexion polyvalent cations by organic acids, followed by slower dissolution being attributed to ion
release involving dislocation of structural bonds in the silicate structure .
3.4 Conclusions
Results of this dissolution experiment were closely associated with the structural properties of the minerals in the rocks as affected by the milling
treatments. The general trends of rock dissolution in this present study were consistent with those found by other researchers Huang and Keller 1970; Welch
and Ullman 1996; Blake and Walter 1999; Oelkers and Scott 2001; Oelkers and Gislason 2001; Harley 2002. However, pretreatment by removing amorphous
constituents from ground rocks or minerals as applied by the above workers results in different dissolution mechanisms. In particular the high initial dissolution rate was
not so important for “cleaned” materials. In this present study, the dilute organic acid mixture dissolved more amorphous than crystalline material, whereas the
mechanism suggested by the cited authors involves an initial higher dissolution rate associated with the detachment of outmost structural elements from ordered
structures. The enhanced solubility of the milled rocks encourages the view that high-
energy ball milled basalt and dolerite may be used as Ca and Mg fertilizers and milled K-feldspar as a K fertilizer. However, only relatively minor percentages of
total plant nutrients were rapidly released from milled rocks. In marked contrast most commercial chemical Ca, Mg, and K fertilizers dissolve rapidly and to a major
extent shortly after application. Dissolution of milled silicate rocks must be
59 evaluated for the soil environment where dissolution of SRF relies partly on soil
acidity. There may be substantial modifications of soil chemistry in the rhizosphere due to plant roots enhancing the availability of plant nutrients from poorly soluble
sources through the evaluation of inorganic and organic solvents Hinsinger and Gilkes 1995; Hinsinger et al. 2001. Furthermore plant growth experiments should
be carried out to identify if the large increase in solubility of nutrient elements created by milling is accompanied by large increased in plant uptake of these
elements. These topics are investigated in subsequent chapters.
60
Chapter 4
DISSOLUTION OF MILLED-SILICATE ROCKS IN THE SOIL
4.1. Introduction
