34 rock powders to eliminate fines as mentioned above was avoided as such treatments
may cause loss of the readily soluble nutrients from the materials and furthermore such fractionation will be not applicable to commercially manufactured fertilizers.
In stead of using individual acid as the solvent, mixture of the acids is simple and presumed will be more representative for various combination of main organic acids
in soils. The central objective of this experiment was to identify the effects of high-
energy milling on the dissolution kinetics of several silicate rocks in a mixture of 0.01M acetic and citric acids. The results of this experiment were subsequently used
to provide an indication of the appropriate application rates of SRFs for laboratory, glasshouse, and field experiments.
3.2. Materials and Methods
The silicate rocks investigated in this experiment were basalt, dolerite, gneiss, and K-feldspar milled for 0, 10, 60, and 120 min as described in Chapter 2.
A mixture of 0.01M acetic acid + 0.01M citric acid at a ratio of 1:1 was used as the solvent. Four hundred milligrams of each rock powder and 400 mL of solvent were
transferred into a 500mL plastic container, tightly capped, then continuously agitated on a rotary shaker at 130 rpm and an ambient temperature of 25
o
C. Subsamples of suspension were taken periodically from 1 hour up to 56 days by
removing the containers from the shaker, the suspension was homogenised by hand- shaking for 5 seconds, and immediately 20 mL of suspension was taken by pipette.
The samples of suspension were centrifuged at 4000 rpm for 20 min using an Eppendorf 5810 Centrifuge maximum capacity 6 x 125 g and speed 4000 rpm, and
about 17 mL of clear solution was transferred by a pipette into a plastic tube. The concentrations of Ca, Mg, and Al in the solution were measured using AAS, Na and
K were determined using flame emission, and Si using a colorimetric method described by Rayment and Higginson 1992. The pH and EC of solutions were
measured with pH and EC meters Cyberscan 2000. At the end of the dissolution experiment, the remaining rock powder was centrifuged, oven-dried at 60
o
C for two weeks and used for XRD and transmission electron microscopy TEM
analyses to provide explanations of dissolution mechanisms.
35 The XRD patterns of each rock powder before and after dissolution were
collected using the methods described in chapter 2, and the diffraction intensity was recorded between 5 and 40° 2
θ. For TEM analysis, well-dispersed powder of the rocks milled under the dry condition for 120 min samples before and after partial
dissolution were prepared on carbon coated-copper grids. Two sub samples for each rock powder were prepared in this manner. High magnification 250 – 300 k
micrographs for several 10 – 20 particles of ground rock powder and electron diffraction patterns for several 3 – 5 zones of selected particles were obtained
using a JEOL 3000 FEG instrument operated at 300 kV.
3.3. Results and Discussion
