Weathering reaction pathways

As will be illustrated, our own studies on PC film weathering indicate that the major parameter which affects PC degradation is the amount of UV light to which the sample is exposed. This implicates photochemical reactions as principal degradation pathways. 

Fig. 1. Schematic diagram showing major pathways, reactions and stable weathering products for CCB alteration. Heavy solid arrow indicates changes caused by progressive natural weathering of alkaline CCBs at high L/S (liquid/ solid) ratios. Heavy dashed arrow indicates changes caused by progressive alteration of CCBs by acidic leachate solutions at low L/S ratios.

Fig. 2 shows the different pathways in which chemical elements contained in rocks are released to the different environmental compartments. Five main processes are responsible for their dispersion into the different ecosystems (1) Weathering, either directly by rain water on rock outcrops, by soil percolation water or by root exsu-dates, which interact with rock fragments, contained in the soil cover (2) Down hill mechanical transport of weathered rock particles, such as creep and erosion and subsequent sedimentation as till material or alluvial river and lake sediments (3) Transport in dissolved or low size colloidal form by surface and groundwater (4) Terrestrial and aquatic plants growing in undisturbed natural situations will take up whatever chemical elements they need and which are available in the surface and shallow groundwater. Trace elements taken up from the soil will accumulate in the leaves and will possibly enrich the soil by litterfall (5) Diffuse atmospheric input by aerosols and rain rock particles from volcanic eruptions, desertic areas (Chester et al., 1996), seaspray and their reaction with rain water. A considerable part of this can be anthropogenic. 

---