Crystal chemistry of suspended matter in a tropical hydrosystem, Nyong basin (Cameroon, Africa)

Suspended matter (SM) from the Nyong basin (Cameroon, Africa), a tropical w atershed, was collected by tangential flow ultrafiltration to separate part iculate (> 0.45 mu m) and colloidal (< 0.45 mu m; > 20 kDa) fractions. In t his basin, two distinctive systems in a selected small catchment (Nsimi-Zoe tele) of the Nyong river basin have been considered: (i) colourless water ( groundwater and spring) with a low suspended load(< 3 mg/l) and a low total organic carbon content (TOC < 1 mg/l) and (ii) coloured water (Mengong bro ok and Nyong river), which is organic rich (TOC > 10 mg/l) and contains hig her amounts of SM (10-20 mg/l) than the colourless water. Freeze-dried samp les of SM have been analysed by X-ray diffraction (XRD), transmission elect ron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), elect ron paramagnetic resonance spectroscopy (EPR), and visible diffuse reflecta nce spectroscopy (DRS). Colourless water mainly contains mineral phases, such as poorly ordered kao linite, plus quartz and goethite in the particulate fraction, and euhedral kaolinite plus amorphous iron oxyhydroxides in the colloidal fraction. In c ontrast, the SM in coloured water is mainly organic in nature. The mineral phases in the particulate fraction are similar to those from clear water, b ut with additional phytoliths and diatom frustules composed of biogenic opa l. In the colloidal fraction, complexation of Fe3+ and Mn2+ with organic ma tter is evidenced by EPR, together with significant occurrence of Fe oxyhyd roxides associated with organic matter. The sites of Al, Si, Fe, Mn in colloidal fractions derived from spectroscop ic analyses are discussed with reference to chemical analyses performed by inductively coupled plasma mass spectrometry. Most of the observed solid ph ases or species correspond to those expected from published thermodynamic c alculations for the same hydrosystem, except the colloidal iron oxyhydroxid es in the coloured water. The presence of such iron phases is emphasised si nce they are expected to have large sorption capacities for numerous trace elements. The crystal chemistry of SM is used to discuss the origin of the mineral pa rticles transported from the soil to the main rivers in terms of mechanical and chemical erosion processes. (C) 2000 Elsevier Science B.V. All rights reserved.