CARACTERIZAÇÃO QUÍMICA E MINERALÓGICA DOS SUSPENSATOS DOS RIOS PURUS E JURUÁ NO ESTADO DO ACRE

Abstract

The state of Acre, located in the southwest of the Brazilian Amazon, is drained by two large river basins, the Purus and Juruá rivers, which have their sources on the sub-Andean slopes of Peru and cross the state in a path almost parallel to each other and a southwest-northeast direction. They are meandering rivers, with a large amount of suspended sediment (suspended solids or load materials), which gives them their white or muddy color, in addition to transporting a lot of silty-sandy sediments by traction, forming significant beach bodies (pointed bars). The geological terrains drained by these rivers are made up of rocks from the Solimões Formation and sediments from the Alluvial Terraces. The present work sought to characterize the granulometry, mineralogy and chemical composition of suspensions from rivers in the State of Acre. These were subjected to granulometric analysis, mineral identification using X-ray diffraction (XRD) and scanning electron microscopy (SEM/SED) and total chemical analysis of trace elements. The highest concentration of suspended solids was found in the central part of the State both during the flood and dry periods, while the lowest are in the Moa and Paraná dos Mouras rivers. They are predominantly made up of silt with little fine sand and clay, averaging 12 microns. Mineralogically, they are made up of quartz, mica, smectite, illite, kaolinite, as well as albite and K-feldspar. These are sediments rich in SiO2 (55.69% +/- 3.89%), Al2O3 (18.68% +/- 1.76%), Fe2O3 (5.89% +/- 0.41%), with significant K2O values ​​(1, 93% +/- 0.07%), MgO (1.17% +/- 0.22%), Na2O (0.41% +/- 0.06%), in addition to TiO2 (0.85% +/- 0.10%). The values ​​of P2O5 (0.10% +/- 0.014%), CaO (0.14% +/- 0.17%) and MnO (0.10% +/- 0.021%) are at crustal levels. In terms of trace elements Rb, Sr, Cs, Ba, Th, U, Zr, Hf, Y, Nb, Cr, Co, Ni, Cu, Sc and V, are the elements that presented the highest concentrations. The average concentrations of these elements in the suspended materials when compared with beach sediments show enrichment in Al2O3, Fe2O3, MnO, MgO, Rb, Cs, Sc, and V. On the other hand, with the upper earth’s crust they show enrichment in Cs, Y, Cr, Co, Ni, Cu, Sc and V and with post-Archaean Australian shales only slightly enriched in Y. The presence of clay minerals (smectite, illite and kaolinite) and feldspars (albite and K-feldspars) are responsible for the high concentrations not only Al2O3 and Fe2O3, but also K2O, MgO and Na2O. The mineralogical and chemical composition shown by these suspensions, in addition to their granulometry, contribute to fertilizing both the floodplains (“varzeas”) as well as the beaches exposed by the rivers during the dry period, which are intensely cultivated with corn (Zea-mays) and beans (Vigna unguiculata (L) Walp) by riverside dwellers.