Hexavalent chromium is frequently found in industrial effluents as a result of the industrial applications of this compound and its anti-corrosive features. However, hexavalent chromium is extremely toxic, and its discharge in water is regulated, with a maximum limit of 0.1 mg/L in accordance with legislation established by CONAMA-Brazil (no. 397, April 3, 2008). To achieve lower discharge values, it is necessary to reduce from Cr(VI) to Cr(III), which is less toxic, and an economic alternative involves biological removal of this compound. Residence time distributions (RTDs) were measured to evaluate the behavior of actual biofilter operation conditions in a biofilter flow. The medium residence time distributions used were 8 and 24 h (recommended by the legislation). To optimize this process, a central composite design was used, considering the initial chromium concentration and pH as the independent variables and the removal of hexavalent chromium as the response. The boundary curves and surface response showed optimal behavior at 3.94 mg/L [Cr(0)] and a pH of 6.2. The removal process of hexavalent chromium is mathematically described by the Michaelis-Menten kinetic model. This model appropriately represents the variation of chromium concentration along the bioreactor.

The nanotechnology has become an important area, either because the special features of nanometric particles confer to products, or due to a maximum limit of emission of such particles in air. However, the reduced size of this kind of particle limit them to be separated by conventional techniques, and a good way to ensure an aerosol stream with monodispersed particles is to use the electrical mobility technique. This theory is based on the ability of a charged particle to cross an electrical field, thus depending of the size of particles, the bigger ones will arrive later in the central electrode than the smaller ones, and only a narrow band of sizes will be collected in a slit located in the bottom of the equipment. This work aimed to project and construct a Differential Mobility Analyzer (DMA) in order to produce an aerosol containing NaCl nanoparticles monodispersed, from a aerosol polydispersed produced by an atomizer. The operating principle is to charge NaCl nanoparticles with a neutralizer so that they can be collected according to the voltage applied to the DMA in a specific range of size distribution. The unit comprises a compressor for generating air current, high efficiency filters in order to generate ultrapure air, atomizer, reservoir of NaCl solution to generate aerosol, a diffusion dryer to remove moisture silica current gas, a source of X-rays to charge nanoparticles, a source of high voltage to be applied to the DMA, a DMA to classify the nanoparticles and a particle counter to perform the counting of particles before and after DMA. Preliminary results indicated the dependence of the particle size with the applied electric field. The results were very promising.