Sorção e redução de cromo hexavalente [Cr(VI)] em solos tropicais

Abstract

Depending on its oxidation states, Cr is a potentially toxic element (PTE). Cr (VI) is a well- known toxic form for organisms. In the soil, many factors control redox reactions of Cr and its

bioavailability. Thus, this work aimed to evaluate the sorption and reduction of Cr (VI) in typical soils from Brazil. Surface samples (A horizon) and subsurface samples (B Horizon) from Cambisol, Argisol and four Oxisols were used. The following conditions were assessed: pH, ionic strength (IS), shaking time, and soil properties as well. The sorption capacity of Cr (VI) was evaluated by the batch method. For that, 0.3 g of soil (air-dried and passed through 2 mm sieve) were reacted with 30 mL of solution containing, individually, different concentrations of Cr (VI) (from 0 to 1.5 mmol L-1

). The background solution was NaCl (IS =

15 or 45 mmol L-1

); pH (4, 5, and 6); and shaking time (0.5, 4, and 24 hours). After shaking time, the amount of Cr (VI) remained in the soil (Q) was calculated by the difference between initial concentration (Ci) and equilibrium concentration (Ce). The Q values were plotted as function of Ce values and adjusted to the Langmuir model. From this model, the maximum sorption capacity (Qmax) of Cr (VI) was estimated. Qmax was correlated to soil properties and multiple linear regression (MLR) was employed for prediction of Qmax. For Cr (VI) reduction test, under laboratory conditions (using a BOD), 40 g of soil were packed into 50 mL plastic cups and artificially contaminated with Cr (VI) at the following concentrations: 350 mg/kg for A soils horizons; 200 mg/kg for B soils horizons. The samples of the A soils horizons were kept incubated for 0, 12, 24, and 36 hours for A horizon, and for 0, 1, 2, 4, 8 and 16 days for B horizons. After each incubation time, the remained Cr (VI) in the soil was extracted and quantified according to USEPA 3060a method. The pH, IS and shaking time significantly influenced the Cr (VI) sorption capacity. In general, higher sorption of Cr (VI) was found at lowest pH values and IS, and higher shaking times. Overall, the Oxisols showed Qmax up to 2,000 mg/kg. The contents of Al2O3, Fe2O3, TiO2, clay and organic matter (OM) influenced both sorption and reduction of Cr (VI). Samples from A horizon (organic rich) showed high capacity to reduce the Cr (VI). Considering the very unstable condition of Cr (VI) in soils, from this work it is concluded that the batch method, commonly employed for sorption studies, does not reflect the true sorption of Cr (VI), mainly for soil samples with higher OM contents. The Cr speciation in the equilibrium solution is strongly recommended in further works. The artificial contamination of soil with Cr (VI) and kinetic studies of its reduction allow a proper way to investigate the behavior of Cr (VI) in soils. Additionally, from this work it is suggested that the investigation of Cr (VI) contaminated areas needs the soil sampling, maintenance of original condition, incubation and repetition of analysis along the time, or the soil sampling in the same place along the time.