Modelling the kinetics of toxic metal desorption in sediment affected by a dam breakdown disaster in Doce River - Brazil
Description
On November 5$^{th}$, 2015, Brazil's biggest socio-environmental disaster occurred. The mining dam spilled a huge brown plume of mining waste in the Doce River. Considering the hypothesis that each metal travels in a different way, the question is, how long will a toxic metal remain in the basin until the natural balance is achieved? Therefore, samples of sludge, sediment, and water were collected along the Doce River Basin, to characterise physical and chemically the sludge and assess the elements' total leaching by kinetic modelling. The sludge is composed mainly of quartz, hematite, and goethite. Toxic elements such as Al, Mn, Ag, Cd, Pb, Cu, and As were also found in the sediments. Manganese concentration and turbidity exceeded the maximum allowed by the Brazilian regulation for freshwater. A proposed sorption/desorption factor (F$_{SD}$) indicated that Mn$^{2+}$, Ag$^{+}$, and Cd$^{2+}$ can be mobilised about 80, 90, and 57 times more than its initial concentration. However, Pb, Al, As, Fe, and Cu ions remained immobilised on the sediments. The desorption kinetics showed lower rate constant (k) and higher initial desorption constant (h) for Mn$^{2+}$ than Cd$^{2+}$ and Ag$^{+}$, which can suggest both high- and low-affinity interaction sites for Mn$^{2+}$. The exponential decay from half-life demonstrated that metals can leach for months or years. The long-lasting releasing metal-rich tailing waste, in concentrations that endanger ecosystem and human health, makes it clear that long-term monitoring is required. It's worth mentioning that is necessary to strengthen the laws and Brazilian regulation on environmental issues.