Assessment of the Physical Environment of the Plumbum Mining and Metallurgy Co. in Santo Amaro - Bahia, Brazil

Abstract

The purpose of this investigation was to evaluate the efficiency of a wetland that occurs on the premises of a lead industrial plant located in Santo Amaro da Purificação, Bahia, as a measure to control contamination from a slag dam contaminated with heavy metals. Initially, an annual survey was proposed with weekly sampling of rainwater and surface water from the wetland, in order to assess the efficiency of the wetland, by surveying the concentrations of metals from the leaching and/or slag solubilization processes in their entry and exit points from the flooded system and concentrations and potential availability of the metals retained in the sediments of the floodplain. The following parameters were selected: for rainwater, pH and volume; for surface water, the concentrations of the metals Lead (Pb), Cadmium (Cd), Zinc (Zn), Copper (Cu), Aluminum (Al), Manganese (Mn), Iron (Fe), Magnesium (Mg) and Calcium ( Ca) and support parameters pH, Eh, conductivity, Dissolved Oxygen (OD) and temperature. For the soil and sediment of the wetland, the concentrations of metals like Pb, Cd, Zn and Cu e, determination by analyses of the sequential extraction of the total removed by the system in its different phases and the potential availability of heavy metals. However, a modification of the investigation strategy had to be made due to a court decision that determined the covering of the slag and closing of the access to the wetland. The work was then divided into three phases, considering the surveys carried out before, during and after the coating. The data collected in the five months before the coating showed that the metals cadmium, lead, copper and zinc have been retained by the wetland and that this system was 100% efficient for the copper and zinc metals, 82% for the lead and 73 % for cadmium. The support parameters that influence the removal of these metals were the pH, between neutral to alkaline, and Eh, in the surface water oxidation range, besides the high cation exchange capacity of the montmorillonite present in the sediment. As for the potential availability of metals, cadmium, lead and zinc present high values, while copper is preferentially concentrated in the residual phase. The second stage of the survey carried out during the slag coating showed that there was little migration of metals from the wetland area, although the company did not comply with the technical standards for slag coating. In the third stage, a survey was carried out at the exit of the wetland and the drainage near the Subaé River. The analyses showed the great availability of cadmium in the flooded system, a mechanism triggered by the erosion of the contaminated soil disposed on the slag and the great solubility of cadmium. It can thus be concluded that the wetlands are efficient in retaining metals. The reduction of its area is undesirable, as it tends to decrease its efficiency, as a surface water pollution control system. It is recommended to build a new wetland downstream of the existing one.