Occurrence and Removal of Copper and Aluminum in a Stream Confluence Affected by Acid Mine Drainage

Carolina Rodríguez, Enzo Leiva-Aravena, Jennyfer Serrano, Eduardo Leiva
2018 Water  
Acid mine drainage (AMD) is an environmental concern characterized by low pH and high concentrations of dissolved metals and sulfate. Yerba Loca Creek in Santiago, Chile, is an AMD-affected water stream that originates in a glacier and, therefore, has a season-dependent flow. This water course is characterized by low pH (3.75 ± 0.13) and high concentrations of aluminum (2.2-2.6 mg/L) and copper (4.8-6.5 mg/L). A field campaign was carried out to study the geochemical behavior around the
more » ... around the confluence of the Yerba Loca Creek with the San Francisco River, which has a neutral pH and low concentration of dissolved metals. The results show that the geochemical parameters after the confluence are very similar to those registered for the Yerba Loca Creek, due to its great flow in relation to the San Francisco River. The pH after the mixing zone was controlled by the geochemical conditions and flow of the Yerba Loca Creek; however, the turbidity decreases and stabilizes downstream. We found that, despite the low impact of pH on the precipitation of aluminum and copper phases due to poor neutralization, the dissolved aluminum and copper concentrations are slightly decreased after the mixing zone by natural microscale removal processes or suspended solids formation. Scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDX) analysis of suspended solids indicates the presence of various oxides, hydroxy-sulfates and aluminosilicates, which have a great affinity for adsorption and co-precipitation with dissolved metals (i.e., Al and Cu). A pH-neutralization would favor the formation of more minerals and, therefore, the immobilization of the heavy metals found in these waters. These results contribute to a better understanding of the effect of the confluence of water courses related to pollution by AMD. It is possible that the seasonal variation of the flows has an impact on the composition of water and minerals formed. River (pH~7.2) [4]. At high elevations of the Yerba Loca Basin, there is a large deposit of copper porphyry, which have long influenced the surface water quality of the area, particularly in terms of pH, sulfate content and mineral concentrations [5] . In particular, in the Yerba Loca Creek acidic conditions and metal content, mainly copper (Cu) and aluminum (Al) [6] , have a substantial impact on the vegetation of the area since these waters are highly toxic for most plant species [5] . Furthermore, the presence of Cu in excessive concentrations is harmful to a variety of living organisms such as microorganisms, fish and humans [7] . In the environment, Cu is commonly presented as a divalent cation and is generally more mobile in acidic conditions, while at pH above 7, it tends to form minerals like Cu carbonates and hydroxyl-carbonates [8, 9] . Al is the third most abundant element in the Earth's crust and is commonly present as Al oxide and Al silicate [10, 11] . At pH less than 6, Al can be leached from the soil and sediments in the water [10], but the solubility increases when pH is less than 4.5 [12]. In addition, high Al concentrations, low pH and high levels of sulfate generate toxicity in the aquatic environment, which can be fatal for animal and plant life [13] . Several studies have been carried out about the effects of the confluence of water courses related to pollution by AMD. The Azufre River-Caracarani River Confluence (Chile) presents a change in particle sizes and a decrease in As concentration after the confluent point [14] . Agrio River-Odiel River Confluence (Spain) shows a strong seasonal variation in water composition [15] . For its part, Cement Creek-Animas River Confluence (USA) is a mixing zone of two water courses with acidic pH (Cement Creek) and neutral pH (Animas River). The complete mixing results in a circumneutral pH and metals in solution exhibit transformations to colloidal forms [16] . The mixing zone receiving acid drainage is a reactive zone where changes in pH and the chemical environment promotes the formation and precipitation of minerals [14] . Metal partitioning between the solid phase and solution can be modified along the flow path by changes in pH, redox and biotic factors downstream of the mixing zone [17, 18] . The description and quantification of non-conservative processes that control the fate of metals in natural environments are critical for the development of environmental remediation in systems affected by acid mine drainage [19, 20] . Reactive confluences form when acid drainage discharges into a circumneutral river are an opportunity to study how the geochemical processes in the solid/liquid interface modify the aqueous/solid partitioning of heavy metals. In this study, on-site measurements and water samples analysis were carried out to determine the geochemical behavior in the Yerba Loca Creek-San Francisco River confluence in a radius of 50 m around the confluence point. The objective of this study was to determine the effect of the confluence on pH and the fate of Cu and Al and the formation of minerals. This study will help to improve our knowledge about the processes that control the transport, removal and environmental fate of Al and Cu in acid mine drainage waters and thus contribute to the development of new alternatives for enhanced natural attenuation of these pollutants. Materials and Methods Study Site
doi:10.3390/w10040516 fatcat:x2grk2iqovcz5kcuhfj2dxfoku