Abstract
The red mud (RM) has been used as an alternative low-cost adsorbent to remove trace elements, with the adsorption onto sodalite surface described as the main removal mechanism for trace elements. However, recent studies have shown that precipitation might be of great importance for some trace metals removal using natural and thermal activated RM. Therefore, the aim of this study was to identify the main mechanism responsible for Cd(II), Pb(II) and Zn(II) removal from aqueous solutions using natural and activated forms of RM, based on sequential extractions and a precipitation kinetic model was developed. Results showed that the carbonate fraction was responsible for the highest trace elements removal (ca. 85%), with the minerals assemblages precipitated: otavite – CdCO3, cerussite - PbCO3, smithsonite - ZnCO3 and anglesite - PbSO4. The kinetic model showed that the mineral precipitation was limit due to the HCO3− consumption during the formation of new minerals. Hence, this study showed that precipitation was the central mechanism on trace elements removal, regardless the natural or activated forms of RM. This finding raise doubt about the effectiveness of the traditional adsorption isotherms and kinetics models to describe trace metals removal using RM, contributing with new insights for future researches involving these hazardous materials.
Highlights
Brazilian mining activities contribute significantly to global mineral production, including the third-largest global production of bauxite (Brasil, 2018)
Kinetics studies showed a maximum amount of trace elements removed using red mud (RM), RM400, RMHCl and RMCa of 0.95, 0.99, 0.37 and 0.35 and mmol g−1 for Cd(II), 1.27, 1.39, 0.51 and 0.50 mmol g−1 for Pb(II) and 0.90, 0.94, 0.28 and 0.30 mmol g−1 for Zn(II)
These results clearly indicated that the chemical treatments decreased the Cd(II), Pb(II) and Zn(II) removal whilst the thermal treatments increased up to 10% the trace elements removal in relation to RM
Summary
Brazilian mining activities contribute significantly to global mineral production, including the third-largest global production of bauxite (Brasil, 2018). The Brazilian RM can be considered a hazardous material due to presence of different oxides and toxic trace elements mixed in a highly alkaline matrix (Antunes et al, 2012; Souza et al, 2013). The disposal of this residue usually occurs in tailing dams, producing a high financial and environmental cost, leading to problems related to contamination of soil, groundwater and surface water and damage to flora and fauna (Silva Filho et al, 2007; Jones and Haynes, 2011). In Brazil, an environmental disaster caused by high rainfall occurred in a RM tailing dam located in Barbacena (Pará State) in February 2018, affecting thirteen riverside communities, which depends on the natural resources of the Pará River basin in this municipality (Amazônia Real, 2018)
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