Characterization of adsorption processes for the removal of metal ions from waste effluents using biosorbents and graphene-based sorbents. Studies in batch and in fixed-bed column

Resumen

Wastewater discharged from industrial and agricultural activities contains relatively large amounts of toxic metal ions, especially including Cd(II), Cu(II), Pb(II), and Cr(VI). The removal of these pollutants is of great interest from both health and environmental perspectives. Arsenic contamination, generally associated with the geochemical environment, is a global threat due to its acute toxicity and carcinogenicity. Conventional technologies for wastewater treatment and water purification such as precipitation, coagulation-flocculation, membrane processes, electrodialysis and ion-exchange are of limited utility due to their high cost, inefficiency in removing low metal concentrations, and sometimes also because they can generate large volumes of sludge. Adsorption is an attractive alternative due to its simplicity, its ability to remove trace amounts of metal ions, low cost, short operation time, and for the capacity for the material to be reused. In the search for highly efficient, eco-friendly and economic adsorbents, the type of functional groups and chemical components of lignocellulosic materials makes them a good alternative to treat contaminated effluents. Hence, in our case, we have evaluated lignocellulosic agro-industrial wastes that are available in the Mediterranean region, such as olive stones and pine cones, as efficient biosorbents for the removal of toxic metal ions such as Pb(II), Cu(II), Cd(II) and Cr(VI). Milled olive stones and, specially, milled pine cone show high capacities in removing Pb(II), Cd(II) and Cu(II) at pH 5.5 and Cr (VI) at pH= 2. Pine cone was selected as the biosorbent to evaluate the removal of toxic metal ions in dynamic conditions in a fixed bed column showing a good performance for the removal of Pb(II), Cu(II), Cd(II) and Cu(II), Ni(II) and Cr(VI) from aqueous effluents such as of those of electroplating industry. Conventional adsorbents and bioadsorbents have certain constraints such as low adsorption capacities, new sorbents in nano dimensions, engineered nano-adsorbents, such as graphene oxides (GOs) and metal organic frameworks (MOFs). GOs have been investigated for the adsorption of Cu(II), Cd(II) and Cr(VI) and iron-based MOFs for the adsorption of As(III) and As(V). High removal percentages were for Cu(II) (> 90%) and Cd(II) ( > 88%) at pH=6 with different GOs whereas the maximum removal percentage for Cr(VI) at pH 2 was 68%. The commercial-based MOF, Basolite ®F300, was compared with a synthetized nano Fe-BTC MOF resulting in no differences between both adsorbents in the case of As(V) with removal percentages of 90% and 86.7% for Basolite F300 and NanoFe-BTC at pH 5.5, respectively. However, the removal percentage for As(III) at pH 9.5 by Basolite ®F300 (47%) was higher than for the synthetic Fe-BTC (36%). In all these studies, the physico-chemical properties of the adsorbents have been characterized as well as the isotherm adsorption processes.