Evaluation of the adsorption capacity of antimony from aqueous solutions by using cork as bio-adsorbent

Nowadays many products are dumped into waters causing serious pollution problems, such as the spill of heavy metals like iron, lead, cadmium and others. Adsorption is one of the most useful methodology to remove metals and metalloids from wastewater. In recent years many researchers have studied the capabilities of low cost biomass materials as biological adsorbents such as food wastes, leathers, cork, hairs, chicken feathers, degreased wool, biochar obtained from different original biomasses, and sponges. Cork (Quecur suber L.) has been chosen in the present work as bio(adsorbent) to remove antimony (Sb) from aqueous solutions. This low cost material is the outer bark of the oak tree and it is industrially used for several purposes, principally the manufacturing of wine stoppers. Cork has high biosorption efficiencies because of its physical characteristic like elasticity and impermeability as well as its chemical composition (a complex mixture of fatty acids and heavy organic alcohols ≈45% w/w, tannins ≈6% w/w, polysaccharides ≈12% w/w, lignin ≈27% w/w, alkanes, mineral content ≈5% and the most abundant element Ca 0.038–0.625% w/w). Antimony is a metalloid, naturally present in the environment but also introduced by human activities. It is classified as a minor metal, growing in strategic importance because of its use in flame-retardants, plastics and semiconductors. Sb is toxic and carcinogenic with no known biological function. So, its removal from water is of unquestionable importance as well as its recovery due to the growing interest in antimony by the industry. The aim of this work is to evaluate the adsorption capacity of Sb (III) and Sb (V) by this innovative bio(adsorbent). In order to evaluate the cork adsorption capacity, kinetics and isotherm behavior is checked. Corresponding batch experiments have been performed, also to determine the influence of the aqueous antimony speciation. So, 25 mg of cork were stirred in presence of 2.5 mL of Sb (III) and/or Sb (V) synthetic solutions at pH 5. At this pH the Sb(III) is dissolved in its corresponding neutral form, Sb(OH)3, and Sb(V) is present as its anionic form, Sb(OH)-6. First, cork has been shown as a good bio(adsorbent) for antimony from weak acidic aqueous solutions. The kinetic study shows that cork adsorbs Sb(III) faster than Sb(V), reaching the equilibrium uptake at 6 h in both cases. The form of the active sites of the cork adsorbent surface, with carboxylic and phenolic groups, could explain this phenomenon. So, cork preferably interacts with the neutral form of Sb(III) instead with the anionic form of Sb(V). These results are complemented by the isotherm profile, which shows higher total sorption capacity for Sb(III) than Sb(V).