Recovery and valorization of tannins from a forest waste as an adsorbent for antimony uptake

Abstract

Abstract In the current context, it is imperative to seek for a sustainable management and an efficient use of natural resources. Pinus pinaster bark is a forest and industrial waste whose chemical richness is commonly ignored. In this work, tannins were extracted from P. pinaster bark and converted into adsorbents through polymerization. Aqueous (alkaline) extraction yielded more formaldehyde-condensable phenols than an organic extraction using ethanol (Soxhlet) (53 ± 8 vs. 13 ± 4 mg of gallic acid equivalents per g of bark). The polymerization reaction was optimized and higher amounts of adsorbent were produced using 6.0 mL of 0.25 mol L−1 sodium hydroxide solution and 0.40 mL of formaldehyde (36 % wt) per g of extract. The performance of the produced adsorbent was assessed on the sequestration of Sb(III) and Sb(V) species from water. The adsorbent was effective for both species, in diluted and heavily-contaminated waters, providing maximum adsorption capacities (Langmuir model) of 24 ± 3 mg g−1 (pH 6) and 27 ± 7 mg g−1 (pH 2), respectively for Sb(III) and Sb(V). No significant effect was observed due to the presence of arsenic, chloride, nitrate, sulfate or phosphate and little influence was obtained when a tailings water from a mine site was used as aqueous matrix. Electrostatic attraction and Sb(III) and Sb(V) complexation with polyol groups of tannin-adsorbents were the suggested adsorption mechanisms. Moreover, tannin-adsorbents were stable at different pH (no color leaching; total dissolved carbon ≤16 mg L−1) and their production does not require high energy or expensive chemicals.