Removal of arsenic from water by an iron-loaded resin prepared from Pinus pinaster bark tannins

Abstract

Arsenic is a toxic and carcinogenic metalloid naturally present in water bodies. However, its concentration can be exacerbated by anthropogenic activities (e.g. mining, industry, agriculture). Thus, removal of arsenic from contaminated waters is a crucially important goal. Tannins are ubiquitous and inexpensive natural biopolymers that are easy to extract and convert into insoluble matrices. The aim of this work was to assess the potential of Pinus pinaster bark tannin resins to take up arsenic from aqueous solutions. Methods of tannin extraction and polymerization, oxidation and iron-loading were optimized. An extraction efficiency of 24.2 ± 0.1% was achieved and 166 ± 2 mg of formaldehyde-condensable phenols were extracted per gram of bark used. Polymerization of tannins, under the best conditions, presented an efficiency around 80%. Oxidation and iron loading yielded a tannin resin with an iron content of approx. 20 mg g–1. Adsorption of As(V) was optimal at pH 3 with a removal of approx. 90% from an initial solution containing 5 mg L–1. Adsorption assays with As(III) presented negligible results. As(V) adsorption was somewhat fast, with equilibrium being achieved in 1–4 h. Equilibrium data obtained for As(V) adsorption showed a maximum adsorption capacity of 0.72 ± 0.03 mg g–1 (pH 3, smaller than 150 µm). Iron leaching was high at low pH but could be drastically reduced by limiting the contact time. This work attempts to show that a forest residue, usually undervalued, can be converted into an adsorbent and can help in dealing with arsenic-contaminated waters, which is an environmental problem affecting many countries in the world.