Utilización de nanocelulosa en tratamientos avanzados de aguas residuales

Resumen

The dumping of metals generates several environmental effects, especially in natural water bodies. The main source of metals in nature are the mismanaged industrial waste and wastewater (90% of the total). Industries like the fiber cement (FC) sector minimize the water discharge by recycling it. It concentrates metals, like Cr(VI), in process water. Reducing its concentration and keeping production quality is critical. Another origin is the spent Li-ion batteries processing. Hydrometallurgy (acid leaching of black masses) is widely used to recover critical metals, but a selective separation process is needed to purify metals. Many technologies have been applied to treat metals. Among them, adsorption shows low environmental impact and costs and present high efficiency. Nanocelluloses are sustainable nanomaterials from cellulosic sources with excellent mechanical and surface properties as adsorbents, which may be great future adsorbents. In this thesis, a total of four nanocelluloses (NCs) (cellulose nanocrystals (CNCs), cellulose nanofibers (CNFs), cationic cellulose nanocrystals (CCNCs), and bacterial cellulose (BC)) were synthesized and characterized before its application as metal adsorbents. CNFs, CNCs and BC presented a negatively charged surface, with up to 1.72; 0.40; and 0.05 meq/g of cationic demands, respectively. The CCNCs was able to attract up to 0.68 meq/g of anions. Such results matched well with the results in bibliography. Both CCNCs and CNCs presented highly crystalline structure (Cr·I > 81%). TEM images showed the nanosized BC and TEMPO-CNFs fibrils. These last materials had an extremely high transmittance (90-95%), linked to a large yield of nanofibrillation. The NCs application to adsorb different metals was fully studied: Cr(VI), Pb(II), Ni(II), Co(II), Mn(II), Cu(II). The adaptation of the NC to the metal treatment was addressed by optimizing pH, dosing, metal content, time, temperature and operation. After the process optimization, Cr(VI) could be fully abated by adsorption onto CCNCs (44 mg/g) and CNFs-hydrogel (70 mg/g) operating at pH 3. CCNCs adsorbed and reduced it to Cr(III) using 40 mg CCNCs/L and just 60 s of contact time. Up to 1 g/L of CNFs-hydrogel was required for full decay. As well, BC adsorption of Pb(II) (8 mg/g) and Ni(II) (28 mg/g) adsorption onto BC surface was also observed at pH 4. CNCs and TEMPO-CNFs were used to recover Ni(II), Co(II), Mn(II), and Cu(II), simulating acidic black mass leachate (BML). Achieving recoveries over 1 g/g was feasible under optimized conditions and really low NC dosages (10 mg/g). Operating under multiple step batches (MSB), the recoveries rose up to over 10 g/g. The operation was fully modeled through four kinetic and six isotherm models. Pseudo-second order kinetics showed the best fitting. Intraparticle model revealed the presence of mass transfer limitations. Cr(VI) isotherms onto CCNCs and CNFs-hydrogel, and Pb(II) and Ni(II) onto BC were well fitted by Freundlich and Sips models. The isotherm modeling of the recovery of critical metals onto TEMPO-CNFs and CNCs was split into two mechanisms: Langmuir for adsorption and Freundlich for surface precipitation. Cr(VI) treatment with conventional adsorbents for was compared to CCNCs. Activated carbon use required a further membrane post-treatment of backwashing water which increased the costs up to 1.6 US$/m3. An equivalent CCNCs costs below 22 US$/kg would be competitive, being greener and easier to operate.MSB operation was applied for selective separation of Ni(II), Co(II), Mn(II), and Cu(II). Some steps of selective uptake were found for each metal. Co(II) recovery was much higher and selective than the rest, recovering up to 100 g/g. This is the largest Co(II) recovery observed in bibliography.