Waste materials and adsorbents for water treatment: from material characterization to applications in continuous operation systems

Wastewater treatment is a crucial activity that prevents soils and water bodies pollution avoiding the decrease of freshwater availability. Industrial and agricultural sectors are the main responsible for the generation of contaminated water and standard primary or secondary treatments are usually not enough to deal with such effluents. As a matter of fact, a specific tertiary treatment is often required for the removal of recalcitrant compounds that are harmful even at very low concentrations. Among all the technologies used to face such issue (e.g. Advanced Oxidation processes, Membrane processes), Adsorption constitutes a valid alternative for its ease of operation. However, the most commonly adopted solid adsorbents such as Active Carbons or Zeolites have to sustain high activation and synthesis costs respectively. In order to reduce such costs as well as to promote circular economy principles, in this thesis three different waste materials were proposed as bio-adsorbents: Yerba Mate (Ilex paraguarensis, YM) tea, Aloe vera (AV) rind and Pteris Vittata (PV) fern roots. Yerba Mate was used for the removal of anionic and cationic compounds from wastewater. Chromium hexavalent Cr(VI), Remazol brilliant blue (RBB) and methylene blue (MB) were selected as pollutants. A calcination step was performed after the washing and drying steps to evaluate its effectiveness at increasing the adsorption capacity of the solid. Both YM and calcinated YM (CYM) were characterized by means of scanning electron microscopy (FE SEM), Fourier transform infrared spectroscopy (FT-IR) and Brunauer–Emmett–Teller (BET) analysis. Adsorption batch tests revealed that YM was ineffective for the removal of Cr(VI) and RBB, while good results were obtained for MB (up to 80%) without pH dependency of the adsorption process, and CYM was able to remove Cr(VI) (up to 77%) and RBB (up to 65%) but not MB. The adsorption isotherm of MB on YM at 298 K was obtained experimentally and it is well represented by the Langmuir isotherm. YM’s adsorption capacity for MB was estimated to be 59.6 mg/g. Kinetic batch tests were conducted and the experimental results were fitted with a mathematical model. The low influence of temperature compared to the influence of the YM concentration on the adsorption rate was explained. The adsorption properties of Aloe vera (Aloe barbadensis Miller) for the uptake of Methylene Blue (MB) from water were investigated after pre-treating the material with water–ethanol solutions at different ethanol concentrations: 0% v/v (AV0), 25% v/v (AV25), and 50% v/v (AV50). The pretreated materials were characterized as follows: the pH of zero charge was evaluated to be 6, 5.7, and 7.2 for AV0, AV25, and AV50, respectively; from BET-BJH analysis the mesoporous nature of the material and an increase from 108.2 (AV0) to 331.7 (AV50) m2/kg of its solid surface area was observed; TG analysis revealed a significant increase in volatile compounds from the untreated (5.4%) to the treated materials (8.9%, 10.3%, and 11.3% for AV0, AV25, and AV50, respectively). Adsorption batch tests were then 10 performed to investigate the equilibrium, the kinetics, and the thermodynamics of the process. Results suggested that the Langmuir model was in agreement with the experimental results, and the values for the adsorption capacity were evaluated to be 199 mg/g, 311 mg/g, and 346 mg/g for AV0, AV25, and AV50, respectively. The kinetic results were used to develop a mathematical model to estimate the effective diffusion coefficient for each type of Aloe adopted. Effective diffusion coefficients of 5.43·10−7 cm2 /min, 3.89·10−7 cm2 /min, and 5.78·10−7 cm2 /min were calculated for AV0, AV25, and AV50, respectively. It was found that pre-treatment, on the one hand, enhances the adsorption capacity of the material and on the other, reduces its affinity toward MB uptake. The perennial fern Pteris vittata is an Arsenic-hyperaccumulator plant able to grow in hydroponic cultures and hence used for phytoremediation of contaminated water. In order to abate the costs linked to the disposal of As-contaminated biomass, in this work Pteris vittata waste roots were tested for the removal of Methylene Blue (MB). The pH of zero charge was evaluated to be 6.2, hence all the adsorption tests were performed at a neutral pH. Isotherms at 20 °C and 40 °C showed a typical Langmuir trend with a maximum adsorption capacity of 112 mg/g and 154 mg/g respectively. Kinetic tests were also carried out for different solid-liquid ratios and fitted by a mathematical model. The effective diffusivity of MB in the solid was estimated using the maximum likelihood method and the values of 5.99·10−8 ± 9 .6·10−9 cm2/s and 9.56·10−8 ± 4.5·10−9 cm2/s were obtained at 20 °C and 40 °C respectively. After calculating the Biot number it was found out that both the intra particle resistance and the external mass transfer resistance are important for the description of the rate of the dye uptake. Among all the three materials, Pteris vittata roots were selected to perform fixed bed column tests for the removal of Methylene Blue from water. As a matter of fact, swelling phenomena for Aloe vera induced to an excessive packing of the bed, while Yerba Mate tea showed a smaller adsorption capacity than Pteris vittata. Breakthrough curves were obtained experimentally at 20°C and the Linear Driving Force (LDF) model was used to predict their behavior. An empirical expression for the evaluation of the LDF mass transfer coefficient was then derived and validated for Reynolds number less than 0.025. In conclusion, this thesis provides all the steps (from characterization to fixed bed applications) that are required to evaluate the feasibility of adopting an agro-waste material as an adsorbent for real industrial applications. In this sense this work constitutes a guide from both an experimental and modeling point of view setting the basis for further studies in this research field.