Methylene Blue removal from wastewater using renewable adsorbent materials

Regarding wastewater treatment, an important aspect is the removal of organic pollutants to reduce environmental impact and comply with discharge regulations. This thesis focuses on the adsorption of a cationic dye, Methylene Blue (MB), using sustainable innovative materials. The studied adsorbent materials include biochar and hydrochar of organic origin, as well as activated carbon as a reference. The main objective is to evaluate their effectiveness and to understand the key factors influencing the adsorption process, while also modelling it. Biochar was produced from rice husk through slow pyrolysis at 600 °C with 15 °C/min, using both N2 and CO2 as inert gases, leading to two different biochars (biochar N2 and biochar CO2). Hydrochar is the product of hydrothermal carbonization (HTC) of rice husk at 240 °C. The removal of MB depends on several factors: in this work, the effects of adsorbent dosage, contact time and pH were analysed, also including a study on the materials’ Point of Zero Charge (PZC). Adsorption experiments were conducted discontinuously at room temperature with stirred solutions of MB at 50 mg/L, using three different initial adsorbent concentrations (0,25/0,5/1 g/L). Each experiment lasted 6 hours and the residual dye quantification was carried out by measuring the absorbance with a spectrophotometer. As the dosage increased, the rate of MB removal showed an increase due to the greater surface area and number of available adsorption sites, until reaching an equilibrium. To evaluate the effect of pH, each material was studied at pH of 7, 10 and 11,5 for the same initial adsorbent dosage (0,5 g/L) and tests report that, as the pH increases, higher removal efficiencies are obtained the more the pH exceeds the one corresponding to the PZC of the material; above this value, in fact, the adsorbent assumes a negative surface charge, attracting more of the positively charged dye thanks to electrostatic interactions. The effect of contact time was evaluated by sampling at regular intervals. During the first hour there is faster adsorption due to a high concentration gradient acting as a driving force. However, as time proceeds, the concentration gradient decreases because the MB molecules accumulate on the adsorption sites, and the rate of adsorption slows down until equilibrium. After 16 hours, a further sampling was carried out to assess the amount of MB adsorbed at equilibrium. A kinetic and thermodynamic analysis was then conducted using non-linear models, including pseudo-1°order and pseudo-2°order kinetic models, Freundlich and Langmuir isotherms. Particular attention was paid to the validation of the models, testing the assumptions of randomness and normal distribution of the residuals by means of normal probability plots and by evaluating the trend of the residuals as a function of time and predicted variable. Finally, a single-stage batch adsorber was designed based on data from thermodynamic models. In conclusion, the only material comparable to activated carbon in terms of removal efficiency at equal dosage (1 g/L) was biochar CO2; hydrochar has a lower performance, while biochar N2 achieves better MB removal than activated carbon if the pH is increased above the pH of its PZC.