Adsorptive removal of pharmaceuticals from aqueous solution using end-of life magnetic tyre pyrochars: Experimental investigations and numerical modelling

Abstract

In recent decades, occurrence of pharmaceutical pollutants in aquatic environment has become a growing concern due to their low biodegradability, high persistence, and facile bioaccumulation. Different carbonaceous materials such as activated carbon, biochar and pyrochar are widely utilised for removal of pharmaceuticals from water or wastewater. This thesis presents a challenge-based innovation of End of Life Tyres (ELTs) pyrolysis for magnetic pyrochar production and synthesis with evidence of its use as low-cost, novel adsorbent for pharmaceuticals removal from aqueous solutions. Magnetic tyre pyrochar (MTC) derived from ELTs at Technology Readiness Level 3-7 (TRL3- 7), was tested for the removal of ciprofloxacin (CIP), propranolol (PRO) and clomipramine (CLO), from water. The morphological and chemical properties of the adsorbents were assessed using Brunauer Emmett Teller (BET) surface area, Vibrating Sample Magnetometer (VSM), Fourier Transform Infrared (FTIR), Scanning Electron Microscope coupled with Energy Dispersive X-ray (SEM-EDS), elemental analysis and zeta potential measurements. Batch adsorption experiments revealed MTC’s excellent adsorption efficiency of 85%, 90% and 92% for CIP, PRO and CLO respectively, higher than that of the non-magnetic tyre pyrochar (TC), due to the larger surface area, and porosity and lower polarity. Adsorption of the compounds onto MTC was highly pH dependent, and favourable at lower ionic strength. The experimental data were well described by pseudo-second order kinetic and Freundlich isotherm models. Based on FTIR and zeta potential analysis, the interaction mechanisms were explained by cation-π, π-π electron donor acceptor (EDA), cation exchange, electrostatic repulsion and hydrophobic effect. In fixed-bed column experiments, MTC, activated tyre char (ATC) and commercial activated carbon (CAC) were used as packing materials in lab-scale column study for the adsorption of the three pharmaceuticals from aqueous solution. The obtained breakthrough curves (BTCs) suggest that, lower flow rate, greater bed height, higher pH and nano particle size led to increased adsorption of PRO. The lowest adsorption capacity was observed for CIP either from single or ternary solution while it was significantly higher for CLO. Surface area of ATC increased nearly twelve-fold (38.17 to 453.81 m²/g), after thermal and chemical activation and its adsorption capacity was comparable to commercial activated carbon. The suitability of Hydrus-1D model incorporating chemical non-equilibrium process to simulate the pharmaceutical transport and fit experimental BTCs was demonstrated (0.97<R²<0.99) in comparison to other common models (Adams–Bohart, Thomas and Yoon–Nelson). The modelling suggests the existence of non-equilibrium conditions and rate-limited sorption sites and the effect of dispersion and mass transfer mechanisms in the solute transport under dynamic conditions. The competitive effects of different fractions of wastewater treatment plant effluent organic matter (EfOM) on adsorption of PRO were investigated in a fixed-bed column packed with MTC. Based on results, presence of EfOM inhibited adsorption of PRO in real wastewater and decreased the PRO adsorption capacity from 5.86 to 2.03 mg/g due to competitive effects and pore blockage by smaller EfOM fractions. Characterization of EfOM using size exclusion chromatography (LC-OCD) showed that the principal factor controlling EfOM adsorption was pore size distribution. Low molecular weight neutral fraction of EfOM had the highest adsorption onto MTC while humic substances were the least interfering fraction. Effect of important parameters (contact time, linear velocity and bed height/diameter ratio) on MTC Abstract iv performance in large-lab scale columns revealed that linear velocity and contact time were effective in increasing adsorption capacity and delaying breakthrough time. Increase in linear velocity from 0.64 cm/min to 1.29 cm/min increased mass transfer and dispersion, resulting in considerable rise of adsorbed amount (5.86 mg/g to 22.58 mg/g) and increase in breakthrough time (15.8 to 62.7 hours). Efficiency of non-equilibrium Hydrus model considering dispersion and mass transfer mechanism was shown for real wastewater and scale-up purposes. Based on the modelling results, non-equilibrium Hydrus model could describe the experimental data well (R² > 0.98). Ball milling was used for degradation of adsorbed PRO and regeneration of MTC. The adsorbed PRO onto MTC was degraded more than 79% after 5 hours milling at 550 rpm and the addition of quartz sand increased the efficiency to 92%. In the context of the circular economy, this ELTs based low-cost magnetic adsorbent (estimated at 299 US$/tonne) can be potentially used at full-scale industrial wastewater treatment for the removal of pharmaceutical drugs from aqueous solution, offering sustainable environmental remediation. The cost analysis showed that unit cost for treatment of wastewater using fixed-bed columns of tyre char was calculated to be 1.57 US$/m³ which can be deemed as commercially feasible.