Quantification of Selected Pharmaceuticals in Wastewater and Their Removal by S/TiO₂ Doped Carbon Nanotubes Synthesized from Macadamia Nutshells

Abstract

Pharmaceutical contaminants in wastewater have become a significant environmental concern due to their potential impacts on aquatic ecosystems and human health. This study examined the occurrence of four pharmaceutical contaminants, namely ibuprofen (IBU), efavirenz (EFA), diclofenac (DIC), and levonorgestrel (LVG), in wastewater treatment plants (WWTPs) across the Vhembe and Mopani Districts in Limpopo province, South Africa. The study further synthesised S/TiO₂ doped carbon nanotubes for application in the removal of these pharmaceutical contaminants from wastewater. The findings are presented in five sections. The first section investigates the occurrence of IBU, EFA, DIC, and LVG in wastewater from Vhembe and Mopani Districts, while the second section presents the optimisation of solidphase extraction (SPE) conditions for the simultaneous quantification of efavirenz and levonorgestrel using high-performance liquid chromatography (HPLC). The third and fourth sections focus on the synthesis of S-doped and TiO₂-doped carbon nanotubes from macadamia nutshells for application in the removal of efavirenz and levonorgestrel from wastewater, respectively. The fifth section focuses on the synthesis and evaluation of S/TiO₂ carbon nanotube composite for the effective removal of the selected pharmaceutical contaminants from wastewater. The first section of results showed that the concentrations of ibuprofen, efavirenz, diclofenac, and levonorgestrel in wastewater treatment plants ranges from below the limit of detection (LOD) to 114 μg/L, 3.81-11.9 μg/L, 2.21-25.0 μg/L, and 6.2-8.09 μg/L in the influent and LOD-59.9 μg/L, 0.69-6.3 μg/L, 0.36-5 μg/L and 4.25-20.9 μg/L in the effluent, respectively. The presence of these contaminants in wastewater effluents could result in their accumulation in receiving streams and negatively impact aquatic ecosystems. As such, routine monitoring of pharmaceutical contaminants in these regions and their removal is recommended. Efavirenz and levonorgestrel were dominant in all wastewater treatment plants; therefore, the second section of results focused on optimising the SPE conditions, such as pH, solvent concentration, and elution volume, for the preconcentration of EFA and LVG to enhance their simultaneous detection using an HPLC coupled with a photodiode array detector (PDA). The conditions found were a pH of 2, a 100% methanol solvent concentration, and an elution volume of 4 mL using HLB cartridges. These conditions yielded percentage recoveries of 83% and 94.61% for EFA and LVG, respectively. The accuracy of the HPLC method was confirmed through a coefficient of determination (R²) > 0.98, derived from the calibration curves of the respective target contaminants. The limits of detection (LOD) and quantification (LOQ) for efavirenz were determined to be 0.705 μg/L and 0.14 μg/L, respectively, whereas for levonorgestrel, these values were 0.061 μg/L and 0.199 μg/L. The optimised solid phase extraction conditions were employed in subsequent experiments to quantify the residual EFA and LVG concentrations. In the third and fourth sections, S-doped carbon nanotubes (S-CNT) and TiO₂-doped carbon nanotubes (TiO₂-CNT) were synthesised via a pyrolysis process using macadamia nutshells and used for the removal of EFA and LVG. The surface properties of the synthesised materials were determined using Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), Energy dispersive x-ray spectroscopy (EDX), X-ray diffractogram (XRD) techniques, and Brunauer–Emmett–Teller (BET). SEM micrographs confirmed the tubular structures of the materials. Batch experiments were conducted to evaluate the effectiveness of S-CNT and TiO₂-CNT in removing EFA and LVG. Experiments were conducted in the presence of a 30 W T6 tubular 254 nm UV germicidal lamp to evaluate the effect of light on EFA and LVG removal. The maximum removal efficiencies for EFA and LVG were 99.81% and 94.64% using S-CNT and 99.43% and 99.94% using TiO₂ CNT, respectively, at a pH of 2, contact time of 90 minutes, an initial concentration of 2.5 mg/L, and sorbent dosage of 0.3 g in the presence of UV light. The removal of EFA and LVG using TiO₂-CNT and S-CNT under UV irradiation involves a synergistic mechanism of photocatalytic degradation and adsorption. Upon exposure to UV light, TiO₂ acts as a photocatalyst, generating electron-hole pairs that produced reactive oxygen species (ROS), including hydroxyl radicals (•OH) and superoxide anions (O₂•−), which then initiated oxidative degradation of EFA and LVG into smaller intermediates (C14H8ClF3NO± and C13H10ClF3N±; C15H21O3‾ and C21H29O3‾). Concurrently, the carbon nanotube component provided a high surface area and facilitates electron transfer, reducing electron-hole recombination and enhancing photocatalytic efficiency. The resulting intermediates, along with residual parent compounds, were subsequently adsorbed onto the CNT surface, ensuring near-complete removal of EFA and LVG. Although S-CNT and TiO₂ - CNT showed strong EFA and LVG removal potential, their effectiveness was limited to acidic pH, which restricts their practical application. The fifth section presents the synthesis of S/TiO₂-CNT nanocomposite and its effectiveness in removing EFA and LVG. The objective was to combine S, TiO₂, and CNT into a single composite to harness their synergistic properties to improve their adsorption, photocatalytic activity, and electron transfer potency for the removal of the pharmaceuticals under mild conditions. The results obtained showed improved EFA and LVG removal efficiency of >99% for both pharmaceutical contaminants at pH 5 and a contact time of 60 minutes, yielding an adsorption capacity of 6.92 mg/g and 6.35 mg/g. The synthesised material exhibited notable photocatalytic activity as its photodegradation performance followed a Langmuir-Hinshelwood pseudo-first-order photodegradation kinetic model, with a coefficient of determination above 0.8 and rate constants of 0.02 and 0.018 min−1 for the respective contaminants, indicating a strong model fit. Regeneration studies confirmed that the synthesised nanocomposites can be reused for up to six cycles, highlighting their cost-effectiveness. The practical applicability of the nanocomposite was further demonstrated by achieving high removal efficiency during field trials using real water samples. The study revealed three major findings: (1) Wastewater in Vhembe and Mopani districts contains pharmaceutical contaminants that are being released into receiving rivers through the effluents; (2) carbon nanotubes doped with S and TiO₂ for application in the treatment of pharmaceutical contaminants can be synthesized from macadamia nutshells; and (3) co-doping CNTs with S and TiO2 enhances their performance in removing pharmaceutical contaminants from wastewater. This study recommends monitoring pharmaceutical contaminants from wastewater. This study, therefore, recommends monitoring of pharmaceutical contaminants in receiving rivers within the Vhembe and Mopani District municipalities and further assessment of their fate in aquatic ecosystems. The study further recommends pilot-scale testing of S/TiO2 CNTs.