Novel Ultrafine Composite Biochar for Adsorption and Co-adsorption of Potentially Toxic Elements and Pharmaceutical Compound from Aqueous Solution

Abstract

The co-existence of potential toxic elements and pharmaceutical compounds in aquatic environment is a growing global concern due to their low biodegradability, high persistence, and ease of bioaccumulation. Biochar is a carbonaceous solid material produced via pyrolysis process in the absence of or under limited oxygen conditions. Recent studies have shown that biochar can be a potential adsorbent to remove heavy metals and pharmaceutical compounds. Modifying raw biochar by using green method would also potentially enhance the adsorption capacity for single and multi-contaminants. The overarching aim of this study was to investigate the feasibility of two novel engineered biochar composites for the removal of copper (Cu), lead (Pb), and metoprolol (a pharmaceutical compound). The composites were prepared by pyrolysing cabbage leaves at 350°C after pretreating them with anthocyanin, followed by a post-treatment with kaolinite. The results indicated that the kaolinite-biochar composite (KB) exhibited the highest adsorption capacity for Pb (188.67 mg/g) and Cu (48.07 mg/g) at pH 5, while the anthocyanin-biochar composite (AB) exhibited the highest adsorption capacity for metoprolol (41.15 mg/g) at pH 6, compared to raw biochar. X-ray photoelectron spectroscopy (XPS) and Attenuated Total Reflectance-Fourier transform infrared (ATR-FTIR) analysis confirmed that the enhancement of adsorption in KB and AB was due to an increase in specific oxygen functional groups. The pseudo-second-order kinetic model and Langmuir isotherm model provided the best fit for the adsorption of Pb, Cu, and metoprolol in KB and AB composites, respectively. The sorption mechanisms involved electrostatic interaction, ion exchange, and complexation for the metals, while electrostatic interaction, H-bonding, π-π interaction, and hydrophobic bonding were postulated for metoprolol sorption. Furthermore, ball milling was applied to the composite biochars to enhance their sorption abilities. Optimization of the ball milling process, including dosage of biochar, number of balls, speed, time, and size, resulted in improved physicochemical properties of the adsorbent. The surface area of non-milled biochar increased by approximately ten times after ball milling, and additional functional groups were present. Ball-milled kaolinite composite biochar (BMKB) and anthocyanin composite biochar (BMAB) showed significantly higher removal efficiencies, with BMKB achieving 162.9 mg/g for Pb and 48.5 mg/g for Cu, and BMAB achieving 76.3 mg/g for metoprolol. The simultaneous sorption of Pb, Cu, and metoprolol in aqueous solutions revealed that BMKB had a higher adsorption capacity for Pb and Cu, while BMAB had a higher capacity for metoprolol. Fixed-bed column experiments were conducted using ball-milled kaolinite biochar (BMKB) to evaluate the co-adsorption of Pb, Cu, and metoprolol. Parameters such as bed depth, flow rate, and influent concentrations were investigated, and the breakthrough curves were analyzed using the Thomas and Yoon–Nelson models. The Thomas model exhibited good agreement with the experimental data for describing the breakthrough behavior. The preliminary cost analysis showed that unit cost treatment of Pb, Cu, and metoprolol by an ultrafine composite biochar in a fixed-bed column was much lower than other adsorbent such as activated carbon (AC) based on the available literature data. However, further cost analysis at the pilot-scale and eventually at commercial level for application in actual remediation of wastewater could be a focus for future work.