Evaluation of Lightweight Expanded-Clay Aggregates as Bed Material in Constructed Wetlands for Attenuation of Antibiotics

Recently, lightweight expanded-clay aggregates (LECAs) have emerged as a promising material for various geoenvironmental applications. This study aimed to explore the potential of LECAs as an adsorbent in a laboratory-scale constructed wetland to mitigate the impact of antibiotics, specifically erythromycin (ery) and doxycycline (doxy). The physical characteristics of LECA were determined, including hydraulic conductivity of $1.12\times {10}^{-3}\mathrm{m}/\mathrm{s}$, a specific surface area of $\mathrm{2,890}{\mathrm{m}}^2/\mathrm{kg}$, and a pH value of 7.6. In the laboratory-scale batch study, Langmuir ($K_L=3.2565\mathrm{L}/\mathrm{mg}$) and Freundlich [$K_f=0.2376$ ($\mathrm{mg}/\mathrm{g}$) ($\mathrm{L}/\mathrm{mg}$)] isotherm models provided the best fit for doxy and ery, respectively. Additionally, the pseudo-second-order kinetic model exhibited the best fit for both antibiotics. The adsorption of doxy was primarily attributed to $\pi –\pi$ interactions and hydrogen bonding, explaining why it followed the Langmuir isotherm with most sorbents. On the other hand, ery exhibited electrostatic sorption and cation exchange as the dominant mechanisms, potentially demonstrating its multilayer sorption behavior. A shift from an acidic to an alkaline pH significantly enhanced the adsorption of both doxy (16% to 94%) and ery (45% to 89%). Similarly, raising the temperature from 5°C to 45°C increased the sorption capacity to 83% for doxy and 88% for ery. In the one-dimensional vertical-column study using LECA as the adsorbent, the exhaustion time for ery and doxy was determined to be 70 and 84 h, respectively. These results aligned well with the findings obtained from the HYDRUS model. Constructed wetlands employing LECA beds demonstrated remarkable removal efficiencies, with doxy removal from 93% to 96% and ery removal from 92% to 97%. A long-term study on LECA after pouring studied antibiotics solution in cycles revealed an appreciable drop in removal efficiencies at the end of an extensive period. Based on these observations, it can be concluded that LECA possesses a significant adsorption capacity against doxy and ery due to high in situ pH, specific surface area, and hydraulic conductivity. Under acidic conditions, ery exhibited higher sorption capacity in comparison to doxy because of the preponderance of ${\mathrm{H}}^{+}$ ions. However, in an alkaline state, the abundance of ${\mathrm{OH}}^{-}$ ions hindered the release of ${\mathrm{H}}^{+}$ ions, resulting in minimal changes in the adsorption capacity of ery, unlike doxy, which displayed a steady increase. Furthermore, elevated temperatures enhanced the adsorption capacity by accelerating intraparticle diffusion.

The use of LECA in constructed wetlands has emerged as a cost-effective, sustainable, and highly efficient strategy for mitigating the release of antibiotic-laden wastewater into the environment, particularly water bodies. Extensive research has already established LECA as an exceptional sorbent, and this study further reinforces its remarkable efficacy in adsorbing antibiotics from aqueous solutions. These findings position LECA as a leading candidate for implementation as bed material in scaled-up constructed wetland systems in actual field conditions. The long-term advantages of utilizing LECA are manifold. First, it offers easy production, ensuring a readily available supply. Additionally, LECA exhibits a long life span, durability, and the ability to withstand changes in pH and temperature without degradation or decomposition. These characteristics make it a reliable and robust material for sustained use in constructed wetlands. Moreover, the simplicity of the approach, requiring minimal expertise for setup and maintenance, makes LECA an attractive option for government organizations, pharmaceutical industries, and rural communities alike. The broad applicability of LECA extends beyond antibiotic removal, because it effectively eliminates contaminants from wastewater, thereby safeguarding surface and groundwater bodies from pollution. Thus, LECA is an ideal solution for removing antibiotics and other pollutants from wastewater in constructed wetland systems. Its low-cost production, longevity, resilience, and user-friendly nature make it an appropriate choice for large-scale implementation. By adopting LECA, the impact of antibiotic contamination on the environment can be significantly reduced and can help preserve the integrity of precious water resources.