Efficacy Evaluation of Silty-Sandy Soil and <i>Chrysopogon zizanioides</i> to Attenuate Doxycycline from Wastewater in a Constructed Wetland System

Abstract

In recent years, the emergence of COVID-19 has created disastrous health effects worldwide. Doxycycline, a member of the tetracycline group, has been prescribed as a treatment companion for attending this catastrophe. Due to extensive use and high solubility, a significant amount of un-metabolized doxycycline has been found to reach water bodies within a short time, and consumption of this water may lead to the development of fatal resistance in organisms and create health problems. Therefore, it has become necessary to develop suitable technologies from a geoenvironmental point of view to remove these unwanted antibiotics from wastewater. In this context, locally obtainable silty-sandy soil was explored as a low-cost material in a constructed wetland with Chrysopogon zizanioides (vetiver sp.) for phytoremediation to mitigate doxycycline spiked wastewater. The obtained soil hydraulic conductivity was 1.63×10−7 m/s. Batch adsorption tests conducted on silty-sandy soil, vetiver leaf, and vetiver root provided maximum removal efficiencies of 90%, 72%, and 80% percent, respectively, at optimal sorbent doses of 10 g/L, 17 g/L, and 16 g/L, and contaminant concentrations of 25 mg/L, 20 mg/L, and 23 mg/L, with a 30-min time of contact. The Freundlich isotherm was the best fit, indicative of sufficient sorption capacity of all the adsorbents for doxycycline. The best match in the kinetic research was pseudo-second-order kinetics. A one dimensional vertical column test with the used soil on doxycycline revealed a 90% breakthrough in 24&nbsp;h for a soil depth of 30&nbsp;mm. Studies on a laboratory-scale wetland and numerically modeled yielded removal of around 92% by the selected soil and about 98% combined with Chrysopogon zizanioides for 25 mg/L of initial doxycycline concentration, which is considered quite satisfactory. Simulated results matched the laboratory tests very well. The study is expected to provide insight into remedies for similar practical problems. To explore a cost-effective method for abatement of aqueous doxycycline in the surrounding environment and groundwater, a locally accessible silty-sandy soil was selected. While investigating its suitability, it was found that this soil met current standards (10−4 to 10−5 cm/s) for hydraulic conductivity of wetland adsorbent. The selected soil also had a relatively large surface area (23.502 m2/g), and the presence of higher percentages of finer particles (50%) resulted in an outstanding adsorptive and removal capacity for doxycycline, as indicated by several test findings and validated with numerical modeling. Furthermore, as evidenced by batch experiments, the vegetative cover, primarily provided by vetiver, was also found to be an effective adsorbent. Sorption of a particular contaminant in wastewater to wetland soil is problematic due to the simultaneous presence of various compounds and other wastewater contaminants. The competition effect among these compounds is of concern in any treatment wetland, where thousands of compounds are present. Hence, further studies are required to address the competition between these compounds and their cumulative effect on the efficacy of the wetland system over a long period. This has therefore cemented the fact that silty-sandy soil is a perfect choice as bed material for wetlands. Together with vetiver, it has the potential to become a stable and environment-friendly removal system against doxycycline. However, the investigation was carried out with a spiked doxycycline solution in the laboratory, the results of which are believed to provide a basic understanding and remedial measures for this problem. Based on that, if several field studies are conducted with the studied soil using phytoremediating plants in doxycycline-laden wastewater, a real-life solution to the doxycycline problem may be obtained that is very much needed for assuage of social health problems.