Selective adsorption of tetracycline by b-CD-immobilized sodium alginate aerogel coupled with ultrafiltration for reclaimed water

Abstract

In this work, a novel composite material based on b-cyclodextrin-immobilized sodium alginate aerogel (b-CD/NaAlg) was developed utilizing cross-linker of epichlorohydrin and applied as an adsorbent to remove tetracycline antibiotics from reclaimed wastewater. A series of characterizations were utilized to confirm the successful synthesis of the adsorbent and this b-CD/NaAlg presented a three-dimensional network at the nanoscale or microscale. Under optimal conditions (pH ¼ 4, t ¼ 8 h, b-CD: NaAlg ¼ 9, adsorbent dosage ¼ 1.5 g$L-1), the maximum removal rate of b-CD/NaAlg to tetracycline was 70%. The adsorption behavior of tetracycline on b-CD/NaAlg conformed to the Freundlich isotherm model (R2 ¼ 0.9977) and the pseudo-second-order kinetic model (R2 ¼ 0.9993). Moreover, the adsorbent still removed 55.3% of tetracycline after five cycles. Specially, the adsorbent was integrated with ultrafiltration to adsorb tetracycline antibiotics from simulated reclaimed wastewater, and the removal rate of tetracycline reached 78.9% within 2 h. The existence of Cr (VI) had a negligible impact on tetracycline removal, while the presence of humic acid exhibited a promoting effect. The possible adsorption mechanisms were also elucidated through X-ray photoelectron spectroscopy and density functional theory analysis. In summary, b-CD/NaAlg represents an environmentally friendly, efficient, and sustainable adsorbent for removing tetracycline antibiotics from reclaimed water.

1. Introduction

The presence of even minute quantities of antibiotics in wastewater poses a substantial threat to both human health and ecosystems [1]. Tetracycline (TC) antibiotics, celebrated for their broad-spectrum attributes, have been identified in substantial concentrations within aquatic environments [2]. Their residues can result in digestive issues in humans and interference with the synthesis of interconnected proteins in freshwater algae and duckweed by inhibiting photosynthesis [3]. Hence, it holds significance to alleviate the adverse impacts posed by TC antibiotics in the environment.

Numerous advanced methodologies, encompassing chemical oxidation, microbiological approaches, ultrasonication, and adsorption, have been explored as alternatives for conventional methods to efficiently eliminate TC antibiotics in aquatic settings [4]. In the realm of water treatment, ultrafiltration (UF) membranes have been observed to produce higher flux and consume less energy than nanofiltration or reverse osmosis. However, due to their larger pore size, the rejection of desired antibiotics may not be as effective [5]. Conversely, adsorption, celebrated for its “green material, metal-free, and stable” nature [6], is considered an effective method. This effectiveness is attributed to its operational simplicity, elevated reactivity, cost-effectiveness, and environmental compatibility [7]. Despite these advantages, enhancing the selectivity of adsorbent materials remains a significant challenge. Recent research focus has shifted toward exploring the functionalization and modification of both synthetic and natural adsorbents. Furthermore, while numerous reports exist on water treatment involving adsorbents integrated with membrane filtration, the approach of eliminating TC antibiotics via a hybrid aerogel adsorption-ultrafiltration method has received comparatively limited attention.