Abstract
Effects of pH on adsorption and removal efficiency of ionizable organic compounds (IOCs) by environmental adsorbents are an area of debate, because of its dual mediation towards adsorbents and adsorbate. Here, we probe the pH-dependent adsorption of ionizable antibiotic oxytetracycline (comprising OTCH2 +, OTCH±, OTC−, and OTC2−) onto cyclodextrin polymers (CDPs) with the nature of molecular recognition and pH inertness. OTCH± commonly has high adsorption affinity, OTC− exhibits moderate affinity, and the other two species have negligible affinity. These species are evidenced to selectively interact with structural units (e.g., CD cavity, pore channel, and network) of the polymers and thus immobilized onto the adsorbents to different extents. The differences in adsorption affinity and mechanisms of the species account for the pH-dependent adsorption of OTC. The mathematical equations are derived from the multiple linear regression (MLR) analysis of quantitatively relating adsorption affinity of OTC at varying pH to adsorbent properties. A combination of the MLR analysis for OTC and molecular recognition of adsorption of the species illustrates the nature of the pH-dependent adsorption of OTC. Based on this finding, γ-HP-CDP is chosen to adsorb and remove OTC at pH 5.0 and 7.0, showing high removal efficiency and strong resistance to the interference of coexisting components.
Highlights
Ionizable organic compounds (IOCs) occupy a large fraction of the pre-registered REACH compounds [1]
It is probably due to dual mediation of pH toward antibiotics and adsorbents
Comparison of the KC values (Table 1) demonstrated that molecular recognition of the species was related to cavity size [39] (e.g., b-CD versus c-CD) and structural modification (b-CD versus HPCD/RMCD) of CD (Figure 1)
Summary
Ionizable organic compounds (IOCs) occupy a large fraction of the pre-registered REACH compounds [1]. They have one or more pKa values and are present in the form of a mixture of ionized and unionized species. Conventional natural and synthetic adsorbents themselves (e.g., clays [8,12], humic substances [13,14], activated carbons [15], and CNTs [16]) probably suffer changes in structure and/or chemistry from the variation of pH These changes may render alteration of adsorption sites of adsorbents. The dual mediation of pH complicates adsorption behavior of antibiotics, hereby making an impediment to the understanding of their pH-dependent adsorption
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