Abstract
Iron (Fe) (hydr)oxides control the mobility and bioavailability of tetracycline (TC) in waters and soils. Adsorption of TC on Fe (hydr)oxides is greatly affected by polyvalent metals; however, impacts of molar metal/TC ratios on TC adsorptive behaviours on Fe (hydr)oxides remain unclear. Results showed that maximum TC adsorption on ferrihydrite and goethite occurred at pH 5–6. Such TC adsorption was generally promoted by the addition of Cu2+, Zn2+ and Al3+. The greatest increase in TC adsorption was found in the system with molar Cu/TC ratio of 3 due to the formation of Fe hydr(oxide)–Cu–TC ternary complexes. Functional groups on TC that were responsible for the complexation with Cu2+shifted from phenolic diketone groups at Cu/TC molar ratio < 1 to amide groups at Cu/TC molar ratio ≥ 1. For the addition of Al3+, the complexation only took place with phenolic diketone groups, resulting in the enhanced TC adsorption at a molar Al/TC ratio of 1. However, TC adsorption decreased for Al/TC molar ratio > 1 as excess Al3+ led to the competitive adsorption with Al/TC complexes. For the Zn2+ addition, no significant correlation was found between TC adsorption capacity and molar Zn/TC ratios.
Highlights
Up to 13.5 million kilograms of antimicrobial chemicals were sold in the USA in 2011, and 80% of these chemicals were antibiotics, which were applied in livestock production as non-therapeutical administration [1]
When the system pH was higher than the point of zero charge (PZC) of Feoxides, the emergence of negative charges on the surfaces of Feoxides repulsed the anionic TC, resulting in a decrease in TC adsorption
A favourable electrostatic attraction between TC and Feoxides was observed at the intermediate pH range, causing a bell-shaped adsorption curve as a function of pH with the maximum TC adsorption at the pH close to the pHpzc of the adsorbents [18,25,28,29]
Summary
Up to 13.5 million kilograms of antimicrobial chemicals were sold in the USA in 2011, and 80% of these chemicals were antibiotics, which were applied in livestock production as non-therapeutical administration [1]. Tetracycline (TC) is one of the most widely used antibiotics, which is responsible for around 40% of antimicrobial application in swine production in the USA [2,3]. TC is commonly applied in human infection medicines, veterinary medicines and animal growth promoters. Because only small amounts of TC would be absorbed in the digestive tract, up to 50–80% TC is excreted unmetabolized via faeces and urine [4,5]. Since animal wastes or manures are directly discharged to drainage and applied as fertilizers in agriculture activities, TC might be widely distributed over waters and soils. Owing to the absence of efficient adsorbents, TC is relatively preserved and/or mobile in these environments, resulting in the dissemination of antibiotic-resistant pathogens that threaten biota and disrupt indigenous microbial populations [6,7,8,9]
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