Effects of water components on the advanced oxidation of a veterinary antibiotic, oxytetracycline

Abstract

Antibiotics are an important group of pharmaceuticals in today's medicine. They are used for the treatment of human and animal infections and they are used as growth promoter in animal feeding operations. Human and veterinary antibiotics are continually being released into the environment mainly as a result of manufacturing processes, disposal of unused or expired products, and excreta. Veterinary drugs may enter into the environment more directly than does human drugs. The existence of antibiotics in the environment and their possible effects on living organisms are giving rise to growing concern. Depending upon their physical and chemical properties, many of antibiotic substances or their bioactive metabolites end up in soils and sediments. Surface runoff and leaching cause the transport of the antibiotics from soil to surface and groundwater. In addition, effluent of sewage treatment plant can constitute a source for antibiotic pollution in the surface water. Bacterial resistance is a significant problem related with the presence of antibiotics in the environment. These compounds have also an important exerting toxic effect to aquatic organisms even in the μg L-1– mg L-1 concentration range that change the ecological balance negatively. Conventional treatment processes are unable to eliminate pharmaceuticals in water and wastewater, thus it is necessary to investigate advanced treatment technologies for antibiotic pollution control. Different advanced treatment technologies have been recently evaluated for this purpose, including chemical oxidation using ozone and ozone/hydrogen peroxide, membrane filtration such as nano-filtration and reverse osmosis, and activated carbon adsorption. Among these ozone oxidation and heterogenous photocatalysis can be a promising process for degradation of pharmaceuticals. However, in these studies the effects of water components on the oxidation of antibiotics were not investigated. The tetracyclines (TCs) are broad-spectrum antibacterials widely used in human and animal medicine. Tetracycline, oxytetracycline (OTC), and chlortetracyclines (CTC) are widely used in animal feeds to maintain health and improve growth efficiency in many countries. These chemicals are characterized by a partially conjugated four-ring structure with a carboxyamide functional group. The molecule of tetracycline has several ionizable functional groups of a rather unusual type, and the charge of the molecule depends on the solution pH. The present investigation was aimed to study the treatment of water synthetically contaminated with a tetracycline group antibiotic by TiO2 mediated photocatalytic oxidation and ozone oxidation. While the effects of initial antibiotic concentration, pH, and H2O2 concentration on the performance of photocatalytic degradation were investigated in ozone oxidation experiments, the effects of pH and applied ozone dose on the degradation of antibiotic were studied. The antibiotic treatment performances of both oxidation processes were also investigated in the presence of Ca2+, HCO3 -, NO3 -, PO4 3-, SO4 2-, Cl- ions, and humic acid to observe the effect of water components on the degradation of antibiotic. All investigated anions decreased the photocatalytic degradation rate of OTC depending on the reaction rate constants of them with the hydroxyl radicals. The effect of initial OTC concentration in the photocatalytic process was analyzed using the linear form of the Langmuir–Hinshelwood (L-H) model and k and K values calculated from the slope of straight line and from the intercept with the 1/r0 axis as 6.99x10-6 M min-1 (3.47 mg/L min-1) and 1.613x104 M-1 (0.032 mg-1 L), respectively. In the ozonation process, degradation rate of OTC was increased by increasing the applied ozone dose. The pseudo first-order degradation rate constant of OTC was 1.67 min-1 with an applied ozone dose of 606 mg L-1 h-1and it was increased to 4.18 min-1 by raising the ozone dose to 1,086 mg L-1 h-1 However, further increase in the applied ozone dose led to only a slight enhancement in the OTC degradation rate. The results of the experiments performed at three different pH values (pH 4, 7 and 9) showed that the highest degradation rate of OTC was obtained at pH 7. The assessment of the results showed that, in case of ozone oxidation of OTC, water components significantly decreased the degradation rate and in contrary to the results obtained by photocatalytic oxidation, the effects of ions were not depended on the reaction rates of them with hydroxyl radicals.

Özet

Antibiyotikler insan ve hayvan hastalıklarının tedavisinde ve hayvanlarda büyümeyi destekleyici olarak yaygın bir şekilde kullanılırlar. Organizmaya uygulanan antibiyotiklerin %90’a varan oranları metabolize olmadan vücuttan atılırlar. Bu nedenle, çevredeki antibiyotik kirliliğinin ana kaynağını teşkil eden insan ve hayvan dışkısı yüksek miktarda antibiyotik içerebilir. Yapılan çeşitli çalışmalarla antibiyotiklerin çevrenin çeşitli kompartımanlarının yanısıra hayvan dışkısı ve evsel atıksu arıtma çamurunda bulunduğu tespit edilmiştir. Antibiyotikler, fiziksel ve kimyasal özelliklerine bağlı olarak toprağa, sedimentlere, ve yeraltı sularına ulaşabilmektedirler. Sularda bulunan düşük derişimlerindeki antibiyotiklerin gideriminde konvansiyonel arıtma yöntemlerinin yetersiz kaldığı bilinmektedir. Bu maddelerin yüksek konsantrasyonlarının çevrede bulunması, mikroorganizmalar üzerinde toksik etkiye neden olarak ekolojik dengenin bozulmasına, düşük konsantrasyonları ise patojen ve patojen olmayan bakterilerin antibiyotik direnci kazanmasına neden olabildiğinden antibiyotik kirliliğinin kontrolünde alternatif arıtım metotları gerekmektedir. Bu çalışmada, sulu çözeltide bulunan bir tetrasiklin (TC) grubu antibiyotiğin fotokatalitik ve ozon oksidasyon prosesleri ile arıtımı incelenmiştir. 0.1 mM antibiyotik, ozon oksidasyonu ile birkaç dakikada tamamen giderilirken, aynı sonucun fotokatalitik oksidasyon prosesi ile elde edilmesi için 60 dakikalık bir süre gerekmiştir. Her iki oksidasyon prosesinde pH 7 değerinde daha yüksek antibiyotik oksidasyonu elde edilmiştir. Su bileşenlerinin antibiyotik arıtım verimi etkilerini incelemek amacıyla fotokatalitik ve ozon oksidasyon prosesleri Ca2+, HCO3 -, NO3 -, PO4 3-, SO4 2-, Cl- iyonları, ve hümik asidin, varlığında gerçekleştirilmiştir. Bu çalışmada seçilen katyon ve anyonların fotokatalitik ve ozon oksidasyon verimlerini düşürdükleri saptanmıştır. Antibiyotiğin fotokatalitik oksidasyon hızı, su bileşeni olarak çözeltiye eklenen anyonların hidroksil radikali tutma reaksiyon hız sabitlerine bağlı olarak azalmış, ancak ozon oksidasyon prosesinde bu ilişki gözlenmemiştir.