Removal of hormones and antibiotics by nanofiltration membranes
Introduction
There is a growing awareness of the importance of trace levels of organic compounds as contaminants originating from industrial, agricultural, medical and domestic uses. Compounds used in personal care products, pharmaceuticals and other consumables as well as hormones may enter aquatic environments after passing through wastewater treatment plants, which often are not designed to remove these chemicals [1], [2]. In addition, veterinary pharmaceuticals and growth promoters used in animal husbandry may be released directly to the environment with animal wastes through overflow or leakage from storage facilities or land application [1], [3].
As early as 1973, Norpoth et al. [4] indicated that the use of contraceptives may cause severe long-term problems due to the high persistence and biological activity of those compounds in the environment. Evidence now exists that hormones and pharmaceuticals are widespread in effluents of sewage treatment plants [5]. One of the first results concerning environmental occurrence of pharmaceuticals was reported by several researchers [6], [7], [8] who detected clofibric acid in treated sewage in the US. Further studies were obtained in Great Britain [9] and Canada [10]. However, extensive investigation of the occurrence of hormones and pharmaceuticals in the environment began in the 1990s, when the first analytical methods were developed allowing for the determination of pharmaceuticals in aqueous matrices [11].
Many scientific reports have documented the environmental and health implications of hormones and antibiotics. Although, the concentrations of hormones and antibiotics in drinking water and wastewaters are at low levels (ng/l), these compounds may accumulate in animals. Several studies have suggested a link between environmental exposures to hormones and deteriorating trends in human health including decreases in male sperm count; increase in testicular, prostate, ovarian and breast cancer; and reproductive malfunctions [12], [13], [14], [15], [16], [17]. Desbrow et al. [18] found levels of hormones in domestic effluent samples at concentrations up to 80 ng/l. A recent study conducted by the US Geological Survey on fresh water resources that receive effluents from across the US showed the occurrence of estradiol and estrone in approximately 7–10% of the water samples with maximum concentrations up to 93 and 112 ng/l, respectively [1]. Estradiol concentrations ranging from 6 to 66 ng/l have been found in ground water [19] and in the South Nevada water system at 2.6 μg/l [20], [21]. More than 70 different pharmaceutically active compounds have been detected at concentrations up to the μg/l level in sewage effluents, surface waters, groundwater and drinking water [22]. Recent studies showed that tetracycline as high as 4 μg/l and chlorotetracycline at 1.2 μg/l have been detected in municipal wastewater [23], [24]. According to a study obtained in 144 different water samples collected between April 1999 and April 2001 [25], tetracyclines and sulfanamides were detected in samples from 9 sites (6% detections) in concentrations ranging from 0.07 to 15 μg/l. The majority of these detections were from surface water sites. Only one site had detection in groundwater. This sample was a groundwater site from Washington and contained the sulfamethaxazole. Overall, 7 of the 144 groundwater and surface water sites were found to contain sulfanamides and six sites were found to contain tetracyclines. Chlortetracycline was detected at 0.15 μg/l in one surface water sample, and the most commonly detected tetracycline was oxytetracycline.
The occurrence of these materials in natural waters has led to a search for treatment methods to remove hormones and antibiotics. This concern is particularly critical for water reuse applications where there is a potential for concentration of these contaminants in the course of repeated water recycling. Coagulation alone is generally not effective in removing these trace-level organic compounds. However, activated carbon adsorption, advanced oxidation and membrane filtration can effectively remove trace organic compounds [8]. Oxidation of EDCs and pharmaceuticals can result in reaction and transformation of these compounds [25]. Removal of tetracyclines was investigated using activated sludge at different sludge and hydraulic retention times and removal efficiencies of 80–85% were obtained [23].
Membrane filtration using nanofiltration (NF) and reverse osmosis (RO) membranes is one of the most promising techniques for the removal of hormones [21] and antibiotics [26], [27], [28], [29]. However, there are few data available on the rejection of these chemicals by NF and RO membranes, particularly under conditions present in wastewater treatment plant systems where there may be multiple species interacting in solution and on the membrane surfaces. There is some indication that interaction with naturally occurring solutes such as natural organic matter may enhance the removal efficiency of NF and RO membranes [30], [31], [32].
During the early stages of membrane filtration, adsorption on the membrane may play an important role in reducing the concentration of hormones that move across the membrane. However, after the adsorption capacity has been saturated, the apparent removal efficiency may decrease due to the partitioning and subsequent diffusion of the hormones [21]. Adams et al. [27] evaluated the conventional drinking water treatment processes including RO to determine their effectiveness in the removal of seven common antibiotics. In these experiments, reverse osmosis was shown to be effective in removing all of the studied compounds. Drewes et al. [33], investigated the different treatment technologies (activated sludge, tricking filter, NF and RO) for removing pharmaceuticals at full scale facilities. None of the drugs investigated was detected in tertiary treated effluents after NF and RO. Ngheim et al. [29] investigated the removal of sulfanamides by NF membranes and determined that retention of pharmaceuticals by a tight NF membrane is dominated by size exclusion, whereas both electrostatic repulsion and size exclusion govern the retention by loose NF membranes.
The objective of this study is to elucidate the removal mechanism of antibiotics and hormones by NF membranes in mixed solutions. The effects of solution chemistry, organic matter and salinity were investigated. In addition, the interactive effects of hormones on antibiotic removal and antibiotics on hormones removal were also investigated.
Section snippets
Experimental set-up
Experimental procedures used in this work have been previously described [34] and are summarized here. Experiments were performed using a dead-end filtration cell. The dead-end filtration (DEF) set-up consisted of a 300 ml stirred cell (Sterlitech, HP4750) pressurized with air. The active membrane area of the DEF cell was 14.6 cm2 and a sample volume of 200 ml was used in each experiment. Permeate flux was determined by weight using a Scientech 5200 model electronic balance and the results sent to
Water matrix and chemicals
Synthetic solutions were prepared for the experiments. Antibiotics and hormones were mixed with 10 mM calcium chloride, 10 mg/l humic acid, and 10 mM NaCl. In addition to these synthetic solutions, 10 ppb antibiotics and hormones were mixed with tap water to analyze the effect of slightly more complex water compositions. Tetracyclines, sulfanamides and selected hormones were used for the experiments. A list of chemicals and their characteristics are given in Table 1. pH of the solutions were
Results and discussion
Retention of solutes by NF membranes can be affected by several factors such as adsorption, charge effect, straining, etc. [28]. Two measures of solute/membrane interaction were followed in this study; namely, the amount of hormones and antibiotics adsorbed on membrane surface and apparent retention/rejection of these compounds across the membrane.
Conclusions
The solution chemistry, organic matter and salinity affect the rejection of tetracycline's and sulfanamides and selected hormones by NF membranes. Tetracyclines have a high adsorptive affinity for the membrane while the adsorption rates for hormones are lower. An addition of antibiotics to hormone solution increases the hormone rejections while almost complete rejections were observed for tetracyclines. Not only the humics and other organics but also antibiotics influence the hormone rejection.
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