Clay minerals for pharmaceutical wastewater treatment

CHAPTER

Clay minerals for pharmaceutical wastewater treatment

7

Po-Hsiang Chang1,2, Zhaohui Li3, Wei-Teh Jiang2 and Binoy Sarkar4 1

School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, P.R. China 2Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan 3 Department of Geosciences, University of Wisconsin Parkside, Kenosha, WI, United States 4 Leverhulme Centre for Climate Change Mitigation, Department of Animal and Plant Sciences, The University of Sheffield, Sheffield, United Kingdom

7.1 INTRODUCTION In recent years, with the rapid development of chemical industries, numerous chemicals have become closely linked with human life. Since the late 1970s, chemicals in the environment and their harmful effects have drawn widespread attention from the international environmental science community and even the public. Most of these chemical pollutants are toxic, microcontent, and bioenriched. The Stockholm Convention on persistent organic pollutants (POPs), adopted in Stockholm, Sweden in May 2001, has been signed by 151 countries and ratified by 83 countries, declaring people’s attention and determination to harmful chemicals in the environment, and has managed to put efforts toward improving the environment. With the development of environmental science, this understanding and determination have been continuously strengthened and deepened. However, compared with POPs, people are most often exposed to the presence, migration, and transformation of pharmaceuticals and personal care products (PPCPs) type chemicals in their daily lives, and these may cause negative impacts, but have been always neglected—in particular, antibiotics, which are closely related to people’s lives and widely used in aquaculture, poultry breeding, and food processing. The research on the behavior of antibiotics in the environment started relatively late in the developing countries, especially for China where these compounds have been widely used for a long time. Recently, it has aroused the concern of scholars and the general public. PPCPs include a wide range of chemicals such as various prescription drugs and nonprescription drugs (e.g., antibiotics, steroids, antiinflammatories, sedatives, antiepileptics, developers, painkillers, antihypertensives, contraceptive drugs, hypnotics, diet pills, etc.), cosmetics, refiners, soaps, fragrances, shampoos, hair sprays, hair dyes, and the like (Marta et al., 2004). Although the half-lives of PPCPs are not Modified Clay and Zeolite Nanocomposite Materials. DOI: https://doi.org/10.1016/B978-0-12-814617-0.00011-6 Copyright © 2019 Elsevier Inc. All rights reserved.

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very long, they have been “fake continued” due to their frequent use. More and more data show that PPCPs have become extremely important emerging pollutants in the environment, causing widespread concern in the society. Among them, the environmental pollution caused by the abuse of antibiotics has become one of the current research hotspots (Batchelder, 1981, 1982; Figueroa et al., 2004; Figueroa and Mackay, 2005). According to the Union of Concerned Scientists (UCS, 2001), 30% of the 16,200 tons of antibiotics were used in humans, and 70% in animals in the United States in 2000; 65% of the 13,288 tons of antibiotics were consumed as medicinal drugs, 29% used in veterinary drugs, and 6% used in animal growth promoters in the EU and Switzerland in 1999 (UCS, 2001). The amount of antibiotics used in various countries is also very alarming. Thus, antibiotic contamination in the environment mainly comes from the use of medical drugs and veterinary drugs. In summary, the total amount of antibiotics used in the livestock and poultry industries is huge, and it shows an increasing trend year by year. As analyzed above, with the extensive use of antibiotics, the increasing resistance of bacteria to antibiotics and the feminization of aquatic organisms have become two major health challenges. Abuse of antibiotics in humans can directly cause the individual’s resistance. It has been reported that 25% 75% of the antibiotics used in animals are excreted as their prototype from the excrement (Elmund et al., 1971; Feinman and Matheson, 1978; Addison, 1984), and are in the long term retained in manures and soils (Gavalchin and Katz, 1994; Donoho, 1984). It was reported that antibiotics excreted from animal excrement and urine could even exceed 90% of their doses (Kroker and Aspekte, 1983). Therefore, a considerable proportion of veterinary antibiotics (VAs) enter into the environment as their prototype or metabolites through livestock manure, thus posing a potential threat to the soil environment. Clay minerals are abundant in the nature. Due to the characteristics of their complex and controllable structures, they could be reconstructed effectively via ion exchange resulting in a wide range of applications and high economic value. Clay minerals have applications as environmental remediation agents (e.g., air and water pollution treatments), environmental decontamination agents (e.g., sterilization, disinfection, separation, etc.), and environmental substitution agents (to replace the adsorbent materials with high environmental load). Clay minerals used as adsorbents have a unique role in wastewater treatment. However, activated carbon has been the most conventional adsorbent material for a long time. Activated carbon is widely used because most of the heavy metals, organic compounds, and biopolymers can be adsorbed and removed by this adsorbent, and the adsorption capacity is high. However, activated carbon is more expensive than clay minerals such as montmorillonite or kaolinite. Indeed, this disadvantage limits the wide application of activated carbon for environmental remediation. Therefore, seeking and developing a cheap, high-performance new adsorbent material to replace activated carbon has received more attention. Because of clay minerals’ unique reconstructed structure, they have good adsorption and ion exchange performances. Besides, they have huge reserves in nature, and are cheap

7.2 Pharmaceutical Drugs in the Environment

and environmentally friendly. Thus, clay minerals act as a promising high quality cheap adsorbent. Clay minerals have been used widely in wastewater treatment. There are many published articles on the removal of inorganic and nonmetallic pollutants, colors, heavy metal ions, pesticides, oils and greases from various wastewaters (Booker et al., 1996; Srimurali et al., 1998). In particular, heavy metal laden wastewater treatment is the most studied (Abollino et al., 2003; Bekheit et al., 2011; Zhou et al., 2016). However, medical drugs have become an emerging environmental pollutant, and there are many reports on this research area, such as adsorption, degradation, and plant absorption. Therefore, the discussion and review on the use of clay minerals to remove medical pollutants from wastewater is still insufficient, and it is worth evaluating potential applications of clay minerals in this sector.

7.2 PHARMACEUTICAL DRUGS IN THE ENVIRONMENT 7.2.1 SOURCES Antibiotics pollution in the environment mainly comes from (1) the use of medical drugs, and (2) agricultural veterinary drugs. The use of medical antibiotics mainly comes from iatrogenic prescription slips and at-home self-medication. Among them, the main source of iatrogenic origin drugs is from the hospital, where patients are relatively concentrated, and antibiotics are more frequently used. All kinds of sewage and excrement contain antibiotics, which are the main source of the contaminants in the sewage wastewater. Besides, antibiotics used as a result of family self-medication are also excreted into the domestic sewage through the human body. Multisource data indicate that existing water treatment technologies have no significant effect on the removal of antibiotics contained in wastewaters (Heberer and Stan, 1997; Drewes et al., 2002; Heberer, 2002; Carballa et al., 2004; Seino et al., 2004). For antiepileptic drugs such as carbamazepine, the removal rate was only 8%, while the removal rate of clofibric acid was almost 0; all of these drugs end up in the effluent water of the sewage treatment plant. Therefore, any discharges of hospital effluents or municipal sewage containing antibiotics, whether it is treated or not, can lead to the contamination of surface water, groundwater, and farmland soil environment. On the other hand, VAs are mainly used for the prevention and treatment of animal diseases. The use of subtherapeutic doses in livestock and poultry farming for long-term addition to animal feed has the effect of stimulating animal growth and promoting yield. At present, antibiotics are used in almost all regions of the world to increase production outputs and economic benefits. However, studies showed that only 15% of the antibiotics are available for absorption, about 85% are not metabolized, and are released directly into the environment. For example, 21% of orally administered oxytetracyline (OTC) on sheep is excreted via the

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urine. Furthermore, 17% 75% of chlortetracycline (CTC) is excreted as a prototype form without metabolism (Montforts et al., 1999). According to statistics from the American Institute of Animal Health, there are about 104 110 billion head of livestock, 75 86 billion chickens, 0.60 9.2 billion pigs, and 2.75 2.92 billion turkeys in the United States with different levels of antibiotics (Dresser, 2002; Bloom, 2004). Thus, there are a lot of VAs in the environments where these animals are reared. Antibiotics are also widely used in aquaculture, and commonly used routes of administration include oral or medical dipping. Antibiotics that are not absorbed by aquaculture organisms during the use would be excreted with fecals, eventually sink into the water bodies, or settle with the suspended particles on the sediments. Only 20% 30% of the antibiotics used in aquaculture are absorbed by fish, and 70% 80% enter the aquatic environment (Samuelsen, 1989). Therefore, the extensive use of antibiotics in aquaculture is an important route for antibiotics to enter the aquatic environment. Antibiotics could enter farmland ecosystem through human and animal manures, which is an important way to enter into the soil environment. Antibiotics in the soil environment can contaminate surface water, groundwater, and drinking water sources by leaching, percolation, or other migration pathways; accumulate into the food chain through crop absorption; and would pose a potential hazard to the health of animals and human beings.

7.2.2 TYPES OF ANTIBIOTICS Classified antibiotics, according to the chemical structure of subcategories, can be divided into beta-lactam, quinolone, tetracyclines (TCs), aminoglycosides, macrolides, and sulfonamides. Among them, TCs were made by separation from Streptomyces spp., such as S. viridifaciens or S. aureofaciens (Moellering, 1979), CTCs by separation from S. aureofaciens (Duggar, 1948), and OTCs by separation from S. rimosus (Finlay et al., 1950). Their environmental behavior has been the most researched. Currently, the problems of antibiotics pollution have become an important environmental issue in many advanced countries (such as the European Union and the United States), and related research has rapidly developed. Human consumption of animal food containing antibiotic residues usually manifests as chronic poisoning, further causing deformities, mutations, cancers, and embryotoxicity (Ku¨mmerer, 2001). Many antibiotics are water-soluble, with about 90% excreted from the body via urine and 75% via feces (Halling-Sørensen et al., 1998; Halling-Sørensen, 2001). However, until the late 1990s, there had been no systematic studies to investigate the residues of antibiotics in the aquatic environment and contamination problems. Recent studies showed that more than 80 kinds of antibiotic drugs, for example, macrolide, sulfonamide, and TCs were detected in the waters in Austria, Germany, the United Kingdom, Italy, Spain, Switzerland, the Netherlands, the United States, and Japan (Hirsch et al., 1999; Person and Liglis, 1993; Ternes,1998; Ternes et al., 2004). In medicinal use, TCs had inhibitions in leptospirosis, actinomycetes, rickettsial infections, mycoplasma,

7.2 Pharmaceutical Drugs in the Environment

and many viruses, and thus are called broad-spectrum antibiotics (Moellering, 1979; Oka et al., 2000). TCs are by far the most frequently used antibiotics in animal husbandry, with the largest global production and sales (Sarmah et al., 2006). With agricultural application of livestock manure directly into the soil environment, TCs may have ecological health and safety impacts (Halling-Sørensen et al., 1998), and thus the research on the fate and transport of TCs in the environment has received special attention. A large number of antibiotics are used for the treatment and prevention of human and animal diseases or for the promotion of plant growth. After being taken by the human body or animals, many antibiotics are untransformed through decomposition and deposited directly into the environment through the excrement. Many studies have reported varying degrees of antibiotics contamination detected in water and soil environments (Kolpin et al., 2002; Mackie et al., 2006; Kim et al., 2007), tap water in an urban environment (Peng et al., 2008), both effluent and effluent from wastewater treatment plants (WWTPs) (Karthikeyan and Meyer, 2006; Miao et al., 2004), sediments (Tang et al., 2009), and rivers (Zhou et al., 2011). As antibiotics are not easy to be decomposed by microorganisms and come into the environment continuously, they have become an emerging persistent organic pollutant. The residues, migration, fate, and eco-toxicological effects of antibiotics in the environment have become a hot research area in recent years.

7.2.3 ANTIBIOTIC RESIDUES IN THE AQUATIC ENVIRONMENT At present, the reports on the detection and contamination of antibiotic substances in surface water, drinking water, and sewage water have become more and more common. For example, the United States Geological Survey (USGS) surveyed the drugs, phytosterols, and biocides in 139 rivers in 30 states, resulting in the detection of 21 antibiotic residues (USGS), the vast majority of these antibiotics being animal growth hormones, such as tylosin (TYL), TCs, and sulfonamides. The highest frequency was detected for sulfonamides and lincomycin, followed by TYL. The residual concentration of these antibiotics in the aquatic environment is generally less than 1.0 μg/L (Kolpin et al., 2002). Batt et al. investigated the concentrations of antibiotics of some wastewater treatment plants in the United States and found that trimethoprim, TC, and clindamycin are all present in the water environment with levels of 0.090 6.0 μg/L (Batt et al., 2006). The studies have shown that as an increasingly common contaminants, various antibiotic drugs have been detected in water environments, especially in drinking water, river water, and sediments in Lombardy, north Italy (Zuccato et al., 2000; Zuccato et al., 2005). These antibiotic contaminants mainly include spiramycin, erythromycin (ERY), lincomycin, TYL, and oleandomycin. According to the authors, the antibiotic drug contamination might have originated from incomplete metabolic excretion, and improper sewage treatment or discharge (Zuccato et al., 2000; Zuccato et al., 2005). Antibiotics were detected in surface water, drinking water,

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and sewage in South Korea, and the detection rate was over 80% (Kim et al., 2007). There were also varying degrees of oxytetracycline contamination in the seawater, especially with higher levels of oxytetracycline in marine sediments, reaching 500 4000 μg/kg (Capone et al., 1996). Additionally, antibiotics were also detected in groundwater (Holm et al., 1995; Hirsch et al., 1999; Hamscher et al., 2002). A study by Sarmah et al. (2006) analyzed four groundwater samples collected from Iowa in the United States, and found that many antibiotics exist, including CTC, oxytetracycline, lincomycin, sulfamethazine (SMZ), sulfadimethoxine, and antibiotic metabolites (Sarmah et al., 2006). The majority of antibiotic resistance in groundwater comes from farmland irrigation and aquaculture, with the exception of a few such as sulfonamide residues for which the common content of detection is lower than the minimum detection limit of 0.02 0.05 μg/L (Hirsch et al., 1999).

7.2.4 IMPACTS OF ANTIBIOTIC IN THE ENVIRONMENT ON HUMAN BODIES Residual antibiotics in the environment wreak havoc on the tremendous antimicrobial resistance in humans. Most of the drugs within livestock excreta have a long-term half-life and are very easily accumulated in waters. If these drugs do not easily adsorb on the adsorbents or desorb by other effects like pH, temperature, or ions, their high concentrations in waters will be accumulated in aquatic animals, which causes chronic toxicity to organisms. When microbial long-term exposure inhibits the activity of microorganisms, it may stimulate pathogens to become resistant. The resistant bacteria may infect animals and humans, and resistance genes may spread between bacteria, animals, and humans (Acar and Moulin, 2006; Sengelov et al., 2003; Teuber, 2001). A case was reported of one man in China who was riddled with multiple strains of drug-resistant bacteria (Hvistendahl, 2012). Because the Chinese government allowed 15% profit from drug sales, antibiotics prescriptions by doctors became rampant. The churning out of prescriptions boils down to this perverse incentive, thus, antibiotic resistance is a serious public health threat both in China (Hvistendahl, 2012) and other developing countries. Herein, an astonishing study showed that multinational companies manufactured antibiotics of unapproved fixed dose combination (FDC) in India. These antibiotics will cause bacteria and viruses to rise in resistance and pose a global health crisis (McGettigan et al., 2018). This study also pointed out that there were 118 antibiotic FDC formulations in the market in India (2007 2012) compared with 5 in the United Kingdom and/or United States. It is against the law to sell unapproved new pharmaceuticals in India, but it was verified that up to 64% of the 118 agents were not approved by the National Drug Administration or the Central Pharmaceutical Standards Control Organization of India. These FDC agents were processed by 476 pharmaceutical or pharmaceutical companies to produce 3307 products (McGettigan et al., 2018). Scientists

7.2 Pharmaceutical Drugs in the Environment

believe that these unlicensed or banned antibiotics are the “incubators” of this bacterium and therefore have called on the Indian government to take the toughest measures to limit the illicit trade of antibiotics and to ban the use of such agents for treating bacterial infectious disease (McGettigan et al., 2018). In previous studies, much antibiotics pollution in the marine environment and marine microbial resistance had been also reported (Bjorklund et al., 1990; Krumperman, 1983). The study has pointed out that the OTC-resistant isolates of Aeromonas salmonicida transferred the OTC resistance phenotype to Escherichia coli (Adams et al., 1998), but also transferred between Aeromonas spp. and E. coli (Rhodes et al., 2000). This can also lead to one of the reasons for TC resistance. In 2005, the first isolated resistant bacteria, Campylobacter jejuni, was discovered. This bacteria has resistance to ciprofloxacin, ERY, and cerftriaxone (Moore et al., 2006). The latest research points out that the use of antibiotics affects the ability of the immune system to kill foreign bacteria in mouse; meanwhile, the bacteria also reduce susceptibility to antibiotics and become more resistant (Yang et al., 2017). In addition, the research indicates that antibiotics kill the intestinal probiotics in the human body and endanger the human immune system (Nield, 2017; Moyer, 2017). This latest study also concluded that this new discovery will overthrow the understanding by the academic community that antibiotics only treat the cells of bacteria (Yang et al., 2017). Moreover, some researchers found that the mcr-1 gene in Chinese pigs is resistant to one of the “last resort” antibiotics, colistin (Schwarz and Johnson, 2016). And then, this gene appears one by one in the world. Not only that, the study indicates that mcr-1 has jumped from the bacterial genome into the plasmid (Liu et al., 2016). Mcr-1 or other drug-resistance genes are believed to naturally evolve from bacteria, but it has received much attention in recent years. All these studies show that mcr-1 can occur in many germs. The mcr-1 gene is theoretically easy to get into human, if someone ate undercooked meat or farm workers who had close contact with livestock. In this way, once humans are sick, antibiotics cannot defeat these diseases or superbugs. Fortunately, researchers at the University of Queensland in Australia found that the last resort antibiotic vancomycin can defeat superbugs after modification (Blaskovich et al., 2018). They believe that this new-type vancomycin can treat methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus. On the other hand, limiting the abuse of antibiotics in livestock husbandry can effectively reduce the frequency of occurrence of drug-resistant bacteria up to 39% (Tang et al., 2017). Tang’s report was also used by the WHO as a reference source for the new proposal. Abuse of antibiotics in livestock husbandry is the number one cause of the appearance of superbugs, and causes the last resort antibiotics to fail. According to a WHO report (WHO, 2017), as many as 80% of antibiotics are used in livestock in some countries, the usage far exceeding human consumption. It is recommended that the use of antibiotics in livestock could be prohibited completely and not allowed when they are sick. However, the new WHO proposal is not legally valid. WHO also expects the

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proposal to be unpopular because it will cause economic losses to the industry, but WHO emphasizes the importance of the suggestion to human health (WHO, 2017). Not only that, of four different onion molecules in Iranian cooking ingredients tested, it was found that the combination of these four onions and antibiotics for tuberculosis significantly reduced the numbers of bacteria in multidrugresistant tuberculosis (MDR-TB) and inhibited the growth of the isolated tuberculosis (TB) cells more than 99.9% (Danquah, et al., 2018), to enhance the effect of existing antibiotic treatment of tuberculosis. The researchers said this result could help “reverse the tide” of MDR-TB, which infected 490,000 people in 2016. However, they also said the study is still in its early stages and requires clinical trials. Consequently, the natural products from plants and microorganisms have great potential as a source of new antibiotics (Danquah et al., 2018). Recently, a team of scientists from the University College London (UCL) and the National Physical Laboratory (NPL) of the United Kingdom successfully synthesized “artificial viruses” with 20 nanometers of hollow-textured protein tissue (Santis et al., 2017). Artificial viruses can attach to bacteria larger than themselves and destroy their cell membranes. When the artificial virus falls on the bacteria for a few seconds, it can neatly open a hole in the cell membrane to expose the internal of the bacteria, and the bacteria is nearly attacked to death. Such an outcome has led to new treatments for drug-resistant bacteria and an eagerness to treat more than 700,000 patients worldwide who are sick with drug-resistant bacteria (Santis et al., 2017). One of the advantages of artificial viruses is that bacteria are not resistant to viruses and the second is more convenient than traditional techniques that require the direct delivery of antibiotics into cells to hit a specific target. Not only that, the artificial virus can enter human cells like a normal virus without any adverse effect on the human body, indicating that it can be applied to the medical field in the future. It can be used as a technology to deliver drugs directly into cells and allow gene therapy in cells (Santis et al., 2017). Similarly, one research team found a potential weapon, named PPMO (peptide-conjugated phosphorodiamidate morpholino oligomer), against drug-resistant bacteria (Sully et al., 2017). This molecule is able to fight New Delhi metallo-β-lactamase 1 (NDM-1), which is produced by bacterial secretion. NDM-1 is one of the factors that cause bacteria to evolve resistance genes. Since many different bacteria contain these genes, a PPMO molecule can attack it effectively. Scientists use a widely used antibiotic, meropenem, that belongs to the class of carbapenems. They successfully used meropenem to treat mice infected with E. coli. Human trials will start within 3 years (Sully et al., 2017). An astonishing finding was reported by the research team of Hong Kong University (Wang et al., 2018), which demonstrates that NDM-1 is an important resistance factor responsible for the formation of the superbugs of carbapenemresistant Enterobacteriaceae (CRE). They suggested that an antimicrobial agent used in the treatment of gastric ulcer disease called colloidal bismuth subcitrate was found to be effective as a “suppressant” to inhibit the activity of some highly resistant or multidrug-resistant superbugs, and can delay the emergence of

7.3 Adsorption Studies of Antibiotics on Clay Minerals

drug-resistant bacteria. Furthermore, it can be used to treat patients suffering from severe infections such as bloody diarrhea, septicemia, meningitis, and multiple abscesses caused by CRE and carbapenem-resistant Klebsiella pneumoniae (Wang et al., 2018). Most importantly, the new combination therapy can reduce the use of existing antibiotics by nearly 90% and further prevent the increase of NDM-1 resistance over a long period of time, thus the life expectancy of existing antibiotics has been greatly extended (Wang et al., 2018). A whole new discovery was made by scholars from Rockefeller University, who analyzed more than 1000 soil samples from across the United States and found new antibiotics: malacidins (Hover et al., 2018). It can kill the superbug MRSA without side effects and toxicity. Malacidins destroyed the superbug’s cell wall and eliminated it, and bacterial load was not observed on the wound after dosing of 24 72 h. In the highest concentration dosing experiment, it was found that there was no significant toxicity of malacidins to mammalian cells. Even after 20 days of the limit concentration test, they did not detect any drug-resistant superbugs. The wounds that were infected with superbugs in the rats were then successfully healed (Hover et al., 2018). Similarly, Alexander Fleming, a Scottish microbiologist, discovered the first antibiotic in the world, penicillin, from soil in 1928. Soil is considered a good source of antibiotics because its low nutrient content forces different species of bacteria to compete with each other to survive and make them stronger. In 1961, however, for the first time the superbug MRSA was found in the United Kingdom to have become resistant to all antibiotics. This study opened a new research area for antimicrobial resistance (AMR). Last but not the least, octapeptin was discovered in the late 1970s, but there were more effective antibiotics being developed at that time, so no further investment was made in the study and it was eventually forgotten (Velkov et al., 2018). Because the structure of octapeptin is very similar to one of the last resort antibiotics (colistin), it is expected that octapeptin can replace colistin where effectiveness is reduced by resistance (Velkov et al., 2018). The study also found that octapeptin is more potent against Gram-negative bacteria than colistin before clinical trials, and is believed to serve as a foundation for the development of a new generation of antibiotics (Velkov et al., 2018).

7.3 ADSORPTION STUDIES OF ANTIBIOTICS ON CLAY MINERALS 7.3.1 TETRACYCLINE Adsorption has an important influence on the migration, activity, and bioavailability of antibiotics, which depend largely on antibiotics and soil characteristics. The adsorption studies of TC, CTC, and OTC on soil under different pH, clay contents, soil types, cation exchange capacity (CEC), anion exchange capacity, and organic carbon contents (Sassman and Lee, 2005) showed that these three kinds

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of antibiotics were strongly adsorbed on soils (especially on acidic and high viscosity soils). The study of TC and CTC adsorption on K- and Ca-saturated soil clays, humic substances, and clay-humic complexes indicated that the strength of adsorption was in the order: soil clays . humic substances . clay-humic complexes (Pils and Laird, 2007). In addition, the results suggested that the adsorption mechanism was cation exchange, and the adsorption capacity of Ca-saturated soil was greater than the K-saturated soil. The humic substances could weaken the adsorption of TC on the soil, especially if the soil had high organic matter content (Gu et al., 2007). Davis et al. (2006) investigated the outflow trend of several antibiotics, including TC, CTC, sulfathiazole (STZ), SMZ, ERY, TYL, and monensin, by runoff from rainfall-simulated plots (Davis et al., 2006). Compared with other antibiotic concentrations after runoff for 1 h, the TC and CTC concentrations did not have significant reduction, which further confirmed the TCs have a high adsorption capacity on soil (Davis et al., 2006). This study suggested that the minerals and organic matter of soil were the main adsorption sites for antibiotics. Another study found the adsorption of TC decreased with the removal of organic matter on cinnamon soil (Bao et al., 2009). However, the hydrophobic property, cation exchange, cation bridging, surface complexation, and hydrogen bonding may also play an important role in the process of adsorption (Tolls, 2001). As for adsorption, our research team used seven clay minerals to uptake TC (Chang et al., 2009a,b,c; 2012a; Li et al., 2010a; Fig. 7.1) under different environmental factors like pH value, temperature, or ionic strength. These sequential studies also investigated the adsorption mechanisms. Results showed that the maximum adsorbed amount of TC was 1053 mmol/kg at pH 1.5 for SAz-1, and this value was close to the clay mineral’s CEC (Table 7.1). Thus, cation exchange was the major adsorption mechanism expect for SYn-1 and SWy-2 at low pH and low concentration of the adsorbate (Table 7.2). Besides, the isotherm studies demonstrated that Langmuir model was fitted to the experimental data of the seven clay minerals (Table 7.2 and Fig. 7.2), and most of the adsorption equilibrium times were very fast, except for rectorite (Tables 7.2 and 7.3 and Fig. 7.3). According to published papers, all of the mechanisms were determined by virtue of ionic analysis under unadjusted pH condition (Table 7.3), and the solution pHs were maintained in the range of cation or zwitterion forms after adsorption (Chang et al., 2009b,c; 2012a; Li et al., 2010a,b; Figueroa et al., 2004). TC has one positive electric charge. One mmol of TC adsorbed would be associated with one milli-equivalent (meq) of positively charged cations (may have different valence charges) desorbed in a cation exchange process. The unit is essentially identical to that routinely used for the expression of the CEC values of clays. If the linear relationship with a slope of 0.9 suggests that ca. 90% of TC was adsorbed via cation exchange with the exchangeable cations of the clay, thus, the slope of adsorbed and desorbed cations can be used to decide if the adsorption mechanism is cation exchange or not (Fig. 7.4). Besides, we cannot acquire the mechanism under alkaline or acidic condition, but can obtain it from Fourier

7.3 Adsorption Studies of Antibiotics on Clay Minerals

FIGURE 7.1 Molecular structure of TC on a planar view (A) and speciation under different pH (B). From Chang, P.H., Jiang, W.T., Li, Z., Jean, J.S., Kuo, C.Y., 2015. Antibiotic tetracycline in the environments A review. Res. & Rev.: J. of Pharmaceut. Anal. 4 (3), 86 111 (Chang et al., 2015).

transform infrared spectroscopy (FTIR) data (Figueroa and Mackay, 2005; Sithole and Guy, 1987). Reasonably, the maximum adsorbed amount should happen if TC exists in cation or zwitterion form under the mechanism of cation exchange, but not always. Take palygorskite for instance. The maximum adsorption capacity was 223 mmol/kg at pH 8.7 when TC existed in anion form (Table 7.3). Compare this with 126 mmol/kg at pH 5 6 when TC exists in cation form (Chang et al., 2009b). This study points out that the pH adsorption edge effect showed that H1 was strongly competing with TC on adsorption sites on the clay surface, and further reduced the adsorption capacity when TC existed as cationic form, while the cationic function group of dimethylammonium (NHMe21) (Fig. 7.1) still played a significant role for adsorption to increase the adsorption capacity when TC existed as anionic form (Chang et al., 2009b). The same reason was also

177

Table 7.1 Maximum Adsorption Capacities and CEC Values of Clay Minerals at Four pH Values Clay Minerals

Rectorite

PFL-1

SWy-2

SAz-1

SYn-1

SHCa-1

IMt-2

Highest initial concentration (mg/L) Point of zero charge (pHpzc)a

1000 4.21

800 4.12

3000 8.3538.456

2000 None

2000 8.34

2000 2.45 3.58

Original CEC (meq/kg)a Sm at pH 1.5 (mg/g) CEC at Sm (mmol/kg) Sm at pH 4 6 (mg/g) CEC at Sm (mmol/kg) Sm at pH 8.7 (mg/g) CEC at Sm (mmol/kg) Sm at pH 11 (mg/g) CEC at Sm (mmol/kg)

41011 131 295 140 315 107 241 54 122

1651219514 61 137 56 126 99 223 23 52

8501276413 404 910 340 766 210 473 140 315

3000 8.442 5.887 8.0910.5410 12001123012 468 1053 422 950 302 680 86 194

700 14001 217 489 170 383 122 275 59 133

6612 330 743 350 788 375 845 227 511

14013 — — 32 72 — — — —

Sm, Maximum adsorption amount. 1 Chen et al. (2013). 2 Alexander and Arieh (2000). 3 Stathi et al. (2007). 4 Tombacz (2004). 5 Xia et al. (2009). 6 Stadler and Schindler (1993). 7 Cherlet et al. (2003). 8 Lan et al. (2007). 9 Zysset (1992). 10 Goldberg and Glaubig (1986). 11 Hong et al. (2008). 12 Borden and Giese (2001). 13 Kahle and Stamm (2007). 14 Data from http://www.clays.org/SOURCECLAYS/SCdata.html.

Table 7.2 Adsorption Behaviors of Tetracycline on Clay Minerals Clay Minerals

Rectorite

Kinetic model

Pseudo-second-order model

Elovich model

Equilibrium time (h)

24

8

Adsorption model Maximum adsorption amount (mg/g) Adsorbed TC/desorbed cations Major desorbed cation

Langmuir model pH 4 6 140 1:2.5

pH 8.7 99

Ca21

Ca21

17.3 Å at pH 11

Clay crystallinity

Descend

TC decomposition temperature Adsorption mechanism

, data not available. Nonlinear but positive relation.

a

IMt-2

2

Swelling amount of d-spacing

Interaction between clays and TC (FTIR band shift in cm21

PFL-1

SAz-1

2

Freundlich model

a

8

Langmuir model pH 1.5 468 1:0.9

pH 1.5 404 1:1.5

pH 8.7 375 1:1.7

pH 1.5 217 1:0.2

Ca21

Ca21

Ca21, Na1

Ca21, Na1

Na1

11 Å at pH 5 6

0Å

10.3 Å at pH 8.7 and 11

Unchanged

1 10 15 205 C

SYn-1

pH 5 6 32 1:1.5

0Å

Cation exchange

SHCa-1 Pseudo-second-order model

Descend

Strong interaction 1 5 20 410 C

SWy-2

1 20 50

Cation exchange

Medium interaction 16

Unchanged

Surface complexation (,800 mg/L); cation exchange ( . 800 mg/L)

Unchanged Strong interaction 1 16

Medium interaction Unchanged

Cation exchange

Surface complexation

CHAPTER 7 Clay minerals for pharmaceutical wastewater treatment

500 IMt-2 Rectorite

Amount TC sorbed (mg/g)

180

400

PFl-1 SAz-1 SWy-2

300

SHCa-1 SYn-1

200

100

0 0

200

400

600

800

1000

1200

1400

1600

Equilibrium TC concentration (mg/L)

FIGURE 7.2 TC adsorption on seven clay minerals. Lines are Langmuir fit to the observed data, except IMt 2 (Freundlich). From Chang, P.H., Jiang, W.T., Li, Z., Jean, J.S., Kuo, C.Y., 2015. Antibiotic tetracycline in the environments A review. Res. & Rev.: J. of Pharmaceut. Anal. 4 (3), 86 111.

displayed for SHCa-1 (Chang et al., 2009a). It seems that, if the adsorption mechanism is cation exchange, the maximum adsorption amount is not necessarily located at the same pH condition (ion form), but it is still determined by other effects like pH or ionic strength effect. Another mechanism is surface complexation; if the linear slope between desorbed and adsorbed cations is low and the adsorbate have no lone pairs like hydrogen, oxygen, or fluorine, the mechanism should be surface complexation instead of hydrogen bonding. Take Syn-1 for instance (Li et al., 2010b). The slope was 0.20 and TC has no lone pairs (Figs. 7.1 and 7.3), thus, the mechanism was surface complexation (Table 7.3). The d-spacing change of clays after adsorption depends on their expandable property. Essentially, the PFl-1, IMt-2, SYn-1 are unswelling clays make their dspacing unchanged, but this changed for rectorite, SAz-1, SWy-2, and SHCa-1 (Table 7.2 and Figs. 7.5 and 7.6). Logically, the expandable d-spacing renders more adsorbates into the interlayers as two or three stacks via cation exchange and increases the adsorption amounts on the inner surface. Indeed, the adsorption capacities have the obvious relationship with expandable property (Table 7.1). Take the SYn-1 and PFL-1 for instance; the adsorption amounts just reach to 170 mg/g (1/4 CEC) and 56 mg/g (1/3 CEC) at natural conditions, respectively. However, the adsorption amounts almost reach their CEC for rectorite, SAz-1, SWy-2, and SHCa-1 at neutral conditions (Table 7.1). In fact, these characteristics enhance the adsorption ability more and less, but is not the main control factor.

Table 7.3 The Properties of Adsorption between Clay and Iron-oxide Minerals and TCs at Various pHs Adsorbate

Adsorbent

Sm (mmol/kg)

pH

TC Form

Isotherm Model

Solid/Water

TC

Rectorite PFL-1 IMt-2 SAz-1

315 223 72 1053

4 6 8.7 5B6 1.5

1/

Langmuir

0.1 g/20 mL

1/ 1

Freundlich Langmuir

0.1 g/10 mL 0.1 g/20 mL

SAz-2 SWy-2

1010 910

6 1.5

1/ 1

SHCa-1

845

8.7

SYn-1

489

1.5

1

kaolinite SWy-2

9 800

5B6 1.5, 5

1/ 1, 1/

Freundlich

OTC OTC TC CTC

Na-kaolinite Na-montmorillonite

30 7 112 167

5.5

1/

Langmuir

OTC

Goethite

2.8 3 1024 (mmol/m2) 4.2 3 1024 (mmol/m2) 108 280

OTC

Hematite TC

Na bentonite Ca bentonite

Sm, maximum adsorption amount;

, data not available.

1.0 g/10 mL 1.0 g/1 L 4.76 3 1023 kg/L 4 3 1024 kg/L

0.1 g/10 mL

4.55

Mechanism, TC Form Cation exchange, 1/

Surface complexation, 1/ Cation exchange, 1/ Cation exchange and surface complexation, 1/ Surface complexation, 1/

Reference Chang et al. (2009c) Chang et al. (2009b) Chang et al. (2012a) Chang et al. (2009c) and Li et al. (2010b) Chang et al. (2014) Chang et al. (2009c) and Li et al. (2010b) Chang et al. (2009c) and Li et al. (2010b) Chang et al. (2009c) and Li et al. (2010b) Li et al. (2010a) Kulshrestha et al. (2004) Figueroa et al. (2004)

Figueroa and MacKay (2005)

Sithole and Guy (1987)

CHAPTER 7 Clay minerals for pharmaceutical wastewater treatment 250 Rectorite

SAz-1

SHCa-1

PFl-1

SWy-2

SYn-1

IMt-2

Amount TC sorbed (mg/g)

200

150

100

50

0 0

15 10 Equilibrium time (h)

5

20

25

FIGURE 7.3 Sorption kinetics of TC on seven clay minerals under different initial concentrations. The solid lines are pseudo-second-order fit to the observed data.

1.8

Rectorite y = 1.86x + 0.40

PFl-1

1.6 Amount cation desorbed (meq/g)

182

R2 = 0.96

SAz-1

1.4

SWy-2 SHCa-1

1.2

SYn-1

1.0

IMt-2 y = 2.51x + 0.04 R2 = 0.94

0.8

y = 0.96x + 0.03 R2 = 0.99

0.6 0.4

y = 1.57x + 0.02 R2 = 0.97

0.2

y = 0.21x + 0.04 R2 = 0.98

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Amount TC sorbed (mmol/g)

FIGURE 7.4 Desorption of exchangeable cations as affected by TC uptake by seven clay minerals. The lines are straight line regressed against the total cations desorbed. From Chang, P.H., Jiang, W.T., Li, Z., Jean, J.S., Kuo, C.Y., 2015. Antibiotic tetracycline in the environments A review. Res. & Rev.: J. of Pharmaceut. Anal. 4 (3), 86 111.

7.3 Adsorption Studies of Antibiotics on Clay Minerals

FIGURE 7.5 XRD patterns of three raw clay minerals, and their samples with the highest d-spacing change. The d-spacing change corresponds to Table 7.1.

The CEC is the key factor on adsorption for clay minerals. Nevertheless, there is one unclarified phenomenon, i.e., that the adsorption of antibiotic TC on some clays cannot reach the CEC, and this condition also happened on other adsorbates. Future research should figure out how to identify this mechanism. As for pHpzc (Table 7.1), PZC is an important parameter for characterization of surface charge properties of clay minerals. When the solution pH is lower than PZC, the surface of clay mineral is positively charged due to protonation; when the solution pH . PZC, it is negatively charged; if the solution pH 5 PZC, the surface is uncharged. Therefore, the positively charged clay mineral surface is by virtue of the adhesion of H1 from the acid solution on clay surfaces. With the pH value increase, the concentration descent of H1 weakness this influence and the surface returns to neutral. Once the solution pH is alkali, the surface is negatively charged. Take SAz-1 montmorillonite for instance (Table 7.1). At pH , pHpzc, both the surface of clay mineral and TC were positively charged; while at pH . pHpzc, it is negatively charged. For these two conditions, it should be

183

CHAPTER 7 Clay minerals for pharmaceutical wastewater treatment

d001 = 14.9 Å d001 = 20.8 Å

0.66 CEC SWy-2

d001 = 22.3 Å d001 = 15.5 Å

0.88 CEC

SAz-1

d001 = 23.6 Å

8000 Intensity (cps)

184

0.41 CEC

7000 6000

SHCa-1

5000 4000

d001 = 11.1 Å

0.36 CEC

3000 2000 1000

SYn-1

0 2

4

6

8

10

12

14

16

18

20

2θ (°)

FIGURE 7.6 XRD patterns of four raw clay minerals, and their samples with the highest d-spacing change. The d-spacing change corresponds to Table 7.1.

mutually exclusive between clay surface and TC, and the adsorption amount should be low. But the experimental results displayed moderate adsorption amounts in acid or alkali solution (Table 7.1). Obviously, the adsorption cannot be simply interpreted as electrostatic adsorption, but may be ascribed to other mechanisms such as hydrogen bonding, van der Waals, cation exchange and intermolecular charge distribution, polar functional groups complexation, and hydrophobic interaction. From this systematic investigation, we also confirmed that adsorption ability not only depends on the CEC value of clays (Table 7.1, Goldberg and Glaubig, 1986; Tombacz, 2004; Zysset, 1992), but also on the functional groups of TC or pH values (Chang et al., 2012a; 2009b,c). For example, the maximum adsorbed amount was 210 mmol/kg at pH 8.7 under TC in its anionic form TCH2 (Chang

7.3 Adsorption Studies of Antibiotics on Clay Minerals

et al., 2009b); the presence of the positively charged functional group of dimethylammonium played a significant role, thus enhancing the adsorption capacity. In this case, the swelling or unswelling property of clays seems not to have played an important role on adsorbed amounts. On the other hand, the correlation between adsorbed TC and desorbed metals (Fig. 7.4) was strongly illustrated in the adsorption mechanism; furthermore, the extra desorbed amounts (Table 7.2) suggested that other mechanisms like hydrogen bonding, cation bridging, or complexes also played a second role in adsorption. Take SYn-1 for instance. The slope of 0.2 (Table 7.2) was distinct, deducing that about 20% adsorbed amount was obtained from cationic exchange and about 80% was due to other mechanisms such as surface complexation.

7.3.2 SULFONAMIDES Sulfonamides contain sulfonyl and amine functional groups, including sulfadiazine, sulfafurazole, sulfamethoxazole, sulfadimidine, sulfamerazine, and so on. Sulfonamides have weaker interactions with soil and minerals because they have fewer functional groups than other antibiotics. The concentrations of sulfamethoxazole can reach up to 7.91 μg/L in sewage (Peng et al., 2006). The concentration of sulfamethoxazole in shallow groundwater in Baden-Wurttemberg, Germany was greater than 410 ng/L (Sacher et al., 2001). The concentrations of sulfonamides in pig farms in Beijing were greater than 33 μg/L (Ben et al., 2008). The concentrations of sulfamerazine in pig farms in Vietnam ranged from 18.5 to 19.2 μg/L (Managaki et al., 2007). It can be seen that water environments around the world have varying degrees of sulfonamide residues; therefore, investigation of the environmental behavior of sulfonamides should become an emerging issue. Sulfonamides degrade to polar metabolites easily, and the degradation time is 6 h. Meanwhile, the adsorption amounts of sulfonamides are proportional to mud, clay minerals, and organic matter contents of soils (Fan et al., 2011). The adsorption amounts of sulfamethoxazole and SMZ on montmorillonite, soil, and the mixture of soil mud were 0.07 1.81 L/kg (Fan et al., 2011). The adsorption coefficient is smaller than that of TC antibiotics, the partition coefficient decreases with the increase of pH and ionic strength, and the coefficient of desorption is larger. It also has the same rule in soil colloidal minerals (Fan et al., 2011). This further shows that sulfonamides could easily migrate in soil minerals, thus entering the aquatic system and posing a threat to human health (Avisar et al., 2010; Fan et al., 2011). Additionally, humic acid could promote the adsorption of sulfonamides on montmorillonite, and the adsorption capacity increased with the increase of carboxyl groups in humic acid, and also increased with the carbon numbers of aliphatic groups (Avisar et al., 2010). The FTIR spectra showed that the polar functional groups and aliphatic groups of humic acid preferentially adsorbed on the surface of montmorillonite, and then complexed with sulfonamides to form the complexes (Gao and Pedersen, 2010). Related studies found that the addition of solid pig slurry or surfactant to form ion micelles could

185

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CHAPTER 7 Clay minerals for pharmaceutical wastewater treatment

increase the adsorption capacity of sulfonamides in soils and minerals, and make them less accessible to surface and groundwater (Sukul et al., 2008). On the other hand, an interesting report studied the effect of liquid pig slurry on the adsorption of five sulfonamides in loess, i.e., SMZ, sulfonamides, sulfadiazine, sulfapyridine, and SMZ (Thiele-Bruhn and Aust, 2004). After the addition of liquid pig slurry in the adsorption systems, the adsorption amount of sulfonamides decreased significantly. The adsorption capacity of sulfadiazine in loamy soil with low clay content and low organic carbon content was higher than that in silty loam soil, which might be related to the different chemical speciation of sulfadiazine at different pH (Polubesova and Nir, 2003). The cationic form of STZ is more easily adsorbed on clay minerals than its anion and neutral forms (Kahle and Stamm, 2007). The adsorption amounts of sulfamerazine, sulfamethoxazole, and sulfapyridine on clay minerals decreased with increasing ionic strength of the solution (Gao and Pedersen, 2005). In general, the adsorption of sulfonamides on the soil and minerals was not only related to pH, but also to the content of organic matter, the existing forms, and the adsorption time. Sulfonamides and clay minerals have the anion bridging and cation exchange interactions during adsorption (Gao and Pedersen, 2005). The strong or weak chemical interactions are related to their functional group contents, where the strong interaction depends on the more functional groups. Depending on the few functional groups of sulfonamides, the behavior of their adsorption and desorption on soil is significantly influenced by the interaction of physical and chemical factors, which means the sulfonamides may have better migration ability in the environment, which poses a greater threat to surface waters and even groundwaters than other antibiotics.

7.3.3 MACROLIDES Macrolide antibiotics are weakly alkaline compounds with 12-, 14-, and 16membered lactone rings. They are divided into lactone and polylactone rings, including TYL, ERY, melemomycin, spiramycin and midecamycin, roxithromycin, azithromycin, and clarithromycin. In terms of adsorption research, there have been fewer related articles for this group of antibiotics than for tetracyclines. Therefore, researchers should focus on this group to comprehensively recognize macrolides’ adsorption behaviors. The adsorption of TYL on different adsorption media is very different (Lee and Seo, 2010; Bewick, 1979). The adsorption capacity of TYL on bentonite and montmorillonite could reach 190 and 65 μg/mg, respectively, while the adsorption capacity on illite and kaolinite was only 22 and 6.5 μg/mg, respectively (Bewick, 1979). Besides, the adsorption capacity of TYL on montmorillonite and kaolinite decreased with the increase of pH and ionic strength, and the adsorption capacity decreased significantly when Ca21 was replaced with Na1 as the background electrolyte (Bewick, 1979). These results indicated that the adsorption of TYL on the surface of clay minerals was affected by mineral properties, environmental pH, and ionic strength (Lee and Seo, 2010; Bewick, 1979). The adsorption of roxithromycin and clarithromycin were

7.3 Adsorption Studies of Antibiotics on Clay Minerals

effective on bentonite, iron(III) oxy-hydroxides, and hydrous manganese oxide (MnO2). The order of adsorption ability was bentonite . MnO2 . ferrihydrite, and the pH values had no effect for its adsorption on the ferrihydrite surface. Most importantly, the adsorption mechanism was combined with electrostatic force and cation exchange on Na-bentonite. On the contrary, electrostatic attraction, cation exchange, van der Waals force and hydrogen bonding were the adsorption mechanisms on MnO2, and surface complexation was the mechanism on the goethite surface (Feitosa-Felizzola et al., 2009). The adsorption strength of antibiotics on different soils is greatly influenced by the different soil types and antibiotic types, and is mainly affected by clay and iron oxide contents (Sassman et al., 2007). The results showed that the adsorption behavior of TYL A, TYL D, and A-aldol were similar on the soil, and had a strong adsorption capacity, which was affected by the surface area, clay content, and CEC of the soil (Sassman et al., 2007). For the adsorption mechanism of macrolides, some studies suggested that electrostatic interaction played an important role in the adsorption process and hydrogen bonding also existed (Sibley and Pedersen, 2008). Although the functional groups of macrolides are similar to those of TCs, they contain fewer active functional groups, which may be responsible for their weak adsorption on soil. The adsorption mechanism of macrolide antibiotics on soil and clay minerals are still to be studied in detail.

7.3.4 QUINOLONE The quinolone antibiotics have adjacent carbonyl and carboxyl groups, and the major varieties are ciprofloxacin, ofloxacin, norfloxacin, flumequine, tosufloxacin, and enoxacin. The isothermal adsorption of ciprofloxacin on hydrous oxides of Al (HAO) and Fe (HFO) was best described by the Langmuir model (Gu and Karthikeyan, 2005). The adsorption capacity of ciprofloxacin on the HFO was more than that on the HAO (Gu and Karthikeyan, 2005). Moreover, FTIR analyses indicated that ciprofloxacin had different surface complex types that formed with HAO and HFO (Gu and Karthikeyan, 2005). A monodentate mononuclear complex (with COO ) formed between ciprofloxacin and HAO, keto O, and one O from COO of ciprofloxacin seemed to form a six-membered ring complex between ciprofloxacin and HFO (Gu and Karthikeyan, 2005). Compared with illite, vermiculite, and kaolinite, the adsorption amounts of ciprofloxacin, levofloxacin, and quinolone derivatives on montmorillonite were bigger, and this was mainly due to the specific interlayer structure, which allowed adsorption to increase and intercalate into the clay layers (Nowara et al., 1997). The kinetic study of adsorption and intercalation of ciprofloxacin on montmorillonite showed a high initial rate constant on the montmorillonite surface, and cation exchange was the dominant adsorption mechanism (Wu et al., 2010). In addition, FTIR data further demonstrated that the alkyl amino groups of ciprofloxacin adsorbed on the surface of montmorillonite and carboxyl groups were linked to the silicate layers of montmorillonite via hydrogen bonding (Wang et al., 2010). The soil with

187

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CHAPTER 7 Clay minerals for pharmaceutical wastewater treatment

aluminosilicates had highly effective CEC, and the adsorption amount of zwitterionic ciprofloxacin was greater than that of oxytetracycline, probably because the cationic and anionic groups of ciprofloxacin were far apart, rendering the Coulombic forces to reach the maximum on the soil surface. On the other hand, in a soil with low to moderate CEC, the adsorption amount of ciprofloxacin was less than that of oxytetracycline (Carrera et al., 2001). Thus, it was obvious that the contents of clay minerals in soils played an important role on the adsorption. The bonding between ofloxacin and SiO2 surfaces was via the protonated N of the piperazinyl groups or cation bridging; whereas the adsorption mechanism was coordination exchange on the surface of Al2O3 (Goyne et al., 2005). It showed that the adsorption of quinolone antibiotics was affected by the type and specific surface area of the sorbent media (Goyne et al., 2005). The complexes had been formed with montmorillonite-norfloxacin-Cu(II) at pH 4.5. But at pH 7.0, both montmorillonite norfloxacin Cu(II) and montmorillonite Cu(II) nofloxacin complexes were formed. At pH 9.0, a montmorillonite Cu(II) nitrofloxacin complex was formed, while ciprofloxacin did not follow a similar trend (Pei et al., 2011). Therefore, when Cu(II) and norfloxacin coexisted on the surface of montmorillonite, different complexes were formed at different pH values (Pei et al., 2011).

7.3.5 β-LACTAM β-Lactam antibiotics have a β-lactam ring in their chemical structure, including the most commonly used penicillins and cephalosporins, as well as newly developed cephamycins, thiomycins, monocyclic β-lactams, and other atypical β-lactam antibiotics. The penicillin group includes amoxicillin and ampicillin. The pKa values of penicillin and cephalosporin were 2.6 and 2.8, respectively (Lee et al., 2004). Therefore, the compounds were the anion forms at the pH . pKa in aquatic environment. Consequently, the clay minerals have no obvious adsorption effect for these compounds through cation exchange under neutral or base conditions. But the adsorption might take place on the basis of electrostatic interaction. However, no published articles have discussed the use of unmodified clay minerals as adsorbents for β-lactam compounds.

7.4 CONCLUSIONS In the treatment of environmental pollution, we mainly use adsorption, ion exchange, and surface activity of clay minerals, supplemented by the development of modification technologies. Thus the use of clay minerals has become increasingly widespread as a simple, effective, and economical solution. The problem of water pollution is a very worrying issue, and it presents an increasing trend. This contrasts sharply with the huge demand for water resources from rapid economic development in various countries. It is imperative to strengthen technologies for the treatment and improvement of polluted water and poor quality water.

References

However, large-scale regional water pollution control projects are no longer supported by traditional environmental pollution control technologies. In this case, clay minerals’ advantages of simple processing, good effect, low cost, and no secondary pollution of environmental mineral materials are outstanding. Clay minerals are mainly used for chemical and domestic water filtration in the treatment of heavy metal and metalloid ions, adsorption of cationic dyes and organic pollutants, and MH3N H2PO42 HPO422 PO432 in wastewater. The removal process is mainly achieved through flocculation, adsorption, and ion exchange. Because the clay adsorption technology is simple, easy to operate, fast and efficient, causes no secondary pollution, has a wide range of applications, reduces wastewater treatment costs and reduces sludge production, the development and research of clay mineral adsorbents has become a hot topic in environmental mineralogy. Also, the depth and breadth of research in this area are continuously increasing. Published articles show that contaminant adsorption amounts are very high using clay minerals as adsorbents. The main mechanism of adsorption is cation exchange; however, some articles displayed that hydrogen bonding and surface complexation could also occur. It is well accepted that clay minerals are a potent tool for the removal of cationic or zwitterionic forms of pharmaceutical drugs from water. The residual medical drugs in the environment not only cause environmental harm but also create drug resistance to bacteria in the human body. Moreover, many major antibiotics are no longer available, forcing scientists to keep finding new antibiotics to continue their fight against the evolution of bacteria. Therefore, only solution is to improve existing antibiotics. But once we make a chemical change to antibiotics, the bacteria mutate and become resistant to new antibiotics. Therefore, how to ward off calamitous outbreaks of drug-resistant bacteria has become a momentous issue. This is why we need to remove pharmaceutical drugs from the environment by means of effective methods and materials. However, the removal should consider the cost as well as effectiveness; thus, the clay minerals can act as the candidate adsorbents for the adsorption method due to their low cost and high cation exchange capacities.

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FURTHER READING Al-Ani, T., Sarapa¨a¨, O., 2008. Clay and Clay Mineralogy, first ed. Geologian Tutkuskeskus, Espoo, pp. 1 94. Chang, P.H., Jiang, W.T., Li, Z., Kuo, C.Y., Wu, Q.F., Jean, J.S., et al., 2016. Interaction of ciprofloxacin and probe compounds with palygorskite PFl-1. J. Hazard. Mater. 303, 55 63. Reardon, S., 2018. Resistance to last-Ditch antibiotic has spread farther than anticipated. Nature/News. ,https://www.nature.com/news/resistance-to-last-ditch-antibiotic-hasspread-farther-than-anticipated-1.22140.. Thiele-Bruhn, S., 2003. Pharmaceutical antibiotic compounds in soils—a review. J. Plant Nutr. Soil Sci. 166, 145 167.