The measured concentration of some pharmaceutical active compounds in some of the aquatic environment in European countries.
Carbon-based materials, namely activated carbon, carbon nanotube and graphene, are considered as one of the most effective adsorbents for pollutant removal and wastewater treatment. Due to their high surface area and distinct chemical and physical properties of the carbon-based materials, particularly activated carbon and carbon nanotube are rapidly emerging as one of the most effective adsorbents for wastewater treatment. Various studies have reported the applications of activated carbon, carbon nanotubes and graphene as promising adsorbents for removing organic and inorganic pollutants. In this chapter, an introduction about the activated carbon, carbon nanotubes and graphene and their production, prosperities and usage for the removal of pharmaceutical active materials from aqueous media are highlighted and summarized. Challenges and future opportunities for application of these carbon-based materials as adsorbents in wastewater treatment are also addressed in this chapter.
- aquatic environment
- activated carbon
- carbon nanotube
In recent years, there is great concern about the occurrence and the impact of the pharmaceutical active compounds in water, in addition, development of efficient and cost-effective technologies for the removal of these compounds and treatment of industrial effluent, surface water and ground water. Pharmaceutical active compounds are natural or synthetic chemicals that can be found in over-the-counter therapeutic drugs and veterinary drugs. They induce pharmacological effect and give significant benefits to human beings. A continuous release of these chemical compounds into aquatic environment has been increased due to the increase of general use of pharmaceutical compounds in human and veterinary medicines. Figure 1 illustrates the routes of releasing the pharmaceutical compounds into water. These routes include wastewater effluents, human and animal excreta, sewage sludge, medical and industrial waste and land fill leaching .
Depending on the biodegradability and hydrophobicity of these pharmaceutical active compounds, they are naturally reduced by dilution, degradation and adsorption in the environment. Thus, these compounds in water exist in a trace concentration level .
Some of the pharmaceutical active compounds used for birth control, heart medication and painkilling were detected in wastewater in the United State of America (USA) since more than 40 years ago [3, 4, 5]. Literature shows that the pharmaceutical active compounds enter the surface water through different sources such as excretion, bathing, effluent discharging, improper disposal of these compounds and veterinary facilities [1, 6, 7, 8]. In addition, a study conducted in the United Kingdom by Drinking Water Inspectorate reported that many classes of pharmaceutical active compounds are present in wastewater influent . Table 1 represents several pharmaceutical active compounds that were detected in the aquatic environment of United Kingdom (UK) and other European countries [10, 11].
|Pharmaceutical active compounds||Maximum detected concentration (ng/L)||Aquatic environment type|
|Bleomycin||19 (United Kingdom)||Sewage|
|Clotrimazole||34 (United Kingdom)||Stream or river water|
|Erythromycin||80 (United Kingdom)||River water|
|Fluoxetine||290 (United Kingdom)||Sewage|
|Mefenamic acid||1440 (United Kingdom)||Sewage|
|Paracetamol||< 20 (United Kingdom)||Sewage|
|Propranolol||215 (United Kingdom)||River water|
|Tamoxifen||42 (United Kingdom)||Sewage|
|Tetracycline||1000 (United Kingdom)||River water|
|Trimethoprim||1288 (United Kingdom)||Sewage|
|Pharmaceutical active compound||Carbon-based adsorbent||Adsorption capacity (mg/g)||Reference|
|Clofibric acid||Mesoporous silica SBA-15||70|||
|Cephalexin||Amberlite XAD-16 polymer||116|||
|Nalidixic acid||Polystyrene-divinylbenzene, X16||800|||
|Penicillin||Polymer Amberlite XAD-16||1401|||
|Flurbiprofen||Organophilic montmorillonite clay||240|||
|Tetracycline||NaOH-activated carbon produced from macadamia nut shells||455.33|||
|H3PO4-activated carbon produced from apricot nut shells||308.3|||
|Activated carbons produced by KOH activation of tyre pyrolysis char||312|||
|Commercial activated carbon||471|||
|Ciprofloxacin||Hydrous oxides of Al (HAO)||13.6|||
There is no international standard method for drinking water sampling and method of analysis for pharmaceutical active compounds. In addition, a few systematic monitoring studies on measuring the pharmaceutical active compounds in surface water, drinking water and ground water were conducted. Therefore, limited data are available on their occurrence in these aquatic environments to be used in assessing the potential health risk due to the exposure to a trace concentration level of pharmaceutical compounds. However, literature showed that the surface water and ground water sources affected by wastewater discharges have pharmaceutical active compound concentrations less than 100 ng/L, while these compounds were found in the drinking water with a concentration less than 50 ng/L .
2. Treatment technologies for pharmaceutical compounds’ removal from water
The presence of these compounds at trace concentration levels (nanogram to sub microgram per liter) in the aquatic environment has raised a question concerning the efficiency of wastewater treatment techniques in removing of the pharmaceutical active compounds. Many removal techniques such as chlorination, photocatalysis, adsorption, biodegradation and advanced oxidation or ozonation have been investigated for the removal of pharmaceutical active compounds from the aquatic environment [12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25]. Some of these techniques have different disadvantages such as their high cost, high energy consumption and formation of toxic by-products. Adsorption technique has many advantages over these techniques such as it works at mild operation conditions, requires low energy and is efficient and cost-effective. Therefore, it is a promising technique for the removal of pharmaceutical active compounds.
2.1 Adsorption technique
The removal of pharmaceutical active compounds from water by adsorption is considered as one of the easiest and safest techniques since it is easy to design and operate and this technique does not produce any toxic wastes as a by-product and is capable of removing most forms of organic material. The adsorption process includes the accumulation of pharmaceutical compounds on the adsorbent’s surface. Hence, the selection of adsorbent must be precious. The adsorbent must have a capability to accumulate the pollutant from water with high surface area and high hydrophobicity. The efficiency of this technique is mainly depending on the functional group composition, surface area, pore size and the ash content. It also depends on the chemical parameters like temperature, polarity, pH, concentration of the adsorbate and the availability of other competing solutes. The adsorption process also depends on the mobility of the adsorbate molecules toward the external boundary layer of the adsorbent, active surface sites and surface pore size. Many researchers have studied the adsorption of pharmaceutical active compounds from wastewater using different types of adsorbents. Several materials as an adsorbent have been reported in the literature and listed in Table 2 and were tested and investigated for the pharmaceutical active compounds’ removal from aquatic environment, such as silica-based adsorbents [26, 27, 28, 29, 30], polymeric materials [31, 32, 33, 34, 35, 36, 37], clay [38, 39, 40, 41, 42, 43, 44, 45, 46, 47], carbonaceous materials [48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58] and other materials [59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71]. The next sections focus on carbonaceous materials as adsorbents, namely activated carbon and carbon nanotubes.
2.1.1 Activated carbon
Activated carbon is a pure carbon graphite form with amorphous and highly porous structure. It contains different range of pore sizes starting from cracks to slits of molecular dimensions . The first produced commercially activated carbon was in early nineteenth century from wood as a raw material. It has been used for water odor and taste control in 1930 . Nowadays, activated carbon is produced from a wide range of raw organic materials and sources, such as sugar, shells, refinery coke, rice hulls and different types of wood. The main features of activated carbon that make it good as an adsorbent in the adsorption process are the following: (i) its high surface area, (ii) its porosity and (iii) its surface reactivity.
220.127.116.11 Classifications of activated carbons
Activated carbon can be classified based on its activation process or its properties. Based on the activation process, the following are the main two categories based on the activation process:
Physically or thermally activated carbon: the activation process involves carbonization of organic raw materials at temperature ranging from 500°C to 600°C .
Chemically activated carbon: the activation process involves addition of some inorganic salts such as metallic chloride to activate the surface of carbon .
Mattson et al.  suggested another classification, which categorizes activated carbon to acidic or basic activated carbon:
Carbon activated at low temperature range from 200°C to 400°C: this develops an acidic surface that lowers the pH value of the solution. This activated carbon exhibits negative zeta potential and usually adsorbs basic and hydrophilic compounds.
Carbon activated at a high temperature range from 800°C to 1000°C: this develops basic surface that increases the pH value of the solution. Therefore, this type of activated carbon has a positive zeta potential and is usually used for adsorbing acidic organic compounds.
Commercially, activated carbon can be classified as three main types , and they are the following:
Powdered activated carbon (PAC): it has fine granules or powder with particle size less than 1.0 mm and average diameters ranging between 0.15 and 0.25 mm.
Granular activated carbon (GAC): it combines powdered activated carbon with a binder and forms cylindrical shape activated carbon particles with diameters from 0.8 to 130 mm. The main application for this form is for gas purification.
Impregnated activated carbon (IAC): it is impregnated with different inorganic ions.
Polymeric coated activated carbon, which is used in medical field applications.
18.104.22.168 Physicochemical properties of activated carbon
The properties of activated carbon are influenced by the used raw materials and activation method in its preparation process. The porous graphite and graphene sheets that form the activated carbon are connected together and have π-orbitals in the benzene rings, which enable several modifications to be carried out on activated carbon. For example, cooling the activated carbon in the presence of oxygen can produce activated carbon rich with oxides and acidic functional groups, as a result, alter the positive zeta potential of basic activated carbon to negative to be used for different applications. In addition, the surface chemistry, pore structure (volume and diameter) and surface area of activated carbon depend significantly on the employed temperature in the preparation process [75, 79].
22.214.171.124 Activated carbon production
A wide range of raw materials can be used as a starting material for producing activated carbon as stated in Section 2.1.1. The following activation methods are used in activated carbon production:
Thermal activation: this physical process may involve two main steps: the first one to eliminate the volatile matters in the raw materials by carbonizing them thermally at a temperature ranging from 500°C to 600°C and in the second step the porosity and surface are improved by the gasification process. In the gasification process, a carbon dioxide CO2, methane or steam as an oxidizing gas is used at a high temperature of 800–1000°C .
Chemical activation: in this process, inorganic salts such as metallic chloride are added before the carbonization step to improve the micro-porosity as well as the surface area of the activated carbon .
126.96.36.199 Activated carbon for removal of pharmaceutical active compounds
Activated carbon (AC) is widely used in adsorption processes as filtration and purification materials. For instance, in water treatment, activated carbon is used to control taste and odor and to adsorb undesired suspended metals and pollutants . Due to the high surface area and commercial availability of AC, it was studied for removal of different pharmaceutical active compounds. Table 3 summarizes some of these pharmaceuticals. For example, different types of activated carbon were used for removal of tetracycline (antibiotic drug) from aqueous media. Martins et al.  prepared activated carbon from macadamia shells as precursors, the yield was 19.79% and the prepared activated carbon’s surface area was 1524 m2/g. They used it for the tetracycline removal and it had 455.33 mg/g adsorption capacity. Muthanna et al.  reported that the activated carbon was used for removal of three pharmaceutical active compounds (i.e., tetracycline, penicillins and quinolones) and the used activated carbon has 1340.8 mg/g adsorption capacity for tetracycline. Chen et al.  studied the effect of the adsorption parameters (i.e., pH, contact time, initial concentration and temperature) on the removal of tetracycline from aqueous solution using rice husk ash (RHA). They found the adsorption capacity increased from 1.51 to 3.41 mg/g when the initial tetracycline concentration in the solution increased from 5 to 20 mg/l. Another study showed that activated carbon prepared via a chemical activation of apricot shells using phosphoric acid heated in air at 100 °C for 24 hours has 307.6 m2/g surface area and 308.3 mg/g adsorption capacity . In 2016, an activated carbon (TPC-AC) was prepared from tires waste by their pyrolysis and then activated using potassium hydroxide . The prepared adsorbent was tested for tetracycline removal and it has been found that the adsorption process was spontaneous and has adsorption capacity (312 mg/g) higher than the commercial activated carbon. Carl et al.  reported that the adsorption capacity of the commercial activated carbon for tetracycline is directly related to the density of π electrons in the graphene layers on activated carbon and the aromatic ring in the tetracycline.
2.1.2 Carbon nanotube
Single and multiwall carbon nanotube (CNT) materials are graphene sheets rolled-up tubular individually or more than one inside each other. CNTs were discovered by Sumio Ijima in 1991 at NEC Laboratory in Japan using the Arc discharge production method and then characterized using a transmission electron microscope . CNTs have two different structures based on the rolling direction of graphene sheets: (i) armchair nanotube and (ii) zigzag nanotube structure  as shown in Figure 2.
The cylindrical shape of CNT nanostructure can have a length to diameter ration up to 132,000,000:1, which is significantly higher than any other materials . This property was explained by the sp2 hybridization in the carbon atoms that CNTs are composed of in addition to the natural alignment of CNT into ropes attracted together by Van der Waals interaction .
188.8.131.52 Physical properties and chemical reactivity of carbon nanotubes
CNTs form bundles of a highly complex network . They have electrical conductivity that depends on the arrangement of the hexagonal rings along the tubular surface. Due to their extraordinary properties, such as large geometric aspect ratio, nanocavities and electrical conductivity, CNTs are considered as attractive candidates in many nanotechnological applications, including the removal of pharmaceutical compounds in water treatment processes. One of the main drawbacks of carbon nanotubes is that they do not have good suspension properties in aqueous and organic solvents that in turn has made CNTs’ use in industry limited . This disadvantage can be overridden by modifying CNTs chemically with some hydrophilic functional groups that in turn increase CNTs’ suspension in water.
The main distinct properties of the carbon nanotubes are categorized into the following:
Mechanical properties: due to the covalent sp2 bonds formed between the individual carbon atoms, CNTs have high strength and stiffness. According to the reported results, CNTs have elasticity higher than steel by 10–100 times with an elastic modulus 1Tpa . A comparison between some materials, which have good mechanical properties, with CNTs is shown in Table 3.
|Physical property||Material name|
|Tensile strength (Gpa)||150||150||0.008||0.4||0.005|
|Young’s modulus (Gpa)||1200||1054||0.6||208||3.5|
Thermal conductivity: CNTs have thermal conductivity ranging from 2800 up to 6000 W/m K .
Electrical properties: CNT carbon-based material exhibits extraordinary electrical properties and it can be conducting or semiconducting material. The conductive CNTs are found to carry electrical current thousand times higher than copper material .
Chemical reactivity: CNTs can chemically be modified to make them highly soluble in aqueous and organic solutions as well as more efficient for certain applications. Their reactivity is related to the mismatching of π-orbitals, which are caused by the curvatures in CNTs’ structure. In general, smaller nanotube diameters result in increasing their reactivity. Moreover, the reported results showed that chemical modification of sidewalls or end caps of CNTs are also possible .
Based on the CNTs’ properties that have been discussed above, CNT materials and their modified structures are promising for different applications such as water treatment, environmental protection and pharmaceutical active compound removal, material science, medicinal chemistry and others.
184.108.40.206 Carbon nanotube production
CNTs are produced using different techniques, and the most common and widely used techniques are:
Arc discharge technique. Arc discharge technique is the most common and simplest technique for CNT production. As mentioned earlier, CNTs were firstly discovered using this technique. In arc discharge technique, CNTs are produced at low pressure of helium inert gas or any other neutral gas . They are produced through arc vaporization of two separated carbon rods in an enclosed system filled with inert gas . One of the major disadvantages of CNT production using this technique is that the produced CNTs are not pure containing some of the catalytic metals; therefore, they require purification to remove these metals and get clean CNTs.
Laser ablation technique. In 1995, carbon nanotubes were synthesized using a laser beam to vaporize graphite at 1200°C . The pulsed and continuous laser methods are the main two types of laser ablations. Much higher light intensity (100 kW/cm2) is used in the pulsed laser, compared to 12 kW/cm2 in case of the continuous laser type, which is the main difference between these two laser ablation technique types. In the laser ablation method, CNTs are produced and collected on a cooler surface in the reactor system as the vaporized carbon is condensed. In this technique, SWCNTs can be produced from graphite electrodes by adding metal-based catalysts such as Co, Fe and Ni to the system. However, MWCNTs are the main product when a pure graphite electrode is used .
Chemical vapor deposition (CVD) technique. Chemical vapor deposition technique is a simple process and it is believed to be the easiest technique for industrial production of CNTs. In this method, the desired CNT type and quality can be produced by controlling the system production parameters such as temperature, type of catalyst and type of carbon source gases. CVD technique consists of two main steps (catalyst preparation step and then CNT synthesis). In general, to produce CNTs, methane and carbon monoxide gases are dissociated into reactive carbon atoms using an energy source, and then these reactive atoms diffuse over a substrate that is coated by transition metals as a catalyst and heated at a temperature range from 500 to 1000°C .
A comparison between the previously discussed methods for CNT production is summarized in Table 4.
|Chemical vapor deposition||20–100%||Long tubes with diameters 0.6–4 nm||Long tubes with diameters 10–240 nm||Nets are usually mints and often riddled with defects|
|Arc discharge||30–90%||Short tubes with diameters 0.6–1.4 nm||Short tubes with inner diameter 1–3 nm||Short tubes with random sizes and directions and required purification|
|Laser ablation (Vaporization)||Up to 70%||Long bundles of tubes with diameters 1–2 nm||Not suitable and too expensive||Costly and required high power|
220.127.116.11 Carbon nanotubes for pharmaceutical active compounds’ removal
Carbon nanotubes with their excellent properties show considerable adsorption capability for removal of pharmaceutical active compounds. A study in 2009 found that the single wall carbon nanotubes (SWNTs) are more efficient for removal of tetracycline from aqueous solutions than multiwall carbon nanotubes (MWNTs), graphite and activated carbon . This finding was explained through the molecular sieving effect, whereas the tetracycline is bulky molecules failed to seep through inner pores, which indicates the important role of molecules’ size and their accessibility into pores in the adsorbent materials. In 2016, Yu et al.  studied the adsorption performance of MWNTs for removal of ciprofloxacin and found the maximum adsorption capacity is 20 mg/g, which was obtained at pH 4 and 240 min that was attributed by the π-π interaction rather than hydrogen bonding and interaction with oxygenated functional groups on MWNTs. Another study by Yu et al.  showed that the maximum adsorption capacity of MWNTs for tetracycline was 269.54 mg/g, which achieved at 25°C and pH 5 within 80 min.
In order to improve the performance and adsorption capacity of CNTs, different types of modifications can be performed such as graphitization, hydrolyzation, carboxylation and etching with potassium hydroxide (KOH). For example, Ji et al. [98, 99] modified the SWNTs and MWNTs by etching using KOH and tested the etched CNTs for three pharmaceutical active compounds (i.e., sulfamethoxazole, tetracycline and tylosin). They found the adsorption performance of the KOH modified SWNTs (K-SWNTs) and KOH modified MWNTs (K-MWNTs) for sulfamethoxazole and tetracycline was enhanced by around 56% and 84% compared to the unetched SWNTs and MWNTs, respectively. This has been explained by increasing the surface area of the etched CNTs.
Graphene is a two-dimensional carbonaceous nanomaterial formed from a layer of sp2 hybridized carbon atoms. The graphene nanomaterial has exceptional properties such as high specific surface area [98, 99], high electrocatalytic activity , great thermal conductivity , high stiffness and strength  and high speed electron mobility . These unique physical properties attracted great interest of scientist and introduced it for different potential applications. Among these applications is the adsorptive removal of emerging pollutants such as pharmaceutical active compounds.
18.104.22.168 Types of graphene
The following are the common types of graphene:
Single layer graphene (SLG): it is one thick hexagonally arranged sp2 hybridized bonded carbon atoms. The dimensions of SLG vary from nano- to microscale. It can be suspended in an aqueous solution or adhered on a substrate.
Multilayer graphene (MLG): it consists of few flaks of single layer graphene and it is useful in the preparation of nanomaterial composites.
Graphene oxide: it is a single layer or multilayer graphene that has high oxygenated surface and prepared by exfoliation and chemical oxidation of graphite.
Reduced graphene oxide: it is that same as graphene oxide; however, the oxygenated functional groups are reduced chemically, thermally or biologically.
22.214.171.124 Graphene for pharmaceutical active compounds’ removal
The graphene nanomaterials and their modified forms have extraordinary surface area and catalytic activity, and as a result, they can be used in several applications such as adsorptive removal of pharmaceutical active compounds [104, 105, 106, 107, 108, 109, 110, 111]. Gao et al.  investigated the adsorption performance of graphene oxide for tetracycline antibiotic from aqueous solution. They found that the adsorption of tetracycline achieved mainly through a π-π and cation-π interactions with a maximum monolayer adsorption capacity is 313 mg/g and it decreased with an increase in the solution pH or the sodium ions concentration. In 2017, Danna et al.  modified a graphene oxide with decafluorobiphenyl and then investigated the prepared adsorbent for removal of six pharmaceutical active compounds from water namely, carbamazepine, sulfamethoxazole, sulfadiazine, ibuprofen, paracetamol and phenacetin. They found that the adsorption capacities for these compounds are 340.5 μmol/g, 428.3 μmol/g, 214.7 μmol/g, 224.3 μmol/g, 350.6 μmol/g and 316.1 μmol/g, respectively. A study in 2014 showed that the adsorptive removal of acetaminophen, aspirin and caffeine from aqueous solution using graphene nanoplates (GNPs) was thermodynamically spontaneous and exothermic with adsorption capacities of 18.07 mg/g, 12.98 mg/g and 19.72 mg/g for acetaminophen, aspirin and caffeine, respectively .
The surface area of graphene reduces significantly in solutions due to its aggregation, and as a result, the adsorption capacity of graphene is reduced, which is one of the main disadvantages associated with using graphene as adsorbents. Functionalization or modification of the graphene with certain functional group or metals can be the best solution to overcome that disadvantage as well as increase the adsorption capacity of graphene. Lin et al.  functionalized a graphene oxide with magnetic nanoparticles and then studied its adsorptive removal for four tetracycline (TC) pharmaceutical active compounds (i.e., tetracycline, oxytetracycline, chlortetracycline and doxycycline) from aqueous solution. They found that the solution pH and ionic strength had insignificant effect on the TC adsorption and the maximum adsorption capacity is 39.1 mg/g.
Pharmaceutical active compounds are continuously released into aquatic environment via different routes (i.e., human and animal excreta, medical industry’s waste, wastewater effluent, sewage and landfill leaching). That release increases due to the increase of general use of pharmaceutical compounds in human and veterinary medicines. Therefore, these compounds should be removed from the contaminated water to prevent their accumulation, reduce the environmental pollution and provide an additional source of clean water. Removal of pharmaceutical active compounds from aquatic media can be achieved by either conventional or advanced methods. Among them, the adsorption technique has many advantages over the others. Several materials as adsorbents have been reported and discussed in the literature such as silica-based adsorbents, polymeric materials, clay, carbonaceous materials and other materials. Activated carbon, carbon nanotube and graphene oxide among carbonaceous materials show excellent performance and high adsorption capacity for pharmaceutical active compounds. As discussed in this chapter, the activated carbon can be activated using different methods (i.e., physical or chemical activation), while the carbon nanotube can be produced through using one of the following methods: (i) arc discharge, (ii) laser ablation and (iii) chemical vapor deposition. The physical (surface area and porosity) and chemical (functional groups) properties are significantly affected by the followed production method for these carbonaceous materials. Using freely available raw materials for the activated carbon and carbon nanotubes production and their modification with different nanoparticles and functional groups is the future prospect for the adsorptive removal of pharmaceutical active compounds from the aquatic environment.
The support of the Center for Environment and Water in the research institute of King Fahd University of Petroleum and Minerals is highly acknowledged.
Conflict of interest
The author declares that there are no conflicts of interest.