Improved adsorption performance of activated carbon covalently functionalised with sulphur-containing ligands in the removal of cadmium from aqueous solutions
Cadmium is a heavy metal that has high toxic potential. Biological absorption of cadmium can cause severe disease. Cadmium ions are frequently removed from the aqueous environment via adsorption. In this paper, commercially available granular activated carbon (AC) was covalently functionalised with three sulphur-containing ligands—thiodiglycol, thiourea and cysteine. The ligands were introduced in order to improve adsorption capacity for the removal of cadmium ions from aqueous solutions. The AC functionalised with cysteine and thiourea had twice the adsorption capacity of Cd(II) than the pristine AC. The introduction of sulphur-containing ligands also improved the adsorption kinetics, i.e. the adsorption kinetic rates were ca. 2–10 times higher than the pristine AC. This enhancement of the adsorption performance was systematically studied in terms of morphology, texture, structural features, the content of sulphur-containing ligands and surface chemistry.
KeywordsActivated carbon Functionalisation Cadmium Adsorption kinetics
Cadmium is one of the most toxic elements, and can seriously affect human health. The dispersion of cadmium into the environment results from industrial processes, domestic activities, agricultural practices (e.g. fertilisers), the use of Cd(II)-containing goods and the disposal of waste (Sukreeyapongse et al. 2002). Inhalation of cadmium-containing fumes can cause various severe diseases, including pneumonitis and pulmonary oedema, and can even result death (Fernandez et al. 1996). Moreover, from the epidemiological studies, this element is known to be carcinogenic, and exposure to it should be limited to the lowest possible levels (Hartwig 2013). In addition, the toxicity of has a cumulative character because its biological half-life in the human body is between 10 and 30 years. Most absorbed Cd(II) cadmium accumulates in the liver and kidneys (Lind et al. 1997).
The removal of Cd(II) from aqueous systems is generally realised via different treatment technologies, namely adsorption (Attar et al. 2018; Izidoro et al. 2013; Huang et al. 2013), reverse osmosis (Kheriji et al. 2015; Pirsaheb et al. 2017), nanofiltration (Gao et al. 2016; Liu et al. 2016; Saljoughi and Mousavi 2013) and precipitation (Islamoglu et al. 2006; Tan et al. 2013).
Activated carbon (AC) is commonly used in the adsorptive removal of various heavy metals from aqueous solutions, including Cd(II) (Youssef et al. 2004; Huang et al. 2007; Machida et al. 2012). Its adsorption properties primarily depend on surface chemistry features, while their textural properties are of minor importance (Pyrzyńska 2010). AC is routinely subjected to oxidisation in order to introduce new surface acidic functional groups, and to enhance their adsorption properties. Metal uptake occurs via non-covalent electrostatic interactions between heavy metal cations and negatively charged surface functional groups (Radovic et al. 2001). The introduction of surface oxygen-containing acidic groups obviously improves the adsorption capacity (Rodríguez-Estupiñana et al. 2013); however, selectivity towards a specific heavy metal ion is only moderate (Faur-Brasquet et al. 2002).
This work is focused on the comparison of three sulphur-containing ligands (which are covalently attached to AC) used in the removal of Cd(II) ions from aqueous solutions. It is known that Cd(II) forms stable and strong complexes with sulphur-containing molecules, e.g. cysteine (Jalihenvand et al. 2009) and thiourea (El-Bahy et al. 2003). Recent works have demonstrated that even zero-valent sulphur nanoparticles also have the potential to bind Cd(II) ions (Ghanemi et al. 2011). More complex molecules also form stable complexes with Cd(II). For example, Yola et al. demonstrated that glassy carbon functionalised with 2-thiolbenzimidazole selectively reacts with Cd(II), and can be used for the construction of electrochemical sensors of Cd(II) ions in human plasma (Lütfi Yola et al. 2014). It has also been shown that calixarene and p-aminophenol moieties have a high affinity for Cd(II) (Göde et al. 2017; Gupta et al. 2013). Cadmium ions also form stable complexes (even at low concentrations) with sulfisoxazole. In this case, Cd(II) ions do not directly interact with the sulphur moiety, but form a complex with sulfisoxazole via a coordinative bond with the oxazole group.
Materials and methods
Activated carbon (particle size between 0.3 and 0.5 mm) was purchased from FLUKA, and used as received. Before any chemical treatment or analysis, each sample of AC was thoroughly dried in an oven (typically 2 h, 420–430 K). The mass loss during drying was between 5 and 7%. Dimethylformamide (DMF), thionyl chloride, triethylamine, HNO3, thiodiglycol, thiourea and l-cysteine ethyl ester hydrochloride were purchased from Sigma. All reagents had a purity of greater than 98.5%. The solvents were dried before use.
The obtained AC-COCl was used as the reagent for subsequent grafting with thiourea or l-cysteine. The sulphur-containing ligand (thiourea or l-cysteine ethyl ester hydrochloride, 2 g) was dissolved in anhydrous DMF with a small amount of triethylamine (10 mL). This solution was dripped into a bulb in which AC-COCl was present. Then, the mixture was magnetically stirred under Ar at 330 K for 24 h. Next, the product obtained from the reaction with l-cysteine ethyl ester hydrochloride was treated with 100 mL 2 M HCl for 15 h at 330 K. This treatment was necessary in order to hydrolyse the ester bond. The functionalised AC with l-cysteine and thiourea was labelled AC-cysteine and AC-thiourea, respectively. After functionalisation, the AC-cysteine and AC-thiourea materials were rinsed in an excess of acetone, and dried in air at 380 K.
The functionalisation of AC with thiodiglycol was as follows. Two grams of AC-COOH, 20 mL of thiodiglycol and 4 mL of concentrated sulphuric acid were added to 100 mL DMF. The obtained suspension was refluxed for 10 h, and then filtered and rinsed with an excess of acetone through a filter paper. Next, the sample was dried in air at 380 K.
In addition, the pristine AC was annealed in a tube oven at 1273 K for 1 h under an Ar flow. This material was labelled AC-1273 K.
The morphology of the activated carbons was studied by scanning electron microscopy (SEM, Zeiss Merlin). The infrared spectra were acquired using a Shimadzu 8201 instrument with a spectral resolution of 2 cm−1. The studied carbon sample (ca. 1.0–1.5 mg) was ground in an agate mortar and mixed with KBr (300 mg), and subsequently pressed into a pellet. Low-temperature nitrogen adsorption was carried out at 77 K using an Autosorb iQ analyser. The samples were degassed at 420 K under a vacuum (10−5 mbar) prior to the measurements. Thermogravimetric analyses were carried out using a TA Q-50 instrument under nitrogen, with a heating rate of 5 K min−1. The point of zero charge was measured by the mass titration method (Noh and Shwarz 1990). The pH of the starting 0.01 M NaCl solution (30 mL) was adjusted to 10 by adding small volumes of 0.1 M NaOH and HCl. Then, the desired amount of carbon sample was added, and the pH was measured after it reached the equilibrium value (typically after 30 min). The suspension was continuously purged with Ar to eliminate interference from ambient CO2, if any. The mass titration curves are shown in Figures S1–S6 (Supplementary Data).
The content of the surface functional groups was evaluated by the Boehm titration method (Boehm 2002). About 300 mg of the studied carbon material (the mass was precisely monitored) was placed in a vial containing 50 mL of 0.05 M base (NaOH, Na2CO3 and NaHCO3). The vials were tightly sealed with parafilm tape and shaken for 24 h. Then, a 5 mL aliquot of each filtrate was titrated with 0.05 M HCl using a Metrohm Titrando automatic titrator. The aliquots were bubbled with Ar to prevent absorption of ambient CO2. The titrations were done in triplicate. The content of acidic functional groups was calculated under the assumption that: the NaOH neutralised the carboxylic, phenolic and lactonic groups; the Na2CO3 reacted exclusively with the carboxylic and lactonic groups; and the NaHCO3 neutralised the carboxylic groups only. The ash content was evaluated from the thermogravimetric curve (burning in an O2 atmosphere at a heating rate of 5 K min−1).
Adsorption of Cd(II)
The adsorption of Cd(II) was investigated at pH 7 (ammonium acetate solution) at constant temperature (298 ± 1 K). The stock solution of cadmium nitrate (1000 mg/L) was purchased from Merck. For the adsorption studies, a 50 mg sample of carbon material was added to a 10 mL solution of Cd(II) (initial concentration between 2 and 100 mg/L), with subsequent vigorous shaking for 4 h. Then, the carbon adsorbent was filtered onto a paper filter, and the concentration of Cd(II) in the recovered solution was measured via an atomic absorption spectrometer equipped with a flame atomisation probe (PerkinElmer 3110). The adsorption kinetics studies were carried out as follows. Samples containing 50 mg of the studied material were added to 50 mL of the solution (20 mg/L, pH = 7) of Cd(II). The suspensions were mechanically shaken for 5–240 min. The adsorption capacity for each contact time was determined by individual testing. The operational parameters of the measurements are shown in the Supplementary Data.
Results and discussion
Textural parameters of studied activated carbons
Total pore volume (cm3/g)
Micropore volume (cm3/g)
Mesopore volume (cm3/g)
Surface area (m2/g)
AC 1273 K
Elemental composition and surface chemistry
Elemental composition (wt%), ash content (wt%) and content of sulphur-containing ligand (wt%)
Content of ligand
AC 1273 K
The spectrum of pristine AC has three characteristic bands, located at 1550 cm−1, 1285 cm−1 and 1100 cm−1. A band at 3200–3500 cm−1 was observed in all samples, and originates from adsorbed moisture. Its diagnostic utility is of marginal importance and the changes in this spectral feature during functionalisation were not analysed. The feature at 1550 cm−1 is typical for carbon materials, and results from conjugated stretching vibrations in aromatic C–C bonds (Pakuła et al. 2005). The bands at 1285 cm−1 and 1100 cm−1 can be attributed to the stretching vibrations of C–O moiety in various aliphatic functionalities (ester and ether). The spectrum of annealed AC had the same features as the pristine AC. Note that AC always contains some amount of oxygen, and the presence of oxygen moieties was an expected finding.
The spectrum of the AC-COOH sample revealed two strong features (1195 cm−1 and 1710 cm−1), which are not present in the pristine and annealed AC. Both these bands correspond to the stretching vibrations of C–O and C=O bonds in the carboxylic functional group. This observation proves that the oxidation process was successful, and that surface acidic groups were introduced onto the surface of the AC. The ACs functionalised with sulphur-containing ligands had an intensive band at 1540–1550 cm−1, which originated from the C=C stretching vibration. There were two sharp bands, located at 1120 cm−1 and 1225 cm−1, in the AC-thiourea sample. The first feature had a frequency typical for the C=S stretching vibration (Lin-Vien et al. 1991). The second band may be attributable to C–O vibrations in the free carboxylic groups, i.e. those not used in the conjugation of thiourea molecules. On the other hand, this band could also correspond to C–N symmetric modes (which are typical for the thiourea molecule). Importantly, the C=O stretching vibration was downshifted to 1700 cm−1 (in comparison with the AC-COOH sample), and this observation directly proves that the amide bond between the carboxylic groups on the carbon surface and the amine groups in thiourea was formed. The spectrum of AC-thiodiglycol had several strong bands, i.e. 1115 cm−1, 1210 cm−1, 1710 cm−1 and 2900 cm−1. The last band originates from C–H stretching vibrations. The former two bands are associated with C–O stretching modes. The band located at 1710 cm−1 is obviously from the symmetric C–O vibrations in the ester moiety. There was also a band located at 740 cm−1, which can be ascribed to the C–S stretching vibration. This spectral feature, even in the case of pure thiodiglycol and other compounds containing C–S moieties, was of relatively weak intensity (Socrates 2001).
The following bands appeared on the spectrum of the AC-cysteine sample: 1215 cm−1, 1330 cm−1, 1690 cm−1 and 2900 cm−1. The first and last features originate from C–O and C–H stretching vibrations, respectively. The band at 1330 cm−1 can be attributed to the C–N stretching mode. The spectral feature located at 1690 cm−1 originates from the C=O moiety in the amide group. There was also a weak band at 3430 cm−1, and this feature was distinguished from the broad O–H band. This band is likely due to N–H stretching vibrations. The band from the S–H functionality (ca. 2600 cm−1) was absent. This observation suggests that the cysteine molecule was bound to the surface of the AC via both amide and thioester bonds. Another explanation for this absence may be related to the fact that the S–H mode, even in the case of pure cysteine, has a very low intensity (Figure S1). In other words, even if this functionality is present, it has too low intensity to be discriminated from the background. To conclude, analysis of the FT-IR spectra clearly indicated that the surface of the AC was functionalised, and the observed bands undoubtedly agree with the structural units of the covalently attached surface-containing ligands.
Content of surface acidic groups and point of zero charge (pHZC) for studied activated carbons
Total content of acidic groups (mmol/g)
Carboxylic groups (mmol/g)
Lactonic groups (mmol/g)
Phenolic groups (mmol/g)
AC 1273 K
In the case of AC functionalised with cysteine, the reduction in surface acidity was the smallest. This is because the cysteine molecule, after conjugation to AC, donates an extra carboxylic group. As a consequence, the point of zero charge in the AC-cysteine sample was lower than in the AC functionalised with two other sulphur-containing ligands. The largest reduction in surface acidity (by nearly twice) was observed in AC-thiodiglycol. In this sample, the low content of acidic groups was accompanied by the highest pHZC. The content of thiodiglycol ligands in the AC-thioglycol sample (12.6%) was comparable to that in the AC-thiourea material (14.7%); however, the reduction in surface acidity for the AC-thioglycol sample was much larger. Hence, it is highly likely that thiodiglycol molecules (which contain two O–H groups) are cross-linked onto the surface of AC. Another interesting finding was that the relative content of carboxylic groups in AC-cysteine, AC-thioglycol and AC-thiourea was between 49 and 52%, and this value was of the same order as in the AC-COOH sample. The same observations were made for the relative content of lactonic and phenolic groups in sulphur-functionalised carbons and AC-COOH material.
Adsorption of Cd(II)
Content of sulphur-containing ligand, adsorption capacity of Cd(II), Langmuir adsorption affinity constants (K) and adsorption kinetics rates (k2) for studied activated carbons
Content of ligand (wt%)
Adsorption capacity Γmax (mg/g)
k2 (min−1 mg−1 g)
AC 1273 K
Factors influencing the adsorption of Cd(II)
The thermodynamic predictions showed that the Cd(II) ion is the only component in the pH range between 0 and 7. (Its relative abundance exceeded 99%.) The adsorption studies were carried out at pH 7. This value is higher than the point of zero charge of all the studied carbon adsorbents. Hence, their surface net charge is negative, and the electrostatic attraction of Cd(II) is possible. As shown in many previous works, the adsorption of Cd(II) (and other heavy metal cations) is generally driven by electrostatic interactions due to the presence of negatively charged surface acidic groups, which can compensate for the positive charge of Cd(II). Our findings are in agreement with these arguments. The surface-oxidised carbons (and those subsequently functionalised with sulphur-containing ligands) had substantially better adsorption performance than the pristine material. This behaviour is unquestionably related to the presence of surface functionalities; however, the annealed AC showed a relatively high uptake of Cd(II) (4.52 mg/g), although its surface acidity was very weak (0.05 mmol/g). It is highly likely that the adsorption of Cd(II) is a combination of two parallel and independent mechanisms. The first one is based on the compensation of electric charge on the negatively charged surface groups. The second mechanism likely involves non-covalent interactions with graphene layers, which can act as Lewis bases and form electron donor–acceptor complexes with positively charged Cd(II) ions. Note that AC is a highly disordered material, and does not have long-distance structural ordering. The high disorder implies that boundary hexagons in graphene layers may have a different charge density than ‘internal’ hexagons. This diversity realistically explains the observed uptake of Cd(II) onto annealed AC.
The studied carbon materials had very distinct textural properties; for example, the specific surface area varied between 14 m2/g (AC-thiodiglycol) to 1083 m2/g (pristine AC). A direct comparison of the textural properties with the maximum adsorption capacity did not provide a clear relationship between porosity and adsorption performance. This finding is in agreement with the opinion presented by Goyal et al. (2001).
The determined values of the point of zero charge (Table 3) can be used to predict the effect of pH on the adsorptive removal of Cd(II). The decrease in pH should lower the uptake of Cd(II). This is due to the protonation of the surface acidic groups and suppression of the electrostatic attractions between the negatively charged adsorbent surface and positively charged Cd(II) ions. This behaviour has been observed previously in surface-oxidised AC (Tang et al. 2017) and carbon cloth (Rangel-Mendez and Streat 2002). The uptake of Cd(II) at a pH below the point of zero charge should also be lower. Liu et al. (2008) demonstrated that the uptake of Cd(II) onto carbon nanotubes functionalised with l-cysteine under highly acidic conditions (pH = 2) was substantially reduced. (The adsorption capacity was ca. 15 times lower compared to pH = 7.) On the other hand, increasing the pH to above 8 is associated with the precipitation of cadmium hydroxide (Li et al. 2013). To conclude, the optimal pH for the adsorption of Cd(II) by AC functionalised with sulphur-containing ligands should be between 5 and 7.
Adsorption kinetics of Cd(II)
In order to compare the adsorption kinetics data in a quantitative way, a pseudo-second-order adsorption kinetics model was applied. The corresponding adsorption kinetics rates were evaluated from the linearised form of the model (Ho and McKay 1999). The obtained values are listed in Table 3. (The goodness of fit was greater than 0.98 in each case.) The adsorption kinetics rates for the pristine, annealed and HNO3-treated ACs were at a similar level, i.e. 0.141–0.152 min−1 mg−1 g. The introduction of thiodiglycol and cysteine doubled the kinetics rate (to 0.310–0.368 min−1 mg−1 g). The highest adsorption kinetics rate was found for AC functionalised with thiourea because, in this case, the rate was an order of magnitude higher than pristine AC, AC-1273 and AC-COOH. There was no correlation between the specific surface area and the obtained kinetics rates. This was especially evident for AC-COOH and carbon functionalised with sulphur-containing ligands. These materials had comparable specific surface areas (Table 1); however, AC-thiodiglycol, AC-cysteine and AC-thiourea were characterised by at least a twice as fast adsorption process. This is likely related to the presence of sulphur-containing ligands.
Activated carbon was functionalised with three sulphur-containing ligands—thiodiglycol, thiourea and cysteine. The functionalisation included pre-functionalisation in order to introduce surface acidic groups. The covalent binding of sulphur-containing ligands was achieved via amide or ester bonds. The surface-functionalised carbons had substantially lower porosity (10–15 times lower) than the pristine AC. The presence of introduced ligands was confirmed by FT-IR spectroscopy, elemental analysis and thermogravimetry. The content of ligands was between 8.8 and 14.7 wt%, and the highest value was found for carbon functionalised with thiourea. All of the surface-functionalised ACs had an acidic character, and their point of zero charge was between 5.15 and 5.96. The chemical composition of the surface was tracked by Boehm titration. The content of acidic groups was the highest (5.60 mmol/g) for the HNO3-treated carbon. The surface acidity decreased after functionalisation with sulphur-containing ligands (3.22–4.29 mmol/g). It was found that the adsorption performance was strongly influenced by surface chemistry. The pre-functionalisation with HNO3 resulted in an increase in adsorption capacity from 6.19 to 9.50 mg/g. A further increase in adsorption capacity was obtained after binding the sulphur-containing ligands, and the increase was observed for each ligand. The highest adsorption capacity was found for AC modified with thiourea. Finally, it was shown that the increase in the uptake of Cd(II) was directly related to the presence of sulphur-containing ligands. The introduction of sulphur-containing ligands also improved the adsorption kinetics because the adsorption kinetics rates were an order of magnitude higher than in the pristine, annealed and HNO3-treated AC.
P. Pietrowski thanks the Central Institute of Labour Protection (Łódź, Poland) for low-temperature nitrogen adsorption measurements. Ajaya Bhattarai acknowledges the Erasmus Mundus fellowships for the Postdoctoral research work at the Faculty of Chemistry, University of Warsaw, Warsaw, Poland.
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