1. Introduction
The removal of pollutants from urban and industrial wastewaters is one of the most important issues to be solved by modern research. In fact, water pollution has increased in the last decades by the virtue of the disposal of industrial effluents enriched with toxic species [
1] which has hazardous effects on flora, fauna, and humans. Moreover, the growth of the world population results in a limitation of water supplies and scarcity of water resources, therefore the necessity of clean water has become fundamental for our society [
1,
2].
In the last decades, new chemicals were detected in wastewaters as a result of new industrial processes and increased consumption of pharmaceuticals and personal care products [
3,
4].
Among other pollutants, synthetic dyes [
5] can be found in the effluent of many industries, including textile, plastic, printing, and dye manufacturing companies. In this group of contaminants, the cationic organic aromatic dyes are ones of the most present pollutants, released from industries, causing various harmful influences on aquatic bodies as well as organisms. Therefore their removal has been deeply studied in recent years [
6,
7].
Crystal violet (CV) belongs to this group of contaminants. It has been widely used as a dermatological agent and biological stain as well as a coloring agent for dyeing leather, silk, wool, paper, and cotton [
6,
7,
8,
9].
Several techniques have been used to remove organic pollutants and synthetic dyes from urban and industrial wastewaters such as membrane separation [
2], biological [
10] and electrochemical treatments [
11], flocculation, liquid-liquid extraction [
12], advanced oxidation processes (AOP) [
13] and coagulation. Out of methods listed above, the adsorption technique [
14] has attracted considerable attention because of the simple procedures needed, because no toxic substances are produced, because of ability to treat concentrated forms of the pollutants, and to reuse the spent adsorbent via regeneration. Moreover, it plays a central role in drinking water purification and wastewater treatments [
3,
7,
15,
16].
Mesoporous materials, thanks to their large and tunable porosity find potential applications in catalysis, encapsulation of proteins, filtration and separation of large molecules, membrane technology, drug delivery, dosing, sensing, among many others [
17,
18] Furthermore, their large specific surface area and huge number of adsorbing active sites suggest their possible application as adsorption materials [
19].
In particular, alumina found various technological applications, i.e., as electrical insulator, presenting exceptionally high resistance to chemical agents, as well as giving an excellent performance as catalyst or support for many applications [
20,
21].
Mesoporous γ-Al
2O
3 has been not only widely applied in all the field mentioned above [
17,
21,
22] but in recent years the chance to synthesize hybrid materials to enhance the useful properties of an oxide opens the way to very interesting opportunities. The literature reports that many oxides have been modified using organic moieties able to enhance the adsorption capacity of the original material. Among others, bio-based substances (BBS), extracted from composted green wastes, are very interesting macromolecules with a complex lignin-derived structure and characterized by several functional groups (they contain long aliphatic chains, aromatic rings, and several acid and basic functional groups such as carboxyl, primary and substituted amine and amide, carbonyl, hydroxyl, phenol, ether or ester [
23,
24]), appealing for their importance in the valorization of the organic refuses. BBS have been found to exhibit typical properties of anionic surfactants and polyelectrolytes [
25,
26] and have been used in the formulation of detergents, textile dyeing baths, emulsifiers, auxiliaries for soil/water remediation, flocculants, and dispersants, as binding agents and templates for ceramics manufacture, as well as for application in agriculture and animal husbandry [
27]. BBS have been reported to bear chemical similarities with humic substances, and to exhibit enhanced adsorption capacity towards polar pollutants given the presence of several carboxylate and phenolic groups carrying negative charges at neutral pH, as well as photosensitizing properties [
27] promoting significant mineralization of the organic carbon [
24].
Hence, the aim of this study is to prepare new hybrid materials by surface immobilization of negatively charged BBS molecules on the positively charged surface of γ-Al
2O
3 taking advantage of electrostatic interactions occurring between them. After characterization and stability test, the materials were used as adsorbents and compared with other systems cited in the literature (reported in
Table 1) to evaluate their adsorption capacity towards the cationic dye Crystal Violet (CV). Once identified the best adsorbing material for CV removal, two Contaminants of Emerging Concern (CECs) [
28], namely Atenolol and Carbamazepine, were tested.
2. Experimental
2.1. Materials
γ-alumina was kindly supplied by Centro Ricerche FIAT (Orbassano (TO), Italy), the X-ray diffraction pattern confirms the expected crystalline structure of the material consistent with the reference card 01-075-0921 related to γ-Al2O3. As a simple electrostatic interaction of the support with the BBS molecules is not expected to modify the crystalline structure of the support, the hybrid sample diffractograms are not shown.
BBS were extracted from composted organic refuses (from urban public park trimming and home gardening residues) aged for more than 180 days supplied by ACEA Pinerolese Industriale (Pinerolo (TO), Italy) [
23]. The extraction procedure was described elsewhere [
53].
Crystal violet (CV) was purchased from Merck (Milan, Italy) and used without any further treatment to prepare solution in ultrapure MilliQ water for adsorption tests.
Carbamazepine and Atenolol were provided by Sigma-Aldrich (Milano, Italy) in analytical purity ≥99.0% and used in adsorption experiments.
All aqueous solutions for High-Performance Liquid Chromatography (HPLC) analysis were prepared using ultrapure water Millipore Milli-QTM (resistivity >18 MΩ). All chemicals were used without further purification.
2.2. Preparation of Hybrid Materials
The γ-Al2O3 particles were used as support for different amounts of BBS immobilized at their surface by simple electrostatic interaction occurring between the two components carrying opposite surface charges.
Hybrid materials were prepared by mixing 1 g of γ- Al2O3 in 20 mL of distilled water containing 0.1, 0.2, and 0.4 g of BBS under stirring for 24 h at 25 °C. The pH of solution was about 6.5 during the preparation. The samples were washed with 10 mL of distilled water for 10 min and every time centrifuged at 4000 rpm for 10 min. The washing solution was tested using UV-Vis spectrophotometer to evidence the presence of leached BBS molecules. The procedure was carried out several times, till the washing solution did not evidence the presence of BBS in the UV-Vis spectra. Drying process was performed in the oven at 40 °C for 24 h. Hybrid samples were named A-BBS0.1, A-BBS0.2, and A-BBS0.4, the reference pure alumina sample was indicated as A.
2.3. Characterization Methods
X-ray diffraction (XRD) analyses of alumina support was obtained using a X’Pert PRO MPD diffractometer from PANalytical (Royston, UK), equipped with Cu anode and working at 45 kV and 40 mA in a Bragg-Brentano geometry. In this study, the flat sample-holder configuration was used.
Fourier transform infrared (FTIR) spectra were recorded in transmission mode by means of a Bruker Vector 22 spectrophotometer equipped with Globar source, DTGS detector (Billerica, Massachusetts, USA), and working with 128 scans at 4 cm−1 resolution in the 4000–400 cm−1 range. Samples were dispersed in KBr (approximatively, sample: KBr weight ratio was 0.045).
Nitrogen adsorption-desorption experiments were carried out using an ASAP 2010 Micromeritics volumetric apparatus (Norcross, GA, USA). Before the measurements, the samples were outgassed at 40 °C for 24 h. Specific surface areas (SSA) were calculated using the Brunauer, Emmett and Teller (BET) method. Pore volumes (PV) and Pore Size Distribution (PSD) were determined by the Barrett, Joyner, and Halenda method [
54] applied to the isotherm desorption branch.
Zeta potential measurements were performed on the instrument Zetasizer ZS90 by Malvern (Malvern, UK). 10 mg of BBS and hybrid materials were suspended in 20 mL of deionized water under constant stirring (400 rpm) for 15 min. The zeta potential measurements were performed starting from the natural pH of the suspension then decreasing it point by point by addition of 0.1 M HCl and successively increasing it with 0.1 M NaOH. A digital pH meter (Metrohm, model 827 pH lab, swiss mode, Herisau, Switzerland) was used to measure the pH of the solution.
Thermo-gravimetric analysis (TGA) was carried out using a TA Q600 (New Castle, DE, USA). Thermal analyses were performed with a heating ramp of 10 °C/min from RT to 600 °C under air in order to quantify the amount of BBS immobilized on γ-Al2O3 particles.
2.4. Adsorption Procedures
2.4.1. Analytical Instruments
UV-Vis spectrophotometer (Varian Cary 300 Scans, Agilent, Santa Clara, CA, USA) was used to determine the adsorption of CV (maximum absorbance at 584 nm).
A Merck-Hitachi liquid chromatographer (Knauer, Berlin, Germany) equipped with Rheodyne injector L-6200 and L-6200A pumps for high-pressure gradients, L-4200 UV-Vis detector, and a LiChrocart RP-C18 column (Merck, Milano, Italy, 12.5 cm × 0.4 cm) was used to determine the concentration of atenolol and carbamazepine during the experiments. The detection wavelength was set at 224 nm for atenolol and 284 nm for carbamazepine). Isocratic elution (1 mL min−1 flow rate) was carried out with 60% of phosphate buffer 1 × 10−2 M at pH 2.8 and 40% acetonitrile and retention times were 5 min.
2.4.2. Kinetic of CV Adsorption
The kinetic of the adsorption was followed contacting 10 ppm of CV with 20 mg of adsorbing hybrid materials (total volume 10 mL) at pH 6.5 (therefore, a CV:adsorbent ratio of 1:2 wt was applied to all the preliminary measurements). The mixture was stirred vigorously under isolated orbital mixing plate (rotation at 1000 rpm) keeping the temperature at 15 °C. The adsorption phenomenon was followed measuring every 10 min the absorbance of the dye in the supernatant after centrifugation (at 4000 rpm for 10 min) by means of a UV–Vis spectrophotometer. Although an equilibrium time of less than or equal to 10 min was evidenced, all the experiments were carried out leaving adsorbent and adsorptive in contact for 30 min. All the measurements (data not showed) were carried out in duplicate and the average data are reported.
2.4.3. CV Adsorption Study and Model Application
The adsorption experiments performed on the hybrid absorbents were carried out at 15 °C and pH = 6.5 (natural value of the suspensions in CV) modifying the relative amount of CV and adsorbent in order to explore a wide Ce range (10 to 100 mg of dye were mixed with 20 mg of adsorbents in 10 mL).
Adsorption studies were carried out using 20 mg of adsorbents in contact with 10 batches containing 10 mL of different concentration of CV aquatic solution (from 10 to 100 ppm). The batches were sealed and placed in a shaker for 30 min at 15 °C and pH = 6.5 to obtain the measurement of the adsorption capacity. The experiments were performed in duplicate and average values were reported.
The adsorption capacity was calculated by using Equation (1):
where
C0 (mg/L) is the initial dye concentration and
Ce (mg/L) is the concentration of dye at equilibrium,
V (L) is the volume of dye concentration,
W (g) is the mass of the adsorbent, and
qe (mg/g) is the amount of dye CV adsorbed. The percentage removal of CV was calculated from the formulae given below:
Freundlich and Langmuir models were applied to the experimental data.
Freundlich Model
The Freundlich model considers an adsorption taking place on a heterogeneous surface. The isotherm model can be represented by the following equation:
where
Ce is the adsorbate equilibrium concentration expressed in mg/L,
is the amount of adsorbate in the adsorbent at equilibrium expressed in mg/g,
KF is the Freundlich constant representing the affinity of the adsorptive towards the adsorbing material, 1/n is the Freundlich constant, representing the degree of affinity adsorptive/adsorbing material and indicating how much the adsorption process is favored (
n < 1 indicates a poor adsorption and the desorption as the favored process, 1 <
n < 2 indicates a good equilibrium between adsorption/desorption, 2 <
n < 10 represents a very good adsorption going towards an irreversible phenomenon [
55]).
KF and n can be determined from the linearized plot ln qe vs. Ce (not reported for the sake of brevity).
Langmuir model
The Langmuir model assumes the adsorption reaches a monolayer of coverage [
56].
The general equation is
where
is the adsorbate equilibrium concentration expressed in mg/L,
is the adsorbate initial concentration expressed in mg/L,
is the amount of adsorbate in the adsorbent at equilibrium expressed in mg/g,
is the monolayer coverage capacities expressed in mg/g,
is the Langmuir constant indicating the ratio of reagents and products at the equilibrium.
In the linearized form Equation (4) becomes
or
The plot reporting allows to obtain q0 and KL.
The value,
indicates if the adsorption is unfavored (if
), if it shows a linear trend (if
) or if the adsorption is favored (if
).
indicates that the reaction is irreversible.
2.4.4. Contaminants of Emerging Concern (CECs) Adsorption Study
Kinetic studies were performed using 10 ppm of Atenolol and Carbamazepine in contact with 20 mg of A-BBS0.4 in a volume of 10 mL, at 20 °C and pH 6.5. Adsorption studies were performed using different amounts of Atenolol and Carbamazepine in contact with 20 mg of A-BBS0.4 in duplicate. The mixtures were kept under stirring at 20 °C, pH 6.5 and the residual amount of contaminants was measured at 30 min-contact time in order to obtain the adsorption isotherms.
4. Removal of CECs
Figure 7 reports the kinetic of adsorption relative to two different CECs, Carbamazepine and Atenolol, whose structures are shown in
Scheme 2. These CECs were chosen on the basis of their chemical structure: both molecules show positively charged structure at pH 6.5 [
58], but while Atenolol possessed a branched aliphatic chain, Carbamazepine is a polyaromatic and more compact molecule.
The efficiency of the material remained acceptable with the larger polar molecule atenolol which is more easily adsorbed with 51% of removal in 30 min thanks to its polar structure and consequently stronger interaction with adsorbent, whereas in the case of carbamazepine only 32% of removal was reached in 30 min suggesting a worse affinity between the material and the polyaromatic substrate.
Langmuir and Freundlich models were also applied to CEC adsorption and the most significant results are reported in
Table 5. Good fittings were observed applying both Langmuir and Freundlich models as indicated by very good values of r
2.
qm obtained by these models defines the amount of Carbamazepine and Atenolol saturating the samples (monolayer amount): the values obtained for the Carbamazepine and Atenolol correspond to maximum adsorption of about 3.06 and 3.26 mg/g respectively. A comparison study of capacity of adsorption for different adsorbents was performed and the results are reported in
Table 6. Considering the literature, we have reached promising results in particular in the adsorption of Carbamazepine.
5. Conclusions
New hybrid absorbents were prepared following a very easy procedure and characterized via TGA, XRD, Zeta potential, FTIR spectroscopy, and nitrogen adsorption then, they were tested towards the removal of selected pollutants. BBS can functionalize the alumina surface given the electrostatic attraction of opposite charges carried by the two components. In this way, cationic pollutants can be captured by the negatively charged surface of the adsorbent. The electrostatic interaction is, therefore, the major driving force for the adsorption process, but the polarity of the pollutant molecules affects both kinetics and adsorbed amounts as less polar molecules interact at lower extent and in longer time.
The strength of these new hybrid materials is not so much in photocatalytic efficiency, (the comparison in CV removal is reported in
Figure 8), as in the simplicity of preparation and in its definitely “green” way. In fact, not only the efficiency of the materials should be taken in account but also other aspects deserve to be considered, i.e., the easiness and the green aspects related to the materials preparation procedure and the energy and economic savings. In fact, several materials with very good performances reported in
Table 1 are prepared following complex procedures and use of non-green processes. In order to perform this comparison, we considered only the materials with better performance in CV adsorption with respect to the
A-BBS hybrid system and we classified them on the basis of the following aspects: energy consumption (calcination and/or thermal treatment at temperature higher than 200 °C), multistep/complex synthesis, use of non-green compounds (acids, bases, solvents and so on). In the classification we assigned a color code from green to orange to red considering the absence of these negative aspects or their simultaneous presence during the preparation. As evidenced in
Figure 8, several materials possessing much better adsorption capacity with respect to the
A-BBS system show an orange or red color, whereas only few, as
A-BBS0.4, show the green color.
From this perspective, the A-BBS materials can be developed further and their performance optimized in view of their use in in-field applications.