Facile synthesis of Al2O3/Sodium dodecyl sulphate/2-aminophenol composite for efficient removal of Pb(II), Cd(II), and Co(II) ions from aqueous media

ABSTRACT In this paper, Al2O3 nanoparticles with an average crystallite size of 35.56 nm are facilely synthesised using the combustion procedure in the presence of glutamine as organic fuel. Besides, Al2O3/Sodium dodecyl sulphate/2-aminophenol composite is synthesised and applied as a novel adsorbent for the removal of Pb(II), Cd(II), and Co(II) ions from aqueous media. The synthesised samples are characterised using several tools such as X-ray diffraction (XRD), elemental analysis, Fourier-transform infrared spectroscopy (FT-IR), and field emission scanning electron microscopy (FE-SEM). In the case of Al2O3, the XRD peaks, which were appeared at 2θ of 38.11, 46.21, and 66.72°, are attributed to the (311), (400), and (440) planes of γ-Al2O3, respectively. In the case of Al2O3/Sodium dodecyl sulphate/2-aminophenol composite, the existence of a wide XRD peak at 2Ɵ = 25° confirming that the crystal assembly of Al2O3 combined or interfered over amorphous organic surroundings. Elemental analysis of the Al2O3/Sodium dodecyl sulphate/2-aminophenol composite displayed that the % C, % H, and % N are 17.23, 4.21, and 1.58%, respectively. The maximum adsorption capacity, which was obtained from Langmuir isotherm, of the composite towards Pb(II), Cd(II), and Co(II) ions is 119.19, 101.21, and 86.51 mg/g, respectively. The best description for the experimental results is given by the pseudo-second-order kinetic model and Langmuir adsorption isotherm. Also, the thermodynamic parameters confirmed that the removal of studied ions is spontaneous, exothermic, and chemical. After three adsorption-desorption cycles, 25%V/V (Methanol/HNO3) is the suitable desorbing medium needed for the recapture of the adsorbed metal ions on the composite. Hence, the synthesised composite could be reutilised without notable losses of its effectiveness.


Introduction
Heavy metals, such as Pb(II), Cd(II), and Co(II) ions, which are discharged into the environment, have presented a major risk to public health and the environment due to their persistence and toxicity in the environment [1][2][3][4][5].The pollution of surface waters and wastewaters by poisonous heavy metals is international environmental trouble.These poisonous metal ions usually exist in metal-plating facilities, waste streams due to mining processes, power generation facilities, tanneries, and electronic manufacturing parts.Lead poisoning occurs when lead accrues in the body, often for months or years.Even small amounts of lead can cause serious health problems.Children under the age of 6 are more prone to lead poisoning, which can seriously affect mental and physical development.At extremely high levels, lead poisoning can be fatal.Paints containing lead and lead-contaminated dust in old buildings are among the most common sources of lead poisoning in children.Other sources include polluted air, water, and soil.Adults who use batteries for work, make repairs in their homes, or work in auto repair shops may also be injured.Although children are primarily at risk, lead poisoning is also dangerous for adults.Signs and symptoms in adults may include hypertension, Joint or muscle pain, difficulties with memory or concentration, headache, and low sperm count or deformation.Cadmium causes toxic effects on the kidneys and bone skeleton or respiratory system.It is present in the environment at low levels, but human activity has increased those levels considerably.Exposure to a high amount of cobalt can damage the heart muscles, cause an overproduction of red blood cells, or damage the thyroid gland.An overdose can also negatively affect a man's fertility [6][7][8][9].Due to environmental and economic considerations, the recovery and removal of metal ions from wastewater occupy noteworthy importance in almost all industrial sections [10][11][12][13].The most usually utilised physicochemical procedures are: (I) precipitation of ions in carbonate forms, sulphides, or hydroxides, and successive liquidsolids removal by filtration, gravity settling, or flotation, (II) sorption (adsorption, ion exchange), (III) electrolytic recovery (IV) membrane routes, and (V) solvent extraction [14][15][16][17][18][19][20].Nevertheless, each procedure has its limitations and merits in the application and they are frequently limited by economical and technical issues.Also, the adsorption procedure is debatably one of the more widespread approaches for the uptake of heavy metals due to its high removal efficiency, convenience, and simplicity [21][22][23][24][25].The adsorption procedure using activated carbon as an adsorbent is appealing to several scientists due to the efficiency of the removal of trace quantities of heavy metals [26][27][28].Nevertheless, the procedure has not been operated widely due to its high cost.Consequently, the utilisation of low-cost substances as sorbents for the removal of heavy metals from polluted water has been a concentrate.Operating low-cost biosorbents, for example, clay materials, agricultural wastes, seafood wastes, and biomass may be an alternate wastewater procedure since they are low-cost and able to remove trace quantities of metals [29][30][31].Nevertheless, to increase their adsorption capacity and improve the uptake rate, the exploration and design of novel adsorbent substances are still required.Lately, the application of nanosized substances for the uptake of contaminants has come up as an attractive area of scientific research [32][33][34][35][36].The distinctive properties of nanosized materials are supplying unmatched opportunities for the uptake of heavy metals in cost-effective and highly efficient methods.Various nanosized substances and dendrimers were utilised for this goal.Nanoparticles display excellent adsorption efficacy particularly because of greater active centres for interaction with metal ions and higher surface area.Additionally, adsorbents with particular functional groups were exploited to enhance the adsorption capacity [37][38][39].Aluminium oxide is a traditional adsorbent.Al 2 O 3 nanoparticles are a favourable substance as an adsorbent for pollutants due to their high adsorption capacity, large specific surface area, low-temperature modification, and mechanical strength [40][41][42].But, some heavy metals are weakly adsorbed on their surfaces because adsorption is physical.To fix this problem, physical or chemical modification of the Al 2 O 3 surface with particular functional groups comprising some donor atoms, for example, nitrogen, oxygen, phosphorus, and sulphur is necessary [43][44][45].If the surface of Al 2 O 3 is modified by a modifier, the pathway of the removal mechanism has changed.In this case, the metal ions are removed by adsorption as well as chemical bonding/surface attraction on the newly added functional groups.What distinguishes the use of 2-aminophenol as a modifier rather than other chemicals bearing amino functional groups is its ability to form stable 5-membered ring chelates with metal ions due to the presence of the hydroxyl group next to the amino group.A vastly usual technique to precipitate an organic substance on certain metal oxide, for example, iron oxide, silica, and alumina is to dissolve it in an appropriate solvent, mix the resultant solution with metal oxide for a certain time, evaporate the solvent, and air drying of the adsorbent.So, in this work, Al 2 O 3 nanoparticles were synthesised using glutamine as organic fuel.Also, Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite was synthesised.There are no papers that have prepared this mixture to the best of our knowledge.The synthesised composite was utilised for the efficient uptake of Pb(II), Cd(II), and Co(II) ions from aqueous solutions by forming chelates.Therefore, this research is considered a new addition in the environmental field due to its reliance on the use of effective and low-cost adsorbents in the uptake of pollutants.

Synthesis of Al 2 O 3 nanoparticles
Firstly, 3.00 g (7.99 mmoles) of aluminium(III) nitrate nonahydrate was dissolved in 60 mL of distilled water.Besides, 60 mL aqueous solution of glutamine (0.73 g, 4.99 mmoles) was added to the previous aluminium solution drop by drop with constant stirring for 60 min using a magnetic stirrer at room temperature.Moreover, the temperature of the used magnetic stirrer was raised to 170 °C for the whole evaporation of the water.Furthermore, the yielded powder was calcined at 650 °C for 180 min to produce Al 2 O 3 nanoparticles.

Synthesis of Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite
2.00 g of Al 2 O 3 sample was suspended in about 50 mL of distilled water and was mixed with 0.10 g of sodium dodecyl sulphate.Also, 20 mL of 2-aminophenol solution, which was prepared by dissolving 1.00 g of 2-aminophenol in 1 mL of concentrated HCl and 19 mL of acetonitrile solvent, was added.Moreover, the formed suspension was magnetically stirred at about 60°C for 180 min.Additionally, the solvent was evaporated then the product was carefully washed numerous times utilising distilled water, and dried in air.

Removal of Pb(II), Cd(II), and Co(II) ions from aqueous media
The adsorption of Pb(II), Cd(II), or Co(II) ions by Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite was studied via a batch operation using a magnetic stirrer as the following; 0.10 g of the composite was suspended in 100 mL aqueous solution of 150 mg/L of Pb(II), Cd(II), or Co(II) ions at different pH (2.50-7.50),temperature (298-328 kelvin), and time (10-90 min) values.Besides, the effect of the initial concentration of studied metal ions was studied as previously described but in the range of 100-300 mg/L.
C d (mg/L) is regarded as the concentration of Pb(II), Cd(II), or Co(II) ions in the desorption reagent whereas V d (L) is regarded as the consumed volume of desorption reagent.

Characterisation
FT-IR spectrum of the Al

Characterisation
The synthesis of Al 2 O 3 nanoparticles was accomplished according to Equation (4).In this scheme, the aluminium(III) nitrate nonahydrate reacts with glutamine fuel to form aluminium oxide as well as gases (nitrogen, carbon dioxide, and water).In the case of Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite, the existence of a wide XRD peak at 2Ɵ = 25° confirming that the crystal assembly of Al 2 O 3 combined or interfered over amorphous organic surroundings [23,37].The intensity of the wide XRD peak of organic surroundings is much higher than that of the peaks of aluminium oxide.Hence, the aluminium oxide XRD peaks are almost hardly seen in the case of the composite.This observation is in agreement with similar works in which Inorganic/Organic composites have been prepared by Abdelrahman et al [23,37].Figure 2(a,b) displays the FT-IR spectra of Al 2 O 3 nanoparticles and Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite, respectively.In the case of Al 2 O 3 nanoparticles, the bands, which were observed at 570, 760, and 1020 cm −1 , are attributed to bending, symmetric, and asymmetric vibrations of Al-O-Al, respectively.Also, the bands, which were observed at 1635 and 3433 cm −1 , are attributed to bending and stretching vibrations of adsorbed H-O-H, respectively [37][38][39][46][47][48][49].In the case of Al 2 O 3 /Sodium dodecyl sulphate/2-amino phenol composite, the bands, which were observed at 630, 833, and 1063 cm −1 , are attributed to bending, symmetric, and asymmetric vibrations of Al-O-Al, respectively.The shift in the positions of these bands is due to the formation of the composite.The bands, which were observed in the range 1151-1256 cm −1 , are attributed to the stretching vibration of C-N.Also, the band, which was observed at 708 cm −1 , is due to the out-of-plane bending vibration of CH aromatic.Besides, the bands, which were observed at 1386, 2928, and 3050 cm −1 , are attributed to bending vibration of CH, stretching vibration of CH aliphatic, and stretching vibration of CH aromatic, respectively.Moreover, the bands, which were observed in the range 1425-1515 cm −1 , are attributed to the stretching vibration of C = C aromatic.Additionally, the bands, which were observed at 1622 and 3446 cm −1 , are attributed to bending and stretching vibration of NH and/or OH, respectively [37][38][39][46][47][48][49]. Figure 3

Removal of Pb(II), Cd(II), and Co(II) ions from aqueous media
Figure 4(a,b) demonstrates the plotting of pH against % R and Q, respectively.The data proved that there is a minor change in the value of both Q and % R when the pH increased from 6.50 to 7.50.Consequently, the optimum pH, which will be chosen in additional studies, is 6.5.% R of Pb(II), Cd(II), and Co(II) ions at pH = 6.5 is 69.83, 59.95, and 49.89%, respectively.Q of the composite towards Pb(II), Cd(II), and Co(II) ions at pH = 6.5 is 104.74,89.93, and 74.84 mg/g, respectively.% R of Pb(II), Cd(II), and Co(II) ions using Al 2 O 3 at pH = 6.5 is 18.53, 12.26, and 8.75%, respectively.Also, % R of Pb(II), Cd(II), and Co(II) ions using Al 2 O 3 /Sodium dodecyl sulphate at pH = 6.5 is 3.51, 3.00, and 2.78%, respectively.Hence, Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite has high removal efficiency as a result of the formation of chelates with metal ions under study through the hydroxyl and amino groups.
The point of zero charge (pH PZC ), which was determined as reported by Shah et al. [49], is 5.6.The surface of the composite is surrounded by H + ions if the pH of the solution is less than pH PZC .Thus, the positive hydrogen ions prevent other metal ions from approaching the surface, which leads to a decrease in the % removal.The surface of the composite is surrounded by OH − ions if the pH of the solution is more than pH PZC .Thus, the negative hydroxyl ions allow other metal ions from approaching the surface, which leads to an increase in the % removal.
Figure 5(a,b) displays the plotting of time against % R and Q, respectively.The data proved that there is a minor change in the value of both Q and % R when the time increased from 60 to 70 min.Consequently, the optimum time, which will be chosen in additional effects, is 60 min.At this optimum time, the active sites on the surface of the adsorbent were saturated.So, when the time exceeds the optimum time, there is no noticeable change in the % removal.% R of Pb(II), Cd(II), and Co(II) ions at 60 min is 70.33, 60.63, and 50.52%, respectively.Q of the composite towards Pb(II), Cd(II), and Co(II) ions at 60 min is 105.50, 90.94, and 75.78 mg/g, respectively.The experimental data, which were obtained from the effect of time, were investigated using the pseudo-first-order (Equation ( 5)), pseudo-second-order (Equation ( 6)), and intra-particle diffusion models (Equation ( 7)) [37,[49][50][51].where, Q e (mg/g) is the adsorption capacity of the Al 2 O 3 /Sodium dodecyl sulphate/ 2-aminophenol composite at the equilibrium towards Pb(II), Cd(II), or Co(II) ions.Additionally, Q t (mg/g) is the adsorption capacity of the Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite towards Pb(II), Cd(II), or Co(II) ions at time t.Besides, P 2 (g/mg.min) is the rate constant of the pseudo-second-order.P 1 (1/min) is the rate constant of the pseudo-first-order.P 3 (mg/g min 0.5 ) is the rate constant of the intraparticle diffusion model.Z (mg/g) is the thickness of the boundary layer.Also, Figure 6(a,b) represents the plot of time versus log (Q e -Q t ) and t/Q t , respectively.The data proved that the correlation coefficient values (R 2 ) of the pseudo-second-order model are larger than that of the pseudo-first-order model.Accordingly, the best explanation for the data was given by the pseudo-second-order model.The gotten constants of operated models are listed in Table S1. Figure 6(c) represents the plot of Q t versus t 0.5 .Straight lines with a high correlation coefficient were obtained.The straight lines did not go across the origin and thus this proves that the intra-particle diffusion model is not the lone pathway that organises the removal of Pb(II), Cd(II), and Co(II) ions.
Figure 7(a,b) displays the plotting of temperature against % R and Q, respectively.The data proved that there is a major decrease in the value of both Q and % R when the temperature increased from 298 to 328 Kelvin.This can be explained by the fact that the high temperature releases the adsorbed ions and returns them to the solution again.Consequently, the optimum temperature value, which will be chosen in additional studies, is 298 Kelvin.% R of Pb(II), Cd(II), and Co(II) ions at 328 Kelvin is 32.71, 23.57, and 18.41%, respectively.Q of the composite towards Pb(II), Cd(II), and Co(II) ions at 328 Kelvin is 49.07, 35.36, and 27.62 mg/g, respectively.The thermodynamic factors, for example, change in enthalpy (ΔH o ), change in the entropy (ΔS o ), and change in free energy (ΔG o ) were determined utilising Equations ( 8) and ( 9) [37,[49][50][51].where, T (Kelvin) is the temperature of adsorption.Also, R (KJ/mol K) and K d (L/g) are the gas constant and distribution constant, respectively.Besides, the distribution constant (K d ) was determined utilising Equation (10) [37,[49][50][51].S2.Also, the data confirmed that the removal of Pb(II), Cd(II), and Co(II) ions is exothermic as a result of the negative sign of enthalpy.Also, the removal of Pb(II), Cd(II), and Co(II) ions is chemical because enthalpy is more than 40 KJ/mole (i.e.−42.82, −44.96, and −42.02KJ/mol, respectively).Moreover, the removal of Pb(II), Cd(II), or Co(II) ions is spontaneous as a result of the negative sign of free energy as clarified in Table S2.Additionally, the removal of Pb(II), Cd(II), or Co(II) ions takes place in a disordered approach at the solution boundary/composite as a result of the positive sign of entropy as clarified in Table S2.
Figure 9(a,b) displays the plotting of concentration against % R and Q, respectively.The data proved that there is a decrease in the value of % R and an increase in the value of Q when the concentration increased from 100 to 300 mg/L.% R of Pb(II), Cd(II), and Co(II) ions at 300 mg/L is 39.91, 33.55, and 28.18%, respectively.Q of the composite towards Pb(II), Cd(II), and Co(II) ions at 300 mg/L is 119.72,100.66, and 84.53 mg/g, respectively.The experimental data, which were obtained from the effect of concentration, were investigated using the Langmuir (Equation ( 11)) and Freundlich (Equation ( 12)) isotherms [37,[49][50][51].where, Q max (mg/g) is the maximum adsorption capacity of the Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite.Besides, P 4 (L/mg) is the Langmuir constant.P 5 (mg/g)(L/ mg) 1/n ) is the Freundlich constant.Moreover, 1/n is the heterogeneity constant.Additionally, the Q max for Freundlich isotherm was calculated using Equation ( 13) [37,[49][50][51].than that of the Freundlich isotherm.Accordingly, the best explanation for the data was given by the Langmuir isotherm.Besides, the gotten constants of operated isotherms are scheduled in Table S3.The maximum uptake capacity of the composite towards Pb(II), Cd(II), and Co(II) ions is 119.19,101.21, and 86.51 mg/g, respectively.The coordination number with hydrated ionic sizes could be accountable for the high adsorption of Pb(II) or Cd(II) than Co(II) [52][53][54][55][56][57][58].

Conclusions
Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite was synthesised and employed as a novel adsorbent for the uptake of Pb(II), Cd(II), and Co(II) ions from aqueous media.The aluminium(III) nitrate nonahydrate reacts with glutamine fuel to produce aluminium oxide by the combustion method.The synthesised composite was characterised using some apparatuses such as XRD, CHN elemental analysis, FT-IR spectrophotometer, and FE-SEM.In the case of Al 2 O 3 , the peaks, which were appeared in the XRD pattern at 2θ of 38.11, 46.21, and 66.72°, are attributed to the (3 1 1), (4 0 0), and (4 4 0) planes of γ-Al 2 O 3 as clarified from JCPDS card no.10-0425, respectively.In the case of Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite, the existence of a wide XRD peak at 2Ɵ = 25° confirming that the crystal assembly of Al 2 O 3 combined or interfered over amorphous organic surroundings.The maximum uptake capacity of the composite towards Pb(II), Cd(II), and Co(II) ions is 119.19,101.21, and 86.51 mg/g, respectively.The best explanation for the adsorption data was given by the Langmuir isotherm and pseudo-second-order model.The synthesised composite could be reutilised for three adsorption-desorption cycles without notable losses of its effectiveness.
where, C e (mg/L) and C o (mg/L) are the final and initial concentrations of Pb(II), Cd(II), or Co(II) ions, respectively.Besides, V (L) is the volume of Pb(II), Cd(II), or Co(II) solution.Moreover, m (g) is the mass of the Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite.

Figure 1 ( 3 Table 1 .Figure 1 .
Figure 1(a,b) shows the XRD pattern of Al 2 O 3 nanoparticles and Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite, respectively.In the case of Al 2 O 3 , the peaks, which were appeared in the XRD pattern at 2θ of 38.11, 46.21, and 66.72°, are attributed to the (3

2 -
aminophenol composite, respectively.In the case of Al 2 O 3 , spherical and irregular shapes have an average size of 120 nm were observed.The irregular shapes increased in the composite and hence this clarified that the crystal assembly of Al 2 O 3 interfered or combined over amorphous organic environments.Elemental analysis of the Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite displayed that the % C, % H, and % N are 17.23, 4.21, and 1.58%, respectively.So, previous analyses clarified the successful formation of the composite as illustrated in Scheme 1 where 2-aminophenol is trapped into the sodium dodecyl sulphate aggregates on Al 2 O 3 particles.

Figure 6 .
Figure 6.Plotting of time against log (Q e -Q t ) (a) and t/Q t (b).

Figure 8
Figure 8 demonstrates the plotting of 1/T against lnK d .The gotten thermodynamic factors are scheduled in TableS2.Also, the data confirmed that the removal of Pb(II), Cd(II), and Co(II) ions is exothermic as a result of the negative sign of enthalpy.Also, the removal of Pb(II), Cd(II), and Co(II) ions is chemical because enthalpy is more than 40 KJ/mole (i.e.−42.82, −44.96, and −42.02KJ/mol, respectively).Moreover, the removal of Pb(II), Cd(II), or Co(II) ions is spontaneous as a result of the negative sign of free energy as clarified in TableS2.Additionally, the removal of Pb(II), Cd(II), or Co(II) ions takes place in a disordered approach at the solution boundary/composite as a result of the positive sign of entropy as clarified in TableS2.Figure9(a,b) displays the plotting of concentration against % R and Q, respectively.The data proved that there is a decrease in the value of % R and an increase in the value of Q when the concentration increased from 100 to 300 mg/L.% R of Pb(II), Cd(II), and Co(II) ions at 300 mg/L is 39.91, 33.55, and 28.18%, respectively.Q of the composite towards Pb(II), Cd(II), and Co(II) ions at 300 mg/L is 119.72,100.66, and 84.53 mg/g, respectively.The experimental data, which were obtained from the effect of concentration, were investigated using the Langmuir (Equation (11)) and Freundlich (Equation (12)) isotherms[37,[49][50][51].

Figure 8 .
Figure 8. Plotting of ln K d against 1/T.

Figure 10 (
Figure 10(a) represents the plot of C e /Q e versus C e (i.e.Langmuir isotherm) whereas Figure 10(b) represents the plot of ln Q e versus ln C e (i.e.Freundlich isotherm).The data proved that the correlation coefficient values (R 2 ) of the Langmuir isotherm are larger
XRD patterns of the Al 2 O 3 and Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite were obtained using a Bruker (Cu K α = 0.154 nm) X-ray diffractometer (Malvern, United Kingdom).CHN analysis of the Al 2 O 3 /Sodium dodecyl sulphate/2-aminophenol composite was determined using 2400 CHN Perkin Elmer Elemental Analyser (Waltham, United States).Inductively Coupled Plasma Optical Emission spectroscopy (ICP-OES, 7300 DV, Perkin Elmer, Waltham, United States) was used to determine the concentration of Pb(II), Cd(II), and Co(II) ions.Instrumental parameters for the determination of Pb(II), Cd(II), and Co(II) ions by ICP-OES are given in Table1.