A study of Zinc ions adsorption by native and chemically modified leaves of earleaf acacia ( Acacia auriculiformis )

. Isotherms of Zn 2+ ions adsorption by native and 3% H 2 SO 4 solution-treated leaves of Acacia auriculiformis were plotted. The resulting adsorption isotherms were processed using the Langmuir, Freundlich, Dubinin-Radushkevich and Temkin models. It was found that the isotherms of these Zn 2+ ions adsorption with native leaves of earleaf acacia are most accurately described by the Temkin model and with modified leaves by the Langmuir model. It was found that the modification of acacia leaves with a 3% H 2 SO 4 solution contributes to an increase in the maximum sorption capacity for Zn 2+ ions from 9.5 mg/g to 30.5 mg/g. It was found that the kinetics of Zn 2+ ions adsorption by native and modified Acacia auriculiformis leaves is most accurately described by a pseudo-second order model. The calculated values of the Biot's coefficient are in the range of 1 < B i < 20, which indicates mixed diffusion in all cases. Thermodynamic parameters of the process of Zn 2+ ions adsorption by native and acid-modified Acacia auriculiformis leaves were determined. The calculat ed adsorption (E) and Gibbs (∆G 0 ) energies, as well as enthalpy and entropy indicate spontaneous, self-induced endothermic physical adsorption process in both cases.


Introduction
Adsorption is an effective method to purify aqueous environments from heavy metal ions (HMI), due to its significant advantages over other methods, such as stability, ease of equipment operation, and high productivity [1][2][3].However, the use of activated carbons in industrial production is limited by the high cost of the latter and the need for regeneration, which increases the cost and reduces the economic efficiency of the process [4].
In this regard, a new innovative environmental safety sector is currently emerging.This involves the use of plant processing waste as an absorbent for the removal of various contaminants from aquatic environments.Renewable tree biomass (foliage, needles, bark, fruit shells and cones, etc.) and wood processing waste (sawdust, wood chips, etc.) are of particular interest as cellulose-containing sorption materials (SM) for pollutants removal from natural and waste water [5][6][7].Plant leaves are one of the inexpensive tree biomass materials widely used to remove pollutants from aquatic environments.The reviews [8][9][10][11] provide information on the use of tree leaves as adsorbents of various pollutants from aqueous environments.One of the most widespread tree species in the world are Acacia trees.
Based on the above, the Zn 2+ ions sorption characteristics of native and 3% H2SO4 solution-modified Acacia auriculiformis leaves have been studied.This tree species is widespread in Vietnam, where wood is used for cellulose production [12].

Experimental methodology
The crushed Acacia auriculiformis leaf was treated with 3% H2SO4 solutions at 20 o C for 5 hours by mixing the latter in a reaction vessel.At the end of the contact period, the sorption material was filtered, washed with distilled water, and dried up to a constant weight.Adsorption isotherms were plotted for native and acid-modified samples of Acacia auriculiformis leaf at 20 °C.1g leaf samples were placed in 250 cm 3 flat-bottomed flasks.Then 100 cm 3 solutions with 10 to 1500 mg/dm 3 of Zn 2+ ions were poured into the flasks.The contents of the flasks were intensively mixed for 3 hours.After this time, the sorption material was filtered out and the residual content of Zn 2+ ions was determined in the filtrates.Based on the resulting data, adsorption isotherms were plotted.
Kinetic dependences were built based on the results of zinc (II) ions adsorption by native and modified Acacia auriculiformis leaf at a 25°C, initial ion concentrations in the solution from 10 to 1500 mg/dm 3 , 1g of the sorption material and 0-3 hours of sorption time.
The C, H, N content in sorption materal samples was defined using a «Vario EL» element analyzer by burning the samples and separating the combustion gases in adsorption columns.
The IR spectra of the tested Acacia auriculiformis leaf samples were taken in the frequency range of 400-4000 cm -1 using an «Avatar-360» IR Fourier spectrometer.
The sorption material specific surface area was defined using a «Sorbi-MS» instrument designed to measure the textural characteristics of dispersed and porous materials by lowtemperature inert gas adsorption.
X-ray diffraction analysis of the samples was performed using an «Ultima IV RIGAKU» X-ray diffractometer with copper cathode tubes.The changes in the structural makeup of the samples were found by comparing the intensity of the corresponding lines of the X-ray image.

Results and discussion
Zn 2+ ion absorption isotherms plotted for native and acid-modified acacia leaves samples are shown in Figure 1.As appears from the isotherm of Zn 2+ ion adsorption by native Acacia auriculiformis leaves, they are classified as type I isotherms according to the IUPAC classification and describe the monomolecular adsorption of metal ions on the adsorbent surface.The adsorption isotherms in Figure 1 show that the maximum sorption capacity of native Acacia auriculiformis leaves for Zn 2+ ions is ~ 9.5 mg/g, after modification -30.5 mg/g.
To describe the adsorption mechanism, the resulting isotherms were processed using four models: those of Langmuir, Freundlich, Dubinin-Radushkevich, and Temkin [13,14].The Langmuir sorption model is often used to describe monolayer adsorption on homogeneous adsorption surfaces in which all active sites have equal energy and enthalpy, while the Freundlich model is used to describe adsorption on a heterogeneous SM layer with an indefinite number of active binding sites.The Dubinin-Radushkevich model is more general than the Langmuir model, since it does not involve surface homogeneity or the adsorption potential constancy.The Temkin model describes adsorption occurring on an inhomogeneous surface with a uniform distribution of adsorption sites by adsorption energies [13,14].The generalized data on the obtained regression equations and approximation coefficients (R 2 ) are given in Table 1.As appears from the data given in Table 1, the process of Zn 2+ ions adsorption by native acacia leaves is best described by the Temkin model, i.e. the heat of adsorption of all molecules in the layer decreases linearly during the adsorption layer filling due to the mutual repulsion of Zn 2+ ions, while the process runs with a homogeneous distribution of the maximum binding energy.In addition, the adsorption heat decreases mainly according to the linear law.
As Table 1 data shows, after treating the studied native biosorbent with a lowconcentrated sulfuric acid solution, the Zn 2+ ions adsorption isotherm is most accurately described by the Langmuir model.
The next stage involved determination of the thermodynamic process parameters using the sorption materials under study for Zn 2+ ions.
The calculated thermodynamic process parameters are given in Table 2. shows that the process of Zn 2+ ions adsorption by native acacia leaves is a spontaneously developing process (∆G 0 < 0) in the studied temperature range.Besides, negative ∆G 0 values ranging from -2 to 0 kJ/mol indicate spontaneous physical adsorption in all cases [15][16][17].The equilibrium constant (KL), which describes the composition of the system in equilibrium, is determined on the basis of the Langmuir equation.KL > 1 values indicate the predominance of direct process products in the system.This circumstance means that the adsorption process becomes more preferable [15][16][17].Coefficient n determined on the basis of the Freundlich equation characterizes the adsorption process intensity and the distribution of active sites.The n < 1 coefficients values mean that the binding energy increases as the surface of acacia leaves is filled with the studied HMI [15][16][17].
Kinetic dependences of Zn 2+ ions adsorption by Acacia auriculiformis leaves were obtained to identify the process limiting stage.Adsorption was carried out on simulated solutions containing Zn 2+ ions at a concentration of 100 mg/dm 3 and a sorption time of 180 min at temperatures of 293, 298, 313K.Figure 2 shows the resulting graphical dependences of the change in sorption capacity (A) on the time of sorption interaction.Based on the above, the dependencies were plotted: -lg(1-F) = f(t), A = f(t 1/2 ), Bt = f(t).The sorption process rate can be limited by both the adsorption stage and the diffusion of the sorbent.At the same time, transport processes (sorbtive molecules from the solution moving to the active sites of the sorbent) play an important role in the sorption system.The contribution of the diffusion process to sorption kinetics can be estimated using the Boyd diffusion model [18].The results of processing kinetic dependencies as part of the Boyd diffusion model are given in Table 4.The study also involved the determination of the correspondence of the resulting kinetic dependences to the pseudo-first and pseudo-second order models.As can be seen from the resulting data, higher values of the determination coefficients R 2 (Table 5) allow us to favor the applicability of the pseudo-second order model for describing the adsorption process, as well as for the possibility of taking into account intermolecular interactions in the systems under study.The values of the pseudo-second order velocity constant are the highest.
The calculated kinematic parameters of the pseudo-first and pseudo-second order model equations on native and modified Acacia auriculiformis leaves at different temperatures for Zn 2+ ions are shown in Table 5  Based on the R 2 value and the difference between the experimental and calculated values, it was found that the kinetic equation of the pseudo-second order more closely matches the experimental data.The k2 value increases as the experiment temperature rises.However, the calculated Aꝏ value differs from the experimental Aꝏ value at a low temperature, and at a higher temperature (298 and 313 K), the adsorption process completely follows the pseudo-second order kinetics model.This can be accounted for by the fact that at higher temperatures, Zn 2+ ions are transferred to the SM surface and capillaries faster, thus resulting in an increased adsorption rate.
As follows from the above data, a 3% H2SO4 solution treatment of leaves ensures an increase in the sorption capacity from 9.5 mg/g to 30.5 mg/g, i.e. more than 3 times.This circumstance is accounted for by several reasons.The results of elemental analysis revealed that acid-treated leaf samples showed a slight decrease in the mass fraction of carbon, hydrogen and nitrogen.An increase in the oxygen content in the modified leaf samples compared to the native material indicates an increase in the number of hydrophilic groups in the biosorbent structure and, accordingly, an increase in the adsorption capacity for Zn 2+ ions.The results of the elemental composition of native and Acacia auriculiformismodified leaves are given in Table 6.The IR spectra of native and treated (with H2SO4 low concentration solution) Acacia auriculiformis leaves confirm the above, which indicates that acid-modified acacia leaves show an increase in the intensity of peaks with a characteristic wavenumber for the OH group (3139 cm -1 ) and the carbonyl group (1719 cm -1 ).
The increase in the hydrophilicity of the modified biosorbent is indirectly evidenced by the values of Acacia auriculiformis leaves surface marginal angle of wetting with a drop of distilled water (Fig. 3).As Figure 3 shows, treating the leaves with a 3% H2SO4 solution results in a decrease in the marginal wetting angle from 70.In addition, the increase in the modified leaves sorption capacity for Zn 2+ ions is contributed to by an increase in the specific surface area calculated by the BET method.It was found that native acacia leaves have a total area of 0.432 m 2 /g, which after treating with a low-concentration acid solution increases to 0.740 m 2 /g, i.e. by 1.71 times.
X-ray phase analysis data showed that the X-ray diffraction spectra of native and acidmodified acacia leaves samples contained the main peaks at 2θ = 21.54o and 26.55 o .The comparison of diffractograms of native and modified Acacia auriculiformis leaves showed that they have a look typical of cellulose diffractograms; well-defined halide reflexes evidence an amorphous structure of acacia leaf samples.Some narrowing of modified biosorbents diffraction reflexes in comparison with native leaf samples indicates an increase in the size of crystallized areas in the SM matrix.It was found that Acacia auriculiformis native leaves have a degree of crystallinity γ = 0.25 and after treating with a low-concentration H2SO4 solution, the degree of crystallinity increased to γ = 0.28.

Conclusion
The results showed that the maximum adsorption capacity of native Acacia auriculiformis leaves increased after being modified with 3% H2SO4 solution.The process of Zn 2+ ions adsorption by native Acacia auriculiformis leaves is best described by the Temkin model and after treating the studied native biosorbent with 3% H2SO4 solution, the Zn 2+ ions adsorption isotherm is most accurately described by the Langmuir model.The thermodynamic parameters of the process show that the Zn 2+ ion adsorption process of native and Acacia auriculiformis-modified leaves is a spontaneously developing process (∆G 0 < 0) in the studied temperature range.Besides, negative ∆G 0 values ranging from -2 to 0 kJ/mol indicate spontaneous physical adsorption in all cases.∆H 0 > 0 values indicate that the adsorption process is endothermic.∆S 0 > 0 values indicate that the process is chaotic and increases when Zn 2+ ions are adsorbed on the modified biosorbent.The R 2 value and the difference between the experimental and calculated values, it was found that the kinetic equation of the pseudo-second order more closely matches the experimental data.Based on the results elemental analysis, IR spectroscopy, specific surface area and X-ray diffraction analysis of samples of native and modified Acacia auriculiformis leaves, it was shown that the Zn 2+ ions adsorption capacity of modified Acacia auriculiformis leaves with 3% H2SO4 solution is much better than native Acacia auriculiformis leaves samples.

Fig. 3 .
Fig. 3. Values of the marginal angle wetting with a drop of distilled water: a) native Acacia auriculiformis leaves; b) Acacia auriculiformis leaves treated with 3% H2SO4 solution

Table 1 .
Regression equations and approximation coefficients of isotherms of Zn 2+ ions adsorption by native and 3% sulfuric acid solution-modified Acacia auriculiformis leaves

Table 2 .
Thermodynamic parameters of Zn 2+ ions adsorption by native and modified Acacia ∆H 0 > 0 values indicate that the adsorption process is endothermic.∆S 0 0 values indicate that the process is chaotic and increases when Zn 2+ ions are adsorbed on the modified biosorbent.Values of sorption energies (E) less than 2 kJ/mol indicate physical adsorption.Other constants of the Langmuir and Freundlich equations are given in Table3.

Table 3 .
Thermodynamic constants of the processes of Zn 2+ ions adsorption by native Acacia auriculiformis leaves

Table 4 .
Results of processing kinetic dependences of Zn 2+ ions adsorption by native Acacia auriculiformis leaves as part of the diffusion model

Table 5 .
Kinetic parameters of the pseudo-first and pseudo-second order equations of Zn 2+ ion adsorption by native and modified Acacia auriculiformis leaves