The present study is based on the investigation of performance of C,N-bipyrazole receptor grafted onto silica surface (SG2P) of adsorption Arsenic (AS) from aqueous solutions. The effects of operating parameters that include pH, contact time, concentration of As and dosage of adsorbent on adsorption were accomplished. The results clearly showed that the removal efficiency of As was decreased with an increasing of As concentration, pH, and temperature, while it was continuously increasing with time and adsorbent dose. Moreover, the removal efficiency of Cr (VI) adsorption was 75% corresponding to pH; temperature (°C), initial concentration (ppm) and weight of dose (g) were 6, 25, and 0.04 respectively at 24 hours. The adsorption capacity of the synthesized sorbent (SG2P) for arsenic at pH < 7 from pseudo second order in batch experiments was efficient at 20 minutes. Furthermore, the results obtained in this study presented that the antimicrobial activity was limited where the Escherichia coli (ATCC25922) and Staphylococcus aureus (ATCC25932) were as a reference strains, while, the SG2P was able to inhibit growth only at high concentration (MIC = 1.5625 mg/ml).
Arsenic (As) is the 20th most abundant element present in the earth’s crust; it is considered as a toxic trace element present in natural waters (ground and surface water), and has become a major unavoidable threat for the life of human beings and useful microorganisms. Arsenic concentration in soils and water can become high due to several reasons like, mineral dissolution, use of arsenical pesticides, disposal of fly ash, mine drainage, and geothermal discharge [
Recently, the environmental fate and behavior of arsenic (As) are receiving increased attention due to the arsenic (As) pollution in groundwater. It has been a serious health threat to the human beings in the world. Occurrence of arsenic in groundwater above the permissible value (>10 µ g/L WHO) is one of the wide spread problem owing to its toxicity and carcinogenicity [
The entire chemicals used were of AR grade and were used without further purification. All aqueous solutions were prepared in double distilled water. The distilled water was inspected for arsenic concentration. The concentration was lower than 10 ppb, the detection limit that can be easily accomplished by the analytical methods used in this study [
The instruments used for the experimental work are Electronic Balance, pH meter; Spectrophotometer, Gutzeit apparatus, incubator and Gyro shaker which were used from the college laboratory and all the glass that used were made up of Borosilicate. On the other hand, the amount of metal ion that was sorbed and the percent removal of metal ion by adsorbent were calculated by applying Equation (1) and Equation (2) respectively:
q = (Co − Cf) ÷ .V (1)
%Removal = (Co − Cf) ÷ Co 100 (2)
where q is the amount of metal ion sorbed by the adsorbent (mg/g); Co is the initial metal ion concentration; (mg/L), Cf is the final ion concentration (mg/L) after the chemo-sorption occurred, V is the volume of aqueous solution (L) in contact with the adsorbent, and m is the mass (g) of adsorbent.
After converting the hydroxy-bipyrazolic ligand 4 to the alcoolate derivative using sodium metal in tetrahydrofuran, the resulting salt (103 mol) was added to a suspension of 3-glycidoxypropyl-functionalized silica (SiPz) (1.00 g) in 30 ml of dimethylformamide (DMF). The mixture was stirred and refluxed under nitrogen for 24 h. The solid material was filtered, and the residue was washed with DMF, toluene, water (distilled and deionized), methanol, dichloromethane (150 ml of each) and finally dried [
To determine the optimum dosage of SG2P, it was added to the conical flask in different dosage varying from (0.02 g, 0.04 g, and 0.1 g up to 0.15 g), containing 10 mL concentration of arsenic solution (10 µg/L) at pH 6. The solution in the conical flask was subjected to stirring for optimum contact time, filtered and analyzed for residual arsenic concentration. The dosage which provides minimum residual concentration is chosen as optimum dosage.
The adsorption is strongly influenced by the contact time. To study the effect of contact time, 10 mL of 10 µg/L arsenic solution was mixed with 0.02 g of adsorbant (SG2P), stirred at different contact times varying from (5 mins, 15 mins up-to 180 mins). Then filtrate was analyzed for arsenic concentration using uv visible spectrophotometer.
It was reported in prior studies that pH is a central factor that affects the performance of adsorption process. The effect of pH on arsenic adsorption was studied by performing equilibrium adsorption tests at different initial pH values. i.e. from 1.0 to 12.0. The pH of solution was adjusted by using 0.1N H2SO4 or 0.1N NaOH. The pH which gives minimum residual concentration is chosen as optimum pH.
The effect of temperature was investigated at different degrees, 25˚C, 35˚C, 45˚C and 55˚C. For each temperature, a 0.02 g adsorbent (SG2P) samples was added to 10 mL of As(III) solutions with concentration 10 µg/L at pH around 6. The temperature which gives minimum residual concentration is chosen as optimum temperature.
Al of these concentrations of 10, 20, 30, 40 and 60 ppb solutions were prepared from the stock solution to find out the optimum concentration, 0.02 g of adsorbent (SG2P) was added to a number of tubes contains 10 ml of the previous concentrations of As(III) solutions under optimized temperature (25˚C) and pH 6 for 30 min, the absorbance of the solution above the solid residue was measured by Flame Atomic Absorption at 193.7 nm.
Antibacterial activity of SG2P was determined by broth micro-dilution method in which SG2P was serially diluted and examined against the bacteria. Minimum inhibitory concentration (MIC) was considered the lowest concentration of substance that inhibited visible growth of bacteria.
Effect of contact time was investigated and graph of percentage of arsenic removal versus time in minutes was schemed as shown in
Effect of adsorbent dosage was examined and graph of percentage of arsenic removal versus dosage was plotted as shown in
The adsorption of arsenic from aqueous solution using GS2P in this research was found to be a lower pH (pH 3) dependent process. The graph of percentage of arsenic removal versus pH was plotted as shown in
This is because the solution contains protons that bind with electrons from oxygen, so the negative charge on the surface attract the positive ions of As, where at high pH in alkaline conditions the result showed is not favorable for arsenic sorption because carboxyl, hydroxyl, and amide groups of the adsorbent become negatively charged and a high density of OH− at alkaline conditions would compete with all anionic species of As(III) [
Effect of temperature was investigated and graph of percentage of arsenic removal versus temperature was plotted as shown in
The effect of arsenic concentration on the adsorption was studied under optimized
pH found from our previous results. Concentrations of Arsenic varied from 10 - 40 ppb. 10 ml of each concentration of arsenic was treated with the adsorbent at pH value of 3.0. The results are illustrated in
Adsorption isotherms are used to explain the equilibrium of metal ions that occurs between the solid phase of adsorbent and the aqueous solution. Freundlich and Langmuir isotherms are the most models that widely used to determine some kinetic and thermodynamic parameters that can give a clearer image about the binding mechanism.
The Freundlich isotherm is an empirical equation employed to describe heterogeneous systems. This model is specified with the following equation [
Q e = K F C e 1 / n (5)
The linear form of this equation can be written as:
ln Q e = 1 n ln C e + ln K F (6)
where, KF and n are Freundlich constants with KF is an approximate indicator of adsorption capacity of the sorbent and n giving an indication of how favorable the adsorption process. The magnitude of the exponent, 1/n, gives an indication of the favorability of adsorption. If value of 1/n is below one, it indicates a normal
adsorption. If n lies between one and ten, this indicates a favorable sorption process [
From
This describes quantitatively the formation of a monolayer adsorbate on the outer surface of the adsorbent, and after that no further adsorption takes place. Thereby, the Langmuir represents the equilibrium distribution of MB dye between the solid and liquid phases. The Langmuir isotherm is valid for monolayer adsorption onto a surface containing a finite number of identical sites. The model assumes uniform energies of adsorption onto the surface and no transmigration of adsorbate in the plane of the surface. Based upon these assumptions, Langmuir represented the following equation [
C e Q e = 1 Q m C e + 1 Q m K L (7)
where:
Ce = the equilibrium concentration of adsorbate (mg/L);
Qe = the amount of As(III) adsorbed per gram of the adsorbent (mg/g);
Qm = maximum monolayer coverage capacity (mg/g);
KL = Langmuir isotherm constant (L/mg).
The values of Qm and KL were computed from the slope and intercept of the Langmuir plot of Ce/Qe versus Ce. From Langmuir plots which is shown in
The essential characteristics of the Langmuir isotherm can be expressed in
Freundlich isotherm model parameters | ||||
---|---|---|---|---|
Adsorbate | Parameters | |||
1 n | n | KF = (mg?g) | R2 | |
SG2P | 0.59 | 1.6949 | 2.1 | 0.911 |
Langmuir isotherm model parameters | ||||
---|---|---|---|---|
Adsorbate | Parameters | |||
Qm (mg/g) | KL = (L/mg) | RL | R2 | |
SG2P | 4.61 | 0.578 | 0.166 | 0.958 |
terms of a dimensionless constant separation factor RL that is given by the following equation [
R L = 1 ( 1 + K L C o ) (8)
where, Co is the highest initial concentration of adsorbate (mg/L).
The value of RL indicates the shape of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0). The RL values between 0 and 1 indicate favorable adsorption. The value of RL in the present investigation was found to be 0.166 at 25˚C indicating that the adsorption of As(III) dye on (SG2P) is favorable [
Uptake of each metal ion from different initial concentrations of the metal can be used to study the dependency of the rate of adsorption on the concentration of metal ion left in solution. Thus, the reaction order of the adsorption process can be determined. Pseudo first-order equation was applied for evaluation the adsorption kinetics for As(III) onto SG2P, the rate constant for the adsorption K1 was evaluated. The pseudo first-order equation expressed as Equation (4):
log ( Q e − Q t ) = log Q e − ( K 1 2.303 ) t (9)
where Qe is the adsorption capacity of the SG2P at equilibrium (mg/g), Qt is the amount of As(III) adsorbed at time t (mg/g) and K1 is the pseudo first order rate constant (min−1).
A linear plot of log(Qe − Qt) against time allows obtaining the rate constant (
From
The Lagergren pseudo first-order rate constant (K1) and (Qe) values are represented in
The pseudo second order kinetics may be expressed in a linear form as integrated second order rate law [
t Q t = 1 Q e t + 1 K 2 Q e 2 (10)
where K2 is the pseudo second order rate constant (g∙mg−1∙min−1).
Adsorbent | Qe (exp) (mg/g) | pseudo first-order | ||
---|---|---|---|---|
K1 min−1 | Qe (calculated) (mg/g) | R2 | ||
SG2P | 2.3 | 4.6 × 10−s | 0.51 | 0.408 |
Pseudo second-order adsorption model for As(III) adsorption onto SG2P was applied and the rate constant for the adsorption K2 was evaluated as shown in
The results obtained shows that the value of linear regression coefficient R2 is 0.999, the values of Qe experimental, K2, R2 and Qe calculated were listed in
The results shows that the Pseudo second-order kinetic model perfect fit with experimental data and the value of R2 = 0.9998, by comparing (Qe) experimental and (Qe) calculated values from
Adsorption thermodynamics were determined using the thermodynamic equilibrium coefficients obtained at different temperatures and concentrations to verify possible adsorption mechanisms. The adsorption characteristics of a material can be expressed in terms of thermodynamic parameters such as ΔG (Gibbs free energy change), which can be calculated by the following equation [
Δ G = − R T ln K d (10)
where Kd is the thermodynamic equilibrium constant (L∙g−1).
According to thermodynamics, the Gibbs free energy is the difference between
Adsorbent | Qe (exp) (mg/g) | pseudo second-order | ||
---|---|---|---|---|
K2 min−1 | Qe (calculated) (mg/g) | R2 | ||
SG2P | 2.3 | 171.1 × 10−s | 2.1 | 0.9998 |
the adsorption enthalpy (ΔH) and adsorption entropy (ΔS) multiplied by the temperature. In this manner, by applying this concept to Equation (9), the thermo chemical parameters ΔH and ΔS can be determined using Van’t Hoff’s plot (
ln K d = − Δ H R T + Δ S R (12)
Δ H ˚ and Δ S ˚ were calculated from the slope and intercept of the linear plot of lnKd versus 1/T respectively. The results show that the enthalpy of adsorption Δ H ˚ was −3.7886 kJ∙mol−1 and Δ S ˚ was 9.9 J∙mol−1∙K−1. Δ G ˚ was calculated at different temperatures from the following equation:
Δ G ˚ = Δ H ˚ − T Δ S ˚ (13)
The obtained thermodynamic values are given in
The negative Δ G ˚ values indicate that the adsorption is spontaneous at these temperatures. The negative value of Δ H ˚ reflects an exothermic adsorption and indicates that the adsorption is favored at low temperature. In the other hand, the positive value of Δ S ˚ suggests that some structural changes occur on the adsorbent and the randomness at the solid/liquid interface in the adsorption system increases during the adsorption process.
The results of the minimum inhibitory concentration of micro dilution tray of the examined microorganism are shown in
Adsorbent | Δ H ˚ (KJ/mol) | Δ S ˚ (J/mol∙K) | Δ G ˚ (KJ/mol) | |||
---|---|---|---|---|---|---|
289 K | 308 K | 318 K | 328 K | |||
SG2P | −3.7886 | 9.9 | −6.6 | −6.8 | −6.9 | −7.0 |
substance | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
S. aureus + 25 mg/ml of SG2P | A | − | − | − | − | + | + | + | + | + | + | − | + |
S. aureus + 25 mg/ml of SG2P | B | − | − | − | − | + | + | + | + | + | + | − | + |
E. coli + 25 mg/ml of SG2P | C | − | − | − | − | + | + | + | + | + | + | − | + |
E. coli + 25 mg/ml of SG2P | D | − | − | − | − | + | + | + | + | + | + | − | + |
S. aureus | E | − | − | + | + | + | + | + | + | + | + | − | + |
S. aureus | F | − | − | + | + | + | + | + | + | + | + | − | + |
E. coli | G | − | − | + | + | + | + | + | + | + | + | − | + |
E. coli | H | − | − | + | + | + | + | + | + | + | + | − | + |
Abbreviation: −, no visible bacterial growth; +, visible bacterial growth.
As shown in
To well number 1 in the first four rows (A, B, C, D), 100 µl of 25 mg/ml adsorbent (SG2P) dissolved in 50% DMSO was add, thus this well contained 12.5% of adsorbent (SG2P). This was further serially diluted as described in materials and method section. On the other hand, to the first well (number 1) in the remaining four rows (E, F, G, H) 100 µl of 50% DMSO was added and a serial dilution of DMSO was prepared to detect antibacterial activity of DMSO and to ensure that the bacterial growth inhibition was due to SG2P and not DMSO.
The results shown in
According to the results obtained in this study, the adsorption capacity of arsenic onto SG2P was efficient. We can be concluded that the removal efficiency of arsenic from aqueous solution has related robustly to operating parameters. The optimum adsorption capacity of arsenic was 50% at pH 5.0, initial concentration 10 µg/L, temperature of 35˚C - 40˚C and contact time 20 minutes. The adsorption isotherms of arsenic were Freundlich model parameters value 1/n and n, showing that the adsorption of As(III) onto SG2P is promising. While, Lagergren pseudo second order model has been applied to obtain the amount of As(III) adsorbed per unit mass of SG2P, the qe (cal.) was agreement with the experimental value qe (exp.), which explains the exchange between adsorbent and adsorbate. Regarding to antimicrobial activity, SG2P has a low antibacterial activity against both Staphylococcus aureus, and Escherichia coli. In this paper, we have proposed that it is possible to investigate more studies to explain the adsorption of toxic heavy metals as well as antimicrobial activity onto silica gel compound.
The authors declare no conflicts of interest regarding the publication of this paper.
Obaid, A.A., Al-Masri, M., Deghles, A., Taha, N., Jodeh, S. and Smail, R. (2019) Functionalized C,N-Bipyrazole Receptor Grafted onto Silica Surface for Arsenic Adsorption and Its Antibacterial Activity. American Journal of Analytical Chemistry, 10, 38-53. https://doi.org/10.4236/ajac.2019.101004