In this work, Coffee husk (CH) was used as a solid phase extractor (SPE) for removal and/or minimization of Zn 2+ and Ni 2+ ions in aqueous media. XRD, FESEM and FTIR analysis of the SPE were performed for surface morphology and function groups characterisation. Batch mode adsorption studies were performed by varying the operational parameters such as adsorbent dose, solution pH, initial analyte concentration and contact time. The equilibrium data of both analytes was found a better fit with the Langmuir and Freundlich isotherm models. The q m of Langmuir for Zn 2+ and Ni 2+ ions were 12.987 and 11.11 mg/g, respectively. The adsorption capacities of the CH adsorbent towards Zn 2+ and Ni 2+ resulted of 12.53 and 10.33 mg/g, respectively. In addition, the kinetic data of Zn 2+ and Ni 2+ ions uptake revealed that the present system fitted well with pseudo-second-order kinetic model (R 2 > 0.99). Thermodynamic studies showed that the retention step was exothermic, and spontaneous in nature. The results indicated that the coffee husk provides an effective and economical approach in highly reducing or almost eradication of both metals Zn 2+ and Ni 2+ from the aqueous solution.
Pollution of the aquatic environment by heavy metal ions is progressing and becoming a major challenge due to its silent toxicity and pathological impact to human’s general health [
In aquatic environment, released metal ions accumulate in the living tissues through food chains. Thus, minimization of their concentrations to acceptable tolerable levels into the aquatic environment is of prime importance [
Recently, many articles have studied the utilization of solid waste with two fold approaches: i) reduction of solid waste generation and ii) use as a low cost solid phase extractor (SPE) for removal of phenols, heavy metal ions [
Based on the International Coffee Organization (ICO), the estimated number of coffee consumed was 151.3 million of 60 kg bags of coffee/year in 2015/2016 [
Zn(CH3CO2)2∙2H2O and Ni(CH3CO2)2 were purchased from BDH chemicals (Poole, England) and utilized. Stock solutions (1000 mg/L) of Zn2+ and Ni2+ ions were prepared individually in deionized water from their acetate salts. More diluted solutions of Zn2+ and Ni2+ ions were prepared in deionized water.
A Perkin Elmer Inductively Coupled Plasma―Optical Emission Spectroscopy (ICP-OES) model 7000 DV was used for analysis of trace metal concentrations. A JEOL-JSM-6380 model Field Emission Scanning Electron Microscopy (FESEM) and Philips PW1398 model X-ray powder diffractometer (XRD,) with a Cu-K radiation source were used for studying the surface morphology and degree of crystallinity of the coffee husk, respectively. Recording the characterization spectra of the SPE was performed by the usage of A Shimadzu FTIR spectrometer (IRAffinity-1, Japan) equipped with ATR-8200H (PIKE Technologies). The samples were grinded with high speed FOSS Cyclone mill (CT 193 Cyclotec™). A digital pH meter (Mettler Toledo MP220) and micropipette (Transferpette 0.5 - 5 mL) were used for pH measurements and preparation of more diluted solutions, respectively.
In the solid phase extractor (SPE) the coffee husk (CH) was washed initially with tap water until the washing solutions were found clear and colourless. The SPE finally washed with distilled water, dried at room temperature for 10 days due to the thickness of the CH, then ground and sieved to obtain a fraction of the particle size (<180 µm). The CH dried powder was stored in a plastic bag for it to be used.
Batch mode of separation was critically applied to study the uptake of Zn2+and Ni2+ from the test aqueous solutions by the proposed CH. The different adsorption parameters were performed at various metal concentrations (50 - 400 mg/L) at a shaking time of 120 min for Zn2+ and Ni2+ at 300 rpm in a 25 mL aqueous solution at pH 6 ± 0.1, and 25˚C ± 0.1˚C. After establishment of the equilibrium, SPE was separated out by filtration through Whatman No. 2 filter paper. Zn and Ni in the aqueous phase was analysed by ICP-OES. Following these procedures, the influence of adsorbent dose (0.3 - 2.0 ± 0.01 g) was also studied. The collected samples were analysed in triplicate (average ± standard deviation 5%) for metal ion concentrations. The adsorbate analyte retained (qe) and the extraction percentage (%E) on the SPE was evaluated as reported [
SEM micrographs for the unloaded coffee husk and loaded Zn2+, and Ni2+ ions are presented in Figures 1(a)-(c). It shows the substantial extent of crevasses
and bumps, due to the heterogeneous, rough, and spiked nature of the CH [
FTIR spectra of coffee husk before and after adsorption of Zn2+ and Ni2+ ions are demonstrated in
groups, respectively. On metal uptake by the used sorbent, these peaks began to decay (
The influence of extraction media pH on the analyte (400 mg/L) uptake by the coffee husk was performed in a range of pH (2.0 to 9.0 ± 0.10). The adsorption (% removal) achieved maximum at a pH of 7.0 and 6.0 for the Zn2+ and Ni2+ ions respectively, while the minimum uptake was achieved at lower pH
of the SPE is associated with hydroxonium ions (H3O+) at low pH which hindered the analyte uptake by the adsorbent surfaces [
The influence of SPE varying dose (0.3 - 2.0 ± 0.05 g) on analyte retention was also investigated at 400 mg/L initial concentrations of Zn2+ and Ni2+ with a controlled 90 min shaking time as demonstrated in
The residence time has an impact on analyte uptake by the SPE. The data are demonstrated in
The influence of metal ions concentration on the removal efficiency is illustrated in
Zn2+ and Ni2+ ions uptake from the aqueous solution onto the CH were subjected to kinetic models to understand the dynamic of the retention process. The pseudo-first-order and pseudo-second-order kinetic models were applied in this study. The linearized form of pseudo-first-order model is expressed by the following equation [
ln ( q e − q t ) = ln q e − k 1 t (1)
The values of qe and qt represent the analyte (mg/g) adsorbed capacity at equilibrium and at time t (min), respectively. k1 is the overall rate constant (1/min) of pseudo-first-order. The plot of log ( q e − q t ) vs. (t) shown in
t q t = 1 k 2 q e 2 + 1 q e t (2)
where k2 represents the rate constant (g/mg min) of pseudo-second-order rate model [
Pseudo second order | Pseudo first order | |||||||
---|---|---|---|---|---|---|---|---|
R2 | k2 g/mg∙min | qe,cal mg/g | R2 | k1 1/min | qe,cal mg/g | qe,exp mg/g | Metal ions | |
0.999 | 0.056 | 12.53 | 0.813 | 0.036 | 2.254 | 12.13 | Ni(II) | |
0.999 | 0.023 | 10.33 | 0.743 | 0.080 | 4.305 | 9.18 | Zn(II) | |
agreement for (R2 > 0.99) and the theoretical (qe,cal) values agreed with the actual (qe,exp) values for both Zn2+ (9.18 mg/g, 10.33 mg/g) and Ni2+ (12.13 mg/g, 12.53 mg/g). This indicated that the sorption of Zn2+ and Ni2+ fitted well by pseudo- second-order kinetic model. In the previous study the same indication was found using coffee husk as effective SPE for eliminating Pb (II) in water purification [
The equilibrium adsorption isotherm is fundamental in describing the retention of a substance from the aqueous media to a solid phase. Describing the regression correlation coefficient variations (R2) of Ni2+ and Zn2+ adsorbed onto the CH by using the Langmuir and Freundlich isotherm models [
q e = ( q m k L C e ) 1 + k L C e (3)
C e q e = 1 K L q m + ( 1 q m ) C e (4)
where KL = Langmuir equilibrium constant (L/mg) related to the free energy of analyte uptake, qm = Langmuir maximum adsorption capacities (mg/g), and Ce is the instantaneous concentration (mg/L) [
q e = k F C e 1 / n (5)
log q e = ( 1 n ) log C e + log K F (6)
The KF ((mg/g(L/mg))1/n and 1/n are related to the adsorption constant capacity, and the heterogeneity factor related to the intensity of the adsorption, respectively [
Langmuir and Freundlich isotherm parameters were determined from the plots of Ce/qe vs. Ce, and logqe vs. logCe
To calculate the essential energy involved during analyte uptake and the nature of the adsorption the thermodynamic parameters were applied.
The activation energy (Ea, J/mol), the Arrhenius equation for analyte uptake can be expressed as follow [
Zn2+ | Ni2+ | Isotherm | ||||||
---|---|---|---|---|---|---|---|---|
R2 | KL l/mg | qm mg/g | R2 | KL l/mg | qm mg/g | Langmuir | ||
0.997 | 0.101 | 11.111 | 0.998 | 0.113 | 12.936 | |||
R2 | KF ((mg/g(L/mg))1/n | 1/n | R2 | KF ((mg/g(L/mg))1/n | 1/n | Freundlih | ||
0.993 | 1.230 | 0.204 | 0.987 | 1.611 | 0.234 | |||
ln K 2 = ln A − E a R T (7)
where k2 = the rate constant of pseudo-second-order model for adsorption (g/mg∙min), A = the temperature-independent Arrhenius factor (g/mg∙min), T = the solution temperature (K), and R = the gas constant (8.314 J/mol∙K). The plot of lnk2 vs. 1/T is shown in
Vant Hoff equation [
ln K D = ( Δ S 0 R ) − ( Δ H 0 R T ) (8)
Δ G 0 = Δ H 0 − T Δ S 0 (9)
where KD is the Langmuir equilibrium constant of the adsorption. The plots of lnKD vs. 1/T were linear (
positive value of ΔS0 was 53.457 J/mol K for Zn2+ and 53.189 J/mol K for Ni2+, indicating increase of the randomness at the solid/solution interface [
The experimental data revealed that adsorption process for Zn2+ and Ni2+ adsorbates onto CH SPE was found to be a predominantly electrostatic interaction between the SPE and adsorbate. Moreover, thermodynamic studies exposed that the retention step is also random, exothermic, physisorption and spontaneous in nature [
Coffee husk has great potentials in being used as an efficient, cost-effective, and green SPE for Zn2+ and Ni2+ removal from environmental water samples. An excellent adsorption capacity of Zn2+ (12.13 mg/g) and Ni2+ (9.18 mg/g) was achieved at 0.5 g mass dose, 30 min time, 25˚C temperature, and pH 7. Analyte uptake is satisfactorily described by pseudo-second-order kinetic model. As a
References | qmax (mg/g) | Biosorption | Metal ion |
---|---|---|---|
This study | 12.98 | Coffee husk | Ni2+ |
[ | 0.125 | Dicerocaryum eriocarpum | |
[ | 5.86 | Coconut dregs residue | |
[ | 6.0753 | Moringa pods | |
[ | 23.63 | Pigeon peas hulls | |
[ | 27.78 | Citrus limetta peels | |
[ | 30.30 | Sophora japonica | |
This study | 11.11 | Coffee husk | Zn2+ |
[ | 0.01 | Dicerocaryum eriocarpum | |
[ | 5.965 | Cassava | |
[ | 8.970 | cabbage | |
[ | 14.852 | Jatropha curcas L. | |
[ | 25.71 | Sophora japonica |
result, the Langmuir and Freundlich adsorption isotherms (R2 > 0.99) fitted well for analytes uptake. The sorbent process was exothermic and spontaneous in nature depending on the value of ΔH0 and ΔG0. Therefore, CH SPE offers an optimal green alternative approach for the removal of Zn2+ and Ni2+ ions from environmental effluents before their discharge into the environment. The significant results suggest a rationale techno-economic sense for an organization to a prudent environmental management program to assess the environmental impacts of global coffee industries are of great importance.
The authors would like to thank King Abdulaziz University, Jeddah, Saudi Arabia for the facilities provided.
Alhogbi, B.G. and Al-Enazi, Z.F. (2018) Retention Profile of Zn2+ and Ni2+ Ions from Wastewater onto Coffee Husk: Kinetics and Thermodynamic Study. Journal of Encapsulation and Adsorption Sciences, 8, 1-17. https://doi.org/10.4236/jeas.2018.81001