Studies were carried out on Palm Kernel Shell, an agricultural waste available in large quantity in Nigeria, to evaluate its ability to remove phosphorus from wastewater. The adsorbents, which were prepared from Palm Kernel Shells (PKN), were characterized using Fourier Transform Infrared (FT-IR), Energy dispersive X-ray (EDX) and Scanning Electron Microscopy (SEM). Batch mode experiments were conducted to study the effects of adsorbent dosage and contact time on phosphorus adsorption. Equilibrium and Kinetic studies of the process were also carried out. Results obtained show that, FT-IR spectrum of the activated carbon displays a number of absorption peaks, reflecting the complex bio-mass structure and a variety of functional groups which explains its improved adsorption behaviour on the colloidal particles. SEM shows the spherical shape of the carbon particles with a wide range of sizes, EDX indicated the constituent elements in the adsorbent in which C and O were found to be the most abundant. Equilibrium data fitted well to the Freundlick and Langmuir models but the data were best described by Langmuir Isotherm model at the temperature of 313 K. Pseudo second order best described the kinetics of the adsorption process. Removal efficiency ( E%) of 97% was attained within 120 minutes at 50 g/l adsorbent concentration, pH6 and 0.2mm particle size of the adsorbent.
Phosphorus pollution is a major problem emanating from natural decomposition of rocks and minerals, agricultural runoff, erosion and sedimentation, improper disposal of wastes generated especially from indigenous chemical industries such as fertilizers, soap and detergent companies [
Phosphorus is a common constituent of agricultural fertilizers, manure and organic wastes in sewage and industrial effluent. It is an essential element for plant life but when there is too much of it in water, it results in the contamination of groundwater and eutrophication of lakes, rivers and canals. Eutrophication results in reductions in aquatic fish and other animal populations as it promotes excessive growth of algae [
Given the severity of environmental impact of phosphorus pollution to our local, national and global communities, it is imperative that a technologically feasible, economically viable and socially acceptable solution be found to solve the problem. Against this background, adsorption process has been adopted using Palm Kernel Shell, a bio degradable non-toxic agricultural waste available in large quantity in Nigeria.
Palm Kernel Shells are the shell fractions left after the nut has been removed in the palm oil mill. In the palm oil value chain, the utilization rate of palm kernel shell is comparatively very low.
Palm Kernel Shells (PKN) were obtained from Umuoma village, near Anambra State University Campus, Uli. The shells were cleaned and dried in a Memmert oven at 110˚C for 24 hrs. The dried sample was then carbonized in a muffle furnace at a temperature of 800˚C for 3 hrs. The charred material was allowed to cool to room temperature, ground and sieved using 0.2 mm mesh. The sieved 0.2 mm particle size material was weighed and impregnated with 1M H2SO4 at the ratio of 1:2 (wt%) for 12 hrs. The impregnated sample was washed with de-io- nized water until pH 7.0, filtered and dried in an oven at 110˚C for 24hrs before being packed in an air tight sample bags for use.
Proximate analysis was carried out on the activated PKN to determine the % weight loss, bulk density (g/cm3), % ash content, iodine number, % volatile matter, % moisture content and % fixed carbon using standard methods [
The effluent used in the experiment was prepared by first dissolving 500 g of phosphate rock sample in 1000 cm3 of de-ionized water. The solution was thoroughly stirred and filtered off the silts, organic matters and insoluble phosphate rock particles. The filtrate was further diluted with 3000 cm3 deionized water before it was tested for pH level with digital pH meter and its phosphorous content concentration was measured in UV spectrophotometer set at a wavelength of 650 nm. The initial pH was 2.8 and phosphorus concentration was 373 mg/l.
Batch Adsorption Experiment
1.0 g of PKN was added to 100ml of the synthesized effluent in a conical flask and placed on a magnetic stirrer.
The stirring was done at different temperatures of 30˚C, 35˚C, 40˚C and at different stirring periods (contact time) of 30 mins., 60 mins., 120 mins., 180 mins., 240 mins. and 300 mins. respectively. Upon the completion of each stirring period, the solution was filtered using what man filter paper. The residual concentration of the filtrate was determined using UV-spectrophotometer set at wavelength of 650 nm. The same procedure was repeated for 2.0 g, 3.0 g, 4.0 g and 5.0 g of the adsorbent respectively. Removal efficiency E% was calculated using Equation (1).
where Co and C1 are respectively the initial and residual concentrations of phosphorus in the effluent (mg/l).
Physico-chemical characteristics of PKN derived activated carbon are presented in
The surface morphology of the PKN before and after sorption is measured and presented in Plate 1 and Plate 2 respectively. Plate 1 clearly reveals the surface texture of the materials with a wide range of sizes before adsorption. Plate 2 shows the morphological changes with respect to shape and size of the activated carbon after adsorption. It can be clearly observed that, the surface of the activated carbon has been changed into a new shining bulky particles and whitish patches structure.
Parameters | PKN |
---|---|
Weight loss % | 44.79 |
Bulk density g/cm3 | 0.48 |
Ash content, % | 6.33 |
Iodine number, mg/g | 558.06 |
Volatile matter, % | 20.87 |
Moisture content, % | 7.17 |
Fixed carbon, % | 75.28 |
Surface area, m2/g | 635.73 |
Plate 1. PKN before adsorption
Plate 2. PKN after adsorption.
The qualitative elements composition of the activated carbon was analyzed and presented in
FTIR spectrum of the activated PKN displays a number of absorption peaks, reflecting the complex biomass structure. The broad stretching absorption peak between 4288 cm−1 and 4638 cm−1 indicates C-H group of alkanes and the low
Element | App. Conc. | Intensity | Wt.% | Wt.%. Sigma | Atomic% |
---|---|---|---|---|---|
C | 7.39 | 0.4037 | 10.68 | 0.48 | 18.67 |
O | 26.27 | 0.3787 | 40.49 | 0.50 | 53.12 |
Na | 0.53 | 0.7282 | 0.42 | 0.06 | 0.39 |
Mg | 0.36 | 0.6991 | 0.30 | 0.05 | 0.26 |
Al | 1.21 | 0.8162 | 0.86 | 0.05 | 0.67 |
P | 30.97 | 1.3386 | 13.50 | 0.17 | 9.15 |
Cl | 2.41 | 0.7985 | 1.76 | 0.06 | 1.04 |
Ca | 54.24 | 0.9987 | 31.70 | 0.32 | 16.60 |
Fe | 0.39 | 0.8083 | 0.28 | 0.08 | 0.11 |
Total | 100 |
bands of between 509.22 cm−1 and 596.99 cm−1 indicate C-I aromatic ring vibration as presented in
Effects of Adsorbent Dosage and Contact Time
The removal efficiency of PKN activated with H2SO4 as a function of time for varying adsorbent dosages is presented in
Adsorption Kinetics
Kinetics of sorption describes the solute uptake rate, which in turn governs the residence time of sorption. It is one of the important characteristics in defining the efficiency of sorption. In the present study, the kinetics of the phosphorus removal was carried out to understand the behaviour of PKN.
Pseudo first order kinetic model
The rate constant of adsorption was determined from the Pseudo first order rate expression given by Largergren [
where
The rate constants were determined from the slopes and intercept of the plots
Pseudo Second Order Kinetic Model
The adsorption Kinetics may also be described by a pseudo second order equation [
Integrating Equation (4) and applying the boundary conditions
Equation (5) can be rearranged to obtain a linear form
where
Second Order Kinetic Model
The kinetic rate constants for second order kinetic model were evaluated from the plots of
Weber and Morris Kinetic Model
Weber and Morris plot was used to investigate the intra-particle diffusion mechanism. The equation used in this case is as follows:
where ẟ is Weber and Morris constant and
The deviation of straight lines from the origin indicates that intra-particle transport is not the rate limiting steps and that, pore diffusion is the only con-
Kinetic model | Parameter | Temperatures, K | Remarks | ||
---|---|---|---|---|---|
303 K | 308 K | 313 K | |||
Pseudo first order | K1 (min−1) qe (mg/g) R2 | 0.0270 5.9835 0.8144 | 0.0133 2.9274 0.8043 | 0.0082 1.7579 0.7619 | Data did not fit well to the model |
Second order | K2 (mg/g∙mn) qe (mg/g) R2 | 0.0110 2.5536 0.9417 | 0.0228 1.2973 0.9595 | 0.0483 0.5759 0.9501 | Data fitted well to second order kinetic model |
Pseudo second order | K2 (mg/g∙min) qe (mg/g) R2 | 0.0032 33.7838 1.0000 | 0.0034 34.2465 1.0000 | 0.0039 34.8432 1.0000 | Data fitted well at all the three chosen experimental temperatures |
Weber and Morris | Kd (mg/g∙min½) ẟ R2 | 0.5220 24.526 0.8852 | 0.5130 25.1990 0.8760 | 0.4765 26.543 0.8524 | Data did not conform to Weber and Morris |
Bhatacharya Venkobachor | KB (min−1) DB (m2/s) R2 | 0.0064 | 0.0064 | 0.0340 | Data fitted well at the temperature of 313 K |
trolling step and not film diffusion [
Bhattacharya-Venkobachor Kinetic Model
Bhattacharya-Venkobachor model is expressed as [
where,
The effective diffusion coefficient
where,
The values of
Adsorption Isotherms Studies
Adsorption isotherms are characterized by certain constants and described the mathematical relationship between the quantity of adsorbate and concentration of adsorbate remaining in the solution at equilibrium. In this work, Langmuir, Freundlick and Temkin isotherm models have been used to analyze adsorption data at different temperatures.
Langmuir Isotherm Model
The linearized form of Langmuir Isotherm Model is expressed as:
where
Langmuir constants
To confirm the favourability of an adsorption process to Langmuir Isotherm, the essential features of the isotherm can be expressed in terms of a dimension- less constant or separation factor
where
Freundlick Isotherm Model
The Freundlick adsorption isotherm is expressed as
The linearized form of the above expression is
Isotherm | Parameter | Temperatures, K | Remarks | ||
---|---|---|---|---|---|
303 K | 308 K | 313 K | |||
Langmuir | qmax (mg/g) KL (1/mg) RL R2 | 4.2158 0.0196 0.1203 0.9509 | 4.4169 0.0180 0.1296 0.9632 | 3.8109 0.0166 0.1399 0.9904 | |
Freundlick | n KF (1/g) R2 | 0.2328 | 0.2436 | 0.2230 | |
Temkin | B (J/mg) Kt (1/g) R2 | 36.59 0.0334 0.844 | 39.82 0.0315 0.8123 | 37.77 0.0276 0.8039 | R2 values show that data did not fit to Temkin Isotherm |
The Freundlick constants “n” giving an indication of how favourable the adsorption process is and
Temkin Isotherm Model
The linear form of Temkin Isotherm Model for liquid adsorbate is expressed as:
Temkin constants
Activated carbon prepared from palm kernel shell was used to remove up to 97% phosphorus from wastewater through adsorption process. Equilibrium data fitted well to Langmuir and Freundlick Isotherm Models. Kinetics of adsorption was best described by Pseudo Second Order Kinetic Model.
Conclusively, this work has demonstrated that, palm kernel shell, an agricultural waste, non-toxic and bio-degradable material could be used as an effective adsorbent for the removal of phosphorus from wastewater.
Babayemi, A.K. (2017) Performance Evaluation of Palm Kernel Shell Adsorbents for the Removal of Phosphorus from Wastewater. Advances in Chemical Engineering and Science, 7, 215- 227. https://doi.org/10.4236/aces.2017.72016