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 [1] .

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 [1] [2] . As the algae die and decompose, high levels of organic matter and the decomposing organism deplete the water of oxygen. Even some algal blooms are harmful to humans because they produce elevated toxins and bacterial growth that can make people sick if they come in contact with the polluted water or consume the contaminated aquatic foods.

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 [3] [4] . Surface area of the PKN was estimated using Sear’s method [5] [6] . PKN was also characterized using: Fourier Transform Infrared to identify the presence of functional groups, Energy dispersive X-ray for the identification of the constituent elements and Scanning Electron Microscopy for surface morphologies and pores diameters distribution.

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 C_o 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 Table 1.

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.

Plate 1. PKN before adsorption

Plate 2. PKN after adsorption.

The qualitative elements composition of the activated carbon was analyzed and presented in Table 2. The EDX patterns for PKN reveals that C and O are the main constituents, an indication of PKN suitability for adsorption process.

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

bands of between 509.22 cm⁻¹ and 596.99 cm⁻¹ indicate C-I aromatic ring vibration as presented in Figure 1.

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 Figure 2. The figure shows that the removal efficiency of PKN increases very fast within the first 60 mins after which the rate of adsorption began to decrease gradually with time. As the system approaches equilibrium stage between 180 mins and 240 mins, no significant changes were observed in the rate of removal. The sharp increase in removal efficiency at the early stages of adsorption may be due to the availability of the active sites of PKN which were yet to be saturated by the adsorbates [7] However, as the process proceeds most of the available sites get occupied by the adsorbates thereby reducing the possibility of contact between the surface area of the solute ion and the adsorbent [8] . Also, increase in the adsorbent dosage brings about an increase in the fresh and unsaturated adsorbent hence, the increase in the active sites for sorption.

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 [9] .

where and (mg/g) are the amounts of phosphorus adsorbed at equilibrium and at time t(mins) respectively and (mins⁻¹) is the rate constant of adsorption. After integration and applying boundary conditions, to and to, the integrated form of Equation (2) becomes

The rate constants were determined from the slopes and intercept of the plots versus t in Figure 3.

Pseudo Second Order Kinetic Model

The adsorption Kinetics may also be described by a pseudo second order equation [10]

Integrating Equation (4) and applying the boundary conditions to and to gives

Equation (5) can be rearranged to obtain a linear form

where is the rate constant of Pseudo second order adsorption (g∙mg⁻¹∙min⁻¹). The plot of versus t as presented in Figure 4 gives a linear relationship, from the slope and intercept of which and can be determined respectively.

Second Order Kinetic Model

The kinetic rate constants for second order kinetic model were evaluated from the plots of versus t as presented in Figure 5 and the values of the rate constants are presented in Table 3. The correlation coefficients () showed that, the data conformed to the second order kinetic model.

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 (mg/gmin^1/2) is intra-particle diffusion rate constant.

and ẟ are determined from the plots of adsorbate uptake versus the square root of time as shown in Figure 6 and their values are presented in Table 3.

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-

trolling step and not film diffusion [11] [12] [13] .

Bhattacharya-Venkobachor Kinetic Model

Bhattacharya-Venkobachor model is expressed as [14] .

where,

The effective diffusion coefficient, is obtained from the equation (Joseph and Philomena, 2011).

where,

is the Bhattacharya Venkobachor constant (min⁻¹)

is the initial concentration (mg/l)

is the concentration at time t(mg/l)

is the particle radius

The values of are determined from plots of versus t as presented in Figure 7.

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 is the equilibrium value of adsorbate per unit mass of adsorbent (mg/g), is the equilibrium concentration of the adsorbate.

Langmuir constants and are determined from the intercepts and slope of the linear plots of versus as presented in Figure 8.

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 which is expressed as (Babayemi and Onukwuli, 2016; Gueu et al, 2006)

where is the initial adsorbate concentration. The value of indicates whether the isotherm is irreversible (), favourable (), linear () or unfavourable ().

values for phosphorus adsorption on PKN are less than 1 and greater than zero indicating favourable adsorption under the chosen experimental conditions. The values are presented in Table 4.

Freundlick Isotherm Model

The Freundlick adsorption isotherm is expressed as

The linearized form of the above expression is

The Freundlick constants “n” giving an indication of how favourable the adsorption process is and which is the adsorption capacity of the adsorbent are determined from the slope and intercept of plots of versus as presented in Figure 9.

Temkin Isotherm Model

The linear form of Temkin Isotherm Model for liquid adsorbate is expressed as:

Temkin constants and are obtained from the intercepts and slope of the plots of versus as shown in Figure 10 and the values are presented in Table 4.

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