Characterization and equilibrium studies for the removal of methylene blue from aqueous Solution using activated bone char

The surface characteristics as well as adsorption potential of activated cow bone char for the removal of methylene blue (MB) from aqueous solution were investigated. Physical characteristics of the adsorbent revealed a large surface area, low pore volume, reduced ash and moisture contents, which have been identified as good adsorption characteristics. The surface of the adsorbent was predominated by mesopores with a few microporous structures as well as the presence of carbonates, phosphates, silicates and hydroxyl groups which are characteristic of the apatite phase. Adsorption efficiency for the removal of MB was observed to be influenced by pH, adsorbent dosage as well as initial dye concentrations. Equilibrium adsorption data was best described by the Freundlich isotherm with a good correlation coefficient suggesting multilayer adsorption of the dye molecules on the surface of the adsorbent. Based on the drive for reduced cost, removal efficiency and availability, activated carbon from cow bone could be a promising adsorbent for methylene blue-laden effluent that could be utilized in small and large industrial applications.


Introduction
The quest for cost effective, readily available and highly efficient adsorbents has led to the search for materials of biological and chemical origin, for the removal of organic and inorganic contaminants from aqueous solution. This is an effort made to overcome the hurdles of high energy requirement and other limitations posed by routine methods like precipitation, reverse osmosis, coagulation, chemical oxidation and so on, which have been employed in the treatment of effluents [1,2].
Dyes have an array of industrial applications and methylene blue is a thiazine cationic dye used as a redox indicator and has wide applications as paper and temporary hair colourant as well as dyeing of fabrics and coatings for paper stock [3]. Challenges of removing residual dye from effluents have been the major setback particularly in small scale textile industries which are prevalent in most developing countries. Within these regions, unavailability of sophisticated treatment facilities due to high cost have often been the reason for the direct discharge of dye effluents into the environment, which is not only aesthetically displeasing, but also hinders the penetration of light which interferes with photosynthetic processes in aquatic matrices [4]. Other health concerns including mutagenic and carcinogenic effects, jaundice, cyanosis quadriplegia, damage to the liver, reproductive systems and kidneys have been reported [5,4,6]. It is therefore necessary to develop readily available and cost effective materials that have excellent adsorption potentials and can be utilized for the removal of chromophoric organic contaminants from influents before disposal.
Among the various available solid substances known to possess this remediating activity, activated carbon has been reported to possess the highest adsorption potential due to their high specific surface and controlled chemical affinity [7,8]. It can be derived from organic based materials like coconut shells, animal bones, rice husks, banana pith and wheat straw, among others. In a study to investigate the adsorption efficiency of different animal bones for the bleaching of palm oil, activated carbons prepared from cow bone was reported to possess the highest decolourising effect over other animal sources due to its wide surface area, low pore volume, reduced moisture and ash contents as well as high yield of charcoal [9]. Sequel to these excellent properties as reported, the focus of this research was to investigate the surface chemistry of activated carbon from cow bone using relevant spectroscopic techniques to give insight to the chemical characteristics as well as the porous nature of the adsorbent.
Also, to evaluate the efficiency of the adsorbent for the removal of methylene blue from aqueous solution in order to assess its suitability for use in designing sorption beds for the removal of methylene blue from industrial effluents, which could serve as excellent treatment alternative especially to small scale textile industries.

Pre-treatment, carbonization and activation
These stages were carried out according to the following procedure. Cow bone samples were collected from an abattoir and washed in distilled water to remove sand and dirt. Flesh The dried sample was sieved using a 1 mm pore size sieve to obtain a uniform sized adsorbent [9].

Characterization of the activated carbon
Surface characteristics including appearance, moisture content, ash, bulk density and particle size were investigated using standard procedures reported elsewhere [9]. Total surface area as well as the porous nature of the adsorbent was analysed using Brunnauer-Emmett-Teller (BET) and t-plot methods respectively, using micrometrics analyser (Tristar 3000). Surface functionalities of the adsorbent were evaluated using Fourier transform infrared spectrophotometer (Fischer Thermoscientific, USA). X-ray diffraction patterns were obtained using X'Pert PRO MPD diffractometer at 40mA and 40kV, using Cu Kα radiation with a speed of 10 o /min. [10]. Change in pH was noted at each instance and the point of zero charge was evaluated.

Batch adsorption studies
The mixture was centrifuged and the supernatant was analysed using UV spectrophotometer

Textural characterization of activated carbon
The surface area, porous nature and other physical characteristics of the adsorbent are presented in table 1. Figure    reported that cationic adsorption is highly favourable when pH>pH pzc, while anionic dye adsorption is favoured at pH<pH pzc , where the surface of the sorbent is positively charged [6]. Therefore, this implies that the adsorption process will best be favoured in alkaline medium, in which the hydrated surface of the adsorbent becomes deprotonated, thereby acquiring a negative charge which attracts the positively charged surface of the adsorbate. On

Equilibrium adsorption isotherms
Langmuir and Freundlich isotherm models ( Figure 5) were used to study the mechanism of adsorption of MB on activated bone char. The parameters of each model are presented in activated bone char had a higher monolayer capacity than class fly ash, raw beach sawdust, sheep bone and cotton stalk, but lower than Pork bone and raw clay mineral.
On the other hand, the Freundlich isotherm model describes a multilayer adsorption where adsorbate molecules are retained at more than a single adsorption site on the surface of the adsorbent. The high correlation coefficient of the isotherm suggest that the mechanism of removal is more of a multilayer adsorption than adherence of dye molecules to a single adsorption site. The good correlation coefficient of both isotherms which suggest different mechanisms of adsorption is expected of materials with heterogeneous porosity as shown in Figure 3. The slope of the graph represented by the value of 1/n (table 2) suggests a heterogeneous surface which favours multilayer adsorption confirmed by the dimensionless constant (R L ) which is greater than unity [14].

Conclusions
The adsorption characteristics of activated carbon from cow bone char was investigated for its surface properties and adsorption efficiency for use in the treatment of a cationic dyeladen effluent. The surface characteristics indicated a heterogeneous surface predominated by mesoporous structure. Equilibrium adsorption studies indicated increasing adsorption potential at high pH due to the negatively charged surface of the adsorbent in this medium which enhances attraction of dye molecules to the surface of the adsorbent. Equilibrium adsorption data suggest a multilayer adsorption promoted by the heterogeneous surface of the adsorbent. This study revealed that activated carbon from cow bone has good adsorption properties and could be utilized for the treatment of methylene blue dye waste water before discharge into the environments.