Kinetic Modeling for Removal of Pb, Cd, Ni, and Cr Ions from Petrochemical Effluent using Termite Soil

Increase in concentration of heavy metals ions in our water bodies is of great concern. This work is aimed at providing an environmental and cost effective adsorbent for the removal of Pb, Ni, Cr and Cd from petrochemical effluent. Termite soil was pretreated and used for the sorption process and process parameters such as contact time and particle size of the adsorbent was varied. Optimum contact time of 60 minutes was obtained for Pb and Cd ions while 40 minutes was obtained for Ni and Cr ions for all particle sizes of adsorbent used. Initial reaction rate H(mg/g.min) was of the order for the different particle sized adsorbent, 75mm: Pb>Cd>Cr>Ni, 212mm: Cd>Pb>Cr>Ni and 300mm: Cd>Pb>Ni>Cr. Pseudo first order kinetics had less difference between experimental and theoretical adsorption capacity (Qa Qb) with R close to that obtained for Pseudo-second order kinetics. Hence it is considered as the kinetic order for the adsorption process. Percentage of the studied ions removed from the wastewater solution shows the suitability of the adsorbent sequestering of heavy metal ions from industrial wastewater.


Introduction
Water is an indispensable natural resource that is vital for life. However, it is difficult to find it in pure form due to human activities. European Public Health Alliance (2009) reported that greater percentage of the world's populations cannot find clean water. Rain water and sewage water pollution, industrial discharged effluents are major sources of water pollution.Polluted water consists of industrial discharged effluents, sewage water and rain water pollution (Ashraf et al., 2010). In Nigeria, today,it has been reported that wastewater from industrial effluents containing heavy metals most especially liquid in the form of wastewater or effluents into water ways or bodies (Ashraf et al., 2010). The deterioration of aquatic animals has occurred due to improper discharge of industrial effluents containing heavy metals have been a global concern. Heavy metals can easily enter the food chain because of their high solubility in water. These nonbiodegradable specieshave been proven hazardous and tend to cause a number of health problems, diseases and disorders (WHO, 2006). Different convectional techniques have been adoptedfor sequestering of heavy metal ions in wastewater, such as chemical precipitation, coagulation, ion exchange, oxidation, electro dialysis, membrane separation, solventextraction, photocatalytic reduction and adsorption methods (El-Ashtoukly et al., 2008). Application of the above conventional methods can be expensive, prohibitive for developing economy and most times do noteffectively remove polluting metals (Bassey et al., 2014), thus it becomes imperative to search for cheap and alternative means and bio-sorbents lately have become of considerable interest. The use of indigenous biodegradable resources for treating hazardous waste would be less expensive and more effective (Okieimen and Wuana, 2007;Tarawouet al., 2007 andDilek andOznur, 2008).Adsorption becomes a preferredchoice than other physicochemical techniques of heavy metal remediation due to its simplicity, cheap, easy to scale-up and mostimportantly able to remove low concentration substance even at part per million levels with high efficiency (Kurniawan et al., 2006). Recently, different researcher such asMuhammad and Wahabi (2011), Ademiluyi and Ujile (2013), Dada et al., (2013) and Agbozu and Emoruwa (2014) had applied adsorption as a technique for heavy metal removal. Hence this research is aimed at investigating the adsorption capacity of different particle sized termite hill biomass for the removal of toxic heavy metal ions: Pb, Fe, Ni, and Cr ions from a petroleum industry effluent.

Materials and Method
Termite moundsoil was sampled in Federal University of Petroleum Resources Effurun at Uvwie Local Government Area of Delta State, Nigeria. The soils samples were collected using soil auger 5.0cm deep into the surface soil. The sampled soil was then air-dried in the laboratory at room temperature for 5 days and crushed in a mortar. The milled mound was then sieved into different particle sizes (75mm 212mm and 300mm) and was used for subsequent experiments. This fraction was pretreated to remove non-clay material such as carbonate and quartz minerals in order to concentrate the active minerals and improve the sorption property of the adsorbent. The biomass was treated with 0.1M HCl for removal of any trace concentration of the studied heavy metal ions.

Wastewater Sampling
Wastewater effluent was collected from Warri Refinery and Petrochemical Company (WRPC) and was stored in Five Liters (5L) polyethylene container and 0.1 M Trioxonitrate (V) acid was added to the mixture.
Batch adsorption technique as reported by Agbozu and Emoruwa (2014) was adopted and modified for all process parameters in this study. Batch adsorption process was carried out at room temperature. Process parameters such as particle size, contact time, and adsorbent dose were varied in order for optimization.40ml of wastewater was introduced and allowed to remain in the packed column as shown in Figure 1 for the studied contact time (20, 40 and 60 minutes).Thereafter, the filtrate was collected for metal analysis determination.

Figure 1: Batch Adsorption Process
The sorption capacity Qe (mg/g) and removal efficiency Q were obtained according to equations 1 and 2 respectively.
Where v (L) is the volume of the solution, m (g) is the amount of adsorbent; Co and Ce are the initial and final concentration of the metal ions in the solution before and after adsorption (Nassar, 1997).

Kinetics Modeling
These include; pseudo-first order and pseudo-second order modelswhich considers that the rate of occupation of the biosorption sites is proportional to the unoccupied site (Ertugay and Bayhan, 2008).

The expression for the Lagergren pseudo-first order model
Where, K1 is the Lagergren rate constant for adsorption (min -1 ) Qtis equilibrium concentration of metal ion in solution at a particular time in (mg/g) The Lagergren pseudo-second order kinetic model K2 is equilibrium rate constant of second order kinetics model (g/mg/min).

Qt = K√t + C…………………………5
Where C is the boundary layer or thickness of the adsorbent  Figures 2, 3 and 4 gives plot for various percentages of metal ions sequestered. Initial increase in metal ion removal is due to the availability of adsorption sites at the start of the reaction. Optimum percentage removal of Ni and Cr ions occurred for all the particle size used at 40 minutes contact time while for Pb and Cd ions occurred at 60minutes. This is not surprising as in adsorption process, metal ions from the bulk solution travels to the thin film surrounding the adsorbent, during the process; the thin film in the liquid generates an impermeable membrane for the metal ion to pass through (Kannan and Sundaram, 2001 Tables 1 and 2 give parameters for pseudo-first and pseudo-second order kinetics of Pb, Ni, Cr and Cd adsorption. The slopes and intercepts of plots of log (Qe-Qt) versus twere used to determine the first-order rate constant K1 and equilibrium adsorption capacity (theoretical adsorption capacity)Qb.Qa-Qb(EABS) values obtained showed the closeness of experimental adsorption capacity using 300mm particle mesh size adsorbent. This probably occurred due to better surface area of the adsorbent. Intercept from the plot of t/Qtversus twas used in estimating the value of h (initial reaction rate). The results revealed that Pb and Cd tend to have higher initial reaction rate compared to other ions. Although this process tends to be high initially, but gradually became slower with passage of time as the reaction proceeded. This can be seen from their various rate constant k (s -1 ) for Pb and Cd: (0.230 x 10 -3 and 0 x 10 -3 for 75mm particle size), (9.212 x 10 -3 and 0 x 10 -3 for 212mm particle size) and (4.606 x 10 -3 and 0 x 10 -3 for 300mm particle size). Rate constant k (s -1 ) for Ni and Cr is appreciable higher for particle size studied causing positive change of their initial concentration, depicting increase in their movement to the surface of the adsorbent.  Table3 give parameters for intra-particle diffusion of metal ions.Intra-particle kinetics gives an idea of the penetrating capacity and movement of ions into the micropores structure of the adsorbent. It is observed that for all the metal ions studied boundary layer C is negligible. These greatly influence the uptake of the metal ions through the boundary layer of the adsorbent as shown in figures 2, 3 and 4

Results and Discussion
above.According to Itodo et al., (2009),non deviation of plot of Qt against √tfrom the origin signifies superiority ofintra-particle process over film adsorption. Since the value of C is insignificant it shows there was intra-particle transport of the metal ions and movement occurred through the particlesample interface. Similar result was reported by Badmus et al. (2007). Cd>Cr>Pb>Ni, Cd>Pb>Ni>Cr and Cd>Pb>Cr>Ni. Percentage removal of the studied metal ions at various optimum time is of the order Cd>Pb>Ni>Cr for 75mm particle size, Cd>Pb>Cr>Ni for 212mm particle size and Cd>Cr>Pb>Ni for 300mm particle size. Particle size of 300mm used showed better removal of the ions due to large surface area. Percentages of various metal ions sequestered from the natural wastewater solution shows a great potential for it use in industrial fixed beds reactor for treatment of their wastewater.