Kinetic modelling of dumpsite leachate treatment using M usa sapientum peels as bio-sorbent

Kinetics models are very vital to dumpsite operators and planners as they provide relevant information for effective treatment of leachates. The aim of this work is to model the kinetic process of treatment of Lagos dumpsite leachate using Musa sapientum peels as bio-sorbent with a view of establishing the kinetic parameters of the treatment process. Musa sapientum peels which were collected from Ayetoro market in Epe Local Government area of Lagos State were used to prepare the bio-sorbent. Kinetic process was carried out using 1 g of the prepared bio-sorbent in 100 ml Lagos dumpsite leachate in different conical flasks and at various contacting time. The kinetic data obtained were fitted to different kinetics models. The kinetics models tested were Fractional power model, Lagregren pseudo first – order model, Pseudo second – order model, Kuo – Lotse kinetic model, Blanchard kinetic model and Elovich kinetic model. Other kinetics models considered were Sobkowsk – Czerwi kinetic model, Intraparticle diffusion (IPD) model, Behnajady – Modirshahla – Ghanbery (BMG) model and Diffusion – Chemisorption model. Coefficient of determination (R 2 ) values and the expected nature of the plots of the models were used to screen the tested models. The results revealed that the Pseudo second – order kinetic model has the best R 2 value of 0.99996 and the graph followed the expected nature of the plot hence it was adopted in this work. It was concluded that Pseudo second – order kinetic model can be used to navigate the treatment process of Lagos dumpsite using Musa sapientum peels as bio-sorbent.


Introduction
The poor management of solid wastes generated by human activities is really contributing immensely to the environmental problems. The operation of dumpsites instead of engineered landfill sites is a major practice in developing countries (Nigeria inclusive). The leachates generated from the dumpsites pollute our environment and as a result, the leachates need to be treated before discharging to the environment or water bodies. Leachates can be treated by the use of activated carbons produced from agricultural wastes such as banana peels, woods, coconut shell, orange peels, rice straw, soya cake, cactus leaves, beech leaves and moss [1].
Several researchers have used agricultural wastes in the treatment of wastewater and leachates [2 -13]. Saima et al. (2008) [2] evaluated banana peels in the treatement of arsenic contaminated water. The evaluation revealed that banana peels have a very high potential for the removal of arsenic from groundwater. El -Nafaty et al. (2013) [3]studied the biosorption and kinetic of oil removal from polluted water using banana peels. The study showed that banana peels obeyed Pseudo second -order kinetic model in the removal of oil from polluted water. Deshmukh et al. (2017) [11] investigated the removal of cadmium from aqueous solution using banana peels as adsorbent. The investigation pointed out that the adsorption capacity of cadmium on dried banana peels was 5.91 mg/g. It was also shown that the experimental data fitted into second order kinetic model. Olafedehan et al., (2018 [12] worked on the equilibrum, kinetic and thermodynamic studies of biosorption of zinc ions from industrial wastewater using derived composite bio-sorbent from walnut shell. The work showed that the removal efficiencies of Zn (11) ions were effective in the acidic range. The experimental data correlated well with Pseudo second -order kinetic model. Sumanik et al. (2019) [13] researched on the use of banana peels for removal of lead in leachate. The research revealed the banana peels adsorbent has a capacity of 23 mg/g and could be used for treatment of leachates. Therefore the aim of this work is to model the kinetic process of treatment of Lagos dumpsite leachate using Musa sapientum peels as bio-sorbent with a view of establishing the kinetic parameters of the treatment process which gives vital information for the design of the treatment process [14] 2. Methodology

Preparation of Adsorbent
Muse sapientum peels were collected from a market in Ayetoro located in Epe Local Government, Lagos State, Nigeria. The peels were washed with water to remove any unwanted materials. The peels sample were carbonised for a duration of 1 hr in a furnace. The char product from the furnace was cooled with cold water after which it was tranferred into the oven for further drying at 110 o C. The activated carbon was impregnated with tetraoxosulphate (vi) (H2SO4) (10 % by weight) and heated in the absence of air. The resultant moist paste was charged into the furnace followed by heating for 1 hr at a temperature of 110 o C until a constant weight of activated carbon was achieved. The chemical activation was done to remove the tar in the pores of the activated carbon. Distilled water was used to rinse the activated carbon thoroughly to remove the remaining H2SO4. The activated carbon was then dried with the aid of an oven at a temperature of 110 o C for 3 hrs. A mortal was then used to crush the activated carbon to size of 100 mesh.

Kinetic of Treatment Process
100 ml of leachate collected from Soluos dumpsite in Lagos State, Nigeria, having a pH of 7.2 was measured into 250 ml conical flasks containing 1 g each of the prepared bio-sorbent. The mixture was agitated with a shaker at 150 rpm for 2 hrs after which it was left undisturbed for 20 mins and then filtered. The concentration of the total dissolved solid (TDS) in the filtrate was determined at 100

Effect of Contact Time
The effect of contact time on the treatment of Lagos dumpsite leachate using Musa sapientum peels as bio-sorbent is shown in Figure 1 which depicts the graph of percentage removal against contact time. In Figure 1

Kinetic Modelling of Leachate Treatment
Various kinetic models presented in Equations (4), (6), (8), (10), (12) and (14) [16]were used to test the kinetic data obtained during the kinetic process for the treatment of Lagos dumpsite leachate. Other kinetic models used to fit the experimantal data were shown in Equations (17), (19), (20) and (23) [16]. The Fractional power model which is of the form of Freundlich equation, indicates the adsorbate (or solute) uptake increases exponentially with time and it is given by: Where kF and N are the fractional power kinetic model constant [=] mg/ (g min N ). Taking the natural logarithm of Equation (4), it yields: A plot of t q ln against t ln was done to obtain the parameters inherent in Equation (5).
Lagergren Pseudo first -order kinetic model is presented in Equation (6).
Where k1 is the Lagergren Pseudo first -order rate constant [=] min -1 . Integrating Equation (6), it gives: ln ln   A plot of ln qtagainst ln t was carried out to determine the apparent adsorption rate constant and Kuo -Lotse constant.
Blanchard kinetic model which is presented in Equation (12) was also applied to the kinetic data. Elovich kinetic model is of the form in Equation (14)    The kinetic parameter and R 2 values are presented in Table 1. The closer the R 2 value to unity, the better the kinetic model fitted the experimental data. The N value in the Fractional power model was less than unity which indicated time dependent of adsorption on Musa sapientum peels adsorbent [12]. The R 2 value was approaching unity however it was not the best R 2 among the models considered in this work hence Fractional power model was ignored. Lagergren pseudo first -order model, Blanchard kinetic model and Behnajady -Modirshahla -Ghanbery model were jettisoned as their R 2 values were not close to unity. Elovich kinetic model can be taken to fit kinetic experimental data provided the R 2 value was greater than unity. In this work, the R 2 value was 0.9690 which was less than unity hence the model was rejected. Sobkowsk -Czerwi kinetic model will fit kinetic data if the intercept on the vertical axis is unity but in this work, the intercept of the plot on vertical axis was 1.4623 and on this basis, it was ignored. IPD model can be used to identify the mechanism involved in the adsorption process system. Using the model, the plot must pass through the origin which was not in line with the plot obtained in this work and as a result, it was jettisoned. Pseudo second -order kinetic model, Kuo -Lotse kinetic model, and IPD model have their R 2 values close to unity. However, Pseudo second -order kinetic model has the best R 2 value of 0.99996 among these three models. On the basis of this, Pseudo second -order kinetic model was adopted as the best model that fitted the experimental data obtained in this work. Previous works of El -Nafatyet al. (2013) [3] and Deshmukh et al. (2017) [11] also revealed that using Musa sapientum peels as bio-sorbent in the removal of cadmium and oil respectively from aqueous solution followed Pseudo second order kinetic model.

Conclusion
The kinetic process of dumpsite leachate treatment using Musa sapientum peels as bio-sorbent has been modeled. The percentage removal of TDS increased as the contact time between the adsorbent and the leachate increased up to 150 mins after which there was no significant removal in the TDS. Various kinetic models were tested with the kinetic process data. Lagergren pseudo first -order, Blanchard kinetic model and BMG models were rejected as their R2 values were not close to unity. Elovich kinetic model was ignored because the R2 value was not greater than unity. IPD model was jettisoned as the plot did not pass through the origin. Fractional power model, Pseudo second -order kinetic model, Kuo -Lotse kinetic model and Diffusion -Chemisorption model have their R2 values very close to unity however Pseudo second -order kinetic model has the best value in term of coefficient of determination hence it was adopted in this work as the best kinetic model that fitted the kinetic data of treatment of Lagos dumpsite leachate using Musa sapientum peels as bio-sorbent.