Kinetic and Adsorption Studies of a Thiazine and Triarylmethane Dye onto Bamboo Impregnated Nanoscale Manganese

In this study, bamboo impregnated with nanoscale manganese (BMn) was prepared by the aqueous phase borohydride reduction method and characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and PIXE analysis. The synthesized nMn-bamboo (BMn) was subsequently applied for the sorption of methylene blue (MB) and acid violet 19 (AV 19) from aqueous solution representing a thiazine and a triarylmethane class of dyes, respectively. Batch adsorptions of MB and AV 19 dyes were investigated under various experimental conditions such as pH, contact time, initial concentration of dyes and adsorbent dosage. The results showed that the synthesized BMn is an effective adsorbent with a high MB and AV 19 dyes adsorption capacity above 180 mg/L for both dyes. At concentration 120 mg/L, usage of 0.02 g of BMn resulted to 263.5 and 187.2 mg/g removal efficiency for MB and AV 19, respectively at 165 rpm for a period of 120 min and at a solution pH of 7.6. The equilibrium data were best represented by Freundlich isotherm model and the pseudo-second order kinetic model better explained the kinetic data


ISSN 2474-8811
A malaise that has captured the world's attention over the past 20 years is the spate of water pollution around the world with textile industry being one of the largest polluters globally; the second after agriculture (put reference) (www.un.org). The World Bank estimates that almost 20% of global industrial water pollution comes from the treatment and dyeing of textiles (www.un.org). Industrial dyes are highly coloured synthetic organic substances with complex structures, high colour intensity and low biodegradability. They persist for long distances in flowing water, inhibit the biological activities of aquatic biota and generally impact negatively on the domestic and economic values of water bodies. The annual consumption of dyes has been on the increase globally due to their diverse applications. It is estimated that more than 10% of the total 7 x 10 5 metric tonnes of dyes produced annually finds their way into water bodies via direct discharge as aqueous effluents or loss during colouration processes (Balasubramanian, 2009). Adsorption is established over the years to be a viable alternative among remediation methods for the treatment of dye contaminated wastewater owing to its cost efficiency, versatility, and ease of operation (Garg et al., 2003 This study evaluates the efficiency of bamboo impregnated nanoscale manganese (BMn) as an adsorbent for the removal of MB and AV 19 from aqueous solution representing thiazine and triarylmethane dyes, respectively. The effects of pH, contact time, initial dye concentration, temperature and adsorbent dosage on the adsorption capacity were investigated. The experimental data were also fitted into different equilibrium and kinetic models and deductions made.

Experimental
Chemicals and Reagents Analytical grade manganese chloride (Minimum assay 99.9%, Tianjin Kermel, Hebei, China), sodium borohydride (BDH 95%, prd no. 30114, Sigma Aldrich, Diegem, Belgium), MB dye (BDH prd no. 340484B, Sigma Aldrich, Diegem, Belgium), absolute ethanol (BDH Analar, 95% UN No. 1097), hydrochloric acid (purity 37%, density 1.1kg/cm3, Riedel-deHaen, Buffalo, NY, USA) and sodium hydroxide (BDH prod. No. 30167, Sigma Aldrich, Diegem, Belgium) were used without further purification. Adsorbent (Bamboo Impregnated Nanoscale Manganese) Preparation The BMn was prepared by borohydride reduction method with MnCl 2 .H 2 O and bamboo in the ratio 1:1. The bamboo was washed, dried at room temperature and then chopped into small pieces for easy grinding. The chopped bamboo was pulverized into very fine particles with the help of mortar and pestle and soaked in HCl solution for 4 hrs, washed several times with deionized water and finally dried at room temperature prior to impregnation with nanoscale manganese.

Characterization of Bmn
The scanning electron micrographs (SEM) of bamboo, nMn and nMn-bamboo were viewed under a FEITM scanning electron microscope (Nova Nano SEM 230, FEI, Hillsboro, OR, USA). A 1.7 MV tandem pelletron accelerator (model 5SDH) was used to determine the concentrations of the elements present while fourier transform infrared spectroscopy (FTIR) absorption spectra were obtained using the potassium bromide (KBr) pellet method and recorded over the range 4000 -400 cm-1 using Shimadzu FTIR-8400s. Adsorption Studies Batch dye removal experiments with the prepared BMn were car-ried out in 100 mL flasks under vigorous agitation at 165 rpm. The experiment involved preparing 50 mL of dye solutions with desired initial concentration and contacting with a known mass of the adsorbent (BMn) at a predefined time and speed. The pH of the solution was adjusted using 0.1 M HCl or 0.1 M NaOH solutions with a pH meter model (pH 211 microprocessor). A 0.02 g of BMn powder was added to the solution and the obtained suspension immediately agitated for 120 min. After the contact time elapsed, the suspension was centrifuged and the supernatant syringed and analyzed using a UV/visible spectrophotometer (UV/vis DU 730, Beckman Coulter, Pasadena, CA, USA) at maximum absorption wavelengths of 665 and 480 nm for MB and AV 19 dyes, respectively. The amount of MB and AV 19 adsorbed, qe (mg/g) was obtained using Equation 1: where C 0 and C f are the initial and final concentrations of dyes(mg/L), respectively, V is the volume of the solution (L) and W is the amount of adsorbent used (g). All tests were performed in triplicates to ensure reproducibility; the mean of the measurements was reported. Furthermore, all experiments were performed at room temperature (29 ± 2 0 C). The investigated ranges of the experimental variables were as follows: MB dye concentration 10 -120 mg/L, initial pH of solution 3 -11, adsorbent dosage 0.01-0.05 g and contact time of 10 -180 min.

Characterisation
The surface morphologies of BMn studied using SEM are shown in Figs 1a and b. It is observed that the surface of the BMn is non-uniform, rough and characterized with pores. Presence of deep fissures, numerous openings and folds on the surface of the bamboo fiber houses the manganese nanoparticles (nMn) and consequently leads to a synergistic adsorption and trapping of the dye molecules during the removal process. FTIR spectra of bamboo, nMn and BMn in Fig. 2, show prominent absorption bands with a strong broad O-H, prominent C-H, non-conjugated C-O stretching and an aromatic skeletal vibration at around 3417.98 cm -1 , 2939.61 cm-1, 1728.28 cm -1 and 1604.83 cm -1 , respectively, which are attributed to the presence of the bamboo in the BMn matrix. The concentration of elements present ranges from Si to Ni as shown in Table 1. Many of the elements detected had values above 4000 mg/L except for Ni, Ti, Ca and K which were below 500 mg/L. The concentrations of the Mn and Cl in BMn were evidently higher than other elements present at about 109879 and 46723 mg/L respectively due to the composition of the starting materials used for the preparation.  Effect of Initial Concentration The initial dye concentration plays a vital role in surmounting the mass transfer resistance of the dyes between the aqueous and the solid phase. The quantity of dyes adsorbed per gram increased with increase in concentration for both MB and AV 19 (Fig 3), suggesting that at lower MB concentration, there were many vacant adsorption sites available for the dye molecules to attach to until the surface of the adsorbent was saturated at 120 mg/L where the amount of dyes adsorbed did not increase significantly. In addition, similar to the report by Aniba & Haris, 2010 and Samiey & Farhadi 2014, the sorption capacities of BMn increased from 23.071-252.91 mg/g and 15.2-187.2 mg/g for MB and AV 19, respectively as the initial concentration of the dyes increased from 10-120 mg/L.   Adsorption Isotherm Langmuir and Freundlich adsorption isotherm models were used to test the equilibrium data of MB and AV 19 adsorption onto BMn. The Langmuir equation (Eq. 2), which is a widely used model, is valid for monolayer sorption on a surface containing a finite number of adsorption sites, predicting a homogeneous distribution of sorption energies while the heterogeneity of adsorption sites and sorption energy are deduced from Freundlich equation. This equation is also applicable to multilayer adsorption (Eq. 3).
where Ce is the equilibrium concentration of the adsorbate (mg/L), qe the amount of adsorbate adsorbed per unit mass of adsorbate (mg/L), Qo and b are Langmuir constants related to adsorption capacity and rate of adsorption, respectively. KF and n are Freundlich constants, n is an indication of how favourable the adsorption process and KF is the adsorption capacity of the adsorbent. The data for the adsorption of MB and AV 19 onto BMn fitted more to Freundlich isotherm as shown in Fig. 7 with regression coefficient of 0.986 and 0.982, respectively. This indicates that the dyes are physisorbed on the surface of the adsorbent. The good agreement of Freundlich's isotherm with the adsorption data may be due to heterogeneous distribution of active sites on BMn. The model (Eq. 2) assumes the surface of adsorbents to be non-uniform and characterized by multilayer adsorption. The Freundlich constant n (1.3348) indicates that the adsorption is favorable. However, Langmiur isotherm (Fig. 8) poorly interpretes the adsorption data for MB and AV 19 with a regression coefficients of 0.185 and 0.278, respectively. To study the kinetics of the adsorption of MB and AV 19 onto BMn, three different kinetic models were used for the analysis of the data of each of the adsorbents, viz: pseudo-first order, pseudo-second order and Elovich models. Evidently, the adsorption data fitted more to the pseudo-second order with a regression coefficient of 0.9995 and 0.998 for MB and AV 19, respectively as shown in Figs. 9a -9c and Table 3.

Conclusion
This study confirmed that the nMn-bamboo prepared by the borohydride reduction method was effective for the removal of MB and AV 19 dyes from aqueous solution. The results showed that the adsorption is highly influenced by the initial concentration, solution's pH, adsorbent dosage and contact time. The adsorption data were adequately interpreted by Freundlich adsorption isotherm while the kinetic data were better explained by pseudo-second order kinetic model.