Removal of Pb (II) from Synthetic Solution and Paint Industry Wastewater Using Activated Carbon Derived from African Arrowroot (Canna indica) Stem

Department of Environmental Engineering, College of Biological and Chemical Engineering, Addis Ababa Science and Technology University, Addis Ababa 16417, Ethiopia Bioprocess and Biotechnology Center of Excellence, Addis Ababa Science and Technology University, Addis Ababa 16417, Ethiopia Department of Biotechnology, College of Biological and Chemical Engineering, Addis Ababa Science and Technology University, Addis Ababa 16417, Ethiopia


Introduction
Water and land contamination by heavy metals discharged from industrial wastes has become a global problem during the current years [1]. e rapid development of various industrial activities and technologies discharged heavy metals into the environment which highly affected the environment and human health due to their toxicity, bioaccumulation, and bioaugmentation in the food chain and persistence in nature [2]. Heavy metals like arsenic, chromium, copper, mercury, nickel, and silver are among the most widely known toxins found in modern effluents [3].
However, lead is a substantial heavy metal found in wastewater from the paint industry which is toxic to life, even at low concentrations, and can affect the nervous and reproductive system [3,4]. To remove heavy metals from industrial effluent, precipitation, ion exchange, coagulation, electrodialysis, etc. are the most commonly used technologies [5]. ese technologies have numerous disadvantages such as incomplete metal ion removal, high energy and reagent costs, and toxic sludge [6]. However, adsorption techniques look to be more attractive due to their simplicity, ease of use, high efficiency, and being economical in the removal of heavy metals from wastewater [7].
Commercial activated carbon is the most commonly used adsorbent but not cost-effective [8]. erefore, many researchers were prompted to search for low cost and equally workable adsorbents. Conversion of lignocellulose content to activated carbon is a possible and feasible approach [1]. Some previous studies have reported the production of activated carbon from agricultural residues and other materials for removing Pb (II) ions from wastewater, for instance, Banana steam [9], Apricot stone [10], Almond shell [11], winemaking waste [12], and pine cone [13]. However, the Canna indica found in Ethiopia (which is planted to decorate homes and public parks) is not yet studied for its potential as an adsorbent.
is research investigated the potential of Canna indica stem activated carbon to determine its Pb (II) ion adsorption capacity from aqueous solution and paint wastewater.

Preparation of the Adsorbent.
e experiment was conducted in an environmental engineering laboratory at Addis Ababa Science and Technology University. e Canna indica stems were collected from the locally available canna indica garden in Addis Ababa city, Ethiopia. en, the stems were reduced from 1.5 to 3 cm using a knife and sun-dried for 5 days. e dirt was removed by washing with distilled water and dried in an oven at 105°C for 24 h. en, it was ground using pestle and mortar, and a 100 g sample was taken and mixed with 100 ml of concentrated phosphoric acid (85% w/w) for 12 h. After that, it was dried in an oven at 105°C for 24 h and carbonized using a rectangular electrical muffle furnace (Nabertherm F 330) at 500°C with a heating rate of 25°C/min for 1 h [14]. e carbonized sample was then taken out of the furnace and cooled in a desiccator and continuously washed with distilled water until a neutral solution was achieved. Finally, it was dried in an oven at 105°C for 24 h, ground and sieved using 125 µm sieve, and stored in plastic bottles for further use.

Characterization of the Adsorbent.
e proximate analysis (moisture, volatile matter, ash, and fixed carbon content) and iodine number were determined with ASTM (D 2866-2869) and (D4607-94) method, respectively. e functional groups on the prepared adsorbent were determined using a Fourier Transform Infrared (Shimadzu IRAffinity-1s) spectrometer in the spectral range of 4000-400 cm −1 and the data were analyzed by using standard software (Origin 2018 Version 9.55). e X-ray diffraction analysis was also determined using X-ray diffraction (Rigaku Miniflex 600 diffractometer). e XRD was operated at Cu Ka, 40 kV/40 mA, and a current of 15 mA. e X-ray diffraction patterns were collected with a scan rate of 4.2°C/min and the results were analyzed using standard software (Origin 2018 Version 9.55).

Preparation of Aqueous Solution.
e lead (II) stock solution (1000 mg/L) was prepared by dissolving 1.5985 g of Pb (NO 3 ) 2 (99.8%) with 1000 ml distilled water. e lead standard working solution was prepared by using the following equation: where C 1 is the initial concentration, C 2 is the final concentration, V 1 is the initial volume, and V 2 is the final volume

Collection and Processing of the Paint Industry Effluent.
Polythene bottle (1500 ml) was washed with tap water, thoroughly cleaned with hydrochloric acid, and resined with distilled water to make it acid-free, being used to collect the samples. e effluent samples were collected from the paint industry in Addis Ababa (the subcity of Nefas Silk Lafto (latitude: 8°58′50.29″, longitude: 38°44′57.6″)). e effluent was taken in the morning and afternoon for three days during the first week of July, as there was no further treatment. e sample was immediately transported to the laboratory allowed to settle the solids for 1 h and analyzed within 24 h. e heavy metals (Pb (II), Cu (II)) and physicochemical characteristics (BOD 5 , COD, TSS, turbidity, pH, and temperature) of the effluent before and after adsorption were characterized using the APHA (2010) method.

Experimental Methods.
e experiments were done to optimize the influence of experimental factors such as pH (3, 5.5, and 8), adsorbent dose (0.5, 1, and 1.5 g), and initial concentration of Pb (II) ions (50, 100, and 150 mg/ L) for the removal efficiency of Pb (II) ion from aqueous solution. Using the optimal experimental conditions, the real paint industry wastewater was examined for the removal efficiency of Pb (II) ion. e experiments were conducted in a 250 ml Erlenmeyer flask with a constant speed of 250 rpm and a contact time of 120 min at room temperature [15] on a batch basis. en, it was filtered using 0.45 μm Whatman filter paper to separate from the adsorbent. e residual Pb (II) ion concentration was determined by using Microwave Plasma Atomic Emission Spectrometry (Agilent mp-aes 4200). All of the experiments were performed in triplicate. e removal efficiency and adsorption capacity qe (mg/g) were calculated using the following equations, respectively: where C o and C e are the initial and the final Pb (II) ion concentration (mg/L), respectively, V is the adsorbate volume (L), and m is the mass of adsorbent (g).

Experiment Design and
Optimization. e experimental design for process optimization and its statistical analysis was carried out using Design Expert ® software version 7.0.0. e relationship between dependent response variables and a set of quantitative experimental factors (independent 2 Advances in Materials Science and Engineering variables) was analyzed by using the Box-Behnken factorial surface (3 levels and 3 factors). is design was utilized to determine the effect of three factors initial Pb (II) ion concentration, adsorbent dosage, and pH on the removal efficiency of Pb (II) ion using Canna indica-activated carbon over three levels. e ranges and levels of the experimental parameters are shown in Table 1. e total number of experiments was determined using the following equation: where N is the number of experimental runs, F represents the factor number, and x o is the number of replicates at the central point. In this study, the values of N, F, and xo were 17, 3, and 5, respectively. e relationship between coded and actual values of variables was measured using the following equation: wherex i is the dimensionless coded value of the i th independent variable, X o is the value of X i at the center point, and Δx is the step change value. A second-order polynomial response surface model for the fitting of experimental data was calculated using the following equation: where y, x i , b o , and b i represent the predicted response, independent variables, constant offset term, and linear coefficients, respectively. Furthermore, b ii represents the regression coefficients for the quadratic and b ij represents interaction effects.

Adsorption Isotherms.
Langmuir and Freundlich isotherm models were used in this work: the Langmuir isotherm theory assumes monolayer adsorbent distribution over a homogeneous adsorbent surface. e Langmuir isotherm is presented in the following equation: where q m is the maximum amount of metal ion adsorbed capacity (mg/g), qe is the amount of metal ion per unit mass of adsorbent at equilibrium (mg/g), K L is a constant related to the binding energy of adsorption, and the other constants can be estimated by plotting Ce versus qe. e Freundlich isotherm is an empirical equation describing heterogeneous surface adsorption. e Freundlich isotherm is commonly presented as shown in the following equation: where k f is the Freundlich constant related to adsorption capacity (mg/g) and n is the Freundlich exponent (dimensionless). By taking the logarithmic function of equation (8), it is simplified to the following equation:

Proximate and Iodine Number
Analysis. e results of proximate and iodine number analysis values are presented in Table 2. In observation of the data of proximate analysis, CISAc shows low ash content (5%), medium content of volatile matter (26.7%), low moisture content (5.4%), and a high percentage of fixed carbon (62.9%). e moisture content value was lower than the values reported by Olugbenga et al. [16] and Ozdemir et al. [36] while studying the activated carbon from Pawpaw (Carica papaya) leaf and grape stalk, respectively. is indicates that CISAC may have better removal potential due to its lower moisture content [17]. However, the volatile matter contents of this were more than the value obtained from Banana (Musa paradisiaca) stalk-based activated carbon [18]. e fixed carbon for CISAC was better than the activated carbons made from Banana empty fruit bunch, Delonix regia fruit pod [19], and pumpkin seed shell [20]. e medium content of volatile matter and the low ash content usually increase the solid yield of the carbon and produce high fixed carbon [21]. Better performance was obtained when there is a high microstructure which is directly correlated with the iodine value. e higher number of carbons in iodine was due to the presence of a large structure of micropores and the high likelihood of carbons having a large surface area due to the enlargement of their pore structure [22]. e iodine value is significantly affected by activation temperature and time (Mopoung et al. [14]) and Kumar et al. [23] reported an activation time of 1 to 2 hr, and the activation temperature within 500 to 600°C increases the microstructure. e iodine number of the CISAC was greater than a value obtained from activated carbon from Cassava peels [24] and Lapsi (Choerospondias axillaris) seed stone [25].
is shows that CISAC has a better removal capacity as a higher iodine number of carbons credited to the nearness of large micropore shape and to have expansive surface vicinity due to the broadening of their pore structure [26]. Table 3 presents the FTIR spectral characteristics of CISAC based on pH (7 and 5.5), initial Pb (II) ion concentration (0 and 50 mg/L), and adsorbent dosage (1 and 1.5) before and after Pb (II) ion adsorption, respectively. e FTIR spectrum of the activated carbon shows a significant difference in peak frequencies due to the binding of Pb (II) ion with active sites of the activated carbon indicating the presence of ionizable functional groups on the activated carbon which has the potential to interact with other cations [4]. e FTIR spectra of CISAC before and after adsorption of Pb (II) ion are revealed in Figure 1. e recorded spectra give different adsorption peaks which represent the presence of various functional groups in the CISAC. e spectra of CISAC before adsorption of Pb (II) ion band at 3450 cm −1 representing stretching vibrations of O-H in hydroxyl group and band at 2374 cm −1 assigned to C-H stretching indicate the methyl and methylene [27]. Besides band at 1620 cm −1 stretching of the acetyl group in hemicellulose, band at 1383 cm −1 indicated stretching in the aromatic ring, band at 1313 cm −1 attributed to N-O stretching vibration, and band at 1166 cm −1 and 1040 cm −1 associated with the C-O stretching of the aryl group in lignin, respectively [21]. All of the assigned wave numbers of CISAC after adsorption are different from before adsorption except for one of them where many wavenumbers shift during the adsorption process.

XRD
Analysis. X-ray diffraction was used to assess the presence of amorphous and crystalline between the matrixes of carbon. e X-ray powder diffraction spectra are presented in Figure 2. According to the powder diffraction results, the CISAC shows two broad intense peaks at 2ϴ � 23°and 2ϴ � 24°.
ese were a sign of crystalline graphite formation inside the carbon [28]. e spectra pattern exhibits a persistent decrease in crest concentration as a result collapse of the graphite layers. is can be an ordinary amorphous carbon arrangement [29].

Paint Industry Untreated and Treated Wastewater
Characteristics. Table 4 presents the untreated and treated wastewater characteristics collected from the paint industry. e concentrations of most parameters for the untreated wastewater, except for pH and temperature, were above the WHO standard. Particularly, the Pb (II) ions concentration was 46 times higher than the standard; hence, it must be removed to avoid environmental risks. e treated wastewater characteristics were below the standard set by WHO (2017) except for COD and TSS. Hence, further treatment is needed to remove the excess COD and TSS before disposing to the environment. ough the research was concentrated on Pb (II) removal, [30] reported the biochar made from Canna indica has the potential to remove cadmium from an aqueous solution which shows that it might have a potential to remove other heavy metals.

Effects of pH.
e influences of pH were investigated in the range of 3-8 under constant initial Pb (II) concentration of 100 mg/L, a contact time of 120, and an adsorbent dose of 1.5 g. Figure 3(a) displays the Pb (II) ions removal efficiency was increased when the solution of pH increased from 3 to 5.5. e better removal of Pb (II) ion was achieved at pH 5.5 while the removal decreased under highly acidic and moderate basic conditions. A similar finding was reported by Gundogdu et al. [31]. At a highly acidic pH, the overall surface charge on the active site becomes positive and metal cations and protons compete for the binding site of the adsorbent [32]. e removal efficiency was decreased when the values of pH increased from 5.5 to 8. In this condition, Pb (II) ions were precipitated in the form of Pb (OH) 2 [33]. Related trends were reported for the adsorption of Pb (II) ions on the activated carbon prepared from coconut shell [34].

Effects of Initial Pb (II) Ions Concentration.
e better removal was achieved at the initial Pb (II) ion concentration of 50 mg/L (Figure 3(b)). e elimination of Pb (II) ion was decreased with the increase in concentration from 50 to 150 mg/L; because of low concentration, there is a low number of lead ions to the ration of the surface-active site found in the adsorbent surface. erefore, all of the lead ions may interact with the active site. In contrast when higher initial concentration proved more lead ion for being attached on the adsorbent surface, as a result, the active site is not sufficient and saturation in the adsorbent has happened which resulted in the reduction of removal efficiency.

Effects of Adsorbent Dosage.
e effect of an adsorbent dose was investigated by altering the adsorbent in the range of 0.5-1.5 g/100 ml. e fixed parameters were pH of 5.5, a contact time of 120 min, and an initial Pb (II) ion concentration of 100 mg/L. Figure 3(c) shows the removal of Pb (II) ion was increased while the adsorbent dose increased  Advances in Materials Science and Engineering from 0.5 to 1.5 g. It is due to a constant initial concentration whilst growing the adsorbent dose gives a higher adsorption surface area. However, a reverse trend was observed for adsorption capacity. A similar result was reported in the elimination of Pb (II) ion from aqueous solution using bamboo-based activated carbon as adsorbent [35]. e elimination of lead (II) increased with an increase in the adsorbent dosage and then it remains almost constant which leads to better removal at 1.5 g of adsorbent.

Process Optimization and Effects of Factors Interaction
Response Surface Methodology. 3D plots can be drawn for a different combination of parameters which show the trend of Advances in Materials Science and Engineering variation of response within the selected range of input factors ((pH, initial Pb (II) ions concentration, and an adsorbent dose) and the influence of each parameter over the other parameters. Few such typical plots are shown in Figure 4. Figure 4(a) shows the 3D plot, the combined effects of pH (x 1 ), and initial Pb (II) ion concentration (x 2 ) on the elimination efficiency of Pb (II) ion by keeping the adsorbent dose constant. e result from Figure 4(a) indicates that the elimination efficiency was increased with an increase in the solution of pH from 3 to 5.5 with decreasing the Pb (II) ion concentration from 150 to 50 mg/L and then decreased when further increasing pH from 5.5 to 8. e 3D surface plot in Figure 4(b) displays the Pb (II) ion removal efficiency as the combined effects of adsorbent dose (x 3 ) and pH (X 1 ) in maintaining the initial Pb (II) ion concentration constant. e removal of Pb (II) ion decreased when the pH decreased from 5.5 to 3 with the decrease in the adsorbent dose from 1.5 to 0.5 g. en, the removal efficiency increased when the pH decreased from 8 to 5.5 and the adsorbent dose increased from 0.5 to 1.5 g. e 3D plot which was constructed to show the most significant two factors of adsorbent dose (X 3 ) and initial Pb (II) ion concentration (x 2 ) with keeping pH constant is revealed in Figure 4(c). From Figure 4(c), it can be observed  that the adsorption capacity increased when the adsorbent dose increased from 0.5 to 1.5 g and the initial Pb (II) ion concentration decreased from 150 to 50 mg/L. e increase in the adsorbent dose provided a greater surface area or increased available adsorptions sites which increased the amount of Pb (II) adsorbed. e optimization process was performed by numerical optimization defined in the Design-Expert software. In numerical optimization, the program seeks to maximize the desirability function to create the optimal condition. All the three factors and Pb (II) removal efficiency were set in experimental ranges for the maximum desirability. Optimization was done by considering the values of parameters for better removal for Pb (II) ion from aqueous solution using CISAC, that is, pH value of 5.5, adsorbent dose of 1.35 g, and initial Pb (II) ions concentration of 102.37 mg/L with the desirability of 1 ( Figure 5). erefore, using these values, the removal efficiencies of Pb (II) ion from aqueous solution and paint industry wastewater were found to be 98% and 70%. Pb (II) ion removal efficiency of CISAC in paint wastewater was much lower than in aqueous solution because paint industry wastewater contained different types of heavy metals (Cu +2 , Cr +3 , and Zn +2 ), binders, additives, biological oxygen demand (BOD 5 ), chemical oxygen demand (COD), total suspended solids and turbidity that affected the Pb (II) ion removal efficiency by competing one another in the adsorbent site.

Adsorption Equilibrium Isotherm Studies.
e equilibrium isotherm experiments were carried out using 1.5 g/ 100 ml of Canna indica activated carbon with pH of 5.5 and the initial Pb (II) ions concentrations (50, 100, and150 mg/L) under room temperature. e adsorbate-adsorbent solutions were mixed at a constant speed of 250 rpm for an equilibrium time of 120 min. Figures 6(a) and 6(b) represent the Langmuir and Freundlich isotherm adsorption of Pb (II) ion on CISAC, and Table 5    isotherms. e correlation coefficients (R 2 ) of Langmuir and Freundlich isotherms models are 0.9884 and 0.9461, respectively. us, the adsorption of Pb (II) on CISAC fits the model of Langmuir very well.

Conclusion
e present study shows that CISAC is a good adsorbent for the removal of Pb (II) ions from aqueous solution and paint industry wastewater. Better removal (98%) was achieved at an initial Pb (II) ion concentration of 102.4 mg/L, an adsorbent dosage of 1.4 g, and pH of 5.5. With the same condition, 69.7% of Pb (II) ion was removed from the paint industry wastewater. e performance of the CISAC for the removal of Pb (II) ion in the paint industry wastewater was much lower than in aqueous because the paint industry wastewater contains various types of pollutants. Adsorption isotherm showed that the Langmuir isotherm model provides the best correlation of R 2 (0.9884). e treated wastewater characteristics were below the standard set by WHO (2017) except for COD and TSS. Hence, further treatment is needed to remove the excess COD and TSS before disposing to the environment. e findings of the current study suggest that the adsorption process using   Data Availability e information used in this study can be obtained from the corresponding author upon request.

Conflicts of Interest
e authors have no conflicts of interest.