115 Journal of Multidisciplinary Applied Natural Science Vol. 2 No. 2 (2022) Research Article Removal of Toxic Cations from Aqueous Solutions using Ginger Root Waste Jude Chinedu Onwuka*, Stephen Igberi Azubuike, and Timothy M. Akpomie Received : April 4, 2022 Revised : May 26, 2022 Accepted : May 28, 2022 Online : May 29, 2022 Abstract Recently, the harmful impact of toxic metals in the aquatic environment cannot be over emphasized again. This work investigated the potential application of ginger root waste (GRW) to remove toxic cations (Cd 2+ and Pb 2+ ) from the aqueous medium. Batch adsorption examination was carried out as a function of sorbent dose, initial metal ion concentration, contact time, and temperature. The sorption equilibrium of the metal ions onto the GRW was subjected to Langmuir, Freundlich, Elovich and Redlich-Peterson isotherm models over concentration ranges of 10-50 mg/L. Sorption information was used for kinetic and thermodynamic modeling. The GRW materials before and after sorption was characterized using FTIR and SEM. Results showed higher removal percentage of Cd 2+ over Pb 2+ ions in all the factors studied. The Redlich – Peterson isotherm model affirmed that sorption of Cd 2+ and Pb 2+ occurred in a heterogenous surface of the sorbent which is strongly influenced by multiple micropores and caves. Kinetic studies revealed that the sorption was controlled through intra-particle diffusion model aided by surface and chemical reactions. Meanwhile, thermodynamic parameters indicated that the Cd 2+ and Pb 2+ sorption process was endothermic, however, non- spontaneous at temperature of 303 and 313 K. The FTIR and SEM data showed the evidence of successful sorption of the toxic cations on to the sorbent material. Keywords Ginger root waste, sorption, toxic cations, kinetics, thermodynamics 1. INTRODUCTION Heavy metal contamination is a serious challenge to the human race and the natural ecosystem. The environmental hazard and their effects on human existence are very serious [1] due to their accumulation in the human body and inability to biodegrade into harmless waste [2]. Today, the dangerous global impact of these metals on the ecosystem has become a non-push-over reality that is very difficult to be circumvented. Environmental pollution by heavy metals occurs by means of industrialization and extraction of natural resources [3]. Most industrial discharge and effluent contain high amounts of heavy metals which consequently constitute serious concern due to the high toxicity it portends to the living organism [4]. Organic wastes are quite susceptible to biodegradation, but heavy metals are not simply degraded by any biological means into harmless by- products [2]. The global increase in industrialization has accelerated industrial applications of heavy metals mainly as the catalyst and thus, increased their presence in wastewater giving rise to environmental pollution and wastewater contamination. Among the heavy metals, cadmium and lead, which are quite carcinogenic, are often found in industrial effluents such as those related to mining, metallurgy, petrochemicals, electroplating, batteries, pharmaceutical industries, paper, pulp, plastic, and paint manufacturing [5]. These metals are presumed toxic since they are not metabolized by the human body rather, they keep accumulating in the human tissues [6]-[9]. Thus, it becomes incumbent to search for the possible way of removing these toxic metals from the aqueous medium. As a result of the many health challenges associated with contamination of water bodies by heavy metals, a lot of techniques such as chemical precipitation, ion exchange, electro-dialysis, membrane separation, and nanofiltration have been implored in preventing the presence of metal cations in our drinking water samples. However, there are obvious limitations inherent with the application of these techniques [10] including less efficiency and sensitivity, generation of sludge, high maintenance cost, high energy and reagent requirement, and inability to sequestrate metal ions at low concentration [11]. These limitations Copyright Holder: © Onwuka, J. C., Azubuike, S. I., and Akpomie, T. M. (2022) First Publication Right: Journal of Multidisciplinary Applied Natural Science Publisher’s Note: Pandawa Institute stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Article is Licensed Under: https://doi.org/10.47352/jmans.2774-3047.126 OPEN ACCESS https://creativecommons.org/licenses/by-sa/4.0/deed.id https://doi.org/10.47352/jmans.2774-3047.126 https://crossmark.crossref.org/dialog/?doi=10.47352/jmans.2774-3047.126&domain=pdf&date_stamp=2022-06-14 J. Multidiscip. Appl. Nat. Sci. 116 Table 1. Data isolated from equilibrium adsorption isotherm plots. Toxic cation Langmuir Isotherm Freundlich Isotherm Elovich Isotherm Redlich-Peterson qmax b RL R 2 Kf n R 2 qm Ke R 2 β A R 2 Cd 2+ -0.9832 -0.1709 0.8562 0.7691 0.0463 0.3493 0.9640 * -3.2927 1.5095 0.8868 1.8622 0.0463 0.9187 Pb 2+ 4.0016 0.0646 0.7946 0.0045 11.1970 -1.0092 0.0495 1.6292 5.4016 0.9817 * 1.9906 11.1931 0.1735 Note: * means highest R 2 values. R 2 values that are moderate are bolded prompted individuals and research institutes to engage themselves in finding the best viable technique to adequately bring this menace into limits acceptable by standard organizations and one of such options is well provided in the adsorption process [12]. The adsorption process is cheap, eco- friendly and cost-effective as this process does not produce sludge and does not require high energy and reagents [10][11]. Globally, safe drinking water is everyone's concern, as it is difficult to find anybody who has not been affected by waterborne ailment [13]. Termed as a “universal solvent”, a higher percentage of the global population depends on water for drinking and domestic uses [10]. Due to the indispensable daily need and use of water across the globe, it has been made vulnerable and susceptible to being contaminated by the heavy metal discharge of domestic and industrial effluents, radioactive waste, mining waste, leakages from oil pipelines and atmospheric deposition [14]. Consequently, this has made water one of the key sources of global infection and diseases [15]. Statistics show that 80% of diseases and infections globally are waterborne and there is the propensity that it will be doubled if nothing is done in the near future [16]. At the moment, the global concern on heavy metal accumulation in drinking water and its effects on the human body is on the increase. The apprehension is that no biological means of degradation have been identified for these set of elements [3]. Health experts opined those natural products such as ginger drink are of great benefit to the human body [17]. Recent discoveries showed that it prevents nausea and vomiting, muscle pain and curbs cancer growth, soothes sore throat, and prevents common cold [18][19]. Nutritionally, the ginger drink is rich in antioxidants which are necessary for combating activities of free radicals in the body [12]. According to Adanlawao and Dairo [20], the ginger drink is quite rich in potassium, an important mineral for biochemical reactions in the body. Chronic intake of synthetic beverages majority of which are soft drinks is susceptible to dental erosion, obesity with a high risk of cardiovascular dysfunction and type 2-diabetes [18]. Due to these obvious health concerns on synthetic drink consumption, the focus has been shifted towards the consumption of natural drinks such as ginger root drinks [21]. Unfortunately, after the aqueous extraction of the ginger root, the wastes are dumped indiscriminately, thus, constituting an environmental nuisance. Previous research [12] utilized ginger root in removing selected toxic metal cations. However, this research aims to study the potential of using ginger root waste (GRW) to specifically remove Cd 2+ and Pb 2+ from the aqueous solution. 2. MATERIALS AND METHODS 2.1. Sample Collection and Preparation The fresh ginger roots used for this research (201 g) were obtained from the Lafia main market, in Nasarawa State. It was transported in a polyethylene bag to the Department of Chemistry Laboratory, Federal University of Lafia. It was washed and rinsed severally with deionized water to remove dust and adhering soil particles. The thin outer covers were carefully peeled using stainless steel knife. The sample was washed again and sliced into bits and pounded using a ceramic mortar. The pounded sample was placed in a stainless basin, and 2 L of deionized water was added. The mixture was allowed to stand for 10 min, stirred and then filtered. The residue obtained after the juice extraction was re-soaked in 2 L of deionized water for another 10 min. This process was repeated for further three consecutive times and filtered after each soaking. The chaff obtained after this series of extraction is referred to as the ginger root waste. The waste was air-dried for 24 h and then oven- dried for 72 h at 80 °C. The dried sample was J. Multidiscip. Appl. Nat. Sci. 117 ground into fine powder using ceramic mortar and pestle and sieved to less than 1 mm fine particle size with a sieve and stored in a plastic sealable bag ready for the chemical analysis. 2.2. Batch Sorption Studies The experimental solution was prepared by diluting the stock solution earlier prepared with deionized water. A 50 mL portion of the prepared 10 mg/L metal ion solution was measured into 250 mL plastic container and 0.5 g of the sorbent (GRW) was added. The plastic container was closed and then placed on a shaker (Bernareggio MI, PID system type M 428-Bd) with a temperature- controlled water bath. It was shaken continuously for 60 min at 27 °C and 120 rpm. The supernatant was filtered using 0.2 mm Whatman filter paper. The filtrate was analyzed using atomic absorption spectrophotometry (PG990) in order to ascertain the equilibrium metal ion concentrations. Various parameters of the adsorption process were studied as a function of sorbent dose, initial metal ion concentration, contact time and temperature of the solution. The effect of sorbent dose (0.5, 1.0, 1.5, 2.0, and 2.5 g) was studied by fixing the volume (50 mL) and concentration (10 mg/L) of toxic cations as well as the contact time (60 min) and temperature (27 °C). The effect of initial metal ion concentration (10, 20, 30, 40, and 50 mg/L) was carried out by metal ions sorption at a constant sorbent dose, contact time and temperature. The contact time effect (30, 60, 90, 120, and 150 min) was studied under various temperatures (303, 313, 323, and 343 K) while keeping other factors constant. Similarly, the variation of temperature (303, 313, 323, and 343 K) was carried out at various contact times (30, 60, 90, 120 and 150 min). The amount of metal ion sorbed per unit mass was calculated using a mass balance as shown Eq. 1. (1) Where qe is heavy metal ion concentration absorbed onto the sorbent at equilibrium (mg/g), Ci and Ce are initial and equilibrium concentrations (mg/L) of the metal ions, respectively, m is mass (mg) of sorbent, and V is the volume (L) of sorbate solution. The adsorption or removal percentage (%) was calculated using Eq. 2. (2) 2.3. Adsorption Isotherm Modeling This experiment was used to establish the relationship between the amount of the toxic cations (Cd 2+ and Pb 2+ ) that was adsorbed by the GRW sorbent GRW and the equilibrium concentrations of the toxic cations at a constant temperature [12]. Langmuir, Freundlich, Elovich and Redlich- Peterson isotherm models were used to evaluate how the toxic cations (adsorbate) interacted with the GRW sorbent. This study was conducted by fitting the obtained data from the effect of initial metal ion concentration into the isotherm models. 2.3.1. Langmuir Isotherm The equation was developed by Irving Langmuir (1916). The linearized form of the Langmuir model as shown in Eq. 3, was used in this study. Figure 1. Effect of sorbent dose on Cd 2+ and Pb 2+ sorption. Figure 2. Effect of initial metal ion concentration on Cd 2+ and Pb 2+ sorption. J. Multidiscip. Appl. Nat. Sci. 118 (3) Where Ce is concentration at equilibrium or final concentration of the metal in solution (mg/L), qe is adsorption efficiency or the amount of the adsorbate adsorbed at equilibrium (mg/g), qmax is the maximum adsorption capacity (mg/g), and b is the Langmuir constant related to the energy of adsorption (L/mg). A plot of the graph Ce/qe against Ce, produces 1/qmax as the slope, and thus, qmax was calculated. The value of b which is the Langmuir constant was evaluated from the intercept of the graph which is 1/b x qmax [22]. Dimensionless equilibrium function known as RL value was evaluated using Eq. 4 expressed as: (4) When RL = 1, a linear adsorption occurred. If RL = 0, there is a irreversible adsorption. But when 0< RL < 1, then a favorable adsorption occurred. 2.3.2. Freundlich Isotherm The linear form of Freundlich isotherm demonstrated in Eq. 5, was applied. (5) A plot of logqe against logCe produced 1/n as the slope, from where n was evaluated, while from the intercept (logkf), kf was estimated. It should be noted that kf and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively [23]. Figure 3. Effect of contact time for Cd 2+ sorption at various temperatures. Figure 4. Effect of contact time for Pb 2+ sorption at various temperatures. 2.3.3. Elovich Isotherm The linear form of Elovich isotherm used is given as Eq. 6. (6) The plot of against gives a straight line. Thus, Elovich maximum adsorption capacity (qm) and Elovich constant (Ke) were calculated from the slope ( )and intercept ( ), respectively. 2.3.4. Redlich-Peterson Isotherm The linear form of the Redlich-Peterson isotherm used is shown in Eq. 7. (7) Where A is the Redlich-Peterson isotherm constant (L/g), β (L/mg) is the exponent, Ce (mg/L) is the liquid-phase equilibrium concentration of the sorbent (GRW) while qe is the sorbate (Cd 2+ and Pb 2+ ) equilibrium onto the GRW sorbent. When the graph of InCe/qe vs InCe is plotted, the slope is β while the intercept is lnA. When β value is below or equal to 1, the Redlich-Peterson isotherm is confined to Langmuir isotherm, however, when β value is above 1, the Redlich-Peterson isotherm tends to Freundlich isotherm. The constant “A” indicates the sorption capacity [24][25]. 2.4. Kinetic Studies This was studied to measure the uptake of the toxic cations as a function of time at a constant concentration so as to determine the mechanism or reaction pathway involved in the toxic cation uptake by the sorbent. Thus, data obtained from the J. Multidiscip. Appl. Nat. Sci. 119 effect of contact time were analyzed using pseudo– first order, pseudo–second order, intra–particle diffusion and liquid film diffusion models. 2.4.1. Pseudo First–Order kinetic Pseudo first-order kinetic was carried out to establish the contribution of physisorption or surface reaction, to the uptake of the toxic cations by the sorbent. According to Amadi et al. [26], the linear form of pseudo first–order kinetics is given below. (8) The plot of log(qe-qt) against time ‘t’, gave - as the slope and logqe as the intercept [27]. 2.4.2. Pseudo Second–Order Kinetic The contribution of chemisorption was evaluated by analyzing the data with pseudo-second order kinetic model. The linear form of the pseudo-second order kinetic is stated Eq. 9. (9) The plot of against t, gives k2 as the slope and as the intercept. 2.4.3. Intra–Particle Diffusion The equation for the linear form of intra-particle diffusion kinetics is given Eq 10. qt = kd × t 1/2 + C (10) Thus, when qt is plotted against t 1/2 , the slope is the rate constant kd in (mg/(g min 1/2 )) and the intercept ‘C’ in (mg/g). The importance of the intercept is that it explains the thickness of the boundary of the layer. The characteristic significance of the value of the constant ‘C’ is that the larger the value, the greater the boundary layer thickness [22][27]. 2.4.4. Liquid–Film Diffusion This was used to investigate the rate at which the toxic cations in the form of a film glide over one another [25]. The linear form of liquid film diffusion model is given below. In(1-α) = klft + C (11) Where α = qt/qe, klf is the liquid film diffusion rate constant. A plot of In(1-α) against time (t) yields the constant klf (min -1 ) as the slope and a dimensionless constant C as the intercept [27]. 2.5. Thermodynamic Studies Thermodynamic parameters such as enthalpy change (∆H), entropy change (∆S), and Gibbs’ free energy (∆G), of the adsorption processes were determined. The Arrhenius equation described by Onwuka et al. [27], was used for the thermodynamic modeling, the equation is expressed in Eq. 12. (12) The enthalpy change (∆H) was determined from the slope of the plot of versus using Eq.12. The heat capacity (Cp) and entropy change, (∆S), were determined from Eqs. (13) and (14) respectively. (13) (14) The Gibbs’ free energy was determined using Eq. 15. Source of Variation SS df MS Fcal P-value Fcrit Cd 2+ Between Groups 10.46042 4 2.615106 0.005798 0.999927 2.866081 Within Groups 9021.483 20 451.0741 Total 9031.943 24 Pb 2+ Between Groups 2178.819 4 544.7049 1.127699 0.371616 2.866081 Within Groups 9660.465 20 483.0232 Total 11839.28 24 Table 2. ANOVA for effect of contact time on the sorption of Cd 2+ and Pb 2+ at various temperatures. J. Multidiscip. Appl. Nat. Sci. 120 Source of Variation SS df MS F P-value Fcrit Cd 2+ Between Groups 175150.1 5 35030.03 1247.75 3.31705E-22 2.772853 Within Groups 505.3421 18 28.07456 Total 175655.5 23 Pb 2+ Between Groups 189251.5 5 37850.31 277.7577 2.22831E-16 2.772853 Within Groups 2452.877 18 136.2709 Total 191704.4 23 Table 3. ANOVA for effect of temperature for Cd 2+ and Pb 2+ sorption at different time intervals. (15) Where is the amount adsorbed at temperature T, is the amount adsorbed at temperature , R is the universal gas constant, is the heat capacity of the system at pressure p, and are the initial and final pressures respectively, and are the initial and final temperatures, respectively. 2.6. Characterization 2.6.1. Fourier Transform Infra-Red (FT-IR) Analysis The KBr pellet was prepared by mixing about 1 mg of the sorbent with 250 mg KBr (FT-IR grade), pressed and placed for acquisition of IR spectrum on SHIMADZU model (8400S) of the FT-IR equipment. The spectra were recorded between 4000–400 cm -1 resolutions of 4 cm -1 . This was carried out at the National Research Institute for Chemical Technology (NARICT) Zaria, Kaduna State. 2.6.2. Scanning Electron Microscope (SEM) Analysis The morphology of the GRW sorbent before and after metal ion sorption, was investigated using Scanning Electron Microscope (SEM; JSM- 5610LV). The micrographs were taken at 1000 times magnification. 3. RESULTS AND DISCUSSIONS 3.1. Effect of Sorbent Dose The result of the effect of sorbent dose on the metal ions (Cd 2+ and Pb 2+ ) removal efficiency is presented in Fig. 1. The result shows that the sorption capacity is greatly influenced by the amount of sorbent material in the aqueous solution since the percentage sorption of the toxic metal ions increased in a higher sorbent dose. This suggests that there are more active binding sites, pores and caves at higher amount of the sorbent as similarly reported by Kampalanonwat and Supaphol [28] in a related investigation of heavy metal ion adsorption by animated polyacrylonitrile nanofibre mats. Increasing the sorbent dose also reduces the distance metal ions needed to travel in order to be entrapped by active pores and caves. A higher amount of the sorbent in solution results in a proportionate increase in the surface area which enhanced the sorption of more metal cations. A similar observation was reported by Vijayakurma et al. [29] and Shooto et al. [12] on natural pelite and ginger root, respectively. It was also observed in Fig 1 that increment in sorbent dose leads to higher Cd 2+ adsorption through a maximum while Pb 2+ adsorption increased continuously with the range of sorbent dose studied. This suggests that Cd 2+ adsorption attained equilibrium faster than Pb 2+ adsorption within the studied sorbent dosages. This can be attributed to the smaller ionic size of Cd 2+ (compared to Pb 2+ ) which makes it more mobile and consequently results in more interaction with the active sites on the sorbent. 3.2. Effect of Initial Metal ion Concentrations Fig. 2 shows the effect of initial metal ions concentration on the sorption of the toxic metal ions from the aqueous medium. It is observed that the removal of the metal ions increased with increase in the initial metal ions concentration. This suggests that at lower metal ion concentration, desorption of these metal cations from the sorbent occurs more than their adsorption on the sorbent. It also indicates that the metal ions interacted more with J. Multidiscip. Appl. Nat. Sci. 121 Temp (K) Pseudo First–order Pseudo Second–order Intra–particle Diffusion Liquid Film Diffusion qexp K1 R 2 qcal K2 R 2 qcal Kd R 2 C Klf R 2 C Cd 2+ 303 0.012 0.086 0.132 0.540 0.282 -0.458 0.006 0.583 a 1.769 0.011 0.069 2.506 1.847 313 0.009 0.072 0.145 0.264 0.381 0.278 0.006 0.540 a 1.769 0.009 0.051 2.447 1.862 323 0.007 0.053 0.129 0.301 0.178 0.205 0.005 0.383 a 1.794 0.008 0.040 2.542 1.874 333 0.007 0.050 0.132 0.291 0.157 0.204 0.005 0.340 a 1.798 0.007 0.034 2.558 1.881 Pb 2+ 303 0.000 0.004 0.205 0.026 0.002 0.081 0.040 0.772 a 0.954 0.000 0.000 2.002 1.456 313 0.021 0.470 1.189 0.221 0.480 -0.135 0.065 0.633 a 0.993 0.021 0.375 0.275 1.749 323 0.014 0.173 0.308 0.296 0.291 -0.879 0.022 0.590 a 1.548 0.015 0.139 1.627 1.810 333 0.002 0.017 0.252 0.018 0.062 0.229 0.008 0.113 1.744 0.002 0.010 1.900 1.927 Table 4. Data Isolated from Kinetic Plots. Note: a means highest R 2 values. R 2 values that are moderate are bolded the sorbent active binding sites and caves at their higher metal ion concentrations compared to lower metal ion concentrations [30]. This can also imply that the ionic orientation favours sorption at higher initial metal ions concentration which is ascribed to the fact that the force of repulsion at higher concentrations has a minimum barrier to the sorption of the metal cations [31]. It was also observed in Fig. 2 just as in Fig. 1, that the sorption of Cd 2+ from an aqueous medium was higher than that of Pb 2+ . This disparity can be attributed to the differences in the atomic/ionic radius between the two toxic cations. Egila et al. [32], in similar research on Amarathus hydridus L. stalk waste, reported that atoms with lower atomic radii show more tendencies to be sorbed easily than atoms with larger atomic radii. The atomic radii of Cd 2+ is 0.97 Å while that of Pb 2+ is 1.20 Å. Thus, this made it relatively difficult for Pb 2+ ions to infiltrate the lignocellulosic matrix component of the GRW sorbent and hence, result in lower Pb 2+ sorption compared to Cd 2+ sorption. Chukwuemeka et al. (2015), Onwuka et al. [27] and Shooto et al. [12] reported similar findings in their works on ginger, Delonix regia pods and ginger root, respectively. 3.3. Adsorption Isotherm Studies The nature of the interaction between the toxic cations and the sorbent was assessed using Langmuir, Freundlich, Elovich and Redlich– Peterson isotherm models. Parameters evaluated and isolated from the isotherm plots in Equations 3, 5, 6 and 7 are summarized in Table 1. Generally, the value of coefficient of determination (R 2 ) in Table 1 shows that the order of best fit isotherm model for Cd 2+ sorption onto the GRW sorbent is Freundlich > Redlich–Peterson > Elovich > Langmuir isotherm while that of Pb 2+ sorption is Elovich > Redlich–Peterson > Freundlich > Langmuir isotherm. RL value is the most important feature of the Langmuir adsorption isotherm that helps to explain and predict the degree of interaction and compatibility between the sorbate and sorbent in the aqueous solution [22][33]. Favourable Langmuir isotherm modeling is such that 0