Adsorption Isotherms, Kinetics and Thermodynamic Studies of the removal of Manganese from Aqueous Solutions using Activated Carbon Produced from Lantana camara stem

This study investigated the equilibrium adsorption mechanism and thermodynamics of Manganese (II) ion (Mn) present in wastewater onto the activated carbon produced from Lantana camara stem as a low cost adsorbent. The Lantana camara stem was carbonized at 300  ̊C for 2 h, ground and chemical-activated. The Chemical-Activated Lantana camara (CALC) stem carbon was characterized using Scanning Electron Microscope (SEM) and Fourier Transform Infrared (FTIR) Spectrophotometry before and after adsorption of Mn. Batch experiments were conducted at different temperature (35 65  ̊C) to study the effects of pH, contact time, adsorbent dosage, initial concentration and temperature of Manganese ion. The FTIR bands at 3425, 2366, 1635, 1446, 1288, and 1116 cm were shifted to 3903, 3331, 2378, 2339, 2090, 1845, 1398 and 1342 cm after Mn adsorption. Similarly, the Scanning Electron Microscopy (SEM) analysis indicated that the average pore size on the activated carbon was 50 μm this shows that H3PO4 was effective in creating well-developed pores on the surface morphology of the precursor. The Manganese (II) ion (Mn) uptake increased with increase in pH from 2 to 6. However the uptake of Manganese (II) ion decreased slightly from pH of 8 to 12. The adsorption of Manganese (II) ion having the maximum percentage removal at pH 6. Similarly, there was an increase in the uptake of Mn as the dosage increases from 0.5 to 1.0 g∙L and shows slight decreased from 1.5 to 2.5 g∙L, at different metal ion concentrations (50 – 200 mg∙L) The percentage removal decreased with increasing initial Mn concentration for CALC. The Mn adsorption increased as contact time increased, and reached equilibrium at 120 minutes. The adsorption capacity decreases more as temperature of the reaction medium increases from 35 to 65 °C, which indicates the exothermic adsorption process of Mn onto CALC. The equilibrium adsorption data were analyzed using different model equations such as Langmuir, Freundlich, Temkin and DubininRadushkevich (D-R) isotherms. The experimental results were reasonably correlated by Langmuir (R = 0.9915) than other three isotherm models. The maximum adsorption capacity (qm), intensity of adsorption (b) and separation factor (RL) were calculated from Langmuir plot and activation energy of adsorption (Ea) was determined from D-R isotherm. The adsorption kinetic data were analyzed using pseudoInternational journal of Chemistry, Mathematics and Physics (IJCMP) [Vol-4, Issue-6, Nov-Dec, 2020] https://dx.doi.org/10.22161/ijcmp.4.6.1 ISSN: 2456-866X http://www.aipublications.com/ijcmp/ Page | 96 Open Access first-order, pseudo-second-order, intraparticle diffusion and Elovich models. It was found that the pseudo-second order kinetic model was the most appropriate model, describing the adsorption kinetics model (R = 0.99). Thermodynamic parameters such as changes in the free energy of adsorption (ΔG), enthalpy (ΔH) and entropy (ΔS) were calculated. The negative values of ΔG indicate that the Manganese (II) ion adsorption process is spontaneous in nature and the negative value of ΔH shows the exothermic nature of the process. The adsorption capacity was evaluated as 147.65 mg/g at 35 °C showing that Lantana camara stem is a promising adsorbent and can be used effectively for the adsorption of metal ions in wastewater. Keywords— Adsorption, Lantana camara, Manganese, Chemical Activation, Thermodynamic.


INTRODUCTION
The Naturally occurring Manganese (II) (Mn +2 ) is commonly found in drinking water sources and is essential for human health at low concentrations. On the other hand, excessive ingestion of Mn +2 can result in respiratory, reproductive, and especially neurological adverse effects for human beings [1]. According to World Health Organization (WHO), a Mn +2 concentrations of 0.05 mg/L in drinking water are usually acceptable to consumers. The permissible limit of Mn +2 in drinking water as set by the World Health Organization (WHO) is 0.4 mg/l [2]. Pollution due to the presence of heavy metal ions in water and wastewaters has been a major cause of concern for environmental engineers. There are numerous metals which are significantly toxic to human beings and ecological environments; they include Chromium, Copper, Lead, Cadmium, Mercury, Zinc, Manganese, Nickel, [3]. Heavy metals constitute an important part of environmental pollutants and source of poisoning [4]. Heavy metals are toxic to humans and they persist in the environment. The need for processes to remove heavy metals has received increasing attention [5,6,7].
The presence of heavy metal ions is a major concern due to their toxicity to many life forms such as humans, aquatic animals and other living organisms in the environment. Heavy metal contamination exists in aqueous wastes of many industries, such as metal plating, mining operations, tanneries, chloralkali, radiator manufacturing, smelting, alloy industries and storage batteries industries, etc. [8]. Treatment processes for heavy metal removal from wastewater include precipitation, membrane filtration, ion exchange, adsorption, and co-precipitation/ adsorption. Studies on the treatment of effluent bearing heavy metal have revealed adsorption to be a highly effective technique for the removal of heavy metal from waste stream and activated carbon has been widely used as an adsorbent [9].
Several researchers have reported the potential use of agricultural by-products as good adsorbents for the removal of metal ions from aqueous solutions and wastewaters. This process attempts to put into use the principle of using waste to treat waste and become even more efficient because these agricultural by-products are readily available and often pose waste disposal problems. Hence, since they are waste products, they are more cost-effective when compared with the conventional adsorbents like activated carbon. Also, the use of agricultural by-products for wastewater treatment does not involve complicated regeneration process [10]. Many attempts to convert carbonaceous materials into activated carbon for heavy metals removal have been reported in the literature [11]. These include, coconut shell [12], rubber wood sawdust [13], Shells of palm tree [14], Chrysophyllum albidum seed shell [15] Cocoa pod husk Cocoa Pod [16], Lantana camara stem [17],Cocoa pod husk [18], Lantana camara stem [19] etc. The aim of this present work is to investigate the use of Lantana camara stem as an adsorbent for removing heavy metals from aqueous solutions.

Preparation of chemical activated carbon from Lantana camara stems (CALC)
The fresh Lantana camara stem used in this research were obtained locally, from the premises of Obada farmer's market at Ipapo area of Oke-Ogun in Oyo State, Nigeria. The stems were separated from the leaves, seeds and flower. The stem were cut into a small pieces and washed with tap water and distilled water to remove sand, debris and other impurities. The sample then sundried for 48 hours until constant weight.
It was carbonized at 300 ˚C for 2 h, ground with mortar and pestle and sieved to 2 mm mesh size. The Lantana camara stem powder were activated using Phosphoric acid (H3PO4) for the synthesis of activated carbon from Lantana camara Open Access stem (CALC) in order to improve the surface properties of the raw powder. The activated carbon sample were then soaked overnight in 1% sodium bicarbonate solution, washed with 0.1M NaOH followed by distilled water repeatedly to remove residual minerals and acid until reached pH 7 after which it oven dry at 110 °C for 24 h and kept in desiccators until further use [20].

Preparation of the Adsorbate
All the chemicals used in this study were of analytical grade and were used without further purification. The adsorbate was prepared by weighing 1 g of MnCl4 powder and dissolving it in small quantity of distilled water in 1000 mL capacity volumetric flask. Distilled water was then added to make up to the mark of the 1000 mL volumetric flask and then stirred rigorously for 5 min to ensure homogeneity and this makes the stock solution (1000 mg/L). Varying concentrations (50 -200 mg/L) of the stock solution were prepared by serial dilution. The pH of Manganese (II) (Mn +2 ) solutions was adjusted with 0.1 M NaOH or HCl using Equation (1). The initial concentrations of the metal ions were determined using atomic adsorption spectrophotometer AAS (FS-244AA).
C1V1 ≡ C2V2 (1) where C1 is the initial dye concentration; C2 is the final dye concentration; V1 is the initial volume of solution sand V2 is the final volume of solution [21]

Batch Adsorption Experiments
1.0 g of the adsorbent (CALC) was equilibrated with 100 mL of the Manganese (II) (Mn +2 ) solution of known concentration (50 -200 mg/L) in stoppered borosil glass flask at a different temperature (35 -65 °C) in an orbital shaker at a constant speed of 145 rpm for 30-120 min. After equilibrium, 30 mL sample was collected from each flask in time intervals of (30, 45, 60 and 120 min); the suspension of the adsorbent was separated from the solution by filtration using centrifugation (EUTECH Instrument) at 2000 rpm for 5 minutes. The concentration of Manganese (II) ion (Mn +2 ) in the solution was measured by atomic absorption spectrophotometer (FS-244AA). The temperature of the solution was maintained at 35°C. The metal ion concentration was measured until the equilibrium was reached. The removal efficiency (% of adsorption or rate of adsorption) of the Manganese (II) ion (Mn +2 ) was calculated using the equation below: where C 0 (mg/L) is the initial metal ion concentration, C e (mg/L) is the metal ion concentration at equilibrium.

Batch adsorption studies
The procedures of kinetic experiments were basically identical to those of equilibrium tests. The aqueous samples were taken at preset time intervals, and the concentrations of Mn 2+ were similarly measured. The amount of adsorption at time t, t (mg g −1) was calculated using eqn 3 where C 0 and C (mg l −1 ) are the liquid-phase concentrations of dye at initial and any time t , respectively. V is the volume of the solution and W is the mass of dry adsorbent (g).

III. ADSORPTION ISOTHERMS
The adsorption isotherm indicates how the adsorption molecules distribute between the liquid and solid phases when the adsorption process reaches an equilibrium state. The analysis of equilibrium adsorption data, by fitting them to different isotherm models. Moreover, analysis of adsorption Isotherm is an important step in environmental pollution control, the key step is to develop an equation which will accurately represent the result and which can be used for design purposes, [22,23]. Adsorption isotherm in this study was carried out on four well-known isotherms, such as Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R). The applicability of the isotherm equation was compared by judging the correlation coefficients, (R 2 ).

Langmuir Adsorption Isotherm (Model)
The Langmuir equation is valid for monolayer adsorption onto a completely homogenous surface with a finite number of identical sites and with the negligible interaction between adsorbed molecules. The linearised form of Langmuir adsorption model. The Langmuir adsorption isotherm model is successfully used to explain the metal ions adsorbed from the aqueous solutions [24]. The expression of the Langmuir model is given by equation.
Rearranging equation 4, to give equations 5 and 6, makes the equation linearized,

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Equation (6) could also be rewritten as expressed in equation Where, (mg/g) and b (L/mg) are Langmuir constants. is the amount of solute removed or adsorbed at equilibrium (mg/g).
is equilibrium concentration of the mixture (mg/L).The values of and b can be determined respectively from the slope and intercept of the plot of versus [25].
It has been well documented that the essential characteristics of the Langmuir Isotherm may be expressed in terms of the dimensionless equilibrium parameter, , [26,27] also known as the separation factor, given by: has been defined as the isotherm shape that predicts if an adsorption system is favourable or unfavourable.

Freundlich Adsorption Isotherm Mode)
The Freundlich Isotherm is an empirical relationship which often gives a more satisfactory model of experimental data [30]. This isotherm is derived from the assumption that the adsorption sites are distributed exponentially with respect to the heat of adsorbed and given by Freundlich [31]. The Freundlich isotherm is an empirical equation employed to describe heterogenous systems. The Freundlich equation is expressed as: Where, is the measure of adsorption capacity and n is the adsorption intensity.
However, the Linearized form of Freundlich equation can be expressed in the form; and are equilibrium concentration and adsorption capacity at equilibrium stage. While and n are Freundlich constants which incorporate all factors affecting the adsorption process (adsorption capacity and intensity). Values of and n can be obtained from the intercept and slope of a plot of adsorption capacity, against equilibrium concentration, . Both parameters and n affect the adsorption Isotherm. The larger the values and n, the higher the adsorption capacity. Also, the magnitude of the exponent n gives an indication of the favourability of the adsorption process. When the value of n is between 2 and 10 it indicates good adsorption value [32].

Dubinin-Radushkevich (D-R)
isotherm model was also investigated to evaluate the mean energy of adsorption. It is represented in the linear form by the equation [33].
where q is amount adsorbed per gram of the adsorbent (mg/g), is equilibrium adsorption capacity (mg/g) using model, β is Polanyi potential, ε is activity coefficient, is concentration of metal ion in solution at equilibrium(mg/L), R is universal gas constant (8.314J/K/mol) and T is temperature in Kelvin. The values of β and are evaluated from the slope and intercept of the graph of ln versus £ 2 (figs 11-3). The mean energy of adsorption E (kJ mol −1 ) is calculated from the relation [34]:

Temkin adsorption isotherm
Temkin adsorption isotherm contains a factor that is explicitly entered into the adsorbent adsorbate interactions and characterized by a uniform distribution of the binding energy since it assumes that the heat of adsorption of all molecules in the layer decreases linearly with the coverage owning to adsorbent-adsorbate interactions [35]. The fitting was carried out by plotting the quantity adsorbed against ln . The constants were determined from the slope and the intercept.

Adsorption Kinetics
The study of the kinetics of adsorption is desirable as it provides information about the mechanism and characteristics of adsorption, which is important for efficiency of the process [36]. Adsorption kinetic modeling is very useful for better understanding of Manganese (II) adsorption mechanisms onto Lantana camara such as the importance of the chemical reactions and the intra-particle diffusion processes. The kinetics of Mn 2+ adsorption onto CALC was investigated using four different models: pseudo-first-order kinetic, pseudo-second-order, Elovich Equation and Intra-particle diffusion equation kinetic models were tested at different concentrations in this study to determine which model is in good agreement with experimental (adsorption capacity) value, thus suggesting which model the adsorption system.

Thermodynamic Studies
The Gibbs free energy (ΔG°), the entropy (ΔS°) and the enthalpy (ΔH°) were estimated to determine the feasibility and nature of adsorption process. For determination of ΔH° and ΔS°, the relationship between adsorption equilibrium constant b and Gibbs free energy was calculated at all temperature: using eqns (20) and (21) = ΔH°+ ΔS° (20) The plot of as a function of 1 should give a linear relationship with slope of ΔH° and an intercept of ΔS°. Then ΔG° is obtained at any temperature from the following equation:

Fourier Transform Infrared (FTIR) analysis
The FTIR spectra give information about the characteristic functional groups on the surface of the sample before and after the adsorption of Mn 2+ to ascertain the possible involvement of the functional groups on the surface of CALC in the adsorption of Mn 2+ presented in Figure. 1a and 1b.  Plate 1a and 1b: shows the scanning electron micrograph (SEM) of Lantana camara Activated Carbon before (a) and after (b) adsorption of Manganese (II) ion. There was a significantly microscopic morphology change in both plates.

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. From the plate 1a, it is clear that CALC surface was rough, uneven and the pores were not properly developed in shape, whereas, in Plate 1(b), there are several pores formed on the CALC. This shows that H3PO4 was effective in creating welldeveloped pores on the surface morphology of the precursor, thus, leading to activated carbon with large surface area and porous surface structure. These pores provided a good surface for Mn 2+ to be trapped and adsorbed [20,30].

Effect of pH on Adsorption of Mn 2+ onto ACLC
The pH of the aqueous solution is an important controlling parameter in the adsorption process.
It was observed that the pH was affected by the amount of Manganese (II) ion as shown in fig.2. The percentage removal for the adsorption of Manganese (II) ion increased with increase in pH from 2 to 6. However the uptake of Manganese (II) ion decreased slightly from pH of 8 to 12. The adsorption of Manganese (II) ion having the maximum percentage Open Access removal at pH 6. The minimum percentage removal observed at low pH was due to large amount of hydroxonium ion (H3O + ) which competed with the positively charged metal ions (Mn 2+ ) for the adsorbent surface sites, therefore, the available surface area and subsequent adsorption of the metal ion was reduced. The increase in metal ion removal as pH increases could be explained on the basis of a decrease in competition between hydroxonium ions and metal species for the surface sites and also by the decrease in positive surface charge on the adsorbents, which resulted in a lower electrostatic repulsion between the surface and the metal ions. and hence uptake of metal ions increased. A similar report was proposed by several earlier workers for metal adsorption on different adsorbents [20,37].  Fig. 3 shows that the limited number of the adsorbing species is present for a relatively larger number of available surface sites on the adsorbent at higher dosages. This support the fact that at higher dosages of the adsorbent there would be higher availability of exchangeable sites from metals ions uptake [29]. The highest adsorption was observed when the dose was increased from 0.5 to 1.0 g. further addition of the adsorbent beyond 1g dose uptake; this is probably due to overlapping of adsorption sites a consequence of overcrowding of adsorbent particles. The optimum removal of Mn 2+ (88.80%) was obtained in the adsorbent dose of 1.0 g.

Effect of Initial Concentrations on Adsorption of Mn 2+ onto CALC
The effect of initial concentration of the metal ions on the adsorption rate of Mn 2+ from the aqueous solution was studied for the metal ion concentrations ranging from 50 -200 ppm. Figure 4. Shows the effect of initial concentrations of Mn 2+ on % removal by CALC. The % removal decreased with increasing initial Mn 2+ concentrations. The lower uptake at higher concentration resulted from an increased ratio of initial adsorption number of moles of the metal ions to the available surface area; hence fractionally becomes dependent on initial concentration. The initial metal ions concentration provides an important driving force to overcome the mass transfer resistance of the metal ions between the aqueous and solid phases. Therefore, at higher initial metal ions concentration, the number of ions competing for the available sites on the surface of CALC was high, resulting in higher Mn 2+ adsorption capacity [38]. Similar results were reported by [28]. With 1.0 g of adsorbent at pH 4 the maximum adsorption is obtained for concentrations 50 -100 ppm.

Effect of contact time on adsorption of Mn 2+ onto CALC
The contact time is one the effective factor in batch adsorption process and it is necessary to determine the contact time to reach equilibrium. The Mn 2+ adsorption uptake was increased as contact time increased from 35-120 mins and reaches equilibrium at 120 minutes (Figure .5). The result suggests that, adsorption takes place rapidly at the initial stage on the external surface of the adsorbent followed by a slower internal diffusion process, which may be the rate determining step [20,39]. In addition, the fast adsorption at the initial stage may be due to the fact that a large number of surface sites are available for adsorption but with time, the remaining surface sites are difficult to be occupied. This is because of the repulsion between the solute molecules of the solid and bulk phases, thus, taking longer time to reach equilibrium [40].   [41]; this phenomenon could be explained by the adsorption of Manganese (II) ions caused by an increase in the thermal energy available. Increasing the temperature would induce greater mobility of the adsorbate, leading to the adsorption of Manganese (II) ions [42].   Table 1.  Table 1.  Table 1.   [43]. In the present study n value was found to be lesser than 1. Accordingly, it can be inferred that CALC is applicable for removal of Mn 2+ from aqueous solutions. Also, the activation energy (Ea), resulting from Dubinin-Radushkevich equation, was found as 13.984, which indicates that the adsorption occurred with ion exchange since this value falls within 8-16 kJ/mol interval. R 2 was found as 0.9578 for Temkin isotherm, indicating that adsorption also conforms to Temkin model. b value was found as 0.098 g.J/mg.mol. This low b value is an indication of exothermic adsorption [44].

Effect of temperature and adsorption thermodynamics
The optimum temperature for manganese (II) ion adsorption of CALC was 35 °C and fall within the temperature range studied. Thermodynamic parameters Gibbs free energy change, (ΔG o ) was calculated using Langmuir constants ( Table 2). The enthalpy change, (ΔH o ) and the entropy change,( ΔS o ), for the adsorption process were calculated to give the values −3.13 and 11.23 J/mol K, respectively. The negative values of ΔG° confirm the feasibility of the process and the spontaneous nature of adsorption due to high preference for Mn 2+ onto CALC. The value of (ΔH o ) is negative, indicating that the adsorption reaction is exothermic. The positive value of (ΔS°) reflects the affinity of the CALC for Mn 2+ and suggests some structural changes in Mn 2+ and CALC interaction [34]. The thermodynamics parameters ΔG°, ΔH o and ΔS° of different temperature was tabulated in table 2. The Pseudo-first-order-model can be expressed [45]; [46]; [47]; [48].
Where, , (mg/g) are the mass of the metal ion adsorbed at equilibrium (adsorptive capacity), and mass adsorbed at any time't', 1 , (min -1 ) is the equilibrium constant of the pseudofirst-order adsorption. The value of 1 and are determined respectively from the slope and intercept of the plot of ( − ) versus t.

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The corresponding pseudo-second-order model is expressed thus [34]; Where, 2 (gmg -1 min -1 ), t, and , are the Pseudo-secondorder rate constant, time, and adsorption capacity at equilibrium respectively. The value of is determined from the plot of versus t. The intraparticle diffusion equation can be written as follows [49]: where is the amount of the adsorbate adsorbed at time t, C is the intercept, and is the intraparticle diffusion rate constant (mg g −1 min −1 ). The kinetic results were tested by the intraparticle model to elucidate the diffusion mechanism. The values of correlated linearly with the values of 0.5 (Shown in Figure: 13), and the rate constant was directly evaluated from the slope of the regression line. The intercept of the plot reflects the boundary layer effect. The larger the intercept, the greater the contribution of the surface adsorption in the rate controlling step. The calculated intraparticle diffusion coefficient values are listed in Table 3. If the regression of vs. 0.5 is linear and passes through the origin, then intraparticle diffusion is the sole rate-limiting step. [49].

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where α is the initial adsorption rate (mg/g.min) and β is the adsorption constant of surface coverage and activation energy for chemisorption (g/mg) during any experiment. The slope and intercept of the plot of vs. ln can be used to calculate the values of the constants α and β, respectively. There was increased in initial adsorption rate α and decreased in desorption constant β with an increase in initial dye concentration. A similar result was reported by [51]. The kinetic parameters calculated using four different models pseudo first order, pseudo second order, intra particle diffusion and elovich were tabulated in table 3. From the comparative analysis of all the four tested models, it was observed that the pseudo second order model have the highest R 2 0.99 for all the concentration and temperature used for the experimental studies the predictions of for all the tested conditions was closed to the experimental value of which show that these was a good agreement between the experimental and predicted values. These indicate that the adsorption system is probably best described by the pseudosecond order kinetic model. Manganese ions were adsorbed on the surface of activated carbon via chemical interaction such as ionic or covalent bonds [52]. Similar phenomena for biosorption of heavy metal ions were reported in previous studies [53,54,55]. Kinetic models such as intra-particle diffusion model and Elovich equation demonstrate an important role of the pore diffusion process in the adsorption mechanism. As seen in Figure 13 and 14 respectively the Mn 2+ adsorption on CALC coincides with the origin for all pH values and it occurs in two steps. The rate constants for two resultant steps were in the order 1 < 2 , and the C values at each step increased with increasing pH. This is an indication of increased in boundary layer thickness with increasing pH, which in turn results in a decrease in the diffusion rate. The first and the second linear sections can be respectively attributed to the external mass transfer and intra particle diffusion mechanisms [51,53].

Test of kinetic models
Besides the value of R 2 , the applicability of both pseudo-first order kinetic model and pseudo-second order kinetic model out of the four model used are verified through the sum of error squares (SSE, %). The adsorption kinetics of Mn 2+ on activated carbon derived from CALC was tested at different initial concentrations. The validity of each model was determined by the sum or error squares (SSE, %) given by: where N is the number of data points. The higher is the value of R 2 and the lower is the value of SSE; the better will be the goodness of fit. Table 3 lists the calculated results. It is found that the adsorption of manganese (II) ion onto CALC produced from Lantana camara stem can be best described by the second-order kinetic model. Similar phenomena processes have been observed in the adsorption of methylene blue on activated carbon prepared from Chrysophyllum albidum seed shell [41] and Scavenging Rhodamine B dye using Moringa Oleifra seed pop [44].

V. CONCLUSION
This present study described the adsorption of manganese (II) onto activated carbon produced from Lantana camara stem. The suggested adsorbent showed promising results in removing metal ions from synthetic wastewater under the studied conditions. In batch adsorption studies, data showed that activated Lantana camara stem had relatively considerable potential for the removal of manganese (II) ion from aqueous solution. Langmuir isotherm fitted very well with studied temperature and concentration ranges. The RL values showed that activated Lantana camara stem was favorable for the adsorption of manganese (II) ion. The suitability of the kinetic models for the adsorption of manganese (II) onto activated Lantana camara stem was also investigated. It was clear that the adsorption kinetics of manganese (II) onto activated Lantana camara stem followed